arXiv subjects

Sugata Chowdhury

Publications and source records attributed to Sugata Chowdhury.

At least 19 recordsLinked to original sources

arXiv2026

The Origin of the Observed Raman Peaks in α-MnTe

The Raman spectrum of the room-temperature altermagnet $α$-MnTe is reported to contain unassigned peaks at 120(3) cm$^{-1}$ and 140(3) cm$^{-1}$, absent from the predicted phonon spectrum of the material. This has generated considerable debate within the altermagnet community and necessitates urgent resolution. This work establishes that these peaks, together with the 90(5) cm$^{-1}$ peak that matches a theoretically predicted mode, are all extrinsic, originating from elemental tellurium formed during air exposure of the surface. Raman spectra of MBE-grown thin films show that these peaks closely match those of elemental tellurium, emerge soon after air exposure, and are absent in AlO$_x$-capped films. X-ray photoelectron spectroscopy shows that air exposure breaks Mn--Te bonds and oxidizes Mn within a minute. Cross-sectional scanning transmission electron microscopy with energy-dispersive X-ray spectroscopy reveals that the oxidation leads to Mn out-diffusion, forming a few-nanometer-thick oxide layer above a buried, Te-enriched region, likely responsible for the anomalous Raman peaks. Additionally, no sample exhibited the 175 cm$^{-1}$ Raman peak which is commonly attributed to MnTe$_2$. The aggressive surface oxidation and associated elemental Te formation in MnTe have important implications for any surface-sensitive and optical characterization of MnTe (and other similar Te-containing materials) involving even the briefest air exposure.

cond-mat.mtrl-sci↗

arXiv2026

Giant Kohn anomaly and chiral phonons in the charge density wave phase of 1H-NbSe$_2$

Despite extensive investigations, many aspects of charge density waves (CDWs) remain elusive, especially the relative roles of electron-phonon coupling and Fermi surface nesting as the underlying driving mechanisms responsible for the emergence of the CDW vector $Q_{CDW}$. It is puzzling that even though electrons interact strongly with optical phonons in many correlated systems, the actual mode softening is of an acoustic mode. Here we consider monolayer 1H-NbSe$_2$ as an exemplar system, and through an accurate computation of the phonon self-energy, including its off-diagonal components, we provide compelling evidence that the relevant mode is a longitudinal optical phonon that softens by anti-crossing several intervening phonon bands, i.e. a Kohn ladder which has been only observed previously in high temperature superconductors. We also show that $Q_{CDW}$ is fixed by the convolution of the susceptibility and electron-phonon coupling, and that the softened phonons are circularly polarized.

cond-mat.str-el↗

arXiv2026

Engineering Nonlinear Optical Responses via Inversion Symmetry Breaking in Bilayer Bi2Se3

Paucity of naturally occurring noncentrosymmetric materials is stimulating growing interest in engineered two-dimensional systems for nonlinear optical applications. Here, we show that breaking inversion symmetry in centrosymmetric bilayer Bi$_2$Se$_3$ through twisting, point-defect insertion, or the application of an external electric field unlocks rich nonlinear optical responses. In twisted bilayer Bi$_2$Se$_3$ at the first commensurate angle of 21.78$^\circ$, we find peak shift and injection current conductivities of -14 $ nm.μAV^{-2}$ and 104 $\times 10^8$ $nm.A V^{-2}s^{-1}$, respectively, which lie in the visible spectrum and enable efficient THz applications. The external electric field and point-defect insertion both transform the bilayer into C$_ {3v}$ symmetry, with the selenium vacancy (V$_{Se}$) achieving peak shift and injection current conductivities of -190 nm.$μAV^{-2}$ and -170 $\times 10^8$ $nm.A V^{-2}s^{-1}$. In all three cases, the peak nonlinear optical responses are found to be comparable to those of benchmark 2D materials such as GeS, and the broadband responses, including helicity-dependent current generation, make these engineered bilayers viable candidates for next-generation 2D photovoltaics.

