arXiv subjects

Sushmita Saha

Publications and source records attributed to Sushmita Saha.

7 recordsLinked to original sources

arXiv2026

Lorentz-Covariant Landau Levels of Tilted Dirac Fermions in Nonuniform Fields

We develop an analytical theory of Landau quantization for tilted anisotropic Dirac fermions in an exponentially decaying magnetic field. Using anisotropy scaling and a Lorentz transformation, we recast the laboratory-frame problem into the isotropic Dirac equation in the boosted frame, where the exponentially decaying magnetic-field problem admits an exact solution. Transforming the boosted-frame spectrum back to the laboratory frame yields an implicit quantization condition with an intrinsically energy-dependent guiding-center parameter. We identify the conditions for physically admissible states. We further show that the formalism recovers the known uniform-field spectrum of tilted anisotropic Dirac fermions in the appropriate limit. Our results extend the Lorentz-covariant treatment of tilted anisotropic Dirac fermions to an exponentially decaying magnetic field and reveal the energy-dependent guiding-center structure generated by the inverse transformation to the laboratory frame.

cond-mat.mes-hall↗

arXiv2025

A Quantum Framework for Negative Magnetoresistance in Multi-Weyl Semimetals

We develop a fully quantum-mechanical theory of negative magnetoresistance in multi-Weyl semimetals in the ${\bf E}\parallel{\bf B}$ configuration, where the chiral anomaly is activated. The magnetotransport response is governed by Landau quantization and the emergence of multiple chiral Landau levels associated with higher-order Weyl nodes. These anomaly-active modes have unidirectional dispersion fixed by the node's monopole charge and dominate charge transport. As the magnetic field increases, individual chiral branches successively cross the Fermi energy, producing discrete slope changes in the longitudinal conductivity and a step-like negative magnetoresistance. This quantized evolution provides a direct experimental signature of multi-Weyl topology. Bulk Landau levels contribute only at very low fields due to strong disorder scattering and do not affect the anomaly-driven regime. Our results establish a unified, fully quantum-mechanical framework in which negative magnetoresistance arises from the discrete Landau-quantized spectrum and microscopic impurity scattering, beyond semiclassical anomaly descriptions.

cond-mat.mes-hall↗

arXiv2025

Enhanced Gilbert Damping via Cubic Spin-Orbit Coupling at 2DHG/Ferromagnetic Insulator Interface

We investigate the enhancement of Gilbert damping at 2DHG/ferromagnetic insulator (FI) interfaces, where spin pumping from the FI layer injects spins into the 2DHG, and cubic Rashba spin-orbit coupling (RSOC) significantly boosts spin relaxation and spin-pumping efficiency compared to 2DEG systems. The dominant contribution to spin damping arises from interband transitions which does exhibits conductivity-like behavior as the temperature, $ T \to 0 $. Our results reveal that damping remains stronger than in 2DEG due to the persistent influence of cubic RSOC. The interplay between RSOC and magnon absorption broadens the spectral response, with the damping peak shifting more notably at higher temperatures. Stronger RSOC expands the magnon interaction phase space, thus widening the damping spectrum. A key observation emerges with the Fermi level ($E_f$): a finite $E_f$ sustains spin imbalance and enhances damping, whereas $E_f = 0$ suppresses it, unlike in 2DEG. The electric field tunability of RSOC enables real-time control over spin relaxation and angular momentum transfer, offering a pathway toward voltage-controlled spintronic devices. These findings highlight the superior potential of 2DHG for tailoring spin dynamics via electric and thermal effects.

cond-mat.mes-hall↗

arXiv2024

Unveiling the Chiral States in Multi-Weyl Semimetals through Magneto-Optical Spectroscopy

