arXiv subjects

Souvik Roy

Publications and source records attributed to Souvik Roy.

At least 37 records · Page 2Linked to original sources

arXiv2025

Spin-Selective Thermoelectric Transport in a Triangular Spin Ladder

We theoretically investigate spin-resolved thermoelectric transport in a triangular ladder geometry hosting antiferromagnetic spin alignment, where lattice topology and magnetic ordering jointly enable highly efficient spin-selective energy conversion. The inherent geometric frustration of the ladder, together with intrinsic spin-filtering mechanisms, is shown to promote a pronounced separation between spin channels. Implementing spin-dependent onsite modulations, such as binary asymmetric potentials, induces pronounced spin splitting in the transmission spectrum, enabling controlled spin-selective transport and highlighting the role of lattice engineering in tailoring spin-dependent thermoelectric response. Additional control is achieved through modulation of the hopping amplitudes, which activates multiple transport pathways and allows fine tuning of spin-dependent conduction. A detailed evaluation of charge and spin thermoelectric coefficients reveals a strong enhancement of the thermoelectric performance, with the dimensionless figure of merit ZT reaching large values in optimized parameter regimes. Notably, the spin figure of merit systematically surpasses its charge counterpart, underscoring the decisive role of lattice geometry and antiferromagnetic order in amplifying spin thermoelectric efficiency. Our findings provide a versatile theoretical platform for designing low-dimensional spin-caloritronic devices with enhanced functionality.

cond-mat.mes-hall↗

arXiv2025

A characterization of strategy-proof probabilistic assignment rules

We study the classical probabilistic assignment problem, where finitely many indivisible objects are to be probabilistically or proportionally assigned among an equal number of agents. Each agent has an initial deterministic endowment and a strict preference over the objects. While the deterministic version of this problem is well understood, most notably through the characterization of the Top Trading Cycles (TTC) rule by Ma (1994), much less is known in the probabilistic setting. Motivated by practical considerations, we introduce a weakened incentive requirement, namely SD-top-strategy-proofness, which precludes only those manipulations that increase the probability of an agent's top-ranked object. Our first main result shows that, on any free pair at the top (FPT) domain (Sen, 2011), the TTC rule is the unique probabilistic assignment rule satisfying SD-Pareto efficiency, SD-individual rationality, and SD-top-strategy-proofness. We further show that this characterization remains valid when Pareto efficiency is replaced by the weaker notion of SD-pair efficiency, provided the domain satisfies the slightly stronger free triple at the top (FTT) condition (Sen, 2011). Finally, we extend these results to the ex post notions of efficiency and individual rationality. Together, our findings generalize the classical deterministic results of Ma (1994) and Ekici (2024) along three dimensions: extending them from deterministic to probabilistic settings, from full strategy-proofness to top-strategy-proofness, and from the unrestricted domain to the more general FPT and FTT domains.

econ.TH↗

arXiv2025

Fibonacci-Engineered Spin and Charge Thermoelectrics in a Long Range Su-Schrieffer-Heeger Chain: A Pathway to Giant Figure of Merit

In this work, we present a novel investigation into the spin-dependent thermoelectric performance of an extended Su-Schrieffer-Heeger (SSH) model, showcasing for the first time how its intrinsic spin filtration mechanism can be strategically harnessed to function as an efficient spin thermoelectric generator. By introducing a Fibonacci-type aperiodic modulation in the onsite energies, we engineer a deterministic disorder that mimics realistic aperiodic systems and profoundly influences transport characteristics. Furthermore, we incorporate both nearest-neighbor (NN) and next-nearest-neighbor (NNN) hopping amplitudes with tunable cosine dependencies, enabling us to meticulously explore the intricate interplay between these hopping processes and its implications on thermoelectric behavior. Our analysis reveals a remarkable enhancement in the dimensionless thermoelectric figure of merit ZT for both charge and spin transport channels, under carefully optimized conditions. Notably, the spin thermoelectric response exhibits distinct advantages, opening a new frontier in the design of next-generation thermoelectric materials and devices. This qualitative study not only deepens our understanding of aperiodic topological systems but also establish a foundational framework for exploiting spin-based thermoelectricity in low-dimensional platforms.

