arXiv subjects

Shubhankar Roy

Publications and source records attributed to Shubhankar Roy.

14 recordsLinked to original sources

arXiv2026

Unconventional topological Hall response and anisotropic magnetotransport properties of a helical magnet EuAuAs

Topological magnets with nontrivial spin texture have attracted considerable interest because they display a rich spectrum of emergent quantum phenomena. Here, we present a combined experimental and theoretical investigation of the magnetic and magnetotransport properties of EuAuAs, an antiferromagnet with N\'eel temperature ($T_\mathrm{N}$) $\sim$ 6 K. The temperature and magnetic field dependence of electrical resistivity and magnetization demonstrate that the charge transport in EuAuAs is strongly influenced by the spin configuration of local Eu moments. Below $T_\mathrm{N}$, both longitudinal magnetoresistance (LMR) and transverse magnetoresistance (TMR) are positive at low fields but large and negative at high fields. With increasing temperature, TMR becomes positive above 60 K, whereas LMR remains negative up to 100 K. The low-field positive LMR and TMR originate from weak antilocalization (WAL). The WAL contribution in TMR is well captured by the Hikami-Larkin-Nagaoka model, whereas the LMR data are described by a generalized Altshuler-Aronov framework. Moreover, we observe a giant topological Hall effect arising from the scalar spin chirality, which is further supported by the helical magnetic structure obtained from the ab-initio calculations. The observed anisotropy in longitudinal resistivity and magnetoresistance underscores the very nature of the Fermi surface of the EuAuAs, as confirmed by first-principles calculations. These results establish EuAuAs as a unique platform for exploring the interplay between electronic structure and noncoplanar spin texture in a centrosymmetric helical magnet.

cond-mat.mtrl-sci↗

arXiv2026

Topological and Planar Hall Effect in Monoclinic van der Waals Ferromagnet NbFeTe$_2$

Two-dimensional (2D) van der Waals (vdW) ferromagnets have emerged as a critical class of quantum materials for next-generation, low-dimensional spintronic devices. In this study, we report a comprehensive study of the transport properties of the layered soft ferromagnet $\text{NbFeTe}_2$. We report the first observation of the topological Hall effect (THE) and the planar Hall effect (PHE) in metallic $\text{NbFeTe}_2$. THE signatures persist up to 45 K, while PHE remains evident well above Curie temperature ($T_C$). The observed negative longitudinal magnetoresistance, along with the PHE, provides strong evidence for a nontrivial electronic band structure. The coexistence of perpendicular magnetic anisotropy and a substantial THE: two key properties that are highly desirable for future spintronics applications, makes monoclinic vdW ferromagnetic $\text{NbFeTe}_2$ a promising platform to advance spintronics applications.

cond-mat.supr-con↗

arXiv2024

Kramers nodal line in the charge density wave state of YTe$_3$ and the influence of twin domains

Recent studies have focused on the relationship between charge density wave (CDW) collective electronic ground states and nontrivial topological states. Using angle-resolved photoemission and density functional theory, we establish that YTe$_3$ is a CDW-induced Kramers nodal line (KNL) metal, a newly proposed topological state of matter. YTe$_3$ is a non-magnetic quasi-2D chalcogenide with a CDW wave vector ($q_{\rm cdw}$) of 0.2907c$^*$. Scanning tunneling microscopy and low energy electron diffraction revealed two orthogonal CDW domains, each with a unidirectional CDW and similar YTe$_3$. The effective band structure (EBS) computations, using DFT-calculated folded bands, show excellent agreement with ARPES because a realistic x-ray crystal structure and twin domains are considered in the calculations. The Fermi surface and ARPES intensity plots show weak shadow bands displaced by $q_{\rm cdw}$ from the main bands. These are linked to CDW modulation, as the EBS calculation confirms. Bilayer split main and shadow bands suggest the existence of crossings, according to theory and experiment. DFT bands, including spin-orbit coupling, indicate a nodal line along the $\Sigma$ line from multiple band crossings perpendicular to the KNL. Additionally, doubly degenerate bands are only found along the KNL at all energies, with some bands dispersing through the Fermi level.

cond-mat.mtrl-sci↗

arXiv2023

Detection of nontrivial topology driven by charge density wave in a semi-Dirac metal

