arXiv subjects

Soumik Mukhopadhyay

Publications and source records attributed to Soumik Mukhopadhyay.

At least 19 recordsLinked to original sources

arXiv2026

In-plane anomalous and third-harmonic Hall response in an easy-plane trigonal magnet

We report the emergence of an in-plane anomalous Hall effect (IPAHE) and a pronounced third-harmonic Hall response in an easy-plane trigonal magnetic system achieved via tuning of magnetic anisotropy. Angle-dependent Hall measurements reveal a clear departure from conventional sinusoidal behavior, revealing a strong sin(3θ) component at elevated fields, which increases as the magnetic anisotropy is reduced, accompanied by a finite in-plane Hall signal. Consistently, the observed nonlinear Hall signal exhibits a cubic field dependence, indicative of a leading third-order contribution to the transverse conductivity. The enhancement of the third harmonic reflects the evolution toward coherent in-plane magnetization and symmetry-governed nonlinear transport. These observations support an intrinsic, symmetry-controlled transport mechanism enabled by reduced magnetic anisotropy and establish a direct link between anisotropy, crystal symmetry, and higher-order Hall response.

cond-mat.mes-hall↗

arXiv2026

Junction-free polarity-tunable superconducting diode effect enables XNOR logic operation

The realization of the superconducting diode effect (SDE), characterized by non-reciprocal supercurrent, is a major step towards dissipationless logic circuits. A polarity-tunable superconducting diode with high diode efficiency primarily consists of Josephson-junction architectures that couple two superconducting regions via a weak link. Realizing an SDE without a weak-link junction would reduce fabrication complexity, lower electrical noise, and minimize dependence on junction quality. Here, we report an all-vdW, highly efficient, junction-free SDE in NbSe2, enabled by proximity-induced symmetry breaking from a strong uniaxial ferromagnet Fe3GeTe2 (F3GT). Superconducting diode efficiency of up to 30% is achieved at a small magnetic field of 50 mT. A field-free diode rectification effect is realized with diode polarity that is tunable by the F3GT spin polarization. Furthermore, by employing the active-field dependence of the SDE, a two-input exclusive-NOR (XNOR) logic gate operation is demonstrated. The findings highlight the potential utilization of magnetic proximity effect towards realizing junction-free superconducting diodes for high-performance, energy-efficient logic circuits.

cond-mat.supr-con↗

arXiv2026

Selective Timestep Weighting and Advantage-Based Replay for Sample-Efficient Diffusion RLHF

Reinforcement learning from human feedback (RLHF) has emerged as a powerful paradigm for aligning generative models with human preferences. However, applying RLHF to diffusion models remains highly feedback inefficient, as existing approaches typically require large amounts of human or reward model evaluations. This limitation reduces the practicality of diffusion RLHF in realworld settings where feedback is the primary bottleneck. In this paper, we propose two complementary strategies that substantially improve the feedback efficiency of diffusion RLHF while preserving generalization to unseen prompts. Our key observation is that reward information in diffusion trajectories is unevenly distributed: not all denoising timesteps or trajectories contribute equally to learning from a reward signal. By emphasizing informative timesteps and trajectories during optimization, we obtain more effective gradient updates. First, we introduce a per-timestep weighting scheme that reweights denoising steps during policy optimization. We theoretically connect this weighting to the optimal convergence properties of proximal policy optimization (PPO) and approximate the resulting weighting trend empirically. Second, we introduce a replay mechanism that prioritizes informative trajectories, enabling the model to reuse past samples instead of repeatedly querying new rewards. Together, these strategies significantly improve the feedback efficiency of diffusion RLHF. Under identical hyperparameter settings, our approach achieves up to a 6$\times$ improvement in sample efficiency compared to widely used diffusion RLHF baselines.

