arXiv subjects

Subhajit Roychowdhury

Publications and source records attributed to Subhajit Roychowdhury.

10 recordsLinked to original sources

arXiv2026

Magneto-optic phonon resonances in magnetic topological EuCd2As2 via helical Raman spectroscopy

EuCd2As2 materials have two magnetic ordering states: antiferromagnetic (AFM) and ferromagnetic (FM) when their chemical tunability is utilized. While AFM-EuCd2As2 has a nonzero magnetoelectric response due to its symmetry breaking with spin configuration, FM-EuCd2As2 is an ideal candidate for studies of Weyl physics because of its minimum number of Weyl points with opposite chirality. In this article, we examine cryogenic low-frequency Raman spectroscopy of phonon modes in FM-EuCd2As2 crystals using circular polarization configurations, with support from density functional theory calculations, and investigate in-plane magneto-anisotropy by linear polarization configuration below the Curie temperature (Tc = 26 K). We attribute the anomalous enhancements in Raman intensities below the Curie temperature are due to spin-phonon coupling. Furthermore, we see that A-mode peaks can be distinguished by magneto-helical Raman spectroscopy through the magneto-optic effect and that the degree of circular polarization (DCP) of 12.5 meV peak reaches 60% at 4.2 K and becomes saturated. We also examine AFM-EuCd2As2 below Néel temperature (TN = 9 K) to compare with FM-EuCd2As2, but we hardly observe spin-phonon coupling and find negligible DCP values due to almost zero net magnetization. Our results contribute to the understanding of the phonon dynamics and the interplay between topology and magnetism in FM-EuCd2As2, through helical light and external magnetic fields. This lays the foundation for utilizing state-of-the-art Weyl systems for applications in thermoelectrics, phononic devices, and topological quantum computing.

cond-mat.mtrl-sci↗

arXiv2026

Plasmon-driven exciton formation in a non-equilibrium Fermi liquid

Collective modes in Fermi liquids are usually regarded as dissipation channels that relax electronic excitations through Landau damping. Whether such modes can instead mediate the formation of correlated electronic states under non-equilibrium conditions remains an open question. Here we show that, under optical photo-doping, a bulk plasmon can drive correlated inter-band transfer within a transient electronic continuum. Using time- and angle-resolved photoemission spectroscopy (Tr-ARPES) on EuCd$_2$As$_2$ supported by electronic structure calculations, we observe that at high excitation density, plasmons transfer energy from a weakly dispersing bulk band into unoccupied surface states. This bulk-to-surface redistribution stabilizes a long-lived, energy-localized spectral feature consistent with a Mahan exciton. Our results uncover a non-equilibrium regime of Fermi-liquid physics in which collective modes do not merely dissipate energy, but also stabilize correlated bound states.

cond-mat.mes-hall↗

arXiv2026

Anisotropic anomalous Hall effect in distorted kagome GdTi3Bi4

Topological kagome magnets offer a rich landscape for exploring the intricate interplay of quantum interactions among geometry, topology, spin, and correlation. GdTi3Bi4 crystallizes in layered Ti based kagome nets intertwined with zigzag Gd chains along the a axis and orders antiferromagnetically below 15 K. Here, we present the temperature and field dependent electrical transport of GdTi3Bi4 in different directions. The material exhibits anomalous Hall conductivity (AHC) of 410 S cm-1 at 2 K for B parallel c and it is completely absent for B parallel a, despite the similar magnetization observed in both orientations. This behavior is quite contradictory, as anomalous Hall effect (AHE) typically scales with the magnetization. Through first principles calculations, it is demonstrated that in the presence of time reversal symmetry broken by the Gd 4f sublattice and spin orbit coupling, the magnetization direction controls the orbital mixing in the Ti t2g bands, relocating Berry curvature hot spots and producing the observed orientation selective AHC. The results establish GdTi3Bi4 as platform for investigating new avenues of AHE, such as directional AHE, and thus shed new light on the intricate coupling between magnetic and electronic structures, paving the way for exploring novel quantum phenomena.

