arXiv subjects

Sourav Dey

Publications and source records attributed to Sourav Dey.

At least 19 recordsLinked to original sources

arXiv2026

Spin dynamics and polarization in relativistic systems: recent developments

We review recent theoretical and experimental developments in spin dynamics and polarization phenomena in relativistic systems, with a particular focus on heavy-ion collisions. The large angular momentum and magnetic field generated in non-central collisions induce vorticity in the quark-gluon plasma, leading to observable spin polarization of emitted hadrons. We discuss the theoretical foundations of spin polarization arising from spin-vorticity coupling, including formulations based on relativistic hydrodynamics, kinetic theory, and quantum statistical approaches such as the Zubarev density operator. A central theme of the review is the role of pseudo-gauge freedom and its implications for defining energy-momentum and spin tensors, which can influence theoretical predictions of polarization observables. We further examine different formulations of spin hydrodynamics, emphasizing the impact of gradient expansions, spin chemical potential, and entropy-current analysis on the structure of the theory and associated transport coefficients. In addition, we discuss the recent developments in heavy flavor spin dynamics within the framework of rotational Brownian motion, where spin degrees of freedom undergo stochastic evolution due to interactions with the medium. This framework provides a complementary perspective on spin relaxation and diffusion by incorporating the effects of strong initial magnetic fields and establishes connections between spin polarization and the initial geometry through the definition of polarization harmonics. This review provides a comprehensive overview of relativistic spin hydrodynamics as well as non-equilibrium spin dynamics, and outlines future directions toward a consistent and predictive description of spin phenomena in strongly interacting matter.

nucl-th↗

arXiv2026

Quantum spin dynamics of heavy quarks and polarization observables in relativistic heavy-ion collisions

We develop a quantum spin-density-matrix framework for heavy-quark spin dynamics in relativistic heavy-ion collisions. Starting from an initial polarization induced along the magnetic-field direction, we derive the evolution equation for spin polarization within this framework and obtain analytic solutions. The evolved polarization is connected to open heavy-flavor observables via a fragmentation-based hadronization prescription. For vector mesons, the spin-alignment parameter $\rho_{00}$ is constructed by coupling the heavy-quark spin to that of the light antiquark produced during fragmentation. We confront our results with recent ALICE measurements of prompt $D^{*+}$ spin alignment in Pb--Pb collisions at $\sqrt{s_{\rm NN}}=5.02~{\rm TeV}$ and extract an effective depolarization strength that determines the spin-relaxation time scale. Using this fitted parameter, we provide benchmark estimates for $\Lambda_c^+$ and $\bar{\Lambda}_c^-$ polarization, up to an overall spin-transfer normalization. We further estimate the recently proposed elliptic polarization harmonic arising from path-length-dependent depolarization in an anisotropic fireball.

hep-ph↗

arXiv2026

Dissipative spin hydrodynamics in Bjorken flow and thermal dilepton production

We investigate the boost-invariant expansion of a recently developed first-order spin hydrodynamic framework in which the spin chemical potential is treated as a leading-order hydrodynamic variable. Considering a symmetric energy-momentum tensor and a separately conserved spin tensor, we derive the coupled evolution equations for the medium temperature and the independent components of the spin chemical potential in the presence of both viscous and spin-diffusive transport coefficients. For a boost-invariant system, only the magnetic-like components of the spin chemical potential survive, and their evolution is shown to depend sensitively on the spin transport coefficients. The transverse spin components decay more rapidly due to spin dissipation, while the longitudinal component survives for a longer duration. We further demonstrate that the evolution of the spin degrees of freedom modifies the temperature profile of the expanding medium. Using the resulting temperature profiles, we calculate thermal dilepton production rates from quark-antiquark annihilation. We find that the presence of spin dynamics enhances the dilepton yield relative to standard dissipative hydrodynamics, with the magnitude of the enhancement depending on the spin transport coefficients. Our results indicate that thermal dileptons can provide an indirect probe of spin dynamics and spin transport in the quark-gluon plasma.

nucl-th↗

arXiv2025

Switchable Magnonic Crystals Based on Spin Crossover/CrSBr Heterostructures

The progress of magnonics ultimately depends on material platforms that offer precise control of spin waves propagation. Here, we put forward a chemical strategy to create locally tunable magnonic crystals by integrating switchable spin-crossover (SCO) molecules with 2D van der Waals magnets. Specifically, we investigate from first principles a hybrid molecular/2D heterostructure formed by [Fe((3,5-(CH3)2Pz)3BH)2] molecules (Fe-pz) deposited on a single-layer of semiconducting CrSBr. We show that Fe-pz molecules are stable on CrSBr while preserving its SCO bistability, particularly in densely packed molecular arrays. By patterning Fe-pz into periodic stripes separated by pristine CrSBr regions, the interface becomes a magnonic crystal that filters spin waves at selected frequencies. Crucially, light-driven excited spin-state trapping (LIESST) enables LS-HS switching and induces up to ~1.3% local strain in CrSBr, which in turn reshapes the magnonic band structure in a dynamic and reversible manner. These results establish Fe-pz@CrSBr as a switchable platform for on-chip, programmable magnonic devices.