cs.LG↗

arXiv2026

Angle dependent hysteretic magnetotransport in MnBi2Te4 nanoflakes

Controlling magnetic phases in two-dimensional systems, where charge transport is highly sensitive to real-space spin inhomogeneities, is central to understanding emergent magnetic states in reduced dimensions. In this context, thickness-dependent magnetotransport provides access to irreversible magnetic processes that are not captured by reversible transport or bulk magnetization alone. Here we report an extensive study of hysteretic magnetoresistance in single-crystalline nanoscale thin flakes of the layered antiferromagnet MnBi2Te4. The multi-step hysteresis exhibits a pronounced non-monotonic dependence on thickness and displays nontrivial angular anisotropy. The transport signatures rule out surface-dominated magnetism and simple bulk metamagnetic transitions as the primary origin. We argue that the magnetic irreversibility is possibly governed by domain wall pinning and de-pinning processes within a spatially non-uniform magnetic landscape. These results suggest that reduced dimensionality is a key driver of magnetic irreversibility in MnBi2Te4.

cond-mat.mtrl-sci↗

arXiv2026

Scale Space Diffusion

Diffusion models degrade images through noise, and reversing this process reveals an information hierarchy across timesteps. Scale-space theory exhibits a similar hierarchy via low-pass filtering. We formalize this connection and show that highly noisy diffusion states contain no more information than small, downsampled images - raising the question of why they must be processed at full resolution. To address this, we fuse scale spaces into the diffusion process by formulating a family of diffusion models with generalized linear degradations and practical implementations. Using downsampling as the degradation yields our proposed Scale Space Diffusion. To support Scale Space Diffusion, we introduce Flexi-UNet, a UNet variant that performs resolution-preserving and resolution-increasing denoising using only the necessary parts of the network. We evaluate our framework on CelebA and ImageNet and analyze its scaling behavior across resolutions and network depths. Our project website ( https://prateksha.github.io/projects/scale-space-diffusion/ ) is available publicly.

cs.CV↗

arXiv2026

Chiral orbital current driven topological Hall effect in Mn3Si2Te6

Chiral orbital current (COC) plays a crucial role in governing the magnetization and transport behaviour in the layered ferrimagnetic nodal-line semiconductor Mn3Si2Te6. Here, we observe that the topological Hall effect (THE), typically attributed to Berry curvature from chiral spin textures, originates from COC, which produces an emergent magnetic field for conduction electrons due to its real-space orbital textures. We find that the THE signal strengthens as we move down from bulk to nanoflakes, but tends to disappear with increasing current, along with the disappearance of the COC state. We also demonstrate a strong correlation between the colossal magnetoresistance (CMR) and the observed THE, suggesting that large Berry curvature and topological transport can arise purely from orbital degrees of freedom, providing a new platform for engineering dissipationless transport in 2D magnets.

cond-mat.other↗

arXiv2026

Probing Interfacial Spin Dynamics and Temperature Dependent Asymmetry in Spin Pumping Across Ni80Fe20/Cu/Cr1.12Te2 Interfaces

Spin transfers in magnetic multilayers offers a promising pathway toward ultrafast, energy-efficient spintronic devices. In this study, we investigate the interfacial spin pumping and temperature-dependent spin current exchange in a Cr1.12Te2/Cu/Ni80Fe20 (Py)(FM1/NM/FM2) trilayer structure. Using broadband and cryogenic ferromagnetic resonance (FMR) measurements, we investigate key magnetization dynamical parameters, including the effective Gilbert damping factor, effective magnetic fields, interfacial spin mixing conductance, and spin current density. Efficient spin angular momentum transfers from Py to Cr1.12Te2 are observed at room temperature. At lower temperatures, the enhanced linewidth reflects temperature dependent spin pumping effects occurring at distinct precession frequencies of the ferromagnetic layers. Notably, the absence of interfacial Damping indicates that spin pumping can be modulated by controlling the net spin current flow. These findings offer critical insight into temperature-dependent tunable spin transport mechanisms in magnetic multilayers, highlighting their potential for next-generation spintronic applications.

