arXiv subjects

Arun Bansil

Publications and source records attributed to Arun Bansil.

At least 127 records · Page 7Linked to original sources

arXiv2020

$f$-electron and magnetic ordering effects in nickelates LaNiO$_2$ and NdNiO$_2$: remarkable role of the cuprate-like $3d_{x^2-y^2}$ band

Recent discovery of superconductivity in the doped infinite-layer nickelates has renewed interest in understanding the nature of high-temperature superconductivity more generally. The low-energy electronic structure of the parent compound NdNiO$_{2}$, the role of electronic correlations in driving superconductivity, and the possible relationship betweeen the cuprates and the nickelates are still open questions. Here, by comparing LaNiO$_2$ and NdNiO$_2$ systematically within a parameter-free density functional framework, all-electron first-principles framework, we reveal the role Nd 4$f$ electrons in shaping the ground state of pristine NdNiO$_2$. Strong similarities are found between the electronic structures of LaNiO$_2$ and NdNiO$_2$, except for the effects of the 4$f$-electrons. Hybridization between the Nd 4$f$ and Ni 3$d$ orbitals is shown to significantly modify the Fermi surfaces of various magnetic states. In contrast, the competition between the magnetically ordered phases depends mainly on the gaps in the Ni $d_{x2-y2}$ band, so that the ground state in LaNiO$_2$ and NdNiO$_2$ turns out to be striking similarity to that of the cuprates. The $d-p$ band-splitting is found to be much larger while the intralayer 3$d$ ion-exchange coupling is smaller in the nickelates compared to the cuprates. Our estimated value of the on-site Hubbard $U$ is similar to that in the cuprates, but the value of the Hund's coupling $J_H$ is found to be sensitive to the Nd magnetic moment. The exchange coupling $J$ in NdNiO$_2$ is only half as large as in the curpates, which may explain why $T_c$ in the nickelates is half as large as the cuprates.

cond-mat.str-el↗

arXiv2020

Observation of gapped state in rare-earth monopnictide HoSb

The rare-earth monopnictide family is attracting an intense current interest driven by its unusual extreme magnetoresistance (XMR) property and the potential presence of topologically non-trivial surface states. The experimental observation of non-trivial surface states in this family of materials are not ubiquitous. Here, using high-resolution angle-resolved photoemission spectroscopy (ARPES), magnetotransport, and parallel first-principles modeling, we examine the nature of electronic states in HoSb. Although we find the presence of bulk band gaps at the G and X-symmetry points of the Brillouin zone (BZ), we do not find these gaps to exhibit band inversion so that HoSb does not host a Dirac semimetal state. Our magnetotransport measurements indicate that HoSb can be characterized as a correlated nearly-complete electron-hole-compensated semimetal. Our analysis reveals that the nearly perfect electron-hole compensation could drive the appearance of non-saturating XMR effect in HoSb.

cond-mat.mes-hall↗

arXiv2020

Saddle-point von Hove singularity and dual topological insulator state in Pt$_2$HgSe$_3$

Saddle-point van Hove singularities in the topological surface states are interesting because they can provide a new pathway for accessing exotic correlated phenomena in topological materials. Here, based on first-principles calculations combined with a $\mathbf {k \cdot p}$ model Hamiltonian analysis, we show that the layered platinum mineral jacutingaite (Pt$_2$HgSe$_3$) harbours saddle-like topological surface states with associated van Hove singularities. Pt$_2$HgSe$_3$ is shown to host two distinct types of nodal lines without spin-orbit coupling (SOC) which are protected by combined inversion ($I$) and time-reversal ($T$) symmetries. Switching on the SOC gaps out the nodal lines and drives the system into a topological insulator state with nonzero weak topological invariant $Z_2=(0;001)$ and mirror Chern number $n_M=2$. Surface states on the naturally cleaved (001) surface are found to be nontrivial with a unique saddle-like energy dispersion with type II van Hove singularities. We also discuss how modulating the crystal structure can drive Pt$_2$HgSe$_3$ into a Dirac semimetal state with a pair of Dirac points. Our results indicate that Pt$_2$HgSe$_3$ is an ideal candidate material for exploring the properties of topological insulators with saddle-like surface states.