The presence of electron correlations in a system with topological order can lead to exotic ground states. Considering single crystals of LaAgSb2 which has a square net crystal structure, one finds multiple charge density wave transitions (CDW) as the temperature is lowered. We find large planar Hall (PHE) signals in the CDW phase, which are still finite in the high temperature phase though they change sign. Optimising the structure within first-principles calculations, one finds an unusual chiral metallic phase. This is because as the temperature is lowered, the electrons on the Ag atoms get more localized, leading to stronger repulsions between electrons associated with atoms on different layers. This leads to successive layers sliding with respect to each other, thereby stabilising a chiral structure in which inversion symmetry is also broken. The large Berry curvature associated with the low temperature structure explains the low temperature PHE. At high temperature the PHE arises from the changes induced in the tilted Dirac cone in a magnetic field. Our work represents a route towards detecting and understanding the mechanism in a correlation driven topological transition through electron transport measurements, complemented by ab-initio electronic structure calculations.

cond-mat.mtrl-sci↗

arXiv2022

Charge density wave induced nodal lines in LaTe$_3$

LaTe$_3$ is a noncentrosymmetric (NC) material with time reversal (TR) symmetry in which the charge density wave (CDW) is hosted by the Te bilayers. Here, we show that LaTe$_3$ hosts a Kramers nodal line (KNL), a twofold degenerate nodal line that connects the TR invariant momenta in NC achiral systems, using angle resolved photoemission spectroscopy (ARPES), density functional theory (DFT), effective band structure (EBS) calculated by band unfolding, and symmetry arguments. DFT incorporating spin-orbit coupling (SOC) reveals that the KNL -- protected by the TR and lattice symmetries -- imposes gapless crossings between the bilayer-split CDW-induced shadow bands and the main bands. In excellent agreement with the EBS, ARPES data corroborate the presence of the KNL and show that the crossings traverse the Fermi level. Furthermore, spinless nodal lines - entirely gapped out by the SOC - are formed by the linear crossings of the shadow and main bands with a high Fermi velocity.

cond-mat.mtrl-sci↗

arXiv2021

Signature of topological non-trivial band structure in Ta$_{3}$SiTe$_{6}$

The study of topology protected electronic properties is a fascinating topic in present day condensed matter physics research. New topological materials are frequently being proposed and explored through various experimental techniques. Ta$_{3}$SiTe$_{6}$ is a newly predicted topological semimetal with fourfold degenerate nodal-line crossing in absence of spin-orbit coupling (SOC) and an hourglass Dirac loop, when SOC is included. Recent angle-resolved photoemission spectroscopy study in this material, has also confirmed Dirac like dispersions and two nodal-lines near the Fermi energy, protected by nonsymmorphic glide mirror symmetry. In this work, we present the detailed magnetotransport properties of single crystalline Ta$_{3}$SiTe$_{6}$. A nonsaturating magnetoresistance has been observed. Hall measurements reveal hole type charge carriers with high carrier density and a moderate value of carrier mobility. Furthermore, we report a robust planar Hall effect, which persists up to high temperatures. These results validate the nontrivial nature of the electronic band structure.

cond-mat.mtrl-sci↗

arXiv2020

Lattice dynamics of topological Dirac semimetal LaAgSb$_{2}$ with charge density wave ordering

LaAgSb$_{2}$ is a rare material, which offers the opportunity to investigate the complex interplay between charge density wave (CDW) ordering and topology protected electronic band structure. As both of these phenomena are governed by the structural symmetries, a comprehensive study of the lattice dynamics is highly desirable. In this report, we present the results of temperature and pressure dependent Raman spectroscopy and x-ray diffraction in single crystalline LaAgSb$_{2}$. Our results confirm that Raman spectroscopy is a highly sensitive tool to probe CDW ordering phenomenon, particularly the low-temperature second CDW transition in LaAgSb$_{2}$, which appears as a very weak anomaly in most experiments. The crystal orientation-dependent measurements provide the evolution of Raman modes with crystallographic symmetries and can be further studied through group symmetry analysis. The low-temperature x-ray diffraction data show the emergence of structural modulations corresponding to the CDW instability. The combined high-pressure Raman spectroscopy and synchrotron x-ray diffraction reveal multiple structural phase transitions through lowering of crystalline symmetries, which are also expected to lead to electronic topological transitions.

cond-mat.mtrl-sci↗

arXiv2020

Anomalous Hall effect in half-metallic Heusler compound Co$_{2}$Ti$X$ ($X$=Si, Ge)

