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At least 649 records · Page 36
All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

Orbital gyrotropic magnetoelectric effect and its strain engineering in monolayer NbX 2

Electrical control of the orbital degrees of freedom is an important area of research in the emerging field of "orbitronics.'' Orbital gyrotropic magneto-electric effect (OGME) is the generation of an orbital magnetization in a nonmagnetic metal by an applied electric field. Here, we show that strain induces a large GME in the monolayer NbX 2 (X = S, Se) normal to the plane, primarily driven by the orbital moments of the Bloch bands as opposed to the conventional spin magnetization, without any need for spin-orbit coupling. The key physics is captured within an effective two-band valley-orbital model and it is shown to be driven by three key ingredients: the intrinsic valley orbital moment, broken $C_{3z}$ symmetry, and strain-induced Fermi surface changes. The effect can be furthermore switched by changing the strain condition, with potential for future device applications.

2D materials↗
DOE OSTI2020

Interfacial Dzyaloshinskii-Moriya interaction of antiferromagnetic materials

The interface between a ferromagnet (FM) or antiferromagnet (AFM) and a heavy metal (HM) results in an antisymmetric exchange interaction known as the interfacial Dzyaloshinskii-Moriya interaction (iDMI) which favors non-collinear spin configurations. The iDMI is responsible for stabilizing noncollinear spin textures such as skyrmions in materials with bulk inversion symmetry. Interfacial DMI values have been previously determined theoretically and experimentally for FM/HM interfaces, and, in this work, they are calculated for the metallic AFM MnPt and the insulating AFM NiO. The heavy metals considered are W, Re, and Au. Values for the iDMI, exchange, and anisotropy constants are determined for different AFM and HM thicknesses. The iDMI values of the MnPt heterolayers are comparable to those of the common FM materials, and those of NiO are lower. In few-layer films of (001) MnPt, the high spin orbit coupling of the Pt layers can give rise to a small DMI in the absence of a HM layer.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Flat-band topology of magic angle graphene on a transition metal dichalcogenide

In this work, we consider twisted bilayer graphene on a transition metal dichalcogenide substrate, where proximity-induced spin-orbit coupling significantly alters the eight flat bands which occur near the magic angle. The resulting band structure features a pair of extremely flat bands across most of the mini-Brillouin zone. Further details depend sensitively on the symmetries of the heterostructure; we find semiconducting band structures when all twofold rotations around the in-plane axis are broken, and semimetallic band structures otherwise. We calculate the Chern numbers of the different isolated bands, and identify the parameter regimes and filling factors where valley Chern insulators and topological insulators are realized. Interestingly, we find that for realistic values of the proximity-induced terms, it is possible to realize a topological insulator protected by time-reversal symmetry by doping two holes or two electrons per superlattice unit cell into the system.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Role of element-specific damping in ultrafast, helicity-independent, all-optical switching dynamics in amorphous (Gd,Tb)Co thin films

Ultrafast control of the magnetization in ps timescales by fs laser pulses offers an attractive avenue for applications such as fast magnetic devices for logic and memory. However, ultrafast helicity-independent all-optical switching (HI-AOS) of the magnetization has thus far only been observed in Gd-based, ferrimagnetic amorphous (a-) rare earth-transition metal (a-RE-TM) systems, and a comprehensive understanding of the reversal mechanism remains elusive. Here, we report HI-AOS in ferrimagnetic a-Gd 22-x Tb x Co 78 thin films, from x=0 to 18, and elucidate the role of Gd in HI-AOS in a-RE-TM alloys and multilayers. Increasing Tb content results in increasing perpendicular magnetic anisotropy and coercivity, without modifying magnetization density, and slower remagnetization rates and higher critical fluences for switching but still shows picosecond HI-AOS. Simulations of the atomistic spin dynamics based on the two-temperature model reproduce these results qualitatively and predict that the lower damping on the RE sublattice arising from the small spin-orbit coupling of Gd (with L=0) is instrumental for the faster dynamics and lower critical fluences of the Gd-rich alloys. Annealing a-Gd 10 Tb 12 Co 78 leads to slower dynamics which we argue is due to an increase in damping. These simulations strongly indicate that accounting for element-specific damping is crucial in understanding HI-AOS phenomena. The results suggest that engineering the element-specific damping of materials can open up new classes of materials that exhibit low-energy, ultrafast HI-AOS.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Striped electron fluid on (111) KTaO 3

