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D. Hong, N. Anand, C. Liu, H. Liu, I. Arslan, J. E. Pearson, A. Bhattacharya, J. S. Jiang. Large anomalous Nernst and inverse spin-Hall effects in epitaxial thin films of Kagome semimetal Mn3Ge. Physical Review Materials 4, 094201 (2020).

Fabrication of crystallographically well-defined thin films of topological materials is important for unraveling their mesoscale quantum properties and for device applications. Mn3Ge, an antiferromagnetic Weyl semimetal with a chiral spin texture on a Kagome lattice, is expected to have enhanced Berry curvature around Weyl nodes near the Fermi energy, leading to large anomalous Hall / Nernst effects and spin-Hall effect in transport. Using magnetron sputtering, we have grown epitaxial thin films of hexagonal D019 Mn3Ge that are highly crystalline, flat and continuous. Large anomalous Nernst and inverse spin-Hall effects are observed in thermoelectric and spin pumping devices. The anomalous Nernst coefficient in the Mn3Ge films is estimated to be 0.1 {\mu}V/K, and is comparable to that of Fe, despite Mn3Ge having a weak magnetization of ~ 0.003 {\mu}B / Mn atom. The spin mixing conductance and spin-Hall angle in Mn3Ge are estimated to be 3.5 and 7 times of those in Pt, respectively.

N. Bultinck, E. Khalaf, S. Liu, S. Chatterjee, A. Vishwanath, M. P. Zaletel. Ground state and hidden symmetry of magic-angle graphene at even integer fillingPhysical Review X 10, 031034 (2020).

In magic angle twisted bilayer graphene (TBG), electron-electron interactions play a central role, resulting in correlated insulating states at certain integer fillings. Identifying the nature of these insulators is a central question, and it is potentially linked to the relatively high-temperature superconductivity observed in the same devices. Here, we address this question using a combination of analytical strong-coupling arguments and a comprehensive Hartree-Fock numerical calculation, which includes the effect of remote bands. The ground state we obtain at charge neutrality is an unusual ordered state, which we call the Kramers intervalley-coherent (K-IVC) insulator. In its simplest form, the K-IVC order exhibits a pattern of alternating circulating currents that triples the graphene unit cell, leading to an “orbital magnetization density wave.” Although translation and time-reversal symmetry are broken, a combined “Kramers” time-reversal symmetry is preserved. Our analytic arguments are built on first identifying an approximate U(4)× U(4)symmetry, resulting from the remarkable properties of the TBG band structure, which helps select a low-energy manifold of states that are further split to favor the K-IVC state. This low-energy manifold is also found in the Hartree-Fock numerical calculation. We show that symmetry-lowering perturbations can stabilize other insulators and the semimetallic state, and we discuss the ground state at half-filling and give a comparison with experiments.

N. H. Jo, L.-L. Wang, R.-J. Slager, J. Yan, Y. Wu, K. Lee, B. Schrunk, A. Vishwanath, A. Kaminski. Intrinsic axion insulating behavior in antiferromagnetic MnBi6Te10Physical Review B 102, 045130 (2020).

A striking feature of time reversal symmetry (TRS) protected topological insulators (TIs) is that they are characterized by a half integer quantum Hall effect on the boundary when the surface states are gapped by time reversal breaking perturbations. While time reversal symmetry (TRS) protected TIs have become increasingly under control, magnetic analogs are still largely unexplored territories with novel rich structures. In particular, topological magnetic insulators can also host a quantized axion term in the presence of lattice symmetries. Since these symmetries are naturally broken on the boundary, the surface states can develop a gap without external manipulation. In this work, we combine theoretical analysis, density functional calculations and experimental evidence to reveal intrinsic axion insulating behavior in MnBi6Te10. The quantized axion term arises from the simplest possible mechanism in the antiferromagnetic regime where it is protected by inversion symmetry and a fractional translation symmetry. The anticipated gapping of the Dirac surface state at the edge is subsequently experimentally established using Angle Resolved Spectroscopy. As a result, this system provides the magnetic analogue of the simplest TRS protected TI with a single, gapped Dirac cone at the surface.

A. Bouhon, Q.S. Wu, R.-J. Slager, H. Weng, O.V. Yazyev, T. Bzdušek. Non-Abelian reciprocal braiding of Weyl points and its manifestation in ZrTeNature Physics (2020).

