Session E10: Dirac/Weyl Semimetals -- Thin Films, Surfaces and Interfaces

Towards topological electronics: Epitaxial thin films of topological Dirac semimetal Na3Bi

I will discuss our experiments on epitaxial thin films of topological Dirac semimetal Na₃Bi as a route to novel topological electronic phenomena and devices, including the conventional-to-topological quantum phase transition (QPT) as a function of layer thickness and a topological transistor based on an electric field-tuned QPT. Na₃Bi thin films are grown by molecular beam epitaxial and transferred in UHV to a low-temperature STM capable of magnetotransport at 5 K. Thin films (20 nm) of Na₃Bi on α-Al₂O₃(0001) substrates have low temperature charge carrier mobilities exceeding 6000 cm²V^-1s^-1 with n-type carrier densities below 1 x 10¹⁸ cm^-3[1], comparable to single crystal values. Mapping the local Dirac point via scanning tunneling spectroscopy reveals a high degree of spatial uniformity, with rms variations in Dirac point energy less than 5 meV[2], comparable to the best graphene samples on hexagonal boron nitride. Chemical doping[3] and electrostatic gating[4] (using SiO₂/Si substrates) can be used to tune the carrier density, first steps toward a topological transistor.

[1] J. Hellerstedt et al., Nano Letters 16, 3210 (2016).
[2] M.T. Edmonds et al., ArXiv:1612.03385.
[3] M.T. Edmonds et al., ACS Appl. Mater. Interfaces 8, 16412 (2016).
[4] J. Hellerstedt et al., arXiv:1707.09286.   

Electrostatic Gating and Transport Studies of Thin Films of the 3D Dirac Semimetal Cd3As2

3D Dirac semimetals are an unusual class of quantum materials with unique electronic states. Topological transitions have been predicted as a result of confinement, symmetry breaking, and by the application of external electric fields. Here, we present transport studies of Cd₃As₂ films under external electric and magnetic fields.
To fabricate devices for electrostatic gating, 20 nm Al₂O₃ gate dielectrics were deposited on epitaxial Cd₃As₂ films using atomic layer deposition (ALD). An increase in sheet carrier density and decrease in mobility were observed after ALD. Capacitance-voltage characteristics show a small change in capacitance with gate bias, suggesting that the carrier modulation is occurring only at the Cd₃As₂/Al₂O₃ interface. The modulated carrier density is ~3x10¹³ cm^-2. Field effect devices show a 12% modulation in drain current.
Hall measurements show a decrease in sheet carrier density with negative gate bias. At large negative gate bias, the Hall resistance suggests a two-carrier transport, as expected if the Fermi level is near the Dirac node. Moreover, a negative to positive magnetoresistance transition is observed when gate voltage is changed from positive to negative voltage. We discuss the results in terms of the nature of the electronic states in this system.   

Investigating Quantum Quasiparticle Interference on the surface of Weyl Semimetal by Scanning Tunneling Microscopy

We report work in progress for measurements of atomic structure and quantum quasiparticle interference (QPI) on the surface of Weyl semimetal (TaAs) using a low temperature scanning tunneling microscopy/spectroscopy equipped with vector magnets. The sample surface that is being studied is prepared by UHV in situ cleavage. Ta- and As- terminated surfaces are observed with atomic scale. QPI patterns that arise from quasiparticle scattering of the surface states including the topological Fermi arcs in TaAs are observed on As- and Ta- terminated surfaces respectively and compared without/with external magnetic field. The electron scattering near the atomic steps are also investigated. The results should help us to understand the electron scattering behaviors of the topological Fermi arcs of the Weyl semimetal, more comprehensively.   

