Session W60: Weyl Semimetal, Transport II

Planar Hall effect in thin films of the 3D Dirac semimetal Cd3As2

Cd₃As₂ is a 3D Dirac semimetal with doubly degenerate band crossings at isolated points in the bulk Brillouin zone. It can also host gapless surface states. In Dirac and Weyl semimetals, a giant planar Hall effect (PHE) has been predicted to be associated with the chiral anomaly. In this talk, we will discuss our measurements of the PHE in (001) and (112) oriented epitaxial thin films of Cd₃As₂ having a wide range of film thicknesses. We show that the PHE depends sensitively on the Fermi level. Studies of films of different thickness allow us to distinguish the contributions of bulk and surface states. We will discuss the different mechanisms that give rise to the PHE in these films.
  

NMR Investigation of Field-Induced Energy Gap in Topological Node-Line Semimetal ZrGeSe

The discovery of topological semimetals provides opportunities to explore the exotic properties of relativistic fermions in condensed matter. Among those materials, the nodal-line semimetal represents a new type of topological quantum state which displays Dirac cones along a one-dimensional line or a closed circle protected by combined symmetry of inversion and time-reversal, in contrast with the Dirac or Weyl semimetals with discrete Dirac or Weyl cones. Here we report NMR investigation of field-induced energy gap in topological nodal-line semimetals of ZrGeSe single crystals, with high magnetic field up to 16 T and high hydrostatic pressure up to 50 GPa. We found that the opening of the energy gap is driven by the magnetic field-induced anti-ferromagnetic (AFM) order, and the gap opening is highly restricted under high pressure.   

Violation of Ohm`s law in a Weyl metal

Weyl metal is one of the topological non-trivial materials holding Weyl fermions which are massless and have a chirality. The Weyl metal has been described in terms of axion electromagnetism rather than in Maxwell electromagnetism, and has peculiar properties such as chiral anomaly, the presence of magnetic monopole in the reciprocal lattice space and negative longitudinal magneto resistance. In this presentation, by transportation experiment besides negative longitudinal magneto resistance, we observed ohm’s law was broken in the Weyl metal and carried experimental and theoretical analysis of the violation of ohm`s law [1].

REFERENCES:
[1] D. W. Shin and Y. W. Lee et. al., Nat. Mater 16, 1096-1099 (2017).
  

Interplay of Dirac Nodes and Volkov-Pankratov Surface States in Compressively Strained HgTe

With the advent of topological materials, the Weyl fermion, a massless particle once proposed to describe neutrinos, can now be investigated in condensed matter systems as Weyl semi-metals, and their close cousins Dirac semi-metals. The HgTe material system, as a prototypical topological insulator for transport studies, is ideally suited, as the tunability of the details of its band structure through strain engineering, and its Fermi level by gating provide unparalleled control for the investigation of Weyl/Dirac semi-metals [1].
We can clearly disentangle surface and bulk contributions by using gate voltage to tune the Fermi energy precisely to the Weyl/Dirac nodes and thus convincingly identify the chiral anomaly. We find no evedince for Fermi-arcs, but topological surface states, that are responsible for the transport in the electron regime. These states are created by the same band inversion leading to the formation of the Dirac/Weyl nodes, and therefore must always be present. Additionally, in the hole regime surface states induced by the electric field, so called massive Volkov-Pankratov states [2, 3], are observed.

[1] D. M. Mahler, et al. Phys. Rev. X 9, 031034 (2019)
[2] B. A. Volkov and O. A. Pankratov, JETP Lett. 42, 4 (1985)
[3] A. Inhofer, et al. PRB 96, 195104 (2017)   

Exploring broken time-reversal symmetry in Cd3As2/(Ga,Mn)Sb Dirac semimetal/ferromagnetic semiconductor heterostructures

Cd₃As₂ has attracted attention as a canonical Dirac semimetal. In this study, we aim to break time reversal symmetry (TRS) in Cd₃As₂ by interfacing with a ferromagnetic semiconductor, (Ga,Mn)Sb. We have grown epitaxial Cd₃As₂ /(Ga,Mn)Sb bilayers using molecular beam epitaxy on GaSb (111) buffer layers deposited on GaAs (111)B substrates. High-resolution x-ray diffraction shows good crystalline quality of GaSb and Cd₃As₂ layers with full width half maximum of rocking curves, 0.04° and 0.11°, respectively. Atomic force microscopy shows smooth surfaces of (Ga,Mn)Sb and Cd₃As₂ with root mean square roughness ~ 1.3 nm. SQUID magnetometry reveals that (Ga,Mn)Sb films are ferromagnetic, while low temperature magnetoresistance data show that they are highly resistive for the Mn composition used. We report the temperature dependent magneto-transport properties in these Cd₃As₂ /(Ga,Mn)Sb heterostructures and investigate the magnetic proximity effect by varying the thickness of the Cd₃As₂ layer. We are also exploring interfacing Cd₃As₂ with other ferromagnetic semiconductors in order to find an efficient material for breaking TRS in Cd₃As₂.   

