Showing 1–50 of 61 results for author: Bose, A

Symmetry-Guided Computational Screening of Two-Dimensional Altermagnets with ab initio Hubbard Corrections

Authors: Anumita Bose, Nataliia Manko, Marco Gibertini, Antimo Marrazzo

Abstract: Altermagnets combine compensated antiferromagnetic order with momentum-dependent spin splitting, offering a promising platform for spintronic applications without macroscopic magnetization or stray magnetic fields. Although a wide range of three-dimensional (3D) materials have been identified as altermagnets, two-dimensional (2D) altermagnets remain comparatively limited. In this work, we perform… ▽ More Altermagnets combine compensated antiferromagnetic order with momentum-dependent spin splitting, offering a promising platform for spintronic applications without macroscopic magnetization or stray magnetic fields. Although a wide range of three-dimensional (3D) materials have been identified as altermagnets, two-dimensional (2D) altermagnets remain comparatively limited. In this work, we perform a high-throughput computational search for altermagnetism across 2710 materials in the Materials Cloud 2D Crystals (MC2D) database. Our approach combines symmetry-based screening with first-principles density functional theory calculations, including self-consistent Hubbard-$U$ corrections, to reliably capture magnetic ground states. Through a systematic exploration of magnetic configurations and their energetic stability, we identify 42 materials exhibiting altermagnetic ground states for at least one value of $U$, of which 24 remain robust upon determination of the Hubbard-$U$ parameters from first principles--including 4 materials previously reported in the literature and 20 newly predicted candidates. These comprise promising monolayers such as metallic Fe$_2$Si$_2$SbO$_9$, and insulating CoBrO, with spin splittings about 294 meV and 330 meV, respectively. Our results significantly expand the pool of potential 2D altermagnet candidates with favorable exfoliation energetics and provide valuable guidance for experimental efforts. In addition, this work establishes a high-throughput computational framework for reproducible discovery and characterization of altermagnetic materials. △ Less

Submitted 3 August, 2026; originally announced August 2026.

Comments: 12 pages, 5 figures

Orbital and Spin Nernst Effects in Monolayers of Transition Metal Dichalcogenides

Authors: Saikat Saha, Arnab Bose, Sayantika Bhowal

Abstract: In recent years, orbitronic effects have attracted growing attention as complementary counterparts to the well-established spintronic phenomena. In this work, we demonstrate that monolayers of transition metal dichalcogenides provide an excellent platform for the observation of the orbital Nernst effect, a relatively less explored phenomenon describing the generation of a transverse orbital curren… ▽ More In recent years, orbitronic effects have attracted growing attention as complementary counterparts to the well-established spintronic phenomena. In this work, we demonstrate that monolayers of transition metal dichalcogenides provide an excellent platform for the observation of the orbital Nernst effect, a relatively less explored phenomenon describing the generation of a transverse orbital current in response to an applied temperature gradient. We show that, similar to its electrical counterpart, viz., the orbital Hall effect, the orbital Nernst effect does not require the presence of spin-orbit coupling. Analytical results based on a low-energy valley model offer key insights into the underlying mechanisms, highlighting in particular the crucial role of electronic states at the Fermi energy for the emergence of this effect. The inclusion of spin-orbit coupling further gives rise to a spin Nernst effect, which scales with the strength of spin-orbit coupling and vanishes in its absence. We substantiate our analytical findings with full Brillouin-zone tight-binding results for two representative systems, monolayer 2H MoS$_2$ and 2H NbS$_2$. Our results show that while both orbital and spin Nernst conductivities in MoS$_2$ require electron or hole doping, both effects are intrinsically present in metallic NbS$_2$. Our work reveals the central role of orbital and spin Berry curvatures, identifies doping as an effective route for tuning orbital and spin Nernst responses, and proposes a possible experimental setup for detecting these effects in monolayer transition metal dichalcogenides. △ Less

Submitted 11 May, 2026; originally announced May 2026.

Comments: 13 pages, 9 figures

Multimer Embedding for Molecular Crystals Utilizing up to Tetramer Interactions

Authors: Alexander List, A. Daniel Boese, Johannes Hoja

Abstract: Molecular crystals possess a highly complex crystallographic landscape which in many cases results in the experimental observation of multiple crystal structures for the same compound. Accurate results can often be obtained for such systems by employing periodic density functional theory using hybrid functionals; however, this is not always computationally feasible. One possibility to circumvent t… ▽ More Molecular crystals possess a highly complex crystallographic landscape which in many cases results in the experimental observation of multiple crystal structures for the same compound. Accurate results can often be obtained for such systems by employing periodic density functional theory using hybrid functionals; however, this is not always computationally feasible. One possibility to circumvent these expensive periodic calculations is the utilization of multimer embedding methods. Therein, the fully periodic crystal is described at a lower level of theory, and subsequently monomer energies, dimer interaction energies, etc. are corrected via high-level calculations. In this paper, we further extend such a multimer embedding approach by one multimer order for all investigated properties, allowing us to compute lattice energies up to the tetramer embedding level, and atomic forces, the stress tensor, and harmonic phonons up to the trimer level. We test the significance of including these higher-order multimers by embedding PBE0+MBD multimers into periodic PBE+MBD calculations utilizing the X23 benchmark set of molecular crystals and comparing the results to explicit periodic PBE0+MBD calculations. We show that tetramer interactions systematically improve the lattice energy approximation and explore multiple possibilities for multimer selection. Furthermore, we confirm that trimer interactions are crucial for the description of the stress tensor, yielding cell volumes within 1 % of those of PBE0+MBD. Subsequently, this also results in an improvement of the description of vibrational properties, giving on average gamma point frequencies within 1.3 wave numbers and vibrational free energies within 0.3 kJ/mol of the PBE0+MBD results. △ Less

Submitted 18 December, 2025; originally announced December 2025.

OXtal: An All-Atom Diffusion Model for Organic Crystal Structure Prediction

Authors: Emily Jin, Andrei Cristian Nica, Mikhail Galkin, Jarrid Rector-Brooks, Kin Long Kelvin Lee, Santiago Miret, Frances H. Arnold, Michael Bronstein, Avishek Joey Bose, Alexander Tong, Cheng-Hao Liu

Abstract: Accurately predicting experimentally realizable 3D molecular crystal structures from their 2D chemical graphs is a long-standing open challenge in computational chemistry called crystal structure prediction (CSP). Efficiently solving this problem has implications ranging from pharmaceuticals to organic semiconductors, as crystal packing directly governs the physical and chemical properties of orga… ▽ More Accurately predicting experimentally realizable 3D molecular crystal structures from their 2D chemical graphs is a long-standing open challenge in computational chemistry called crystal structure prediction (CSP). Efficiently solving this problem has implications ranging from pharmaceuticals to organic semiconductors, as crystal packing directly governs the physical and chemical properties of organic solids. In this paper, we introduce OXtal, a large-scale 100M parameter all-atom diffusion model that directly learns the conditional joint distribution over intramolecular conformations and periodic packing. To efficiently scale OXtal, we abandon explicit equivariant architectures imposing inductive bias arising from crystal symmetries in favor of data augmentation strategies. We further propose a novel crystallization-inspired lattice-free training scheme, Stoichiometric Stochastic Shell Sampling ($S^4$), that efficiently captures long-range interactions while sidestepping explicit lattice parametrization -- thus enabling more scalable architectural choices at all-atom resolution. By leveraging a large dataset of 600K experimentally validated crystal structures (including rigid and flexible molecules, co-crystals, and solvates), OXtal achieves orders-of-magnitude improvements over prior ab initio machine learning CSP methods, while remaining orders of magnitude cheaper than traditional quantum-chemical approaches. Specifically, OXtal recovers experimental structures with conformer $\text{RMSD}_1<0.5$ Å and attains over 80\% packing similarity rate, demonstrating its ability to model both thermodynamic and kinetic regularities of molecular crystallization. △ Less

Submitted 20 April, 2026; v1 submitted 7 December, 2025; originally announced December 2025.

Non-relativistic spin splitting: Features and Functionalities

Authors: Sayantika Bhowal, Arnab Bose

Abstract: Recently, spin splitting of non-relativistic origin in compensated antiferromagnets has drawn growing attention in condensed matter research. Although many materials, now known to exhibit such spin splitting, have been studied for decades, their manifestation along non-high-symmetry momentum directions initially hindered their recognition. In recent years, significant progress has been made in unc… ▽ More Recently, spin splitting of non-relativistic origin in compensated antiferromagnets has drawn growing attention in condensed matter research. Although many materials, now known to exhibit such spin splitting, have been studied for decades, their manifestation along non-high-symmetry momentum directions initially hindered their recognition. In recent years, significant progress has been made in uncovering the symmetry principles that allow non-relativistic spin splitting in the absence of net magnetization, revealing the unconventional physics arising from their coexistence. In this review, we provide a concise overview of non-relativistic spin splitting in compensated antiferromagnets with various spin configurations, including collinear, coplanar, and non-coplanar spin arrangements. We summarize practical identification guidelines, highlight characteristic features in electronic band structures, and discuss the emerging functionalities, with an emphasis on promising directions for future exploration. △ Less

Submitted 23 October, 2025; originally announced October 2025.

Comments: 17 pages, 9 figures

Unlocking Doping Effects on Altermagnetism in MnTe: Emergence of Quasi-altermagnetism

Authors: Nayana Devaraj, Anumita Bose, Arindom Das, Md Afsar Reja, Arijit Mandal, Awadhesh Narayan, B. R. K. Nanda

Abstract: Governed by specific symmetries, altermagnetism is an emerging field in condensed matter physics, characterized by unique spin-splitting of the bands in the momentum space co-existing with the compensated magnetization as in antiferromagnets. As crystals can have tailored and unintended defects, it is important to gain insights on how altermagnets are affected by the defects-driven symmetry-breaki… ▽ More Governed by specific symmetries, altermagnetism is an emerging field in condensed matter physics, characterized by unique spin-splitting of the bands in the momentum space co-existing with the compensated magnetization as in antiferromagnets. As crystals can have tailored and unintended defects, it is important to gain insights on how altermagnets are affected by the defects-driven symmetry-breaking which, in turn, can build promising perspectives on potential applications. In this study, considering the widely investigated MnTe as a prototype altermagnet, defects are introduced through substitutional doping to create a large configuration space of spin space groups. With the aid of density functional theory calculations, symmetry analysis, and model studies in this configuration space, we demonstrate the generic presence of spin-split of the antiferromagnetic bands in the momentum space. This is indicative of a wider class of quasi-altermagnetic materials, augmenting the set of ideal altermagnetic systems. Furthermore, we show that while pristine MnTe does not show anomalous Hall conductivity (AHC) with out-of-plane magnetization, suitable doping can be carried out to obtain finite and varied AHC. Our predictions of quasi-altermagnetism and doping-driven tailored AHC have the potential to open up as-yet-unexplored directions in this developing field. △ Less

Submitted 27 March, 2026; v1 submitted 27 August, 2025; originally announced August 2025.