cond-mat.mtrl-sci↗

arXiv2026

Layer-dependent antiferromagnetic Chern and axion insulating states in UOTe

Magnetic topological insulators have received significant interest due to their dissipationless edge states, which promise advances in energy-efficient electronic transport. However, the magnetic topological insulator state has typically been found in ferromagnets (FMs) that suffer from low magnetic ordering temperatures and stray fields. Identifying an antiferromagnetic topological insulator that exhibits the quantum anomalous Hall effect (QAHE) with a relatively high Néel temperature has been a longstanding challenge. Here, we focus on the recently discovered van der Waals (vdW) antiferromagnet (AFM) UOTe, which not only features a high Néel temperature ($\sim$150K) but also exhibits intriguing Kondo interaction and topological characteristics. Our systematic analysis of the layer-dependent topological phases based on \textit{ab} initio computations predicts the two-layer UOTe film to be an ideal 2D AFM Chern insulator in which the Hall conductivity is quantized with a fully compensated spin magnetization. By applying an in-plane strain or electric field, we show how the itinerancy of U-5f electrons can be manipulated to trigger a transition between the nontrivial ($C = 1$) and trivial ($C = 0$) phases. Interestingly, the 3-layer UOTe film is found to have zero charge conductance but it hosts a quantized spin Hall conductivity (SHC) with finite magneto-electric coupling, suggesting the presence of an axion insulator-like state. The unique magnetic structure of UOTe supports a layer-tunable topology in which films with an odd number of layers are axion-like insulators, while films with an even number of layers are Chern insulators, and the bulk material is a Dirac semimetal. Our study offers a new intrinsic AFM materials platform for realizing correlated topological phases for next-generation spintronics applications and fundamental science studies.

cond-mat.mtrl-sci↗

arXiv2025

Physics-Guided Dual Implicit Neural Representations for Source Separation

Significant challenges exist in efficient data analysis of most advanced experimental and observational techniques because the collected signals often include unwanted contributions--such as background and signal distortions--that can obscure the physically relevant information of interest. To address this, we have developed a self-supervised machine-learning approach for source separation using a dual implicit neural representation framework that jointly trains two neural networks: one for approximating distortions of the physical signal of interest and the other for learning the effective background contribution. Our method learns directly from the raw data by minimizing a reconstruction-based loss function without requiring labeled data or pre-defined dictionaries. We demonstrate the effectiveness of our framework by considering a challenging case study involving large-scale simulated as well as experimental momentum-energy-dependent inelastic neutron scattering data in a four-dimensional parameter space, characterized by heterogeneous background contributions and unknown distortions to the target signal. The method is found to successfully separate physically meaningful signals from a complex or structured background even when the signal characteristics vary across all four dimensions of the parameter space. An analytical approach that informs the choice of the regularization parameter is presented. Our method offers a versatile framework for addressing source separation problems across diverse domains, ranging from superimposed signals in astronomical measurements to structural features in biomedical image reconstructions.

cs.CV↗

arXiv2025

Magnetism-Enhanced Strong Electron-Phonon Coupling in Infinite-Layer Nickelate

Intriguing analogies between the nickelates and the cuprates provide a promising avenue for unraveling the microscopic mechanisms underlying high-$T_c$ superconductivity. While electron correlation effects in the nickelates have been extensively studied, the role of electron-phonon coupling (EPC) remains highly controversial. Here, by taking pristine LaNiO$_2$ as an exemplar nickelate, we present an in-depth study of EPC for both the non-magnetic (NM) and the $C$-type antiferromagnetic ($C$-AFM) phase using advanced density functional theory methods without invoking $U$ or other free parameters. The weak EPC strength $λ$ in the NM phase is found to be greatly enhanced ($\sim$4$\times$) due to the presence of magnetism in the $C$-AFM phase. This enhancement arises from strong interactions between the flat bands associated with the Ni-3$d_{z^2}$ orbitals and the low-frequency phonon modes driven by the vibrations of Ni and La atoms. The resulting phonon softening is shown to yield a distinctive kink in the electronic structure around 15 meV, which would provide an experimentally testable signature of our predictions. Our study highlights the critical role of local magnetic moments and interply EPC in the nickelate.