Negative longitudinal magnetoresistance, anisotropic magnetoresistance, and planar Hall effect in epitaxial thin films of elemental Bismuth

Observations of negative longitudinal magnetoresistance (NLMR), anisotropic magnetoresistance (AMR), and the planar Hall effect (PHE) under an in-plane magnetic field are often used as indications of non-trivial topology in non-magnetic material systems. We show NLMR, AMR, and PHE in crystalline epitaxial thin films (< 50 nm) of elemental Bismuth (111) grown by molecular beam epitaxy on intrinsic GaAs (111) substrates. Films were characterized in-situ using scanning tunneling microscopy to confirm topography and density of states. Furthermore, the thickness and temperature dependence of these phenomena are investigated, demonstrating a high degree of tunability. We posit that these observations may be due to the Rashba effect in the surface states of Bi (111), rather than any non-trivial topological effects.   

Quantum Hall effect due to surface states in (001)-plane cadmium arsenide thin films

We report transport studies of gated Hall bar structures fabricated from (001)-oriented epitaxial thin films of cadmium arsenide, a three-dimensional Dirac semimetal, in magnetic fields up to 45 T. This orientation does not host Fermi arc-type surface states because the Dirac nodes project onto the same point in the surface Brillouin zone. Recent developments in the growth of these layers have resulted in films with quantum mobilities exceeding 4,000 cm²/Vs. We explain the quantum Hall effect, which prominently features plateaus at even and odd filling factors across a range of carrier density, in terms of a surface state different from the Fermi arc-type states thought to exist on other surfaces, underlining differences between these measurements and those in the more widely studied (112)-plane films.   

Anomalous Hall Effect induced by extremely low field in ultra pure ZrTe5

ZrTe₅ has gathered interest in recent years due to its non-trivial topology. In monolayer form, it is predicted to be a quantum spin hall insulator. In bulk, it is predicted to reside extremely close to a phase transition between a strong and weak topological insulator, with a 3D Dirac semimetal state at the boundary between these two phases. We report detailed measurements of the anomalous hall effect (AHE) in ultra-pure, high mobility bulk ZrTe₅. We find the hall resistance saturates at an extremely low magnetic field ( B << 1T ), and remains at the saturation value for fields up to 32T. This AHE is present despite no evidence of magnetism in ZrTe₅. We investigate the AHE effect in ZrTe5 as a function of temperature, magnetic field angle, strain, and doping, and discuss the origins of this effect.   

Epitaxial Growth and in-situ magneto-transport of Na3Bi – a 3D Dirac semimetal

3D topological Dirac semimetals (TDS) such as Na₃Bi and Cd₃As₂ possess Dirac nodes located outside the time-reversal invariant momentum points in the Brillouin zone. Recent theoretical and experimental work shows that quantum confinement of 3D TDS can open a tunable bulk bandgap and give rise to thickness-dependent alternate transitions between trivial and quantum spin Hall insulator states. In this talk, we will present our results on ultra-thin growth of Na₃Bi films on insulating substrates using molecular beam epitaxy (MBE). We will also discuss our efforts towards angle-dependent high-field in-situ magnetotransport measurements on back-gated thin films of Na₃Bi.   

Evolution of the electronic structure of the Weyl semimetal TaAs under pressure

Weyl semimetals provide ideal condensed matter platform to study Weyl fermions. When current and magnetic field are parallel in such systems, the chiral anomaly is expected to induce a negative longitudinal magnetoresistance. To observe this special feature the Fermi level must be close to the band crossing points (Weyl nodes) where quasiparticles behave like Weyl fermions [1].

We studied whether hydrostatic pressure can tune the Weyl node energy in TaAs closer to the Fermi level by the Shubnikov-de Haas effect at pressures up to 2.5 GPa. Analysis of the quantum oscillations (QO) showed that the big electron and hole Fermi surface pockets in TaAs are shrinking in size by 40% while the small electron pocket does not. Therefore some of the Weyl nodes are moving closer to the Fermi level with pressure.

Two-band model fits of the magnetoconductivity reveal that the electron and hole densities and mobilities in TaAs do not show a significant evolution with pressure. This seeming disagreement with QO can be explained by the smaller electron pocket being probably the main contributor to the magnetoresistance in TaAs. Our experimental data are further supplemented by theoretical calculations.

[1] A. Johansson et al., PRB 99, 075114 (2019)   

The chiral qubit: quantum computing with chiral anomaly

The quantum chiral anomaly enables a nearly non-dissipative current in the presence of chirality imbalance. We propose to utilize the chiral anomaly for the designs of qubits potentially capable of operating at THz frequency and at room temperature with a coherence time to gate time ratio of about 10⁴. The proposed “Chiral Qubit” is a micron-scale ring made of a Weyl or Dirac semimetal, with the |0〉 and |1〉 states corresponding to the symmetric and antisymmetric superpositions of chiral currents circulating along the ring clockwise and counter-clockwise. A fractional magnetic flux through the ring induces a quantum superposition of the |0〉 and |1〉quantum states. The entanglement of qubits can be implemented through the near-field THz frequency electromagnetic fields (EMF). We show that the Hamiltonian of the chiral qubit is similar to that of the superconducting qubit. This means that quantum gates can be implemented in a traditional way, and the algorithms developed for superconducting quantum processors will apply. Light-driven (THz) ultrafast topology switching, demonstrated experimentally in Dirac/Weyl semimetal recently, will be discussed.

The author is grateful for collaborations with Dmitri Kharzeev and Jigang Wang.