cond-mat.mtrl-sci↗

arXiv2025

Anisotropic Phonon Dynamics and Directional Transport in Actinide van der Waals Semiconductor USe$_3$

Direction-dependent charge transport and optical responses are characteristic of van der Waals (vdW) materials with strong in-plane anisotropy. While transition-metal trichalcogenides (TMTCs) exemplify this behavior, heavier analogs remain largely unexplored. In this study we examine USe$_3$ as an anisotropic vdW material and a heavier analog of the well-studied TMTCs. We reveal strong in-plane anisotropy using polarization-resolved Raman spectroscopy, investigate strain-induced shifts of phonon modes, and quantify direction-dependent charge-carrier mobility through transport measurements on field-effect devices. First-principles calculations based on density-functional theory corroborate our findings, providing a theoretical basis for our experimental observations. Casting USe$_3$ as an actinide analog of a TMTC establishes a platform for exploring low-dimensional semiconductors that combine strong in-plane anisotropy with f-electron physics.

cond-mat.mtrl-sci↗

arXiv2025

Optimizing rake-links independently of timetables in railway operations

This study addresses optimal rake-link formation in large-scale timetabled rail operations by modeling the problem as a directed acyclic graph and solving it via the minimum path cover algorithm. It enables efficient rake-to-service assignment while minimizing fleet size. Crucially, it decouples rake-link optimization from the timetable planning process, allowing planners to evaluate feasible rake configurations independently. The model incorporates operational constraints such as deadhead limits, service balance, and slack allowances. Applied to real-world data from Indian Railways, the results reveal clustered Pareto fronts in the decision space, indicating robust and redundant solutions. The approach lays a foundation for resilient, adaptive rail management via digital twin systems.

math.OC↗

arXiv2025

Tunable Itinerant Ferromagnetism in the Two-Dimensional FePd$_2$Te$_2$ Hosting 1D Spin Chains

One-dimensional (1D) magnetism offers unidirectional spin interactions that allow unique tunable properties and unconventional spin phenomena. However, it often suffers from poor stability, limiting practical applications. In this regard, integrating 1D magnetism into two-dimensional (2D) materials enables a promising route to stabilize these systems while preserving their anisotropic magnetic characteristics. Here, we focus on the 2D ferromagnet FePd$_2$Te$_2$ (T$_C$ = 183K), which hosts 1D spin chains and strong in-plane anisotropy. Our first-principles calculations reveal highly anisotropic magnetic exchange interactions, confirming its 1D ferromagnetic nature. We modulate this behavior by Co and Ni substitution and introduce two new members of this family, CoPd$_2$Te$_2$ -- a ferromagnet -- and NiPd$_2$Te$_2$. Our results unveil the microscopic mechanisms governing the behaviour of FePd$_2$Te$_2$ and CoPd$_2$Te$_2$. Furthermore, we also demonstrate that the variation of the chain length is key to modulate magnetism. Finally, we determine the magnon dispersion, showcasing a pronounced anisotropy that enables unidirectional magnon propagation.

cond-mat.mtrl-sci↗

arXiv2025

Virial theorem for rigidly rotating matter

The scaling property of the thermodynamic free energy ($\Phi$) of a system at global equilibrium has been examined using a real-time method known as the virial theorem. We demonstrate these scaling properties through a derived relation based on the general structure of equal-time commutators among Poincare charges and their densities. This relation is applicable to any renormalizable fields with spin $\leq 1$, excluding gauge fields. In this particular study, we investigate a rigidly rotating solution ($\Omega = \text{const}$) at global equilibrium for massless fermionic matter. It has been shown that the applicability of a hydrodynamic description requires a hierarchy $\Omega R \gg \Omega \beta_{0}$, where $R$ is the radius of the cylindrical-shaped rotating matter and $\beta_{0} = 1/T_{0}$ is the inverse temperature on the rotation axis. Consequently, the thermodynamic free energy $\Phi$ depends on the angular velocity through the product $\Omega R$, a dependency that extends to other thermodynamic variables as well. These findings are consistent with recent lattice QCD simulation results. Furthermore, we compute the moment of inertia for massless fermions and estimate the light quark contribution to the total moment of inertia of the Quark-Gluon Plasma (QGP) produced in heavy-ion collisions.