cond-mat.mtrl-sci↗

arXiv2025

Vector Spin Chirality Switching in Noncollinear Antiferromagnets

Spin chirality provides a powerful route to control magnetic and topological phases in materials, enabling next-generation spintronic and quantum technologies. Coplanar noncollinear antiferromagnets with Kagome lattice spin geometries host vector spin chirality (VSC), the handedness of spin arrangement, and offer an excellent platform for chirality-driven phase control. However, the microscopic mechanisms governing VSC switching and its coupling to magnetic order, electronic structure, and quantum geometry remain elusive, with experimental evidence still lacking. Here, we present conclusive experimental evidence of temperature-driven VSC switching in an archetypal noncollinear antiferromagnetic manganese chromium nitride (Mn3CrN) epitaxial thin films. The VSC switching induces a concomitant quantum-geometric and Lifshitz transition, manifested through a pronounced peak in anomalous Hall conductivity remanence, a metal-insulator-like crossover in longitudinal resistivity, and a distinct evolution of x-ray magnetic circular dichroic signal. The reversal of VSC reconstructs the spin configuration, Fermi surface topology and Berry curvature, marking a unified magnetic-electronic-quantum geometric transition. This emergent behaviour, captured through magneto-transport and magneto-optic measurements, and supported by first-principles theory establish VSC as an active control knob for chirality-driven phase engineering and the design of multifunctional quantum devices.

cond-mat.mtrl-sci↗

arXiv2024

Pressure-induced superconductivity in monoclinic RhBi$_2$

RhBi$_2$ is a polymorphic system that exhibits two distinct phases. RhBi$_2$ in the triclinic phase has been identified as a weak topological insulator with a van Hove singularity point close to the Fermi energy. Thus, triclinic RhBi$_2$ is expected to exhibit exotic quantum properties under strain or pressure. In this study, we report on the emergence of superconductivity in the monoclinic RhBi$_2$ under external pressures. The electrical resistivity behavior of the monoclinic RhBi$_2$ single crystal is studied at a wide range of applied external pressures up to 40 GPa. We observe a pressure-induced superconductivity with a dome-shaped dependence of the critical temperature on pressure at pressures above 10 GPa. A maximum critical temperature ($T_\mathrm{c}$) value of $T_\mathrm{c}$ = 5.1 K is reached at the pressure of 16.1 GPa. Furthermore, we performed detailed ab initio calculations to understand the electronic band structures of monoclinic RhBi$_2$ under varying pressures. The combination of topology and pressure-induced superconductivity in the RhBi$_2$ polymorphic system may provide us with a new promising material platform to investigate topological superconductivity.

cond-mat.supr-con↗

arXiv2024

Revealing the hidden Dirac gap in a topological antiferromagnet using Floquet-Bloch manipulation

Manipulating solids using the time-periodic drive of a laser pulse is a promising route to generate new phases of matter. Whether such `Floquet-Bloch' manipulation can be achieved in topological magnetic systems with disorder has so far been unclear. In this work, we realize Floquet-Bloch manipulation of the Dirac surface-state mass of the topological antiferromagnet (AFM) MnBi$_2$Te$_4$. Using time- and angle-resolved photoemission spectroscopy (tr-ARPES), we show that opposite helicities of mid-infrared circularly polarized light result in substantially different Dirac mass gaps in the AFM phase, despite the equilibrium Dirac cone being massless. We explain our findings in terms of a Dirac fermion with a random mass. Our results underscore Floquet-Bloch manipulation as a powerful tool for controlling topology even in the presence of disorder, and for uncovering properties of materials that may elude conventional probes.

