arXiv subjects

Bishal Das

Publications and source records attributed to Bishal Das.

6 recordsLinked to original sources

arXiv2026

A Unified Theory of Collective Magnon and Orbiton Excitations in Altermagnets

Altermagnetism has recently emerged as a distinct collinear magnetic phase exhibiting momentum-dependent spin-splitting despite vanishing net magnetization, as a consequence of inequivalent non-magnetic environments. Lately, it has been proposed that strong electronic correlations may yield spontaneous altermagnetism due to orbital ordering even for equivalent non-magnetic environments. While previous studies have largely focused on the electronic structure, a unified understanding of the collective excitations associated with these two different microscopic mechanisms stabilizing altermagnetism remains absent. Here, we develop an extended Kugel'-Khomski\u{i} spin-orbital model on a decorated square lattice that simultaneously incorporates inequivalent non-magnetic environments and correlation-driven orbital ordering within a common theoretical framework. Employing a self-consistent mean-field spin-wave orbital-wave formalism, we demonstrate the emergence of mutually unhybridized but interdependent magnon and orbiton excitations exhibiting characteristic chiral-splitting. We show that the splitting originates from two distinct microscopic contributions: a lattice-dependent term arising from inequivalent non-magnetic environments and an orbital-order-induced exchange-anisotropy term that survives even for equivalent non-magnetic environments. The proposed framework therefore unifies the collective excitations associated with both types of altermagnetism. We further investigate the finite-temperature evolution of the coupled spin-orbital system, revealing the breakdown of spin-wave and orbital-wave approximations through spurious 1st-order transitions, while complementary classical Monte Carlo simulations recover the expected continuous 2nd-order behaviour. Our work establishes a unified microscopic framework for understanding collective magnon and orbiton excitations in altermagnets.

cond-mat.str-el↗

arXiv2026

Topological Weyl Phase of an Ideal Spin-Gapless Semiconductor KCrSe

The coexistence of topological and spin-polarized electronic states within a single material platform provides an attractive route toward emergent quantum phenomena and spintronic functionalities. However, materials simultaneously exhibiting spin-gapless semiconducting (SGS) behavior and Weyl semimetallicity remain exceedingly rare. Here, using first-principles calculations, we identify the half-Heusler compound KCrSe as an ideal spin-gapless Weyl semimetal. Transport calculations reveal a weak temperature dependence of the longitudinal conductivity and relatively small Seebeck coefficients, providing further evidence of its SGS nature. KCrSe hosts a single pair of Weyl nodes-the minimum number permitted in a Weyl semimetal-located in close proximity to the Fermi level (E$_\text{F}$), resulting in exceptionally clean bulk and surface electronic spectra. The nontrivial Berry curvature associated with these Weyl nodes gives rise to sizable anomalous transport responses, including an anomalous Hall conductivity of $\sigma_{xy}^{A}\sim 90.76~\mathrm{S\,cm^{-1}}$ and an anomalous Nernst conductivity of $\alpha_{xy}^{A}\sim 0.15~\mathrm{A\,m^{-1}K^{-1}}$ at E$_\text{F}$, with substantially enhanced values at lower energies. The combination of an ideal Weyl topology, fully spin-polarized low-energy states, and finite anomalous transport establishes KCrSe as a promising platform for designing high-efficiency topological spintronic devices.

cond-mat.mtrl-sci↗

arXiv2026

Topological Surface States and Anisotropic Magnetotransport in SnSb$_6$Te$_{10}$

We have investigated the electronic structure and magnetotransport properties of SnSb$_6$Te$_{10}$ single crystals using density functional theory (DFT), synchrotron-based angle-resolved photoemission spectroscopy (ARPES), and quantum transport measurements. Our DFT calculations reveal a clear spin-orbit coupling driven band inversion between the Sb-$p$ and Te-$p$ states together with a non-trivial $\mathbb{Z}_2$ topological invariant. The calculated surface-state dispersion and hexagonally warped Fermi surface contours agree well with the ARPES measurements. Temperature-dependent transport measurements indicate dominant electron-phonon scattering, while Hall measurements confirm hole-type carriers with carrier density of the order of $10^{21}$ cm$^{-3}$. Both transverse and longitudinal magnetotransport exhibit weak antilocalization behavior, while Shubnikov-de Haas oscillations observed for $H \parallel c$ yield a Berry phase close to $\pi$, consistent with Dirac-like surface states. Furthermore, angle-dependent magnetotransport measurements reveal pronounced anisotropy associated with an anisotropic Fermi surface topology and mixed bulk-surface transport behavior. Our combined theoretical and experimental results establish SnSb$_6$Te$_{10}$ as a strong topological insulator and a promising platform for investigating topological transport phenomena in layered telluride systems.