Though Weyl fermions have recently been observed in several materials with broken inversion symmetry, there are very few examples of such systems with broken time reversal symmetry. Various Co$_{2}$-based half-metallic ferromagnetic Heusler compounds are lately predicted to host Weyl type excitations in their band structure. These magnetic Heusler compounds with broken time reversal symmetry are expected to show a large momentum space Berry curvature, which introduces several exotic magneto-transport properties. In this report, we present systematic analysis of experimental results on anomalous Hall effect (AHE) in Co$_2$Ti$X$ ($X$=Si and Ge). This study is an attempt to understand the role of Berry curvature on AHE in Co$_2$Ti$X$ family of materials. The anomalous Hall resistivity is observed to scale quadratically with the longitudinal resistivity for both the compounds. The detailed analysis indicates that in anomalous Hall conductivity, the intrinsic Karplus-Luttinger Berry phase mechanism dominates over the extrinsic skew scattering and side-jump mechanism.

cond-mat.mtrl-sci↗

arXiv2020

Fascinating interplay between Charge Density Wave Order and magnetic field in Non-magnetic Rare-Earth Tritelluride LaTe$_{3}$

Charge density wave (CDW) states in solids bear an intimate connection to underlying fermiology. Modification of the latter by a suitable perturbation provides an attractive handle to unearth novel CDW states. Here, we combine extensive magnetotransport experiments and first-principles electronic structure calculations on a non-magnetic tritelluride LaTe$_{3}$ single crystal to uncover phenomena rare in CDW systems: $(i)$ hump-like feature in the temperature dependence of resistivity at low temperature under application of magnetic field, which moves to higher temperature with increasing field strength, $(ii)$ highly anisotropic large transverse magnetoresistance (MR) upon rotation of magnetic field about current parallel to crystallographic c-axis, (iii) anomalously large positive MR with spike-like peaks at characteristic angles when the angle between current and field is varied in the bc-plane, (iv) extreme sensitivity of the angular variation of MR on field and temperature. Moreover, our Hall measurement reveals remarkably high carrier mobility $\sim$ 33000 cm$^{2}$/Vs, which is comparable to that observed in some topological semimetals. These novel observations find a comprehensive explication in our density functional theory (DFT) and dynamical mean field theory (DMFT) calculations that capture field-induced electronic structure modification in LaTe$_{3}$. The band structure theory together with transport calculations suggest the possibility of a second field-induced CDW transition from the field-reconstructed Fermi surface, which qualitatively explains the hump in temperature dependence of resistivity at low temperature. Thus, our study exposes the novel manifestations of the interplay between CDW order and field-induced electronic structure modifications in LaTe$_{3}$, and establishes a new route to tune CDW states by perturbations like magnetic field.

cond-mat.mtrl-sci↗

arXiv2019

Complex exchange mechanism driven ferromagnetism in half-metallic Heusler Co$_{2}$TiGe: Evidence from critical behavior

We have investigated the critical phenomenon associated with the magnetic phase transition in the half-metallic full-Heusler Co$_2$TiGe. The compound undergoes a continuous ferromagnetic to paramagnetic phase transition at the Curie temperature $T_{C}$=371.5 K. The analysis of magnetization isotherms in the vicinity of $T_{c}$, following modified Arrott plot method, Kouvel-Fisher technique, and critical isotherm plot, yields the asymptotic critical exponents $β$=0.495, $γ$=1.324, and $δ$=3.67. The self-consistency and reliability of the obtained exponents are further verified by the Widom scaling relation and scaling equation of states. The mean-field-like value of the critical exponent $β$ suggests long-range nature of the exchange interactions, whereas the values of the critical exponents $γ$ and $δ$, imply sizeable critical spin fluctuations. The half-metallic itinerant character of Co$_{2}$TiGe in the presence of magnetic inhomogeneity may result in such a strong deviation from the three-dimensional Heisenberg values ($β$=0.369, $γ$=1.38 and $δ$=4.8) of the critical exponents towards the mean field values ($β$=0.5, $γ$=1 and $δ$=3). The results suggest complex nature of exchange couplings that stabilize the long-range ferromagnetic ordering in the system and are consistent with the earlier theoretical studies on the exchange mechanism in Co$_2$TiGe.

cond-mat.mtrl-sci↗

arXiv2019

Magnetotransport properties and giant anomalous Hall angle in half-Heusler compound TbPtBi

