arXiv subjects

Ratnadwip Singha

Publications and source records attributed to Ratnadwip Singha.

36 records · Page 2Linked to original sources

arXiv2021

TaCo$_{2}$Te$_{2}$: An air-stable, magnetic van der Waals material with high mobility

Van der Waals (vdW) materials are an indispensable part of functional device technology due to their versatile physical properties and ease of exfoliating to the low-dimensional limit. Among all the compounds investigated so far, the search for magnetic vdW materials has intensified in recent years, fueled by the realization of magnetism in two dimensions (2D). However, metallic magnetic vdW systems are still uncommon. In addition, they rarely host high-mobility charge carriers, which is an essential requirement for high-speed electronic applications. Another shortcoming of 2D magnets is that they are highly air sensitive. Using chemical reasoning, we introduce TaCo2Te2 as an air-stable, high-mobility, magnetic vdW material. It has a layered structure, which consists of Peierls distorted Co chains and a large vdW gap between the layers. We find that the bulk crystals can be easily exfoliated and the obtained thin flakes are robust to ambient conditions after four months of monitoring using an optical microscope. We also observe signatures of canted antiferromagntic behavior at low-temperature. TaCo2Te2 shows a metallic character and a large, non-saturating, anisotropic magnetoresistance. Furthermore, our Hall data and quantum oscillation measurements reveal the presence of both electron- and hole-type carriers and their high mobility.

cond-mat.mtrl-sci↗

arXiv2021

Evolving Devil's staircase magnetization from tunable charge density waves in nonsymmorphic Dirac semimetals

While several magnetic topological semimetals have been discovered in recent years, their band structures are far from ideal, often obscured by trivial bands at the Fermi energy. Square-net materials with clean, linearly dispersing bands show potential to circumvent this issue. CeSbTe, a square-net material, features multiple magnetic field-controllable topological phases. Here, it is shown that in this material, even higher degrees of tunability can be achieved by changing the electron count at the square-net motif. Increased electron filling results in structural distortion and formation of charge density waves (CDWs). The modulation wave-vector evolves continuously leading to a region of multiple discrete CDWs and a corresponding complex "Devil's staircase" magnetic ground state. A series of fractionally quantized magnetization plateaus are observed, which implies direct coupling between CDW and a collective spin-excitation. It is further shown that the CDW creates a robust idealized non-symmorphic Dirac semimetal, thus providing access to topological systems with rich magnetism.

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↗

arXiv2021

Topological Hall effect in the antiferromagnetic Dirac semimetal EuAgAs

The non-trivial magnetic texture in real space gives rise to the intriguing phenomenon of topological Hall effect (THE), which is relatively less explored in topological semimetals. Here, we report large THE in the antiferromagnetic (AFM) state in single crystals of EuAgAs, an AFM Dirac semimetal. EuAgAs hosts AFM ground state below $T_N$ = 12 K with a weak ferromagnetic component. The in-plane isothermal magnetization below $T_N$ exhibits a weak metamagnetic transition. We also observe chiral anomaly induced positive longitudinal magnetoconductivity which indicates a Weyl fermion state under applied magnetic field. The first-principles calculations reveal that EuAgAs is an AFM Dirac semimetal with a pair of Dirac cones, and therefore, a Weyl semimetailic state can be realized under time-reversal symmetry breaking via an applied magnetic field. Our study establishes that EuAgAs is a novel system for exploiting the interplay of band topology and the topology of the magnetic texture.

cond-mat.str-el↗

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

Effect of hydrostatic pressure on ferromagnetism in two-dimensional CrI$_3$

We have investigated the magnetic properties of highly anisotropic layered ferromagnetic semiconductor CrI$_3$ in presence of hydrostatic pressure ($P$). At ambient pressure, magnetization exhibits a clear anomaly below 212 K along with a thermal hysteresis over a wide temperature range (212-180 K), where a first-order structural transition is observed. CrI$_3$ undergoes a second-order ferromagnetic-paramagnetic phase transition with Curie temperature $T_C$=60.4 K. With application of pressure, the transition becomes sharper and $T_C$ is found to increase from 60.4 to 64.9 K as $P$ increases from 0 to 1.0 GPa. $T_C$ increases with $P$ in a sublinear fashion. The thermal hysteresis in magnetization and the increase of $T_C$ with pressure suggest that the spin and lattice degrees of freedom are coupled. The observed increase in $T_C$ has been explained on the basis of change in inter-layer coupling and Cr-I-Cr bond angle with pressure.