cond-mat.mes-hall↗

arXiv2025

Non-Hermitian comb effect in coupled clean and quasiperiodic chains

We study localization properties in a system of non-Hermitian quasiperiodic chain coupled to a uniform chain or clean chain by inter-chain hopping. We find that in the limit of weak inter-chain coupling, such a coupled system exhibits transitions from delocalized to intermediate phase with increase in the non-Hermiticity parameter. However, for stronger inter-chain coupling strengths, the delocalized phase undergoes a transition to localized phase and then to an intermediate phase. Interestingly, the intermediate phase in this case exhibits the non-Hermitian comb effect (NHCE), i.e., the coexistence of localized and extended states rather than being well separated from each other by any mobility edge which is conventional in any intermediate phase. We further show that such a NHCE originates from the isolated site limit of the quasiperiodic chain and provide an analytical explanation supporting the numerical signatures.

cond-mat.quant-gas↗

arXiv2025

Flux-driven charge and spin transport in a dimerized Hubbard ring with Fibonacci modulation

We study quantum transport in a one-dimensional Hubbard ring with dimerized nearest-neighbor hoppings and a Fibonacci-modulated onsite potential. For non-interacting case our analysis reveals that at half-filling, the charge current along with the Drude weight decreases with increasing onsite potential when inter-cell hopping dominates over the intra-cell hopping, while for dominating intra-cell hopping it shows non-monotonic behavior with sharp peak at certain critical modulation strength, indicating enhanced transport. Moving away from half-filling gives rise to re-entrant features in both quantities at fillings associated with Fibonacci numbers. On the other hand, in spin-imbalanced systems, both spin and charge current shows multiple peaks and re-entrant behavior, tunable via hopping dimerization and filling. Including the on-site Hubbard interaction preserves the re-entrant behavior in current and moreover favors finite transport which is absent in the non-interacting ring. These results reveal rich interplay among Fibonacci modulated potential, electron fillings, hopping dimerization and interaction.

cond-mat.other↗

arXiv2025

The Game of Graph Nim on graphs with four edges

This work is concerned with the study of the Game of Graph Nim -- a class of two-player combinatorial games -- on graphs with $4$ edges. To each edge of such a graph is assigned a positive-integer-valued edge-weight, and during each round of the game, the player whose turn it is to make a move selects a vertex, and removes a non-negative integer edge-weight from each of the edges incident on that vertex, such that the remaining edge-weight on each of these edges is a non-negative integer, and the total edge-weight removed during a round is strictly positive. The game continues for as long as the sum of the edge-weights remaining on all edges of the graph is strictly positive, and the player who plays the last round wins. An initial configuration of edge-weights is considered winning if the player who plays the first round wins the game, whereas it is defined as losing if the player who plays the second round wins. In this paper, we characterize, almost entirely, all winning and losing configurations for this game on all graphs with precisely $4$ edges each. Only one such graph defies our attempt to fully characterize the winning and losing configurations of edge-weights on its edges -- we are still able to provide a significant set of partial results pertaining to this graph.

math.CO↗

arXiv2025

Flux-Driven Circular Current in a Non-Hermitian Dimerized Aharonov-Bohm Ring: Impact of Physical Gain and Loss

In the present theoretical work, we numerically explore magnetic response of a tight-binding dimerized ring subjected to Aharonov-Bohm (AB) flux and environmental interactions. Specifically, we introduce an imaginary site potential on the odd lattice sites to represent physical gain and loss, while the even lattice sites remain unperturbed. We investigate the induced current resulting from the AB flux in both real and imaginary eigenspaces, aiming to enhance this current significantly by adjusting the gain/loss parameter ($d$). Our analysis focuses on how exceptional points in the real and imaginary eigenenergy spaces contribute to notable increases in current at specific $d$ values, and the emergence of purely real current when the imaginary current vanishes. We discuss how the dual behavior of energy spectrum (real and imaginary), converging to and diverging from zero energy, affects the enhancement of the current. Additionally, we study the interplay between the correlations of dimerized hopping integrals and the gain-loss parameter, which affects the current and highlights key features associated with these physical parameters. Furthermore, we consider how system size impacts our findings. Our study may reveal unconventional characteristics in various loop configurations, potentially paving the way for new research directions.