Magnetic lanthanide half-Heuslers ($R$PtBi; $R$ being the lanthanide) represent an attractive subgroup of the Heusler family and have been identified as ideal candidates for time reversal symmetry breaking topological Weyl semimetals. In this paper, we present the detailed analysis of the magnetotransport properties of frustrated antiferromagnet TbPtBi. This material shows large, non-saturating magnetoresistance (MR) with unusual magnetic field dependence. The MR of TbPtBi is significantly anisotropic with respect to the magnetic field, applied along different crystallographic directions and indicates the anisotropic nature of the Fermi surface. The chiral anomaly induced negative longitudinal magnetoresistance confirms the presence of Weyl fermions. At low temperature, Berry phase driven large anomalous Hall conductivity has been observed. The calculated anomalous Hall angle is the largest reported so far.

cond-mat.mtrl-sci↗

arXiv2018

Planar Hall effect in type II Dirac semimetal VAl$_{3}$

The study of electronic properties in topological systems is one of the most fascinating topics in condensed matter physics, which has generated enormous interests in recent times. New materials are frequently being proposed and investigated to identify their non-trivial band structure. While sophisticated techniques such as angle-resolved photoemission spectroscopy have become popular to map the energy-momentum relation, the transport experiments lack any direct confirmation of Dirac and Weyl fermions in a system. From band structure calculations, VAl$_{3}$ has been proposed to be a type II topological Dirac semimetal. This material represents a large family of isostructural compounds, all having similar electronic band structure and is an ideal system to explore the rich physics of Lorentz symmetry violating Dirac fermions. In this work, we present a detailed analysis on the magnetotransport properties of VAl$_{3}$. A large, non-saturating magnetoresistance has been observed. Hall resistivity reveals the presence of two types of charge carriers with high mobility. Our measurements show a large planar Hall effect in this material, which is robust and can be easily detectable up to high temperature. This phenomenon originates from the relativistic chiral anomaly and non-trivial Berry curvature, which validates the theoretical prediction of the Dirac semimetal phase in VAl$_{3}$.

cond-mat.mtrl-sci↗

arXiv2018

Magneto-transport properties of proposed triply degenerate topological semimetal Pd$_{3}$Bi$_{2}$S$_{2}$

We report transport properties of single-crystalline Pd$_{3}$Bi$_{2}$S$_{2}$, which has been predicted to host an unconventional electronic phase of matter beyond three-dimensional Dirac and Weyl semimetals. Similar to several topological systems, the resistivity shows field induced metal-semiconductor-like crossover at low temperature. Large, anisotropic and non-saturating magnetoresistance (MR) has been observed in transverse experimental configuration. At 2 K and 9 T, the MR value reaches as high as $\sim$1.1$\times$10$^{3}$ \%. Hall resistivity reveals the presence of two types of charge carriers and has been analyzed using two-band model. In spite of the large density ($>$ 10$^{21}$ cm$^{-3}$), the mobility of charge carriers is found to be quite high ($\sim$ 0.75$\times$10$^{4}$ cm$^{2}$ V$^{-1}$ s$^{-1}$ for hole and $\sim$ 0.3$\times$10$^{4}$ cm$^{2}$ V$^{-1}$ s$^{-1}$ for electron). The observed magneto-electrical properties indicate that Pd$_{3}$Bi$_{2}$S$_{2}$ may be a new member of the topological semimetal family, which can have a significant impact in technological applications.

cond-mat.mtrl-sci↗

arXiv2017

Anisotropic Fermi surface probed by the de Haas-van Alphen oscillation in proposed Dirac Semimetal TaSb$_{2}$

TaSb$_{2}$ has been predicted theoretically and proposed through magnetotransport experiment to be a topological semimetal. In earlier reports, the Shubnikov-de Haas oscillation has been analyzed to probe the Fermi surface, with magnetic field along a particular crystallographic axis only. By employing a sample rotator, we reveal highly anisotropic transverse magnetoresistance by rotating the magnetic field along different crystallographic directions. To probe the anisotropy in the Fermi surface, we have performed magnetization measurements and detected strong de Haas-van Alphen (dHvA) oscillations for the magnetic field applied along \textbf{b} and \textbf{c} axes as well as perpendicular to \textbf{bc} plane of the crystals. Three Fermi pockets have been identified by analyzing the dHvA oscillations. Hall measurement reveals electron as the only charge carrier, i.e., all the three Fermi pockets are electron type. With the application of magnetic field along different crystal directions, the cross sectional areas of the Fermi pockets have been found significantly different. Other physical parameters, such as the effective mass of the charge carrier and Fermi velocity have also been calculated using the Lifshitz-Kosevich formula.

cond-mat.mtrl-sci↗