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Arun Bansil

Publications and source records attributed to Arun Bansil.

At least 145 records · Page 8Linked to original sources
arXiv2019

Electrical and magnetic properties of thin films of the spin-filter material CrVTiAl

The spin-filter material CrVTiAl is a promising candidate for producing highly spin-polarized currents at room temperature in a nonmagnetic architecture. Thin films of compensated-ferrimagnetic CrVTiAl have been grown and their electrical and magnetic properties have been studied. The resistivity shows two-channel semiconducting behavior with one disordered gapless channel and a gapped channel with activation energy $ΔE$=~0.1~-~0.2~eV. Magnetoresistance measurements to B~=~35~T provide values for the mobilities of the gapless channel, leading to an order of magnitude difference in the carrier effective masses, which are in reasonable accord with our density-functional-theory based results. The density of states and electronic band structure is computed for permutations of the four sublattices arranged differently along the (111) body diagonal, yielding metallic (Cr-V-Al-Ti), spin-gapless (Cr-V-Ti-Al) and spin-filtering (Cr-Ti-V-Al) phases. Robustness of the spin-gapless phase to substitutional disorder is also considered.

cond-mat.mtrl-sci↗
arXiv2019

A first-principles-based study of the thermodynamics of competing low-energy states in correlated materials: Example of cuprates

We demonstrate how first-principles calculations of many competing low-energy states of a correlated material, here a cuprate, can be used to develop a thermodynamic model of Mott and pseudogap transitions in terms of magnetic short-range order. Mott physics is found in this picture to be driven by an unbinding of the antiphase domain walls, while the pseudogap phenomenon represents local moment formation. We provide explanations for nematicity and Fermi arc formation, and find a striking correspondence with many-body perturbation theory predictions.

cond-mat.str-el↗
arXiv2019

Many-body effects in porphyrin-like transition metal complexes embedded in graphene

We introduce a new computational method to study porphyrin-like transition metal complexes, bridging density functional theory and exact many-body techniques, such as the density matrix renormalization group (DMRG). We first derive a multi-orbital Anderson impurity Hamiltonian starting from first principles considerations that qualitatively reproduce GGA+U results when ignoring inter-orbital Coulomb repulsion $U'$ and Hund exchange $J$. An exact canonical transformation is used to reduce the dimensionality of the problem and make it amenable to DMRG calculations, including all many-body terms (both intra, and inter-orbital), which are treated in a numerically exact way. We apply this technique to FeN$_4$ centers in graphene and show that the inclusion of these terms has dramatic effects: as the iron orbitals become single occupied due to the Coulomb repulsion, the inter-orbital interaction further reduces the occupation yielding a non-monotonic behavior of the magnetic moment as a function of the interactions, with maximum polarization only in a small window at intermediate values of the parameters. Furthermore, $U'$ changes the relative position of the peaks in the density of states, particularly on the iron $d_{z^2}$ orbital, which is expected to greatly affect the binding of ligands.

cond-mat.mes-hall↗
arXiv2019

Purely rotational symmetry-protected topological crystalline insulator $α$-Bi4Br4

Recent theoretical advances have proposed a new class of topological crystalline insulator (TCI) phases protected by rotational symmetries. Distinct from topological insulators (TIs), rotational symmetry-protected TCIs are expected to show unique topologically protected boundary modes: First, the surface normal to the rotational axis features unpinned Dirac surface states whose Dirac points are located at generic k points. Second, due to the higher-order bulk boundary correspondence, a 3D TCI also supports 1D helical edge states. Despite the unique topological electronic properties, to date, purely rotational symmetry-protected TCIs remain elusive in real materials. Using first-principles band calculations and theoretical modeling, we identify the van der Waals material $α$-Bi4Br4 as a TCI purely protected by rotation symmetry. We show that the Bi4Br4's (010) surface exhibits a pair of unpinned topological Dirac fermions protected by the two-fold rotational axis. These unpinned Dirac fermions show an exotic spin texture highly favorable for spin transport and a band structure consisting of van Hove singularities due to Lifshitz transition. We also identify 1D topological hinge states along the edges of an $α$-Bi4Br4 rod. We further discuss how the proposed topological electronic properties in $α$-Bi4Br4 can be observed by various experimental techniques.

cond-mat.mtrl-sci↗
arXiv2019

Structural and electronic properties of the spin-filter material CrVTiAl with disorder