cond-mat.mtrl-sci↗

arXiv2025

The Fundamental Lemma of Altermagnetism: Emergence of Alterferrimagnetism

Recent years have seen a proliferation in investigations on Altermagnetism due to its exciting prospects both from an applications perspective and theoretical standpoint. Traditionally, altermagnets are distinguished from collinear antiferromagnets using the central concept of halving subgroups within the spin space group formalism. In this work, we propose the Fundamental Lemma of Altermagnetism (FLAM) deriving the exact conditions required for the existence of altermagnetic phase in a magnetic material on the basis of site-symmetry groups and halving subgroups for a given crystallographic space group. The spin group formalism further clubs ferrimagnetism with ferromagnetism since the same-spin and opposite-spin sublattices lose their meaning in the presence of multiple magnetic species. As a consequence of FLAM, we further propose a class of fully compensated ferrimagnets, termed as Alterferrimagnets (AFiMs), which can show alternating momentum-dependent spin-polarized non-relativistic electronic bands within the first Brillouin zone. We show that alterferrimagnetism is a generalization of traditional collinear altermagnetism where multiple magnetic species are allowed to coexist forming fully compensated magnetic-sublattices, each with individual up-spin and down-spin sublattices.

cond-mat.mtrl-sci↗

arXiv2025

Intrinsic Berry Curvature Driven Anomalous Hall and Nernst Effect in Co$_2$MnSn

Magnetic topological semimetals often exhibit unusual electronic and thermal transport due to nontrivial bulk band crossings, enabling simultaneous realization of large anomalous Hall and Nernst conductivities ($\sigma_{xy}$ and $\alpha_{xy}$). Here, a comprehensive experimental and theoretical study of the anomalous transport properties of ferromagnetic Co$_2$MnSn is reported. First-principles calculations reveal topological Weyl points producing significant Berry curvature, driving dominant intrinsic anomalous Hall/Nernst effects. Electronic and thermal transport measurements demonstrate robust anomalous transport with substantial conductivity values that persist at room temperature ($\sigma_{xy}\sim$ 500 S/cm, $\alpha_{xy}\sim$ 1.3 A/m/K). We also show how the chemical substitution (via tuning Fermi level) can boost these effects (up to $\sigma_{xy}\sim$ 1376 S/cm, $\alpha_{xy}\sim$ 1.49 A/m/K at 150 K). These findings position Co$_2$MnSn as a compelling platform for exploring topological transport phenomena and advancing next-generation thermoelectric and spintronic technologies.

cond-mat.mtrl-sci↗

arXiv2023

GdAlSi: An antiferromagnetic topological Weyl semimetal with non-relativistic spin splitting

Spintronics has emerged as a viable alternative to traditional electronics based technologies in the past few decades. While on one hand, the discovery of topological phases of matter with protected spin-polarized states has opened up exciting prospects, recent revelation of intriguing non-relativistic spin splitting in collinear antiferromagnetic materials with unique symmetries facilitate a wide possibility of realizing both these features simultaneously. In this work, we report the co-existence of these two intriguing properties within a single material: GdAlSi. It crystallizes in a body-centered tetragonal structure with a non-centrosymmetric space group $I4_{1}md$ ($109$), which is confirmed using detailed structural analysis through X-ray diffraction (XRD) and optical second harmonic generation (SHG) measurements. The magnetization data indicates AFM ordering with an ordering temperature ($T_N$) $\sim$ 32 K. Ab-initio calculations reveal GdAlSi to be a collinear antiferromagnetic Weyl semimetal with an unconventional, momentum-dependent spin splitting, also referred to as altermagnet. Angle-resolved photoemission spectroscopy measurements on GdAlSi single crystals subsequently confirm the presence of Fermi arcs, a distinctive hallmark of Weyl semimetals. Electric and magnetic multipole analysis provides a deeper understanding of the symmetry-mediated, momentum-dependent spin splitting, which has strictly non-relativistic origin. To the best of our knowledge, such co-existence of unconventional antiferromagnetic order and non-trivial topology is unprecedented and has never been observed before in a single material, rendering GdAlSi a special and promising candidate material. We propose a device harnessing these features, poised to enable practical and efficient topotronic applications.

cond-mat.str-el↗