Session Y43: Emerging Magnetic Topological Materials

Topology from band (co-)representation theory in spinful systems: spin groups and magnetic line groups

Topological Quantum Chemistry (TQC) has been successful in diagnosing band topology using only the symmetries of the system [1, 2]. We begin extending TQC to magnetic systems in two new ways. First, we derive the elementary band (co-)representations of magnetic line groups. The (magnetic) line groups are often overlooked despite corresponding to symmetries of quasi-1D systems, which are of fundamental importance due to their use as illustrative models as well as being building blocks of higher-dimensional systems [3]. Second, we begin extending TQC to the spin groups – generalizations of magnetic groups – which apply for systems of any degree of spin-orbit coupling. Since the magnetic groups apply only for the case of strong spin-orbit coupling, this work opens the door to examining weak spin-orbit coupled magnetic materials and excitations in them [4-7].

[1] B. Bradlyn et al Topological quantum chemistry Nature 547 298305 (2017)

[2] L. Elcoro et al Magnetic Topological Quantum Chemistry Nat. Commun. 12 5965 (2021)

[3] M. Damnjanovic et al Line Groups in Physics: Theory and applications to nanotubes and polymers. Springer Berline, Heidelberg (2010)

[4] D. Litvin Spin Point Groups Acta. Cryst. (1975)

[5] P. Liu et al Spin-Group Symmetry in Magnetic Materials with Negligible Spin-Orbit Coupling. PRX 12 021016 (2022)

[6] A. Corticelli et al Spin-space groups and magnon band topology. PRB 105, 064430 (2022)

[7] N. Lazic et al Spin line groups. Acta. Cryst. A69, 611-619 (2013)

    

Symmetry indicators in commensurate magnetic flux

We derive a framework to apply topological quantum chemistry in systems subject to magnetic flux. We start by deriving the action of spatial symmetry operators in a uniform magnetic field, which extends Zak's magnetic translation groups to all crystal symmetry groups. Ultimately, the magnetic symmetries form a projective representation of the crystal symmetry group. As a consequence, band representations acquire an extra gauge invariant phase compared to the non-magnetic theory. Thus, the theory of symmetry indicators is distinct from the non-magnetic case. We give examples of new symmetry indicators that appear at π flux. Finally, we apply our results to an obstructed atomic insulator with corner states in a magnetic field. The symmetry indicators reveal a topological-to-trivial phase transition at finite flux, which is confirmed by a Hofstadter butterfly calculation. The bulk phase transition provides a new probe of higher order topology in certain obstructed atomic insulators.   

Vortex Excitations in Dirac Bose-Einstein Condensates

We explore vortices in non-equilibrium Dirac Bose-Einstein condensates (Dirac BEC) described by a multi-component Gross-Pitaevskii equation. We find that the linear kinetic energy of the Dirac equation enables a difference in phase winding of the two condensate components. We observe three classes of vortex states distinguished by their far-field behavior: A locally constrained ring soliton on either of the two components in combination with a vortex profile on the other component, and a vortex profile on both components if inter-component interactions are sufficiently strong. We also address the role of a Haldane gap on these vortices, which encourages the occupation of the component with the larger winding number, and thereby facilitates the creation of the dual vortex state. We employ a numerical shooting method to identify vortex solutions and use it to scan large parts of the parameter space. A classification algorithm on the integrated wavefunctions allows us to establish a phase diagram of the distinct vortex states in Dirac BEC.   

Multi-state stripe domain racetrack memory devices utilizing Weyl magnetoresistance

Magnetoresistive effects, such as the tunneling magnetoresistance seen in Magnetic Tunnel Junctions (MTJs), are foundational to the development of spintronic nanodevices.  Despite the reliable and efficient write performance, along with great promise for MTJ-based spintronic devices, the readout in CoFeB/MgO/CoFeB MTJs is bounded by the room temperature spin-polarization of CoFe and defects in the MTJ heterostructure. Recently, groups have predicted a massive (>10⁵%) magnetoresistance across magnetic Weyl Semimetal (MWSM) domain walls or in MWSM MTJs. In this work, we instantiate the Dirac equation on a lattice and model NEGF transport through magnetic stripe domains in a toy MWSM Hamiltonian. We show that the device conductance is strongly modulated by the number of domain walls in the active region of the device, providing a means to encode multiple resistance states in a single racetrack device. Additionally, we consider the effects of these periodic magnetic patterns on the electronic structure of the MWSM lattice and show tunable flat bands in the direction of transport. This work elucidates both the physical behavior and computing applications of domain wall-MWSM devices.   

