arXiv subjects

Vipul Upadhyay

Publications and source records attributed to Vipul Upadhyay.

8 recordsLinked to original sources

arXiv2026

Phonon chirality as an additive control of CISS: a symmetry-protected law

Chirality-induced spin selectivity (CISS) is usually associated with molecular handedness. The possible contribution of chiral phonons is less established. We study a helical tight-binding model in which local phonon angular momentum modulates spin-dependent nearest-neighbor hopping. Fewest-switches surface hopping calculations give the transmitted spin polarization $\mathrm{SP}=aC+b\mathrm{PH}$. Here $C$ is the molecular chirality and $\mathrm{PH}$ is the phonon chirality. A mirror symmetry reverses $C$, $\mathrm{PH}$, and $\mathrm{SP}$ simultaneously. This symmetry excludes both a chirality-independent offset and a $C\cdot\mathrm{PH}$ term. The phonon contribution can therefore enhance, cancel, or reverse the molecular CISS signal.

physics.chem-ph↗

arXiv2026

Weak Electron-Phonon Coupling Is Insufficient to Generate Significant CISS in Two-Terminal Transport

A central open question in chiral-induced spin selectivity (CISS) is whether weak electron-phonon coupling in a helical molecular junction can generate a sizable spin polarization in two-terminal transport without invoking additional strong symmetry-breaking ingredients. We address this question by implementing a self-consistent nonequilibrium Green's function (NEGF) calculation for a helical tight-binding model with spin-orbit coupling and electron-phonon interactions. The electron-phonon self-energies are evaluated self-consistently, and the transport signal is extracted using the standard magnetization-reversal protocol with a spin-polarized analyzer lead. We benchmark a fully self-consistent NEGF within the self-consistent Born approximation (SCBA) treatment for both global and local electron-phonon couplings against commonly used approximations, including diagonal self-energy schemes. We quantify how the resulting transport regime and spin polarization depend on phonon frequency, coupling strength, bias, temperature, and system size. In contrast to large polarizations and anomalous size trends reported under approximate treatments, the fully self-consistent calculation yields negligible spin polarization, additionally the electron-phonon coupling mainly renormalizes the spectrum, and transport remains quasi-ballistic across the explored parameter range.

cond-mat.mes-hall↗

arXiv2026

Modified Quantum Wheatstone Bridge based on current circulation

We investigate a simple fermionic system designed to detect an unknown hopping rate between two sites by analyzing current circulation. The system exploits geometric asymmetry and utilizes the connection between the additional energy degeneracy point (AEDP) and current circulation for precise parameter detection. In the low-temperature, low-bias regime, with baths chemical potentials aligned near the degenerate energy, we find that a balanced Wheatstone bridge condition emerges when the direction of current circulation reverses, providing a direct means to determine the unknown hopping strength. We further examine the impact of environmental interactions, demonstrating that the device remains functional under moderately strong dephasing and particle losses, though extreme environmental effects eventually degrade performance. Extending the analysis to general operating conditions, we show that the device continues to function effectively at higher voltages and temperatures. Finally, an analysis of the quantum Fisher information qualitatively supports our findings, revealing a sharp increase in the coherence contribution and a corresponding decrease in the population contribution near the AEDP. Our results highlight geometric asymmetry as a robust and practical tool for quantum metrology.

cond-mat.mes-hall↗

arXiv2025

Quantum thermometry for ultralow temperatures using probe and ancilla qubit chains

We propose a scheme to enhance the range and precision of ultralow temperature measurements by employing a probe qubit coupled to a chain of ancilla qubits. Specifically, we analyze a qubit chain governed by Heisenberg $XX$ and Dzyaloshinskii-Moriya (DM) interactions. The precision limits of temperature measurements are characterized through the evaluation of quantum Fisher information (QFI). Our findings demonstrate that the achievable precision bounds, as well as the number of peaks in the QFI as a function of temperature, can be controlled by adjusting the number of ancilla qubits and the system's model parameters. These results are interpreted in terms of the influence of energy transitions on the range and the number of QFI peaks as a function of temperature. This study highlights the potential of the probe qubit-ancilla chain system as a powerful and precise tool for quantum thermometry in the ultralow temperature regime.

quant-ph↗

arXiv2024

Current circulation near additional energy degeneracy points in quadratic Fermionic networks

