Showing 1–50 of 122 results for author: Roy, D

Symmetry Breaking by Interfacial Dead Layers: Observation of Forbidden Self-Induced Spin-Orbit Torque in Symmetric Ferromagnets

Authors: Athira Ravindran K, Raghvendra Posti, Manish Kumar Mohanta, Abhishek Kumar, Damanpreet Kaur, Alberto Anadón, Sebastien Petit-Watelot, Juan-Carlos Rojas-Sánchez, Michel Hehn, Puru Jena, Debangsu Roy

Abstract: Conventionally, spin-orbit torques (SOTs) in ferromagnets require heavy-metal layers or engineered structural asymmetry to break inversion symmetry. In this work, we report the observation of robust, self-generated SOTs in a nominally symmetric, heavy-metal-free MgO/NiFe/MgO trilayer - a geometry where such torques are theoretically forbidden. By combining harmonic Hall measurements with SQUID mag… ▽ More Conventionally, spin-orbit torques (SOTs) in ferromagnets require heavy-metal layers or engineered structural asymmetry to break inversion symmetry. In this work, we report the observation of robust, self-generated SOTs in a nominally symmetric, heavy-metal-free MgO/NiFe/MgO trilayer - a geometry where such torques are theoretically forbidden. By combining harmonic Hall measurements with SQUID magnetometry and X-ray photoelectron spectroscopy, we identify the symmetry-breaking origin: a 1.8 nm magnetic dead layer at the bottom interface. Crucially, we demonstrate a quantitative agreement between our data and the drift-diffusion theory predicted by Kim and Lee, yielding a theoretically extracted dead-layer thickness (1.2 nm) which matches structural characterization. Furthermore, density-functional calculations confirm that NiFe possesses sufficient intrinsic spin Hall conductivity to support the observed spin currents. These results reframe the parasitic dead layer as a functional spintronic component, establishing a universal, all-ferromagnetic route to SOTs in standard magnetic heterostructures. △ Less

Submitted 10 August, 2026; originally announced August 2026.

Harnessing Native Chromium Oxidation for Giant Orbital Torque and Field-Free Magnetization Switching in NiFe/Cr Bilayers

Authors: Chirag Kalouni, Abhishek Kumar, Manish Kumar Mohanta, Kanupriya, Athira Ravindran K, Damanpreet Kaur, Preet Kamal, Puru Jena, Debangsu Roy

Abstract: Orbital currents offer charge-to-spin conversion beyond the efficiency limit of conventional heavy-metal Spin Hall sources. However, harnessing them has so far required either thick orbital-Hall materials or additional heavy-metal conversion layers. Here, we show that the native oxide of chromium, typically regarded as parasitic, transforms a simple NiFe\Cr bilayer into a self-contained dual-chann… ▽ More Orbital currents offer charge-to-spin conversion beyond the efficiency limit of conventional heavy-metal Spin Hall sources. However, harnessing them has so far required either thick orbital-Hall materials or additional heavy-metal conversion layers. Here, we show that the native oxide of chromium, typically regarded as parasitic, transforms a simple NiFe\Cr bilayer into a self-contained dual-channel orbital-current source without the need for any conversion layer. First-principles calculations predict a nearly threefold enhancement of the orbital Hall conductivity upon surface oxygenation, driven by Cr(3d)-O(2p) hybridization. Experimentally, naturally oxidized NiFe\Cr heterostructures exhibit a giant damping-like torque efficiency of $3.9 \times 10^{6}$ $Ω^{-1}$ m$^{-1}$, exceeding Pt (Ta) by one (two) orders of magnitude. The torque depicts a non-monotonic Cr-thickness dependence which cannot be explained by a conventional model. We have developed a drift-diffusion model with an oxidation-gated interfacial source which quantitatively reproduces the data, revealing that the Cr-CrO$_x$ interface generates orbital currents over an order of magnitude stronger than the bulk orbital Hall channel with an orbital transport length of $\approx 4$ nm. The enhanced torque enables field-free magnetization switching at $1.58 \times 10^{11}$ A m$^{-2}$, outperforming heavy-metal and CuO$_x$ benchmarks. These results establish native oxidation as a scalable strategy for realizing efficient orbital-torque devices. △ Less

Submitted 29 July, 2026; originally announced July 2026.

Comments: 21 pages ,8 figures

Revival of transport reciprocity via quantum interference in asymmetric nonlinear devices

Authors: Rupak Bag, Dibyendu Roy

Abstract: Structural asymmetry combined with optical nonlinearity often leads to nonreciprocal light transport. We explore the mechanism by which 1-photon interference effects can revive reciprocity in such nonlinear models. To this end, we study correlated 2-photon scattering where an artificial atom is asymmetrically (a) side-coupled to an infinite waveguide at two spatially separated points, and (b) dire… ▽ More Structural asymmetry combined with optical nonlinearity often leads to nonreciprocal light transport. We explore the mechanism by which 1-photon interference effects can revive reciprocity in such nonlinear models. To this end, we study correlated 2-photon scattering where an artificial atom is asymmetrically (a) side-coupled to an infinite waveguide at two spatially separated points, and (b) direct-coupled to two semi-infinite waveguides. The setup (a) gives robust reciprocal transport for the two photons. However, the setup (b) shows a transition from a nonreciprocal to a reciprocal regime by tuning the interference effect via an additional tunneling path for photons between the two waveguides. △ Less

Submitted 29 July, 2026; originally announced July 2026.

Observation of Non-linear hall effect in Polycrystalline magnetic multilayes

Authors: Preet Kamal, Chirag kalouni, Amiya Mondal, Awadesh Narayan, Debangsu Roy

Abstract: The Nonlinear Hall effect(NLHE) driven principally by the Berry curvature dipole has been established in non-centrosymmetric vander Waals crystals, topological semimetals, and moiré superlattices, but its extension to technologically mature heavy-metal/ferromagnet multilayer platforms remains largely unexplored. Here, we report the observation of a robust NLHE in polycrystalline magnetic multilaye… ▽ More The Nonlinear Hall effect(NLHE) driven principally by the Berry curvature dipole has been established in non-centrosymmetric vander Waals crystals, topological semimetals, and moiré superlattices, but its extension to technologically mature heavy-metal/ferromagnet multilayer platforms remains largely unexplored. Here, we report the observation of a robust NLHE in polycrystalline magnetic multilayers, persisting from 2 K to room temperature. The second-harmonic transverse voltage is independent of both excitation frequency and applied out-of-plane magnetic field, while the vanishingly small third-harmonic response confirms that the observed signal is not dominated by a quantum-metric contribution and instead reflects a genuine second-order electronic response. A scaling analysis of the second-order Hall conductivity against the longitudinal conductivity identifies a dominant, conductivity-independent term establishing the intrinsic berry curvature dipole. Our theoretical analysis, supported by first-principles DFT calculations, further satisfies and corroborates the experimental results. These results establish sputter-deposited polycrystalline thin film as the first engineered magnetic multilayer platform for BCD-driven nonlinear Hall transport, extending the NLHE material landscape beyond van der Waals systems into scalable, industry-compatible thin-film spintronic architectures suitable for high frequency rectifications and nonlinear sensors. △ Less

Submitted 6 July, 2026; originally announced July 2026.

Inviscid scaling in the Kuramoto-Sivashinsky equation from functional renormalization group and direct numerical simulations

Authors: Liubov Gosteva, Dipankar Roy, Nicolás Wschebor, Léonie Canet

Abstract: We show that the one-dimensional Kuramoto-Sivashinsky (KS) equation features a scaling regime characterized by the dynamical exponent $z=1$ at intermediate scales between the large-scale Kardar-Parisi-Zhang (KPZ) scaling with $z=3/2$ and the small-scale non-universal behavior. This scaling regime is intrinsic to the KS dynamics since it arises from the vanishing of the effective viscosity when evo… ▽ More We show that the one-dimensional Kuramoto-Sivashinsky (KS) equation features a scaling regime characterized by the dynamical exponent $z=1$ at intermediate scales between the large-scale Kardar-Parisi-Zhang (KPZ) scaling with $z=3/2$ and the small-scale non-universal behavior. This scaling regime is intrinsic to the KS dynamics since it arises from the vanishing of the effective viscosity when evolving from its microscopic negative KS value, to its macroscopic effective positive KPZ value. This vanishing of the viscosity deeply imprints the behavior of correlations at intermediate scales, which exhibit a universal $z=1$ scaling. This behavior pertains to the inviscid-Burgers universality class, which corresponds to the zero-viscosity fixed point of the KPZ equation. We evidence and characterize this so-far-overlooked scaling regime using both functional renormalization group and direct numerical simulations. △ Less

Submitted 22 May, 2026; originally announced May 2026.

Comments: 4 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

Ergodic and Discrete Time Crystal Phases in Periodically Kicked Many-Body Quantum Systems: An Analytical Study

Authors: Vijay Kumar, Dibyendu Roy

Abstract: We analytically study the time evolution of the expectation values of observables in periodically kicked many-body quantum systems. Starting from an initial state, we compute both the transient and the long-time properties of the observables. Our derivation explains the criteria and the mechanism that lead to the infinite-temperature statistical average of observables at long times, irrespective o… ▽ More We analytically study the time evolution of the expectation values of observables in periodically kicked many-body quantum systems. Starting from an initial state, we compute both the transient and the long-time properties of the observables. Our derivation explains the criteria and the mechanism that lead to the infinite-temperature statistical average of observables at long times, irrespective of the initial state. When the criteria are violated, the observables oscillate with time. These oscillations are subharmonic and robust to small perturbations, suggesting the emergence of a discrete time crystal phase. We demonstrate these features explicitly in periodically kicked nonintegrable spin chains. For a spin chain with two kicks per cycle, we show that the kicked chain can exhibit an ergodic or a discrete-time crystal phase for the same kicking strengths, depending on the initial state preparation. We complement our time-evolution study of observables with the spectral form factor of these kicked models. △ Less

Submitted 3 May, 2026; originally announced May 2026.

Symmetry Protected Bulk-Boundary Correspondence in Interacting Topological Insulators

Authors: Kiran Babasaheb Estake, Dibyendu Roy

Abstract: We establish a quantitative bulk-boundary correspondence in interacting topological insulators by relating many-body topological invariants to characteristic degeneracy structures in the entanglement spectrum. Focusing on generalized Su-Schrieffer-Heeger chains with higher winding number, we construct a gauge-invariant many-body winding invariant based on Pancharatnam geometric phases that remains… ▽ More We establish a quantitative bulk-boundary correspondence in interacting topological insulators by relating many-body topological invariants to characteristic degeneracy structures in the entanglement spectrum. Focusing on generalized Su-Schrieffer-Heeger chains with higher winding number, we construct a gauge-invariant many-body winding invariant based on Pancharatnam geometric phases that remains well defined in the presence of interactions. We show that this invariant uniquely determines the low-lying entanglement-spectrum degeneracy, which exhibits a universal $4^ν$ scaling with the winding number $ν$, providing a concrete formulation of bulk-boundary correspondence beyond single-particle topology. Using exact diagonalization, we demonstrate the robustness of this correspondence under interactions and symmetry-preserving disorder, and identify inversion symmetry as a minimal protecting symmetry that stabilizes both the quantization of the invariant and the associated entanglement degeneracies. Our results unify geometric-phase invariants and entanglement diagnostics within a many-body framework and provide a route to identifying interacting topological phases beyond band theory. △ Less

Submitted 14 April, 2026; v1 submitted 10 April, 2026; originally announced April 2026.