Weyl semimetals in three-dimensional crystals provide the paradigm example of topologically protected band nodes. It is usually taken for granted that a pair of colliding Weyl points annihilate whenever they carry opposite chiral charge. In stark contrast, here we report that Weyl points in systems that are symmetric under the composition of time reversal with a π rotation are characterized by a non-Abelian topological invariant. The topological charges of the Weyl points are transformed via braid phase factors, which arise upon exchange inside symmetric planes of the reciprocal momentum space. We elucidate this process with an elementary two-dimensional tight-binding model that is implementable in cold-atom set-ups and in photonic systems. In three dimensions, interplay of the non-Abelian topology with point-group symmetry is shown to enable topological phase transitions in which pairs of Weyl points may scatter or convert into nodal-line rings. By combining our theoretical arguments with first-principles calculations, we predict that Weyl points occurring near the Fermi level of zirconium telluride carry non-trivial values of the non-Abelian charge, and that uniaxial compression strain drives a non-trivial conversion of the Weyl points into nodal lines.

M. Thakurathi, A. A. Burkov. Theory of the fractional quantum Hall effect in Weyl SemimetalsPhysical Review B 101, 235168 (2020).

We develop a hydrodynamic field theory of the three-dimensional fractional quantum Hall effect, which was recently proposed to exist in magnetic Weyl semimetals, when the Weyl nodes are gapped by strong repulsive interactions. This theory takes the form of a BF theory, which contains both one-form and two-form gauge fields, coupling to quasiparticle and loop excitations correspondingly. It may be regarded as a generalization of the Chern-Simons theory of two-dimensional fractional quantum Hall liquids to three dimensions.

C. Vaswani, L.-L. Wang, D. H. Mudiyanselage, Q. Li, P. M. Lozano, G. Gu, D. Cheng, B. Song, L. Luo, R. H. J. Kim, C. Huang, Z. Liu, M. Mootz, I. E. Perakis, Y. Yao, K. M. Ho, J. Wang. Light-Driven Raman Coherence as a Nonthermal Route to Ultrafast Topology Switching in a Dirac SemimetalPhysical Review X 10, 021013 (2020).

A grand challenge underlies the entire field of topology-enabled quantum logic and information science: how to establish topological control principles driven by quantum coherence and understand the time dependence of such periodic driving. Here we demonstrate a few-cycle THz-pulse-induced phase transition in a Dirac semimetal ZrTe5 that is periodically driven by vibrational coherence due to excitation of the lowest Raman active mode. Above a critical THz-pump field threshold, there emerges a long-lived metastable phase, approximately 100 ps, with unique Raman phonon-assisted topological switching dynamics absent for optical pumping. The switching also manifests itself by distinct features: nonthermal spectral shape, relaxation slowing near the Lifshitz transition where the critical Dirac point occurs, and diminishing signals at the same temperature that the Berry-curvature-induced anomalous Hall effect magnetoresistance vanishes. These results, together with first-principles modeling, identify a mode-selective Raman coupling that drives the system from strong to weak topological insulators with a Dirac semimetal phase established at a critical atomic displacement controlled by the phonon coherent pumping. Harnessing of vibrational coherence can be extended to steer symmetry-breaking transitions, i.e., Dirac to Weyl ones, with implications for THz topological quantum gate and error correction applications.

P. Swatek, Y. Wu, L.-L. Wang, K. Lee, B. Schrunk, J. Yan, A. Kaminski. Gapless Dirac surface states in the antiferromagnetic topological insulator MnBi2Te4Physical Review B 101, 161109(R) (2020).

We used angle-resolved photoemission spectroscopy (ARPES) and density functional theory calculations to study the electronic properties of MnBi2Te4, a material that was predicted to be an intrinsic antiferromagnetic (AFM) topological insulator. In striking contrast to earlier literature showing a full gap opening between two surface band manifolds on the (0001) surface, we observed a gapless Dirac surface state with a Dirac point sitting at EB = −280 meV. Furthermore, our ARPES data revealed the existence of a second Dirac cone sitting closer to the Fermi level. Surprisingly, these surface states remain intact across the AFM transition. The presence of gapless Dirac states in this material may be caused by different ordering at the surface from the bulk or weaker magnetic coupling between the bulk and surface. Whereas the surface Dirac cones seem to be remarkably insensitive to the AFM ordering most likely due to weak coupling to magnetism, we did observe a splitting of the bulk band accompanying the AFM transition. With a moderately high ordering temperature and interesting gapless Dirac surface states, MnBi2Te4 provides a unique platform for studying the interplay between magnetic ordering and topology.

