Showing 1–50 of 132 results for author: Shashank

Spin-Orbital Hall Nano-Oscillators using PtCr/NiFe

Authors: Utkarsh Shashank, Akash Kumar, Daegeun Jo, Thi Ngoc Anh Nguyen, Jong-Guk Choi, Sambit Ghosh, Michal Strach, Lunjie Zeng, Andrew B. Yankovich, Roman Khymyn, Ahmad A. Awad, Eva Olsson, Peter M. Oppeneer, Johan Åkerman

Abstract: The orbital Hall effect provides a promising route for generating angular-momentum currents beyond conventional spin Hall physics. PtCr alloys exhibit unusually large current-induced torques, but the contribution of orbital transport and the ability of these torques to sustain coherent nonlinear magnetization dynamics remain unresolved. Here we demonstrate spin-orbital Hall nano-oscillators by exp… ▽ More The orbital Hall effect provides a promising route for generating angular-momentum currents beyond conventional spin Hall physics. PtCr alloys exhibit unusually large current-induced torques, but the contribution of orbital transport and the ability of these torques to sustain coherent nonlinear magnetization dynamics remain unresolved. Here we demonstrate spin-orbital Hall nano-oscillators by exploiting a homogeneous heavy-metal/light-metal alloy in which orbital Hall currents generated by Cr are converted by Pt into spin currents, producing giant spin-orbit torques. Using PtCr/NiFe heterostructures, the effective torque efficiency increases from ~0.14 in Pt/NiFe to ~0.40 in Pt0.38Cr0.62/NiFe despite substantial Pt dilution, enabling coherent auto-oscillations with the threshold current density reduced from ~ 1.07 x 10^12 to ~ 4.4 x 10^11 A m^-2. First-principles calculations show that Cr alloying suppresses the intrinsic spin Hall conductivity while enhancing the orbital Hall conductivity, and reproduce the observed torque enhancement only when orbital transport is included. Our combined experimental and first-principles results show that alloy engineering enables giant spin-orbit torques through an intrinsic orbital-mediated contribution, enabling coherent auto-oscillations without engineered multilayers and establishing a scalable materials platform for low-power nonlinear spintronic and orbitronic devices. △ Less

Submitted 9 August, 2026; originally announced August 2026.

Comments: 20 pages, 4 figures

Unconventional Superconductivity in the Chiral Topological Semimetal Ag2Pd3S

Authors: Roshan Kumar Kushwaha, Dibyendu Samanta, Sudarshan Sharma, Mathew Pula, Shashank Srivastava, Poulami Manna, Arushi, Sajilesh K. P., Suhani Sharma, Priya Mishra, Prabin Kumar Naik, James Beare, Yipeng Cai, Kenji M. Kojima, Amit Kanigel, Graeme M. Luke, Sudeep Kumar Ghosh, Ravi Prakash Singh

Abstract: Chiral crystals provide a unique setting where broken inversion symmetry, strong spin-orbit coupling, and electronic topology intertwine, yet superconductivity in intrinsically chiral materials remains rare. Here, we report unconventional superconductivity in the chiral topological semimetal Ag$_2$Pd$_3$S, an enantiomorphic analog of natural mineral coldwellite, crystallizing in the right-handed s… ▽ More Chiral crystals provide a unique setting where broken inversion symmetry, strong spin-orbit coupling, and electronic topology intertwine, yet superconductivity in intrinsically chiral materials remains rare. Here, we report unconventional superconductivity in the chiral topological semimetal Ag$_2$Pd$_3$S, an enantiomorphic analog of natural mineral coldwellite, crystallizing in the right-handed space group $P4_132$. Bulk superconductivity with a transition temperature $T_C = 1.1(2)$ K is confirmed by electrical resistivity, magnetization, and specific-heat measurements. Muon spin rotation and relaxation ($μ$SR) experiments reveal a fully gapped superconducting state that spontaneously time-reversal symmetry (TRS) breaking establishing Ag$_2$Pd$_3$S as the first chiral topological semimetal superconductor exhibiting intrinsic TRS breaking. First-principles calculations uncover multiple multifold band crossings near the Fermi level, hosting Kramers-Weyl, double spin-1, and spin-3/2 quasiparticles with large topological charges. These unconventional fermions generate symmetry-protected topological surface states and underscore the nontrivial topology of the normal state. Symmetry analysis based on the Ginzburg-Landau theory suggests a loop-supercurrent-ordered superconducting state, yielding a full gap alongside spontaneous TRS breaking. The coexistence of TRS-breaking superconductivity and chiral multifold fermions identifies Ag$_2$Pd$_3$S as a platform for realizing intrinsic superconducting diode effects and chirality-induced spin selectivity, offering a transformative pathway toward dissipationless topological quantum technologies. △ Less

Submitted 29 June, 2026; originally announced June 2026.

Comments: 11 pages, 4 figures,

From Knowledge to Action: Outcomes of the 2025 Large Language Model (LLM) Hackathon for Applications in Materials Science and Chemistry

Authors: Aritra Roy, Kevin Shen, Andrew MacBride, Awwal Oladipupo, Mudassra Taskeen, Wojtek Treyde, Ruaa A. E. A. Abakar, Ahmad D. Abbas, Elsayed Abdelfatah, Abbas A. Abdullahi, Seham S. Abyah, Chahd Rahyl Adjmi, Fariha Agbere, Savyasanchi Aggarwal, Muhammad Ahmed, Tasnim Ahmed, Motasem Ajlouni, Mattias Akke, Hussein AlAdwan, Anwaar S. Alazani, Zahra A. Alharbi, Wajd A. Aljulyhi, Mohammed A. AlKubaish, Fatima A. Almahri, Sayed A. Almohri , et al. (328 additional authors not shown)

Abstract: Large language models (LLMs) are rapidly changing how researchers in materials science and chemistry discover, organize, and act on scientific knowledge. This paper analyzes a broad set of community-developed LLM applications in an effort to identify emerging patterns in how these systems can be used across the scientific research lifecycle. We organize the projects into two complementary categori… ▽ More Large language models (LLMs) are rapidly changing how researchers in materials science and chemistry discover, organize, and act on scientific knowledge. This paper analyzes a broad set of community-developed LLM applications in an effort to identify emerging patterns in how these systems can be used across the scientific research lifecycle. We organize the projects into two complementary categories: Knowledge Infrastructure, systems that structure, retrieve, synthesize, and validate scientific information; and Action Systems, systems that execute, coordinate, or automate scientific work across computational and experimental environments. The submissions reveal a shift from single-purpose LLM tools toward integrated, multi-agent workflows that combine retrieval, reasoning, tool use, and domain-specific validation. Prominent themes include retrieval-augmented generation as grounding infrastructure, persistent structured knowledge representations, multimodal and multilingual scientific inputs, and early progress toward laboratory-integrated closed-loop systems. Together, these results suggest that LLMs are evolving from general-purpose assistants into composable infrastructure for scientific reasoning and action. This work provides a community snapshot of that transition and a practical taxonomy for understanding emerging LLM-enabled workflows in materials science and chemistry. △ Less

Submitted 4 May, 2026; originally announced May 2026.

Comments: This paper reflects contributions from hundreds of researchers worldwide through an event, follow-on discussions, and project development exploring LLM applications in materials science and chemistry. While unconventional, it captures a timely, broad, and efficient community exploration of a rapidly evolving field and offers value to the arXiv community

Discovery of Quasi One Dimensional Superconductivity in PtPb3Bi

Authors: Shashank Srivastava, Yash Vardhan, Anshu Kataria, Pradyumna Bawankule, Poulami Manna, Prabin Kumar Naik, Rahul Verma, Rhea Stewart, James S. Lord, Adrian D. Hillier, Mathias S. Scheurer, D. T. Adroja, Bahadur Singh, Ravi Prakash Singh

Abstract: Quasi one dimensional materials provide a compelling platform where reduced dimensionality stabilizes intertwined topological and superconducting phases. Here we report superconductivity in a new Bi based quasi 1D compound, PtPb3Bi, which hosts a nontrivial electronic structure. It exhibits type II superconductivity below 3.01(1) K. Heat capacity and transverse field muon spin rotation relaxation… ▽ More Quasi one dimensional materials provide a compelling platform where reduced dimensionality stabilizes intertwined topological and superconducting phases. Here we report superconductivity in a new Bi based quasi 1D compound, PtPb3Bi, which hosts a nontrivial electronic structure. It exhibits type II superconductivity below 3.01(1) K. Heat capacity and transverse field muon spin rotation relaxation (muSR) measurements demonstrate a fully gapped isotropic s wave state with moderate electron phonon coupling, while zero field muSR confirms the preservation of time reversal symmetry (TRS). Transport measurements reveal low carrier mobility with diffusive normal state transport. Electronic structure calculations show strong dispersion along the quasi 1D direction and relatively flatter bands in the transverse plane, giving rise to pronounced Fermi surface nesting in the kx-ky plane. Consistent with this, the compound undergoes a charge density wave transition at 280(1) K. The flow of Wannier charge centers, together with surface state dispersion, establishes nontrivial band topology. These results identify PtPb3Bi as a new quasi 1D superconductor with nontrivial electronic structure and a promising candidate for topological superconductivity. △ Less

Submitted 6 April, 2026; originally announced April 2026.

Comments: 8 pages, 4 figures

Hourglass Dirac chains enable intrinsic topological superconductivity in nonsymmorphic silicides

Authors: Shashank Srivastava, Dibyendu Samanta, Pavan Kumar Meena, Poulami Manna, Priya Mishra, Suhani Sharma, Prabin Kumar Naik, Rhea Stewart, Adrian D. Hillier, Sudeep Kumar Ghosh, Ravi Prakash Singh

Abstract: Nonsymmorphic crystalline symmetries provide a robust route to symmetry-protected electronic topology, yet their role in stabilizing intrinsic topological superconductivity remains largely unexplored. Here, we report \ch{TaPtSi} as a new member of the superconducting nonsymmorphic silicide family, characterized via AC transport, magnetization, heat capacity, and muon spin rotation/relaxation ($μ$S… ▽ More Nonsymmorphic crystalline symmetries provide a robust route to symmetry-protected electronic topology, yet their role in stabilizing intrinsic topological superconductivity remains largely unexplored. Here, we report \ch{TaPtSi} as a new member of the superconducting nonsymmorphic silicide family, characterized via AC transport, magnetization, heat capacity, and muon spin rotation/relaxation ($μ$SR) measurements. Zero field $μ$SR reveals spontaneous internal magnetic fields below $T_{\rm c}$, establishing time reversal symmetry breaking in \ch{TaPtSi}. First principles calculations on \ch{TaPtSi} and its isostructural nonsymmorphic superconducting analogues reveal the presence of symmetry-protected hourglass dispersions. The "necks" of these dispersions form Dirac nodal rings and chains that reside near or intersect the Fermi level. Guided by Ginzburg Landau symmetry analysis, we identify an internally antisymmetric non unitary triplet pairing state as the unique ground state consistent with the experimental phenomenology. Based on Bogoliubov de Gennes calculations, we further demonstrate that this state supports Majorana surface modes, establishing its intrinsically topological nature. These results reveal a systematic route by which nonsymmorphic symmetry drives the interplay between hourglass Dirac chain topology and unconventional triplet pairing, positioning equiatomic silicides as a unified materials platform for intrinsic topological superconductivity. △ Less

Submitted 26 February, 2026; originally announced February 2026.