cond-mat.mes-hall↗

arXiv2020

Magnetotransport properties of the topological nodal-line semimetal CaCdSn

Topological nodal-line semimetals support protected band crossings which form nodal lines or nodal loops between the valence and conduction bands and exhibit novel transport phenomena. Here we address the topological state of the nodal-line semimetal candidate material, CaCdSn, and report magnetotransport properties of its single crystals grown by the self-flux method. Our first-principles calculations show that the electronic structure of CaCdSn harbors a single nodal loop around the $Γ$ point in the absence of spin-orbit coupling (SOC) effects. The nodal crossings in CaCdSn are found to lie above the Fermi level and yield a Fermi surface that consists of both electron and hole pockets. CaCdSn exhibits high mobility ($μ\approx 3.44\times 10^4$ cm$^2$V$^{-1}$s$^{-1}$) and displays a field-induced metal-semiconductor like crossover with a plateau in resistivity at low temperature. We observe an extremely large and quasilinear non-saturating transverse as well as longitudinal magnetoresistance (MR) at low temperatures ($\approx 7.44\times 10^3 \%$ and $\approx 1.71\times 10^3\%$, respectively, at 4K). We also briefly discuss possible reasons behind such a large quasilinear magnetoresistance and its connection with the nontrivial band structure of CaCdSn.

cond-mat.mtrl-sci↗

arXiv2020

K$_2$CoS$_2$: A new two-dimensional in-plane antiferromagnetic insulator

The recent discovery of two-dimensional (2D) magnetic materials has brought magnetism to the flatland. This has opened up exciting opportunities for the exploration of fundamental physics as well as for novel device applications. Here, we predict a new thermodynamically stable 2D magnetic material, K$_2$CoS$_2$, which retains its in-plane anti-ferromagnetic order down to the monolayer and bilayer limits. We find that the magnetic moments ($2.5 μ_B/$Co) are aligned along the intra-Co chains, from monolayer to bulk. The non-magnetic electronic spectrum of both the monolayer and bilayer films is found to host flat bands and van-Hove singularities, which in instrumental in giving rise to the the magnetic ground state. Based on classical Monte-Carlo simulations, we estimate the Neel temperature for the antiferromagnetic monolayer to be $\approx 15$K. Our study thus establishes that K$_2$CoS$_2$ hosts a robust antiferromagnetic state which persists from the monolayer limit to the bulk material.

cond-mat.mes-hall↗

arXiv2020

Topological Dirac Semimetal Phase in Bismuth Based Anode Materials for Sodium-Ion Batteries

Bismuth has recently attracted interest in connection with Na-ion battery anodes due to its high volumetric capacity. It reacts with Na to form Na$_3$Bi which is a prototypical Dirac semimetal with a nontrivial electronic structure. Density-functional-theory based first-principles calculations are playing a key role in understanding the fascinating electronic structure of Na$_3$Bi and other topological materials. In particular, the strongly-constrained-and-appropriately-normed (SCAN) meta-generalized-gradient-approximation (meta-GGA) has shown significant improvement over the widely used generalized-gradient-approximation (GGA) scheme in capturing energetic, structural, and electronic properties of many classes of materials. Here, we discuss the electronic structure of Na$_3$Bi within the SCAN framework and show that the resulting Fermi velocities and {\it s}-band shift around the $Γ$ point are in better agreement with experiments than the corresponding GGA predictions. SCAN yields a purely spin-orbit-coupling (SOC) driven Dirac semimetal state in Na$_3$Bi in contrast with the earlier GGA results. Our analysis reveals the presence of a topological phase transition from the Dirac semimetal to a trivial band insulator phase in Na$_{3}$Bi$_{x}$Sb$_{1-x}$ alloys as the strength of the SOC varies with Sb content, and gives insight into the role of the SOC in modulating conduction properties of Na$_3$Bi.