N.H. Jo, B. Kuthanazhi, Y. Wu, E. Timmons, T.H. Kim, L. Zhou, L.L. Wang, B.G. Ueland, A. Palasyuk, D.H. Ryan, R.J. McQueeney, K. Lee, B Schrunk, A.A. Burkov, R. Prozorov, S.L. Bud'ko, A. Kaminski, P.C. Canfield. Manipulating magnetism in the topological semimetal EuCd2As2Physical Review B 101, 140402(R) (2020).

Magnetic Weyl semimetals are expected to have extraordinary physical properties such as a chiral anomaly and large anomalous Hall effects that may be useful for future, potential, spintronics applications. However, in most known host materials, multiple pairs of Weyl points prevent a clear manifestation of the intrinsic topological effects. Our recent density functional theory (DFT) calculations study suggest that EuCd2As2 can host Dirac fermions in an antiferromagnetically (AFM) ordered state or a single pair of Weyl fermions in a ferromagnetically (FM) ordered state. Unfortunately, previously synthesized crystals ordered antiferromagnetically with TN\,≃\,9.5\,K. Here, we report the successful synthesis of single crystals of EuCd2As2 that order ferromagnetically (FM) or antiferromagnetically (AFM) depending on the level of band filling, thus allowing for the use of magnetism to tune the topological properties within the same host. We explored their physical properties via magnetization, electrical transport, heat capacity, and angle resolved photoemission spectroscopy (ARPES) measurements and conclude that EuCd2As2 is an excellent, tunable, system for exploring the interplay of magnetic ordering and topology.

D. Wang, F. Tang, H. C. Po, A. Vishwanath, X. Wan. XFe4Ge2 (X = Y, Lu) and Mn3Pt: Filling-enforced magnetic topological metalsPhysical Review B 101, 115122 (2020).

Magnetism, coupled with nontrivial band topology, can bring about many interesting and exotic phenomena, so that magnetic topological materials have attracted persistent research interest. However, compared with non-magnetic topological materials (TMs), the magnetic TMs are less studied, since their magnetic structures and topological phase transitions are usually complex and the first-principles predictions are usually sensitive on the effect of Coulomb interaction. In this work, we present a comprehensive investigation of XFe4Ge2 (X = Y, Lu) and Mn3Pt, and find these materials to be filling-enforced magnetic topological metals. Our first-principles calculations show that XFe4Ge2 (X = Y, Lu) host Dirac points near the Fermi level at high symmetry point S. These Dirac points are protected by PT symmetry (P and T are inversion and time-reversal transformations, respectively) and a 2-fold screw rotation symmetry. Moreover, through breaking PT symmetry, the Dirac points would split into Weyl nodes. Mn3Pt is found to host 4-fold degenerate band crossings in the whole high symmetry path of A-Z. We also utilize the GGA+U scheme to take into account the effect of Coulomb repulsion and find that the filling-enforced topological properties are naturally insensitive on U.

C. Wang, L. Gioia, A. A. Burkov. Fractional Quantum Hall Effect in Weyl SemimetalsPhysical Review Letters 124, 096603 (2020).

Weyl semimetal may be thought of as a gapless topological phase protected by the chiral anomaly, where the symmetries involved in the anomaly are the U(1) charge conservation and the crystal translational symmetry. The absence of a band gap in a weakly-interacting Weyl semimetal is mandated by the electronic structure topology and is guaranteed as long as the symmetries and the anomaly are intact. The nontrivial topology also manifests in the Fermi arc surface states and topological response, in particular taking the form of an anomalous Hall effect in magnetic Weyl semimetals, whose magnitude is only determined by the location of the Weyl nodes in the Brillouin zone. Here we consider the situation when the interactions are not weak and ask whether it is possible to open a gap in a magnetic Weyl semimetal while preserving its nontrivial electronic structure topology along with the translational and the charge conservation symmetries. Surprisingly, the answer turns out to be yes. The resulting topologically ordered state provides a nontrivial realization of the fractional quantum Hall effect in three spatial dimensions in the absence of an external magnetic field, which cannot be viewed as a stack of two dimensional states. Our state contains loop excitations with nontrivial braiding statistics when linked with lattice dislocations.

S.-Y. Xu, Q. Ma, Y. Gao, A. Kogar, G. Zong, A. M. Mier Valdivia, T. H. Dinh, S.-M. Huang, B. Singh, C.H. Hsu, T.-R. Chang, J. R. C. Ruff, K. Watanabe, T. Taniguchi, T.-R. Chang, H. Lin, G. Karapetrov, D. Xiao, P. Jarillo-Herrero, N. Gedik. Spontaneous gyrotropic electronic order in a transition-metal dichalcogenideNature 578, 545-549 (2020). 