Comments: 12 pages, 5 figures

When electrons meet ferroelastic domain walls in Strontium Titanate

Authors: Shashank Kumar Ojha, Jyotirmay Maity, Srimanta Middey

Abstract: Strontium titanate (SrTiO$_3$), famously described by Nobel laureate K. A. Müller as the "drosophila of solid-state physics", has been extensively investigated over the last seventy five years for its intricate coupling of structural, electronic, and dielectric properties and continues to serve as a foundational platform for advancing oxide electronics. In its pristine form, SrTiO$_3$ exhibits qua… ▽ More Strontium titanate (SrTiO$_3$), famously described by Nobel laureate K. A. Müller as the "drosophila of solid-state physics", has been extensively investigated over the last seventy five years for its intricate coupling of structural, electronic, and dielectric properties and continues to serve as a foundational platform for advancing oxide electronics. In its pristine form, SrTiO$_3$ exhibits quantum paraelectric behavior below 35 K and undergoes an antiferrodistortive phase transition near 105 K. This transition generates ferroelastic twin domains separated by a dense network of domain walls, which function as nanoscale structural defects with far-reaching consequences. While the static influence of ferroelastic domain walls on carrier transport in electron-doped SrTiO$_3$ is well established, recent experimental results show that the emergence of polarity at these walls, combined with strain fields and inherent quantum fluctuations, induces correlated dynamical phenomena such as glass-like relaxations of electrons and memory effects. In this review, we highlight these recent advances, focusing on the subtle interplay between the emergence of nanoscale polar order, quantum fluctuations, and long-range strain fields. We propose that understanding charge carrier dynamics in the background of these complex ferroelastic domain wall landscapes offers a new paradigm for exploring electronic transport in the presence of local polar order and quantum fluctuations, with broad implications for correlated oxides. △ Less

Submitted 20 January, 2026; originally announced January 2026.

Comments: Invited review article, comments and suggestions are welcome

Topological textures and emergent altermagnetic signatures in ultrathin BiFeO3

Authors: George Fratian, Maya Ramesh, Xinyan Li, Evangelos Golias, Yousra Nahas, Sebastian Maria Ulrich Schultheis, Julian Skolaut, Marti Checa, Arundhati Ghosal, Jan Priessnitz, F. C. Fobasso Mbognou, Shashank Kumar Ojha, Shiyu Zhou, Alexander Qualls, Kai Litzius, Christoph Klewe, Peter Meisenheimer, Laurent Bellaiche, Libor Šmejkal, Darrell G. Schlom, Yimo Han, Sergei Prokhorenko, Ramamoorthy Ramesh, Paul Stevenson, Angela Wittmann , et al. (1 additional authors not shown)

Abstract: Magnetoelectric multiferroics, materials with intrinsically coupled electric polarization and magnetic order, promise ultralow-power switching, nonvolatile memory, and energy-efficient signal transduction. Yet practical deployment demands ultrathin films down to the atomic limit, where both orders typically degrade. Maintaining both order parameters at the thinnest scales in complex oxides remains… ▽ More Magnetoelectric multiferroics, materials with intrinsically coupled electric polarization and magnetic order, promise ultralow-power switching, nonvolatile memory, and energy-efficient signal transduction. Yet practical deployment demands ultrathin films down to the atomic limit, where both orders typically degrade. Maintaining both order parameters at the thinnest scales in complex oxides remains a tremendous challenge, as uncompensated bound charge drives nanoscale depolarization in most ferroelectrics, while off-stoichiometry, reduced anisotropy, and charge transfer can produce magnetic dead layers in ultrathin oxides at substrate interfaces. Here, we realize a multiferroic phase of BiFeO3 that not only sustains both order parameters at room temperature with no dead layer but also exhibits signatures of emergent altermagnetism in the four-unit-cell, ultrathin limit. First-principles calculations, spin symmetry analysis, atomic-resolution imaging, and angle-resolved magnetic imaging reveal that short-circuit electrostatic boundary conditions, together with epitaxial strain, drive a continuous second-order, thickness-driven phase transition that enables the formation of multiferroic topological textures. Moreover, the imposed boundary conditions stabilize a d-wave altermagnetic time-reversal symmetry breaking, with corresponding signatures observed in magnetic circular dichroism. Collectively, these results establish a pathway to stabilize unconventional multiferroicity at device-relevant thicknesses, reframing scaling limits for oxide electronics. △ Less

Submitted 14 January, 2026; originally announced January 2026.

Observation of Time-Reversal Symmetry Breaking in the Type-I Superconductor YbSb$_2$

Authors: Anshu Kataria, Shashank Srivastava, Dibyendu Samanta, Pushpendra Yadav, Poulami Manna, Suhani Sharma, Priya Mishra, Joel Barker, Adrian D. Hillier, Amit Agarwal, Sudeep Kumar Ghosh, Ravi Prakash Singh

Abstract: The spontaneous breaking of time-reversal symmetry is a hallmark of unconventional superconductivity, typically observed in type-II superconductors. Here, we report evidence of time-reversal symmetry breaking in the type-I superconductor YbSb$_2$. Zero-field $μ$SR measurements reveal spontaneous internal magnetic fields emerging just below the superconducting transition, while transverse-field… ▽ More The spontaneous breaking of time-reversal symmetry is a hallmark of unconventional superconductivity, typically observed in type-II superconductors. Here, we report evidence of time-reversal symmetry breaking in the type-I superconductor YbSb$_2$. Zero-field $μ$SR measurements reveal spontaneous internal magnetic fields emerging just below the superconducting transition, while transverse-field $μ$SR confirms a fully gapped type-I superconducting state. Our first-principles calculations identify YbSb$_2$ as a ${\mathbb Z}_2$ topological metal hosting a Dirac nodal line near the Fermi level. Symmetry analysis within the Ginzburg Landau framework indicates an internally antisymmetric nonunitary triplet (INT) state as the most probable superconducting ground state. Calculations based on an effective low-energy model further demonstrate that this INT state hosts gapless Majorana surface modes, establishing YbSb$_2$ as a topological superconductor. Our results highlight YbSb$_2$ as a unique material platform where type-I superconductivity coexists with triplet-pairing and nontrivial topology. △ Less

Submitted 12 January, 2026; originally announced January 2026.

Comments: 9 pages, 3 figures,

Anisotropic magnon transport in an antiferromagnetic trilayer heterostructure: is BiFeO$_3$ an altermagnet?

Authors: Sajid Husain, Maya Ramesh, Qian Song, Sergei Prokhorenko, Shashank Kumar Ojha, Surya Narayan Panda, Xinyan Li, Yousra Nahas, Yogesh Kumar, Pushpendra Gupta, Tenzin Chang, Alan Ji-in Jung, Rogério de Sousa, James G. Analytis, Lane W. Martin, Zhi Yao, Sang-Wook Cheong, Laurent Bellaiche, Manuel Bibes, Darrell G. Schlom, Ramamoorthy Ramesh

Abstract: Magnons provide a route to ultra-fast transport and non-destructive readout of spin-based information transfer. Here, we report magnon transport and its emergent anisotropic nature in BiFeO$_3$ layers confined between ultrathin layers of the antiferromagnet LaFeO$_3$. Due to the confined state, BiFeO$_3$ serves as an efficient magnon transmission channel as well as a magnetoelectric knob by which… ▽ More Magnons provide a route to ultra-fast transport and non-destructive readout of spin-based information transfer. Here, we report magnon transport and its emergent anisotropic nature in BiFeO$_3$ layers confined between ultrathin layers of the antiferromagnet LaFeO$_3$. Due to the confined state, BiFeO$_3$ serves as an efficient magnon transmission channel as well as a magnetoelectric knob by which to control the stack by means of an electric field. We discuss the mechanism of the anisotropic spin transport based on the interaction between the antiferromagnetic order and the electric field. This allows us to manipulate and amplify the spin transport in such a confined geometry. Furthermore, lower crystal symmetric and suppression of the spin cycloid in ultrathin BiFeO$_3$ stabilizes a non-trivial antiferromagnetic state exhibiting symmetry-protected spin-split bands that provide the non-trivial sign inversion of the spin current, which is a characteristic of an altermagnet. This work provides an understanding of the anisotropic spin transport in complex antiferromagnetic heterostructures where ferroelectricity and altermagnetism coexist, paving the way for a new route to realize electric-field control of a novel state of magnetism. △ Less

Submitted 7 January, 2026; originally announced January 2026.

Comments: 8 pgaes, 5 figure

Coexistence of inequivalent time-crystalline orders in a Floquet collective spin system

Authors: Shashank Mishra, Sayan Choudhury

Abstract: We investigate the dynamical phases that emerge in collective spin models subjected to a spatially non-uniform periodic drive. Taking the paradigmatic Lipkin-Meshkov-Glick (LMG) model as a concrete platform, we establish that a rich landscape of dynamical phases emerges when two regions of the system are driven with different field strengths, $h_1$ and $h_2$. Remarkably, despite the `all-to-all' n… ▽ More We investigate the dynamical phases that emerge in collective spin models subjected to a spatially non-uniform periodic drive. Taking the paradigmatic Lipkin-Meshkov-Glick (LMG) model as a concrete platform, we establish that a rich landscape of dynamical phases emerges when two regions of the system are driven with different field strengths, $h_1$ and $h_2$. Remarkably, despite the `all-to-all' nature of the interactions, the system can be driven into dynamical phases characterized by distinct kinds of discrete time crystal (DTC) orders in different parts of the system. Apart from these coexisting DTCs, tuning the driving field leads to the emergence of phases where DTCs coexist with Floquet-synchronized or oscillatory phases; the former has been dubbed a chimera DTC. Finally, we demonstrate that a tunable set of global DTC phases emerges when $h_1$ and $h_2$ are proximate. Crucially, these dynamical regimes can be observed both for experimentally relevant finite-size systems and in the thermodynamic limit. Our results establish spatially structured driving as a powerful route to realize non-equilibrium phase coexistence in collective spin systems. △ Less

Submitted 21 June, 2026; v1 submitted 23 December, 2025; originally announced December 2025.

Comments: 7+6 pages, 3+4 figures

The chiral gyrating H'-T surface family: construction from the dual qtz--qzd nets and existence proof using a toroidal Weierstrass method

Authors: Hao Chen, Shashank G. Markande, Matthias Saba, Gerd E. Schröder-Turk, Elisabetta A. Matsumoto

Abstract: This paper provides a construction and existence proof for a 1-parameter family of chiral unbalanced triply-periodic minimal surfaces of genus 4. We name these {\textit{gyrating H'-T} surfaces, because they are related to Schoen's H'-T surfaces in a similar way as the Gyroid is to the Primitive surface. Their chirality is manifest in a screw symmetry of order six. The two labyrinthine domains on e… ▽ More This paper provides a construction and existence proof for a 1-parameter family of chiral unbalanced triply-periodic minimal surfaces of genus 4. We name these {\textit{gyrating H'-T} surfaces, because they are related to Schoen's H'-T surfaces in a similar way as the Gyroid is to the Primitive surface. Their chirality is manifest in a screw symmetry of order six. The two labyrinthine domains on either side of the surface are not congruent, rather one representing the quartz net (\texttt{qtz}) and the other one the dual of the quartz net (\texttt{qzd}). The family tends to the Scherk saddle tower in one limit and to the doubly periodic Scherk surface in the other. The motivation for the construction was to construct a chiral tunable unbalanced surface family, originally as a template for photonic materials. The numeric construction is based on reverse-engineering of the tubular surface of two suitably chosen dual nets, using the \textit{Surface Evolver}} to minimize area or curvature variations. The existence is proved using Weierstrass parametrizations defined on the branched torus. △ Less

Submitted 20 December, 2025; originally announced December 2025.