Comments: 17 pages, 11 figures, and 1 table

Journal ref: Phys. Rev. B, 113, 104438 (2026)

Symmetry constrained field theories for chiral spin liquid to spin crystal transitions

Authors: Anjishnu Bose, Andrew Hardy, Naren Manjunath, Ramanjit Sohal, Arun Paramekanti

Abstract: We consider the spin rotationally invariant Kalmeyer-Laughlin chiral spin liquid (CSL) in systems with broken time-reversal symmetry and explore symmetry constraints on possible conventional spin crystal states accessible via a direct transition. These constraints provide a framework to identify topological invariants of the magnetically ordered state. We show that the existence of a direct transi… ▽ More We consider the spin rotationally invariant Kalmeyer-Laughlin chiral spin liquid (CSL) in systems with broken time-reversal symmetry and explore symmetry constraints on possible conventional spin crystal states accessible via a direct transition. These constraints provide a framework to identify topological invariants of the magnetically ordered state. We show that the existence of a direct transition from a CSL requires a precise compatibility condition between the topological invariants of the ordered state and the anomaly of the CSL. The lattice symmetries also constrain the functional form of the low-energy theory to describe these transitions. This allows us to construct explicit Chern-Simons-matter field theories for the transition into a class of noncoplanar orders identified as candidates directly accessible from the CSL, including the octahedral spin crystal on the kagomé lattice, and the tetrahedral order on the triangular and honeycomb lattice. These transitions can either be described using coupled fractionalized $ \mathbb{CP}^1 $ theories or fractionalized matrix principal chiral models. We also discuss extensions to more general magnetic ordering transitions out of the CSL. △ Less

Submitted 6 November, 2025; v1 submitted 2 May, 2025; originally announced May 2025.

Comments: 26 pages, 3 figures, 4 tables

Modified large-$N$ approach to gapless spin liquids, magnetic orders, and dynamics: Application to triangular lattice antiferromagnets

Authors: Anjishnu Bose, Kathleen Hart, Ruairidh Sutcliffe, Arun Paramekanti

Abstract: Recent work has shown that the triangular lattice spin-$1/2$ $J_1$-$J_2$ Heisenberg and XXZ antiferromagnets may exhibit coplanar or supersolid orders proximate to a gapless Dirac spin liquid phase. We explore a distinct $SU(2N)\!\!\times\!\!SU(M)$ fermionic parton approach, complemented by variational Monte Carlo calculations for the spin-$1/2$ model, to study the phase diagram of these models. W… ▽ More Recent work has shown that the triangular lattice spin-$1/2$ $J_1$-$J_2$ Heisenberg and XXZ antiferromagnets may exhibit coplanar or supersolid orders proximate to a gapless Dirac spin liquid phase. We explore a distinct $SU(2N)\!\!\times\!\!SU(M)$ fermionic parton approach, complemented by variational Monte Carlo calculations for the spin-$1/2$ model, to study the phase diagram of these models. We also calculate their dynamical spin response including parton interactions within a random phase approximation, and discuss implications for neutron scattering on triangular lattice cobaltates Ba$_3$CoSb$_2$O$_9$, Na$_2$BaCo(PO$_4$)$_2$, K$_2$Co(SeO$_3$)$_2$, Rb$_2$Co(SeO$_3$)$_2$, and Yb-based magnet KYbSe$_2$. △ Less

Submitted 4 June, 2025; v1 submitted 12 March, 2025; originally announced March 2025.

Comments: 16 pages, 14 figures

Journal ref: Phys. Rev. B 111, 214410 Published 4 June, 2025

Identification of orbital pumping from spin pumping and rectification effects

Authors: Nils Keller, Arnab Bose, Nozomi Soya, Elias Hauth, Fabian Kammerbauer, Rahul Gupta, Hiroki Hayashi, Hisanobu Kashiki, Gerhard Jakob, Sachin Krishnia, Kazuya Ando, Mathias Kläui

Abstract: The recently predicted mechanism of orbital pumping enables the generation of pure orbital current from a precessing ferromagnet (FM) without the need for electrical current injection. This orbital current can be efficiently injected into an adjacent nonmagnetic material (NM) without being hampered by electrical conductivity mismatch. However, experimentally identifying this novel effect presents… ▽ More The recently predicted mechanism of orbital pumping enables the generation of pure orbital current from a precessing ferromagnet (FM) without the need for electrical current injection. This orbital current can be efficiently injected into an adjacent nonmagnetic material (NM) without being hampered by electrical conductivity mismatch. However, experimentally identifying this novel effect presents significant challenges due to the substantial background contributions from spin pumping and spin rectification effects (SREs). In this work, we disentangle the effects of orbital pumping from spin pumping in bilayer structures composed of Nb/Ni and Nb/$\mathrm{Fe_{60}Co_{20}B_{20}}$ by observing a sign reversal of the measured voltage. This reversal arises from the competing signs of the spin and orbital Hall effects in the Nb. We establish methods to differentiate the pumping signal from SREs by analyzing the distinct angular dependence of the measured voltage and its spatial dependence relative to the radio frequency excitation source. △ Less

Submitted 12 February, 2025; originally announced February 2025.

Spin dynamics of an easy-plane Dirac spin liquid in a frustrated XY model: Application to honeycomb cobaltates

Authors: Anjishnu Bose, Arun Paramekanti

Abstract: Recent work has shown that the honeycomb lattice spin-$1/2$ $J_1$-$J_3$ XY model, with nearest-neighbor ferromagnetic exchange $J_1$ and frustration induced by third-neighbor antiferromagnetic exchange $J_3$, may be relevant to a wide range of cobaltate materials. We explore a variational Monte Carlo study of Gutzwiller projected wavefunctions for this model and show that an easy-plane Dirac spin… ▽ More Recent work has shown that the honeycomb lattice spin-$1/2$ $J_1$-$J_3$ XY model, with nearest-neighbor ferromagnetic exchange $J_1$ and frustration induced by third-neighbor antiferromagnetic exchange $J_3$, may be relevant to a wide range of cobaltate materials. We explore a variational Monte Carlo study of Gutzwiller projected wavefunctions for this model and show that an easy-plane Dirac spin liquid (DSL) is a viable `parent' state for the competing magnetic orders observed in these materials, including ferromagnetic, zig-zag, spiral, and double zig-zag orders at intermediate frustration, and show that such broken symmetry states can be easily polarized by a weak in-plane magnetic field consistent with experiments. We formulate a modified parton theory for such frustrated spin models, and explore the potential instabilities of the DSL due to residual parton interactions within a random phase approximation (RPA), both at zero magnetic field and in a nonzero in-plane field. The broken symmetry states which emerge in the vicinity of this Dirac spin liquid include ferromagnetic, zig-zag, and incommensurate spiral orders, with a phase diagram which is consistent with VMC and density matrix renormalization group studies. We calculate the dynamical spin response of the easy-plane DSL, including RPA corrections, near the boundary of the ordered states, and present results for THz spectroscopy and inelastic neutron scattering, at zero field as well as in an in-plane magnetic field, and discuss experimental implications. △ Less

Submitted 5 December, 2024; originally announced December 2024.

Comments: 18 pages, 18 figures

Anomalous and parallel Hall effects in ferromagnetic Weyl semimetal Cr$_3$Te$_4$

Authors: Anumita Bose, Shubham Purwar, Setti Thirupathaiah, Awadhesh Narayan

Abstract: Recently, time-reversal symmetry broken magnetic Weyl semimetals (WSMs) have attracted extensive attention and have provided an intriguing platform for exploring fundamental physical phenomena. The study of chromium telluride-based systems has also drawn significant interest towards spintronics applications owing to their high Curie temperatures. Here, using \textit{ab initio} calculations, we pro… ▽ More Recently, time-reversal symmetry broken magnetic Weyl semimetals (WSMs) have attracted extensive attention and have provided an intriguing platform for exploring fundamental physical phenomena. The study of chromium telluride-based systems has also drawn significant interest towards spintronics applications owing to their high Curie temperatures. Here, using \textit{ab initio} calculations, we propose the emergence of multiple Weyl points (WPs) near the Fermi level in such an intrinsic ferromagnetic system, Cr$_3$Te$_4$. The large, well-separated, nontrivial Fermi arcs and surface states, suggest that the WPs are highly robust and resilient to perturbations. A substantial Berry curvature contribution in the vicinity of the Fermi energy not only serves as the origin of large conventional anomalous Hall conductivity (AHC), but also produces unconventional parallel AHC in this material, owing to the low structural symmetry. In addition to the charge Hall conductivity, we also find significant anomalous Nernst conductivities originating from the Berry curvature. Alongside our theoretical predictions, we present complementary experimental results, including X-ray diffraction (XRD) analysis and an examination of the magnetic properties, which demonstrate a Curie temperature of 327 K. Our study advances the understanding of magnetic WSMs, and also encourages further studies in the context of topological properties of our proposed material. △ Less

Submitted 25 November, 2024; originally announced November 2024.

Comments: 10 pages, 6 figures. Comments are welcome!