cond-mat.mtrl-sci↗

arXiv2020

Superconducting and Antiferromagnetic Properties of Dual-Phase V$_3$Ga

The binary compound V$_3$Ga can exhibit two near-equilibrium phases, consisting of the A15 structure that is superconducting, and the Heusler D0$_3$ structure that is semiconducting and antiferromagnetic. Density functional theory calculations show that the two phases are closely degenerate, being separated by only ~10 meV/atom. Magnetization measurements on bulk-grown samples show superconducting behavior below 14 K. These results indicate the possibility of using V$_3$Ga for quantum technology devices utilizing both superconductivity and antiferromagnetism at the same temperature.

cond-mat.mtrl-sci↗

arXiv2020

Two-dimensional MX family of Dirac materials with tunable electronic and topological properties

We propose a novel class of two-dimensional (2D) Dirac materials in the MX family (M=Be, Mg, Zn and Cd, X = Cl, Br and I), which exhibit graphene-like band structures with linearly-dispersing Dirac-cone states over large energy scales (0.8~1.8 eV) and ultra-high Fermi velocities comparable to graphene. The electronic and topological properties are found to be highly tunable and amenable to effective modulation via anion-layer substitution and vertical electric field. The electronic structures of several members of the family are shown to host a Van-Hove singularity (VHS) close to the energy of the Dirac node. The enhanced density-of-states associated with these VHSs could provide a mechanism for inducing topological superconductivity. The presence of sizable band gaps, ultra-high carrier mobilities, and small effective masses makes the MX family promising for electronics and spintronics applications.

cond-mat.mtrl-sci↗

arXiv2020

First-Principles Calculation of Spin and Orbital Contributions to Magnetically Ordered Moments in Sr$_{2}$IrO$_{4}$

We show how an accurate first-principles treatment of the canted-antiferromagnetic ground state of Sr$_2$IrO$_4$, a prototypical $5d$ correlated spin-orbit coupled material, can be obtained without invoking any free parameters such as the Hubbard U or tuning the spin-orbit coupling strength. Our theoretically predicted iridium magnetic moment of 0.250 $μ_B$, canted by 12.6$^{\circ}$ off the a-axis, is in accord with experimental results. By resolving the magnetic moments into their spin and orbital components, we show that our theoretically obtained variation of the magnetic scattering amplitude $ $ as a function of the polarization angle is consistent with recent non-resonant magnetic x-ray scattering measurements. The computed value of the band gap (55 meV) is also in line with the corresponding experimental values. A comparison of the band structure to that of the cuprates suggests the presence of incommensurate charge-density wave phases in Sr$_{2}$IrO$_{4}$.

cond-mat.str-el↗

arXiv2020

Discovery of topological Weyl fermion lines and drumhead surface states in a room temperature magnet

Topological matter is known to exhibit unconventional surface states and anomalous transport owing to unusual bulk electronic topology. In this study, we use photoemission spectroscopy and quantum transport to elucidate the topology of the room temperature magnet Co$_2$MnGa. We observe sharp bulk Weyl fermion line dispersions indicative of nontrivial topological invariants present in the magnetic phase. On the surface of the magnet, we observe electronic wave functions that take the form of drumheads, enabling us to directly visualize the crucial components of the bulk-boundary topological correspondence. By considering the Berry curvature field associated with the observed topological Weyl fermion lines, we quantitatively account for the giant anomalous Hall response observed in our samples. Our experimental results suggest a rich interplay of strongly correlated electrons and topology in this quantum magnet.

cond-mat.mtrl-sci↗

arXiv2020

Exceptionally large anomalous Hall effect due to anticrossing of spin-split bands in the antiferromagnetic half-Heusler compound TbPtBi

We have investigated magnetotransport properties and the topological electronic structure of the half-Heusler compound TbPtBi. Our experiments reveal an exceptionally large anomalous Hall effect (AHE) in the canted antiferromagnetic state of TbPtBi with the anomalous Hall angle (AHA) reaching ~0.68-0.76, which is a few times larger than the previously reported record in GdPtBi. First-principles electronic structure and the associated anomalous Hall conductivity were computed in order to interpret the experimental results. Our analysis shows that the AHE in TbPtBi does not originate from the Weyl points but that it is driven by the large net Berry curvature produced by the anticrossing of spin-split bands near the Fermi level in TbPtBi.

