Session B55: Dirac and Weyl Semimetals: Experiment

Transport properties of topological semimetal candidate ReSbTe (Re = Sm, Nd, Pr)

The breakthrough in the discovery of topological semimetals provides opportunities to explore the exotic properties of relativistic fermions in condensed matter. Recently, the nonsymmorphic compound ReSbTe (Re = rare earth) has attracted interested owing to its non-trivial topological electronic state. Here we present the magnetic transport properties of these topological semimetal candidates that uncovers it’s electronic and magnetic properties. The coupling of magnetism and electronic properties, plus the variation of magnetism for different rare earth compound, make this material family a tunable platform for investigating topological fermion physics and for further exploration of electronic and spintronic science and applications
  

Anomalous Hall effect in epitaxial WTe2/VTe2 heterostructures

The Td (orthorhombic) phase of WTe₂, a transition metal dichalcogenide (TMD), is a type-II Weyl semimetal (WSM) when a few monolayer thick. Topological semimetals with broken time-reversal symmetry have recently garnered attention due to the ability to manipulate Weyl nodes, causing interesting electro-magnetic responses. Realizing a magnetic WSM in the thin film form, however, is challenging. In this talk, we present our studies on the magnetic properties of WTe₂/VTe₂ thin film heterostructures. Using molecular beam epitaxy, we grow WTe₂ in its Td phase confirmed by in situ reflection high energy electron diffraction. Ultra-smooth film showing atomic terraces and Raman spectroscopy confirm film quality. We perform systematic transport measurements on WTe₂/VTe₂ heterostructures with vanadium doping to form (W_1-xV_x)Te₂, varying x from 0% to 20%. V-doping also changes the Fermi level in WTe₂ and induces a carrier type change. An anomalous Hall effect (AHE) signal is observed, whose magnitude varies with the doping level, reaching a minimum near the charge compensation point. The sign of AHE does not change with carrier type. The origin of the AHE will be discussed.   

Surface chiral metal in the bulk half-integer quantum Hall effect of BaMnSb2

The study of Quantum Hall effect (QHE) in two-dimensional (2D) systems such as 2D electron gas and graphene has led to important breakthroughs in the development of many new concepts in modern physics. Although the QHE is not generally expected for bulk materials due to the band dispersion along the magnetic field direction, bulk QHE has been observed in a few materials such as Cd₃As₂ (1), ZrTe₅ (2) and EuMnBi₂ (3). In this talk, we report a unique bulk half-integer QHE observed in a layered Dirac semimetal BaMnSb₂. In the extreme quantum limit, its quantum Hall state is accompanied by two-dimensional chiral metal at the surface, which represents a novel quantum liquid, never observed in any other bulk single crystal materials [4]. This finding establishes a promising experimental platform which not only provides access to 3D interacting topological states, but also offers opportunities to investigate the novel physics of 2D chiral metal.
References:
1. C. Zhang et al., Nature. 565, 331 (2019).
2. F. Tang et al., Nature. 569, 537 (2019).
3. H. Masuda et al., Science Advances. 2, e1501117 (2016).
4. J.Y. Liu et al., arXiv:1907.06318   

Towards Weyltronics: Realization of epitaxial NbP and TaP Weyl Semimetal thin films

Weyl Semimetals (WSMs), a recently discovered topological state of matter, exhibit an electronic structure governed by linear band dispersions and degeneracy (Weyl) points leading to rich physical phenomena, which are yet to be exploited in thin film devices. While WSMs were established in the monopnictide compound family several years ago, the growth of thin films has remained a challenge. Here, we report the growth of epitaxial thin films of NbP and TaP by means of molecular beam epitaxy. Single crystalline films are grown on MgO (001) substrates using thin Nb (Ta) buffer layers, and are found to be tensile strained (1%) and with slightly P-rich stoichiometry with respect to the bulk crystals. The resulting electronic structure exhibits topological surface states characteristic of a P-terminated surface and linear dispersion bands in agreement with the calculated band structure, along with a Fermi-level shift of -0.2 eV with respect to the Weyl points. The growth of epitaxial thin films opens up the use of strain and controlled doping to access and tune the electronic structure of Weyl Semimetals on demand, paving the way for the rational design and fabrication of electronic devices ruled by topology.   