Computational Design of Metal-Free Porphyrin Dyes for Sustainable Dye-Sensitized Solar Cells Informing Energy Informatics and Decision Support

Authors: Md Mahmudul Hasan, Chiara Bordin, Fairuz Islam, Tamanna Tasnim, Md. Athar Ishtiyaq, Md. Tasin Nur Rahim, Dhrubo Roy

Abstract: This study aims to evaluate the optoelectronic properties of metal free porphyrin-based D-$π$-A dyes via in-silico performance investigation notifying energy informatics and decision support. To develop novel organic dyes, three acceptor/anchoring groups and five donating groups were introduced to strategic positions of the base porphyrin structure, resulting in a total of fifteen dyes. The single… ▽ More This study aims to evaluate the optoelectronic properties of metal free porphyrin-based D-$π$-A dyes via in-silico performance investigation notifying energy informatics and decision support. To develop novel organic dyes, three acceptor/anchoring groups and five donating groups were introduced to strategic positions of the base porphyrin structure, resulting in a total of fifteen dyes. The singlet ground state geometries of the dyes were optimized utilizing density functional theory (DFT) with B3LYP and the excited state optical properties were explored through time-dependent DFT (TD-DFT) using the PCM model with tetrahydrofuran (THF) as solvent. Both DFT and TD-DFT calculations were carried out using the 6-311G(d,p) basis set. The HOMO energy levels of almost all the modified dyes are lower than the redox potential of I$^-$/I$3^-$ and LUMO energy levels are higher than the conduction band of TiO$2$. The absorption maxima values ranged from 690.64 to 975.55 nm. The dye N1 using triphenylamine group as donor and p-ethynylbenzoic acid group as acceptor, showed optimum optoelectronic properties ($ΔG{reg}=-9.73$ eV, $ΔG{inj}=7.18$ eV, $V_{OC}=1.47$ V and $J_{SC}=15.03$ mA/cm$^2$) with highest PCE 14.37%, making it the best studied dye. This newly modified organic dye with enhanced PCE is remarkably effective for the dye-sensitized solar cells (DSSC) industry. Beyond materials discovery, this study highlights the role of high-performance computing in enabling predictive screening of dye candidates and generating performance indicators (HOMO-LUMO gaps, absorption spectra, charge transfer free energies, photovoltaic metrics). These outputs can serve as key parameters for energy informatics and system modelling. △ Less

Submitted 22 December, 2025; originally announced December 2025.

Comments: 33 pages, 5 figures, 6 tables

Emergence of a hidden-order phase well below the charge density wave transition in a topological Weyl semimetal (TaSe$_4$)$_2$I

Authors: Sk Kalimuddin, Sudipta Chatterjee, Arnab Bera, Satyabrata Bera, Deep Singha Roy, Soham Das, Tuhin Debnath, Ashis K. Nandy, Shishir K. Pandey, Mintu Mondal

Abstract: The emergence of a charge density wave (CDW) in a Weyl semimetal -- a correlated topological phase, is exceptionally rare in condensed matter systems. In this context, the quasi-one-dimensional type-III Weyl semimetal (TaSe$_4$)$_2$I undergoes a CDW transition at $T_{\mathrm{CDW}} \approx 263$~K, providing an exceptional platform to investigate correlated topological CDW states. Here, we uncover a… ▽ More The emergence of a charge density wave (CDW) in a Weyl semimetal -- a correlated topological phase, is exceptionally rare in condensed matter systems. In this context, the quasi-one-dimensional type-III Weyl semimetal (TaSe$_4$)$_2$I undergoes a CDW transition at $T_{\mathrm{CDW}} \approx 263$~K, providing an exceptional platform to investigate correlated topological CDW states. Here, we uncover an additional hidden-order phase transition at $T^* \sim 100$ K, well below the CDW onset, using low-frequency resistance noise spectroscopy, electrical transport, and thermoelectric measurements. This transition is characterized by a sharp enhancement in the noise exponent ($α$) and variance of resistance fluctuations. Analysis of higher-order statistics of resistance fluctuations reveals the correlated dynamics underlying the transition. A pronounced anomaly in the Seebeck coefficient near $T^*$ further suggests a Fermi surface reconstruction. First-principles calculations reveal a structural distortion from the high-symmetry $I422$ phase to a low-symmetry $C2$ phase, via an intermediate $I4$ symmetry. This leads to renormalization of the electronic structure near the Fermi level and opening of a bandgap in the hidden-order phase. These findings demonstrate a previously unidentified correlated phase transition in the topological CDW-Weyl semimetal (TaSe$_4$)$_2$I, enriching the phase diagram of this material and establishing it as an ideal platform for studying intertwined electronic and structural orders. △ Less

Submitted 9 January, 2026; v1 submitted 19 December, 2025; originally announced December 2025.

Comments: 15 pages including 11 figures (6 main and 5 supporting)

Journal ref: Phys. Rev. B 113, 045114 (2026)

Stretching and breaking of particles in compressible random flows

Authors: Dipankar Roy, Marco Martins Afonso, Jason R. Picardo, Dario Vincenzi

Abstract: A key feature of turbulent suspensions that involve floating particles on the surface or inertial particles in the bulk is the compressibility of the effective particle-phase velocity field. Little, however, is known about the effects of small-scale flow compressibility on the stretching and breaking of particles. Here, we gain insight into the nature of these effects by studying the deformation o… ▽ More A key feature of turbulent suspensions that involve floating particles on the surface or inertial particles in the bulk is the compressibility of the effective particle-phase velocity field. Little, however, is known about the effects of small-scale flow compressibility on the stretching and breaking of particles. Here, we gain insight into the nature of these effects by studying the deformation of tiny particles in model fluctuating flows. We consider a generic particle with extensional dynamics that are governed by a vector model, which accounts for elasticity, internal viscosity, and non-affine deformation. Applying the dynamical systems approach of Balkovsky, Fouxon & Lebedev (2000), we first obtain general results for the stationary statistics of particle extension in compressible chaotic flows. We then specialize to a time-decorrelated Gaussian random flow and derive an exact solution for the Batchelor regime of the compressible Kraichnan model. We also perform numerical simulations for a time-correlated renewing flow. While straining is suppressed on the average in compressible flows, our results show that large deviations of the strain rate strongly stretch particles and give rise to a power-law distribution of extensions. Extreme straining events are particularly important for stiff particles and, in the examples considered here, give rise to a counter-intuitive effect: stiff particles stretch more and break faster in flows of increasing compressibility. Highly-elastic particles, whose deformation is dictated by the mean straining, stretch less and break slower. Though based on specific random flows, our work shows how compressibility can affect the extensional dynamics of particles by altering the fluctuations of the strain rate, including its large deviations. △ Less

Submitted 9 December, 2025; originally announced December 2025.

Comments: 23 pages, 6 figures

Nonlinearity-induced transition in heat conduction through a topological metamaterial of rotors

Authors: T. R. Vishnu, Dibyendu Roy

Abstract: We investigate heat conduction in a one-dimensional chain of rigid rotors. The rotors are constrained to rotate in a plane about fixed pivot points and coupled by springs, such that in equilibrium, the neighboring rotors lie on opposite sides of the chain axis. The linearized limit of this model valid for small angular displacements, was first introduced by Kane and Lubensky (KL) as a topological… ▽ More We investigate heat conduction in a one-dimensional chain of rigid rotors. The rotors are constrained to rotate in a plane about fixed pivot points and coupled by springs, such that in equilibrium, the neighboring rotors lie on opposite sides of the chain axis. The linearized limit of this model valid for small angular displacements, was first introduced by Kane and Lubensky (KL) as a topological mechanical insulator hosting zero-energy vibrational edge modes. We show that the linearized KL chain behaves as a thermal insulator at low temperature in both the topological phases with a finite band gap, and the heat current falls exponentially with the chain length. When the gap vanishes at the topological phase transition, the KL chain becomes a good thermal conductor and conducts heat ballistically. The chain of rotors for arbitrary angular displacements hosts nonlinear solitary waves and distinct topological mechanical phases. Our numerical analysis shows normal (diffusive) heat conduction in all topological phases of the nonlinear chain. Nevertheless, a finite thermal conductivity is achieved for different system sizes in different topological phases of this nonlinear chain. △ Less

Submitted 31 May, 2026; v1 submitted 18 November, 2025; originally announced November 2025.

Comments: 12 pages, 6 figures

Journal ref: Phys. Rev. E 113, 054141 (2026)

Detection of long-range coherence in driven hot atomic vapors by spin noise spectroscopy

Authors: Rupak Bag, Sayari Majumder, Saptarishi Chaudhuri, Dibyendu Roy

Abstract: We study intriguing dynamical features of hot Rubidium atoms driven by two light fields. The fields resonantly drive multiple Zeeman states within two hyperfine levels, yielding a cascaded-$Λ$ like structure in the frequency space. A non-Hermitian Floquet tight-binding lattice with imaginary hopping between the nearest states effectively describes the coherence dynamics between Zeeman states withi… ▽ More We study intriguing dynamical features of hot Rubidium atoms driven by two light fields. The fields resonantly drive multiple Zeeman states within two hyperfine levels, yielding a cascaded-$Λ$ like structure in the frequency space. A non-Hermitian Floquet tight-binding lattice with imaginary hopping between the nearest states effectively describes the coherence dynamics between Zeeman states within the ground hyperfine manifold. By performing spin noise spectroscopy, we observe higher harmonic peaks in the noise spectrum that capture multi-photon transitions in the ground manifold. Moreover, the peak amplitudes reveal an exponential decay of long-range coherence with increasing separation between the ground states. △ Less

Submitted 6 October, 2025; v1 submitted 22 September, 2025; originally announced September 2025.

Signatures of emergent surface states across a displacive topological phase transition in Bi$_4$I$_4$

Authors: Deep Singha Roy, Sk Kalimuddin, Subrata Pachhal, Saikat Mondal, Soham Das, Sukanya Jana, Arnab Bera, Satyabrata Bera, Tuhin Debnath, Ankan Bag, Souvik Pramanik, Sudipta Chatterjee, Sanjib Naskar, Shishir Kumar Pandey, Adhip Agarwala, Mintu Mondal

Abstract: Topological phase transitions involving crystalline symmetry breaking provide a fertile ground to explore the interplay between symmetry, topology, and emergent quantum phenomena. Recently discovered quasi-one-dimensional topological material, Bi$_4$I$_4$, has been predicted to host topologically non-trivial gapless surfaces at high temperature, which undergo a finite temperature phase transition… ▽ More Topological phase transitions involving crystalline symmetry breaking provide a fertile ground to explore the interplay between symmetry, topology, and emergent quantum phenomena. Recently discovered quasi-one-dimensional topological material, Bi$_4$I$_4$, has been predicted to host topologically non-trivial gapless surfaces at high temperature, which undergo a finite temperature phase transition to a low temperature gapped phase. Here we present experimental signatures of this room temperature phase transition from a high-temperature $β$-phase with a surface state to a gapped $α$-phase hosting hinge states. Using real-space current mapping and resistance fluctuation spectroscopy, we identify signatures of a displacive topological phase transition mediated by a first-order thermodynamic structural change. Near the emergence of $β$-phase, we observe pronounced telegraphic noise, indicating fluctuating phase domains with topological surface states. The spatially resolved current map reveals electron transport via the gapless surface states in the $β$-phase, which vanishes upon transitioning to the $α$-phase with localized conduction channels (or hinge modes). Our experimental results, supported by first principles estimates and effective theory of a topological displacive phase transition, establish Bi$_4$I$_4$ as a candidate material showing intricate interplay of classical thermodynamic phase transitions with topological quantum phenomena. △ Less

Submitted 3 September, 2025; originally announced September 2025.