Charge ordering and spontaneous topological Hall effect in bilayer skyrmion crystals

Authors: Andrew Hardy, Anjishnu Bose, Tanmay Grover, Arun Paramekanti

Abstract: Magnetic skyrmion crystals with zero net skyrmion charge and zero topological Hall response are interesting candidate phases which can occur at a vanishing magnetic field in centrosymmetric systems. We study a minimal bilayer model of skyrmion crystals having opposite chirality and topological charge in the two layers, and show that it can host nearly flat electronic bands with quasi-uniform Berry… ▽ More Magnetic skyrmion crystals with zero net skyrmion charge and zero topological Hall response are interesting candidate phases which can occur at a vanishing magnetic field in centrosymmetric systems. We study a minimal bilayer model of skyrmion crystals having opposite chirality and topological charge in the two layers, and show that it can host nearly flat electronic bands with quasi-uniform Berry curvature and quantum metric. Using Hartree-Fock theory, we show that weak to moderate short-range electron interactions induce two distinct types of symmetry breaking patterns depending on the band dispersion: an intra-unit-cell charge density modulation from Chern band mixing or a layer-imbalanced phase with a nonzero ferroelectric polarization. Both phases break inversion symmetry leading to a spontaneous and large net topological Hall effect, with the phase diagram tunable by external electric fields. Our results may be relevant to centrosymmetric skyrmion materials such as Gd$_2$PdSi$_3$ and Gd$_3$Ru$_4$Al$_{12}$ as well as artificially engineered heterostructures. We also discuss its relation to recent work on twisted transition metal dichalcogenide bilayers. △ Less

Submitted 7 August, 2025; v1 submitted 6 September, 2024; originally announced September 2024.

Journal ref: Phys. Rev. B 111, 115115, 5 March, 2025

Interplay of altermagnetism and pressure in hexagonal and orthorhombic MnTe

Authors: Nayana Devaraj, Anumita Bose, Awadhesh Narayan

Abstract: Alternative magnetic materials or ``altermagnets", characterized by their non-relativistic, momentum-dependent spin-split states, represent a cutting-edge advancement in the field of magnetism, offering promising avenues for spintronic applications. Among these materials, hexagonal MnTe has emerged as a standout material candidate for its substantial spin-splitting. In this study, employing first-… ▽ More Alternative magnetic materials or ``altermagnets", characterized by their non-relativistic, momentum-dependent spin-split states, represent a cutting-edge advancement in the field of magnetism, offering promising avenues for spintronic applications. Among these materials, hexagonal MnTe has emerged as a standout material candidate for its substantial spin-splitting. In this study, employing first-principles electronic structure calculations and spin group symmetry analysis, we delve into the interplay of altermagnetism and pressure in two main phases of MnTe. Our relativistic calculations demonstrate the presence of tunable anomalous Hall effect (AHE) in hexagonal MnTe. In addition, our results underscore the pivotal role of pressure as a tuning parameter for the alternative magnetic traits in the system. Furthermore, we identify another phase of MnTe with orthorhombic structure, namely $γ$-MnTe, hosting altermagnetic characteristics. We study, in detail, its response in AHE and spin-spliting due to magnetization and pressure variations, respectively. Our study highlights the substantial impact of pressure on the properties of alternative magnetic materials, particularly emphasizing the pronounced tuning effect observed in the hexagonal and orthorhombic MnTe. △ Less

Submitted 3 December, 2024; v1 submitted 15 July, 2024; originally announced July 2024.

Journal ref: Phys. Rev. Mater, 8, 2024, 104407

Altermagnetism and superconductivity in a multiorbital t-J model

Authors: Anjishnu Bose, Samuel Vadnais, Arun Paramekanti

Abstract: Motivated by exploring doped multi-orbital antiferromagnets (AFMs) and altermagnets (ALMs) we explore minimal $t$-$J$ models on the square-octagon lattice which favor such collinear magnetic orders in the regime where spin exchange dominates. While the AFM order breaks translational and time-reversal symmetries, the ALM state (equivalently, a `$d$-wave ferromagnet') features multipolar order which… ▽ More Motivated by exploring doped multi-orbital antiferromagnets (AFMs) and altermagnets (ALMs) we explore minimal $t$-$J$ models on the square-octagon lattice which favor such collinear magnetic orders in the regime where spin exchange dominates. While the AFM order breaks translational and time-reversal symmetries, the ALM state (equivalently, a `$d$-wave ferromagnet') features multipolar order which separately breaks time-reversal and crystal rotation symmetries but preserves their product leading to spin-split bands with zero net magnetization. We study the mean field phase diagram of these models as we vary doping and interactions, discovering regimes of weak and strong ALM order, superconductivity including uniform $s$-wave and $d$-wave pairing states, incipient $d$-wave pair density wave order, and phases with coexisting singlet-triplet pairing and AFM/ALM orders which appear unstable to phase separation and could host stripe order with longer-range interactions. We study the mean field phase diagram of these multiorbital models as we vary doping and interactions, discovering two types of ALM order: (i) itinerant weak-coupling ALM metals driven by quasi-1D van Hove singularities, as well as (ii) strong ALM order at half-filling. We also find regimes of superconductivity including uniform $s$-wave and $d$-wave pairing states, incipient $d_{xy}$-wave pair density wave order, and uniform phases with coexisting singlet-triplet pairing and ALM order. Our inhomogeneous mean field theory approach reveals that the coexistence phases are unstable to phase separation, but longer-range interactions could lead to stripe order. Our results may be relevant to doping or pressure studies of multiorbital ALM materials. △ Less

Submitted 19 February, 2025; v1 submitted 25 March, 2024; originally announced March 2024.

Comments: 14 pages, 13 figures

Journal ref: Physical Review B 110, 205120 2024

Analysis of Time-Evolution of Gaussian Wavepackets in Non-Hermitian Systems

Authors: Amartya Bose

Abstract: Simulation and analysis of multidimensional dynamics of a quantum non-Hmeritian system is a challenging problem. Gaussian wavepacket dynamics has proven to be an intuitive semiclassical approach to approximately solving the dynamics of quantum systems. A Gaussian wavepacket approach is proposed for a continuous space extension to the Hatano-Nelson model that enables transparent analysis of the dyn… ▽ More Simulation and analysis of multidimensional dynamics of a quantum non-Hmeritian system is a challenging problem. Gaussian wavepacket dynamics has proven to be an intuitive semiclassical approach to approximately solving the dynamics of quantum systems. A Gaussian wavepacket approach is proposed for a continuous space extension to the Hatano-Nelson model that enables transparent analysis of the dynamics in terms of complex classical trajectories. We demonstrate certain cases where the configuration space trajectory can be made fully real by transforming the initial conditions to account for the non-Hermiticity appropriately through the momentum coordinates. However, in general the complex phase space is unavoidable. For the cases where the trajectory is real, the effective force can be decomposed into that due to the potential energy surface and that due to the imaginary vector potential. The impact of the vector potential on the trajectory of the wavepacket is directly proportional to both the strength of the vector potential and the width of the wavepacket. △ Less

Submitted 30 January, 2024; originally announced January 2024.

Comments: 9 pages, 6 figures

Fluctuation-mediated spin-orbit torque enhancement in the noncollinear antiferromagnet Mn3Ni0.35Cu0.65N

Authors: Arnab Bose, Tom G. Saunderson, Aga Shahee, Lichuan Zhang, Tetsuya Hajiri, Adithya Rajan, Dongwook Go, Hidefumi Asano, Udo Schwingenschlögl, Aurelien Manchon, Yuriy Mokrousov, Mathias Kläui

Abstract: The role of spin fluctuations near magnetic phase transitions is crucial for generating various exotic phenomena, including anomalies in the extraordinary Hall effect, excess spin-current generation through the spin-Hall effect (SHE), and enhanced spin-pumping, amongst others. In this study, we experimentally investigate the temperature dependence of spin-orbit torques (SOTs) generated by Mn3Ni0.3… ▽ More The role of spin fluctuations near magnetic phase transitions is crucial for generating various exotic phenomena, including anomalies in the extraordinary Hall effect, excess spin-current generation through the spin-Hall effect (SHE), and enhanced spin-pumping, amongst others. In this study, we experimentally investigate the temperature dependence of spin-orbit torques (SOTs) generated by Mn3Ni0.35Cu0.65N (MNCN), a member of the noncollinear antiferromagnetic family that exhibits unconventional magnetotransport properties. Our work uncovers a strong and nontrivial temperature dependence of SOTs, peaking near the Néel temperature of MNCN, which cannot be explained by conventional intrinsic and extrinsic scattering mechanisms of the SHE. Notably, we measure a maximum SOT efficiency of 30%, which is substantially larger than that of commonly studied nonmagnetic materials such as Pt. Theoretical calculations confirm a negligible SHE and a strong orbital Hall effect that can explain the observed SOTs. We propose a previously unidentified mechanism wherein fluctuating antiferromagnetic moments trigger the generation of substantial orbital currents near the Néel temperature due to the emergence of scalar spin chirality. Our findings present an approach for enhancing SOTs, which holds promise for magnetic memory applications by leveraging antiferromagnetic spin fluctuations to amplify both orbital and spin currents. △ Less

Submitted 29 January, 2024; originally announced January 2024.

The "visible" Wigner matrix

Authors: Arup Bose, Soumendu Sundar Mukherjee

Abstract: We consider the ``visible'' Wigner matrix, a Wigner matrix whose $(i, j)$-th entry is coerced to zero if $i, j$ are co-prime. Using a recent result from elementary number theory on co-primality patterns in integers, we show that the limiting spectral distribution of this matrix exists, and give explicit descriptions of its moments in terms of infinite products over primes $p$ of certain polynomial… ▽ More We consider the ``visible'' Wigner matrix, a Wigner matrix whose $(i, j)$-th entry is coerced to zero if $i, j$ are co-prime. Using a recent result from elementary number theory on co-primality patterns in integers, we show that the limiting spectral distribution of this matrix exists, and give explicit descriptions of its moments in terms of infinite products over primes $p$ of certain polynomials evaluated at $1/p$. We also consider the complementary ``invisible'' Wigner matrix. △ Less

Submitted 19 December, 2023; originally announced December 2023.