Experimental electronic structure of the switchable, topological, antiferromagnet CuMnAs

Electrical switching and read out of the AFM Néel vector orientation in tetragonal CuMnAs has
been experimentally demonstrated to be scalable to THz speeds, far exceeding speeds in state-
of-the-art memory devices today. Additionally, CuMnAs is predicted to host two Dirac points
protected by a nonsymorphic, glide mirror plane symmetry, which may or may not exist
depending on the orientation of the Néel vector, offering the possibility of opening and closing a gap at the Dirac point at THz speeds. Until now, we lacked experimental confirmation of the predicted electronic band structure, but here we report the electronic structure of tetragonal CuMnAs experimentally measured with ARPES, which we compare to DFT. We performed experiments on the (001) surface of tetragonal CuMnAs thin films, measuring Fermi surfaces, kz dispersion, and high symmetry cuts.   

Temperature dependent infrared spectroscopy of PrAlSi

PrAlSi has been shown recently to provide a new platform for studying Weyl fermions and related effects, such as the anomalous Hall effect. In order to better understand the electronic and magnetic properties of its ground state, we performed temperature dependent optical reflectance measurements of single crystals of PrAlSi. The reflectance was measured between 80 cm^-1 to 50,000 cm^-1, at temperatures between 300 K and 5 K. The Drude peak sharpens with decreasing temperature, consistent with metallic behavior and in good agreement with dc resistivity measurements. From Kramers-Kronig analysis, we obtained various optical functions and here we focus in particular on the evolution of the spectral weight with temperature and energy in PrAlSi.   

Giant magneto-optical effect in the magnetic Weyl semimetal Co3Sn2S2

The search for the topological materials has rapidly developed and novel phase of matter has been extensively proposed in recent years. In particular, the discovery of the Weyl semimetal (WSM), which has a pair of the Wey points with intense Berry curvature, is the important advance in this field. While the WSM shows various giant/functional electromagnetic phenomena such as the nonlinear optical effects and anomalous Hall effect (AHE), the direct evidence that the Weyl point plays the decisive role for those phenomena is still lacking. In this presentation, we will report the magneto-optical study on the recently discovered magnetic WSM Co₃Sn₂S₂ with the giant AHE. The magneto-optical Kerr effect (MOKE) and first-principles calculations reveal that the optical Hall conductivity spectra are significantly dominated by the interband transition upon the nodal ring structures and the Weyl points. This observation demonstrates that those electronic structures play the decisive roles for the giant intrinsic AHE. We will also discuss the MOKE, which is exceptionally large compared with the conventional ferromagnetic metals.   

Extreme Electron-Phonon Coupling in a New Weyl Semimetal

Weyl semimetals are topological systems that have generated considerable interest due to their transport properties. One of the mechanisms that may play an important role in the manifestation of these transport properties is electron-phonon coupling. Previous optical experiments done on various Weyl semimetals have already shown phonon linewidths deviating from the usual model of anharmonic decay. Here we report on Raman measurements performed on a new Weyl semimetal. Careful analysis reveals the lifetime is consistent with electron-phonon coupling far in excess of the anharmonic decay channels.   

Doping induced topological phase transition in WTe2

WTe2 exibits diverse phenomena including non-saturating magnetoresistance in its bulk with predicted type-II Weyl semimetal electronic structure, superconductivity under hydrostatic pressure, quantum spin hall insulator (QSHI) in its monolayer limit, and superconductivity in the gated monolayer. In many of these transformations, the tuning of the carrier density plays an important role. We report two non-monotonic changes in the electronic structure of WTe2 upon in-situ electron doping, realizing occupation near the chemical potential as a pathway for tuning structure and behavior of 2D materials. The first phase transition is understood in terms of the extra charge inducing a shear mode that triggers the recombination of the Weyl points, lifting the topological nature of the bands. This topological switching behavior is relevant for gate-controlled topological transistors or for producing lateral interfaces within a single material host via differential doping. The second phase transition is most consistent with a electrons in a 2D K-overlayer hybridizing with the WTe2 host, broadly relevant to understanding interactions with metallic contacts in devices constructed from 2D materials.   