cond-mat.mes-hall↗

arXiv2020

Correlation in noncollinear antiferromagnetic $α$-Mn

We have investigated the interplay between magnetic and structural degrees of freedom in elemental Mn. The equilibrium volume is shown to depend critically on the magnetic interactions between the Mn atoms. While the standard generalized-gradient-approximation underestimates the equilibrium volume, a more accurate treatment of the effects of electronic localization and magnetism is found to solve this longstanding problem. We capture well the complexity of the large 58 atoms per unit cell $α$-Mn system for the first time, including its charge and spin patterns and the canting of spins with respect to the average magnetization direction.

cond-mat.mtrl-sci↗

arXiv2019

Identification of ferrimagnetic orbitals preventing spinel degradation by charge ordering in Li$_x$Mn$_2$O$_4$

Spinel Li$_x$Mn$_2$O$_4$ is a key cathode material that is used extensively in commercial Li-ion batteries. A challenge with this material has been that the capacity of the battery fades with cycling, an effect that can be traced to the presence of an anti-ferromagnetic insulator phase in the fully lithiated LiMn$_2$O$_4$ (LMO) and the associated charge disproportionation that drives distortions of the MnO$_6$ octahedra. Here, by combining x-ray magnetic Compton scattering experiments with parallel first-principles computations, we show that the anti-ferromagnetic phase of LMO is surrounded by a robust ferrimagnetic metallic phase, which becomes stable when even a small amount of Li is removed from or added to the charge-ordered LMO. In this surprising ferrimagnetic state, charge-ordering and octahedral distortions are found to be strongly suppressed. We identify the nature of the ferrimagnetic orbitals involved through theoretical and experimental analyses of the magnetic Compton scattering spectra.

cond-mat.mtrl-sci↗

arXiv2019

Topological crystalline insulator state with type-II Dirac fermions in transition metal dipnictides

The interplay between topology and crystalline symmetries in materials can lead to a variety of topological crystalline insulator (TCI) states. Despite significant effort towards their experimental realization, so far only Pb$_{1-x}$Sn$_x$Te has been confirmed as a mirror-symmetry protected TCI. Here, based on first-principles calculations combined with a symmetry analysis, we identify a rotational-symmetry protected TCI state in the transition-metal dipnictide RX$_2$ family, where R = Ta or Nb and X = P, As, or Sb. Taking TaAs$_2$ as an exemplar system, we show that its low-energy band structure consists of two types of bulk nodal lines in the absence of spin-orbit coupling (SOC) effects. Turning on the SOC opens a continuous bandgap in the energy spectrum and drives the system into a $C_2T$-symmetry-protected TCI state. On the (010) surface, we show the presence of rotational-symmetry-protected nontrivial Dirac cone states within a local bulk energy gap of $\sim$ 300 meV. Interestingly, the Dirac cones have tilted energy dispersion, realizing a type-II Dirac fermion state in a topological crystalline insulator. Our results thus indicate that the TaAs$_2$ materials family provides an ideal setting for exploring the unique physics associated with type-II Dirac fermions in rotational-symmetry-protected TCIs.

cond-mat.mes-hall↗

arXiv2019

Temperature Dependent Electronic Structure in a Higher Order Topological Insulator Candidate EuIn$_2$As$_2$

The higher order topological insulator (HOTI) has enticed enormous research interests owing to its novelty in supporting gapless states along the hinges of the crystal. Despite several theoretical predictions, enough experimental confirmation of HOTI state in crystalline solids is still lacking. It has been well known that interplay between topology and magnetism can give rise to various magnetic topological states including HOTI and Axion insulator states. Here using the high-resolution angle-resolved photoemission spectroscopy (ARPES) combined with the first-principles calculations, we report a systematic study on the electronic band topology across the magnetic phase transition in EuIn2As2 which possesses an antiferromagnetic ground state below 16 K. Antiferromagnetic EuIn2As2 has been predicted to host both the Axion insulator and HOTI phase. Our experimental results show the clear signature of the evolution of the topological state across the magnetic transition. Our study thus especially suited to understand the interaction of higher order topology with magnetism in materials.