Transport in Coherently Strained Thin Films of a 3D Dirac Semimetal

Cd₃As₂ is a 3D Dirac semimetal with degenerate band crossings at isolated points in k-space
(Dirac nodes). Most prior studies have focused on the electronic states of bulk single crystals.
Thin films of Cd₃As₂ allow for using heteroepitaxy approaches to control their electronic states.
Here, we present our studies of epitaxial thin films of Cd₃As₂ grown by molecular beam epitaxy
on (111) GaAs with relaxed In_xGa_1-xSb buffer layers. The composition of the In_xGa_1-xSb buffer
layers was varied to obtain layers that are lattice matched with the Cd₃As₂, as well as to obtain
tensile and compressively strained Cd₃As₂. We measured the strain in the Cd₃As₂ films using
high-resolution x-ray diffraction and showed that in-plane strains can be varied between 0.7%
to -0.9%, depending on the In-content of the buffer layers. The electrical transport properties
systematically vary with the magnitude of the film strain. We discuss the utility of strain
engineering to control the electronic states.   

Interfaces between Weyl semimetals

We study junctions of two Weyl semimetals and derive an analytical closed-form condition for the existence of localized modes at the interface. In certain cases, stable interface modes are obtained even when the two Weyl semimetals have identical low energy properties and differ only in their Fermi arc connectivities. This provides further support for the notion that a complete characterization of Weyl semimetals requires not only the nodal configuration in the bulk, but also the connectivity of the Fermi arcs on the surface. These interface modes also have experimental signatures in the quantum oscillation of the density of states.   

Growth and characterization of Cd3As2 epitaxial thin films on a II-VI substrate

Cd₃As₂ is a representative system of the Dirac semimetal, characterized by an inverted band structure with a linear Dirac dispersion. A lot of quantum transport phenomena originating from the unique band structure have been reported so far. Among them, magnetotransport measurements on Cd₃As₂ thin films are highly limited, because there are many difficulties to fabricate thin films with high crystallinity and surface flatness. To challenge this problem, we have investigated the growth mechanisms of Cd₃As₂, especially during the initial process of the epitaxial growth. By optimizing the growth conditions, single-crystalline Cd₃As₂ films with step and terrace structures have been successfully synthesized in-situ on a II-VI substrate. Detailed transport properties of the low carrier density films will be also presented.   

Epitaxial growth and transport studies of [001]-oriented cadmium arsenide, a 3D Dirac semimetal

In the 3D Dirac semimetal cadmium arsenide, the Dirac nodes fall along the k_z axis and are stabilized by the fourfold rotation symmetry of the tetragonal crystal structure. However, the easy growth direction is along [221], a lower-symmetry direction. In [221]-oriented thin films, epitaxial strain and finite thickness reduce the symmetry of the k_z axis and can cause the Dirac nodes to vanish. Here, we report the first growth and electronic transport studies of [001]-oriented thin films (i.e. those grown parallel to k_z), in which the fourfold axis is preserved by the sample geometry. These films are grown by molecular beam epitaxy on a III–V substrate with a tunable buffer layer. Low-temperature mobilities in these films greater than 17,000 cm²/Vs permit a comparison with the more widely studied [221]-oriented films to elucidate the role of surface states in the electronic transport.   

Topolectrical circuits

First developed by Alessandro Volta and Felix Savary in the early 19th century, circuits consisting of resistor, inductor and capacitor (RLC) components are now omnipresent in modern technology. The behavior of an RLC circuit is governed by its circuit Laplacian, which is analogous to the Hamiltonian describing the energetics of a physical system. We show that “topolectrical” boundary resonances (TBRs) appear in the impedance read-out of a circuit whenever its Laplacian bandstructure resembles that of topological semimetals - materials with extensive degenerate edge modes known as Fermi arcs that also harbor enigmatic transport properties. Due to the versatility of electronic circuits, our topological semimetal construction can be generalized to topolectrical phases with any desired lattice symmetry, spatial dimension, and even quasiperiodicity. Topolectrical circuits establish a bridge between electrical engineering and topological states of matter, where the accessibility, scalability, and operability of electronics promises to synergize with the intricate boundary properties of topological phases.   