Nature Cover, Made to Order

Chirality is ubiquitous in nature, and populations of opposite chiralities are surprisingly asymmetric at fundamental levels. Examples range from parity violation in the subatomic weak force to homochirality in biomolecules. The ability to achieve chirality-selective synthesis (chiral induction) is of great importance in stereochemistry, molecular biology and pharmacology. In condensed matter physics, a crystalline electronic system is geometrically chiral when it lacks mirror planes, space-inversion centres or rotoinversion axes. Typically, geometrical chirality is predefined by the chiral lattice structure of a material, which is fixed on formation of the crystal. By contrast, in materials with gyrotropic order, electrons spontaneously organize themselves to exhibit macroscopic chirality in an originally achiral lattice. Although such order—which has been proposed as the quantum analogue of cholesteric liquid crystals—has attracted considerable interest, no clear observation or manipulation of gyrotropic order has been achieved so far. Here we report the realization of optical chiral induction and the observation of a gyrotropically ordered phase in the transition-metal dichalcogenide semimetal 1T-TiSe2. We show that shining mid-infrared circularly polarized light on 1T-TiSe2 while cooling it below the critical temperature leads to the preferential formation of one chiral domain. The chirality of this state is confirmed by the measurement of an out-of-plane circular photogalvanic current, the direction of which depends on the optical induction. Although the role of domain walls requires further investigation with local probes, the methodology demonstrated here can be applied to realize and control chiral electronic phases in other quantum materials.

M. Nakagawa, R.-J. Slager, S. Higashikawa, T. Oka. Wannier representation of Floquet topological statesPhysical Review B 101, 075108 (2020).

A universal feature of topological insulators is that they cannot be adiabatically connected to an atomic limit, where individual lattice sites are completely decoupled. This property is intimately related to a topological obstruction to constructing a localized Wannier function from Bloch states of an insulator. Here we generalize this characterization of topological phases toward periodically driven systems. We show that nontrivial connectivity of hybrid Wannier centers in momentum space and time can characterize various types of topology in periodically driven systems, which include Floquet topological insulators, anomalous Floquet topological insulators with micromotion-induced boundary states, and gapless Floquet states realized with topological Floquet operators. In particular, nontrivial time dependence of hybrid Wannier centers indicates impossibility of continuous deformation of a driven system into an undriven insulator, and a topological Floquet operator implies an obstruction to constructing a generalized Wannier function which is localized in real and frequency spaces. Our results pave a way to a unified understanding of topological states in periodically driven systems as a topological obstruction in Floquet states.

E. Lachman, R. A. Murphy, N. Maksimovic, R. Kealhofer, S. Haley, R. D. McDonald, J. R. Long, J. G. Analytis. Exchange biased Anomalous Hall Effect driven by frustration in a magnetic Kagome latticeNature Communications 11, 560 (2020).

Co3Sn2S2 is a ferromagnetic Weyl semimetal that has been the subject of intense scientific interest due to its large anomalous Hall effect. We show that the coupling of this material's topological properties to its magnetic texture leads to a strongly exchange biased anomalous Hall effect. We argue that this is likely caused by the coexistence of ferromagnetism and spin glass phases, the latter being driven by the geometric frustration intrinsic to the Kagome network of magnetic ions.

N. H. Jo, L.-L. Wang, P. P. Orth, S. L. Bud’ko, and P. C. Canfield. Magnetoelastoresistance in WTe2: Exploring electronic structure and extremely large magnetoresistance under strainPNAS 51, 25524 (2019).

Strain describes the deformation of a material as a result of applied stress. It has been widely employed to probe transport properties of materials, ranging from semiconductors to correlated materials. In order to understand, and eventually control, transport behavior under strain, it is important to quantify the effects of strain on the electronic bandstructure, carrier density, and mobility. Here, we demonstrate that much information can be obtained by exploring magnetoelastoresistance (MER), which refers to magnetic field-driven changes of the elastoresistance. We use this powerful approach to study the combined effect of strain and magnetic fields on the semimetallic transition metal dichalcogenide WTe2. We discover that WTe2 shows a large and temperature-nonmonotonic elastoresistance, driven by uniaxial stress, that can be tuned by magnetic field. Using first-principle and analytical low-energy model calculations, we provide a semiquantitative understanding of our experimental observations. We show that in WTe2, the strain-induced change of the carrier density dominates the observed elastoresistance. In addition, the change of the mobilities can be directly accessed by using MER. Our analysis also reveals the importance of a heavy-hole band near the Fermi level on the elastoresistance at intermediate temperatures. Systematic understanding of strain effects in single crystals of correlated materials is important for future applications, such as strain tuning of bulk phases and fabrication of devices controlled by strain.