cond-mat.mes-hall↗

arXiv2026

Room temperature intrinsic anomalous Hall effect in disordered half-metallic ferromagnetic quaternary Heusler alloy CoRuFeSi

Quaternary Heusler alloys offer a versatile platform for engineering magnetic and topological transport phenomena through chemical flexibility and tunable disorder. Here, we report a comprehensive experimental and theoretical investigation of the magnetic, magnetotransport, and anomalous Hall properties of the quaternary Heusler alloy CoRuFeSi. The compound crystallizes in the LiMgPdSn-type structure with significant Co--Ru antisite disorder and exhibits soft ferromagnetism with a saturation magnetization of $4.21~μ_{\mathrm{B}}/\mathrm{f.u.}$ at low temperature and a Curie temperature well above room temperature. Hall measurements reveal a robust anomalous Hall effect persisting up to 300~K, with an anomalous Hall conductivity of $\sim 74$~S/cm that is nearly temperature independent. Scaling analysis demonstrates that the anomalous Hall response is dominated by the intrinsic Berry-curvature mechanism. First-principles calculations identify CoRuFeSi as a topologically nontrivial nodal-line semimetal in its ordered phase. Incorporation of experimentally relevant Co--Ru antisite disorder redistributes the Berry curvature and quantitatively reproduces the experimentally observed anomalous Hall conductivity, while preserving half-metallicity. These results establish CoRuFeSi as a disorder-tolerant half-metallic ferromagnet with a sizable intrinsic anomalous Hall effect at room temperature, highlighting its potential for spintronic and Hall-based device applications.

cond-mat.mtrl-sci↗

arXiv2025

Disorder-driven Weyl-Kondo Semimetal Phase in WTe$_2$

In this Letter, we report the observation of disorder-driven anisotropic Kondo screening and spontaneous Hall effect in bulk WTe${_2}$, a nonmagnetic type-II Weyl semimetal. We show that Kondo scattering emerges more prominently in disordered samples and produces magnetoresistance that is strongly anisotropic with respect to both current and magnetic field orientation, reflecting the underlying type-II Weyl dispersion. Strikingly, we find a spontaneous Hall effect in zero magnetic field, whose magnitude is enhanced with disorder, together with a large second-harmonic Hall signal exhibiting quadratic current scaling. Our analysis indicates that disorder-driven Kondo interactions pin the Fermi level near the Weyl nodes. This enhances the Berry curvature-driven nonequilibrium transport, accounting for both the second-order and spontaneous Hall responses. These findings establish disordered WTe${_2}$ as a platform hosting Weyl-Kondo fermions and highlight disorder as an effective control knob for inducing correlated topological phases in weakly correlated Weyl semimetals.

cond-mat.mes-hall↗

arXiv2025

Common topological origin of longitudinal and transverse magnetoresistance in Fe3GeTe2

In this work, we reveal the coexistence and correlation of a topological cusp anomaly in the planar Hall signal and a spin-flip scattering dominated positive longitudinal magnetoresistance (MR) across the entire temperature range below the Curie point in Fe3GeTe2. This correlation dies out exponentially as the magnetic field is directed away from the ab-plane, resulting in unusually sharp polar angle dependence of longitudinal MR and the Hall response.

cond-mat.mtrl-sci↗

arXiv2025

Enhanced Spin Pumping and Magnetization dynamics in Ni$_{80}$Fe$_{20}$/MoS$_2$ stack via interface modification