Comments: 17 pages. 11 figures

Probing the intermediate state of type-I superconductor SnAs using Muon Spin Spectroscopy

Authors: Shashank Srivastava, Omkar Kulkarni, Arushi, Deepak Singh, Poulami Manna, Priya Mishra, Suhani Sharma, Pabitra Kumar Biswas, Rhea Stewart, Adrian D. Hillier, Ravi Prakash Singh

Abstract: Superconductivity with non-trivial band topology provides a novel platform for exploring topological superconductivity and its quantum applications. A detailed microscopic understanding of the superconducting ground state in such materials is crucial. Here, we report the results of a muon spin rotation/relaxation study ($μ$SR) of the topologically non-trivial superconductor SnAs, which exhibits su… ▽ More Superconductivity with non-trivial band topology provides a novel platform for exploring topological superconductivity and its quantum applications. A detailed microscopic understanding of the superconducting ground state in such materials is crucial. Here, we report the results of a muon spin rotation/relaxation study ($μ$SR) of the topologically non-trivial superconductor SnAs, which exhibits superconductivity below 3.74(1) \si{K}. Zero-field (ZF) $μ$SR data reveal that this system is a time-reversal invariant superconductor, and systematic transverse-field (TF) $μ$SR measurements unveil the type-I nature of the SnAs superconductor. We have established the superconducting phase diagram to understand the intermediate state of type-I superconductors. Moreover, ab \textit{initio} band structure and phonon calculations are performed, which correlate with the experimental characterization. △ Less

Submitted 19 December, 2025; originally announced December 2025.

Comments: 8 pages, 6 figures

Magnetic transition in B2 Al-Cr-Co alloys

Authors: Haireguli Aihemaiti, Esmat Dastanpour, Shashank Chaturvedi, Shuo Huang, Anders Bergman, Levente Vitos

Abstract: Using Density Functional Theory (DFT) calculations and Monte-Carlo (MC) simulations, we investigate the recently reported magnetic transition in B2 Al-Cr-Co alloys. The Cr sublattice is alloyed with different amounts of Co in the antiferromagnetic (AFM) B2 AlCr binary alloy and the resulting exchange interactions are analyzed within the Heisenberg Hamiltonian framework. DFT results reveal that at… ▽ More Using Density Functional Theory (DFT) calculations and Monte-Carlo (MC) simulations, we investigate the recently reported magnetic transition in B2 Al-Cr-Co alloys. The Cr sublattice is alloyed with different amounts of Co in the antiferromagnetic (AFM) B2 AlCr binary alloy and the resulting exchange interactions are analyzed within the Heisenberg Hamiltonian framework. DFT results reveal that at low Co concentrations the system favors the AFM order, while at high Co contents a transition to the ferromagnetic (FM) state is observed. Within the FM stability field, the Curie temperature (TC), obtained within the mean-field approximation, is below ~160 K and decreases with Co concentration. The calculated exchange parameters evolve systematically with Co content, and the trends are consistent with the DFT total energies. The magnetic configurations obtained from MC simulations follow the DFT results at low Cr levels but predict a spin-glass behavior for alloys containing more than 40 at.% Co on Cr sublattice. These findings provide a fundamental understanding of how the chemistry-driven changes in exchange interactions affect magnetism in the B2 Al-Cr-Co alloys. △ Less

Submitted 24 October, 2025; originally announced October 2025.

Roles of Electron-Magnon Cross Diffusion in Unidirectional Magnetoresistance of Metallic Magnetic Bilayers

Authors: Shashank Gupta, Steven S. -L. Zhang

Abstract: Unidirectional magnetoresistance (UMR) in metallic bilayers arises from nonlinear spin-charge transport mediated by broken time-reversal and inversion symmetries, yet the role of magnons remains unsettled. We develop a theoretical framework that incorporates coupled electron-magnon dynamics, revealing cross diffusion and spin-angular-momentum transfer between the two subsystems, which renormalize… ▽ More Unidirectional magnetoresistance (UMR) in metallic bilayers arises from nonlinear spin-charge transport mediated by broken time-reversal and inversion symmetries, yet the role of magnons remains unsettled. We develop a theoretical framework that incorporates coupled electron-magnon dynamics, revealing cross diffusion and spin-angular-momentum transfer between the two subsystems, which renormalize the characteristic electron and magnon spin-diffusion lengths. We show that nonequilibrium magnons, indirectly excited by the electric field, can suppress UMR by absorbing spin angular momentum from conduction electrons. We also analyze the magnetic-field, thickness, and temperature dependencies and identify distinct features that constitute experimental fingerprints of magnonic contributions to UMR in metallic bilayers, providing qualitative to semiquantitative guidance for elucidating the underlying physical mechanisms. △ Less

Submitted 16 March, 2026; v1 submitted 11 October, 2025; originally announced October 2025.

Comments: Published in Phys. Rev. B

Journal ref: Phys. Rev. B 113, 094434 (2026)

Nonsymmorphic symmetry protected hourglass Dirac chain topology and conventional superconductivity in ZrIrGe

Authors: Pavan Kumar Meena, Dibyendu Samanta, Shashank Srivastava, Poulami Manna, Sudeep Kumar Ghosh, Ravi Prakash Singh

Abstract: Ternary transition-metal germanide superconductors with nonsymmorphic symmetries offer promising platforms for symmetry-protected topological phases. In this work, we investigate ZrIrGe, which crystallizes in the nonsymmorphic TiNiSi-type structure. Electrical, magnetic, and specific heat measurements confirm bulk type-II superconductivity with a full gap and a transition temperature of 2.84(7) K,… ▽ More Ternary transition-metal germanide superconductors with nonsymmorphic symmetries offer promising platforms for symmetry-protected topological phases. In this work, we investigate ZrIrGe, which crystallizes in the nonsymmorphic TiNiSi-type structure. Electrical, magnetic, and specific heat measurements confirm bulk type-II superconductivity with a full gap and a transition temperature of 2.84(7) K, consistent with weak-coupling BCS behavior. First-principles calculations reveal hourglass-shaped bulk band dispersions and a Dirac chain composed of symmetry-protected fourfold-degenerate Dirac points, leading to drumhead-like surface states near the Fermi level. Additionally, ZrIrGe exhibits a nontrivial $\mathbb{Z}_2$ topological character, resulting in helical surface states that cross the Fermi level, making it a strong candidate for proximity-induced topological superconductivity. The coexistence of conventional superconductivity and topological band features establishes ZrIrGe as a rare stoichiometric system for exploring intrinsic topological superconductivity. △ Less

Submitted 3 October, 2025; originally announced October 2025.

Comments: 9 pages, 4 figures, Accepted in Physical Review B

First principles band structure of interacting phosphorus and boron/aluminum $δ$-doped layers in silicon

Authors: Quinn T. Campbell, Andrew D. Baczewski, Shashank Misra, Evan M. Anderson

Abstract: Silicon can be heavily doped with phosphorus in a single atomic layer (a $δ$ layer), significantly altering the electronic structure of the conduction bands within the material. Recent progress has also made it possible to further dope silicon with acceptor-based $δ$ layers using either boron or aluminum, making it feasible to create devices with interacting $δ$ layers with opposite polarity. Usin… ▽ More Silicon can be heavily doped with phosphorus in a single atomic layer (a $δ$ layer), significantly altering the electronic structure of the conduction bands within the material. Recent progress has also made it possible to further dope silicon with acceptor-based $δ$ layers using either boron or aluminum, making it feasible to create devices with interacting $δ$ layers with opposite polarity. Using Density Functional Theory, we calculate the electronic structure of a phosphorus-based $δ$ layer interacting with a boron or aluminum $δ$ layer, varying the distances between the $δ$ layers. At separations 1 nm and smaller, the dopant potentials overlap and largely cancel each other out, leading to an electronic structure closely mimicking intrinsic silicon. At separations greater than 1 nm, the two $δ$ layers behave independently of one another, with an equivalent electronic structure to a p-n diode with an intrinsic layer taking the place of the depletion region. One mechanism for charge transfer between $δ$ layers at larger distances could be tunneling, where we see a tunneling probability exceeding what would be seen for a standard silicon 1.1 eV triangular barrier, indicating that the interaction between delta layers may enhance tunneling compared to a traditional junction. △ Less

Submitted 24 February, 2026; v1 submitted 23 September, 2025; originally announced September 2025.

Report number: SAND2026-17837J

Journal ref: Quinn T. Campbell et al 2026 J. Phys.: Condens. Matter 38 075502

Predicting temperature-dependent failure and transformation zones in 2D silica glass through quasistatic Gaussian Phase Packets

Authors: Miguel Spínola, Shashank Saxena, Franz Bamer, Dennis M. Kochmann

Abstract: The athermal quasistatic (AQS) method is a powerful technique to study the mechanical behavior of disordered systems. However, its applicability is limited to temperatures near zero, where thermal activation is unlikely. In this work, we extend the AQS method to finite temperatures, based on a formulation that describes atoms as temperature-dependent Gaussian packets (GPPs) in phase space under qu… ▽ More The athermal quasistatic (AQS) method is a powerful technique to study the mechanical behavior of disordered systems. However, its applicability is limited to temperatures near zero, where thermal activation is unlikely. In this work, we extend the AQS method to finite temperatures, based on a formulation that describes atoms as temperature-dependent Gaussian packets (GPPs) in phase space under quasistatic conditions, thus equivalent to minimum free energy conditions. This framework is used to study the effect of temperature on the onset of inelasticity and fracture in amorphous two-dimensional silica glass approaching quasistatic conditions under uniaxial tensile loading. An important characteristic of this formulation is the directional dependence of the variance of each Gaussian packet in configuration space, making this formulation an inexpensive and accurate predictor of zones prone to atomic-scale rearrangements, both in the undeformed state and (with increasing accuracy) as the deformation progresses. This method is also shown to accurately capture the thermal expansion of the disordered material. Furthermore, combining the GPP description with Metropolis sampling predicts the effect of temperature on the onset of fracture of the material, which is validated through MD simulations at strain rates as low as $10^{4}$s$^{-1}$. The presented framework therefore provides a valuable technique for studying the nonlinear mechanics of disordered materials at finite temperature and for predicting local rearrangement zones in disordered solids efficiently without the need for expensive MD simulations. △ Less

Submitted 18 July, 2025; originally announced July 2025.

Comments: 22 pages, 10 figures

Low-energy domain wall racetracks with multiferroic topologies

Authors: Arundhati Ghosal, Alexander Qualls, Yousra Nahas, Shashank Ojha, Peter Meisenheimer, Shiyu Zhou, Maya Ramesh, Sajid Husain, Julia Mundy, Darrell Schlom, Zhi Yao, Sergei Prokhorenko, Laurent Bellaiche, Ramamoorthy Ramesh, Paul Stevenson, Lucas Caretta

Abstract: Conventional racetrack memories move information by pushing magnetic domain walls or other spin textures with spin-polarized currents, but the accompanying Joule heating inflates their energy budget and can hamper scaling. Here we present a voltage-controlled, magnetoelectric racetrack in which transverse electric fields translate coupled ferroelectric-antiferromagnetic walls along BiFeO3 nanostri… ▽ More Conventional racetrack memories move information by pushing magnetic domain walls or other spin textures with spin-polarized currents, but the accompanying Joule heating inflates their energy budget and can hamper scaling. Here we present a voltage-controlled, magnetoelectric racetrack in which transverse electric fields translate coupled ferroelectric-antiferromagnetic walls along BiFeO3 nanostrips at room temperature. Because no charge traverses the track, the switching dissipates orders of magnitude less energy than the most efficient spin-torque devices with more favourable scaling, making the scheme significantly more attractive at the nanoscale. We further uncover noncollinear topological magnetoelectric textures that emerge at domain walls in BiFeO3, where the nature of these topologies influences their stability upon translation. Among these are polar bi-merons and polar vertices magnetoelectrically coupled with magnetic cycloid disclinations and previously unobserved, topological magnetic cycloid twist topologies. We observe domain wall velocities of at least kilometres per second - matching or surpassing the fastest ferrimagnetic and antiferromagnetic racetracks and approaching the acoustic-phonon limit of BiFeO3 - while preserving these topologies over tens of micrometres. The resulting high velocity, low-energy racetrack delivers nanosecond access times without the thermal overhead of current-driven schemes, charting a path toward dense, ultralow-power racetrack devices which rely on spin texture translation. △ Less

Submitted 16 July, 2025; originally announced July 2025.