Comments: 12 pages, 5 figures, 1 table

MSC Class: 60B20

Weak Electronic Correlations Observed in Magnetic Weyl Semimetal Mn$_3$Ge

Authors: Susmita Changdar, Susanta Ghosh, Anumita Bose, Indrani Kar, Achintya Low, Patrick Le Fevre, François Bertran, Awadhesh Narayan, Setti Thirupathaiah

Abstract: Using angle-resolved photoemission spectroscopy (ARPES) and density functional theory (DFT) calculations, we systematically studied the electronic band structure of Mn$_3$Ge in the vicinity of the Fermi level. We observe several bands crossing the Fermi level, confirming the metallic nature of the studied system. We further observe several flat bands along various high symmetry directions, consist… ▽ More Using angle-resolved photoemission spectroscopy (ARPES) and density functional theory (DFT) calculations, we systematically studied the electronic band structure of Mn$_3$Ge in the vicinity of the Fermi level. We observe several bands crossing the Fermi level, confirming the metallic nature of the studied system. We further observe several flat bands along various high symmetry directions, consistent with the DFT calculations. The calculated partial density of states (PDOS) suggests a dominant Mn $3d$ orbital contribution to the total valence band DOS. With the help of orbital-resolved band structure calculations, we qualitatively identify the orbital information of the experimentally obtained band dispersions. Out-of-plane electronic band dispersions are explored by measuring the ARPES data at various photon energies. Importantly, our study suggests relatively weaker electronic correlations in Mn$_3$Ge compared to Mn$_3$Sn. △ Less

Submitted 1 December, 2023; originally announced December 2023.

Comments: 11 pages, 3 figures. To appear in the Journal of Physics: Condensed Matter

Origin of flat bands and non-trivial topology in coupled kagome lattices

Authors: Anumita Bose, Arka Bandyopadhyay, Awadhesh Narayan

Abstract: We propose an exact analytical decimation transformation scheme to explore the fascinating coexistence of flat bands and Dirac fermions in three-dimensional coupled kagome systems. Our method allows coarse-graining of the parameter space that maps the original system to an equivalent low-level lattice. The decimated system enables defining a quantity in the tight-binding parameter space that predo… ▽ More We propose an exact analytical decimation transformation scheme to explore the fascinating coexistence of flat bands and Dirac fermions in three-dimensional coupled kagome systems. Our method allows coarse-graining of the parameter space that maps the original system to an equivalent low-level lattice. The decimated system enables defining a quantity in the tight-binding parameter space that predominantly controls the emergence of a flat band (FB) and provides a specific criterion for absolute flatness. Likewise, in terms of atomic separations, we develop a quantity that primarily controls the FB width in real materials and thus can be helpful in predicting new systems hosting FB as well as in tuning the FB width. Our predictions on the emergence of the flat band and Dirac fermions are confirmed for M$_3$X (M= Ni, Mn, Co, Fe; X= Al, Ga, In, Sn, Cr,...) family of materials, leveraging materials databases and first-principles calculations. Our work provides an analytical formalism that enables accurate predictions of FBs in real materials. △ Less

Submitted 7 November, 2023; v1 submitted 27 October, 2023; originally announced October 2023.

Comments: v2: 6+7 pages, 3+7 figures. Supplementary information and new references added

Investigation of the Anomalous and Topological Hall Effects in Layered Monoclinic Ferromagnet Cr$_{2.76}$Te$_4$

Authors: Shubham Purwar, Achintya Low, Anumita Bose, Awadhesh Narayan, S. Thirupathaiah

Abstract: We studied the electrical transport, Hall effect, and magnetic properties of monoclinic layered ferromagnet Cr$_{2.76}$Te$_4$. Our studies demonstrate Cr$_{2.76}$Te$_4$ to be a soft ferromagnet with strong magnetocrystalline anisotropy. Below 50 K, the system shows an antiferromagnetic-like transition. Interestingly, between 50 and 150 K, we observe fluctuating magnetic moments between in-plane an… ▽ More We studied the electrical transport, Hall effect, and magnetic properties of monoclinic layered ferromagnet Cr$_{2.76}$Te$_4$. Our studies demonstrate Cr$_{2.76}$Te$_4$ to be a soft ferromagnet with strong magnetocrystalline anisotropy. Below 50 K, the system shows an antiferromagnetic-like transition. Interestingly, between 50 and 150 K, we observe fluctuating magnetic moments between in-plane and out-of-plane orientations, leading to non-coplanar spin structure. On the other hand, the electrical resistivity data suggest it to be metallic throughout the measured temperature range, except a $kink$ at around 50 K due to AFM ordering. The Rhodes-Wohlfarth ratio $\frac{μ_{eff}}{μ_{s}}=1.89 (>1)$ calculated from our magnetic studies confirms that Cr$_{2.76}$Te$_4$ is an itinerant ferromagnet. Large anomalous Hall effect has been observed due to the skew-scattering of impurities and the topological Hall effect has been observed due to non-coplanar spin-structure in the presence of strong magnetocrystalline anisotropy. We examined the mechanism of anomalous Hall effect by employing the first principles calculations. △ Less

Submitted 16 September, 2023; originally announced September 2023.

Comments: 9 pages, 6 figures, To appear in Physical Review Materials

Nematic order in topological SYK models

Authors: Andrew Hardy, Anjishnu Bose, Arun Paramekanti

Abstract: We study a class of multi-orbital models based on those proposed by Venderbos, Hu, and Kane which exhibit an interplay of topology, interactions, and fermion incoherence. In the non-interacting limit, these models exhibit trivial and Chern insulator phases with Chern number $C \geq 1$ bands as determined by the relative angular momentum of the participating orbitals. These quantum anomalous Hall i… ▽ More We study a class of multi-orbital models based on those proposed by Venderbos, Hu, and Kane which exhibit an interplay of topology, interactions, and fermion incoherence. In the non-interacting limit, these models exhibit trivial and Chern insulator phases with Chern number $C \geq 1$ bands as determined by the relative angular momentum of the participating orbitals. These quantum anomalous Hall insulator phases are separated by topological transitions protected by crystalline rotation symmetry, featuring Dirac or quadratic band-touching points. Here we study the impact of Sachdev-Ye-Kitaev (SYK) type interactions on these lattice models. Given the random interactions, these models display `average symmetries' upon disorder averaging, including a charge conjugation symmetry, so they behave as interacting models in topological class $\mathbf{D}$ enriched by crystalline rotation symmetry. The phase diagram of this model features a non-Fermi liquid at high temperature and an `exciton condensate' with nematic transport at low temperature. We present results from the free-energy, spectral functions, and the anomalous Hall resistivity as a function of temperature and tuning parameters. Our results are broadly relevant to correlated topological matter in multiorbital systems, and may also be viewed, with a suitable particle hole transformation, as an exploration of strong interaction effects on mean-field topological superconductors. △ Less

Submitted 6 December, 2023; v1 submitted 25 August, 2023; originally announced August 2023.

Comments: 15 pages, 6 figures. PRB Accepted version

Journal ref: PhysRevB.108.235169 (2023)

Higher-order Hall response arises from octupole order and scalar spin chirality in a noncollinear antiferromagnet

Authors: Adithya Rajan, Tom G. Saunderson, Fabian R. Lux, Rocío Yanes Díaz, Hasan M. Abdullah, Arnab Bose, Beatrice Bednarz, Jun-Young Kim, Dongwook Go, Tetsuya Hajiri, Gokaran Shukla, Olena Gomonay, Yugui Yao, Wanxiang Feng, Hidefumi Asano, Udo Schwingenschlögl, Luis López-Díaz, Jairo Sinova, Gerhard Jakob, Yuriy Mokrousov, Aurélien Manchon, Mathias Kläui

Abstract: Noncollinear antiferromagnets can generate a transverse electrical response known as the anomalous Hall effect, even though they possess almost no net magnetization. The microscopic origin of this behaviour, however, has remained unclear because conventional measurement geometries mix different contributions to the measured response. Here, we show that applying magnetic fields in selected in-plane… ▽ More Noncollinear antiferromagnets can generate a transverse electrical response known as the anomalous Hall effect, even though they possess almost no net magnetization. The microscopic origin of this behaviour, however, has remained unclear because conventional measurement geometries mix different contributions to the measured response. Here, we show that applying magnetic fields in selected in-plane directions allows us to disentangle the mechanisms underlying the Hall effect in a representative noncollinear antiferromagnet. By suppressing any dipole-related signal, we isolate a purely octupole-driven Hall response that exhibits a characteristic three-fold angular symmetry. At low magnetic fields, we further observe an additional Hall-like contribution that arises from the scalar spin chirality associated with noncoplanar spin textures. Combining symmetry analysis, first-principles calculations, and transport measurements, we reveal that octupole order, dipole moments, and chirality coexist and contribute in distinct field regimes. These findings establish a framework for identifying and controlling complex magnetic order parameters for spintronic applications. △ Less

Submitted 9 February, 2026; v1 submitted 21 April, 2023; originally announced April 2023.

Effect of triplet correlation on the equation of pair correlation function in a weakly coupled inhomogeneous plasma system

Authors: Anirban Bose

Abstract: It is observed that retaining the triplet correlation to derive the equation of pair correlation function from the first two members of BBGKY hierarchy, modifies the structure of the equation and the pair correlation function significantly. This equation may be used to explore the thermodynamic properties of the weakly coupled inhomogeneous plasma systems. This study may also be relevant for homog… ▽ More It is observed that retaining the triplet correlation to derive the equation of pair correlation function from the first two members of BBGKY hierarchy, modifies the structure of the equation and the pair correlation function significantly. This equation may be used to explore the thermodynamic properties of the weakly coupled inhomogeneous plasma systems. This study may also be relevant for homogeneous plasmas. △ Less

Submitted 11 August, 2023; v1 submitted 12 April, 2023; originally announced April 2023.

Reentrant condensation transition in a model of driven scalar active matter with diffusivity edge

Authors: Jonas Berx, Aritra Bose, Ramin Golestanian, Benoît Mahault

Abstract: The effect of a diffusivity edge is studied in a system of scalar active matter confined by a periodic potential and driven by an externally applied force. We find that this system shows qualitatively distinct stationary regimes depending on the amplitude of the driving force with respect to the potential barrier. For small driving, the diffusivity edge induces a transition to a condensed phase an… ▽ More The effect of a diffusivity edge is studied in a system of scalar active matter confined by a periodic potential and driven by an externally applied force. We find that this system shows qualitatively distinct stationary regimes depending on the amplitude of the driving force with respect to the potential barrier. For small driving, the diffusivity edge induces a transition to a condensed phase analogous to the Bose-Einstein-like condensation reported for the nondriven case, which is characterized by a density-independent steady state current. Conversely, large external forces lead to a qualitatively different phase diagram since in this case condensation is only possible beyond a given density threshold, while the associated transition at higher densities is found to be reentrant. △ Less

Submitted 6 March, 2023; v1 submitted 3 March, 2023; originally announced March 2023.