Giant anomalous Hall effect in a spin orbit coupled ferromagnet MnBi

Large anomalous Hall effects (AHE) on the order of 10³ Scm^-1 have been found in various ferromagnets, mainly originating from intrinsic Berry curvature with the broken time reversal symmetry. However, further enhancement has rarely been reported, which is limited by the nature of the intrinsic mechanism. Extrinsic effects, particularly the skew scattering, promise higher AHE beyond 10⁴ Scm^-1 but requires high electrical conductivity that few ferromagnets satisfy. In this presentation, we report the giant AHE and anomalous Nernst effect (ANE) in a simple binary ferromagnet MnBi. High quality single crystals with high longitudinal electrical conductivity over 3×10⁶ Scm^-1 are for the first time achieved, which satisfies the skew scattering criterion. Thanks to the large spin-orbit coupling from Bi, the observation of a pure skew scattering AHE signal below is observed below 10 K, with an AHE conductivity orders of magnitude higher than the Berry curvature originated AHE, with a profound anomalous Nernst conductivity over 100 AK^-1m^-1. We highlight that the role of skew scattering mechanism in enhancing the AHE and ANE for highly conductive spin orbit coupled ferromagnets.

    

Evolution of Structure and Magnetism in the Square Net Series TbTe2-xSbx

Square net materials have attracted attention as platforms for studying the interplay between electronic topology, charge density order, superconductivity, and magnetism.  A recent example was seen in GdSb_xTe_2-x-δ, where Sb substitution results in structural modulations that coincide with modification of the electronic state and magnetic order [1]. Motivated by this, we investigated TbTe_2-xSb_x (0 < x < 0.55)_, which is structurally similar - but additionally has an anisotropic f-state with a non-zero orbital quantum number. For the parent compound, we show that the crystal structure is consistent with the UAs₂ prototype, but there is evidence for superstructure formation that evolves as a function of x. TbTe₂ also exhibits an antiferromagnetically ordered state that is modified with increasing x. Trends in the series will be examined using bulk thermodynamic measurements, where we provide evidence for a complex evolution of structure and ground state ordered behaviors.

[1] Lei S., et al.  Advanced Quantum Technologies 2.10 (2019): 1900045

 

    

High-efficiency quantum rectenna for millimeter wave and terahertz technologies

We propose a new concept for rectenna that utilizes the nonlinear transverse current-voltage characteristic of junction-free quantum materials to rectify millimeter and terahertz waves into DC electricity. We analyze the rectification efficiency defined by the ratio of the DC output power to the AC input power. We show that the efficiency can be determined from the measurement of the DC transverse voltage as a function of the AC input current. High efficiency on the order of unity can be achieved when the photoconductivity is small, as we calculate explicitly for time-reversal-breaking semiconductors with non-parabolic bands and Rashba systems. Our proposed rectenna also has an advantage in impedance matching between the rectifier and the antenna, and the rectifier and the external load.   

Disorder Effects in Planar Topological Josephson Junctions

Disorder has emerged as one of the main obstacles toward the realization, and unambiguous detection, of Majorana bound states in superconductor-semiconductor heterostructures. In recent years a lot of work, both theoretical and experimental, has been done to address the effects of disorder in quasi one-dimensional superconductor-semiconductor nanowires. Planar Josephson junctions based on InAs and Al are another promising platform for the realization of Majorana bound states for which the effects of disorder, so far, have not been studied in depth. In this talk I will present some of the recent results that we have obtained to quantitatively estimate the dominant sources of disorder in planar topological Josephson junctions, and their effects on the robustness and detection of the Majorana bound states that can be realized in such junctions.

    

Anomalous Magnetotransport of Single Crystal CsV3Sb5

We report on measurements of the in-plane transverse magnetoresistance (MR) of the Kagome superconductor CsV₃Sb₅. The MR approaches 200 % at 1.8 K and in c-axis fields of 8.5 T. The observation of quantum oscillations in fields as low as 5 T underlines the high quality of the samples. At low temperatures, the MR is unusual: it displays a quadratic field dependence in fields less than ~1 kOe, followed by a linear dependence. In the field range of 15-40 kOe, the linear dependence gives way to a downwards curvature. Above 50 kOe the dependence becomes linear again but with smaller slope.  At temperatures above 100 K, the MR is very small, <0.3 %, and quadratic. In fact, the MR and its low-field features disappear sharply at the CDW transition temperature of 95 K, suggesting that it is associated with the reconstructed Fermi surface arising due to CDW order. Our analysis indicates that the observed MR features are caused by a star-shaped Fermi surface sheet around the G point of the reconstructed Brillouin zone.