D. Wang, F. Tang, J. Ji, W. Zhang, A. Vishwanath, H. C. Po, X. Wan. Two-dimensional topological materials discovery by symmetry-indicator methodPhysical Review B 100, 195108 (2019).

Two-dimensional (2D) topological materials (TMs) have attracted tremendous attention due to the promise of revolutionary devices with nondissipative electric or spin currents. Unfortunately, the scarcity of 2D TMs holds back the experimental realization of such devices. In this work, based on our recently developed, highly efficient TM discovery algorithm using symmetry indicators, we explore the possible 2D TMs in all nonmagnetic compounds in four recently proposed materials databases for possible 2D materials. We identify hundreds of 2D TM candidates, including 205 topological (crystalline) insulators and 299 topological semimetals. In particular, we highlight MoS, with a mirror Chern number of −4, as a possible experimental platform for studying the interaction-induced modification to the topological classification of materials. Our results winnow out the topologically interesting 2D materials from these databases and provide a TM gene pool for further experimental studies.

S. L. Zhang, A. A. Burkov, I. Martin, O G. Heinonen. Spin-to-charge conversion in magnetic Weyl semimetalsPhysical Review Letters 122, 197401 (2019).

Weyl semimetals (WSMs) are a newly discovered class of quantum materials which can host a number of exotic bulk transport properties, such as the chiral magnetic effect, negative magneto-resistance, and the anomalous Hall effect. In this work, we investigate theoretically the spin-to-charge conversion in a bilayer consisting of a magnetic WSM and a normal metal (NM), where a charge current can be induced in the WSM by an spin current injection at the interface. We show that the induced charge current exhibits a peculiar anisotropy: it vanishes along the magnetization orientation of the magnetic WSM, regardless of the direction of the injected spin. This anisotropy originates from the unique band structure of magnetic WSMs and distinguishes the spin-to-charge conversion effect in WSM/NM structures from that observed in other systems, such as heterostructures involving heavy metals or topological insulators. The induced charge current depends strongly on injected spin orientation, as well as on the position of the Fermi level relative to the Weyl nodes and the separation between them. These dependencies provide additional means to control and manipulate spin-charge conversion in these topological materials.

F. N. Ünal, A. Eckardt, R.-J. Slager. Hopf characterization of two-dimensional Floquet topological insulatorsPhysical Review Research 1, 022003(R) (2019).

We present a topological characterization of time-periodically driven two-band models in 2+1 dimensions as Hopf insulators. The intrinsic periodicity of the Floquet system with respect to both time and the underlying two-dimensional momentum space constitutes a map from a three-dimensional torus to the Bloch sphere. As a result, we find that the driven system can be understood by appealing to a Hopf map that is directly constructed from the micromotion of the drive. Previously found winding numbers are shown to correspond to Hopf invariants, which are associated with linking numbers describing the topology of knots in three dimensions. Moreover, after being cast as a Hopf insulator, not only the Chern numbers but also the winding numbers of the Floquet topological insulator become accessible in experiments as linking numbers. We exploit this description to propose a feasible scheme for measuring the complete set of their Floquet topological invariants in optical lattices.

L.-L. Wang, N. H. Jo, B. Kuthanazhi, Y. Wu, R. J. McQueeney, A. Kaminski, P. C. Canfield. Single Pair of Weyl Fermions in Half-metallic Semimetal EuCd2As2Physical Review B 99, 245147 (2019).

Materials with the ideal case of a single pair of Weyl points (WPs) are highly desirable for elucidating the unique properties of Weyl fermions. EuCd2As2 is an antiferromagnetic topological insulator or Dirac semimetal depending on the different magnetic configurations. Using first-principles band-structure calculations, we show that inducing ferromagnetism in EuCd2As2 can generate a single pair of WPs from splitting the single pair of antiferromagnetic Dirac points due to its half-metallic nature. Analysis with a low-energy effective Hamiltonian shows that a single pair of WPs is obtained in EuCd2As2 because the Dirac points are very close to the zone center and the ferromagnetic exchange splitting is large enough to push one pair of WPs to merge and annihilate at Γ while the other pair survives. Furthermore, we predict that alloying with Ba at the Eu site can stabilize the ferromagnetic configuration and generate a single pair of Weyl points without application of a magnetic field.