Electronic Correlations in Nodal-line Semimetals

Dirac fermions with highly-dispersive linear bands are usually considered weakly correlated, due to relatively large bandwidths (W) compared to Coulomb interactions (U). With the discovery of nodal-line semimetals, the notion of Dirac point has been extended to lines and loops in the momentum space. The anisotropy associated with nodal-line structure gives rise to greatly reduced kinetic energy along the line. However, experimental evidence for anticipated enhanced correlations in nodal-line semimetals is sparse. Here we report on prominent correlation effects in a nodal-line semimetal compound ZrSiSe through a combination of optical spectroscopy and density-functional-theory calculations. We observed two fundamental spectroscopic hallmarks of electronic correlations: strong reduction (1/3) of the free carrier Drude weight and of the Fermi velocity compared to predictions of density functional band theory. The renormalization of Fermi velocity can be further controlled with external magnetic field. ZrSiSe therefore offers the rare opportunity to investigate correlation-driven physics in a Dirac system.   

Correlated Dirac Semimetal State with Highly Mobile Electrons in Perovskite CaIrO3

The interplay between electron correlation and quantum topology of Dirac electrons have been a subject of great interest. The perovskite AIrO₃ (A=Ca, Sr, Ba) is a candidate of Dirac semimetal with the sufficiently strong correlation on the verge of Mott transition[1]. Although the Dirac band dispersion has been observed by the angle resolved photoemission spectroscopy[2], the quantum transport originating from highly mobile Dirac fermions has been rarely observed in this class of strongly correlated topological semimetals.
In this study, we report the unique quantum transport phenomena of correlated Dirac electrons in perovskite CaIrO₃ with the Mott criticality[3]. We found that the electron mobility exceeds 60,000 cm²/Vs and the giant magnetoresistance emerges in the quantum limit of the Dirac electrons. Combined with ab-initio calculations, we conclude that the interplay between electron correlation and spin-orbit interaction causes the remarkable proximity of Dirac node to the Fermi energy, yielding the highly mobile electrons. In the presentation, the origin of the giant magnetoresistance will also be discussed.
[1] M. Zeb and H. Kee, PRB 86, 085149 (2012)
[2] Y. F. Nie, et al., PRL 114, 016401 (2015)
[3] J. Fujioka, et al., Nat. Commun. 10, 362(2019)   

Unraveling the Topological Phase of ZrTe5 via Magneto-infrared Spectroscopy

We have systematically studied the 3D band structure of ZrTe$_5$ up to its parabolic band components using magneto-infrared spectroscopy. By comparing the Landau level transitions in different magnetic field orientations, we find that the band velocities are highly anisotropic and the dominant contribution along the layer stacking direction is parabolic with a record-low band velocity. More interestingly, with such a low band velocity, we have both theoretically and experimentally demonstrated that the band inversion can lead to a second extremum (band gap), allowing for a direct probe of the material’s topological nature. Our work not only provides an alternative way to identify the band inversion in 3D layered topological materials, but also yields important implications for understanding the exotic behavior of ZrTe$_5$ as well as that in similar (layered) materials.   

Observation of fractional states in the the three-dimensional quantum Hall regime of HfTe5

Interacting electrons in two dimensions can bind magnetic flux lines to form composite quasiparticles with fractional electric charge. Existence of such states manifests itself by the occurrence of the Fractional Quantum Hall Effect (FQHE). Although similar composite quasiparticles have been predicted to occur in three dimensions, their experimental realization remained elusive. In this talk, we report the observation of fractional plateaus in the Hall conductivity of the bulk semimetal HfTe₅. The plateaus are accompanied by Shubnikov-de-Haas minima of the longitudinal electrical resistivity. The height of the Hall plateaus is given by twice the Fermi wave vector in the direction of the applied magnetic field and scales with integer and particular fractional multiples of the conductance quantum. Our findings are consistent with the 3D FQHE, suggesting the existence of quasiparticles with fractional electric charge in a 3D crystal.   

Fermi Surface Topology and Evidence of Non-trivial Berry Phase in the Flat-band Semimetal Pd3Pb

We present a study of the Fermi surface of the putative triple point topological semimetal Pd₃Pb carried out by measuring Shubnikov-de Haas (SdH) oscillations in fields of up to 60 T. A multi-sheet Fermi surface has been reconstructed from the oscillation data and is largely in agreement with DFT calculations, albeit with a Fermi level shifted slightly (~30 meV) by small electron doping. A Berry-phase analysis of the SdH oscillations reveals a non-trivial phase for two bands along high symmetry directions, confirming the topological nature of Pd₃Pb. Importantly, the phase of the oscillation shows an angular dependence with respect to the direction of the applied magnetic field. With the signature of a topological phase transition to a trivial phase, this angular dependence may signify a unique identity of the evolution of triple point fermionic states under field.