The effects of chemical disorder on the transport properties of the spin-filter material CrVTiAl are investigated experimentally and theoretically. Synchrotron X-ray diffraction experiments on bulk CrVTiAl and the associated Rietveld analysis indicate that the crystal structure consists primarily of a mixture of a partially ordered B2 phase, a fully disordered A2 phase and a small component of an ordered L2\textsubscript{1} or Y phase. High temperature resistivity measurements confirm the existence of a band gap. First-principles, all-electron, self-consistent electronic structure computations show that the chemically disordered A2 and B2 phases are metallic, while the spin-filter properties of the ideal Y-type phase are preserved in the presence of L2\textsubscript{1} disorder. The Hall coefficient is found to decrease with increasing temperature, similar to the measured increase in the conductivity, indicating the presence of thermally activated semiconductor-like carriers.

cond-mat.mtrl-sci↗
arXiv2019

Oxygen-induced in-situ manipulation of the interlayer coupling and exciton recombination in Bi2Se3/MoS2 2D heterostructures

2D heterostructures are more than a sum of the parent 2D materials, but are also a product of the interlayer coupling, which can induce new properties. In this paper we present a method to tune the interlayer coupling in Bi2Se3/MoS2 2D heterostructures by regulating the oxygen presence in the atmosphere, while applying laser or thermal energy. Our data suggests the interlayer coupling is tuned through the diffusive intercalation and de-intercalation of oxygen molecules. When one layer of Bi2Se3 is grown on monolayer MoS2, an influential interlayer coupling is formed that quenches the signature photoluminescence (PL) peaks. However, thermally annealing in the presence of oxygen disrupts the interlayer coupling, facilitating the emergence of the MoS2 PL peak. DFT calculations predict intercalated oxygen increases the interlayer separation ~17%, disrupting the interlayer coupling and inducing the layers to behave more electronically independent. The interlayer coupling can then be restored by thermally annealing in N2 or Ar, where the peaks will re-quench. Hence, this is an interesting oxygen-induced switching between "non-radiative" and "radiative" exciton recombination. This switching can also be accomplished locally, controllably, and reversibly using a low-power focused laser, while changing the environment from pure N2 to air. This allows for the interlayer coupling to be precisely manipulated with submicron spatial resolution, facilitating site-programmable 2D light-emitting pixels whose emission intensity could be precisely varied by a factor exceeding 200x. Our results show that these atomically-thin 2D heterostructures may be excellent candidates for oxygen sensing.

cond-mat.mtrl-sci↗
arXiv2018

Observation of topological nodal-loop state in RAs3 (R = Ca, Sr)

Topological nodal-line semimetals (NLSs) are unique materials, which harbor one-dimensional line nodes along with the so-called drumhead surface states arising from nearly dispersionless two dimensional surface bands. However, a direct observation of these drumhead surface states in the currently realized NLSs has remained elusive. Here, by using high-resolution angle-resolved photoemission spectroscopy (ARPES) along with parallel first principles calculations, we examine the topological characteristics of SrAs3 and CaAs3. SrAs3 is found to show the presence of a topological nodal-loop, while CaAs3 is found to lie near a topologically trivial phase. Our analysis reveals that the surface projections of the bulk nodal-points in SrAs3 are connected by drumhead surface states. Notably, the topological states in SrAs3 and CaAs3 are well separated from other irrelevant bands in the vicinity of the Fermi level. These compounds thus provide a hydrogen-like simple platform for developing an in-depth understanding of the quantum phase transitions of NLSs.

cond-mat.mes-hall↗
arXiv2018

Topological Hourglass Dirac Semimetal in the Nonpolar Phase of Ag$_2$BiO$_3$

Materials with tunable charge and lattice degrees of freedom provide excellent platforms for investigating multiple phases that can be controlled via external stimuli. We show how the charge-ordered ferroelectric oxide Ag$_2$BiO$_3$, which has been realized experimentally, presents a unique exemplar of a metal-insulator transition under an external electric field. Our first-principles calculations, combined with a symmetry analysis, reveal the presence of a nearly ideal hourglass-Dirac-semimetal state in the nonpolar structure of Ag$_2$BiO$_3$. The low-energy band structure consists of two hourglasslike nodal lines located on two mutually orthogonal glide-mirror planes in the absence of spin-orbit coupling (SOC) effects. These lines cross at a common point and form an interlinked chainlike structure, which extends beyond the first Brillouin zone. Inclusion of the SOC opens a small gap in the nodal lines and results in two symmetry-enforced hourglasslike Dirac points on the $\tilde{\mathcal{C}}_{2y}$ screw rotation axis. Our results indicate that Ag$_2$BiO$_3$ will provide an ideal platform for exploring the ferroelectric-semiconductor to Dirac-semimetal transition by the application of an external electric field.