M. Goyal, H. Kim, T. Schumann, L. Galletti, A. A. Burkov, S. Stemmer. Surface states of strained thin films of the Dirac semimetal Cd3As2Physical Review Materials 3, 064204 (2019).

We report on the growth and transport properties of strained thin films of the three-dimensional Dirac semimetal Cd3As2. Epitaxial heterostructures, consisting of (112)-oriented Cd3As2 films, are grown on nearly lattice matched (Ga1−xInx)Sb buffer layers on (111) GaAs substrates by molecular beam epitaxy. The epitaxial coherency strain breaks the fourfold rotational symmetry, which protects the bulk Dirac nodes in Cd3As2. All strained films exhibit the quantum Hall effect with most carriers residing in the two-dimensional states, irrespective of the sign of the biaxial stress. The Hall mobility monotonically increases as the biaxial stress is changed from compressive towards tensile. Furthermore, pronounced anisotropy is seen in the transport properties. The results show that the quantum Hall effect, which is quite similar to that of unstrained (112)-oriented films, is independent of the presence of bulk Dirac nodes. Its appearance is consistent with the topological surface states that are a characteristic of the topological Z2 invariant.

N. Sirica, R. I. Tobey, L. X. Zhao, G. F. Chen, B. Xu, R. Yang, B. Shen, D. A. Yarotski, P. Bowlan, S. A. Trugman, J.-X. Zhu, Y. M. Dai, A. K. Azad, N. Ni, X. G. Qiu, A. J. Taylor, R. P. Prasankumar. Tracking ultrafast photocurrents in the Weyl semimetal TaAs using THz emission spectroscopyPhysical Review Letters 122, 197401 (2019).

We investigate polarization-dependent ultrafast photocurrents in the Weyl semimetal TaAs using terahertz (THz) emission spectroscopy. Our results reveal that highly directional, transient photocurrents are generated along the non-centrosymmetric c-axis regardless of incident light polarization, while helicity-dependent photocurrents are excited within the ab-plane. This is consistent with static photocurrent experiments, and provides additional insight into their microscopic origin by way of the dynamical information gained from the emitted THz waveform. THz emission spectroscopy is thus a powerful, contact-free approach for distinguishing between injection and shift photocurrents by unraveling the polarization dependence, directionality, and intrinsic timescales that underlie their generation and decay.


 Lin-Lin Wang, Hoi Chun Po, Robert-Jan Slager, Ashvin Vishwanath. Topological Descendants of Multicritical EuTl2ArXiv September 2020.

The past decades have witnessed a transformation in characterizing condensed matter systems with topology. Aided by a refined understanding of topological band structures with crystalline symmetries that has emerged recently, many electronic phases have been identified and a plethora of materials have been predicted to host novel properties and functionalities. A key underlying question, also with respect to future application, is to what extent the related physical features can be manipulated, especially in the context of magnetic order. Here we describe a paradigmatic semimetal that simultaneously incorporates multiple, and sometimes conflicting topology which guarantees gaplessness and leads to an exceptionally rich family of descendent phases on lowering symmetry. We predict that this multicritical phase is realized in EuTl2. Starting from the parent semimetallic state, which already separates two topological insulating regimes, the interplay of inherent magnetism and strain allows for an exceptionally rich phase diagram of topological descendant states.

Santanu Pakhira, Thomas Heitmann, S. X. M. Riberolles, B. G. Ueland, R. J. McQueeney, D. C. Johnston, David Vaknin. Zero field magnetic ground state of EuMg2Bi2ArXiv September 2020.

Layered trigonal EuMg2Bi2 is reported to be a topological semimetal that hosts multiple Dirac points that may be gapped or split by the onset of magnetic order. Here, we report zero-field single-crystal neutron-diffraction and bulk magnetic susceptibility measurements versus temperature χ(T) of EuMg2Bi2 that show the intraplane ordering is ferromagnetic (Eu2+,S=7/2) with the moments aligned in the ab-plane while adjacent layers are aligned antiferromagnetically (i.e., A-type antiferromagnetism) below the Néel temperature.

Zhao Huang, Christopher A. Lane, Chao Cao, Guo-Xiang Zhi, Yu Liu, Christian Matt, J. E. Hoffman, Dmitry Yarotski, A. J. Taylor, Jian-Xin Zhu. Signatures of Spin Polarized Fermi Arcs in the Quasiparticle Interferences of CeBiArXiv August 2020.