Journal ref: EPL 142, 67004 (2023)

Sn$_{0.06}$Cr$_3$Te$_4$: A Skyrmion Superconductor

Authors: Shubham Purwar, Anumita Bose, Achintya Low, Satyendra Singh, R. Venkatesh, Awadhesh Narayan, Setti Thirupathaiah

Abstract: Topological superconductors are an exciting class of quantum materials from the point of view of the fundamental sciences and potential technological applications. Here, we report on the successful introduction of superconductivity in a ferromagnetic layered skyrmion system Cr$_3$Te$_4$, obtained by the Sn intercalation, below a transition temperature of $T_c$$\approx$3.5 K. We observe several int… ▽ More Topological superconductors are an exciting class of quantum materials from the point of view of the fundamental sciences and potential technological applications. Here, we report on the successful introduction of superconductivity in a ferromagnetic layered skyrmion system Cr$_3$Te$_4$, obtained by the Sn intercalation, below a transition temperature of $T_c$$\approx$3.5 K. We observe several interesting physical properties, such as superconductivity, magnetism, and the topological Hall effect, simultaneously in this system. Despite the magnetism and Meissner effects being anisotropic, the superconductivity observed from the in-plane electrical resistivity ($ρ_{\it{bc}}$) is nearly isotropic between $H\parallel \it{bc}$ and $H\parallel \it{a}$, suggesting separate channels of conduction electrons responsible for the superconductivity and magnetism of this system, which is also supported by our spin-resolved DFT calculations. We identify two orders of higher carrier density in superconducting Sn$_{0.06}$Cr$_{3}$Te$_4$ than the parent Cr$_3$Te$_4$. A jump in the specific heat is noticed around the $T_c$ with a volume fraction of 33\%, confirming the bulk superconductivity in Sn$_{0.06}$Cr$_{3}$Te$_4$. In addition to the introduction of superconductivity, tuning of topological Hall properties is noticed with Sn intercalation. Our observation of superconductivity in a skyrmion lattice brings up a new class of topological quantum materials. △ Less

Submitted 29 July, 2024; v1 submitted 10 February, 2023; originally announced February 2023.

Comments: 7 figures, 12 pages, Accepted for publication in Applied Materials Today

Journal ref: Applied Materials Today 39, 102328 (2024)

Proximate Dirac spin liquid in honeycomb lattice $J_1$-$J_3$ XXZ model: Numerical study and application to cobaltates

Authors: Anjishnu Bose, Manodip Routh, Sreekar Voleti, Sudeep Kumar Saha, Manoranjan Kumar, Tanusri Saha-Dasgupta, Arun Paramekanti

Abstract: Recent theoretical and experimental work suggest that the honeycomb cobaltates, initially proposed as candidate Kitaev quantum magnets, are in fact described by a pseudospin-$1/2$ easy-plane spin Hamiltonian with nearest neighbor ferromagnetic (FM) exchange $J_1$ being frustrated by antiferromagnetic third-neighbor exchange $J_3$ and weaker compass anisotropies. Using exact diagonalization and den… ▽ More Recent theoretical and experimental work suggest that the honeycomb cobaltates, initially proposed as candidate Kitaev quantum magnets, are in fact described by a pseudospin-$1/2$ easy-plane spin Hamiltonian with nearest neighbor ferromagnetic (FM) exchange $J_1$ being frustrated by antiferromagnetic third-neighbor exchange $J_3$ and weaker compass anisotropies. Using exact diagonalization and density-matrix renormalization group (DMRG) calculations, we show that this model exhibits FM order at small $J_3/J_1$ and zig-zag (ZZ) order at large $J_3/J_1$, separated by an intermediate phase, which we label as $\widetilde{\mathrm{SL}}$. This $\widetilde{\mathrm{SL}}$ phase is shown to exhibit spin-liquid-like correlations in DMRG, although we cannot preclude weak broken symmetries, e.g. weak Ising type Néel order, given the limits on our explored system sizes. Using a modified parton mean field theory and variational Monte Carlo on Gutzwiller projected wavefunctions, we show that the optimal FM and ZZ orders as well as the intermediate $\widetilde{\mathrm{SL}}$ state are proximate to a `parent' Dirac spin liquid (SL). This Dirac SL is shown to capture the broad continuum in the temperature and magnetic field dependent terahertz spectroscopy of BaCo$_2$(AsO$_4$)$_2$, and the reported low temperature metallic thermal conductivity in Na$_2$Co$_2$TeO$_6$ and BaCo$_2$(AsO$_4$)$_2$ upon incorporating disorder induced broadening. △ Less

Submitted 3 November, 2023; v1 submitted 26 December, 2022; originally announced December 2022.

Comments: Final PRB version - 13 pages, 11 figures. Special thanks to Federico Becca, Sasha Chernyshev, Martin Klanjsek

Journal ref: Phys. Rev. B 108, 174422 (2023)

Derivation of an equation of pair correlation function from BBGKY hierarchy in a weakly coupled self gravitating system

Authors: Anirban Bose

Abstract: An equation of pair correlation function has been derived from the first two members of BBGKY hierarchy in a weakly coupled inhomogeneous self gravitating system in quasi thermal equilibrium. This work may be useful to study the thermodynamic properties of the central region of a star cluster which is older than a few or more central relaxation time. An equation of pair correlation function has been derived from the first two members of BBGKY hierarchy in a weakly coupled inhomogeneous self gravitating system in quasi thermal equilibrium. This work may be useful to study the thermodynamic properties of the central region of a star cluster which is older than a few or more central relaxation time. △ Less

Submitted 9 November, 2022; v1 submitted 22 October, 2022; originally announced October 2022.

Thermally-generated spin current in the topological insulator Bi$_2$Se$_3$

Authors: Rakshit Jain, Max Stanley, Arnab Bose, Anthony R. Richardella, Xiyue S. Zhang, Timothy Pillsbury, David A. Muller, Nitin Samarth, Daniel C. Ralph

Abstract: We complete measurements of interconversions among the full triad of thermal gradients, charge currents, and spin currents in the topological insulator Bi$_2$Se$_3$ by quantifying the efficiency with which thermal gradients can generate transverse spin currents. We accomplish this by comparing the spin Nernst magneto-thermopower to the spin Hall magnesistance for bilayers of Bi$_2$Se$_3$/CoFeB. We… ▽ More We complete measurements of interconversions among the full triad of thermal gradients, charge currents, and spin currents in the topological insulator Bi$_2$Se$_3$ by quantifying the efficiency with which thermal gradients can generate transverse spin currents. We accomplish this by comparing the spin Nernst magneto-thermopower to the spin Hall magnesistance for bilayers of Bi$_2$Se$_3$/CoFeB. We find that Bi$_2$Se$_3$ does generate substantial thermally-driven spin currents. A lower bound for the ratio of spin current to thermal gradient is $J_s/\nabla_x T$ = (4.9 $\pm$ 0.9) $\times$ 10$^{6}$ ($\hbar/2e$) A m$^{-2}$ / K $μ$m$^{-1}$, and a lower bound for the magnitude of the spin Nernst ratio is $-$0.61 $\pm$ 0.11. The spin Nernst ratio for Bi$_2$Se$_3$ is the largest among all materials measured to date, 2-3 times larger compared to previous measurements for the heavy metals Pt and W. △ Less

Submitted 11 October, 2022; originally announced October 2022.

Detection of long-range orbital-Hall torques

Authors: Arnab Bose, Fabian Kammerbauer, Rahul Gupta, Dongwook Go, Yuriy Mokrousov, Gerhard Jakob, Mathias Klaeui

Abstract: We report and quantify a large orbital-Hall torque generated by Nb and Ru, which we identify from a strong dependence of torques on the ferromagnets. This is manifested as a sign reversal and strong enhancement in the damping-like torques measured in Nb (or Ru)/Ni bilayers as compared to Nb (or Ru)/FeCoB bilayers. The long-range nature of orbital transport in the ferromagnet is revealed by the thi… ▽ More We report and quantify a large orbital-Hall torque generated by Nb and Ru, which we identify from a strong dependence of torques on the ferromagnets. This is manifested as a sign reversal and strong enhancement in the damping-like torques measured in Nb (or Ru)/Ni bilayers as compared to Nb (or Ru)/FeCoB bilayers. The long-range nature of orbital transport in the ferromagnet is revealed by the thickness dependences of Ni in Nb (or Ru)/Ni bilayers which are markedly different from the regular spin absorption in the ferromagnet that takes place within a few angstroms and thus it uniquely distinguishes the orbital Hall torque from the spin Hall torque. △ Less

Submitted 8 February, 2023; v1 submitted 5 October, 2022; originally announced October 2022.

Report number: 134423

Journal ref: Physical Review B 2023

Chiral Broken Symmetry Descendants of the Kagomé Lattice Chiral Spin Liquid

Authors: Anjishnu Bose, Arijit Haldar, Erik S. Sørensen, Arun Paramekanti

Abstract: The breaking of chiral and time-reversal symmetries provides a pathway to exotic quantum phenomena and topological phases. In particular, the breaking of chiral (mirror) symmetry in quantum materials has been shown to have important technological applications. Recent work has extensively explored the resulting emergence of chiral charge orders and chiral spin liquids on the kagomé lattice. Such ch… ▽ More The breaking of chiral and time-reversal symmetries provides a pathway to exotic quantum phenomena and topological phases. In particular, the breaking of chiral (mirror) symmetry in quantum materials has been shown to have important technological applications. Recent work has extensively explored the resulting emergence of chiral charge orders and chiral spin liquids on the kagomé lattice. Such chiral spin liquids are closely tied to bosonic fractional quantum Hall states and host anyonic quasiparticles; however, their connection to nearby magnetically ordered states has remained a mystery. Here, we show that two distinct non-coplanar magnetic orders with uniform spin chirality, the XYZ umbrella state and the Octahedral spin crystal, emerge as competing orders in close proximity to the kagomé chiral spin liquid. Our results highlight the intimate link between a many-body topologically ordered liquid and broken symmetry states with nontrivial real-space topology. △ Less

Submitted 21 April, 2022; originally announced April 2022.