Bulk spin-orbit torque-driven spin Hall nano-oscillators using PtBi alloys

Authors: Utkarsh Shashank, Akash Kumar, Tahereh Sadat Parvini, Hauke Heyen, Lunjie Zeng, Andrew B. Yankovich, Mona Rajabali, Eva Olsson, Markus Münzenberg, Johan Åkerman

Abstract: Spin-orbit-torque-driven auto-oscillations in spin Hall nano-oscillators (SHNOs) offer a transformative pathway toward energy-efficient, nanoscale microwave devices for next-generation neuromorphic computing and high-frequency technologies. A key requirement for achieving robust, sustained oscillations is reducing the threshold current ($I_{\text{th}}$), strongly governed by spin Hall efficiency (… ▽ More Spin-orbit-torque-driven auto-oscillations in spin Hall nano-oscillators (SHNOs) offer a transformative pathway toward energy-efficient, nanoscale microwave devices for next-generation neuromorphic computing and high-frequency technologies. A key requirement for achieving robust, sustained oscillations is reducing the threshold current ($I_{\text{th}}$), strongly governed by spin Hall efficiency ($θ_{\text{SH}}$). However, conventional strategies to enhance $θ_{\text{SH}}$ face trade-offs, including high longitudinal resistivity, interfacial effects, and symmetry-breaking torques that limit performance. Here, we demonstrate a substantial enhancement of the bulk spin Hall effect in PtBi alloys, achieving over a threefold increase in $θ_{\text{SH}}$, from 0.07 in pure Pt to 0.24 in Pt$_{94.0}$Bi$_{6.0}$ and 0.19 in Pt$_{91.3}$Bi$_{8.7}$, as extracted from DC-bias spin-torque ferromagnetic resonance. The enhanced $θ_{\text{SH}}$ originates from bulk-dominated, extrinsic side-jump scattering across all PtBi compositions. Correspondingly, we observe a 42\% and 32\% reduction in $I_{\text{th}}$ in 100 nm SHNOs based on Co$_{40}$Fe$_{40}$B$_{20}$(3 nm)/Pt$_{94.0}$Bi$_{6.0}$(4 nm) and Co$_{40}$Fe$_{40}$B$_{20}$(3 nm)/Pt$_{91.3}$Bi$_{8.7}$(4 nm), respectively. Structural characterization reveals reduced Pt crystallinity, along with emergence of preferred crystallographic orientations upon introducing higher Bi concentrations. Together, these results position PtBi alloys as a compelling alternative to conventional 5$d$ transition metals, enabling enhanced $θ_{\text{SH}}$ and significantly lower $I_{\text{th}}$, thus opening new avenues for energy-efficient neuromorphic computing and magnetic random access memory. △ Less

Submitted 14 July, 2025; originally announced July 2025.

Comments: 19 pages, 5 figures

Surprisingly High Redundancy in Electronic Structure Data Across Materials Explained by Low Intrinsic Dimensionality

Authors: Sazzad Hossain, Ponkrshnan Thiagarajan, Shashank Pathrudkar, Stephanie Taylor, Abhijeet S. Gangan, Amartya S. Banerjee, Susanta Ghosh

Abstract: Machine learning (ML) models for electronic structure typically rely on large datasets generated by computationally expensive Kohn-Sham density functional theory calculations, as it is not known a priori which portions of the data are essential for accurate learning. Here, we reveal significant redundancies in electronic structure datasets across diverse material systems and attribute them to the… ▽ More Machine learning (ML) models for electronic structure typically rely on large datasets generated by computationally expensive Kohn-Sham density functional theory calculations, as it is not known a priori which portions of the data are essential for accurate learning. Here, we reveal significant redundancies in electronic structure datasets across diverse material systems and attribute them to the low intrinsic dimensionality of the underlying data. We show that even random pruning can substantially reduce dataset size with minimal degradation in predictive accuracy. Moreover, a state-of-the-art coverage-based pruning strategy that samples data across all learning difficulties preserves chemical accuracy and model generalizability while using up to two orders of magnitude less data and reducing training time by a factor of three or more. We further demonstrate that the essential electronic structure information lies on a low-dimensional, non-linear manifold, providing a geometric explanation for the observed prunability. These observations are consistent with the predominance of local atomic environments in determining electronic properties, as suggested by nearsightedness arguments, and indicate that large-scale datasets may contain highly overlapping information. Our findings challenge the prevailing assumption that such extensive datasets are necessary for accurate ML-based electronic structure predictions and open a path toward identifying minimal, representative datasets for each material class. △ Less

Submitted 30 April, 2026; v1 submitted 11 July, 2025; originally announced July 2025.

Long-term atomistic finite-temperature substitutional diffusion

Authors: Shashank Saxena, Prateek Gupta, Dennis M. Kochmann

Abstract: Simulating long-term mass diffusion kinetics with atomic precision is important to predict chemical and mechanical properties of alloys over time scales of engineering interest in applications, including (but not limited to) alloy heat treatment, corrosion resistance, and hydrogen embrittlement. We present a new strategy to bridge from the time scale of atomic vibrations to that of vacancy-mediate… ▽ More Simulating long-term mass diffusion kinetics with atomic precision is important to predict chemical and mechanical properties of alloys over time scales of engineering interest in applications, including (but not limited to) alloy heat treatment, corrosion resistance, and hydrogen embrittlement. We present a new strategy to bridge from the time scale of atomic vibrations to that of vacancy-mediated atomic hops by a combination of statistical mechanics-based Gaussian phase packets (GPP) relaxation and a nudged elastic band (NEB)-facilitated harmonic transition state theory (H-TST) time update. We validate the approach by simulating bulk self-diffusion in copper and the segregation of vacancies and magnesium to a stacking fault and a symmetric tilt grain boundary in aluminum, modeled with an embedded atom method (EAM) potential. The method correctly predicts the kinetics in bulk copper and equilibrium impurity concentrations in aluminum, in agreement with the Langmuir-Mclean solution in the dilute limit. Notably, this technique can reach realistic diffusion time scales of days, weeks, and even years in a computational time of hours, demonstrating its capability to study the long-term chemo-thermo-mechanically coupled behavior of atomic ensembles. △ Less

Submitted 24 June, 2025; originally announced June 2025.

Thermodynamics and Legendre Duality in Optimal Networks

Authors: Amilcare Porporato, Shashank Kumar Anand, Salvatore Calabrese, Luca Ridolfi, Lamberto Rondoni

Abstract: Optimality principles in nonequilibrium transport networks are linked to a thermodynamic formalism based on generalized transport potentials endowed with Legendre duality and related contact structure. This allows quantifying the distance from non-equilibrium operating points, analogously to thermodynamic availability as well as to shed light on optimality principles in relation to different impos… ▽ More Optimality principles in nonequilibrium transport networks are linked to a thermodynamic formalism based on generalized transport potentials endowed with Legendre duality and related contact structure. This allows quantifying the distance from non-equilibrium operating points, analogously to thermodynamic availability as well as to shed light on optimality principles in relation to different imposed constraints. Extremizations of generalized dissipation and entropy production appear as special cases that require power-law resistances and -- for entropy production -- also isothermal conditions. Changes in stability of multiple operating points are interpreted as phase transitions based on non-equilibrium equations of state, while cost-based optimization of transport properties reveals connections to the generalized dissipation in the case of power law costs and linear resistance law, but now with typically unstable operating points which give rise to branched optimal transport. △ Less

Submitted 9 June, 2025; originally announced June 2025.

Ising superconductivity in bulk layered non-centrosymmetric 4H-NbSe2

Authors: Chandan Patra, Tarushi Agarwal, Rahul Verma, Poulami Manna, Shashank Srivastava, Ravi Shankar Singh, Mathias S. Scheurer, Bahadur Singh, Ravi Prakash Singh

Abstract: Transition metal dichalcogenides exhibit multiple polymorphs that enable the exploration of diverse quantum states, including valley-selective spin polarization, the valley Hall effect, Ising superconductivity, and nontrivial topology. Monolayer 2$H$-NbSe$_2$ is a promising candidate for realizing Ising superconductivity due to its spin-split, out-of-plane spin-polarized states arising from invers… ▽ More Transition metal dichalcogenides exhibit multiple polymorphs that enable the exploration of diverse quantum states, including valley-selective spin polarization, the valley Hall effect, Ising superconductivity, and nontrivial topology. Monolayer 2$H$-NbSe$_2$ is a promising candidate for realizing Ising superconductivity due to its spin-split, out-of-plane spin-polarized states arising from inversion symmetry breaking and strong spin-orbit coupling. In contrast, bulk 2$H$-NbSe$_2$ retains inversion symmetry and lacks spin splitting, limiting its suitability for hosting Ising superconductivity. Here, we report the growth of high-quality single crystals of the acentric bulk superconducting polymorph, 4$H$-NbSe$_2$, which intrinsically breaks the inversion symmetry and supports valley-selective spin-polarized states. Magnetization and resistivity measurements reveal anisotropic superconductivity, with the in-plane upper critical field exceeding the Pauli limit, while out-of-plane fields suppress superconductivity more rapidly, before reaching the Pauli limit, which strongly suggests the presence of Ising pairing. First-principles calculations and symmetry analysis confirm significant valley-selective spin splitting with out-of-plane spin polarization, further supporting the emergence of Ising superconductivity in 4$H$-NbSe$_2$. These results establish 4$H$-NbSe$_2$ as a robust bulk platform to investigate Ising superconductivity and valley-selective phenomena in transition-metal dichalcogenides. △ Less

Submitted 11 June, 2025; originally announced June 2025.

Comments: 8 Pages, 4 figures

First-principles dissociation pathways of BCl$_3$ on the Si(100)-2$\times$1 surface

Authors: Quinn T. Campbell, Shashank Misra, Jeffrey A. Ivie

Abstract: One of the most promising acceptor precursors for atomic-precision $δ$-doping of silicon is BCl$_3$. The chemical pathway, and the resulting kinetics, through which BCl$_3$ adsorbs and dissociates on silicon, however, has only been partially explained. In this work, we use density functional theory to expand the dissociation reactions of BCl$_3$ to include reactions that take place across multiple… ▽ More One of the most promising acceptor precursors for atomic-precision $δ$-doping of silicon is BCl$_3$. The chemical pathway, and the resulting kinetics, through which BCl$_3$ adsorbs and dissociates on silicon, however, has only been partially explained. In this work, we use density functional theory to expand the dissociation reactions of BCl$_3$ to include reactions that take place across multiple silicon dimer rows, and reactions which end in a bare B atom either at the surface, substituted for a surface silicon, or in a subsurface position. We further simulate resulting scanning tunneling microscopy images for each of these BCl$_x$ dissociation fragments, demonstrating that they often display distinct features that may allow for relatively confident experimental identification. Finally, we input the full dissociation pathway for BCl$_3$ into a kinetic Monte Carlo model, which simulates realistic reaction pathways as a function of environmental conditions such as pressure and temperature of dosing. We find that BCl$_2$ is broadly dominant at low temperatures, while high temperatures and ample space on the silicon surface for dissociation encourage the formation of bridging BCl fragments and B substitutions on the surface. This work provides the chemical mechanisms for understanding atomic-precision doping of Si with B, enabling a number of relevant quantum applications such as bipolar nanoelectronics, acceptor-based qubits, and superconducting Si. △ Less

Submitted 13 May, 2025; originally announced May 2025.