We report that CeBi in the ferromagnetic state is a Weyl semimetal. Our calculations within density functional theory show the existence of two pairs of Weyl nodes on the momentum path (0,0,kz) at 15 meV above and 100 meV below the Fermi level. Two corresponding Fermi arcs are obtained on surfaces of mirror-symmetric (010)-oriented slabs at E=15 meV and both arcs are interrupted into three segments due to hybridization with a set of trivial surface bands. By studying the spin texture of surface states, we find the two Fermi arcs are strongly spin-polarized but in opposite directions, which can be detected by spin-polarized ARPES measurements. Our study of quasiparticle interference (QPI) for a nonmagnetic impurity at the Bi site also uncovers several features related to the Fermi arcs. Specifically, the spin polarization of the Fermi arcs leads to a bifurcation-shaped feature only in the spin-dependent QPI spectrum, serving as a fingerprint of the Weyl nodes.

S.X.M. Riberolles, T.V. Trevisan, B. Kuthanazhi, T.W. Heitmann, F. Ye, D. C. Johnston, S. L. Bud'ko, D. H. Ryan, P.C. Canfield, A. Kreyssig, A. Vishwanath, R.J. McQueeney, L.L. Wang, P.P. Orth, B.G. Ueland. Magnetic crystalline-symmetry-protected axion electrodynamics and field-tunable unpinned Dirac cones in EuIn2As2ArXiv July 2020.

Knowledge of magnetic symmetry is vital for exploiting nontrivial surface states of magnetic topological materials. EuIn2As2 is an excellent example, as it is predicted to have collinear antiferromagnetic order where the magnetic moment direction determines either a topological-crystalline-insulator phase supporting axion electrodynamics or a higher-order-topological-insulator phase with chiral hinge states. Here, we use neutron diffraction, symmetry analysis, and density functional theory results to demonstrate that EuIn2As2 actually exhibits low-symmetry helical antiferromagnetic order which makes it a stoichiometric magnetic topological-crystalline axion insulator protected by the combination of a 180∘ rotation and time-reversal symmetries: C2×T=2′. Surfaces protected by 2′ are expected to have an exotic gapless Dirac cone which is unpinned to specific crystal momenta. All other surfaces have gapped Dirac cones and exhibit half-integer quantum anomalous Hall conductivity. We predict that the direction of a modest applied magnetic field of H≈1 to 2 T can tune between gapless and gapped surface states.

P.V. Arribi, J.-X. Zhu, T. Schumann, S. Stemmer, A.A. Burkov, O. Heinonen. Topological surface states in strained Dirac semimetal thin filmsArXiv June 2020.

Symmetry plays a central role in conventional and topological phases of matter, making the ability to optically drive symmetry change a critical step in developing future technologies that rely on such control. Topological materials, like the newly discovered topological semimetals, are particularly sensitive to a breaking or restoring of time-reversal and crystalline symmetries, which affect both bulk and surface electronic states. While previous studies have focused on controlling symmetry via coupling to the crystal lattice, we demonstrate here an all-electronic mechanism based on photocurrent generation. Using second-harmonic generation spectroscopy as a sensitive probe of symmetry change, we observe an ultrafast breaking of time-reversal and spatial symmetries following femtosecond optical excitation in the prototypical type-I Weyl semimetal TaAs. Our results show that optically driven photocurrents can be tailored to explicitly break electronic symmetry in a generic fashion, opening up the possibility of driving phase transitions between symmetry-protected states on ultrafast time scales.

N. Sirica, P. P. Orth, M. S. Scheurer, Y.M. Dai, M.-C. Lee, P. Padmanabhan, L. T. Mix, L. X. Zhao, G. F. Chen, B. Xu, R. Yang, B. Shen, C.-C. Lee, H. Lin, T. A. Cochran, S. A. Trugman, J.-X. Zhu, M. Z. Hasan, N. Ni, X. G. Qiu, A. J. Taylor, D. A. Yarotski, R. P. Prasankumar. Photocurrent-driven transient symmetry breaking in the Weyl semimetal TaAsArXiv May 2020.

Symmetry plays a central role in conventional and topological phases of matter, making the ability to optically drive symmetry change a critical step in developing future technologies that rely on such control. Topological materials, like the newly discovered topological semimetals, are particularly sensitive to a breaking or restoring of time-reversal and crystalline symmetries, which affect both bulk and surface electronic states. While previous studies have focused on controlling symmetry via coupling to the crystal lattice, we demonstrate here an all-electronic mechanism based on photocurrent generation. Using second-harmonic generation spectroscopy as a sensitive probe of symmetry change, we observe an ultrafast breaking of time-reversal and spatial symmetries following femtosecond optical excitation in the prototypical type-I Weyl semimetal TaAs. Our results show that optically driven photocurrents can be tailored to explicitly break electronic symmetry in a generic fashion, opening up the possibility of driving phase transitions between symmetry-protected states on ultrafast time scales.