cond-mat.str-el↗

arXiv2025

Tunable superconductivity coexisting with the anomalous Hall effect in 1T'-WS2

Transition metal dichalcogenides are a family of quasi-two-dimensional materials that display a high technological potential due to their wide range of electronic ground states, e.g., from superconducting to semiconducting, depending on the chemical composition, crystal structure, or electrostatic doping. Here, we unveil that by tuning a single parameter, the hydrostatic pressure P, a cascade of electronic phase transitions can be induced in the few-layer transition metal dichalcogenide 1T'-WS2, including superconducting, topological, and anomalous Hall effect phases. Specifically, as P increases, we observe a dual phase transition: the suppression of superconductivity with the concomitant emergence of an anomalous Hall effect at P=1.15 GPa. Remarkably, upon further increasing the pressure above 1.6 GPa, we uncover a reentrant superconducting state that emerges out of a state still exhibiting an anomalous Hall effect. This superconducting state shows a marked increase in superconducting anisotropy with respect to the phase observed at ambient pressure, suggesting a different superconducting state with a distinct pairing symmetry. Via first-principles calculations, we demonstrate that the system concomitantly transitions into a strong topological phase with markedly different band orbital characters and Fermi surfaces contributing to the superconductivity. These findings position 1T'-WS2 as a unique, tunable superconductor, wherein superconductivity, anomalous transport, and band features can be tuned through the application of moderate pressures.

cond-mat.supr-con↗

arXiv2025

Anisotropic Raman scattering and lattice orientation identification of 2M-WS2

Anisotropic materials with low symmetries hold significant promise for next-generation electronic and quantum devices. 2M-WS2, a candidate for topological superconductivity, has garnered considerable interest. However, a comprehensive understanding of how its anisotropic features contribute to unconventional superconductivity, along with a simple, reliable method to identify its crystal orientation, remains elusive. Here, we combine theoretical and experimental approaches to investigate angle- and polarization-dependent anisotropic Raman modes of 2M-WS2. Through first-principles calculations, we predict and analyze phonon dispersion and lattice vibrations of all Raman modes in 2M-WS2. We establish a direct correlation between their anisotropic Raman spectra and high-resolution transmission electron microscopy images. Finally, we demonstrate that anisotropic Raman spectroscopy can accurately determine the crystal orientation and twist angle between two stacked 2M-WS2 layers. Our findings provide insights into the electron-phonon coupling and anisotropic properties of 2M-WS2, paving the way for the use of anisotropic materials in advanced electronic and quantum devices.

cond-mat.mes-hall↗

arXiv2024

UOTe: Kondo-interacting topological antiferromagnet in a van der Waals lattice

Since the initial discovery of two-dimensional van der Waals (vdW) materials, significant effort has been made to incorporate the three properties of magnetism, band structure topology, and strong electron correlations $-$ to leverage emergent quantum phenomena and expand their potential applications. However, the discovery of a single vdW material that intrinsically hosts all three ingredients has remained an outstanding challenge. Here we report the discovery of a Kondo-interacting topological antiferromagnet in the vdW 5$f$ electron system UOTe. It has a high antiferromagnetic (AFM) transition temperature of 150 K, with a unique AFM configuration that breaks the combined parity and time reversal ($PT$) symmetry in an even number of layers while maintaining zero net magnetic moment. Our angle-resolved photoemission spectroscopy (ARPES) measurements reveal Dirac bands near the Fermi level, which combined with our theoretical calculations demonstrate UOTe as an AFM Dirac semimetal. Within the AFM order, we observed the presence of the Kondo interaction, as evidenced by the emergence of a 5$f$ flat band near the Fermi level below 100 K and hybridization between the Kondo band and the Dirac band. Our density functional theory calculations in its bilayer form predict UOTe as a rare example of a fully-compensated AFM Chern insulator.

cond-mat.str-el↗

arXiv2024

3D Heisenberg universality in the Van der Waals antiferromagnet NiPS$_3$

Van der Waals (vdW) magnetic materials are comprised of layers of atomically thin sheets, making them ideal platforms for studying magnetism at the two-dimensional (2D) limit. These materials are at the center of a host of novel types of experiments, however, there are notably few pathways to directly probe their magnetic structure. We report the magnetic order within a single crystal of NiPS$_3$ and show it can be accessed with resonant elastic X-ray diffraction along the edge of the vdW planes in a carefully grown crystal by detecting structurally forbidden resonant magnetic X-ray scattering. We find the magnetic order parameter has a critical exponent of $β\sim0.36$, indicating that the magnetism of these vdW crystals is more adequately characterized by the three-dimensional (3D) Heisenberg universality class. We verify these findings with first-principle density functional theory, Monte-Carlo simulations, and density matrix renormalization group calculations.