cond-mat.mes-hall↗

arXiv2024

Observation of Chiral Surface State in Superconducting NbGe$_2$

The interplay between topology and superconductivity in quantum materials harbors rich physics ripe for discovery. In this study, we investigate the topological properties and superconductivity of the nonsymmorphic chiral superconductor NbGe$_2$ using high-resolution angle-resolved pho-toemission spectroscopy (ARPES), transport measurements, and ab initio calculations. The ARPES data revealed exotic chiral surface states on the (100) surface originating from the inherent chiral crystal structure. Supporting calculations indicate that NbGe$_2$ likely hosts elusive Weyl fermions in its bulk electronic structure. Furthermore, we uncovered the signatures of van Hove singularities that can enhance many-body interactions. Additionally, transport measurements demonstrated that NbGe$_2$ exhibits superconductivity below 2K. Overall, our comprehensive results provide the first concrete evidence that NbGe$_2$ is a promising platform for investigating the interplay between non-trivial band topology, possible Weyl fermions, van Hove singularities, and superconductivity in chiral quantum materials.

cond-mat.mes-hall↗

arXiv2024

Untangle charge-order dependent bulk states from surface effects in a topological kagome metal ScV$_6$Sn$_6$

Kagome metals with charge density wave (CDW) order exhibit a broad spectrum of intriguing quantum phenomena. The recent discovery of the novel kagome CDW compound ScV$_6$Sn$_6$ has spurred significant interest. However, understanding the interplay between CDW and the bulk electronic structure has been obscured by a profusion of surface states and terminations in this quantum material. Here, we employ photoemission spectroscopy and potassium dosing to elucidate the complete bulk band structure of ScV$_6$Sn$_6$, revealing multiple van Hove singularities near the Fermi level. We surprisingly discover a robust spin-polarized topological Dirac surface resonance state at the M point within the two-fold van Hove singularities. Assisted by the first-principle calculations, the temperature dependence of the $k_z$- resolved ARPES spectrum provides unequivocal evidence for the proposed $\sqrt{3}$$\times$$\sqrt{3}$$\times3$ charge order over other candidates. Our work not only enhances the understanding of the CDW-dependent bulk and surface states in ScV$_6$Sn$_6$ but also establishes an essential foundation for potential manipulation of the CDW order in kagome materials.

cond-mat.str-el↗

arXiv2024

Quantum oscillations revealing topological band in kagome metal ScV6Sn6

Compounds with kagome lattice structure are known to exhibit Dirac cones, flat bands, and van Hove singularities, which host numerous versatile quantum phenomena. Inspired by these intriguing properties, we investigate the temperature and magnetic field dependent electrical transports along with the theoretical calculations of ScV6Sn6, a nonmagnetic charge density wave (CDW) compound. At low temperatures, the compound exhibits Shubnikov-de Haas quantum oscillations, which help to design the Fermi surface (FS) topology. This analysis reveals the existence of several small FSs in the Brillouin zone, combined with a large FS. Among them, the FS possessing Dirac band is a non-trivial and generates a non-zero Berry phase. In addition, the compound also shows the anomalous Hall-like behaviour up to the CDW with the CDW phase, ScV6Sn6 presents a unique material example of the versatile HfFe6Ge6 family and provides various promising opportunities to explore the series further.

cond-mat.mtrl-sci↗

arXiv2016

Unexpected superconductivity at nanoscale junctions made on the topological crystalline insulator Pb$_{0.6}$Sn$_{0.4}$Te

Discovery of exotic phases of matter from the topologically non-trivial systems not only makes the research on topological materials more interesting but also enriches our understanding of the fascinating physics of such materials. Pb$_{0.6}$Sn$_{0.4}$Te was recently shown to be a topological crystalline insulator. Here we show that by forming a mesoscopic point-contact using a normal non-superconducting elemental metal on the surface of Pb$_{0.6}$Sn$_{0.4}$Te a novel superconducting phase is created locally in a confined region under the point-contact. This happens while the bulk of the sample remains to be non-superconducting and the superconducting phase emerges as a nano-droplet under the point-contact. The superconducting phase shows a high transition temperature $T_c$ that varies for different point-contacts and falls in a range between 3.7 K and 6.5 K. Therefore, this Letter presents the discovery of a new superconducting phase on the surface of a topological crystalline insulator and the discovery is expected to shed light on the mechanism of induced superconductivity in topologically non-trivial systems in general.

cond-mat.supr-con↗