P. Devi, L. L. Wang, C. Abel, S. L. Bud'ko, P. C. Canfield. Study of single crystalline SrAgSb and SrAuSb semimetalsArXiv May 2020.

Given renewed interest in the electronic properties of semimetallic compounds with varying degrees of spin orbit coupling we have grown single crystals of SrAgSb and SrAuSb, measured their temperature and field dependent electrical resistivity and magnetization and performed density functional theory (DFT) band structure calculations. Magnetization measurements are consistent with a diamagnetic host with a small amount of local moment bearing impurities. Although the residual resistivity ratio (RRR) for all samples studied was relatively low, ranging between 2.4 and 3.4, the compounds had non-saturating magnetoresistance (MR), reaching values of ∼ 17% and ∼ 70% at 4 K and 9 T for SrAgSb and SrAuSb respectively. Band structure calculations, using the experimentally determined Wyckoff positions for the Sr, Ag/Au, and Sb atoms, show that whereas SrAgSb is a topologically trivial, but compensated, semimetal; SrAuSb is a topologically non-trivial, Dirac semimetal.

L. L. Wang. Expansive Open Fermi Arcs and Connectivity Changes Induced by Infrared Phonons in ZrTe5Arxiv April 2020. 

Expansive open Fermi arcs covering most of the surface Brillouin zone (SBZ) are desirable for detection and control of many topological phenomena, but so far has been only reported for Kramers-Weyl points, or unconventional chiral fermions, pinned at time-reversal invariant momentum in chiral materials. Here using first-principles band structure calculations, we show that for conventional Weyl points with the chirality of +1/-1 near the BZ center at general momentum induced by the infrared phonon B1u mode of 44.1 cm-1 for breaking inversion symmetry in ZrTe5, they can also form expansive open Fermi arcs across the SBZ boundary to occupy most of the SBZ when projected on (001) surface. We reveal that such expansive open Fermi arcs are evolved from the topological surface states that connect the three surface Dirac points on the (001) surface of the strong topological insulator phase without lattice distortion in ZrTe5. Furthermore, we find that the connectivity of the induced open Fermi arcs can be changed by the magnitude of the lattice distortion of this infrared phonon mode. Thus, we propose that using coherent optical phonon to modulate lattice parameters can offer ways to induce novel topological features including expansive open Fermi arcs and dynamically control Fermi arcs connectivity in ZrTe5.

D. Hong, C. Liu, H. W. Hsiao, D. Jin, J. E. Pearson, J. M. Zuo, A. Bhattacharya. Molecular beam epitaxy of the magnetic kagome metal FeSn on LaAlO3 (111)ArXiv January 2020.

Materials with a layered Kagome lattice are expected to give rise to novel physics arising from band structures with topological properties, spin liquid behavior and the formation of skyrmions. Until now, most work on Kagome materials has been performed on bulk samples due to difficulties in thin film synthesis. Here, by using molecular beam epitaxy, layered Kagome-structured FeSn films are synthesized on (111) oriented LaAlO3 substrate. Both in-situ and ex-situ characterizations indicate these films are highly crystalline and c-axis oriented, with atomically smooth surfaces. However, the films grow as disconnected islands, with lateral dimensions on the micron scale. By patterning Pt electrodes using a focused electron beam, longitudinal and transverse resistance of single islands have been measured in magnetic fields. Our work opens a pathway for exploring mesoscale transport properties in thin films of Kagome materials and related devices.

B. Kuthanazhi, N. H. Jo, L. Xiang, S. L. Bud'ko, P. C. Canfield. Metamagnetism and magnetoresistance in CeBi single crystalsArXiv December 2019.

We report the synthesis of CeBi single crystals grown out of Bi self flux and a systematic study of the magnetic and transport properties with varying temperature and applied magnetic fields. From these R(T,B)and M(T,B) data we could assemble the temperature-field (T−B) phase diagram for CeBi and visualize the three dimensional M−T−B surface. The magnetoresistance (MR) in the low temperature regime shows a power-law, non-saturated behavior with large MR (∼3×105% at 2 K and 14 T), along with Shubnikov-de Haas oscillations. With increasing temperatures, MR decreases, and then becomes negative for T≳12 K. This crossover in MR seems to be unrelated to any specific metamagnetic transitions, but rather associated with changing from a low-temperature normal metal with an anomalously large MR to increased scattering off of local Ce moments as temperature increases.