Comments: Main text : 9 pages, 4 figures; Supplementary : 12 pages, 11 figures

Journal ref: Phys. Rev. B 107, L020411 (2023)

Pressure-induced magnetic and topological transitions in non-centrosymmetric MnIn$_{2}$Te$_{4}$

Authors: Anumita Bose, Rajdeep Banerjee, Awadhesh Narayan

Abstract: The discovery of time-reversal-invariant topological states has drawn great attention in recent decades. However, despite the potential of displaying a variety of exotic physics, the study of magnetic topological phases lags behind due to underlying added complexity of magnetism. In this work, we predict the interplay of magnetism and topology in the non-centrosymmetric ternary manganese compound… ▽ More The discovery of time-reversal-invariant topological states has drawn great attention in recent decades. However, despite the potential of displaying a variety of exotic physics, the study of magnetic topological phases lags behind due to underlying added complexity of magnetism. In this work, we predict the interplay of magnetism and topology in the non-centrosymmetric ternary manganese compound MnIn$_2$Te$_4$, using first-principles calculations. At ambient pressure, the ground state of the system is an antiferromagnetic insulator. With the application of small hydrostatic pressure ($\sim$0.50 GPa), it undergoes a magnetic transition and the ferromagnetic state becomes energetically favourable. At $\sim$2.92 GPa, the system undergoes a transition into a Weyl semimetallic phase, which hosts multiple Weyl points in the bulk and is associated with non-trivial surface Fermi arcs. Remarkably, we discover that the number of Weyl points in this system can be controlled by pressure and that these manifest in an anomalous Hall conductivity (AHC). In addition to proposing a new candidate magnetic topological material, our work demonstrates that pressure can be an effective way to induce and control topological phases, as well as AHC, in magnetic materials. These properties may allow our proposed material to be used as a novel pressure-controlled Hall switch. △ Less

Submitted 14 February, 2022; v1 submitted 7 February, 2022; originally announced February 2022.

Comments: 6+3 pages, 4+5 figures

Origins of transverse voltages generated by applied thermal gradients and applied electric fields in ferrimagnetic-insulator/heavy-metal bilayers

Authors: Arnab Bose, Rakshit Jain, Jackson J. Bauer, Robert A. Buhrman, Caroline A. Ross, Daniel C. Ralph

Abstract: We compare thermal-gradient-driven transverse voltages in ferrimagnetic-insulator/heavy-metal bilayers (Tm3Fe5O12/W and Tm3Fe5O12/Pt) to corresponding electrically-driven transverse resistances at and above room temperature. We find for Tm3Fe5O12/W that the thermal and electrical effects can be explained by a common spin-current detection mechanism, the physics underlying spin Hall magnetoresistan… ▽ More We compare thermal-gradient-driven transverse voltages in ferrimagnetic-insulator/heavy-metal bilayers (Tm3Fe5O12/W and Tm3Fe5O12/Pt) to corresponding electrically-driven transverse resistances at and above room temperature. We find for Tm3Fe5O12/W that the thermal and electrical effects can be explained by a common spin-current detection mechanism, the physics underlying spin Hall magnetoresistance (SMR). However, for Tm3Fe5O12/Pt the ratio of the electrically-driven transverse voltages (planar Hall signal/anomalous Hall signal) is much larger than the ratio of corresponding thermal-gradient signals, a result which is very different from expectations for a SMR-based mechanism alone. We ascribe this difference to a proximity-induced magnetic layer at the Tm3Fe5O12/Pt interface. △ Less

Submitted 22 February, 2022; v1 submitted 13 December, 2021; originally announced December 2021.

Tilted spin current generated by the collinear antiferromagnet RuO2

Authors: Arnab Bose, Nathaniel J. Schreiber, Rakshit Jain, Ding-Fu Shao, Hari P. Nair, Jiaxin Sun, Xiyue S. Zhang, David A. Muller, Evgeny Y. Tsymbal, Darrell G. Schlom, Daniel C. Ralph

Abstract: We report measurements demonstrating that when the Neel vector of the collinear antiferromagnet RuO2 is appropriately canted relative to the sample plane, the antiferromagnet generates a substantial out of plane damping-like torque. The measurements are in good accord with predictions that when an electric field, E is applied to the spin split band structure of RuO2 it can cause a strong transvers… ▽ More We report measurements demonstrating that when the Neel vector of the collinear antiferromagnet RuO2 is appropriately canted relative to the sample plane, the antiferromagnet generates a substantial out of plane damping-like torque. The measurements are in good accord with predictions that when an electric field, E is applied to the spin split band structure of RuO2 it can cause a strong transverse spin current even in the absence of spin-orbit coupling. This produces characteristic changes in all three components of the E induced torque vector as a function of the angle of E relative to the crystal axes, corresponding to a spin current with a well defined tilted spin orientation s approximately (but not exactly) parallel to the Neel vector, flowing perpendicular to both E and S. This angular dependence is the signature of an antiferromagnetic spin Hall effect with symmetries that are distinct from other mechanisms of spin-current generation reported in antiferromagnetic or ferromagnetic materials. △ Less

Submitted 20 August, 2021; originally announced August 2021.

Journal ref: Nature Electronics 5, 267 (2022)

Strain-induced topological charge control in multifold fermion systems

Authors: Anumita Bose, Awadhesh Narayan

Abstract: Multifold fermion systems feature free fermionic excitations, which have no counterparts in high-energy physics, and exhibit several unconventional properties. Using first-principles calculations, we predict that strain engineering can be used to control the distribution of topological charges in transition metal silicide candidate CoSi, hosting multifold fermions. We demonstrate that breaking the… ▽ More Multifold fermion systems feature free fermionic excitations, which have no counterparts in high-energy physics, and exhibit several unconventional properties. Using first-principles calculations, we predict that strain engineering can be used to control the distribution of topological charges in transition metal silicide candidate CoSi, hosting multifold fermions. We demonstrate that breaking the rotational symmetry of the system, by choosing a suitable strain, destroys the multifold fermions, and at the same time results in the creation of Weyl points. We introduce a low energy effective model to complement the results obtained from density functional calculations. Our findings suggest that strain-engineering is a useful approach to tune topological properties of multifold fermions. △ Less

Submitted 28 June, 2021; originally announced June 2021.

Comments: 23 pages, 8 figures

Journal ref: J. Phys.: Condens. Matter 33, 375002 (2021)

Interfacial and bulk spin Hall contributions to field-like spin-orbit torque generated by Iridium

Authors: Sutapa Dutta, Arnab Bose, A. A. Tulapurkar, R. A. Buhrman, D. C. Ralph

Abstract: We present measurements of spin orbit torques generated by Ir as a function of film thickness in sputtered Ir/CoFeB and Ir/Co samples. We find that Ir provides a damping-like component of spin orbit torque with a maximum spin torque conductivity 1.4e5 in SI unit and a maximum spin-torque efficiency of 0.04, which is sufficient to drive switching in an 0.8 nm film of CoFeB with perpendicular magnet… ▽ More We present measurements of spin orbit torques generated by Ir as a function of film thickness in sputtered Ir/CoFeB and Ir/Co samples. We find that Ir provides a damping-like component of spin orbit torque with a maximum spin torque conductivity 1.4e5 in SI unit and a maximum spin-torque efficiency of 0.04, which is sufficient to drive switching in an 0.8 nm film of CoFeB with perpendicular magnetic anisotropy. We also observe a surprisingly large field like spin orbit torque. Measurements as a function of Ir thickness indicate a substantial contribution to the FLT from an interface mechanism so that in the ultrathin limit there is a non-zero FLT with a maximum torque conductivity -5.0E4 in the SI unit. When the Ir film thickness becomes comparable to or greater than its spin diffusion length, 1.6 nm, there is also a smaller bulk contribution to the fieldlike torque. △ Less

Submitted 6 May, 2021; originally announced May 2021.

Quantum Phase Transitions in Long-Range Interacting Hyperuniform Spin Chains in a Transverse Field

Authors: Amartya Bose, Salvatore Torquato

Abstract: Hyperuniform states of matter are characterized by anomalous suppression of long-wavelength density fluctuations. While most of interesting cases of disordered hyperuniformity are provided by complex many-body systems like liquids or amorphous solids, classical spin chains with certain long-range interactions have been shown to demonstrate the same phenomenon. It is well-known that the transverse… ▽ More Hyperuniform states of matter are characterized by anomalous suppression of long-wavelength density fluctuations. While most of interesting cases of disordered hyperuniformity are provided by complex many-body systems like liquids or amorphous solids, classical spin chains with certain long-range interactions have been shown to demonstrate the same phenomenon. It is well-known that the transverse field Ising model shows a quantum phase transition (QPT) at zero temperature. Under the quantum effects of a transverse magnetic field, classical hyperuniform spin chains are expected to lose their hyperuniformity. High-precision simulations of these cases are complicated because of the presence of highly nontrivial long-range interactions. We perform extensive analysis of these systems using density matrix renormalization group to study the possibilities of phase transitions and the mechanism by which they lose hyperuniformity. We discover first-order QPTs in the hyperuniform spin chains. An interesting feature of the phase transitions in these disordered hyperuniform spin chains is that, depending on the parameter values, the presence of transverse magnetic field may remarkably lead to increase in the order of the ground state as measured by the "$τ$ order metric," even if hyperuniformity is lost. Therefore, it would be possible to design materials to target specific novel quantum behaviors in the presence of a transverse magnetic field. Our numerical investigations suggest that these spin chains can show no more than two QPTs. We further analyze the long-range interacting spin chains via the Jordan-Wigner mapping, showing that under the pairwise interacting approximation and a mean-field treatment, there can be at most two QPTs. Based on these numerical and theoretical explorations, we conjecture that these spin chains can show a maximum of two QPTs at zero temperature. △ Less

Submitted 14 December, 2020; v1 submitted 11 December, 2020; originally announced December 2020.