Comments: 35 pages, 20 figures, research articles

From Chiral Topological Dynamics to Chiral Topological Amplification: Real vs Imaginary Parameters in a Hermitian Bosonic Chain

Authors: Kiran Babasaheb Estake, T. R. Vishnu, Dibyendu Roy

Abstract: We propose a Hermitian quadratic bosonic model (QBH) whose dynamical matrix exhibits distinct topological and dynamical phenomena depending on whether the hopping and pairing amplitudes are real or purely imaginary. In the real-parameter regime, the dynamical matrix is unitarily equivalent to four decoupled copies of the sublattice-symmetric non-Hermitian Su-Schrieffer-Heeger (nSSH2) model, thereb… ▽ More We propose a Hermitian quadratic bosonic model (QBH) whose dynamical matrix exhibits distinct topological and dynamical phenomena depending on whether the hopping and pairing amplitudes are real or purely imaginary. In the real-parameter regime, the dynamical matrix is unitarily equivalent to four decoupled copies of the sublattice-symmetric non-Hermitian Su-Schrieffer-Heeger (nSSH2) model, thereby inheriting its topological phases and energy spectrum-including the Möbius phase, a gapless topological phase with fractional winding number, having no Hermitian counterpart. We show that the dynamics generated by the QBH Hamiltonian naturally reproduce non-Hermitian time evolution, without invoking nonlinear Schrödinger dynamics or ad hoc normalization. It is demonstrated by analytically calculating the Loschmidt amplitude and computing the dynamical topological order parameter under periodic boundary conditions, which displays a distinct chiral response in the Möbius phase. In contrast, when the hopping and pairing terms are taken to be purely imaginary, the dynamical matrix becomes unitarily equivalent to a different version of the sublattice-symmetric non-Hermitian Su-Schrieffer-Heeger (nSSH1) model that supports only two topological phases: trivial and non-trivial, and the Möbius phase disappears. The latter system exhibits sublattice-dependent chiral amplification under open boundary conditions. We show that this amplification arises from the non-trivial topology of the dynamical matrix, establishing a clear link between topological phase and amplification behavior in the imaginary-parameter regime. △ Less

Submitted 19 December, 2025; v1 submitted 20 August, 2025; originally announced August 2025.

Comments: 18 pages, 11 figures

Journal ref: Phys. Rev. B 112, 214314 (2025)

Preliminary experimental investigation on the interaction of a subaqueous dune like granular structure with a turbulent open channel flow

Authors: Durbar Roy, Ikbal Ahmed, Abdul Hakkim, Rama Govindarajan

Abstract: We study the interaction of a subaqueous dune like granular structure with a turbulent open channel flow experimentally using optical diagnostics in the Shields and Froude parameter space ($0.05{\leq}θ{\leq}0.32$, $0.1{\leq}Fr{\leq}0.4$). Interactions between the turbulent flow and the granular structure give rise to transient erosion-deposition dynamics leading to various types of particle transp… ▽ More We study the interaction of a subaqueous dune like granular structure with a turbulent open channel flow experimentally using optical diagnostics in the Shields and Froude parameter space ($0.05{\leq}θ{\leq}0.32$, $0.1{\leq}Fr{\leq}0.4$). Interactions between the turbulent flow and the granular structure give rise to transient erosion-deposition dynamics leading to various types of particle transport. The subaqueous structures in the channel bed evolves due to shear-stress-induced erosion, gravity-driven deposition, and subsequent particle transport. We study the centroid motion and the granular structure shape evolution. At lower end of our $θ-Fr$ parameter space, we observe no erosion and the structure remains at rest. We show that the critical Shields number ($θ_{cr}$) is of the order of $0.1$ beyond which erosion starts to occur. At intermediate values of $θ$ and $Fr$ we observe slow erosion, resulting in a rigid body motion of the granular structure without significant shape deformation. Higher values of $θ$ and $Fr$ causes vortex formation at the upstream of the dune resulting in stronger erosion, rapid shape deformation and relatively higher translation velocity of the centroid. △ Less

Submitted 27 May, 2025; v1 submitted 14 April, 2025; originally announced April 2025.

Fixed Points and Universality Classes in Coupled Kardar-Parisi-Zhang Equations

Authors: Dipankar Roy, Abhishek Dhar, Manas Kulkarni, Herbert Spohn

Abstract: We study coupled KPZ equations with three control parameters $X,Y,T$. These equations are used in the context of stretched polymers in a random medium, for the spacetime spin-spin correlator of the isotropic quantum Heisenberg chain, and for exciton-polariton condensates. In an earlier article we investigated merely the diagonal $X=Y$, $T=1$. Then the stationary measure is delta-correlated Gaussia… ▽ More We study coupled KPZ equations with three control parameters $X,Y,T$. These equations are used in the context of stretched polymers in a random medium, for the spacetime spin-spin correlator of the isotropic quantum Heisenberg chain, and for exciton-polariton condensates. In an earlier article we investigated merely the diagonal $X=Y$, $T=1$. Then the stationary measure is delta-correlated Gaussian and the dynamical exponent is obtained numerically to be close to $z = \tfrac{3}{2}$. We observed that the scaling functions of the dynamic correlator change smoothly when varying $X$. In this contribution, the analysis is extended to the whole $X$-$Y$-$T$ plane. Solutions are stable only if $XY \geq 0$. Based on numerical simulations, the static correlator still has rapid decay. We argue that the parameter space is foliated into distinct universality classes. They are labeled by $X$ and consist of half-planes parallel to the $Y$-$T$ plane containing the point $(X,X,1)$. △ Less

Submitted 1 June, 2025; v1 submitted 5 April, 2025; originally announced April 2025.

Comments: Manuscript revised, figures updated, 29 pages, 8 figures

Coherent suppression and dephasing-induced reentrance of high harmonics in gapped Dirac materials

Authors: Wolfgang Hogger, Alexander Riedel, Debadrito Roy, Angelika Knothe, Cosimo Gorini, Juan-Diego Urbina, Klaus Richter

Abstract: High-harmonic generation in solids by intense laser pulses provides a fascinating platform for studying material properties and ultra-fast electron dynamics, where its coherent character is a central aspect. Using the semiconductor Bloch equations, we uncover a mechanism suppressing the high harmonic spectrum arising from the coherent superposition of intra- vs. inter-band contributions. We provid… ▽ More High-harmonic generation in solids by intense laser pulses provides a fascinating platform for studying material properties and ultra-fast electron dynamics, where its coherent character is a central aspect. Using the semiconductor Bloch equations, we uncover a mechanism suppressing the high harmonic spectrum arising from the coherent superposition of intra- vs. inter-band contributions. We provide evidence for the generality of this phenomenon by extensive numerical simulations exploring the parameter space in gapped systems with both linear dispersion, such as for massive Dirac Fermions, and with quadratic dispersion, as e.g. for bilayer graphene. Moreover, we demonstrate that, upon increasing dephasing, destructive interference between intra- and inter-band contributions is lifted. This leads to reentrant behavior of suppressed high harmonics, i.e. a crossover from the characteristic spectral "shoulder" to a slowly decaying signal involving much higher harmonics. We supplement our numerical observations with analytical results for the one-dimensional case. △ Less

Submitted 19 January, 2026; v1 submitted 14 March, 2025; originally announced March 2025.

Comments: 12 pages, 9 figures, changes following referee reports; 8 pages, 7 figures, updated title, substantial changes to body, new insights on dephasing; 11 pages, 8 figures

Journal ref: Phys. Rev. B 113, 045418, Published 15 January, 2026

Large-scale multifractality and lack of self-similar decay for Burgers and 3D Navier-Stokes turbulence

Authors: Takeshi Matsumoto, Dipankar Roy, Konstantin Khanin, Rahul Pandit, Uriel Frisch

Abstract: We study decaying turbulence in the 1D Burgers equation (Burgulence) and 3D Navier-Stokes (NS) turbulence. We first investigate the decay in time $t$ of the energy $E(t)$ in Burgulence, for a fractional Brownian initial potential, with Hurst exponent $H$, and demonstrate rigorously a self-similar time-decay of $E(t)$, previously determined heuristically. This is a consequence of the nontrivial bou… ▽ More We study decaying turbulence in the 1D Burgers equation (Burgulence) and 3D Navier-Stokes (NS) turbulence. We first investigate the decay in time $t$ of the energy $E(t)$ in Burgulence, for a fractional Brownian initial potential, with Hurst exponent $H$, and demonstrate rigorously a self-similar time-decay of $E(t)$, previously determined heuristically. This is a consequence of the nontrivial boundedness of the energy for any positive time. We define a spatially forgetful \textit{oblivious fractional Brownian motion} (OFBM), with Hurst exponent $H$, and prove that Burgulence, with an OFBM as initial potential $\varphi_0(x)$, is not only intermittent, but it also displays, a hitherto unanticipated, large-scale bifractality or multifractality; the latter occurs if we combine OFBMs, with different values of $H$. This is the first rigorous proof of genuine multifractality for turbulence in a nonlinear hydrodynamical partial differential equation. We then present direct numerical simulations (DNSs) of freely decaying turbulence, capturing some aspects of this multifractality. For Burgulence, we investigate such decay for two cases: (A) $\varphi_0(x)$ a multifractal random walk that crosses over to a fractional Brownian motion beyond a crossover scale $\mathcal{L}$, tuned to go from small- to large-scale multifractality; (B) initial energy spectra $E_0(k)$, with wavenumber $k$, having one or more power-law regions, which lead, respectively, to self-similar and non-self-similar energy decay. Our analogous DNSs of the 3D NS equations also uncover self-similar and non-self-similar energy decay. Challenges confronting the detection of genuine large-scale multifractality, in numerical and experimental studies of NS and MHD turbulence, are highlighted. △ Less

Submitted 11 March, 2025; originally announced March 2025.