This study investigates the transport parameters in multi-Weyl semimetals, focusing on their magneto-optical properties and the role of chiral states. The tilting parameter is identified as a key factor in higher-order Weyl nodes, significantly influencing the magneto-optical response. We obtain a generic Landau-level expression for multi-Weyl semimetals, establishing a robust framework for analyzing their quantum transport properties. A comprehensive expression for the conductivity tensor components is presented, uncovering distinctive low-frequency peaks and other features shaped by the tilting parameter. Our findings reveal that the signatures of chiral states in the conductivity tensors become increasingly pronounced with the Weyl node order. Particularly, the tilting parameter is shown to impact Faraday rotation, at energies near the tilted Dirac cone energies. These results provide critical insights into the magneto-optical behavior of multi-Weyl semimetals and their potential for exploring topological phenomena.

cond-mat.mes-hall↗

arXiv2024

Floquet-Engineered Valley-Topotronics in Kekul\'e-Y Bond Textured Graphene Superlattice

The exquisite distortion in a Kekul\'e-Y (Kek-Y) superlattice merges the two inequivalent Dirac cones (from the $K$- and the $K^\prime$- points) into the highest symmetric $\Gamma$-point in the hexagonal Brillouin zone. Here we report that a circularly polarised light not only opens up a topological gap at the $\Gamma$-point but also lifts the valley degeneracy at that point. Endowed with Floquet dynamics and by devising a scheme of high-frequency approximation, we have proposed that the handedness (left/right) in polarised light offers the possibility to realise the valley-selective circular dichroism in Kek-Y shaped graphene superlattice. Also, the non-vanishing Berry curvature and enumeration of valley resolved Chern number $\mathcal{C}_{K}/\mathcal{C}_{K^\prime}=+1/-1$ enable us to assign two pseudo-spin flavors (up/down) with the two valleys. Thereby, the above observations confirm the topological transition suggesting the ease of realising the valley quantum anomalous Hall (VQAH) state within the photon-dressed Kek-Y. These findings further manifest a non-zero optical valley polarisation which is maximum at the $\Gamma$-point. Our paper thus proposes an optically switchable topological valley filter which is desirous in the evolving landscape of valleytronics.

cond-mat.mes-hall↗

arXiv2023

Magnetoresistive RAM with n-doped AlGaAs/GaAs writing/reading channels

We show that the tunable gate voltage in n-doped AlGaAs/GaAs QW (quantum well) is a key in designing an efficient and ultrafast MRAM (magnetoresistive random access memory). The Rashba spin-orbit coupling in such QWs can be tuned appropriately by the gate voltage to create an intense spin-Hall field which in turns interacts with the ferromagnetic layer of the MRAM through the mechanism of spin orbit torque. The strong spin-Hall field leads to an infinitesimally small switching time of the MRAM. Our proposed MRAM is thus a better alternative to the conventional ferromagnetic/spin-Hall effect bi-layers MRAM for the reason that the switching time can be varied with ease, which is unfeasible in the later. Concisely, not only that this work signals a possibility to design an ultra-fast MRAM, but it also suggests a model to fabricate a tunable switching time MRAM.

cond-mat.mes-hall↗

arXiv2022

Quantum anomalous Hall phase and effective in-plane Lande-g factor in an inverted InAs/GaSb quantum well

The inverted band structure discovered in InAs/GaSb quantum well (QW) is found to host the topological quantum spin Hall (QSH) states. A QSH insulator hosts counterpropagating spin-polarized edge states that are protected by the time-reversal symmetry. The latest experiment reported a robust quantized Hall conductance arising from these QSH states that persists in an in-plane magnetic field as strong as $12$ Tesla. Based on the result of this experiment, we present here a precise calculation of the effective in-plane Lande-\textit{g} factor. We based our calculations on the tight-binding Hamiltonian projected on a square lattice that reproduces a slightly modified Bernevig-Hughes-Zhang (BHZ) Hamiltonian. We also study the topological phase transitions \textit{w.r.t.} a magnetic doping. At suitable doping, one type of spin states penetrate to the bulk of the QW and the system also enters the Quantum Anomalous Hall (QAH) state. We further confirm this through the calculations of quantum Hall conductance which shows a plateau at $e^2/h$ rather than $2e^2/h$ at such a doping state. The paper predicts a certain range of controllable parameters in an inverted QW for enabling a dissipationless charge transport needed for spintronics application.

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