cond-mat.str-el↗

arXiv2024

Topological Hall effect induced by chiral fluctuations in ErMn6Sn6

Topological Hall effect (THE) is a hallmark of scalar spin chirality, which is found in static skyrmion lattices. Recent theoretical works have shown that scalar spin chirality could also emerge dynamically from thermal spin fluctuations. Evidence of such a mechanism was found in the kagome magnet YMn6Sn6 where fluctuations arise from frustrated exchange interactions between Mn kagome layers. In YMn6Sn6, the rare-earth ion Y3+ is non-magnetic. When it is replaced by a magnetic ion (Gd3+-Ho3+), the intrinsically antiferromagnetic Mn-Mn interlayer coupling is overwhelmed by the indirect ferromagnetic Mn-R-Mn one, relieving frustration. This generates interesting anomalous Hall conductivity, but not THE. Here we show that Er lies in an intermediate regime where direct and indirect interactions closely compete, so that ErMn6Sn6 can switch from one regime to the other by temperature, i.e., from a collinear ferrimagnetic ground state to a spiral antiferromagnet at 78 K. The AFM phase forms a dome in the temperature-field phase diagram. Close to the boundary of this dome, we find a sizable fluctuations-driven THE, thus underscoring the universality of this chiral fluctuation mechanism for generating non-zero scalar spin chirality.

cond-mat.mes-hall↗

arXiv2024

Kitaev physics in the two-dimensional magnet NiPSe$_3$

The Kitaev interaction, found in candidate materials such as $α$-RuCl$_3$, occurs through the metal ($M$)-ligand ($X$)-metal ($M$) paths of the edge-sharing octahedra because the large spin-orbit coupling (SOC) on the metal atoms activates directional spin interactions. Here, we show that even in $3d$ transition-metal compounds, where the SOC of the metal atom is negligible, heavy ligands can induce bond-dependent Kitaev interactions. In this work, we take as an example the $3d$ transition-metal chalcogenophosphate NiPSe$_3$ and show that the key is found in the presence of a sizable SOC on the Se $p$ orbital, one which mediates the super-exchange between the nearest-neighbor Ni sites. Our study provides a pathway for engineering enhanced Kitaev interactions through the interplay of SOC strength, lattice distortions, and chemical substitutions.

cond-mat.str-el↗

arXiv2024

Skyrmions: A review on materials perspective for future electronic devices

Recent years have witnessed an enormous rise in research interest in magnetic skyrmions owing to their capability to improve over contemporary spintronic devices. An overview of the various magnetic interactions responsible for the formation of skyrmion together with distinct noncentrosymmetric and centrosymmetric skyrmion candidates is given in this review article. The magnetic interactions known as Dzyaloshinskii-Moriya interactions (DMI) have been extensively studied over the years to better understand the mechanism of skyrmions in chiral magnets that have larger skyrmion sizes. Because of their low skyrmion size, the centrosymmetric frustrated magnets are dwelling to skyrmions controlled by long-range interactions such as the Ruderman-Kittel-Kasuya-Yosida interaction (RKKY), which may be useful in the development of high-density memory devices. To lay a solid foundation for the magnetic interactions involved in skyrmion formations and many other special physical properties, more research in the field of centrosymmetric skyrmions is required. Apart from studying candidates with low skyrmion sizes, one of the main goals for the future is to better understand the dynamics of skyrmion using polarized magnons, which has the potential to be extremely beneficial for spintronic applications.