Comments: 44 pages, 19 figures

Leading and beyond leading-order spectral form factor in chaotic quantum many-body systems across all Dyson symmetry classes

Authors: Vijay Kumar, Tomaž Prosen, Dibyendu Roy

Abstract: We show the emergence of random matrix theory (RMT) spectral correlations in the chaotic phase of generic periodically kicked interacting quantum many-body systems by analytically calculating spectral form factor (SFF), $K(t)$, up to two leading orders in time, $t$. We explicitly consider the presence or absence of time reversal ($\mathcal{T}$) symmetry to investigate all three Dyson's symmetry cl… ▽ More We show the emergence of random matrix theory (RMT) spectral correlations in the chaotic phase of generic periodically kicked interacting quantum many-body systems by analytically calculating spectral form factor (SFF), $K(t)$, up to two leading orders in time, $t$. We explicitly consider the presence or absence of time reversal ($\mathcal{T}$) symmetry to investigate all three Dyson's symmetry classes. Our derivation only assumes random phase approximation to enable ensemble average. For $\mathcal{T}$-invariant systems with $\mathcal{T}^2=1$, we show that beyond the Thouless time $t^*$, the SFF takes the form $K(t)\simeq 2t-2t^2/\mathcal{N}$ up to second order in time, where $\mathcal{N}$ is the Hilbert space dimension. This is identical to the result from circular orthogonal ensemble of RMT. In the absence of $\mathcal{T}$-symmetry, we show that $K(t)\simeq t$ beyond $t^*$, and there is no universal term in the second order, unlike the $\mathcal{T}^2=1$ case, in agreement with the result of circular unitary ensemble. For $\mathcal{T}$-invariant systems with $\mathcal{T}^2=-1$, we show that $K(t)\simeq 2t+2t^2/\mathcal{N}$ up to two orders in time beyond $t^*$, in agreement with the result of circular symplectic ensemble. In all three cases, the system-size, $L$, scaling of $t^*$ is determined by eigenvalues of a doubly stochastic matrix $\mathcal{M}$. For strongly interacting fermionic chains, $\mathcal{M}$ is $SU(2)$ invariant in all three cases, leading to $t^*\propto L^2$ in the presence of $U(1)$ symmetry. In the absence of $U(1)$ symmetry, we find $t^*\propto L^0$, due to gapped non-degenerate second-largest eigenvalue of $\mathcal{M}$ or $t^*\propto \ln(L)$ due to gapped second-largest eigenvalue with degeneracy $\propto L^ζ$. Our calculation of SFF is plausible in higher space dimensions as well, where similar system-size scalings of $t^*$ can be obtained. △ Less

Submitted 6 February, 2025; originally announced February 2025.

Journal ref: Phys. Rev. X 16, 031019 (2026)

High temperature antiferromagnetism in ultrathin SrRu2O6 nanosheets

Authors: Deepak K. Roy, Mukul Kabir

Abstract: The quest for room-temperature nanoscale magnets remains a central challenge, driven by their promising applications in quantum technologies. Layered $4d$ and $5d$ transition metal oxides with high magnetic ordering temperatures offer significant potential in this context. We explore ultrathin \ce{SrRu2O6} nanosheets using first-principles calculations, complemented by the classical Heisenberg Mon… ▽ More The quest for room-temperature nanoscale magnets remains a central challenge, driven by their promising applications in quantum technologies. Layered $4d$ and $5d$ transition metal oxides with high magnetic ordering temperatures offer significant potential in this context. We explore ultrathin \ce{SrRu2O6} nanosheets using first-principles calculations, complemented by the classical Heisenberg Monte Carlo simulations. Remarkably, these nanosheets exhibit robust antiferromagnetic ordering with Néel temperatures exceeding 430 K, despite the enhanced spin fluctuations characteristic of two-dimensional systems. Surface-termination-induced intrinsic charge doping introduces complexity to the magnetism, resulting in an insulator-to-metal transition and renormalized Néel temperatures in doped systems. A detailed microscopic analysis reveals distinct mechanisms underlying the magnetic behavior in electron- and hole-doped nanosheets. These findings provide a foundation for advancing theoretical and experimental studies in the largely unexplored realm of correlated oxides at the two-dimensional limit. △ Less

Submitted 27 December, 2024; v1 submitted 23 December, 2024; originally announced December 2024.

Comments: 12 pages, 7 figures

Journal ref: Physical Review B 111, 134409 (2025)

Insights into Transient Dynamics of Bacteria Laden Liquid Bridges

Authors: Kush Kumar Dewangan, Srinivas Rao S, Durbar Roy, Atish Roy Chowdhury, Dipshikha Chakravortty, Saptarshi Basu

Abstract: We study evaporation and precipitate formation mechanics of bacteria-laden liquid bridge using experimental and theoretical analysis. Aqueous suspension of motile and non-motile Salmonella Typhimurium and Pseudomonas aeruginosa typically found in contaminated food and water were used in liquid bridge configuration between hydrophilic substrates. Using inverse logarithmic evaporation flux model, we… ▽ More We study evaporation and precipitate formation mechanics of bacteria-laden liquid bridge using experimental and theoretical analysis. Aqueous suspension of motile and non-motile Salmonella Typhimurium and Pseudomonas aeruginosa typically found in contaminated food and water were used in liquid bridge configuration between hydrophilic substrates. Using inverse logarithmic evaporation flux model, we study volume regression for cylindrical/catenoid volume models with confinement distance as a parameter. For all confinement distances, the regression is linear on normalizing both volume and time as in the case of pure sessile drop. However, in normalized volume and dimensional time space, we observe non-linearities as the evaporation time scales non linearly with the confinement distance. The non-linearities were captured using the catenoid model. The catenoid model conforms to the experimental volume regression data at all confinement distances, and the transient liquid bridge interface evolution profile at high confinement distance. We also study the precipitate pattern and bacterial distribution using micro/nano characterization techniques. We show the average precipitate pattern for both sessile and higher confinement distance resembles coffee ring type deposits although the underlying bacterial distribution differs. For lower confinement, we observe pattern resulting from a combination of coffee ring effect, stick-slick motion, and thin film instability. The reduction in confinement distance causes an altered bacterial agglomeration, resulting in a multi-pattern network instead of a single circumferential edge deposition. We show the aerial size of motile bacteria increases with decreasing confinement, whereas the size for non-motile bacteria remains constant in the precipitate. △ Less

Submitted 5 January, 2025; v1 submitted 10 December, 2024; originally announced December 2024.

Nonreciprocal electrical transport in linear systems with balanced gain and loss in the bulk

Authors: Rupak Bag, Dibyendu Roy

Abstract: We investigate electrical transport in a quantum wire of $N$ sites connected to an equal number $(N_i/2)$ of sources and drains of charges in bulk. Each source and drain injects and extracts charges at the same rate, respectively. We show that the linear-response electrical current is nonreciprocal in such a system when the arrangement of sources and drains breaks the system's parity. We prove tha… ▽ More We investigate electrical transport in a quantum wire of $N$ sites connected to an equal number $(N_i/2)$ of sources and drains of charges in bulk. Each source and drain injects and extracts charges at the same rate, respectively. We show that the linear-response electrical current is nonreciprocal in such a system when the arrangement of sources and drains breaks the system's parity. We prove that inelastic scattering is essential for nonreciprocity in this system. For this, we invoke a master equation description of classical charge transport in a similar system. The nonreciprocal current in quantum wire matches that in the classical model for $N_i/N \sim 1$, generating a finite scattering length much smaller than the length of the wire. The nonreciprocity in the quantum wire oscillates with wire length when $N_i/N \ll 1$, and it can vanish at specific lengths. △ Less

Submitted 26 February, 2025; v1 submitted 19 September, 2024; originally announced September 2024.

Journal ref: Phys. Rev. B 111, 064309 (2025)

Reconfigurable Spin Logics and High-density Multistate Memory in a Single Spin-orbit Torque Device

Authors: Raghvendra Posti, Dhanajay Tiwari, Debangsu Roy

Abstract: Nonvolatile devices based on the spin-orbit torque (SOT) mechanism are highly suitable for in-memory logic operations. The current objective is to enhance the memory density of memory cells while performing logic operations within the same memory unit. Present study demonstrates that integrating SOT with an out-of-plane magnetic field effectively achieves multiple magnetic states in perpendicularl… ▽ More Nonvolatile devices based on the spin-orbit torque (SOT) mechanism are highly suitable for in-memory logic operations. The current objective is to enhance the memory density of memory cells while performing logic operations within the same memory unit. Present study demonstrates that integrating SOT with an out-of-plane magnetic field effectively achieves multiple magnetic states in perpendicularly magnetized heterostructures. This study further explores this approach, experimentally demonstrating reconfigurable logic operations within a single SOT device using W/Pt/Co/AlOxheterostructures. Our results show that multistate tuning by SOT integration with out-of-plane magnetic field enables reconfigurable logic operations, including AND, OR, NOR, NAND, and Always ON, within a single device. Additionally, we found that careful selection of input logic operations allows multiple configurations to achieve the same logic function within a single memory device. To enhance multistate memory density, we proposed and experimentally verified a two-step writing process, achieving the highest reported multistate memory density in SOT-based memory devices. These findings highlight the potential of integrating SOT and magnetic field effects to realize high-density, multifunctional in-memory logic devices. △ Less

Submitted 19 August, 2024; originally announced August 2024.

Correlation of Magnetic State Configurations in Nanotubes with FMR spectrum

Authors: Abhishek Kumar, Chirag Kalouni, Raghvendra Posti, Vivek K Malik, Dhananjay Tiwari, Debangsu Roy

Abstract: Magnetic nanotubes have garnered immense attention for their potential in high-density magnetic memory, owing to their stable flux closure configuration and fast, reproducible reversal processes. However, characterizing their magnetic configuration through straightforward methodologies remains a challenge in both scope and detail. Here, we elucidate the magnetic state details using Remanence Field… ▽ More Magnetic nanotubes have garnered immense attention for their potential in high-density magnetic memory, owing to their stable flux closure configuration and fast, reproducible reversal processes. However, characterizing their magnetic configuration through straightforward methodologies remains a challenge in both scope and detail. Here, we elucidate the magnetic state details using Remanence Field Ferromagnetic Resonance Spectroscopy (RFMR) for arrays of electrodeposited nanotubes. Micromagnetic simulations revealed distinct spin configurations while coming from saturation, including the edge vortex, onion, uniform and curling states, with chirality variations depending on the preparation field direction. Dynamic measurements, coupled with RFMR spectra analysis, unveiled multiple FMR modes corresponding to these spin configurations. The evolution of spin configurations under bias fields were studied, indicating nucleation within the curling state. Observations revealed opposite RFMR spectra, denoting opposite magnetic spin configurations after removing the positive and negative saturating fields when the magnetic field was applied along {theta_H=0} and perpendicular {theta_H= 90} to the nanotube axis. We observed a mixture of the non-uniform curling states with the end vortex state (onion-like curling state) at the end of the nanotubes for the theta_H=0(90) and uniform magnetization states in the middle of the nanotubes for the theta_H=0 configuration. Building on RFMR information, frequency-swept FMR absorption spectra obtained at different bias fields allowed the characterization of magnetization states. This picture was supported by micromagnetic simulations. These findings were further substantiated with First Order Reversal Curve measurements (FORC). △ Less

Submitted 18 August, 2024; originally announced August 2024.