Materials with strong spin orbit coupling (SOC) are essential for realizing spin orbit torque (SOT) based magnetic memory devices. Transition metal dichalcogenides (TMDs) are promising candidates for such appli cations because of their inherently high SOC strength. In this study, we investigate the spin pumping effect at the interface between a monolayer of molybdenum disulfide (ML-MoS$_2$) and Ni$_{80}$Fe$_{20}$ (Py) thin films using broadband ferromagnetic resonance (FMR) spectroscopy. FMR measurements reveal a notable enhancement in the effective Gilbert damping factor for the ML-MoS$_2$/Py (Pt = 0 nm) interface compared to the reference Py thin films, attributed to spin pumping across the ML-MoS$_2$/Py interface. To further quantify spin pumping efficiency, we introduce a high SOC platinum (Pt) interlayer at the ML-MoS$_2$/Py interface and systematically vary its thickness. This allows us to evaluate key spin transport parameters, including the enhancement in the effective Gilbert damping parameter, the effective spin mixing conductance that reflects the transfer of spin angular momentum from Py to ML-MoS$_2$ and the effective spin current density.

physics.app-ph↗

arXiv2025

Metamagnetic quantum criticality in the antiferromagnetic topological insulator MnBi$_2$Te$_4$

Combining transport and magnetization measurements, we discover the emergence of a tricritical point connecting the antiferromagnetic, metamagnetic and paramagnetic regions in MnBi$_2$Te$_4$, a magnetic topological insulator candidate which can potentially exhibit axion electrodynamics. We observe a unique magnetic field-driven phase diagram where the second-order antiferromagnetic to paramagnetic phase boundary bifurcates into two first-order lines at the tricritical point. The two first-order lines enclosing the metamagnetic phase eventually terminate at the quantum critical endpoints with the application of magnetic field.

cond-mat.mtrl-sci↗

arXiv2025

Tuning the chiral orbital currents in a colossal magnetoresistive nodal line ferrimagnet

The ferrimagnetic nodal-line semiconductor Mn$_3$Si$_2$Te$_6$ exhibits colossal magnetoresistance (CMR) owing to the chiral orbital currents (COC). The COC is developed due to spin-orbit interaction (SOI) attributed to the tellurium (Te) atoms. Here, we observe that on chemical substitution of the Te atoms with selenium (Se), the COC, which runs along the Te-Te edges of the MnTe$_6$ octahedra, becomes weaker and thus affects the angular magnetoresistance (MR) of Mn$_3$Si$_2$Te$_6$. We find that the application of magnetic field along the easy axis leads to a considerable drop in resistance in substituted crystals, which otherwise exhibits weak MR. On the other hand, the CMR effect along the partially polarized magnetization direction is found to be only marginally affected due to the substitution and persists even for a significantly high concentration of Se.

cond-mat.mtrl-sci↗

arXiv2025

Spin-reorientation driven topological Hall effect in Fe4GeTe2

Iron-based van der Waals (vdW) ferromagnets with relatively high ordering temperatures are a current research focus due to their significance in fundamental physics and potential applications in spintronics. Competing magnetic interactions and anisotropies can give rise to nontrivial spin textures in these materials, resulting in novel topological features. Fe4GeTe2 (F4GT) is a nearly room-temperature vdW ferromagnet, well known for hosting a spin-reorientation transition (SRT) arising from the interplay of perpendicular magnetic anisotropy (PMA) and shape anisotropy. In this work, we investigate the angle-dependent magneto-transport properties of F4GT single crystals. We report a large topological Hall effect (THE) in a multi-layer F4GT originating from the SRT-driven non-coplanar spin textures. The THE appears at the in-plane orientation of the external magnetic field and persists over a wide range of temperatures around SRT. Additionally, we find a thickness-sensitive THE signal for the c axis orientation of the magnetic field at a low-temperature regime which is associated with a reentrant Lifshitz transition.

cond-mat.mtrl-sci↗

arXiv2024

Do text-free diffusion models learn discriminative visual representations?