A recent study has revealed that the low carrier density electron gas (2DEG) induced at the interface of EuO and (111) KTaO 3 exhibits a broken symmetry phase with a strong in-plane anisotropy of the resistivity. We present a minimal tight-binding model of this (111) 2DEG, including the large spin-orbit coupling from the Ta ions, which reveals a hexagonal Fermi surface with a highly enhanced 2k F electronic susceptibility. We argue that repulsive electronic interactions, together with a ferromagnetic EuO substrate, favor a magnetic stripe instability leading to a partially gapped Fermi surface. Such a stripe state, or its vestigial nematicity, could explain the observed transport anisotropy. We propose a k · p theory for the low energy j = 3/2 states, which captures the key results from our tight-binding study, and further reveals the intertwined dipolar and octupolar modulations underlying this magnetic stripe order. We conclude by speculating on the relation of this stripe order to the superconductivity seen in this material.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Oxygen vacancy induced electronic structure modification of KTaO 3

The observation of a metallic interface between band insulators LaAlO 3 and SrTiO 3 has led to massive efforts to understand the origin of the phenomenon as well as to search for other systems hosting such two-dimensional electron gases (2-DEGs). However, the understanding of the origin of the 2-DEG is very often hindered as several possible mechanisms such as polar catastrophe, cationic intermixing, and oxygen vacancy (OV) can be operative simultaneously. The presence of a heavy element makes the KTaO 3 (KTO) based 2-DEG a potential platform to investigate spin-orbit coupling driven novel electronic and magnetic phenomena. In this work, we investigate the sole effect of OV in KTO, which makes KTO metallic. O K-edge x-ray absorption spectroscopy measurements find that OV dopes electrons in Ta t * 2g antibonding states. Photoluminescence measurements reveal the existence of a highly localized deep midgap state in oxygen-deficient KTO. Our detailed ab initio calculations demonstrate that such deep midgap state arises due to the linear clustering of OVs around Ta. Finally, our present work emphasizes that we must pay attention to the possible presence of OVs in interpreting emergent behavior of KTO-based heterostructures.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Dynamics of Spin Polarization in Tilted Polariton Rings

We have observed the effect of pseudomagnetic field originating from the polaritonic analog of spin-orbit coupling [transverse electric and transverse magnetic (TE-TM) splitting] on a polariton condensate in a ring-shaped microcavity. The effect gives rise to a stable four-leaf pattern around the ring as seen from the linear polarization measurements of the condensate photoluminescence. This pattern is found to originate from the interplay of the cavity potential, energy relaxation, and TE-TM splitting in the ring. Our observations are compared to the dissipative one-dimensional spinor Gross-Pitaevskii equation with the TE-TM splitting energy, which shows good qualitative agreement.

CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSICS↗
DOE OSTI2021

Higher Chern numbers in multilayer Lieb lattices ( N ≥ 2 ): Topological transitions and quadratic band crossing lines

In this work, we consider a hitherto unexplored setting of a stacked multilayer (N) Lieb lattice which undergoes an unusual topological transition in the presence of intralayer spin-orbit coupling (SOC). The specific stacking configuration induces an effective nonsymmorphic two-dimensional lattice structure, even though the constituent monolayer Lieb lattice is characterized by a symmorphic space group. This emergent nonsymmorphicity leads to multiple doubly degenerate bands extending over the edge of the Brillouin zone (i.e., quadratic band crossing lines). In the presence of intralayer SOC, these doubly degenerate bands typically form three N-band subspaces, mutually separated by two band gaps. We analyze the topological properties of these multiband subspaces, using specially devised Wilson loop operators to compute non-Abelian Berry phases in order to show that they carry a higher Chern number N.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Magnetotransport in semiconductors and two-dimensional materials from first principles

Here we demonstrate a first-principles method to study magnetotransport in materials by solving the Boltzmann transport equation (BTE) in the presence of an external magnetic field. Our approach employs ab initio electron-phonon interactions and takes spin-orbit coupling into account. We apply our method to various semiconductors (Si and GaAs) and two-dimensional (2D) materials (graphene) as representative case studies. The magnetoresistance, Hall mobility, and Hall factor in Si and GaAs are in very good agreement with experiments. In graphene, our method predicts a large magnetoresistance, consistent with experiments. Analysis of the steady-state electron occupations in graphene shows the dominant role of optical phonon scattering and the breaking of the relaxation time approximation. Our paper provides a detailed understanding of the microscopic mechanisms governing magnetotransport coefficients, establishing the BTE in a magnetic field as a broadly applicable first-principles tool to investigate transport in semiconductors and 2D materials.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Superconducting state of Sr 2 RuO 4 in the presence of longer-range Coulomb interactions