Initialization-Free Multistate Memristor: Synergy of Spin-Orbit Torque and Magnetic Fields

Authors: Raghvendra Posti, Chirag Kalouni, Dhananjay K Tiwari, Debangsu Roy

Abstract: Spin-orbit torque (SOT)-based perpendicularly magnetized memory devices with multistate memory have garnered significant interest due to their applicability in low-power in-memory analog computing. However, current methods are hindered by initialization problems such as prolonged writing duration, and limitations on the number of magnetic states. Consequently, a universal method for achieving mult… ▽ More Spin-orbit torque (SOT)-based perpendicularly magnetized memory devices with multistate memory have garnered significant interest due to their applicability in low-power in-memory analog computing. However, current methods are hindered by initialization problems such as prolonged writing duration, and limitations on the number of magnetic states. Consequently, a universal method for achieving multistate in PMA-based stacks remains elusive. Here, we propose a general experimental method for achieving multistate without any initialization step in SOT-driven magnetization switching by integrating an external out-of-plane magnetic field. Motivated by macrospin calculations coupled with micromagnetic simulations, which demonstrate the plausibility of magnetization state changes due to out-of-plane field integration, we experimentally verify multistate behavior in Pt/Co/Pt and W/Pt/Co/AlOx stacks. The occurrence of multistate behavior is attributed to intermediate domain states with Néel domain walls. We achieve repeatable 18 multistate configurations with a minimal reduction in retentivity through energy barrier measurements, paving the way for efficient analog computing. △ Less

Submitted 16 August, 2024; originally announced August 2024.

Physics Informed Deep Learning for Strain Gradient Continuum Plasticity

Authors: Ankit Tyagi, Uttam Suman, Mariya Mamajiwala, Debasish Roy

Abstract: We use a space-time discretization based on physics informed deep learning (PIDL) to approximate solutions of a class of rate-dependent strain gradient plasticity models. The differential equation governing the plastic flow, the so-called microforce balance for this class of yield-free plasticity models, is very stiff, often leading to numerical corruption and a consequent lack of accuracy or conv… ▽ More We use a space-time discretization based on physics informed deep learning (PIDL) to approximate solutions of a class of rate-dependent strain gradient plasticity models. The differential equation governing the plastic flow, the so-called microforce balance for this class of yield-free plasticity models, is very stiff, often leading to numerical corruption and a consequent lack of accuracy or convergence by finite element (FE) methods. Indeed, setting up the discretized framework, especially with an elaborate meshing around the propagating plastic bands whose locations are often unknown a-priori, also scales up the computational effort significantly. Taking inspiration from physics informed neural networks, we modify the loss function of a PIDL model in several novel ways to account for the balance laws, either through energetics or via the resulting PDEs once a variational scheme is applied, and the constitutive equations. The initial and the boundary conditions may either be imposed strictly by encoding them within the PIDL architecture, or enforced weakly as a part of the loss function. The flexibility in the implementation of a PIDL technique often makes for its ready interface with powerful optimization schemes, and this in turn provides for many possibilities in posing the problem. We have used freely available open-source libraries that perform fast, parallel computations on GPUs. Using numerical illustrations, we demonstrate how PIDL methods could address the computational challenges posed by strain gradient plasticity models. Also, PIDL methods offer abundant potentialities, vis-á-vis a somewhat straitjacketed and poorer approximant of FE methods, in customizing the formulation as per the problem objective. △ Less

Submitted 13 August, 2024; originally announced August 2024.

Detecting winding and Chern numbers in topological matter using spectral function

Authors: Kiran Babasaheb Estake, Dibyendu Roy

Abstract: We propose a method to directly probe bulk topological quantum numbers in topological matter by measuring the momentum-space single-particle spectral function (SPSF). Angle-resolved photoemission spectroscopy (ARPES) can detect SPSF and is often used to determine the bulk band structure of quantum materials. Here, we show that while one part of the momentum-space SPSF gives band structure, it also… ▽ More We propose a method to directly probe bulk topological quantum numbers in topological matter by measuring the momentum-space single-particle spectral function (SPSF). Angle-resolved photoemission spectroscopy (ARPES) can detect SPSF and is often used to determine the bulk band structure of quantum materials. Here, we show that while one part of the momentum-space SPSF gives band structure, it also contains the knowledge of winding and Chern numbers of various topological materials. For this, we derive SPSF in different models of topological systems, such as the Kitaev model of topological superconductors, the long-range Su-Schrieffer-Heeger model, the Qi-Wu-Zhang model, the Haldane model on a hexagonal lattice and a four-band model that is a physical realization of the Kitaev chain and explain how to extract the winding or Chern numbers in different topological phases from the SPSF. Such information from SPSF seems experimentally accessible due to the recent advancement of ARPES and scanning tunneling spectroscopy techniques. While the detection of bulk topological quantum numbers using the SPSF works well for one-dimensional systems, it has certain limitations for higher-dimensional systems, where it must be complementary with another measurement to ensure the matter under investigation is topological, i.e., having a non-zero Chern number. △ Less

Submitted 10 February, 2025; v1 submitted 30 July, 2024; originally announced July 2024.

Comments: 15 pages, 11 figures

Journal ref: Phys. Rev. B 111, 085406 (2025)

Modeling of Hot-Carrier Degradation driven by silicon-hydrogen bond dissociation in SPADs

Authors: Mathieu Sicre, Xavier Federspiel, Bastien Mamdy, David Roy, Francis Calmon

Abstract: A novel approach for modeling Dark Count Rate (DCR) drift in Single-Photon Avalanche Diodes (SPADs) is proposed based on Hot-Carrier Degradation (HCD) inducing silicon-hydrogen bond dissociation at the Si/SiO2 interface. The energy and the quantity of hot-carriers are modeled by the interplay of carrier energy distribution and current density. The carrier energy distribution, achieved by a Full-Ba… ▽ More A novel approach for modeling Dark Count Rate (DCR) drift in Single-Photon Avalanche Diodes (SPADs) is proposed based on Hot-Carrier Degradation (HCD) inducing silicon-hydrogen bond dissociation at the Si/SiO2 interface. The energy and the quantity of hot-carriers are modeled by the interplay of carrier energy distribution and current density. The carrier energy distribution, achieved by a Full-Band Monte-Carlo simulation considering the band structure and the scattering mechanisms, establishes a crucial link to the degradation of the top SPAD interface, primarily influenced by hot electrons due to their broader energy spread. The current density is determined by analyzing the generation rates of carriers under dark and photo conditions, along with the multiplication rate, through a combination of experimental data and modeling techniques. Subsequently,these hot carriers are correlated with the distribution of bond dissociation energy, which is modeled by the disorder-induced local variations among the Si-H bond energy at the Si/SiO2 interface. The impact-ionization probability between hot carriers and Si-H bonds is then calculated by differentiating their energies, thereby determining the degradation kinetics. This enables the capture of the rise in dark current density with stress duration by the increasing number of defects, which in turn affects the modeling of degradation rate. For the first time, a direct correlation between the dark current and DCR, along with their drift over stress time, has been established, relying on the carrier generation rate originating from these defects together with the position-dependent breakdown probability Pt.This physic-based model allows to predict DCR for unprecedented long-term stress measurement time up to 10e6s, covering a whole set of characterization and stress conditions for SPAD devices. △ Less

Submitted 3 August, 2024; v1 submitted 23 June, 2024; originally announced July 2024.

Quantum noise induced nonreciprocity for single photon transport in parity-time symmetric systems

Authors: Dibyendu Roy, G. S. Agarwal

Abstract: We show nonreciprocal light propagation for single-photon inputs due to quantum noise in coupled optical systems with gain and loss. We consider two parity-time ($\mathcal{PT}$) symmetric linear optical systems consisting of either two directly coupled resonators or two finite-length waveguides evanescently coupled in parallel. One resonator or waveguide is filled with an active gain medium and th… ▽ More We show nonreciprocal light propagation for single-photon inputs due to quantum noise in coupled optical systems with gain and loss. We consider two parity-time ($\mathcal{PT}$) symmetric linear optical systems consisting of either two directly coupled resonators or two finite-length waveguides evanescently coupled in parallel. One resonator or waveguide is filled with an active gain medium and the other with a passive loss medium. The light propagation is reciprocal in such $\mathcal{PT}$ symmetric linear systems without quantum noise. We show here that light transmission becomes nonreciprocal when we include quantum noises in our modeling, which is essential for a proper physical description. The quantum nonreciprocity is especially pronounced in the $\mathcal{PT}$ broken phase. Transmitted light intensity in the waveguide of incidence is asymmetric for two waveguides even without noise. Quantum noise significantly enhances such asymmetry in the broken phase. △ Less

Submitted 4 January, 2025; v1 submitted 30 June, 2024; originally announced July 2024.

Comments: 12 pages, 4 figures

Journal ref: Phys. Rev. A 111, 013702 (2025)

Geometric Thermodynamics of Collapse of Gels

Authors: Asif Raza, Sanhita Das, Debasish Roy

Abstract: Stimulus-induced volumetric phase transition in gels may be potentially exploited for various bio-engineering and mechanical engineering applications. Since the discovery of the phenomenon in the 1970s, extensive experimental research has helped understand the phase transition and related critical phenomena. Yet, little insight is available on the evolving microstructure. In this article, we aim a… ▽ More Stimulus-induced volumetric phase transition in gels may be potentially exploited for various bio-engineering and mechanical engineering applications. Since the discovery of the phenomenon in the 1970s, extensive experimental research has helped understand the phase transition and related critical phenomena. Yet, little insight is available on the evolving microstructure. In this article, we aim at unravelling certain geometric aspects of the micromechanics underlying discontinuous phase transition in polyacrylamide gels. Towards this, we use geometric thermodynamics and a Landau-Ginzburg type free energy functional involving a squared gradient, in conjunction with Flory-Huggins theory. We specifically exploit Ruppeiner's approach of Riemannian geometry-enriched thermodynamic fluctuation theory, which was previously employed to investigate phase transitions in van der Waals fluids and black holes. The framework equips us with a scalar curvature that is typically indicative of certain aspects of the microstructure during phase transition. Since previous studies have indicated that curvature divergence relates to correlation length divergence, we infer that the gel possesses a heterogeneous microstructure during phase transition, i.e. at critical points. Curvature also provides an insight into the universality class of phase transition and the nature of polymer-polymer interactions. △ Less

Submitted 22 August, 2024; v1 submitted 25 March, 2024; originally announced March 2024.