cond-mat.mes-hall↗

arXiv2025

On Probabilistic Assignment Rules

We study the classical assignment problem with initial endowments in a probabilistic framework. In this setting, each agent initially owns an object and has strict preferences over the entire set of objects, and the goal is to reassign objects in a way that satisfies desirable properties such as strategy-proofness, Pareto efficiency, and individual rationality. While the celebrated result by Ma (1994) shows that the Top Trading Cycles (TTC) rule is the unique deterministic rule satisfying these properties, similar positive results are scarce in the probabilistic domain. We extend Ma's result in the probabilistic setting, and as desirable properties, consider SD-efficiency, SD-individual rationality, and a weaker notion of SD-strategy-proofness -- SD-top-strategy-proofness -- which only requires agents to have no incentive to misreport if doing so increases the probability of receiving their top-ranked object. We show that under deterministic endowments, a probabilistic rule is SD-efficient, SD-individually rational, and SD-top-strategy-proof if and only if it coincides with the TTC rule. Our result highlights a positive possibility in the face of earlier impossibility results for fractional endowments (Athanassoglou and Sethuraman (2011)) and provides a first step toward reconciling desirable properties in probabilistic assignments with endowments.

econ.TH↗

arXiv2025

On the existence of EFX allocations for goods

We consider a set $E$ of $m$ indivisible goods and a set $N$ consisting of $n \geq 2$ agents. The paper shows that if two agents have \textit{arbitrary} set monotonic valuation functions and the remaining agents have size monotonic valuation functions, then EFX allocations always exist.

econ.TH↗

arXiv2025

Estimation of An Infinite Dimensional Transition Probability Matrix Using a Generalized Hierarchical Stick-Breaking Process

Markov chains provide a foundational framework for modeling sequential stochastic processes, with the transition probability matrix characterizing the dynamics of state evolution. While classical estimation methods such as maximum likelihood and empirical Bayes approaches are effective in finite-state settings, they become inadequate in applications involving countably infinite or dynamically expanding state spaces, which frequently arise in domains such as natural language processing, population dynamics, and behavioral modeling. In this work, we introduce a novel Bayesian nonparametric framework for estimating infinite-dimensional transition probability matrices by employing a new class of priors, termed the Generalized Hierarchical Stick-Breaking prior. This prior extends traditional Dirichlet process and stick-breaking constructions, enabling highly flexible modelling of transition probability matrices. The proposed approach offers a principled methodology for inferring transition probabilities in settings characterized by sparsity, high dimensionality, and unobserved state spaces, thereby contributing to the advancement of statistical inference for infinite-dimensional transition probability matrices.

stat.ME↗

arXiv2025

Characterizing Stability in Many-to-One Matching with Non-Responsive Couples

We study many-to-one matching problems between institutions and individuals, where each institution may be matched to multiple individuals. The matching market includes couples, who view pairs of institutions as complementary. Institutions' preferences over sets of individuals are assumed to satisfy responsiveness, whereas couples' preferences over pairs of institutions may violate responsiveness. In this setting, we first assume that all institutions share a common preference ordering over individuals, and we establish: (i) a complete characterization of all couples' preference profiles for which a stable matching exists, under the additional assumption that couples violate responsiveness only to ensure co-location at the same institution, and (ii) a necessary and sufficient condition on the common institutional preference such that a stable matching exists when couples may violate responsiveness arbitrarily. Next, we relax the common preference assumption, requiring institutions to share a common ranking only over the members of each couple. Under this weaker assumption, we provide: (i) a complete characterization of all couples' preferences for which a stable matching exists, and (ii) a sufficient condition on individuals' preferences that guarantees the existence of a stable matching.