Comments: 21 pages, 6 figures

Colossal enhancement of spin transmission through magnon confinement in an antiferromagnet

Authors: Sajid Husain, Maya Ramesh, Xinyan Li, Sergei Prokhorenko, Shashank Kumar Ojha, Aiden Ross, Koushik Das, Boyang Zhao, Hyeon Woo Park, Peter Meisenheimer, Yousra Nahas, Lucas Caretta, Lane W. Martin, Se Kwon Kim, Zhi Yao, Haidan Wen, Sayeef Salahuddin, Long-Qing Chen, Yimo Han, Rogerio de Sousa, Laurent Bellaiche, Manuel Bibes, Darrell G. Schlom, Ramamoorthy Ramesh

Abstract: Since Felix Bloch's introduction of the concept of spin waves in 1930, magnons (the quanta of spin waves) have been extensively studied in a range of materials for spintronics, particularly for non-volatile logic-in-memory devices. Controlling magnons in conventional antiferromagnets and harnessing them in practical applications, however, remains a challenge. In this letter, we demonstrate highly… ▽ More Since Felix Bloch's introduction of the concept of spin waves in 1930, magnons (the quanta of spin waves) have been extensively studied in a range of materials for spintronics, particularly for non-volatile logic-in-memory devices. Controlling magnons in conventional antiferromagnets and harnessing them in practical applications, however, remains a challenge. In this letter, we demonstrate highly efficient magnon transport in an LaFeO$_3$/BiFeO$_3$/LaFeO$_3$ all-antiferromagnetic system which can be controlled electrically, making it highly desirable for energy-efficient computation. Leveraging spin-orbit-driven spin-charge transduction, we demonstrate that this material architecture permits magnon confinement in ultrathin antiferromagnets, enhancing the output voltage generated by magnon transport by several orders of magnitude, which provides a pathway to enable magnetoelectric memory and logic functionalities. Additionally, its non-volatility enables ultralow-power logic-in-memory processing, where magnonic devices can be efficiently reconfigured via electrically controlled magnon spin currents within magnetoelectric channels. △ Less

Submitted 31 March, 2025; originally announced March 2025.

Comments: 12 pages, 4 figures

Giant Spin Pumping at Polymer/Ferromagnet Interfaces for Hybrid Spintronic Devices

Authors: Shiva Gaur, Akash Kumar, Himanshu Bangar, Utkarsh Shashank, Hukum Singh, Saroj P. Dash, Anubhav Raghav, Johan Åkerman

Abstract: While the growing utilization of polymers in flexible electronic devices has sparked significant interest in polymer/metal interfaces, spintronic studies of such interfaces remain limited. Here, we systematically study spin pumping across a polymer/ferromagnet metal interface between hydrogen silsesquioxane (HSQ) oligomer layers ($t_\mathit{HSQ} = 30, 36, 48$ nm) and NiFe (… ▽ More While the growing utilization of polymers in flexible electronic devices has sparked significant interest in polymer/metal interfaces, spintronic studies of such interfaces remain limited. Here, we systematically study spin pumping across a polymer/ferromagnet metal interface between hydrogen silsesquioxane (HSQ) oligomer layers ($t_\mathit{HSQ} = 30, 36, 48$ nm) and NiFe ($t_\mathit{NiFe} = 4, 5, 7, 10$ nm) thin films. Using ferromagnetic resonance measurements, we observe strong spin pumping (large linewidth broadening) and a giant spin mixing conductance, reaching 19.8~${\rm nm^{-2}}$ for HSQ = 48 nm, \emph{i.e.}~comparable to that of heavy metals. Our results suggest efficient spin transfer across the HSQ/NiFe interface, possibly originating from a combination of spin and orbital pumping, and provide valuable insights for designing self-powered and flexible spintronic devices utilizing polymers in combination with ferromagnetic materials. △ Less

Submitted 28 March, 2025; originally announced March 2025.

Comments: 15 pages

Reversal of tracer advection and Hall drift in an interacting chiral fluid

Authors: Erik Kalz, Shashank Ravichandir, Johannes Birkenmeier, Ralf Metzler, Abhinav Sharma

Abstract: Chiral fluids are defined by broken mirror or time-reversal symmetry, giving rise to tensorial transport coefficients with antisymmetric components. A key example is the odd mobility tensor, which governs the response of a chiral tracer to an applied force and induces a characteristic transverse drift. While this response is well understood in the infinite dilution limit, the impact of interpartic… ▽ More Chiral fluids are defined by broken mirror or time-reversal symmetry, giving rise to tensorial transport coefficients with antisymmetric components. A key example is the odd mobility tensor, which governs the response of a chiral tracer to an applied force and induces a characteristic transverse drift. While this response is well understood in the infinite dilution limit, the impact of interparticle interactions on the tracer dynamics remains largely unexplored. Here, we conduct an analytical and computational study of a chiral fluid with interparticle interactions and show that, under an external driving force, a chiral tracer can undergo a complete reversal of both its transverse Hall drift and its advection along the force. This reversal emerges from the interplay between odd mobility and interaction-mediated forces, resulting in a phenomenon reminiscent of absolute negative mobility. △ Less

Submitted 6 March, 2025; originally announced March 2025.

Beyond being free: glassy dynamics of SrTiO$_3$-based two-dimensional electron gas

Authors: Jyotirmay Maity, Shashank Kumar Ojha, Prithwijit Mandal, Manav Beniwal, Nandana Bhattacharya, Andrei Gloskovskii, Christoph Schlueter, Srimanta Middey

Abstract: Electron glasses offer a convenient laboratory platform to study glassy dynamics. Traditionally, the interplay between long-range Coulomb interactions and disorder is deemed instrumental in stabilizing the electron glass phase. Existing experimental studies on electron glass have focused on doped semiconductors, strongly correlated systems, granular systems, etc., all of which are far from the wel… ▽ More Electron glasses offer a convenient laboratory platform to study glassy dynamics. Traditionally, the interplay between long-range Coulomb interactions and disorder is deemed instrumental in stabilizing the electron glass phase. Existing experimental studies on electron glass have focused on doped semiconductors, strongly correlated systems, granular systems, etc., all of which are far from the well-delocalized limit. In this work, we expand the study of electron glasses to a well-known quantum paraelectric SrTiO$_3$ (STO) and unveil a new scenario: how naturally occurring ferroelastic twin walls of STO could result in glassy electrons, even in a metallic state. We show that the emergent two-dimensional electron gas at the $γ$-Al$_2$O$_3$/STO interface exhibits long-lasting temporal relaxations in resistance and memory effects at low temperatures, which are hallmarks of glassiness. We also demonstrate that the glass-like relaxations could be further tuned by application of an electric field. This implies that the observed glassy dynamics is connected with the development of polarity near the structural twin walls of STO and the complex interactions among them, arising from the coupling between ferroelastic and ferroelectric orders. The observation of this glassy metal phase not only extends the concept of electron glasses to metallic systems with multiple order parameters but also contributes to the growing understanding of the fascinating and diverse physical phenomena that emerge near the quantum critical point. △ Less

Submitted 13 January, 2025; originally announced January 2025.

Comments: 20 pages, 5 figures

Journal ref: Commun Mater 6, 84 (2025)

A reduced-temperature process for preparing atomically clean Si(100) and SiGe(100) surfaces with vapor HF

Authors: Luis Fabián Peña, Evan M. Anderson, John P. Mudrick, Samantha G. Rosenberg, David A. Scrymgeour, Ezra Bussmann, Shashank Misra

Abstract: Silicon processing techniques such as atomic precision advanced manufacturing (APAM) and epitaxial growth require surface preparations that activate oxide desorption (typically >1000 $^{\circ}$C) and promote surface reconstruction toward atomically-clean, flat, and ordered Si(100)-2$\times$1. We compare aqueous and vapor phase cleaning of Si and Si/SiGe surfaces to prepare APAM-ready and epitaxy-r… ▽ More Silicon processing techniques such as atomic precision advanced manufacturing (APAM) and epitaxial growth require surface preparations that activate oxide desorption (typically >1000 $^{\circ}$C) and promote surface reconstruction toward atomically-clean, flat, and ordered Si(100)-2$\times$1. We compare aqueous and vapor phase cleaning of Si and Si/SiGe surfaces to prepare APAM-ready and epitaxy-ready surfaces at lower temperatures. Angle resolved X-ray photoelectron spectroscopy (ARXPS) and Fourier transform infrared (FTIR) spectroscopy indicate that vapor hydrogen fluoride (VHF) cleans dramatically reduce carbon surface contamination and allow the chemically prepared surface to reconstruct at lower temperatures, 600 $^{\circ}$C for Si and 580 $^{\circ}$C for a Si/Si$_{0.7}$Ge$_{0.3}$ heterostructures, into an ordered atomic terrace structure indicated by scanning tunneling microscopy (STM). After thermal treatment and vacuum hydrogen termination, we demonstrate STM hydrogen desorption lithography (HDL) on VHF-treated Si samples, creating reactive zones that enable area-selective chemistry using a thermal budget similar to CMOS process flows. We anticipate these results will establish new pathways to integrate APAM with Si foundry processing. △ Less

Submitted 9 January, 2025; originally announced January 2025.

Report number: SAND2025-00199O

Journal ref: Journal of Physical Chemistry C (2025)

Anomalous Magneto-transport and Anisotropic Multigap Superconductivity in Architecturally Misfit Layered System (PbS)$_{1.13}$TaS$_2$

Authors: Tarushi Agarwal, Chandan Patra, Poulami Manna, Shashank Srivastava, Priya Mishra, Suhani Sharma, Ravi Prakash Singh

Abstract: Misfit-layered compounds, naturally occurring bulk heterostructures, present a compelling alternative to artificially engineered ones, offering a unique platform for exploring correlated phases and quantum phenomena. This study investigates the magnetotransport and superconducting properties of the misfit compound (PbS)$_{1.13}$TaS$_2$, comprising alternating PbS and 1$H$-TaS$_2$ layers. It exhibi… ▽ More Misfit-layered compounds, naturally occurring bulk heterostructures, present a compelling alternative to artificially engineered ones, offering a unique platform for exploring correlated phases and quantum phenomena. This study investigates the magnetotransport and superconducting properties of the misfit compound (PbS)$_{1.13}$TaS$_2$, comprising alternating PbS and 1$H$-TaS$_2$ layers. It exhibits distinctive transport properties, including a prominent planar Hall effect and a four-fold oscillatory Butterfly-shaped anisotropic magnetoresistance (AMR). Moreover, it shows multigap two-dimensional superconductivity with an exceptionally high in-plane upper critical field, exceeding the Pauli limit. The coexistence of unconventional superconductivity and anomalous transport - two distinct quantum phenomena, within the same material, suggests that misfit compounds provide an ideal platform for realizing quantum effects in the two-dimensional limit of bulk crystals. This opens the door to the development of simpler and more efficient quantum devices. △ Less

Submitted 6 January, 2025; originally announced January 2025.

Comments: 10 pages, 5 figures

Entropic organization of topologically modified ring polymers in spherical confinement

Authors: Kingkini Roychoudhury, Shreerang Pande, Indrakanty S. Shashank, Debarshi Mitra, Apratim Chatterji

Abstract: It has been shown that under high cylindrical confinement, two ring polymers with excluded volume interactions between monomers, segregate to two halves of the cylinder to maximize their entropy. In contrast, two ring polymers remain mixed within a sphere, as there is no symmetry breaking direction [Nat Rev Microbiol, 8, 600-607 (2010)]. Therefore, in order to observe emergent organization of ring… ▽ More It has been shown that under high cylindrical confinement, two ring polymers with excluded volume interactions between monomers, segregate to two halves of the cylinder to maximize their entropy. In contrast, two ring polymers remain mixed within a sphere, as there is no symmetry breaking direction [Nat Rev Microbiol, 8, 600-607 (2010)]. Therefore, in order to observe emergent organization of ring polymers in a sphere, we can introduce an asymmetric topological modification to the polymer architecture by creating a small loop and a big loop within the ring polymer. We consider the bead-spring model of polymers where there are only repulsive excluded volume interactions between the monomers ensuring that the organization we observe is purely entropy-driven. We find that for a single topologically modified polymer within a sphere, the monomers of the bigger loop are statistically more probable to be found closer to the periphery. However, the situation is reversed when we have multiple such topologically modified polymers in a sphere. The monomers of the small loops are found closer to the walls of the sphere. We can increase this localization and radial organization of polymer segments by increasing the number of small loops in each ring polymer. We study how these loops interact with each other within a polymer, as well as with loops of other polymers in spherical confinement. We compare contact maps of multiple such topologically modified polymers in a sphere. Finally, we discuss the plausible relevance of our studies to eukaryotic chromosomes that are confined within a spherical nucleus. △ Less

Submitted 4 January, 2025; originally announced January 2025.