cond-mat.mes-hall↗
arXiv2018

Landscape of competing stripe and magnetic phases in cuprates

Realistic modeling of competing phases in complex quantum materials has proven extremely challenging. For example, much of the existing density-functional-theory-based first-principles framework fails in the cuprate superconductors. Various many-body approaches involve generic model Hamiltonians and do not account for the couplings between spin, charge, and lattice. Here, by deploying the recently constructed strongly-constrained-and-appropriately-normed density functional, we show how landscapes of competing stripe and magnetic phases can be addressed on a first-principles basis in YBa2Cu3O6 and YBa2Cu3O7 as archetype cuprate compounds. We invoke no free parameters such as the Hubbard U, which has been the basis of much of the cuprate literature. Lattice degrees of freedom are found to be crucially important in stabilizing the various phases.

cond-mat.supr-con↗
arXiv2018

Antiferromagnetic Ground State of La$_{2}$CuO$_{4}$: A Parameter-free Ab Initio Description

We show how an accurate first-principles treatment of the antiferromagnetic (AFM) ground state of La$_2$CuO$_4$ can be obtained without invoking any free parameters such as the Hubbard $U$. The magnitude and orientation of our theoretically predicted magnetic moment of $0.495 μ_{B}$ on Cu-sites along the (100) direction are in excellent accord with experimental results. The computed values of the band gap (1.00 eV) and the exchange-coupling (-138 meV) match the corresponding experimental values. We identify interesting band splittings below the Fermi energy, including an appreciable Hund's splitting of 1.25 eV. The magnetic form factor obtained from neutron scattering experiments is also well described by our calculations. Our study opens up a new pathway for first-principles investigations of electronic and atomic structures and phase diagrams of cuprates and other complex materials.

cond-mat.str-el↗
arXiv2018

Nature of the positron state in CdSe quantum dots

Previous studies have shown that positron-annihilation spectroscopy is a highly sensitive probe of the electronic structure and surface composition of ligand-capped semiconductor Quantum Dots (QDs) embedded in thin films. Nature of the associated positron state, however, whether the positron is confined inside the QDs or localized at their surfaces, has so far remained unresolved. Our positron-annihilation lifetime spectroscopy (PALS) studies of CdSe QDs reveal the presence of a strong lifetime component in the narrow range of 358-371 ps, indicating abundant trapping and annihilation of positrons at the surfaces of the QDs. Furthermore, our ab-initio calculations of the positron wave function and lifetime employing a recent formulation of the Weighted Density Approximation (WDA) demonstrate the presence of a positron surface state and predict positron lifetimes close to experimental values. Our study thus resolves the longstanding question regarding the nature of the positron state in semiconductor QDs, and opens the way to extract quantitative information on surface composition and ligand-surface interactions of colloidal semiconductor QDs through highly sensitive positron-annihilation techniques.

cond-mat.mes-hall↗
arXiv2018

Tunable double-Weyl Fermion semimetal state in the SrSi$_2$ materials class

We discuss first-principles topological electronic structure of noncentrosymmetric SrSi$_2$ materials class based on the hybrid exchange-correlation functional. Topological phase diagram of SrSi$_2$ is mapped out as a function of the lattice constant with focus on the semimetal order. A tunable double-Weyl Fermion state in Sr$_{1-x}$Ca$_{x}$Si$_2$ and Sr$_{1-x}$Ba$_{x}$Si$_2$ alloys is identified. Ca doping in SrSi$_2$ is shown to yield a double-Weyl semimetal with a large Fermi arc length, while Ba doping leads to a transition from the topological semimetal to a gapped insulator state. Our study indicates that SrSi$_2$ materials family could provide an interesting platform for accessing the unique topological properties of Weyl semimetals.

cond-mat.mes-hall↗
arXiv2018

Starfruit-like nodal semimetal to Dirac and Weyl semimetal state in CaAuAs

Band-crossings occurring on a mirror plane are compelled to form a nodal loop in the momentum space without spin-orbit coupling (SOC). In the presence of other equivalent mirror planes, multiple such nodal loops can combine to form interesting nodal-link structures. Here, based on first-principles calculations and an effective $\mathbf{k.p}$ model analysis, we show that CaAuAs hosts a unique starfruit-like nodal-link structure in the bulk electronic dispersion in the absence of SOC. This nodal-link is comprised of three nodal loops, which cross each other at the time-reversal-invariant momentum point $A$. When the SOC is turned on, the nodal loops are gapped out, resulting in a stable Dirac semimetal state with a pair of Dirac points along the $\mathrm{Γ-A}$ direction in the Brillouin zone. The Dirac points are protected by the combination of time reversal, inversion, and $C_3$ rotation symmetries. We show how a systematic elimination of the symmetry constraints yields a Weyl semimetal and eventually a topological insulator state.