Comments: SM was missing in the previous version. Added here

Journal ref: Phys. Rev. B 103, 014118 (2021)

Role of Dirac nodal lines and strain on the high spin Hall conductivity of epitaxial IrO2 thin films

Authors: Arnab Bose, Jocienne N. Nelson, Xiyue S. Zhang, Rakshit Jain, D. G. Schlom, D. C. Ralph, D. A. Muller, K. M. Shen, R. A. Buhrman

Abstract: Since the discovery of a 'giant' spin Hall effect (SHE) in certain heavy metal elements there has been an intense effort to identify and develop new and technologically viable, heavy-metal-based thin film materials that could generate spin currents with even greater efficiency to exert spin-orbit torques (SOT) on adjacent ferromagnetic nanostructures. In parallel, there have been wide ranging fund… ▽ More Since the discovery of a 'giant' spin Hall effect (SHE) in certain heavy metal elements there has been an intense effort to identify and develop new and technologically viable, heavy-metal-based thin film materials that could generate spin currents with even greater efficiency to exert spin-orbit torques (SOT) on adjacent ferromagnetic nanostructures. In parallel, there have been wide ranging fundamental studies of the spin currents that can arise from robust, intrinsic spin-orbit interaction (SOI) effects in more exotic systems including topological insulators, transition metal dichalcogenides with broken crystalline symmetry, Weyl and Dirac semimetals where gapless electronic excitations are protected by topology and symmetry. Here we experimentally study strong SOT from the topological semimetal IrO2 in (001) and (110) normal films, which exhibit distinctly different SHE strengths. Angle resolved photoemission spectroscopy studies have shown IrO2 exhibits Dirac nodal lines (DNL) in the band structure, which could enable a very high spin Hall conductivity (SHC). The (001) films exhibit exceptionally high damping like torque efficiency ranging from 0.45 at 293 K to 0.65 at 30 K which sets the lower bound of SHC that is ten times higher and of opposite sign than the theoretical prediction. We observe a substantial reduction of SHC in anisotropically strained (110) films, which suggests that the DNLs that are present in the (001) films and contribute to SHC, are disrupted and gapped due to the large anisotropic strain in (110) films, which in turn significantly lowers SHC. Very large value of SHC at room temperature of this Dirac semimetal could be very promising for the practical application. △ Less

Submitted 8 June, 2020; originally announced June 2020.

Journal ref: Applied Materials and Interfaces 2020

Electronic Structure Studies of FeSi: A Chiral Topological System

Authors: Susmita Changdar, S. Aswartham, Anumita Bose, Y. Kushnirenko, G. Shipunov, N. C. Plumb, M. Shi, Awadhesh Narayan, B. Büchner, S. Thirupathaiah

Abstract: Most recent observation of topological Fermi arcs on the surface of manyfold degenerate B20 systems, CoSi and RhSi, have attracted enormous research interests. Although an another isostructural system, FeSi, has been predicted to show bulk chiral fermions, it is yet to be clear theoretically and as well experimentally that whether FeSi possesses the topological surface Fermi arcs associated with t… ▽ More Most recent observation of topological Fermi arcs on the surface of manyfold degenerate B20 systems, CoSi and RhSi, have attracted enormous research interests. Although an another isostructural system, FeSi, has been predicted to show bulk chiral fermions, it is yet to be clear theoretically and as well experimentally that whether FeSi possesses the topological surface Fermi arcs associated with the exotic chiral fermions in vicinity of the Fermi level. In this contribution, using angle-resolved photoemission spectroscopy (ARPES) and density functional theory (DFT), we present the low-energy electronic structure of FeSi. We further report the surface state calculations to provide insights into the surface band structure of FeSi near the Fermi level. Unlike in CoSi or RhSi, FeSi has no topological Fermi arcs near the Fermi level as confirmed both from ARPES and surface state calculations. Further, the ARPES data show spin-orbit coupling (SOC) band splitting of 40 meV, which is in good agreement with bulk band structure calculations. We noticed an anomalous temperature dependent resistivity in FeSi which can be understood through the electron-phonon interactions as we find a Debye energy of 80 meV from the ARPES data. △ Less

Submitted 10 December, 2019; originally announced December 2019.

Comments: 9 pages, 7 figures

Journal ref: Phys. Rev. B 101, 235105 (2020)

Determination of the single particle distribution function in a weakly correlated weakly inhomogeneous plasma

Authors: Anirban Bose

Abstract: Single particle distribution function of plasma particles has been derived from the first member of the Bogoliubov-Born-Green-Kirkwood-Yvon (BBGKY) hierarchy utilising the pair correlation function evaluated in \cite{kn:ab1} from the second member of the BBGKY hierarchy. This distribution function may be employed to probe the thermodynamic properties of the weakly inhomogeneous plasma systems. Single particle distribution function of plasma particles has been derived from the first member of the Bogoliubov-Born-Green-Kirkwood-Yvon (BBGKY) hierarchy utilising the pair correlation function evaluated in \cite{kn:ab1} from the second member of the BBGKY hierarchy. This distribution function may be employed to probe the thermodynamic properties of the weakly inhomogeneous plasma systems. △ Less

Submitted 15 November, 2019; originally announced November 2019.

Determination of the second virial coefficient of interacting Bosons using the method of Wigner distribution function

Authors: Anirban Bose

Abstract: In a previous article \cite{kn:anirban1} a method has been introduced to derive the all order Bose-Einstein distribution of the non interacting Bosons as the solution of the Wigner equation. The process was a perturbative one where the Bose-Einstein distribution was taken as the unperturbed solution. In this article it is shown that the same formalism is also applicable in the case of interacting… ▽ More In a previous article \cite{kn:anirban1} a method has been introduced to derive the all order Bose-Einstein distribution of the non interacting Bosons as the solution of the Wigner equation. The process was a perturbative one where the Bose-Einstein distribution was taken as the unperturbed solution. In this article it is shown that the same formalism is also applicable in the case of interacting Bosons. The formalism has been applied to calculate the quantum second virial coefficient of the Bosons interacting pairwise via Lenard-Jones potential and compared with the previous result. △ Less

Submitted 26 May, 2020; v1 submitted 27 September, 2019; originally announced October 2019.

Comments: 10 pages

Determination of pair correlation function in a weakly correlated weakly inhomogeneous plasma

Authors: Anirban Bose

Abstract: In a weakly correlated inhomogeneous plasma an equation of pair correlation function is obtained utilizing the Bogoliubov-Born-Green-Kirkwood-Yvon (BBGKY) hierarchy of equations. In this article the pair correlation function has been calculated from the same equation in the weakly inhomogeneous limit by using a perturbative theory. In a weakly correlated inhomogeneous plasma an equation of pair correlation function is obtained utilizing the Bogoliubov-Born-Green-Kirkwood-Yvon (BBGKY) hierarchy of equations. In this article the pair correlation function has been calculated from the same equation in the weakly inhomogeneous limit by using a perturbative theory. △ Less

Submitted 24 March, 2019; originally announced March 2019.

Self-stresses control stiffness and stability in overconstrained disordered networks

Authors: Anwesha Bose, Mathijs F. J. Vermeulen, Cornelis Storm, Wouter G. Ellenbroek

Abstract: We investigate the interplay between pre-stress and mechanical properties in random elastic networks. To do this in a controlled fashion, we introduce an algorithm for creating random freestanding frames that support exactly one state of self stress. By multiplying all the bond tensions in this state of self stress by the same number---which with the appropriate normalization corresponds to the ph… ▽ More We investigate the interplay between pre-stress and mechanical properties in random elastic networks. To do this in a controlled fashion, we introduce an algorithm for creating random freestanding frames that support exactly one state of self stress. By multiplying all the bond tensions in this state of self stress by the same number---which with the appropriate normalization corresponds to the physical pre-stress inside the frame---we systematically evaluate the linear mechanical response of the frame as a function of pre-stress. After proving that the mechanical moduli of affinely deforming frames are rigourously independent of pre-stress, we turn to non-affinely deforming frames. In such frames, pre-stress has a profound effect on linear response: not only can it change the values of the linear modulus---an effect we demonstrate to be related to a suppressive effect of pre-stress on non-affinity---but pre-stresses also generically trigger bistable mechanical response. Thus, pre-stress can be leveraged to both augment the mechanical response of network architectures on the fly, and to actuate finite deformations. These control modalities may be of use in the design of both novel responsive materials and soft actuators. △ Less

Submitted 3 December, 2018; originally announced December 2018.

Comments: 13 pages, 9 figures

Journal ref: Phys. Rev. E 99, 023001 (2019)

Towards Hybrid Density Functional Calculations of Molecular Crystals via Fragment-Based Methods

Authors: Oleksandr A. Loboda, Grygoriy A. Dolgonos, A. Daniel Boese

Abstract: We introduce and employ two QM:QM schemes (a quantum mechanical method embedded into another quantum mechanical method) and report their performance for the X23 set of molecular crystals. We furthermore present the theory to calculate the stress tensors necessary for the computation of optimized cell volumes of molecular crystals and compare all results to those obtained with various density funct… ▽ More We introduce and employ two QM:QM schemes (a quantum mechanical method embedded into another quantum mechanical method) and report their performance for the X23 set of molecular crystals. We furthermore present the theory to calculate the stress tensors necessary for the computation of optimized cell volumes of molecular crystals and compare all results to those obtained with various density functionals and more approximate methods. Our QM:QM calculations with PBE0:PBE+D3, PBE0:PBE+MBD, and B3LYP:BLYP+D3 yield at a reduced computational cost lattice energy errors close to the ones of the parent hybrid density functional method, whereas for cell volumes, the errors of the QM:QM scheme methods are in between the GGA and hybrid functionals. △ Less

Submitted 3 September, 2018; originally announced September 2018.