Comments: 18 pages, 13 figures

Journal ref: Phys. Rev. E 112, 055409 (2025)

Chemical leaching of Al-Cu-Co decagonal quasicrystals

Authors: Shashank Shekhar Mishra, Thakur Prasad Yadav

Abstract: In the present investigation, the chemical leaching of the poly-grain Al65Cu15Co20 and Al65Cu20Co15 decagonal quasicrystalline alloy have been studied. The polished surfaces of as-cast alloys were leached with 10 mole NaOH solution for 0.5- 8 hours. The x-ray diffraction, scanning electron microscopy and transmission electron microscopy techniques have been used for structural and microstructural… ▽ More In the present investigation, the chemical leaching of the poly-grain Al65Cu15Co20 and Al65Cu20Co15 decagonal quasicrystalline alloy have been studied. The polished surfaces of as-cast alloys were leached with 10 mole NaOH solution for 0.5- 8 hours. The x-ray diffraction, scanning electron microscopy and transmission electron microscopy techniques have been used for structural and microstructural characterization. Energy dispersive x-ray analysis has been carried out for chemical composition analysis. Chemical leaching exclusively removes the Al from the surfaces of the Al65Cu15Co20 and Al65Cu20Co15 decagonal quasicrystalline alloys, consequently by the formation of porous structure containing nano size particles of Cu, Co and their oxides have been observed. Al65Cu15Co20 exhibits high porosity in comparison to Al65Cu20Co15 alloy, however the size of the precipitated nano-particles i.e. Cu, Co, Cu2O and CuO were smaller in the case of Al65Cu20Co15 alloy. The energy dispersive X-ray analysis mapping suggests homogeneous distribution of Cu and Co found on the leached surface and the presence of oxygen was also detected. △ Less

Submitted 29 December, 2024; originally announced December 2024.

Comments: 14 pages, 10 figures

MSC Class: 14J60 ACM Class: F.2.2

Industrial-scale Prediction of Cement Clinker Phases using Machine Learning

Authors: Sheikh Junaid Fayaz, Nestor Montiel-Bohorquez, Shashank Bishnoi, Matteo Romano, Manuele Gatti, N. M. Anoop Krishnan

Abstract: Cement production, exceeding 4.1 billion tonnes and contributing 2.4 tonnes of CO2 annually, faces critical challenges in quality control and process optimization. While traditional process models for cement manufacturing are confined to steady-state conditions with limited predictive capability for mineralogical phases, modern plants operate under dynamic conditions that demand real-time quality… ▽ More Cement production, exceeding 4.1 billion tonnes and contributing 2.4 tonnes of CO2 annually, faces critical challenges in quality control and process optimization. While traditional process models for cement manufacturing are confined to steady-state conditions with limited predictive capability for mineralogical phases, modern plants operate under dynamic conditions that demand real-time quality assessment. Here, exploiting a comprehensive two-year operational dataset from an industrial cement plant, we present a machine learning framework that accurately predicts clinker mineralogy from process data. Our model achieves unprecedented prediction accuracy for major clinker phases while requiring minimal input parameters, demonstrating robust performance under varying operating conditions. Through post-hoc explainable algorithms, we interpret the hierarchical relationships between clinker oxides and phase formation, providing insights into the functioning of an otherwise black-box model. This digital twin framework can potentially enable real-time optimization of cement production, thereby providing a route toward reducing material waste and ensuring quality while reducing the associated emissions under real plant conditions. Our approach represents a significant advancement in industrial process control, offering a scalable solution for sustainable cement manufacturing. △ Less

Submitted 28 December, 2024; v1 submitted 16 December, 2024; originally announced December 2024.

Transport of molecules via polymerization in chemical gradients

Authors: Shashank Ravichandir, Bhavesh Valecha, Pietro Luigi Muzzeddu, Jens-Uwe Sommer, Abhinav Sharma

Abstract: The transport of molecules for chemical reactions is critically important in various cellular biological processes. Despite thermal diffusion being prevalent in many biochemical processes, it is unreliable for any sort of directed transport or preferential accumulation of molecules. In this paper we propose a strategy for directed motion in which the molecules are transported by active carriers vi… ▽ More The transport of molecules for chemical reactions is critically important in various cellular biological processes. Despite thermal diffusion being prevalent in many biochemical processes, it is unreliable for any sort of directed transport or preferential accumulation of molecules. In this paper we propose a strategy for directed motion in which the molecules are transported by active carriers via polymerization. This transport is facilitated by chemical/activity gradients which generate an effective drift of the polymers. By marginalizing out the active degrees of freedom of the system, we obtain an effective Fokker-Planck equation for the Rouse modes of such active-passive hybrid polymers. In particular, we solve for the steady state distribution of the center of mass and its mean first passage time to reach an intended destination. We focus on how the arrangement of active units within the polymer affect its steady-state and dynamic behaviour and how they can be optimized to achieve high accumulation or rapid motility. △ Less

Submitted 25 November, 2024; v1 submitted 19 November, 2024; originally announced November 2024.

Journal ref: (Communication) Soft Matter, 2025, 21, 1835-1840

Morphogenesis of Spin Cycloids in a Non-collinear Antiferromagnet

Authors: Shashank Kumar Ojha, Pratap Pal, Sergei Prokhorenko, Sajid Husain, Maya Ramesh, Peter Meisenheimer, Darrell G. Schlom, Paul Stevenson, Lucas Caretta, Yousra Nahas, Lane W. Martin, Laurent Bellaiche, Chang-Beom Eom, Ramamoorthy Ramesh

Abstract: Pattern formation in spin systems with continuous-rotational symmetry (CRS) provides a powerful platform to study emergent complex magnetic phases and topological defects in condensed-matter physics. However, its understanding and correlation with unconventional magnetic order along with high-resolution nanoscale imaging is challenging. Here, we employ scanning NV magnetometry to unveil the morpho… ▽ More Pattern formation in spin systems with continuous-rotational symmetry (CRS) provides a powerful platform to study emergent complex magnetic phases and topological defects in condensed-matter physics. However, its understanding and correlation with unconventional magnetic order along with high-resolution nanoscale imaging is challenging. Here, we employ scanning NV magnetometry to unveil the morphogenesis of spin cycloids at both the local and global scales within a single ferroelectric domain of (111)-oriented BiFeO$_3$ (which is a non-collinear antiferromagnet), resulting in formation of a glassy labyrinthine pattern. We find that the domains of locally oriented cycloids are interconnected by an array of topological defects and exhibit isotropic energy landscape predicted by first-principles calculations. We propose that the CRS of spin-cycloid propagation directions within the (111) drives the formation of the labyrinthine pattern and the associated topological defects such as antiferromagnetic skyrmions. Unexpectedly, reversing the as-grown ferroelectric polarization from [$\bar{1}$$\bar{1}$$\bar{1}$] to [111] induces a magnetic phase transition, destroying the labyrinthine pattern and producing a deterministic non-volatile non cycloidal, uniformly magnetized state. These findings highlight that (111)-oriented BiFeO$_3$ is not only important for studying the fascinating subject of pattern formation but could also be utilized as an ideal platform for integrating novel topological defects in the field of antiferromagnetic spintronics. △ Less

Submitted 29 October, 2024; originally announced October 2024.

Comments: 20 pages, 15 figures

Symmetry-designed BiFeO3 single domain spin cycloid for efficient spintronics

Authors: Pratap Pal, Jonathon L. Schad, Anuradha M. Vibhakar, Shashank Kumar Ojha, Sajid Hussain Gi-Yeop Kim, Saurav Shenoy, Fei Xue, Kaushik Das, Yogesh Kumar, Paul Lenharth, A. Bombardi, Sayeef Salahuddin, Roger D. Johnson, Si-Young Choi, Mark S. Rzchowski, Long-Qing Chen, Ramamoorthy Ramesh, Paolo G. Radaelli, Chang-Beom Eom

Abstract: Deterministic control of coupled ferroelectric and antiferromagnetic orders remains a central challenge in multiferroics, limiting their integration into functional magnetoelectrics and magnonic-devices. (111)pc BiFeO3 with a robust single spin cycloid, offers direct magnetoelectric-coupling and a platform for efficient spin transport, yet multi-magnetic domains and ferroelectric-fatigue have prev… ▽ More Deterministic control of coupled ferroelectric and antiferromagnetic orders remains a central challenge in multiferroics, limiting their integration into functional magnetoelectrics and magnonic-devices. (111)pc BiFeO3 with a robust single spin cycloid, offers direct magnetoelectric-coupling and a platform for efficient spin transport, yet multi-magnetic domains and ferroelectric-fatigue have prevented reproducible control. Here, we show that anisotropic-compressive in-plane strain stabilizes a single antiferromagnetic domain with unique spin-cycloid vector, by breaking the symmetry of the (111)pc plane. Epitaxial BiFeO3 films grown on orthorhombic NdGaO3 (011)o [(111)pc] substrates impose the required anisotropic in-plane strain and stabilizes single antiferromagnetic domain, as confirmed through direct imaging with scanning NV microscopy and non-resonant-x-ray-magnetic-scattering. Remarkably, these engineered films exhibit deterministic and non-volatile 180° switching of ferroelectric and single antiferromagnetic domains over 1,000 cycles. The monodomain state also enables anisotropic and threefold enhanced magnon transport with reduced scattering. Thus, symmetry-designed (111)pc monodomain BiFeO3 offers a robust platform for advanced magnetoelectric and magnonic applications. △ Less

Submitted 9 October, 2025; v1 submitted 29 October, 2024; originally announced October 2024.

Comments: 51 pages, 33 figures

Exploring transport mechanisms in atomic precision advanced manufacturing enabled pn junctions

Authors: Juan P. Mendez, Xujiao Gao, Jeffrey Ivie, James H. G. Owen, Wiley P. Kirk, John N. Randall, Shashank Misra

Abstract: We investigate the different transport mechanisms that can occur in pn junction devices made using atomic precision advanced manufacturing (APAM) at temperatures ranging from cryogenic to room temperature. We first elucidate the potential cause of the anomalous behavior observed in the forward-bias response of these devices in recent cryogenic temperature measurements, which deviates from the theo… ▽ More We investigate the different transport mechanisms that can occur in pn junction devices made using atomic precision advanced manufacturing (APAM) at temperatures ranging from cryogenic to room temperature. We first elucidate the potential cause of the anomalous behavior observed in the forward-bias response of these devices in recent cryogenic temperature measurements, which deviates from the theoretical response of a silicon Esaki diode. These anomalous behaviors include current suppression at low voltages in the forward-bias response and a much lower valley voltage at cryogenic temperatures than theoretically expected for a silicon diode. To investigate the potential causes of these anomalies, we studied the effects of a few possible transport mechanisms, including band-to-band tunneling, band gap narrowing, potential impact of non-Ohmic contacts, band quantization, impact of leakage, and inelastic trap-assisted tunneling, through semi-classical simulations. We find that a combination of two sets of band-to-band tunneling (BTBT) parameters can qualitatively approximate the shape of the tunneling current at low bias. This can arise from band quantization and realignment due to the strong potential confinement in $δ$-layers. We also find that the lower-than-theoretically-expected valley voltage can be attributed to modifications in the electronic band structure within the $δ$-layer regions, leading to a significant band-gap narrowing induced by the high density of dopants. Finally, we extend our analyses to room temperature operation and predict that trap-assisted tunneling (TAT) facilitated by phonon interactions may become significant, leading to a complex superposition of BTBT and TAT transport mechanisms in the electrical measurements. △ Less

Submitted 15 March, 2025; v1 submitted 22 October, 2024; originally announced October 2024.