econ.TH↗

arXiv2025

Stability in Many-to-One Matching with Couples having Responsive Preferences

This paper studies matching markets where institutions are matched with possibly more than one individual. The matching market contains some couples who view the pair of jobs as complements. First, we show by means of an example that a stable matching may fail to exist even when both couples and institutions have responsive preferences. Next, we provide conditions on couples' preferences that are necessary and sufficient to ensure a stable matching for every preference profile where institutions may have any responsive preference. Finally, we do the same with respect to institutions' preferences, that is, we provide conditions on institutions' preferences that are necessary and sufficient to ensure a stable matching for every preference profile where couples may have any responsive preference.

econ.TH↗

arXiv2025

A Directed Lazy Random Walk Model to Three-Way Dynamic Matching Problem

This paper explores a novel extension of dynamic matching theory by analyzing a three-way matching problem involving agents from three distinct populations, each with two possible types. Unlike traditional static or two-way dynamic models, our setting captures more complex team-formation environments where one agent from each of the three populations must be matched to form a valid team. We consider two preference structures: assortative or homophilic, where agents prefer to be matched with others of the same type, and dis-assortative or heterophilic, where diversity within the team is valued. Agents arrive sequentially and face a trade-off between matching immediately or waiting for a higher quality match in the future albeit with a waiting cost. We construct and analyze the corresponding transition probability matrices for each preference regime and demonstrate the existence and uniqueness of stationary distributions. Our results show that stable and efficient outcomes can arise in dynamic, multi-agent matching environments, offering a deeper understanding of how complex matching processes evolve over time and how they can be effectively managed.

econ.TH↗

arXiv2025

Stellar Magnetic Storm Induced Magnetospheric Polarity Reversals: Distinguishing between Unmagnetised and Magnetised Exoplanets

Exoplanetary and planetary environments are forced by stellar activity which manifest through variable radiation, particle and magnetic fluxes, stellar winds, flares and magnetic storms known as coronal mass ejections (CMEs). Recent studies have shown that (exo)planets with intrinsic magnetic fields and magnetospheres respond differently to this stellar forcing compared to planets which lack an intrinsic magnetism; this is borne out by observations in solar system planets. However, detailed investigations to uncover the subtle ways in which stellar magnetic storms impact exoplanets are still at a nascent stage. Here we utilize 3D magnetohydrodynamic simulations to investigate the impact of stellar CMEs on Earth-like planets with different magnetic fields. Our results show that planetary atmospheric mass loss rates are dependent on the relative orientation of stellar wind and planetary magnetic fields, with significantly higher losses when the CME and planetary magnetic fields are oppositely oriented -- favoring enhanced magnetic reconnections. In contrast, for unmagnetised planets, the mass loss rate do not strongly depend on stellar magnetic field orientation. More significantly, we find that stellar CME induced polarity reversals can distinguish between planets with and without intrinsic magnetism. In unmagnetised or weakly magnetised (exo)planets, the polarity of the externally imposed magnetosphere are prone to global polarity reversals forced by stellar magnetised storms. Our analysis of the magnetotail current density dynamics during polarity reversals aligns with observations of Venus. This distinction in magnetospheric response provides a new paradigm to differentiate between (exo)planets with or without significant (intrinsic) magnetic fields.

astro-ph.EP↗

arXiv2025

Anomalous persistent current in a 1D dimerized ring with aperiodic site potential: Non-interacting and interacting cases

In this work, we investigate the magnetic response by examining flux-driven circular currents in a Su-Schrieffer-Heeger (SSH) tight-binding (TB) ring threaded by an Aharonov-Bohm (AB) flux, $ϕ$. We consider both non-interacting and interacting electrons, where site energies are modulated by a slowly varying cosine form. Repulsive electron-electron interaction is incorporated through an on-site Hubbard term, and we analyze the system using the Hartree-Fock (HF) mean-field (MF) approximation. We discuss the characteristics of flux-driven circular currents to aperiodic potentials, dimerized hopping integrals, and Hubbard interactions. For the chosen aperiodic potential, both the strength and configuration play a crucial role, and we explore these aspects in depth. Interestingly, we observe a counterintuitive delocalizing effect as the aperiodic potential increases, unlike in conventional disordered rings. The effects of system size, filling factor, the presence of circular spin current, and the accuracy of MF results are also discussed. Finally, we provide a brief description of possible experimental realizations of our chosen quantum system. This investigation can be extended to explore additional properties in various loop substructures, promising further insights.