Comments: 13 pages, 10 figures

Myelin figures from microbial glycolipid biosurfactant amphiphiles

Authors: Debdyuti Roy, Vincent Chaleix, Atul N. Parikh, Niki Baccile

Abstract: Myelin figures (MFs) -- cylindrical lyotropic liquid crystalline structures consisting of concentric arrays of bilayers and aqueous media -- arise from the hydration of the bulk lamellar phase of many common amphiphiles. Prior efforts have concentrated on the formation, structure, and dynamics of myelin produced by phosphatidylcholine (PC)-based amphiphiles. Here, we study the myelinization of gly… ▽ More Myelin figures (MFs) -- cylindrical lyotropic liquid crystalline structures consisting of concentric arrays of bilayers and aqueous media -- arise from the hydration of the bulk lamellar phase of many common amphiphiles. Prior efforts have concentrated on the formation, structure, and dynamics of myelin produced by phosphatidylcholine (PC)-based amphiphiles. Here, we study the myelinization of glycolipid microbial amphiphiles, commonly addressed as biosurfactants, produced through the process of fermentation. The hydration characteristics (and phase diagrams) of these biological amphiphiles are atypical (and thus their capacity to form myelin) because unlike typical amphiphiles, their molecular structure is characterized by two hydrophilic groups (sugar, carboxylic acid) on both ends with a hydrophobic moiety in the middle. We tested three different glycolipid molecules: C18:1 sophorolipids and single-glucose C18:1 and C18:0 glucolipids, all in their nonacetylated acidic form. Neither sophorolipids (too soluble) nor C18:0 glucolipids (too insoluble) displayed myelin growth at room temperature (RT, 25 C). The glucolipid C18:1 (G-C18:1), on the other hand, showed dense myelin growth at RT below pH 7.0. Examining their growth rates, we find that they display a linear L $α$ t (L, myelin length; t, time) growth rate, suggesting ballistic growth, distinctly different from the L $α$ t^(1/2) dependence, characterizing diffusive growth such as what occurs in more conventional phospholipids. These results offer some insight into lipidic mesophases arising from a previously unexplored class of amphiphiles with potential applications in the field of drug delivery. △ Less

Submitted 27 February, 2024; originally announced February 2024.

Heat transport through an open coupled scalar field theory hosting stability-to-instability transition

Authors: T. R. Vishnu, Dibyendu Roy

Abstract: We investigate heat transport through a one-dimensional open coupled scalar field theory, depicted as a network of harmonic oscillators connected to thermal baths at the boundaries. The non-Hermitian dynamical matrix of the network undergoes a stability-to-instability transition at the exceptional points as the coupling strength between the scalar fields increases. The open network in the unstable… ▽ More We investigate heat transport through a one-dimensional open coupled scalar field theory, depicted as a network of harmonic oscillators connected to thermal baths at the boundaries. The non-Hermitian dynamical matrix of the network undergoes a stability-to-instability transition at the exceptional points as the coupling strength between the scalar fields increases. The open network in the unstable regime, marked by the emergence of inverted oscillator modes, does not acquire a steady state, and the heat conduction is then unbounded for general bath couplings. In this work, we engineer a unique bath coupling where a single bath is connected to two fields at each edge with the same strength. This configuration leads to a finite steady-state heat conduction in the network, even in the unstable regime. We also study general bath couplings, e.g., connecting two fields to two separate baths at each boundary, which shows an exciting signature of approaching the unstable regime for massive fields. We derive analytical expressions for high-temperature classical heat current through the network for different bath couplings at the edges and compare them. Furthermore, we determine the temperature dependence of low-temperature quantum heat current in different cases. Our study will help to probe topological phases and phase transitions in various quadratic Hermitian bosonic models whose dynamical matrices resemble non-Hermitian Hamiltonians, hosting exciting topological phases. △ Less

Submitted 7 February, 2024; originally announced February 2024.

Comments: 19 pages, 4 figures

Journal ref: J Stat Phys 191, 123 (2024)

Universality in coupled stochastic Burgers systems with degenerate flux Jacobian

Authors: Dipankar Roy, Abhishek Dhar, Konstantin Khanin, Manas Kulkarni, Herbert Spohn

Abstract: In our contribution we study stochastic models in one space dimension with two conservation laws. One model is the coupled continuum stochastic Burgers equation, for which each current is a sum of quadratic non-linearities, linear diffusion, and spacetime white noise. The second model is a two-lane stochastic lattice gas. As distinct from previous studies, the two conserved densities are tuned suc… ▽ More In our contribution we study stochastic models in one space dimension with two conservation laws. One model is the coupled continuum stochastic Burgers equation, for which each current is a sum of quadratic non-linearities, linear diffusion, and spacetime white noise. The second model is a two-lane stochastic lattice gas. As distinct from previous studies, the two conserved densities are tuned such that the flux Jacobian, a $2 \times 2$ matrix, has coinciding eigenvalues. In the steady state, investigated are spacetime correlations of the conserved fields and the time-integrated currents at the origin. For a particular choice of couplings the dynamical exponent 3/2 is confirmed. Furthermore, at these couplings, continuum stochastic Burgers equation and lattice gas are demonstrated to be in the same universality class. △ Less

Submitted 12 January, 2024; originally announced January 2024.

Comments: 34 pages, 9 figures

Anomalous dynamical response of non-Hermitian topological phases

Authors: Ritu Nehra, Dibyendu Roy

Abstract: Composite topological phases with intriguing topology like M${ö}$bius strips emerge in sublattice symmetric non-Hermitian systems due to spontaneous breaking of time-reversal symmetry at some parameter regime. While these phases have been characterized by nonadiabatic complex geometric phases of multiple participating complex bands, the physical properties of these phases largely remain unknown. W… ▽ More Composite topological phases with intriguing topology like M${ö}$bius strips emerge in sublattice symmetric non-Hermitian systems due to spontaneous breaking of time-reversal symmetry at some parameter regime. While these phases have been characterized by nonadiabatic complex geometric phases of multiple participating complex bands, the physical properties of these phases largely remain unknown. We explore the dynamical response of these phases by studying Loschmidt echo from an initial state of the Hermitian Su-Schrieffer-Heeger (SSH) model, which is evolved by a non-Hermitian SSH Hamiltonian after a sudden quench in parameters. Topology-changing quenches display non-analytical temporal behavior of return rates (logarithm of the Loschmidt echo) for the non-Hermitian SSH Hamiltonian in the trivial, M${ö}$bius and topological phase. Moreover, the dynamical topological order parameter appears only at one side of the Brillouin zone for the M${ö}$bius phase case in contrast to both sides of the Brillouin zone for quench by the trivial and topological phase of the non-Hermitian SSH model. The last feature is a dynamical signature of different symmetry constraints on the real and imaginary parts of the complex bands in the M${ö}$bius phase. △ Less

Submitted 19 October, 2023; originally announced October 2023.

Comments: 10 pages, 6 figures

Journal ref: Physical Review B 109, 094311 (2024)

Many-body quantum chaos in mixtures of multiple species

Authors: Vijay Kumar, Dibyendu Roy

Abstract: We study spectral correlations in many-body quantum mixtures of fermions, bosons, and qubits with periodically kicked spreading and mixing of species. We take two types of mixing, namely, Jaynes-Cummings and Rabi, respectively, satisfying and breaking the conservation of a total number of species. We analytically derive the generating Hamiltonians whose spectral properties determine the spectral f… ▽ More We study spectral correlations in many-body quantum mixtures of fermions, bosons, and qubits with periodically kicked spreading and mixing of species. We take two types of mixing, namely, Jaynes-Cummings and Rabi, respectively, satisfying and breaking the conservation of a total number of species. We analytically derive the generating Hamiltonians whose spectral properties determine the spectral form factor in the leading order. We further analyze the system-size $(L)$ scaling of Thouless time $t^*$, beyond which the spectral form factor follows the prediction of random matrix theory. The $L$-dependence of $t^*$ crosses over from $\log L$ to $L^2$ with an increasing Jaynes-Cummings mixing between qubits and fermions or bosons in a finite-sized chain, and it finally settles to $t^* \propto \mathcal{O}(L^2)$ in the thermodynamic limit for any mixing strength. The Rabi mixing between qubits and fermions leads to $t^*\propto \mathcal{O}(\log L)$, previously predicted for single species of qubits or fermions without total number conservation. △ Less

Submitted 10 October, 2023; originally announced October 2023.

Comments: 15 pages, 8 figures

Journal ref: Physical Review E 109, L032201 (2024)

Insights into bubble droplet interactions in evaporating polymeric droplets

Authors: Gannena K S Raghuram, Durbar Roy, D Chaitanya Kumar Rao, Aloke Kumar, Saptarshi Basu

Abstract: Polymer droplets subjected to a heated environment have significance in several fields ranging from spray drying and powder formation to surface coating. In the present work, we investigate the evaporation of a high viscoelastic modulus aqueous polymeric droplet in an acoustically levitated environment. Depending on the laser irradiation intensity, we observe nucleation of a bubble in the dilute r… ▽ More Polymer droplets subjected to a heated environment have significance in several fields ranging from spray drying and powder formation to surface coating. In the present work, we investigate the evaporation of a high viscoelastic modulus aqueous polymeric droplet in an acoustically levitated environment. Depending on the laser irradiation intensity, we observe nucleation of a bubble in the dilute regime of polymer concentration, contrary to the previously observed bubble nucleation in a semi-dilute entangled regime for low viscoelastic modulus polymer droplets. After the bubble nucleation, a quasi steady bubble growth occurs depending on the laser irradiation intensity and concentrations. Our scaling analysis reveals that bubble growth follows Plesset-Zwick criteria independent of the viscoelastic properties of the polymer solution. Further, we establish that the onset of bubble growth has an inverse nonlinear dependence on the laser irradiation intensity. At high concentrations and laser irradiation intensities, we report the expansion and collapse of polymer membrane without rupture, indicating the formation of an interfacial skin with significant strength. The droplet oscillations are primarily driven by the presence of multiple bubbles and, to some extent, by the rotational motion of the droplet. Finally, depending on the nature of bubble growth, different types of precipitate form contrary to the different modes of atomization observed in low viscoelastic modulus polymer droplets. △ Less

Submitted 27 September, 2023; originally announced September 2023.

Exceptionally Slow, Long Range, and Non-Gaussian Critical Fluctuations Dominate the Charge Density Wave Transition

Authors: Sk Kalimuddin, Sudipta Chatterjee, Arnab Bera, Hasan Afzal, Satyabrata Bera, Deep Singha Roy, Soham Das, Tuhin Debnath, Bhavtosh Bansal, Mintu Mondal

Abstract: $(TaSe_4)_2I… ▽ More $(TaSe_4)_2I$ is a well-studied quasi-one-dimensional compound long-known to have a charge-density wave (CDW) transition around 263 K. We argue that the critical fluctuations of the pinned CDW order parameter near the transition can be inferred from the resistance noise on account of their coupling to the dissipative normal carriers. Remarkably, the critical fluctuations of the CDW order parameter are slow enough to survive the thermodynamic limit and dominate the low-frequency resistance noise. The noise variance and relaxation time show rapid growth (critical opalescence and critical slowing down) within a temperature window of $ \varepsilon \approx \pm 0.1$, where $\varepsilon$ is the reduced temperature. This is very wide but consistent with the Ginzburg criterion. We further show that this resistance noise can be quantitatively used to extract the associated critical exponents. Below $|\varepsilon | \lesssim 0.02$, we observe a crossover from mean-field to a fluctuation-dominated regime with the critical exponents taking anomalously low values. The distribution of fluctuations in the critical transition region is skewed and strongly non-Gaussian. This non-Gaussianity is interpreted as the breakdown of the validity of the central limit theorem as the diverging coherence volume becomes comparable to the macroscopic sample size. The large magnitude critical fluctuations observed over an extended temperature range, as well as the crossover from the mean-field to the fluctuation-dominated regime highlight the role of the quasi-one dimensional character in controlling the phase transition. △ Less

Submitted 24 May, 2024; v1 submitted 18 August, 2023; originally announced August 2023.