cond-mat.mtrl-sci↗
arXiv2018

Topological crystalline insulator states in the Ca$_2$As family

Topological crystalline insulators (TCI) are insulating electronic phases of matter with nontrivial topology originating from crystalline symmetries. Recent theoretical advances have provided powerful guidelines to search for TCIs in real materials. Using density functional theory, we identify a class of new TCI states in the tetragonal lattice of the Ca$_2$As material family. On both top and side surfaces, we observe topological surface states protected independently by rotational and mirror symmetries. We show that a particular lattice distortion can single out the newly proposed topological protection by the rotational symmetry. As a result, the Dirac points of the topological surface states are moved to generic locations in momentum space away from any high symmetry lines. Such topological surface states have not been seen before. Moreover, the other family members, including Ca$_2$Sb, Ca$_2$Bi and Sr$_2$Sb, feature different topological surface states due to their distinct topological invariants. We thus further propose topological phase transitions in the pseudo-binary systems such as (Ca$_{1-x}$Sr$_x$)$_2$As and Ca$_2$As$_x$Sb$_{1-x}$. Our work reveals rich and exotic TCI physics across the Ca$_2$As family of materials, and suggests the feasibility of materials database search methods to discover new TCIs.

cond-mat.mtrl-sci↗
arXiv2018

Nonsymmorphic cubic Dirac point and crossed nodal rings across the ferroelectric phase transition in LiOsO$_3$

Crystalline symmetries can generate exotic band-crossing features, which can lead to unconventional fermionic excitations with interesting physical properties. We show how a cubic Dirac point---a four-fold-degenerate band-crossing point with cubic dispersion in a plane and a linear dispersion in the third direction---can be stabilized through the presence of a nonsymmorphic glide mirror symmetry in the space group of the crystal. Notably, the cubic Dirac point in our case appears on a threefold axis, even though it has been believed previously that such a point can only appear on a sixfold axis. We show that a cubic Dirac point involving a threefold axis can be realized close to the Fermi level in the non-ferroelectric phase of LiOsO$_3$. Upon lowering temperature, LiOsO$_3$ has been shown experimentally to undergo a structural phase transition from the non-ferroelectric phase to the ferroelectric phase with spontaneously broken inversion symmetry. Remarkably, we find that the broken symmetry transforms the cubic Dirac point into three mutually-crossed nodal rings. There also exist several linear Dirac points in the low-energy band structure of LiOsO$_3$, each of which is transformed into a single nodal ring across the phase transition.

cond-mat.mtrl-sci↗
arXiv2018

Theoretical prediction of magnetic and noncentrosymmetric Weyl fermion semimetal states in the R-Al-X family of compounds (R=rare earth, Al=aluminium, X=Si, Ge)

Weyl semimetals are novel topological conductors that host Weyl fermions as emergent quasiparticles. While the Weyl fermions in high-energy physics are strictly defined as the massless solution of the Dirac equation and uniquely fixed by Lorentz symmetry, there is no such constraint for a topological metal in general. Specifically, the Weyl quasiparticles can arise by breaking either the space-inversion ($\mathcal{I}$) or time-reversal ($\mathcal{T}$) symmetry. They can either respect Lorentz symmetry (type-I) or strongly violate it (type-II). To date, different types of Weyl fermions have been predicted to occur only in different classes of materials. In this paper, we present a significant materials breakthrough by identifying a large class of Weyl materials in the RAlX (R=Rare earth, Al, X=Ge, Si) family that can realize all different types of emergent Weyl fermions ($\mathcal{I}$-breaking, $\mathcal{T}$-breaking, type-I or type-II), depending on a suitable choice of the rare earth elements. Specifically, RAlX can be ferromagnetic, nonmagnetic or antiferromagnetic and the electronic band topology and topological nature of the Weyl fermions can be tuned. The unparalleled tunability and the large number of compounds make the RAlX family of compounds a unique Weyl semimetal class for exploring the wide-ranging topological phenomena associated with different types of emergent Weyl fermions in transport, spectroscopic and device-based experiments.

cond-mat.mtrl-sci↗
arXiv2017

Topological Hopf and chain link semimetal states and their application to Co2MnGa (Theory and Materials Prediction)

Topological semimetals can be classified by the connectivity and dimensionality of the band cross- ing in momentum space. The band crossings of a Dirac, Weyl, or an unconventional fermion semimet- al are 0D points, whereas the band crossings of a nodal-line semimetal are 1D closed loops. Here we propose that the presence of perpendicular crystalline mirror planes can protect 3D band crossings characterized by nontrivial links such as a Hopf link or a coupled-chain, giving rise to a variety of new types of topological semimetals. We show that the nontrivial winding number protects topolog- ical surface states distinct from those in previously known topological semimetals with a vanishing spin-orbit interaction. We also show that these nontrivial links can be engineered by tuning the mirror eigenvalues associated with the perpendicular mirror planes. Using first-principles band structure calculations, we predict the ferromagnetic full Heusler compound Co2MnGa as a candidate. Both Hopf link and chain-like bulk band crossings and unconventional topological surface states are identified.

cond-mat.mes-hall↗