hep-th↗

arXiv2025

Coupling molecular spin qubits with 2D magnets for coherent magnon manipulation

Magnonics is an emerging field widely considered as a paradigm shift in information technology that uses spin waves for data storage, processing and transmission. However, the coherent control of spin waves in 2D magnets still remains a challenge. Herein, we investigate the interplay between molecular spins and magnons in hybrid heterostructures formed by [CpTi(cot)] and VOPc spin qubits deposited on the surface of the air-stable 2D van der Waals ferromagnet CrSBr using first principles. Our results show that different molecular rotation configurations significantly impact on qubit relaxation time and alter the magnon spectra of the underlying 2D magnet, allowing the chemical coherent control of spin waves in this material. We predict the feasibility of an ultrafast magnon-qubit interface with minimized decoherence, where exchange coupling plays a crucial role. This work opens new avenues for hybrid quantum magnonics, enabling selective tailoring through a versatile chemical approach.

cond-mat.mes-hall↗

arXiv2025

Rotational Brownian motion and heavy quark polarization in QCD medium

We consider the rotational Brownian motion of heavy quark in QCD medium and provide analytical results for polarization of open heavy-flavor hadrons. We calculate expressions for vector and tensor polarization, corresponding to baryon spin polarization and vector meson spin alignment, respectively. Assuming that heavy quarks are initially fully spin polarized along the direction of magnetic field, we compare our results with recent experimental data from ALICE collaboration for $D^{*+}$ meson and provide predictions for spin polarization of open charm baryons. We propose that the transverse momentum dependence of heavy quark polarization may serve as a distinctive signature of the intense initial magnetic field generated in off-central relativistic heavy-ion collisions.

hep-ph↗

arXiv2024

Tailoring the properties of the 2D ferromagnet CrSBr by lanthanide doping

The growing interest in 2D van der Waals (vdW) magnetic materials stems from their unique properties and potential applications in spintronics, magnonics and quantum information technologies. Among them, CrSBr is a semiconductor that stands out, owing to its high Curie temperature (TC ~ 146 K), air stability and tunable electronic and magnetic properties. Here, we present a systematic investigation of the effects of Dy doping (12.5%, 25%, and 50%) on the structural, electronic and magnetic properties of CrSBr monolayer. Our results reveal that Dy incorporation enhances magnetic anisotropy, modulates TC, and can create quasi-1D ferromagnetic chains that arise from competing ferromagnetic and antiferromagnetic interactions. Additionally, we investigate the properties of DySBr, DySI and DySeI monolayers, which are isostructural to the CrSBr. Our results reveal the feasibility of exfoliating them down to the single-layer and the presence of long-range magnetic order at low temperatures, relying on the combination of both weak exchange interactions and large spin-orbit coupling. This work provides insights into tuning the properties of CrSBr through rare earth doping, unlocking new possibilities for advanced applications at the 2D limit.

cond-mat.mtrl-sci↗

arXiv2024

The Mechanism of Spin-Phonon Relaxation in Endohedral Metallofullene Single Molecule Magnets

This study presents the first-ever investigation of spin-phonon coupling mechanisms in fullerene-based single-molecule magnets (SMMs) using ab initio CASSCF combined with DFT calculations. While lanthanide-based SMMs, particularly those with DyIII ions, are known for their impressive blocking temperatures and relaxation barriers, endohedral metallofullerene (EMFs) offer a unique platform for housing low-coordinated lanthanides within rigid carbon cages. We have explored the spin dynamics of in DyScS@C82 exhibiting among the highest blocking temperature (TB) reported. Through our computational analysis, we reveal that while the fullerene cage enhances crystal field splitting and provides structural stability without significantly contributing to spin-relaxation-driving low-energy phonons, the internal ionic motion emerges as the primary factor controlling spin relaxation and limiting blocking temperature. This computational investigation into the spin dynamics of EMF-based SMMs provides key insights into their magnetic behaviour for the first time an

cond-mat.mtrl-sci↗

arXiv2024

Diffusion coefficient matrix for multiple conserved charges: a Kubo approach

The strongly interacting matter created in relativistic heavy-ion collisions possesses several conserved quantum numbers, such as baryon number, strangeness, and electric charge. The diffusion process of these charges can be characterized by a diffusion matrix that describes the mutual influence of the diffusion of various charges. We derive the Kubo relations for evaluating diffusion coefficients as elements of a diffusion matrix. We further demonstrate that in the weak coupling limit, the diffusion matrix elements obtained through Kubo relations reduce to those obtained from kinetic theory with an appropriate identification of the relaxation times. We illustrate this evaluation in a toy model of two interacting scalar fields with two conserved charges.

hep-ph↗

arXiv2024

Kubo formula for spin hydrodynamics: spin chemical potential as leading order in gradient expansion