Comments: 7 pages, 4 figures, supplementary, Will appear in Phys. Rev. Lett

Journal ref: Phys. Rev. Lett. 132, 266504 (2024)

Enhanced Magnetism and Phase Transitions in Ultrathin Quantum Spin Liquid Na2IrO3 Flakes

Authors: Deepak K Roy, Mukul Kabir

Abstract: The quest for quantum spin liquids has garnered significant attention due to their rich physics and disruptive prospects in quantum communication and computation. Spin-orbit coupling, electron correlation, and structural distortion play critical roles in the candidate materials that eventually order antiferromagnetically at low temperatures. We introduce quantum electron confinement to the existin… ▽ More The quest for quantum spin liquids has garnered significant attention due to their rich physics and disruptive prospects in quantum communication and computation. Spin-orbit coupling, electron correlation, and structural distortion play critical roles in the candidate materials that eventually order antiferromagnetically at low temperatures. We introduce quantum electron confinement to the existing complexity and explore the interplay between Heisenberg and Kitaev interactions in ultrathin \ce{Na2IrO3} layers using first-principles calculations. The zigzag antiferromagnetic state in the monolayer is reinforced and pushed further away from the Kitaev spin liquid state due to the increased strength of Heisenberg and off-diagonal exchange interactions. In contrast, the carrier-doped flakes undergo a Mott insulator-to-metal transition accompanied by an antiferromagnetic to ferromagnetic transition. These findings present exciting prospects for comprehending magnetism in a novel two-dimensional framework of non-van der Waals correlated oxide flakes. △ Less

Submitted 24 July, 2023; originally announced July 2023.

Comments: 8 pages, 5 figures

Journal ref: Phys. Rev. B 110, L020403 (2024)

Raman signatures of lattice dynamics across inversion symmetry breaking phase transition in quasi-1D compound, (TaSe$_4$)$_3$I

Authors: Arnab Bera, Partha Sarathi Rana, Suman Kalyan Pradhan, Mainak Palit, Sk Kalimuddin, Satyabrata Bera, Tuhin Debnath, Soham Das, Deep Singha Roy, Hasan Afzal, Subhadeep Datta, Mintu Mondal

Abstract: Structural phase transition can occur due to complex mechanisms other than simple dynamical instability, especially when the parent and daughter structure is of low dimension. This article reports such an inversion symmetry-breaking structural phase transition in a quasi-1D compound (TaSe$_4$)$_3$I at T$_S\sim$ 141~K studied by Raman spectroscopy. Our investigation of collective lattice dynamics r… ▽ More Structural phase transition can occur due to complex mechanisms other than simple dynamical instability, especially when the parent and daughter structure is of low dimension. This article reports such an inversion symmetry-breaking structural phase transition in a quasi-1D compound (TaSe$_4$)$_3$I at T$_S\sim$ 141~K studied by Raman spectroscopy. Our investigation of collective lattice dynamics reveals three additional Raman active modes in the low-temperature non-centrosymmetric structure. Two vibrational modes become Raman active due to the absence of an inversion center, while the third mode is a soft phonon mode resulting from the vibration of Ta atoms along the \{-Ta-Ta-\} chains. Furthermore, the most intense Raman mode display Fano-shaped asymmetry, inferred as the signature of strong electron-phonon coupling. The group theory and symmetry analysis of Raman spectra confirm the displacive-first-order nature of the structural transition. Therefore, our results establish (TaSe$_4)_3$I as a model system with broken inversion symmetry and strong electron-phonon coupling in the quasi-1D regime. △ Less

Submitted 21 July, 2023; originally announced July 2023.

Comments: Main text - 6 figures, 11 pages, supplementary - 10 figures, 13 pages

Nonequilibrium spin transport in integrable and non-integrable classical spin chains

Authors: Dipankar Roy, Abhishek Dhar, Herbert Spohn, Manas Kulkarni

Abstract: Anomalous transport in low dimensional spin chains is an intriguing topic that can offer key insights into the interplay of integrability and symmetry in many-body dynamics. Recent studies have shown that spin-spin correlations in spin chains, where integrability is either perfectly preserved or broken by symmetry-preserving interactions, fall in the Kardar-Parisi-Zhang (KPZ) universality class. S… ▽ More Anomalous transport in low dimensional spin chains is an intriguing topic that can offer key insights into the interplay of integrability and symmetry in many-body dynamics. Recent studies have shown that spin-spin correlations in spin chains, where integrability is either perfectly preserved or broken by symmetry-preserving interactions, fall in the Kardar-Parisi-Zhang (KPZ) universality class. Similarly, energy transport can show ballistic or diffusive-like behaviour. Although such behaviour has been studied under equilibrium conditions, no results on nonequilibrium spin transport in classical spin chains has been reported so far. In this work, we investigate both spin and energy transport in classical spin chains (integrable and non-integrable) when coupled to two reservoirs at two different temperatures/magnetization. In both the integrable case and broken-integrability (but spin-symmetry preserving), we report anomalous scaling of spin current with system size ($\mathbb{J}^s \propto L^{-μ}$) with an exponent, $μ\approx 2/3$, falling under the KPZ universality class. On the other hand, it is noteworthy that energy current remains ballistic ($\mathbb{J}^e \propto L^{-η}$ with $η\approx 0$) in the purely integrable case and there is departure from ballistic behaviour ($η> 0$) when integrability is broken regardless of spin-symmetry. Under nonequilibrium conditions, we have thoroughly investigated spatial profiles of local magnetization and energy. We find interesting nonlinear spatial profiles which are hallmarks of anomalous transport. We also unravel subtle striking differences between the equilibrium and nonequilibrium steady state through the lens of spatial spin-spin correlations. △ Less

Submitted 13 June, 2023; originally announced June 2023.

Comments: 13 pages, 10 figures (including supplementary material)

Quantum light-matter interactions in structured waveguides

Authors: Rupak Bag, Dibyendu Roy

Abstract: We explore special features of quantum light-matter interactions inside structured waveguides due to their finite bandwidth, band edges, and non-trivial topological properties. We model the waveguides as either a tight-binding (TB) chain or a Su-Schrieffer-Heeger (SSH) chain. For unstructured waveguides with infinite bandwidth, the transmission and reflection amplitude of a side-coupled two-level… ▽ More We explore special features of quantum light-matter interactions inside structured waveguides due to their finite bandwidth, band edges, and non-trivial topological properties. We model the waveguides as either a tight-binding (TB) chain or a Su-Schrieffer-Heeger (SSH) chain. For unstructured waveguides with infinite bandwidth, the transmission and reflection amplitude of a side-coupled two-level emitter (2LE) are the same as the reflection and transmission amplitude of a direct-coupled 2LE. We show that this analogy breaks down for structured waveguides with finite bandwidth due to the appearance of Lamb shift only for the direct-coupled 2LE. We further predict a robust light-emitter coupling at zero collective decay width of a single giant 2LE (with two couplings at different points) near the band edges of the structured waveguides where topological features can be beneficial. Finally, we study single-photon dynamics in a heterojunction of a long TB and short SSH waveguide connected to a 2LE at the SSH end. We show the propagation of a photon from the excited emitter to the TB waveguide only when the SSH waveguide is in the topological phase. Thus, the heterojunction acts as a quantum switch or conditional propagation channel. △ Less

Submitted 26 April, 2023; originally announced April 2023.

Comments: 15 pages, 11 figures

Journal ref: Phys. Rev. A 108, 053717 (2023)

Odd symmetry planar Hall effect: A method of detecting current-induced in-plane magnetization switching

Authors: Raghvendra Posti, Abhishek Kumar, Mayank Baghoria, Bhanu Prakash, Dhananjay Tiwari, Debangsu Roy

Abstract: Type-x device attracts considerable interest in the field of spintronics due to its robust spin-orbit torque (SOT) induced magnetization switching, and easy deposition technique. However, universally applicable and straightforward detection of type-X magnetization reversal is still elusive, unlike type-Z switching, which employs DC-based anomalous Hall effect measurement. Here, we, demonstrated th… ▽ More Type-x device attracts considerable interest in the field of spintronics due to its robust spin-orbit torque (SOT) induced magnetization switching, and easy deposition technique. However, universally applicable and straightforward detection of type-X magnetization reversal is still elusive, unlike type-Z switching, which employs DC-based anomalous Hall effect measurement. Here, we, demonstrated that the odd planar Hall signal (O-PHV) exhibits an odd symmetry with the application of an external magnetic field which motivates us to develop a reading mechanism for detecting magnetization switching of in-plane magnetized type-X devices. We verified our DC-based reading mechanism in the Pt/Co/NiFe/Pt stack where a thin Co layer is inserted to create dissimilar interfaces about the NiFe layer. Remarkably, the current-induced in-plane fields are found to be significantly large in Pt/Co/NiFe/Pt stack. Further, we successfully employed the O-PHV method to detect the current-induced magnetization switching. The pure DC nature of the writing and reading mechanism of our proposed type-X detection technique through O-PHV makes it the easiest in-plane magnetization detection technique. Moreover, the high repeatability and easy detection of our proposed method will open new avenues toward in-plane SOT switching based memory devices and sensors. △ Less

Submitted 25 March, 2023; originally announced March 2023.

Out-of-time-ordered correlator in the one-dimensional Kuramoto-Sivashinsky and Kardar-Parisi-Zhang equations

Authors: Dipankar Roy, David A. Huse, Manas Kulkarni

Abstract: The out-of-time-ordered correlator (OTOC) has emerged as an interesting object in both classical and quantum systems for probing the spatial spread and temporal growth of initially local perturbations in spatially extended chaotic systems. Here, we study the (classical) OTOC and its ``light-cone'' in the nonlinear Kuramoto-Sivashinsky (KS) equation, using extensive numerical simulations. We also s… ▽ More The out-of-time-ordered correlator (OTOC) has emerged as an interesting object in both classical and quantum systems for probing the spatial spread and temporal growth of initially local perturbations in spatially extended chaotic systems. Here, we study the (classical) OTOC and its ``light-cone'' in the nonlinear Kuramoto-Sivashinsky (KS) equation, using extensive numerical simulations. We also show that the linearized KS equation exhibits a qualitatively similar OTOC and light-cone, which can be understood via a saddle-point analysis of the linearly unstable modes. Given the deep connection between the KS (deterministic) and the Kardar-Parisi-Zhang (KPZ which is stochastic) equations, we also explore the OTOC in the KPZ equation. While our numerical results in the KS case are expected to hold in the continuum limit, for the KPZ case it is valid in a discretized version of the KPZ equation. More broadly, our work unravels the intrinsic interplay between noise/instability, nonlinearity and dissipation in partial differential equations (deterministic or stochastic) through the lens of OTOC. △ Less

Submitted 7 August, 2023; v1 submitted 14 March, 2023; originally announced March 2023.