Electronic structure prediction of medium and high entropy alloys across composition space

Authors: Shashank Pathrudkar, Stephanie Taylor, Abhishek Keripale, Abhijeet Sadashiv Gangan, Ponkrshnan Thiagarajan, Shivang Agarwal, Jaime Marian, Susanta Ghosh, Amartya S. Banerjee

Abstract: We propose machine learning (ML) models to predict the electron density -- the fundamental unknown of a material's ground state -- across the composition space of concentrated alloys. From this, other physical properties can be inferred, enabling accelerated exploration. A significant challenge is that the number of sampled compositions and descriptors required to accurately predict fields like th… ▽ More We propose machine learning (ML) models to predict the electron density -- the fundamental unknown of a material's ground state -- across the composition space of concentrated alloys. From this, other physical properties can be inferred, enabling accelerated exploration. A significant challenge is that the number of sampled compositions and descriptors required to accurately predict fields like the electron density increases rapidly with species. To address this, we employ Bayesian Active Learning (AL), which minimizes training data requirements by leveraging uncertainty quantification capabilities of Bayesian Neural Networks. Compared to strategic tessellation of the composition space, Bayesian-AL reduces the number of training data points by a factor of 2.5 for ternary (SiGeSn) and 1.7 for quaternary (CrFeCoNi) systems. We also introduce easy-to-optimize, body-attached-frame descriptors, which respect physical symmetries and maintain approximately the same descriptor-vector size as alloy elements increase. Our ML models demonstrate high accuracy and generalizability in predicting both electron density and energy across composition space. △ Less

Submitted 15 January, 2026; v1 submitted 10 October, 2024; originally announced October 2024.

Magnetically Tuned Metal-Insulator Transition in LaAlO$_3$/SrTiO$_3$ Nanowire Arrays

Authors: Ranjani Ramachandran, Shashank Anand, Kitae Eom, Kyoungjun Lee, Dengyu Yang, Muqing Yu, Sayanwita Biswas, Aditi Nethwewala, Chang-Beom Eom, Erica Carlson, Patrick Irvin, Jeremy Levy

Abstract: A wide family of two dimensional (2D) systems, including stripe-phase superconductors, sliding Luttinger liquids, and anisotropic 2D materials, can be modeled by an array of coupled one-dimensional (1D) electron channels or nanowire arrays. Here we report experiments in arrays of conducting nanowires with gate and field tunable interwire coupling, that are programmed at the LaAlO$_3$/SrTiO$_3$ int… ▽ More A wide family of two dimensional (2D) systems, including stripe-phase superconductors, sliding Luttinger liquids, and anisotropic 2D materials, can be modeled by an array of coupled one-dimensional (1D) electron channels or nanowire arrays. Here we report experiments in arrays of conducting nanowires with gate and field tunable interwire coupling, that are programmed at the LaAlO$_3$/SrTiO$_3$ interface. We find a magnetically-tuned metal-to-insulator transition in which the transverse resistance of the nanowire array increases by up to four orders of magnitude. To explain this behavior, we develop a minimal model of a coupled two-wire system which agrees well with observed phenomena. These nanowire arrays can serve as a model systems to understand the origin of exotic behavior in correlated materials via analog quantum simulation. △ Less

Submitted 1 January, 2025; v1 submitted 2 October, 2024; originally announced October 2024.

Electric and Magnetic Field-dependent Tunneling between Coupled Nanowires

Authors: Shashank Anand, Ranjani Ramachandran, Kitae Eom, Kyoungjun Lee, Dengyu Yang, Muqing Yu, Sayanwita Biswas, Aditi Nethwewala, Chang-Beom Eom, Erica Carlson, Patrick Irvin, Jeremy Levy

Abstract: Coupled quasi-one-dimensional (quasi-1D) electron systems host rich emergent physics that cannot be accounted for by understanding isolated 1D electron systems alone. Open questions remain about how transport in these arrays can be manipulated by the application of external electric and magnetic fields. In this theoretical study, we consider a pair of coupled nanowires with non-interacting electro… ▽ More Coupled quasi-one-dimensional (quasi-1D) electron systems host rich emergent physics that cannot be accounted for by understanding isolated 1D electron systems alone. Open questions remain about how transport in these arrays can be manipulated by the application of external electric and magnetic fields. In this theoretical study, we consider a pair of coupled nanowires with non-interacting electrons. We find that a metal-insulator transition is induced by an out-of-plane magnetic field and a transverse potential bias on an array of such coupled wires. We demonstrate the existence of distinct conductance features and highlight the crucial role played by the field dependence of the interwire potential barrier on transport properties. These predictions agree well with transport experiments performed on coupled nanowires sketched on an LaAlO3/SrTiO3 interface. Since our model makes minimal assumptions, we expect our predictions to hold for a wide class of coupled 1D systems. △ Less

Submitted 2 October, 2024; originally announced October 2024.

Finite-temperature grain boundary properties from quasistatic atomistics

Authors: Miguel Spínola, Shashank Saxena, Prateek Gupta, Brandon Runnels, Dennis M. Kochmann

Abstract: Grain boundary (GB) properties greatly influence the mechanical, electrical, and thermal response of polycrystalline materials. Most computational studies of GB properties at finite temperatures use molecular dynamics (MD), which is computationally expensive, limited in the range of accessible timescales, and requires cumbersome techniques like thermodynamic integration to estimate free energies.… ▽ More Grain boundary (GB) properties greatly influence the mechanical, electrical, and thermal response of polycrystalline materials. Most computational studies of GB properties at finite temperatures use molecular dynamics (MD), which is computationally expensive, limited in the range of accessible timescales, and requires cumbersome techniques like thermodynamic integration to estimate free energies. This restricts the reasonable computation (without incurring excessive computational expense) of GB properties to regimes that are often unrealistic, such as zero temperature or extremely high strain rates. Consequently, there is a need for simulation methodology that avoids the timescale limitations of MD, while providing reliable estimates of GB properties. The Gaussian Phase-Packet (GPP) method is a temporal coarse-graining technique that can predict relaxed atomic structures at finite temperature in the quasistatic limit. This work applies GPP, combined with the quasiharmonic approximation for computing the free energy, to the problem of determining the free energy and shear coupling factor of grain boundaries in metals over a range of realistic temperatures. Validation is achieved by comparison to thermodynamic integration, which confirms that the presented approach captures relaxed-energy GB structures and shear coupling factors at finite temperature with a high degree of accuracy. △ Less

Submitted 19 February, 2024; originally announced February 2024.

Journal ref: Computational Materials Science (2024)

Tagged particle behavior in a harmonic chain of direction reversing active Brownian particles

Authors: Shashank Prakash, Urna Basu, Sanjib Sabhapandit

Abstract: We study the tagged particle dynamics in a harmonic chain of direction reversing active Brownian particles, with spring constant $k$, rotation diffusion coefficient $D_{\text{r}}$, and directional reversal rate $γ$. We exactly compute the tagged particle position variance for quenched and annealed initial orientations of the particles. For well-separated time scales, $k^{-1}$, $D_{\text{r}}^{-1}$… ▽ More We study the tagged particle dynamics in a harmonic chain of direction reversing active Brownian particles, with spring constant $k$, rotation diffusion coefficient $D_{\text{r}}$, and directional reversal rate $γ$. We exactly compute the tagged particle position variance for quenched and annealed initial orientations of the particles. For well-separated time scales, $k^{-1}$, $D_{\text{r}}^{-1}$ and $γ^{-1}$, the strength of spring constant $k$ relative to $D_{\text{r}}$ and $γ$ gives rise to different coupling limits and for each coupling limit there are short, intermediate, and long time regimes. In the thermodynamic limit, we show that, to the leading order, the tagged particle variance exhibits an algebraic growth $t^ν$, where the value of the exponent $ν$ depends on the specific regime. For a quenched initial orientation, the exponent $ν$ crosses over from $3$ to $1/2$, via intermediate values $5/2$ or $1$, depending on the specific coupling limits. On the other hand, for the annealed initial orientation, $ν$ crosses over from $2$ to $1/2$ via an intermediate value $3/2$ or $1$ for strong coupling limit and weak coupling limit respectively. An additional time scale $t_N=N^2/k$ emerges for a system with a finite number of oscillators $N$. We show that the behavior of the tagged particle variance across $t_N$ can be expressed in terms of a crossover scaling function, which we find exactly. Finally, we characterize the stationary state behavior of the separation between two consecutive particles by calculating the corresponding spatio-temporal correlation function. △ Less

Submitted 11 March, 2025; v1 submitted 19 February, 2024; originally announced February 2024.

Journal ref: J. Stat. Mech. (2024) 083211

Magnetic Tunnel Junction Random Number Generators Applied to Dynamically Tuned Probability Trees Driven by Spin Orbit Torque

Authors: Andrew Maicke, Jared Arzate, Samuel Liu, Jaesuk Kwon, J. Darby Smith, James B. Aimone, Shashank Misra, Catherine Schuman, Suma G. Cardwell, Jean Anne C. Incorvia

Abstract: Perpendicular magnetic tunnel junction (pMTJ)-based true-random number generators (RNG) can consume orders of magnitude less energy per bit than CMOS pseudo-RNG. Here, we numerically investigate with a macrospin Landau-Lifshitz-Gilbert equation solver the use of pMTJs driven by spin-orbit torque to directly sample numbers from arbitrary probability distributions with the help of a tunable probabil… ▽ More Perpendicular magnetic tunnel junction (pMTJ)-based true-random number generators (RNG) can consume orders of magnitude less energy per bit than CMOS pseudo-RNG. Here, we numerically investigate with a macrospin Landau-Lifshitz-Gilbert equation solver the use of pMTJs driven by spin-orbit torque to directly sample numbers from arbitrary probability distributions with the help of a tunable probability tree. The tree operates by dynamically biasing sequences of pMTJ relaxation events, called 'coinflips', via an additional applied spin-transfer-torque current. Specifically, using a single, ideal pMTJ device we successfully draw integer samples on the interval 0,255 from an exponential distribution based on p-value distribution analysis. In order to investigate device-to-device variations, the thermal stability of the pMTJs are varied based on manufactured device data. It is found that while repeatedly using a varied device inhibits ability to recover the probability distribution, the device variations average out when considering the entire set of devices as a 'bucket' to agnostically draw random numbers from. Further, it is noted that the device variations most significantly impact the highest level of the probability tree, iwth diminishing errors at lower levels. The devices are then used to draw both uniformly and exponentially distributed numbers for the Monte Carlo computation of a problem from particle transport, showing excellent data fit with the analytical solution. Finally, the devices are benchmarked against CMOS and memristor RNG, showing faster bit generation and significantly lower energy use. △ Less

Submitted 27 November, 2023; originally announced November 2023.

Comments: 10 pages, 8 figures, 2 tables

Site-selective polar compensation of Mott electrons in a double perovskite heterointerface

Authors: Nandana Bhattacharya, Arpita Sen, Ke Qu, Arijit Sinha, Ranjan Kumar Patel, Siddharth Kumar, Jianwei Zhang, Prithwijit Mandal, Suresh Chandra Joshi, Shashank Kumar Ojha, Jyotirmay Maity, Zhan Zhang, Hua Zhou, Fanny Rodolakis, Padraic Shafer, Christoph Klewe, John William Freeland, Zhenzhong Yang, Umesh Waghmare, Srimanta Middey

Abstract: Double perovskite oxides (DPOs) with two transition metal ions ($A_2$$BB^\prime$O$_6$) offer a fascinating platform for exploring exotic physics and practical applications. Studying these DPOs as ultrathin epitaxial thin films on single crystalline substrates can add another dimension to engineering electronic, magnetic, and topological phenomena. Understanding the consequence of polarity mismatch… ▽ More Double perovskite oxides (DPOs) with two transition metal ions ($A_2$$BB^\prime$O$_6$) offer a fascinating platform for exploring exotic physics and practical applications. Studying these DPOs as ultrathin epitaxial thin films on single crystalline substrates can add another dimension to engineering electronic, magnetic, and topological phenomena. Understanding the consequence of polarity mismatch between the substrate and the DPO would be the first step towards this broad goal. We investigate this by studying the interface between a prototypical insulating DPO Nd$_2$NiMnO$_6$ and a wide-band gap insulator SrTiO$_3$. The interface is found to be insulating in nature. By combining several experimental techniques and density functional theory, we establish a site-selective charge compensation process that occurs explicitly at the Mn site of the film, leaving the Ni sites inert. We further demonstrate that such surprising selectivity, which cannot be explained by existing mechanisms of polarity compensation, is directly associated with their electronic correlation energy scales. This study establishes the crucial role of Mott physics in polar compensation process and paves the way for designer doping strategies in complex oxides. △ Less

Submitted 17 February, 2025; v1 submitted 27 November, 2023; originally announced November 2023.