cond-mat.mes-hall↗

arXiv2019

Transition from Intrinsic to Extrinsic Anomalous Hall Effect in the Ferromagnetic Weyl Semimetal PrAlGe$_{1-x}$Si$_x$

Recent reports of a large anomalous Hall effect (AHE) in ferromagnetic Weyl semimetals (FM WSM) have led to a resurgence of interest in this enigmatic phenomenon. However, due to a lack of tunable materials, the interplay between the intrinsic mechanism caused by Berry curvature and extrinsic mechanisms due to scattering remains unclear in FM WSMs. In this contribution, we present a thorough investigation of both the extrinsic and intrinsic AHE in a new family of FM WSMs, PrAlGe$_{1-x}$Si$_x$, where $x$ can be tuned continuously. From DFT calculations, we show that the two end members, PrAlGe and PrAlSi, have different Fermi surfaces but similar Weyl node structures. Experimentally, we observe moderate changes in the anomalous Hall coefficient ($R_S$) but significant changes in the ordinary Hall coefficient ($R_0$) in PrAlGe$_{1-x}$Si$_x$ as a function of $x$, confirming a change of Fermi surface. By comparing the magnitude of $R_0$ and $R_S$, we identify two regimes; $|R_0|<|R_S|$ when $x\le0.5$ and $|R_0|>|R_S|$ when $x>0.5$. Through a detailed scaling analysis, we discover a universal anomalous Hall conductivity (AHC) from intrinsic contribution when $x\le0.5$. Such universal AHC is absent when $x>0.5$. Thus, we point out the significance of the extrinsic mechanisms in FM WSMs and report the first observation of a transition from intrinsic to extrinsic AHE in PrAlGe$_{1-x}$Si$_x$.

cond-mat.mtrl-sci↗

arXiv2019

Prediction of three-fold fermions in a nearly-ideal Dirac semimetal BaAgAs

Materials with triply-degenerate nodal points in their low-energy electronic spectrum produce crystalline-symmetry-enforced three-fold fermions, which conceptually lie between the two-fold Weyl and four-fold Dirac fermions. Here we show how a silver-based Dirac semimetal BaAgAs realizes three-fold fermions through our first-principles calculations combined with a low-energy effective $\mathbf{k.p}$ model Hamiltonian analysis. BaAgAs is shown to harbor triply-degenerate nodal points, which lie on its $C_{3}$ rotation axis, and are protected by the $C_{6v}$($C_2\otimes C_{3v}$) point-group symmetry in the absence of spin-orbit coupling (SOC) effects. When the SOC is turned on, BaAgAs transitions into a nearly-ideal Dirac semimetal state with a pair of Dirac nodes lying on the $C_{3}$ rotation axis. We show that breaking inversion symmetry in the BaAgAs$_{1-x}$P$_x$ alloy yields a clean and tunable three-fold fermion semimetal. Systematic relaxation of other symmetries in BaAgAs generates a series of other topological phases. BaAgAs materials thus provide an ideal platform for exploring tunable topological properties associated with a variety of different fermionic excitations.

cond-mat.mtrl-sci↗

arXiv2019

Dirac state switching in transition metal diarsenides

Topological Dirac and Weyl semimetals, which support low-energy quasiparticles in condensed matter physics, are currently attracting intense interest due to exotic physical properties such as large magnetoresistance and high carrier mobilities. Transition metal diarsenides such as MoAs2 and WAs2 have been reported to harbor very high magnetoresistance suggesting the possible existence of a topological quantum state, although this conclusion remains dubious. Here, based on systematic angle-resolved photoemission spectroscopy (ARPES) measurements and parallel first-principles calculations, we investigate the electronic properties of TAs2 (T = Mo, W). Importantly, clear evidence for switching the single-Dirac cone surface state in MoAs2 with the cleaving plane is observed, whereas a Dirac state is not observed in WAs2 despite its high magnetoresistance. Our study thus reveals the key role of the terminated plane in a low-symmetry system, and provides a new perspective on how termination can drive dramatic changes in electronic structures.

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