The symmetry of the superconducting condensate in Sr 2 RuO 4 remains controversial after recent experiments overturned the dominant chiral p-wave paradigm.We perform a theoretical study of pairing by spin- and chargefluctuations in Sr 2 RuO 4 , including the effects of spin-orbit coupling, and both local and longer-range Coulomb repulsion. The latter has important consequences for Sr 2 RuO 4 due to the near-degeneracy of symmetry-distinct pairing states in this material. Here we find leading nodal s', d xy , and helical (p) solutions, while both the g- and d x 2 –y 2 -wave channels remain noncompetitive. This suggests nodal time-reversal symmetry broken s' + id xy or s' + ip phases, promoted by longer-range Coulomb repulsion, as the most favorable candidates for Sr 2 RuO 4 . We analyze the properties of these states and show that the s' + id xy solution agrees with the bulk of available experimental data.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Prediction of double-Weyl points in the iron-based superconductor CaKFe 4 As 4

Employing a combination of symmetry analysis, low-energy modeling, and ab initio simulations, we predict the presence of magnetic-field-induced Weyl points close to the Fermi level in CaKFe 4 As 4 . Depending on the relative strengths of the magnetic field and of the spin-orbit coupling, the Weyl fermions can carry a topological charge of ±1 or ±2, making CaKFe 4 As 4 a rare realization of a double-Weyl semimetal. Here, we further predict experimental manifestations of these Weyl points, both in bulk properties, such as the anomalous Hall effect, and in surface properties, such as the emergence of prominent Fermi arcs. Because CaKFe 4 As 4 displays unconventional fully gapped superconductivity below 30 K, our findings open a route to investigate the interplay between superconductivity and Weyl fermions.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Three-dimensional stacking of canted antiferromagnetism and pseudospin current in undoped Sr 2 IrO 4 : Symmetry analysis and microscopic model realization

Recent optical second-harmonic generation experiments observed unexpected broken spatial symmetries in the undoped spin-orbit Mott insulator Sr2 IrO4, leading to intensive debates on the nature of its ground state. Here we propose that it is a canted antiferromagnetism (CAF) with a hidden order of circulating staggered pseudospin current. Symmetry analysis shows that a proper c-axis stacking of the CAF and the pseudospin current lead to a magnetoelectric coexistence state that breaks the twofold rotation, inversion, and time-reversal symmetries, consistent with experimental observations. We construct a three-dimensional Hubbard model with spin-orbit coupling and structural distortion for the five localized 5d Wannier orbitals centered at Ir sites and demonstrate the microscopic realization of the desired coexistence state in a wide range of band parameters via a combination of self-consistent Hartree-Fock and variational calculations.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Semi-Dirac and Weyl fermions in transition metal oxides

We illustrate that a class of compounds with I4/mcm crystalline symmetry hosts three-dimensional semi-Dirac fermions. Unlike the known two-dimensional semi-Dirac points, the degeneracy of these three-dimensional semi-Dirac points is not lifted by spin-orbit coupling due to the protection by a nonsymmorphic symmetry—mirror reflection in the a–b plane and a translation along the c axis. This crystalline symmetry is found in tetragonal perovskite oxides, realizable in thin films by epitaxial strain that results in a 0 a 0 c - -type octahedral rotation. Interestingly, with broken time-reversal symmetry, two pairs of Weyl points emerge from the semi-Dirac points within the Brillouin zone, and an additional lattice distortion leads to an enhanced intrinsic anomalous Hall effect. The ability to tune the Berry phase by epitaxial strain can be useful in novel oxide-based electronic devices.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Anisotropically large anomalous and topological Hall effect in a kagome magnet

Recently, kagome materials have become an engrossing platform to study the interplay among symmetry, magnetism, topology, and electron correlation. The latest works in RMn6Sn6 (R = rare earth metal) have illustrated that this family could be intriguing to investigate various physical phenomena due to large spin-orbit coupling and strong magnetic ordering. However, combined transport and spectroscopic studies in RMn6Sn6 materials are still limited. Here, we report magnetic, magneto-transport, and angle-resolved photoemission spectroscopy measurements of a kagome magnet ErMn 6 Sn 6 that undergoes antiferromagnetic (T N = 345 K) to ferrimagnetic (T c = 68 K) phase transitions. We observe a large anomalous and topological Hall effect serving as transport signatures of the nontrivial Berry curvature. Our spectroscopic results complemented by theoretical calculations show the multi-orbital kagome fermiology. This work provides new insight into the tunability and interplay of topology and magnetism in a kagome magnet.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Mechanisms of in-plane magnetic anisotropy in superconducting NbSe 2