cond-mat.mes-hall↗

arXiv2025

A Sequential Quadratic Hamiltonian-Based Estimation Method for Box-Cox Transformation Cure Model

We propose an enhanced estimation method for the Box-Cox transformation (BCT) cure rate model parameters by introducing a generic maximum likelihood estimation algorithm, the sequential quadratic Hamiltonian (SQH) scheme, which is based on a gradient-free approach. We apply the SQH algorithm to the BCT cure model and, through an extensive simulation study, compare its model fitting results with those obtained using the recently developed non-linear conjugate gradient (NCG) algorithm. Since the NCG method has already been shown to outperform the well-known expectation maximization algorithm, our focus is on demonstrating the superiority of the SQH algorithm over NCG. First, we show that the SQH algorithm produces estimates with smaller bias and root mean square error for all BCT cure model parameters, resulting in more accurate and precise cure rate estimates. We then demonstrate that, being gradient-free, the SQH algorithm requires less CPU time to generate estimates compared to the NCG algorithm, which only computes the gradient and not the Hessian. These advantages make the SQH algorithm the preferred estimation method over the NCG method for the BCT cure model. Finally, we apply the SQH algorithm to analyze a well-known melanoma dataset and present the results.

stat.CO↗

arXiv2025

Percolation games on rooted, edge-weighted random trees

Consider a rooted Galton-Watson tree $T$, to each of whose edges we assign, independently, a weight that equals $+1$ with probability $p_{1}$, $0$ with probability $p_{0}$ and $-1$ with probability $p_{-1}=1-p_{1}-p_{0}$. We play a game on a realization of this tree, involving two players and a token that is allowed to be moved from where it is currently located, say a vertex $u$ of $T$, to any child $v$ of $u$. The players begin with initial capitals that amount to $i$ and $j$ units respectively, and a player wins if either she is the first to amass a capital worth $κ$ units, where $κ\in \mathbb{N}$ is prespecified, or she is able to move the token to a leaf vertex, from where her opponent cannot move it any farther, or her opponent's capital is the first to dwindle to $0$. This paper is concerned with analyzing the probabilities of the three possible outcomes such a game may culminate in, as well as with finding conditions under which the expected duration of the game is finite. Of particular interest to us is the exploration of criteria that guarantee the probability of draw in such a game to be $0$. The theory we develop is further supported by observations obtained via computer simulations, providing a deeper insight into how the above-mentioned probabilities behave as the underlying parameters and / or offspring distributions are allowed to vary. We include in this paper conjectures pertaining to the behaviour of the probability of draw in our game (including a phase transition phenomenon, in which the probability of draw goes from being $0$ to being strictly positive) as the parameter-pair $(p_{0},p_{1})$ is varied suitably while keeping the underlying offspring distribution of $T$ fixed.

math.PR↗

arXiv2024

A high contrast and resolution reconstruction algorithm in quantitative photoacoustic tomography

A framework for reconstruction of optical diffusion and absorption coefficients in quantitative photoacoustic tomography is presented. This framework is based on a Tikhonov-type functional with a regularization term promoting sparsity of the absorption coefficient and a prior involving a Kubelka-Munk absorption-diffusion relation that allows to obtain superior reconstructions. The reconstruction problem is formulated as the minimization of this functional subject to the differential constraint given by a photon-propagation model. The solution of this problem is obtained by a fast and robust sequential quadratic hamiltonian algorithm based on the Pontryagin maximum principle. Results of several numerical experiments demonstrate that the proposed computational strategy is able to obtain reconstructions of the optical coefficients with high contrast and resolution for a wide variety of objects.

math.OC↗