Comments: 8 pages (including supplemental material), 5 figures, updated

Enhanced coercivity and emergence of spin cluster glass state in 2D ferromagnetic material Fe3GeTe2

Authors: Satyabrata Bera, Suman Kalyan Pradhan, Riju Pal, Buddhadeb Pal, Arnab Bera, Sk Kalimuddin, Manjil Das, Deep Singha Roy, Hasan Afzal, Atindra Nath Pal, Mintu Mondal

Abstract: Two-dimensional (2D) van der Waals (vdW) magnetic materials with high coercivity and high $T_\text{C}$ are desired for spintronics and memory storage applications. Fe$_3$GeTe$_2$ (F3GT) is one such 2D vdW ferromagnet with a reasonably high $T_\text{C}$, but with a very low coercive field, $H_\text{c}$ ($\lesssim$100~Oe). Some of the common techniques of enhancing $H_\text{c}$ are by introducing pi… ▽ More Two-dimensional (2D) van der Waals (vdW) magnetic materials with high coercivity and high $T_\text{C}$ are desired for spintronics and memory storage applications. Fe$_3$GeTe$_2$ (F3GT) is one such 2D vdW ferromagnet with a reasonably high $T_\text{C}$, but with a very low coercive field, $H_\text{c}$ ($\lesssim$100~Oe). Some of the common techniques of enhancing $H_\text{c}$ are by introducing pinning centers, defects, stress, doping, etc. They involve the risk of undesirable alteration of other important magnetic properties. Here we propose a very easy, robust, and highly effective method of phase engineering by altering the sample growth conditions to greatly enhance the intrinsic coercivity (7-10 times) of the sample, without compromising its fundamental magnetic properties ($T_\text{C}\simeq$210K). The phase-engineered sample (F3GT-2) comprises of parent F3GT phase with a small percentage of randomly embedded clusters of a coplanar FeTe (FT) phase. The FT phase serves as both mosaic pinning centers between grains of F3GT above its antiferromagnetic transition temperature ($T_\text{C1}\sim$70~K) and also as anti-phase domains below $T_\text{C1}$. As a result, the grain boundary disorder and metastable nature are greatly augmented, leading to highly enhanced coercivity, cluster spin glass, and meta-magnetic behavior. The enhanced coercivity ($\simeq$1~kOe) makes F3GT-2 much more useful for memory storage applications and is likely to elucidate a new route to tune useful magnetic properties. Moreover, this method is much more convenient than hetero-structure and other cumbersome techniques. △ Less

Submitted 29 December, 2022; originally announced December 2022.

Comments: 12 pages, 11 figures

Single photons versus coherent state input in waveguide quantum electrodynamics: light scattering, Kerr and cross-Kerr effect

Authors: Athul Vinu, Dibyendu Roy

Abstract: While the theoretical studies in waveguide quantum electrodynamics predominate with single-photon and two-photon Fock state (photon number states) input, the experiments are primarily carried out using a faint coherent light. We create a theoretical toolbox to compare and contrast linear and nonlinear light scattering by a two-level or a three-level emitter embedded in an open waveguide carrying F… ▽ More While the theoretical studies in waveguide quantum electrodynamics predominate with single-photon and two-photon Fock state (photon number states) input, the experiments are primarily carried out using a faint coherent light. We create a theoretical toolbox to compare and contrast linear and nonlinear light scattering by a two-level or a three-level emitter embedded in an open waveguide carrying Fock state or coherent state inputs. We identify rules to compare light transport properties, the Kerr, and cross-Kerr nonlinearities of the medium for the two types of inputs. A generalized description of the Kerr and cross-Kerr effect for different types of inputs is formulated to compare the Kerr and cross-Kerr nonlinearity between two photons in these models. △ Less

Submitted 7 September, 2022; originally announced September 2022.

Comments: 10 pages, 1 figure

Detection of field-free magnetization switching through thermoelectric effect in Ta/Pt/Co/Pt with significant spin-orbit torque and competing spin currents

Authors: Raghvendra Posti, Abhishek Kumar, Dhananjay Tiwari, Debangsu Roy

Abstract: Application of sufficient lateral current to a heavy metal (HM) can switch the perpendicular magnetization orientation of adjacent ferromagnetic layer (FM) through spin-orbit torques (SOTs). The choice of the HM and its arrangement plays a major role for the SOT induced magnetization switching in magnetic heterostructures. Here, in asymmetric Pt/Co/Pt heterostructures, anti-damping (AD) SOT prevai… ▽ More Application of sufficient lateral current to a heavy metal (HM) can switch the perpendicular magnetization orientation of adjacent ferromagnetic layer (FM) through spin-orbit torques (SOTs). The choice of the HM and its arrangement plays a major role for the SOT induced magnetization switching in magnetic heterostructures. Here, in asymmetric Pt/Co/Pt heterostructures, anti-damping (AD) SOT prevails. Ta addition to this stack (Ta/Pt/Co/Pt) give rise to several compelling effects viz. competing spin currents (due to opposite spin-Hall angles of adjacent Ta and Pt layers), significant AD-SOT, thermoelectric effects (particularly, anomalous Nernst effect (ANE)), and enhanced perpendicular magnetic anisotropy. For this Ta/Pt/Co/Pt stack, the AD-SOT values are stabilized to that of the Pt/Co/Pt stack, which is significant than what is expected for a stack with competing spin currents. Current-induced field-free magnetization switching was absent in uniformly grown Ta/Pt/Co/Pt stack. It was observed that a thickness gradient is essential to assist the field-free magnetization switching in these heterostructures. Further, the thermoelectric effects are utilized to develop a technique to detect the field-free magnetization switching. This technique detects the second harmonic ANE signal as a reading mechanism. Using ANE symmetry with the applied current, the switching can be detected in a single current sweep which was corroborated to the conventional DC Hall method. △ Less

Submitted 4 July, 2022; originally announced July 2022.

Comments: v1; 32pp, 13 figs

Nonlinear coherent light-matter interaction in 2D MoSe$_2$ nanoflakes for all-optical switching and logic applications

Authors: Sk Kalimuddin, Biswajit Das, Nabamita Chakraborty, Madhupriya Samanta, Satyabrata Bera, Arnab Bera, Deep Singha Roy, Suman Kalyan Pradhan, Kalyan K. Chattopadhyay, Mintu Mondal

Abstract: We report a strong nonlinear optical response of 2D MoSe$_2$ nanoflakes (NFs) through spatial self-phase modulation (SSPM) and cross-phase modulation (XPM) induced by nonlocal coherent light-matter interactions. The coherent interaction of light and MoSe$_2$ NFs creates the SSPM of laser beams, forming concentric diffraction rings. The nonlinear refractive index ($n_2$) and the third-order broadba… ▽ More We report a strong nonlinear optical response of 2D MoSe$_2$ nanoflakes (NFs) through spatial self-phase modulation (SSPM) and cross-phase modulation (XPM) induced by nonlocal coherent light-matter interactions. The coherent interaction of light and MoSe$_2$ NFs creates the SSPM of laser beams, forming concentric diffraction rings. The nonlinear refractive index ($n_2$) and the third-order broadband nonlinear optical susceptibility ($χ^{(3)}$) of MoSe$_2$ NFs are determined from the self diffraction pattern at different exciting wavelengths of 405, 532, and 671 nm with varying the laser intensity. The evolution and deformation of diffraction ring patterns are observed and analyzed by the `wind-chime' model and thermal effect. By taking advantage of the reverse saturated absorption of 2D SnS$_2$ NFs compared to MoSe$_2$, an all-optical diode has been designed with MoSe$_2$/SnS$_2$ hybrid structure to demonstrate the nonreciprocal light propagation. Also a few other optical devices based on MoSe$_2$ and other semiconducting materials such as Bi$_2$Se$_3$, CuPc, and graphene have been investigated. The all-optical logic gates and all-optical information conversion have been demonstrated through the XPM technique using two laser beams. The proposed optical scheme based on MoSe$_2$ NFs has been demonstrated as a potential candidate for all-optical nonlinear photonic devices such as all-optical diodes and all-optical switches. △ Less

Submitted 25 June, 2022; originally announced June 2022.

Comments: 20 pages, 6 figures, accepted in advanced optical materials

Robustness of Kardar-Parisi-Zhang scaling in a classical integrable spin chain with broken integrability

Authors: Dipankar Roy, Abhishek Dhar, Herbert Spohn, Manas Kulkarni

Abstract: Recent investigations have observed superdiffusion in integrable classical and quantum spin chains. An intriguing connection between these spin chains and Kardar-Parisi-Zhang (KPZ) universality class has emerged. Theoretical developments (e.g. generalized hydrodynamics) have highlighted the role of integrability as well as spin-symmetry in KPZ behaviour. However understanding their precise role on… ▽ More Recent investigations have observed superdiffusion in integrable classical and quantum spin chains. An intriguing connection between these spin chains and Kardar-Parisi-Zhang (KPZ) universality class has emerged. Theoretical developments (e.g. generalized hydrodynamics) have highlighted the role of integrability as well as spin-symmetry in KPZ behaviour. However understanding their precise role on superdiffusive transport still remains a challenging task. The widely used quantum spin chain platform comes with severe numerical limitations. To circumvent this barrier, we focus on a classical integrable spin chain which was shown to have deep analogy with the quantum spin-$\frac{1}{2}$ Heisenberg chain. Remarkably, we find that KPZ behaviour prevails even when one considers integrability-breaking but spin-symmetry preserving terms, strongly indicating that spin-symmetry plays a central role even in the non-perturbative regime. On the other hand, in the non-perturbative regime, we find that energy correlations exhibit clear diffusive behaviour. We also study the classical analog of out-of-time-ordered correlator (OTOC) and Lyapunov exponents. We find significant presence of chaos for the integrability-broken cases even though KPZ behaviour remains robust. The robustness of KPZ behaviour is demonstrated for a wide class of spin-symmetry preserving integrability-breaking terms. △ Less

Submitted 8 May, 2022; originally announced May 2022.

Comments: 10 pages, 9 figures (including supplementary material)

Geometric Thermodynamics of Strain-Induced Crystallization in Polymers

Authors: Sanhita Das, Asif Raza, Debasish Roy

Abstract: Going beyond the classical Gaussian approximation of Einstein's fluctuation theory, Ruppeiner gave it a Riemannian geometric structure with an entropic metric. This yielded a fundamental quantity - the Riemannian curvature, which was used to extract information on the nature of interactions between molecules in fluids, ideal gases and other open systems. In this article, we examine the implication… ▽ More Going beyond the classical Gaussian approximation of Einstein's fluctuation theory, Ruppeiner gave it a Riemannian geometric structure with an entropic metric. This yielded a fundamental quantity - the Riemannian curvature, which was used to extract information on the nature of interactions between molecules in fluids, ideal gases and other open systems. In this article, we examine the implications of this curvature in a non-equilibrium thermodynamic system where relaxation is sufficiently slow so as not to invalidate the local equilibrium hypothesis. The non-equilibrium system comprises of a rubbery polymer undergoing strain induced crystallization. The curvature is found to impart information on a spurious isochoric energy arising from the conformational stretching of already crystallized segments. This unphysical component perhaps arises as the crystallized manifold is considered Euclidean with the stretch measures defined via the Euclidean metric. The thermodynamic state associated with curvature is the key to determine the isochoric stretch and hence the spurious energy. We determine this stretch and propose a form for the spurious free energy that must be removed from the total energy in order that the correct stresses are recovered. △ Less

Submitted 3 April, 2022; originally announced April 2022.

Comments: 19 pages, 17 figures