We present a first-order dissipative spin hydrodynamic framework, where the spin chemical potential $ω^{μν}$ is treated as the leading term in the hydrodynamic gradient expansion, i.e., $ω^{μν}\sim \mathcal{O}(1)$. We argue that for the consistency of the theoretical framework, the energy-momentum tensor needs to be symmetric at least up to order $\mathcal{O}(\partial)$. We consider the phenomenological form of the spin tensor, where it is anti-symmetric in the last two indices only. A comprehensive analysis of spin hydrodynamics is conducted using both macroscopic entropy current analysis and microscopic Kubo formalism, establishing consistency between the two approaches. A key finding is the entropy production resulting from spin-orbit coupling, which alters the traditional equivalence between the Landau and Eckart fluid frames. Additionally, we identify cross-diffusion effects, where vector dissipative currents are influenced by gradients of both spin chemical potential and chemical potential corresponding to the conserved charge through off-diagonal transport coefficients. Two distinct methods for decomposing the spin tensor are proposed, and their equivalence is demonstrated through Kubo relations.

nucl-th↗

arXiv2024

Energy-momentum correlators of fermions at finite temperature and density

Equal-time commutators of different components of the energy-momentum tensor at spatially separated points are calculated for a relativistic quantum Fermi gas at finite temperature and density. Different definitions of such components, also known as different pseudogauges, are used and smeared with a Gaussian profile characterized by the width $σ$. In this way, we introduce observables that may represent measurements of energy and momentum in a spatial region of size $σ$. We find that the obtained commutators are sensitive to the pseudogauge chosen if the probed systems or the spatial separation are small. The pseudogauge dependence is expected as different quantum operators are analyzed in this case. On the other hand, we find that for sufficiently large probed systems or with large separation, the studied commutators are pseudogauge independent.

hep-ph↗

arXiv2023

Pseudogauge freedom and the SO(3) algebra of spin operators

The energy-momentum and spin tensors for a given theory can be replaced by alternative expressions that obey the same conservation laws for the energy, linear momentum, as well as angular momentum but, however, differ by the local redistribution of such quantities (with global energy, linear momentum, and angular momentum remaining unchanged). This arbitrariness is described in recent literature as the pseudogauge freedom or symmetry. In this letter, we analyze several pseudogauges used to formulate the relativistic hydrodynamics of particles with spin 1/2 and conclude that the canonical version of the spin tensor has an advantage over other forms as only the canonical definition defines the spin operators that fulfill the SO(3) algebra of angular momentum. This result sheds new light on the results encountered in recent papers demonstrating pseudogauge dependence of various physical quantities. It indicates that for spin-polarization observables, the canonical version is fundamentally better suited for building a connection between theory and experiment.

hep-th↗

arXiv2023

Local Convolutions Cause an Implicit Bias towards High Frequency Adversarial Examples

Adversarial Attacks are still a significant challenge for neural networks. Recent work has shown that adversarial perturbations typically contain high-frequency features, but the root cause of this phenomenon remains unknown. Inspired by theoretical work on linear full-width convolutional models, we hypothesize that the local (i.e. bounded-width) convolutional operations commonly used in current neural networks are implicitly biased to learn high frequency features, and that this is one of the root causes of high frequency adversarial examples. To test this hypothesis, we analyzed the impact of different choices of linear and nonlinear architectures on the implicit bias of the learned features and the adversarial perturbations, in both spatial and frequency domains. We find that the high-frequency adversarial perturbations are critically dependent on the convolution operation because the spatially-limited nature of local convolutions induces an implicit bias towards high frequency features. The explanation for the latter involves the Fourier Uncertainty Principle: a spatially-limited (local in the space domain) filter cannot also be frequency-limited (local in the frequency domain). Furthermore, using larger convolution kernel sizes or avoiding convolutions (e.g. by using Vision Transformers architecture) significantly reduces this high frequency bias, but not the overall susceptibility to attacks. Looking forward, our work strongly suggests that understanding and controlling the implicit bias of architectures will be essential for achieving adversarial robustness.

stat.ML↗

arXiv2021

Long-lived light neutralinos at Belle II

We consider light neutralinos of mass about 1 GeV, produced from $τ$ lepton rare decays at Belle II, in the context of R-parity-violating (RPV) supersymmetry. With large and clean samples of $τ$ leptons produced at the Belle II experiment, excellent sensitivity to such light neutralinos with the exotic signatures of displaced vertices is expected. We focus on two benchmark scenarios of single RPV operators, $λ'_{311} L_3 Q_1 \bar{D}_1$ and $λ'_{312} L_3 Q_1 \bar{D}_2$, which induce both the production and decay of the lightest neutralino. For the reconstruction of a displaced vertex, we require at least two charged pions in the final states. We perform Monte-Carlo simulations for both signal and background events, and find that Belle II can explore regions in the parameter space competitive with other probes. In particular, for the $λ'_{311}$ scenario, it can put limits up to two orders of magnitude stronger than the current bounds.

hep-ph↗