We study heat and particle current circulation (CC) in quadratic Fermionic systems analysed using a general dissipative Lindbladian master equation. It was observed in an earlier study (Upadhyay et al. Phys. Rev. E 107, 034120 (2023)), that CC occurs near the additional energy degeneracy point (AEDP) in Fermionic systems which have some form of asymmetry. We find general analytical expression to support this observation for quadratic Fermionic networks. We then apply these ideas to the Su-Schrieffer-Heeger (SSH) model with periodic boundary conditions and a tight binding model with unequal hopping strengths in the upper and lower branches. In both these cases, we find the specific conditions required for observing CC and study the behavior of these currents with various system parameters. We find that having unequal number of Fermionic sites in the upper and lower branches is enough for generating CC in the SSH model. However, this asymmetry is not adequate for the tight-binding model and we require unequal hopping strengths in the upper and lower branches to induce CC in this model. We also compare our results with the exact results obtained via the Non-Equilibrium Green Function (NEGF) formalism, and observe that the relationship between AEDP and CC also holds for the exact results. Finally, we observe that for certain system parameters, the onset point of particle and heat CC are not the same. Based on all these observations, we describe how carefully examining the energy spectrum of the system gives a great deal of information about the possibility and behavior of CC in Fermionic systems with asymmetries.

cond-mat.stat-mech↗

arXiv2023

Signature of topology via heat transfer analysis in the Su-Schrieffer-Heeger (SSH) model

In this work, we explore the potential of thermodynamics as a tool for identifying the topological phase transition. Specifically, we focus on a one-dimensional Su-Schrieffer-Heeger (SSH) chain sandwiched between two fermionic baths. To investigate distinctive thermodynamic signatures associated with the topological phase, we employ heat flow analysis. Our results, derived using a global master equation, unveil a significant suppression of heat flow as we transition from the trivial to the topological phase. This decline in heat flow can be attributed to the reduction in transmission coefficients of non-zero energy modes within the topological phase. It may serve as an indicator of a phase transition. Furthermore, we investigate the heat flow asymmetry to search for phase transition indicators. Interestingly, no asymmetry is observed when employing fermionic baths. However, upon substituting fermionic baths with bosonic ones, we report a non-zero heat flow asymmetry. For the SSH model with a few fermionic sites, this asymmetry is more pronounced in the topological phase compared to the trivial phase. Therefore, the observed behavior of the heat diode provides an additional means of distinguishing between the topological and trivial phases. Finally, we delve into the contributions from both bulk and edge effects in heat flow and rectification to explore the impact of small system sizes on our findings.

cond-mat.stat-mech↗

arXiv2023

Heat current magnification in Classical and Quantum spin networks

We investigate heat current magnification due to asymmetry in the number of spins in two-branched classical and quantum spin systems. We begin by studying the classical Ising like spin models using Q2R and CCA dynamics and show that just the difference in the number of spins is not enough and some other source of asymmetry is required to observe heat current magnification. Unequal spin--spin interaction strength in the upper and lower branch is employed as a source of this asymmetry and it proves adequate for generating current magnification in both the models. Suitable physical motivation is then provided for current magnification in these systems, along with ways to control and manipulate magnification through various system parameters. We also study a five spin Quantum system with modified Heisenberg XXZ interaction and preserved magnetisation using the Redfield master equation. We show that it is possible to generate current magnification in this model by the asymmetry in the number of spins only. Our results indicate that the onset of current magnification is accompanied by a dip in the total current flowing through the system. On analysis it is revealed that this dip might occur because of the intersection of two non-degenerate energy levels for certain values of the asymmetry parameter in the modified XXZ model. We deduce that the additional degeneracy and the ergodic constraint due to fixed magnetisation in the system are the main reasons for current magnification and other atypical behaviors observed. We then use the concept of `ergotropy' to support these findings. Finally, for both the classical and quantum models, we see that current magnification is only observed when temperature gradient and intra-system interaction strength have similar order of energy.

cond-mat.stat-mech↗

arXiv2021

Heat rectification by two qubits coupled with Dzyaloshinskii-Moriya interaction

We investigate heat rectification in a two-qubit system coupled via the Dzyaloshinskii-Moriya (DM) interaction. We derive analytical expressions for heat currents and thermal rectification and provide possible physical mechanisms behind the observed results. We show that the anisotropy of DM interaction in itself is insufficient for heat rectification, and some other form of asymmetry is needed. We employ off-resonant qubits as the source of this asymmetry. We find the regime of parameters for higher rectification factors by examining the analytical expressions of rectification obtained from a global master equation solution. In addition, it is shown that the direction and quality of rectification can be controlled via various system parameters. Furthermore, we compare the influence of different orientations of the DM field anisotropy on the performance of heat rectification. Finally, we investigate the possible interplay between quantum correlations and the performance of the quantum thermal rectifier. We find that asymmetry in the coherences is a fundamental resource for the performance of the quantum thermal rectifier.

cond-mat.stat-mech↗