This work was supported by the U. S. Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division   

Magnetic Breakdown in the Kagome Superconductor CsV3Sb5 under High Magnetic Field

The recently discovered layered Kagome metals of composition AV₃Sb₅ (A = K, Rb, Cs) exhibit a complex interplay among superconductivity, charge density wave order, topologically non-trivial electronic band structure and geometrical frustration. Here, we probe the electronic band structure underlying these exotic correlated electronic states in CsV₃Sb₅ with quantum oscillation measurements in pulsed fields up to 86 T. The high-field data reveal a sequence of magnetic breakdown orbits that allows the construction of a model for the folded Fermi surface of CsV₃Sb₅. The dominant features are large triangular Fermi surface sheets that cover almost half of the folded Brillouin zone that have not yet been detected in angle resolved photoemission spectroscopy (ARPES). These sheets display pronounced nesting at the charge density wave (CDW) vectors, which may stabilize the CDW state. The Berry phases of the electron orbits have been deduced from Landau level fan diagrams near the quantum limit without the need for extrapolations, thereby unambiguously establishing the non-trivial topological character of several electron bands in this Kagome lattice superconductor.

    

Absence of Edge States in The Valley Chern Insulator in Moire Graphene.

We study the edge spectrum of twisted sheets of single layer and bilayer graphene in cases where the continuum model predicts a valley Chern insulator – an insulating state in which the occupied moire mini-bands from each valley have a net Chern number, but both valleys together have no net Chern number, as required by time reversal symmetry. In a simple picture, such a state might be expected to have chiral valley polarized counter-propagating edge states. We present results from exact diagonalization of the tight-binding model of commensurate structures in the ribbon geometry. We find that for both the single-layer and bilayer moire ribbons robust edge modes are generically absent. We attribute this lack of edge modes to the fact that the edge induces valley mixing. Further, even in the bulk, a sharp distinction between the valley Chern insulator and a trivial insulator requires an exact C3 symmetry that in turn can be related to a new topological state denoted by the shift insulator.   

Towards the quantum spin Hall regime in (Bi1−xSbx)2Te3 ultrathin films

Theory predicts that reducing the thickness of 3D topological insulator thin films opens a hybridization

gap at the Dirac point. Depending on film thickness, the nature of this gap oscillates between trivial and

inverted [1, 2]. When the chemical potential of such a thin film lies within an inverted hybridization gap,

the system will be in the quantum spin Hall (QSH) state, with two counterpropagating spin-momentum

locked channels along each edge.

Ultrathin (Bi_1−xSb_x)₂Te₃ is a candidate material, as tuning the Bi/Sb ratio moves the Dirac point into

the bulk band gap [3]. We optimize the morphology of ultrathin (Bi_1−xSb_x)₂Te₃ on Al₂O₃ substrates

by molecular beam epitaxy, by tuning both substrate temperature and vicinal angle. This leads to film

thicknesses expected to lie within the QSH regime.

The films are structured into micrometer-sized devices to perform both local and non-local transport

measurements at cryogenic temperatures. Due to the morphology of the films, we do not expect

a coherent percolation path between source and drain contacts. However, 1D contributions to the

conductivity can be extracted by comparing the scaling of features as a function of device geometry [4].

Based on this, we study the possibility of both 2D and 1D conductance based on electronic transport

data.

References:

[1] PRB 81, 041307 (2010)

[2] PRB 97, 075419 (2018)

[3] Nat Commun 2, 574 (2011)

[4] PRL 123, 047701 (2019)   

Fermi surface topology and interlayer modulation of charge density waves in the kagome material CsV3Sb5

The recently discovered kagome materials AV₃Sb₅ (A = K, Rb, Cs) attract intense research interest in intertwined topology, superconductivity, and charge density waves (CDW). Although the in-plane 2x2 CDW is well studied, the electronic properties induced by the structural modulation between neighboring kagome layers are less understood. In this work, we investigated the Fermi surface reconstruction and topology in the presence of interlayer CDW (2x2x2) on CsV₃Sb₅ by theoretical calculations. We derived Fermi-energy-resolved and layer-resolved quantum orbits that agree quantitatively with experimental results. We found that Dirac nodal lines and Dirac networks, a topological feature beyond the Z₂ topology, induce a π Berry phase for several quantum orbits. Our work reveals the quasi-two-dimensional nature of Fermi surfaces and the rich topological nature of kagome materials.