While many unsupervised learning models focus on one family of tasks, either generative or discriminative, we explore the possibility of a unified representation learner: a model which addresses both families of tasks simultaneously. We identify diffusion models, a state-of-the-art method for generative tasks, as a prime candidate. Such models involve training a U-Net to iteratively predict and remove noise, and the resulting model can synthesize high-fidelity, diverse, novel images. We find that the intermediate feature maps of the U-Net are diverse, discriminative feature representations. We propose a novel attention mechanism for pooling feature maps and further leverage this mechanism as DifFormer, a transformer feature fusion of features from different diffusion U-Net blocks and noise steps. We also develop DifFeed, a novel feedback mechanism tailored to diffusion. We find that diffusion models are better than GANs, and, with our fusion and feedback mechanisms, can compete with state-of-the-art unsupervised image representation learning methods for discriminative tasks - image classification with full and semi-supervision, transfer for fine-grained classification, object detection and segmentation, and semantic segmentation. Our project website (https://mgwillia.github.io/diffssl/) and code (https://github.com/soumik-kanad/diffssl) are available publicly.

cs.CV↗

arXiv2024

Anisotropic magnetization dynamics in Fe5GeTe2 at room temperature

The determination of Lande's g-factor and the damping constant is central to extracting crucial information about the spin-spin and spin-orbit interactions in magnetically ordered systems. In insulating compounds based on 3d elements, the spin-orbit interaction can be effectively probed by investigating the ground state of the corresponding free 3d ion within the crystalline environment. For metallic systems, the problem is non-trivial because of additional band structure effects. Here, we investigate the anisotropic magnetization dynamics in bulk single crystalline Fe5GeTe2, a van der Waals 2D itinerant ferromagnet at room temperature, using broadband ferromagnetic resonance spectroscopy. Strikingly, contrary to the results of ab-initio calculations, we do not observe any intrinsic anisotropy of magnetization damping close to room temperature, suggesting diminished role of spin-orbit interaction. However, there is a sizable anisotropy in the Lande's g factor near room temperature which is attributed to anisotropic critical spin fluctuations.

cond-mat.mtrl-sci↗

arXiv2024

Enhancement of spin current to charge current conversion in Ferromagnet/Graphene interface

The use of graphene in spintronic devices is contingent on its ability to convert a spin current into a charge current. We have systematically investigated the spin pumping induced spin-to-charge current conversion at the Graphene/FM interface and the effect of interface modification through high spin orbit coupling (SOC) material (Pt) as an interlayer (IL) of varying thicknesses by using broadband FMR spectroscopy. The spin mixing conductance is enhanced from $1.66 \times 10^{18}$ m$^{-2}$ to $2.72 \times 10^{18}$ m$^{-2}$ whereas the spin current density is enhanced from 0.135$\pm $0.003 to 0.242$\pm$0.004 MA/m$^{2}$ at the Graphene/FM interface due to the interface modification using high SOC material Pt as an interlayer. The spin current to charge current conversion efficiency turns out to be $\approx 0.003$ nm for the Graphene/FM interface. These findings support the idea that Graphene in combination with high SOC material (Pt) could be a potential candidate for spintronic applications, specifically for spin-torque-based memory applications.

cond-mat.mes-hall↗

arXiv2023

Diff2Lip: Audio Conditioned Diffusion Models for Lip-Synchronization

The task of lip synchronization (lip-sync) seeks to match the lips of human faces with different audio. It has various applications in the film industry as well as for creating virtual avatars and for video conferencing. This is a challenging problem as one needs to simultaneously introduce detailed, realistic lip movements while preserving the identity, pose, emotions, and image quality. Many of the previous methods trying to solve this problem suffer from image quality degradation due to a lack of complete contextual information. In this paper, we present Diff2Lip, an audio-conditioned diffusion-based model which is able to do lip synchronization in-the-wild while preserving these qualities. We train our model on Voxceleb2, a video dataset containing in-the-wild talking face videos. Extensive studies show that our method outperforms popular methods like Wav2Lip and PC-AVS in Fréchet inception distance (FID) metric and Mean Opinion Scores (MOS) of the users. We show results on both reconstruction (same audio-video inputs) as well as cross (different audio-video inputs) settings on Voxceleb2 and LRW datasets. Video results and code can be accessed from our project page ( https://soumik-kanad.github.io/diff2lip ).

cs.CV↗