Here we present a unifying picture of the magnetic in-plane anisotropies of two-dimensional superconductors based on transition metal dichalcogenides. The symmetry considerations are first applied to constrain the form of the conductivity tensor. We hence conclude that the twofold periodicity of transport distinct from the planar Hall-related contributions requires a tensor perturbation. At the same time, the sixfold periodic variation of the critical field results from the Rashba spin-orbit coupling on a hexagonal lattice. We have considered the effect of a weak tensor perturbation on the critical field, gap function, and magnetoconductivity. The latter is studied using the time-dependent Ginzburg-Landau phenomenology. The common origin of the π periodicity in transport and thermodynamics properties is identified. The scheme constructed here is applied to describe the existing theoretical scenarios from a unified point of view. This allows us to single out the differences and similarities between the suggested approaches.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Clean quantum point contacts in an InAs quantum well grown on a lattice-mismatched InP substrate

Strong spin-orbit coupling, the resulting large g factor, and small effective mass make InAs an attractive material platform for inducing topological superconductivity. The surface Fermi level pinning in the conduction band enables highly transparent ohmic contact without excessive doping. In this study, we investigate electrostatically defined quantum point contacts (QPCs) in a deep-well InAs two-dimensional electron gas. Despite the 3.3% lattice mismatch between the InAs quantum well and the InP substrate, we report clean QPCs with up to eight pronounced quantized conductance plateaus at zero magnetic field. Source-drain dc bias spectroscopy reveals a harmonic confinement potential with a nearly 5 meV subband spacing. We find a many-body exchange interaction enhancement for the out-of-plane g factor |$g^{*}_{⊥}$|=27±1 , whereas the in-plane g factor is isotropic |$g^{*}_{x}$|=|$g^{*}_{y}$|=12±2 , close to the bulk value for InAs.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Gate-defined wires in twisted bilayer graphene: From electrical detection of intervalley coherence to internally engineered Majorana modes

Twisted bilayer graphene (TBG) realizes a highly tunable, strongly interacting system featuring superconductivity and various correlated insulating states. We establish gate-defined wires in TBG with proximity-induced spin-orbit coupling as (i) a tool for revealing the nature of correlated insulators and (ii) a platform for Majorana-based topological qubits. We show that the band structure of a gate-defined wire immersed in an intervalley coherent correlated insulator inherits electrically detectable fingerprints of symmetry breaking native to the latter. Surrounding the wire by a superconducting TBG region on one side and an intervalley coherent correlated insulator on the other further enables the formation of Majorana zero modes—possibly even at zero magnetic field depending on the precise symmetry-breaking order present. Our proposal not only introduces a highly gate-tunable topological qubit medium relying on internally generated proximity effects but can also shed light on the Cooper-pairing mechanism in TBG.

36 MATERIALS SCIENCE↗
DOE OSTI2022

A-type antiferromagnetic order in semiconducting EuMg 2 Sb 2 single crystals

Eu-based Zintl-phase materials EuA 2 Pn 2 (A = Mg, In, Cd, Zn; Pn = Bi, Sb, As, P) have generated significant recent interest owing to the complex interplay of magnetism and band topology. Here, we investigated the crystallographic, magnetic, and electronic properties of the layered Zintlphase single crystals of EuMg 2 Sb 2 with the trigonal CaAl 2 Si 2 crystal structure (space group $P\bar{3}m1$). Electrical resistivity measurements complemented with angle-resolved photoemission spectroscopy (ARPES) studies and density functional theory (DFT) calculations find an activated behavior with intrinsic conductivity at high temperatures indicating a semiconducting electronic ground state with a narrow energy gap of 370 meV. Magnetic susceptibility and zero-field heat capacity measurements indicate that the compound undergoes antiferromagnetic (AFM) ordering at the Néel temperature T N = 8.0(2) K. Here, zero-field neutron-diffraction measurements reveal that the AFM ordering is A-type where the Eu spins (Eu 2+ , S = 7/2) arranged in ab-plane layers are aligned ferromagnetically in the ab plane and the Eu spins in adjacent layers are aligned antiferromagnetically. Eu-moment reorientation within the ab planes in the trigonal AFM domains associated with a very weak inplane magnetic anisotropy is also evident below T N at low fields < 0.05 T. Although isostructural semimetallic EuMg 2 Bi 2 is reported to host Dirac surface states, the observation of narrow-gap semiconducting behavior in EuMg 2 Sb 2 implies a strong role of spin-orbit coupling (SOC) in tuning the electronic states of these materials. Our DFT studies also suggest, besides the SOC, the more electronegative and smaller Sb than Bi shifts the low-lying conduction bands along the Γ-A direction to higher energy, resulting in an indirect bulk band gap between the Γ and M points for EuMg 2 Sb 2 .

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