40 research outputs found
Absence of signatures of Weyl orbits in the thickness dependence of quantum transport in cadmium arsenide
In a Weyl orbit, the Fermi arc surface states on opposite surfaces of the
topological semimetal are connected through the bulk Weyl or Dirac nodes.
Having a real-space component, these orbits accumulate a sample-size-dependent
phase. Following recent work on the three-dimensional Dirac semimetal cadmium
arsenide (Cd3As2), we have sought evidence for this thickness-dependent effect
in quantum oscillations and quantum Hall plateaus in (112)-oriented Cd3As2 thin
films grown by molecular beam epitaxy. We compare quantum transport in films of
varying thickness at apparently identical gate-tuned carrier concentrations and
find no clear dependence of the relative phase of the quantum oscillations on
the sample thickness. We show that small variations in carrier densities,
difficult to detect in low-field Hall measurements, lead to shifts in quantum
oscillations that are commensurate with previously reported phase shifts.
Future claims of Weyl orbits based on the thickness dependence of quantum
transport data require additional studies that demonstrate that these competing
effects have been disentangled
Observation of the quantum Hall effect in confined films of the three-dimensional Dirac semimetal Cd3As2
The magnetotransport properties of epitaxial films of Cd3As2, a paradigm
three-dimensional Dirac semimetal, are investigated. We show that an energy gap
opens in the bulk electronic states of sufficiently thin films and, at low
temperatures, carriers residing in surface states dominate the electrical
transport. The carriers in these states are sufficiently mobile to give rise to
a quantized Hall effect. The sharp quantization demonstrates surface transport
that is virtually free of parasitic bulk conduction and paves the way for novel
quantum transport studies in this class of topological materials. Our results
also demonstrate that heterostructuring approaches can be used to study and
engineer quantum states in topological semimetals.Comment: Accepted, Phys. Rev. Let
Basal-plane growth of cadmium arsenide by molecular beam epitaxy
(001)-oriented thin films of the three-dimensional Dirac semimetal cadmium
arsenide can realize a quantum spin Hall insulator and other kinds of
topological physics, all within the flexible architecture of epitaxial
heterostructures. Here, we report a method for growing (001) cadmium arsenide
films using molecular beam epitaxy. The introduction of a thin indium arsenide
wetting layer improves surface morphology and structural characteristics, as
measured by x-ray diffraction and reflectivity, atomic force microscopy, and
scanning transmission electron microscopy. The electron mobility of 50-nm-thick
films is found to be 9300 cm2/Vs at 2 K, comparable to the highest-quality
films grown in the conventional (112) orientation. This work demonstrates a
simple experimental framework for exploring topological phases that are
predicted to exist in proximity to the three-dimensional Dirac semimetal phase
Negative magnetoresistance due to conductivity fluctuations in films of the topological semimetal Cd3As2
Recently discovered Dirac and Weyl semimetals display unusual
magnetoresistance phenomena, including a large, non-saturating, linear
transverse magnetoresistance and a negative longitudinal magnetoresistance. The
latter is often considered as evidence of fermions having a defined chirality.
Classical mechanisms, due to disorder or non-uniform current injection, can
however, also produce negative longitudinal magnetoresistance. Here, we report
on magnetotransport measurements performed on epitaxial thin films of Cd3As2, a
three-dimensional Dirac semimetal. Quasi-linear positive transverse
magnetoresistance and negative longitudinal magnetoresistance are observed. By
evaluating films of different thickness and by correlating the temperature
dependence of the carrier density and mobility with the magnetoresistance
characteristics, we demonstrate that both the quasi-linear positive and the
negative magnetoresistance are caused by conductivity fluctuations. Chiral
anomaly is not needed to explain the observed features.Comment: Accepted for publication as Rapid Communication in Physical Review
Point group symmetry of cadmium arsenide thin films determined by convergent beam electron diffraction
Cadmium arsenide (Cd3As2) is one of the first materials to be discovered to
belong to the class of three-dimensional topological semimetals. Reported room
temperature crystal structures of Cd3As2 reported differ subtly in the way the
Cd vacancies are arranged within its antifluorite-derived structure, which
determines if an inversion center is present and if Cd3As2 is a Dirac or Weyl
semimetal. Here, we apply convergent beam electron diffraction (CBED) to
determine the point group of Cd3As2 thin films grown by molecular beam epitaxy.
Using CBED patterns from multiple zone axes, high-angle annular dark-field
images acquired in scanning transmission electron microscopy, and Bloch wave
simulations, we show that Cd3As2 belongs to the tetragonal 4/mmm point group,
which is centrosymmetric. The results show that CBED can distinguish very
subtle differences in the crystal structure of a topological semimetal, a
capability that will be useful for designing materials and thin film
heterostructures with topological states that depend on the presence of certain
crystal symmetries.Comment: Accepted for publication in Physical Review Material
Two-Dimensional Dirac Fermions in Thin Films of Cd3As2
Two-dimensional states in confined thin films of the three-dimensional Dirac
semimetal Cd3As2 are probed by transport and capacitance measurements under
applied magnetic and electric fields. The results establish the two-dimensional
Dirac electronic spectrum of these states. We observe signatures of p-type
conduction in the two-dimensional states as the Fermi level is tuned across
their charge neutrality point and the presence of a zero energy Landau level,
all of which indicate topologically non-trivial states. The resistance at the
charge neutrality point is approximately h/e2 and increases rapidly under the
application of a magnetic field. The results open many possibilities for
gate-tunable topological devices and for the exploration of novel physics in
the zero energy Landau level.Comment: Accepted in Phys. Rev.
Dynamics of light-induced anomalous Hall effect in the three-dimensional Dirac semimetal CdAs
We experimentally study the dynamical behavior of the light-induced anomalous
Hall effect in a three-dimensional Dirac semimetal, CdAs. An
ultrashort, circularly polarized, multi-terahertz pump pulse breaks the
time-reversal symmetry of a thin film sample. The resulting anomalous Hall
effect is clearly observed through the polarization rotation of a single-cycle
terahertz probe pulse. Comparing the experimental result with theory, we find
that the field-induced injection current dominates the anomalous Hall effect
during pump irradiation, while the Berry curvature of the Floquet-Weyl
semimetal state does not appreciably contribute. Remarkably, even after pump
irradiation, we observe an anomalous Hall effect that lasts for more than 10
ps. A model fit to the Hall conductivity spectrum reveals a relatively long
scattering time over 400 fs. This result shows that circularly polarized light
creates a polarization of the isospin degree of freedom in the Dirac semimetal,
which labels the crystallographic point group representation of the overlapping
Weyl semimetal bands. Our observation paves the way for conversion of a robust
isospin flow into an electric current at room temperature, being a new analogue
of the inverse spin Hall effect.Comment: 44 pages, 14 figure