Comments: Revised Manuscript accepted in "The Journal of Chemical Physics" (AIP)

Control of magnetization dynamics by spin Nernst torque

Authors: Arnab Bose, Ambika Shankar Shukla, Sutapa Dutta, Swapnil Bhuktare, Hanuman Singh, Ashwin A. Tulapurkar

Abstract: Control of magnetization dynamics is one of the primary goals in spintronics. It has been demonstrated using spin Hall effect i.e charge current to spin current conversion in non-magnetic metal which has large spin-orbit coupling such as Pt, W etc. Recently different groups have shown generation of spin current in Pt, W while thermal gradient is created by virtue of spin Nernst effect. In this wor… ▽ More Control of magnetization dynamics is one of the primary goals in spintronics. It has been demonstrated using spin Hall effect i.e charge current to spin current conversion in non-magnetic metal which has large spin-orbit coupling such as Pt, W etc. Recently different groups have shown generation of spin current in Pt, W while thermal gradient is created by virtue of spin Nernst effect. In this work we show the evidence of magnetization control by spin Nernst torque in Pt/Py bi-layer. We compared relative strength of spin Nernst Torque and spin Hall torque by measuring the systematic variation of magnetic linewidth on application of constant heat or charge current. Spin-torque ferromagnetic resonance (ST-FMR) technique is adopted to excite the magnet and to measure line-width precisely from the symmetric and anti-symmetric voltage component. Control of magnetization dynamics by spin Nernst torque will emerge as an alternative way to manipulate nano-magnets. △ Less

Submitted 5 June, 2018; originally announced June 2018.

Report number: Physical Review B 98, 184412

Journal ref: Phys. Rev. B 98, 184412 (2018)

Exciton-Phonon Coupling and Band-Gap Renormalization in Monolayer WSe$_{2}$

Authors: Himani Mishra, Anindya Bose, Amit Dhar, Sitangshu Bhattacharya

Abstract: Using a fully ab-initio methodology, we demonstrate how the lattice vibrations couple with neutral excitons in monolayer WSe2 and contribute to the non-radiative excitonic lifetime. We show that only by treating the electron-electron and electron-phonon interactions at the same time it is possible to obtain an unprecedented agreement of the zero and finite-temperature optical gaps and absorption s… ▽ More Using a fully ab-initio methodology, we demonstrate how the lattice vibrations couple with neutral excitons in monolayer WSe2 and contribute to the non-radiative excitonic lifetime. We show that only by treating the electron-electron and electron-phonon interactions at the same time it is possible to obtain an unprecedented agreement of the zero and finite-temperature optical gaps and absorption spectra with the experimental results. The bare energies were calculated by solving the Kohn-Sham equations, whereas G$_{0}$W$_{0}$ many body perturbation theory was used to extract the excited state energies. A coupled electron-hole Bethe-Salpeter equation was solved incorporating the polaronic energies to show that it is the in-plane torsional acoustic phonon branch that contributes mostly to the A and B exciton build-up. We find that the three A, B and C excitonic peaks exhibit different behaviour with temperature, displaying different non-radiative linewidths. There is no considerable transition in the strength of the excitons with temperature but A-exciton exhibits darker nature in comparison to C-exciton. Further, all the excitonic peaks redshifts as temperature rises. Renormalization of the bare electronic energies by phonon interactions and the anharmonic lattice thermal expansion causes a decreasing band-gap with increasing temperature. The zero point energy renormalization (31 meV) is found to be entirely due to the polaronic interaction with negligible contribution from lattice anharmonicites. These findings may find a profound impact on electronic and optoelectronic device technologies based on these monolayers. △ Less

Submitted 4 May, 2018; originally announced May 2018.

Comments: 11 pages, 7 figures

Journal ref: Phys. Rev. B 98, 045143 (2018)

A simple method to obtain the all order quantum corrected Bose-Einstein distribution

Authors: Anirban Bose

Abstract: A simple method has been introduced to derive the all order quantum corrected Bose-Einstein distribution as the solution of the Wigner equation. The process is a perturbative one where the Bose-Einstein distribution has been taken as the unperturbed solution. This solution has been applied to calculate the number density of the bosons at finite temperature. The study may be important to investigat… ▽ More A simple method has been introduced to derive the all order quantum corrected Bose-Einstein distribution as the solution of the Wigner equation. The process is a perturbative one where the Bose-Einstein distribution has been taken as the unperturbed solution. This solution has been applied to calculate the number density of the bosons at finite temperature. The study may be important to investigate the properties of bosons and bose condensates at finite temperature. This process can also be applied to obtain the quantum corrected Fermi distribution. △ Less

Submitted 16 February, 2018; originally announced February 2018.

Comments: 14 pages

Direct Observation of the Reciprocity between Spin Current and Phonon Interconversion

Authors: Swapnil Bhuktare, Hanuman Singh, Arnab Bose, Ashwin. A. Tulapurkar

Abstract: Spin current has emerged as a leading candidate for manipulation of spins in a nano-magnet. We here experimentally show another utility of spin current viz. it can be used for generation of phonons. Within the same experimental setup, we also demonstrate the inverse effect of generation of spin current by phonons. To demonstrate them, we measured the scattering-matrix of a two-port device with int… ▽ More Spin current has emerged as a leading candidate for manipulation of spins in a nano-magnet. We here experimentally show another utility of spin current viz. it can be used for generation of phonons. Within the same experimental setup, we also demonstrate the inverse effect of generation of spin current by phonons. To demonstrate them, we measured the scattering-matrix of a two-port device with interdigital transducers as one port and array of Ni/Pt lines as second port on piezoelectric substrate. The off-diagonal elements which correspond to transmission between the ports, were found to have 180 degree relative phase shift. The transmission of electrical signal from port 2 to 1 corresponds to generation of phonons from spin-current, while transmission from port 1 to 2 corresponds to the inverse effect. These results could be useful for designing spin-current based gyrators. △ Less

Submitted 7 December, 2017; originally announced December 2017.

Integer, fractional and side band injection locking of spintronic feedback nano-oscillator to microwave signal

Authors: Hanuman Singh, K. Konishi, S. Bhuktare, A. Bose, S. Miwa, A. Fukushima, K. Yakushiji, S. Yuasa, H. Kubota, Y. Suzuki, A. A. Tulapurkar

Abstract: In this article we demonstrate the injection locking of recently demonstrated spintronic feedback nano oscillator to microwave magnetic fields at integers as well fractional multiples of its auto oscillation frequency. Feedback oscillators have delay as a new degree of freedom which is absent for spin-transfer torque based oscillators, which gives rise to side peaks along with a main peak. We show… ▽ More In this article we demonstrate the injection locking of recently demonstrated spintronic feedback nano oscillator to microwave magnetic fields at integers as well fractional multiples of its auto oscillation frequency. Feedback oscillators have delay as a new degree of freedom which is absent for spin-transfer torque based oscillators, which gives rise to side peaks along with a main peak. We show that it is also possible to lock the oscillator on its side band peaks, which opens a new avenue to phase locked oscillators with large frequency differences. We observe that for low driving fields, side band locking improves the quality factor of the main peak, whereas for higher driving fields the main peak is suppressed. Further, measurements at two field angles provide some insight into the role of symmetry of oscillation orbit in determining the fractional locking. △ Less

Submitted 2 November, 2017; originally announced November 2017.

Comments: 16 pages, 6 figure(Manuscript), 3 figure( supplementary information),Accepted ( Physical Review Applied)

Journal ref: Phys. Rev. Applied 8, 064011 (2017)

Geometry and the onset of rigidity in a disordered network

Authors: Mathijs F. J. Vermeulen, Anwesha Bose, Cornelis Storm, Wouter G. Ellenbroek

Abstract: Disordered spring networks that are undercoordinated may abruptly rigidify when sufficient strain is applied. Since the deformation in response to applied strain does not change the generic quantifiers of network architecture - the number of nodes and the number of bonds between them - this rigidity transition must have a geometric origin. Naive, degree-of-freedom based mechanical analyses such as… ▽ More Disordered spring networks that are undercoordinated may abruptly rigidify when sufficient strain is applied. Since the deformation in response to applied strain does not change the generic quantifiers of network architecture - the number of nodes and the number of bonds between them - this rigidity transition must have a geometric origin. Naive, degree-of-freedom based mechanical analyses such as the Maxwell-Calladine count or the pebble game algorithm overlook such geometric rigidity transitions and offer no means of predicting or characterizing them. We apply tools that were developed for the topological analysis of zero modes and states of self-stress on regular lattices to two-dimensional random spring networks, and demonstrate that the onset of rigidity, at a finite simple shear strain $γ^\star$, coincides with the appearance of a single state of self stress, accompanied by a single floppy mode. The process conserves the topologically invariant difference between the number of zero modes and the number of states of self stress, but imparts a finite shear modulus to the spring network. Beyond the critical shear, we confirm previously reported critical scaling of the modulus. In the sub-critical regime, a singular value decomposition of the network's compatibility matrix foreshadows the onset of rigidity by way of a continuously vanishing singular value corresponding to nascent state of self stress. △ Less

Submitted 8 September, 2017; originally announced September 2017.

Comments: 6 pages, 6 figues

Journal ref: Phys. Rev. E 96, 053003 (2017)

Investigation of thickness dependent composition of boron carbide thin films by resonant soft x-ray reflectivity

Authors: P. N. Rao, R. K. Gupta, K. Saravanan, A. Bose, S. C. Joshi, T. Ganguli, S. K. Rai

Abstract: Boron carbide thin films of different thicknesses deposited by ion beam sputtering were studied. The deposited films were characterized by grazing incidence hard x-ray reflectivity (GIXR), resonant soft x-ray reflectivity (RSXR), x-ray photo electron spectroscopy (XPS), resonant Rutherford backscattering spectrometry (RRBS), and time of flight secondary ion mass spectrometry (TOF-SIMS). An in-dept… ▽ More Boron carbide thin films of different thicknesses deposited by ion beam sputtering were studied. The deposited films were characterized by grazing incidence hard x-ray reflectivity (GIXR), resonant soft x-ray reflectivity (RSXR), x-ray photo electron spectroscopy (XPS), resonant Rutherford backscattering spectrometry (RRBS), and time of flight secondary ion mass spectrometry (TOF-SIMS). An in-depth profile of the chemical elements constitute the films is reconstructed based on analysis of reflectivity curves measured in the vicinity of B K-edge. The composition of films is closely dependent on film thickness. Boron to Carbon (B/C) ratio reaches to ~4 as the thickness of deposited films increases. The B/C ratio estimated from RSXR measurements are in agreement with the RRBS measurements. TOF-SIMS data also suggested that decrease in boron content with decrease in film thickness. XPS measurements confirm the presence of little amount of B atoms on the surface of low thickness film. △ Less

Submitted 24 August, 2017; originally announced August 2017.

Comments: 29 pages, 9 figures and one table