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↗

arXiv2018

Probing lattice dynamics and electron-phonon coupling in topological nodal-line semimetal ZrSiS

Topological materials provide an exclusive platform to study the dynamics of relativistic particles in table-top experiments and offer the possibility of wide-scale technological applications. ZrSiS is a newly discovered topological nodal-line semimetal and has drawn enormous interests. In this report, we have investigated the lattice dynamics and electron-phonon interaction in single crystalline ZrSiS using Raman spectroscopy. Polarization and angle resolved measurements have been performed and the results have been analyzed using crystal symmetries and theoretically calculated atomic vibrational patterns along with phonon dispersion spectra. Wavelength and temperature dependent measurements show the complex interplay of electron and phonon degrees of freedom, resulting in resonant phonon and quasielastic electron scatterings through inter-band transitions. Our high-pressure Raman studies reveal vibrational anomalies, which were further investigated from the high-pressure synchrotron x-ray diffraction (HPXRD) spectra. From HPXRD, we have clearly identified pressure-induced structural transitions and coexistence of multiple phases, which also indicate possible electronic topological transitions in ZrSiS. The present study not only provides the fundamental information on the phonon subsystem, but also sheds some light in understanding the topological nodal-line phase in ZrSiS and other iso-structural systems.

cond-mat.mtrl-sci↗

arXiv2018

Tip-induced Superconductivity Coexisting with Preserved Topological Properties in Line-nodal Semimetal ZrSiS

ZrSiS was recently shown to be a new material with topologically non-trivial band structure which exhibits multiple Dirac nodes and a robust linear band dispersion up to an unusually high energy of 2\,eV. Such a robust linear dispersion makes the topological properties of ZrSiS insensitive to perturbations like carrier doping or lattice distortion. Here we show that a novel superconducting phase with a remarkably high $T_c$ of 7.5\,K can be induced in single crystals of ZrSiS by a non-superconducting metallic tip of Ag. From first-principles calculations we show that the observed superconducting phase might originate from dramatic enhancement of density of states due to the presence of a metallic tip on ZrSiS. Our calculations also show that the emerging tip-induced superconducting phase co-exists with the well preserved topological properties of ZrSiS.

cond-mat.supr-con↗

arXiv2018

Magnetotransport properties and evidence of topological insulating state in LaSbTe

In this report, we present the magnetotransport and magnetization properties of LaSbTe single crystals. Magnetic field-induced turn-on behavior and low-temperature resistivity plateau have been observed. By adopting both metal-semiconductor crossover and Kohler scaling analysis, we have discussed the possible origin of the temperature and magnetic field dependence of resistivity. At 5 K and 9 T, a large, non-saturating transverse magnetoresistance (MR) $\sim$ 5$\times$10$^{3}$ \% has been obtained. The MR shows considerable anisotropy, when the magnetic field is applied along different crystallographic directions. The non-linear field dependence of the Hall resistivity confirms the presence of two types of charge carriers. From the semiclassical two-band fitting of Hall conductivity and longitudinal conductivity, very high carrier mobilities and almost equal electron and hole densities have been deduced, which result in large MR. The Fermi surface properties have been analyzed from de Haas-van Alphen oscillation. From the magnetization measurement, the signature of non-trivial surface state has been detected, which confirms that LaSbTe is a topological insulator, consistent with the earlier first-principles calculations.

cond-mat.mtrl-sci↗

arXiv2017

Probing the Fermi surface and magnetotransport properties in MoAs$_{2}$

Transition metal dipnictides (TMDs) have recently been identified as possible candidates to host topology protected electronic band structure. These materials belong to an isostructural family and show several exotic transport properties. Especially, the large values of magnetoresistance (MR) and carrier mobility have drawn significant attention from the perspective of technological applications. In this report, we have investigated the magnetotransport and Fermi surface properties of single crystalline MoAs$_{2}$, another member of this group of compounds. Field induced resistivity plateau and a large MR have been observed, which are comparable to several topological systems. Interestingly, in contrast to other isostructural materials, the carrier density in MoAs$_{2}$ is quite high and shows single-band dominated transport. The Fermi pockets, which have been identified from the quantum oscillation, are largest among the members of this group and have significant anisotropy with crystallographic direction. Our first-principles calculations reveal a substantial difference between the band structures of MoAs$_{2}$ and other TMDs. The calculated Fermi surface consists of one electron pocket and another 'open-orbit' hole pocket, which has not been observed in TMDs so far.

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↗

arXiv2017

Fermi surface topology and signature of surface Dirac nodes in LaBi

Novel topological state of matter is one of the rapidly growing fields in condensed matter physics research in recent times. While these materials are fascinating from the aspect of fundamental physics of relativistic particles, their exotic transport properties are equally compelling due to the potential technological applications. Extreme magnetoresistance and ultrahigh carrier mobility are two such major hallmarks of topological materials and often used as primary criteria for identifying new compounds belonging to this class. Recently, LaBi has emerged as a new system, which exhibits the above mentioned properties. However, the topological nature of its band structure remains unresolved. Here, using the magnetotransport and magnetization measurements, we have probed the bulk and surface states of LaBi. Similar to earlier reports, extremely large magnetoresistance and high carrier mobility have been observed with compensated electron and hole density. The Fermi surface properties have been analyzed from both Shubnikov-de Haas and de Haas-van Alphen oscillation techniques. In the magnetization measurement, a prominent paramagnetic singularity has been observed, which demonstrates the non-trivial nature of the surface states in LaBi. Our study unambiguously confirms that LaBi is a three-dimensional topological insulator with possible linear dispersion in the gapped bulk band structure.

cond-mat.mtrl-sci↗