cond-mat.mtrl-sci↗

arXiv2023

Unconventional superconducting pairing in a B20 Kramers Weyl semimetal

Topological superconductors present an ideal platform for exploring nontrivial superconductivity and realizing Majorana boundary modes in materials. However, finding a single-phase topological material with nontrivial superconducting states is a challenge. Here, we predict nontrivial superconductivity in the pristine chiral metal RhGe with a transition temperature of 5.8 K. Chiral symmetries in RhGe enforce multifold Weyl fermions at high-symmetry momentum points and spin-polarized Fermi arc states that span the whole surface Brillouin zone. These bulk and surface chiral states support multiple type-II van Hove singularities that enhance superconductivity in RhGe. Our detailed analysis of superconducting pairing symmetries involving Chiral Fermi pockets in RhGe, indicates the presence of nontrivial superconducting pairing. Our study establishes RhGe as a promising candidate material for hosting mixed-parity pairing and topological superconductivity.

cond-mat.supr-con↗

arXiv2023

Machine learning enabled experimental design and parameter estimation for ultrafast spin dynamics

Advanced experimental measurements are crucial for driving theoretical developments and unveiling novel phenomena in condensed matter and material physics, which often suffer from the scarcity of facility resources and increasing complexities. To address the limitations, we introduce a methodology that combines machine learning with Bayesian optimal experimental design (BOED), exemplified with x-ray photon fluctuation spectroscopy (XPFS) measurements for spin fluctuations. Our method employs a neural network model for large-scale spin dynamics simulations for precise distribution and utility calculations in BOED. The capability of automatic differentiation from the neural network model is further leveraged for more robust and accurate parameter estimation. Our numerical benchmarks demonstrate the superior performance of our method in guiding XPFS experiments, predicting model parameters, and yielding more informative measurements within limited experimental time. Although focusing on XPFS and spin fluctuations, our method can be adapted to other experiments, facilitating more efficient data collection and accelerating scientific discoveries.

cond-mat.mtrl-sci↗

arXiv2023

On Ultrafast X-ray Methods for Magnetism

With the introduction of x-ray free electron laser sources around the world, new scientific approaches for visualizing matter at fundamental length and time-scales have become possible. As it relates to magnetism and "magnetic-type" systems, advanced methods are being developed for studying ultrafast magnetic responses on the time-scales at which they occur. We describe three capabilities which have the potential to seed new directions in this area and present original results from each: pump-probe x-ray scattering with low energy excitation, x-ray photon fluctuation spectroscopy, and ultrafast diffuse x-ray scattering. By combining these experimental techniques with advanced modeling together with machine learning, we describe how the combination of these domains allows for a new understanding in the field of magnetism. Finally, we give an outlook for future areas of investigation and the newly developed instruments which will take us there.

cond-mat.mtrl-sci↗

arXiv2023

Capturing dynamical correlations using implicit neural representations

The observation and description of collective excitations in solids is a fundamental issue when seeking to understand the physics of a many-body system. Analysis of these excitations is usually carried out by measuring the dynamical structure factor, S(Q, $ω$), with inelastic neutron or x-ray scattering techniques and comparing this against a calculated dynamical model. Here, we develop an artificial intelligence framework which combines a neural network trained to mimic simulated data from a model Hamiltonian with automatic differentiation to recover unknown parameters from experimental data. We benchmark this approach on a Linear Spin Wave Theory (LSWT) simulator and advanced inelastic neutron scattering data from the square-lattice spin-1 antiferromagnet La$_2$NiO$_4$. We find that the model predicts the unknown parameters with excellent agreement relative to analytical fitting. In doing so, we illustrate the ability to build and train a differentiable model only once, which then can be applied in real-time to multi-dimensional scattering data, without the need for human-guided peak finding and fitting algorithms. This prototypical approach promises a new technology for this field to automatically detect and refine more advanced models for ordered quantum systems.

cond-mat.str-el↗

arXiv2022

Testing the data framework for an AI algorithm in preparation for high data rate X-ray facilities

The advent of next-generation X-ray free electron lasers will be capable of delivering X-rays at a repetition rate approaching 1 MHz continuously. This will require the development of data systems to handle experiments at these type of facilities, especially for high throughput applications, such as femtosecond X-ray crystallography and X-ray photon fluctuation spectroscopy. Here, we demonstrate a framework which captures single shot X-ray data at the LCLS and implements a machine-learning algorithm to automatically extract the contrast parameter from the collected data. We measure the time required to return the results and assess the feasibility of using this framework at high data volume. We use this experiment to determine the feasibility of solutions for `live' data analysis at the MHz repetition rate.

physics.data-an↗