Magnetotransport study of topological phase transition in (Cd1-xZnx)3As2 thin films

Topological Dirac semimetal (DSM) Cd₃As₂, stabilized through the band inversion and the symmetry protection, provides a useful platform not only for investigating Weyl physics such as chiral anomaly, but also for pursuing topological phase transitions originating from DSM. Among others, chemical doping to control strength of the spin-orbit interaction is one effective way to modulate the band structure. In the case of Cd₃As₂, chemical doping of Zn is expected to suppress the band inversion of Cd₃As₂, leading to a transition from the DSM phase to a trivial insulator phase. In this context, we discuss magnetotransport of high quality (Cd_1-xZn_x)₃As₂ thin films under high magnetic fields. Especially when the field is aligned to the current, the longitudinal magnetoresistance exhibits a clear Zn-concentration dependence, changing from negative to positive. Along with analysis of transport properties such as Fermi velocity, effective mass, and scattering times, we have clarified details of the topological phase transition in (Cd_1-xZn_x)₃As₂.   

Metal-insulator Transition in Weyl Semimetal Thin Films

In this talk we demonstrate metal-insulator transition in Weyl semimetal thin films using model Hamiltonians, and introduce special cases in which a quadratic crossing remains even in single-layer limit. The band structure of Weyl semimetals is distinguished by pairs of Dirac points in the 3D Brillouin zone which are sink and source of the Berry curvature. These Weyl nodes act like magnetic monopoles with opposite charges in momentum space. In a slab configuration, this non-trivial topology leads to topological surface states that form unique Fermi arcs connecting two Weyl nodes. Here we show that as the thickness of the slab is reduced, the Fermi arcs shrink with the two magnetic monopoles moving toward each other, distinguishing the top of the Fermi arc as the most stable metallic point on the surface. In the absence of symmetries to protect the metallic crossing at the top of the arc, the metallic surface states disappear at a critical thickness. In special cases in which the metallic state at the top of the Fermi arc is protected by symmetries in the 2D plane, the pair of Fermi arcs on opposite surfaces reduces to a quadratic crossing in the thin film. Finally, we use density functional theory to explore the manifestations of this Fermi arc evolution in real material systems.   

Gap opening and quantum Hall effect in thin films of the three-dimensional Dirac semimetal Cd3As2

Three-dimensional Dirac semimetals, which are zero band gap materials with a linear dispersion relation at the Fermi energy, have attracted tremendous interest. While most studies are being performed on bulk crystals, fewer reports exist for epitaxial thin films. Moreover, predicted transitions into other topological phases induced, e.g., by confinement or symmetry breaking, have not yet been reported.
We report on the magnetotransport properties of epitaxial films of Cd₃As₂, grown by molecular beam epitaxy on III-V substrates. X-ray diffraction and transmission electron microscopy confirm the synthesis of epitaxial films in the low-temperature Dirac semimetal phase, which possesses the tetragonal I4acd structure. We show the emergence of the quantum Hall effect below a critical film thickness, which is a hallmark of a high-mobility, 2D electronic system. We show that an energy gap opens in the bulk states of sufficiently confined films and all carriers reside in 2D states. Sharp quantization of Hall plateaus and a vanishing longitudinal resistance demonstrates transport that is virtually free of parasitic bulk conduction. We discuss the nature of these surface states. The results demonstrate that heterostructure approaches can engineer quantum states in topological semimetals.   

Localization effects in 3D Dirac antiperovskite thin films

We report localization effects in the low temperature magnetotransport of 3D Dirac materials Sr₃PbO and Sr₃SnO. Robust weak antilocalization was observed for Sr₃SnO, whereas weak localization was dominant for Sr₃PbO. The sign change of quantum interference part of the magnetoconductance is related to the position of the Fermi energy with respect to the Dirac nodes, pointing to the role of Berry curvature. Detailed properties of Dirac electrons derived by localization analysis will be discussed.   

Weyl fermion on reconstructed surface of Bi/Si(110) with non-symmorphic lattice symmetry

In two-dimensional crystals, the non-symmorphic lattice symmetry together with time reversal symmetry can stabilize Weyl fermionic states. With 4n+2 electron filling, the states can be tuned to locate near the charge neutral point. Here, using first-principles calculations, we propose a stable Bi adatom induced superstructure on Si(110) surface, characterized by wallpaper symmetry group pg, as a candidate system for the realization of Weyl fermion states with 4n+2 electron filling. Out of various adsorption geometries, the ground structure bears single glide line parallel to [1-10] direction, results in a two-fold degenerate Bloch bands along glide lines. We will also discuss a way of controlling band dispersions where the Weyl fermion reside near the fermi level.