Comments: 16 pages, 5 figures, 61 references

Journal ref: Phys. Rev. Lett. 134, 176201 (2025)

Temperature-Resilient True Random Number Generation with Stochastic Actuated Magnetic Tunnel Junction Devices

Authors: Laura Rehm, Md Golam Morshed, Shashank Misra, Ankit Shukla, Shaloo Rakheja, Mustafa Pinarbasi, Avik W. Ghosh, Andrew D. Kent

Abstract: Nanoscale magnetic tunnel junction (MTJ) devices can efficiently convert thermal energy in the environment into random bitstreams for computational modeling and cryptography. We recently showed that perpendicular MTJs activated by nanosecond pulses can generate true random numbers at high data rates. Here, we explore the dependence of probability bias-the deviations from equal probability (50/50)… ▽ More Nanoscale magnetic tunnel junction (MTJ) devices can efficiently convert thermal energy in the environment into random bitstreams for computational modeling and cryptography. We recently showed that perpendicular MTJs activated by nanosecond pulses can generate true random numbers at high data rates. Here, we explore the dependence of probability bias-the deviations from equal probability (50/50) 0/1 bit outcomes-of such devices on temperature, pulse amplitude, and duration. Our experimental results and device model demonstrate that operation with nanosecond pulses in the ballistic limit minimizes variation of probability bias with temperature to be far lower than that of devices operated with longer-duration pulses. Further, operation in the short-pulse limit reduces the bias variation with pulse amplitude while rendering the device more sensitive to pulse duration. These results are significant for designing TRNG MTJ circuits and establishing operating conditions. △ Less

Submitted 8 November, 2023; v1 submitted 28 October, 2023; originally announced October 2023.

Comments: 6 pages, 4 figures

Electronic Structure Prediction of Multi-million Atom Systems Through Uncertainty Quantification Enabled Transfer Learning

Authors: Shashank Pathrudkar, Ponkrshnan Thiagarajan, Shivang Agarwal, Amartya S. Banerjee, Susanta Ghosh

Abstract: The ground state electron density -- obtainable using Kohn-Sham Density Functional Theory (KS-DFT) simulations -- contains a wealth of material information, making its prediction via machine learning (ML) models attractive. However, the computational expense of KS-DFT scales cubically with system size which tends to stymie training data generation, making it difficult to develop quantifiably accur… ▽ More The ground state electron density -- obtainable using Kohn-Sham Density Functional Theory (KS-DFT) simulations -- contains a wealth of material information, making its prediction via machine learning (ML) models attractive. However, the computational expense of KS-DFT scales cubically with system size which tends to stymie training data generation, making it difficult to develop quantifiably accurate ML models that are applicable across many scales and system configurations. Here, we address this fundamental challenge by employing transfer learning to leverage the multi-scale nature of the training data, while comprehensively sampling system configurations using thermalization. Our ML models are less reliant on heuristics, and being based on Bayesian neural networks, enable uncertainty quantification. We show that our models incur significantly lower data generation costs while allowing confident -- and when verifiable, accurate -- predictions for a wide variety of bulk systems well beyond training, including systems with defects, different alloy compositions, and at unprecedented, multi-million-atom scales. Moreover, such predictions can be carried out using only modest computational resources. △ Less

Submitted 1 May, 2024; v1 submitted 24 August, 2023; originally announced August 2023.

Dense plasma irradiated platinum with improved spin Hall effect

Authors: Sachin Kumar, Sourabh Manna, John Rex Mohan, Utkarsh Shashank, Jospeh Vimal, Mayank Mishra, Surbhi Gupta, Hironori Asada, Yasuhiro Fukuma, Rajdeep Singh Rawat, Rohit Medwal

Abstract: The impurity incorporation in host high-spin orbit coupling materials like platinum has shown improved charge-to-spin conversion by modifying the up-spin and down-spin electron trajectories by bending or skewing them in opposite directions. This enables efficient generation, manipulation, and transport of spin currents. In this study, we irradiate the platinum with non-focus dense plasma to incorp… ▽ More The impurity incorporation in host high-spin orbit coupling materials like platinum has shown improved charge-to-spin conversion by modifying the up-spin and down-spin electron trajectories by bending or skewing them in opposite directions. This enables efficient generation, manipulation, and transport of spin currents. In this study, we irradiate the platinum with non-focus dense plasma to incorporate the oxygen ion species. We systematically analyze the spin Hall angle of the oxygen plasma irradiated Pt films using spin torque ferromagnetic resonance. Our results demonstrate a 2.4 times enhancement in the spin Hall effect after plasma treatment of Pt as compared to pristine Pt. This improvement is attributed to the introduction of disorder and defects in the Pt lattice, which enhances the spin-orbit coupling and leads to more efficient charge-to-spin conversion without breaking the spin-orbit torque symmetries. Our findings offer a new method of dense plasma-based modification of material for the development of advanced spintronic devices based on Pt and other heavy metals. △ Less

Submitted 22 July, 2023; originally announced July 2023.

Quantum fluctuations lead to glassy electron dynamics in the good metal regime of electron doped KTaO3

Authors: Shashank Kumar Ojha, Sankalpa Hazra, Surajit Bera, Sanat Kumar Gogoi, Prithwijit Mandal, Jyotirmay Maity, A. Gloskovskii, C. Schlueter, Smarajit Karmakar, Manish Jain, Sumilan Banerjee, Venkatraman Gopalan, Srimanta Middey

Abstract: One of the central challenges in condensed matter physics is to comprehend systems that have strong disorder and strong interactions. In the strongly localized regime, their subtle competition leads to glassy electron dynamics which ceases to exist well before the insulator-to-metal transition is approached as a function of doping. Here, we report on the discovery of glassy electron dynamics deep… ▽ More One of the central challenges in condensed matter physics is to comprehend systems that have strong disorder and strong interactions. In the strongly localized regime, their subtle competition leads to glassy electron dynamics which ceases to exist well before the insulator-to-metal transition is approached as a function of doping. Here, we report on the discovery of glassy electron dynamics deep inside the good metal regime of an electron-doped quantum paraelectric system: KTaO$_3$. We reveal that upon excitation of electrons from defect states to the conduction band, the excess injected carriers in the conduction band relax in a stretched exponential manner with a large relaxation time, and the system evinces simple aging phenomena - a telltale sign of glassy dynamics. Most significantly, we observe a critical slowing down of carrier dynamics below 35 K, concomitant with the onset of quantum paraelectricity in the undoped KTaO$_3$. Our combined investigation using second harmonic generation technique, density functional theory and phenomenological modeling demonstrates quantum fluctuation-stabilized soft polar modes as the impetus for the glassy behavior. This study addresses one of the most fundamental questions regarding the potential promotion of glassiness by quantum fluctuations and opens a route for exploring glassy dynamics of electrons in a well-delocalized regime. △ Less

Submitted 5 June, 2024; v1 submitted 26 June, 2023; originally announced June 2023.

Comments: 51 pages and 18 figures including supplemental

Journal ref: Nature Communications 15, 3830 (2024)

Orthorhombic distortion drives orbital ordering in an antiferromagnetic 3$d^1$ Mott insulator

Authors: Prithwijit Mandal, Shashank Kumar Ojha, Duo Wang, Ranjan Kumar Patel, Siddharth Kumar, Jyotirmay Maity, Zhan Zhang, Hua Zhou, Christoph Klewe, Padraic Shafer, Biplab Sanyal, Srimanta Middey

Abstract: The orbital, which represents the shape of the electron cloud, very often strongly influences the manifestation of various exotic phenomena, e.g., magnetism, metal-insulator transition, colossal magnetoresistance, unconventional superconductivity etc. in solid-state systems. The observation of the antiferromagnetism in $RE$TiO$_3$ ($RE$=rare earth) series has been puzzling since the celebrated Kug… ▽ More The orbital, which represents the shape of the electron cloud, very often strongly influences the manifestation of various exotic phenomena, e.g., magnetism, metal-insulator transition, colossal magnetoresistance, unconventional superconductivity etc. in solid-state systems. The observation of the antiferromagnetism in $RE$TiO$_3$ ($RE$=rare earth) series has been puzzling since the celebrated Kugel-Khomskii model of spin-orbital super exchange predicts ferromagnetism in an orbitally degenerate $d^1$ systems. Further, the existence of the orbitally ordered vs. orbital liquid phase in both antiferromagnetic and paramagnetic phase have been unsettled issues thus far. To address these long-standing questions, we investigate single crystalline film of PrTiO$_3$. Our synchrotron X-ray diffraction measurements confirm the retention of bulk-like orthorhombic ($D_{2h}$) symmetry in the thin film geometry. We observe similar X-ray linear dichroism signal in both paramagnetic and antiferromagnetic phase, which can be accounted by ferro orbital ordering (FOO). While the presence of $D_{2h}$ crystal field does not guarantee lifting of orbital degeneracy always, we find it to be strong enough in these rare-earth titanates, leading to the FOO state. Thus, our work demonstrates the orthorhombic distortion is the driving force for the orbital ordering of antiferromagnetic $RE$TiO$_3$. △ Less

Submitted 15 June, 2023; originally announced June 2023.

Comments: 37 pages and 17 figures

Interfacial origin of unconventional spin-orbit torque in Py/$γ-$IrMn$_{3}$

Authors: Akash Kumar, Pankhuri Gupta, Niru Chowdhury, Kacho Imtiyaz Ali Khan, Utkarsh Shashank, Surbhi Gupta, Yasuhiro Fukuma, Sujeet Chaudhary, Pranaba Kishor Muduli

Abstract: Angle-resolved spin-torque ferromagnetic resonance measurements are carried out in heterostructures consisting of Py (Ni$_{81}$Fe$_{19}$) and a noncollinear antiferromagnetic quantum material $γ-$IrMn$_{3}$. The structural characterization reveals that $γ-$IrMn$_{3}$ is polycrystalline in nature. A large exchange bias of 158~Oe is found in Py/$γ-$IrMn$_{3}$ at room temperature, while $γ-$IrMn… ▽ More Angle-resolved spin-torque ferromagnetic resonance measurements are carried out in heterostructures consisting of Py (Ni$_{81}$Fe$_{19}$) and a noncollinear antiferromagnetic quantum material $γ-$IrMn$_{3}$. The structural characterization reveals that $γ-$IrMn$_{3}$ is polycrystalline in nature. A large exchange bias of 158~Oe is found in Py/$γ-$IrMn$_{3}$ at room temperature, while $γ-$IrMn$_{3}$/Py and Py/Cu/$γ-$IrMn$_{3}$ exhibited no exchange bias. Regardless of the exchange bias and stacking sequence, we observe a substantial unconventional out-of-plane anti-damping torque when $γ-$IrMn$_{3}$ is in direct contact with Py. The magnitude of the out-of-plane spin-orbit torque efficiency is found to be twice as large as the in-plane spin-orbit torque efficiency. The unconventional spin-orbit torque vanishes when a Cu spacer is introduced between Py and $γ-$IrMn$_{3}$, indicating that the unconventional spin-orbit torque in this system originates at the interface. These findings are important for realizing efficient antiferromagnet-based spintronic devices via interfacial engineering. △ Less

Submitted 8 May, 2023; originally announced May 2023.

Journal ref: Advanced Quantum Technologies 2300092(2023)