203 research outputs found
Three-dimensionality of the bulk electronic structure in WTe2
We use temperature- and field-dependent resistivity measurements
[Shubnikov--de Haas (SdH) quantum oscillations] and ultrahigh resolution,
tunable, vacuum ultraviolet (VUV) laser-based angle-resolved photoemission
spectroscopy (ARPES) to study the three-dimensionality (3D) of the bulk
electronic structure in WTe2, a type-II Weyl semimetal. The bulk Fermi surface
(FS) consists of two pairs of electron pockets and two pairs of hole pockets
along the X-Gamma-X direction as detected by using an incident photon energy of
6.7 eV, which is consistent with the previously reported data. However, if
using an incident photon energy of 6.36 eV, another pair of tiny electron
pockets is detected on both sides of the Gamma point, which is in agreement
with the small quantum oscillation frequency peak observed in the
magnetoresistance. Therefore, the bulk, 3D FS consists of three pairs of
electron pockets and two pairs of hole pockets in total. With the ability of
fine tuning the incident photon energy, we demonstrate the strong
three-dimensionality of the bulk electronic structure in WTe2. The combination
of resistivity and ARPES measurements reveal the complete, and consistent,
picture of the bulk electronic structure of this material.Comment: 6 pages, 3 figure
Observation of Fermi Arcs in Type-II Weyl Semimetal Candidate WTe2
We use ultrahigh resolution, tunable, vacuum ultraviolet laser angle-resolved
photoemission spectroscopy (ARPES) to study the electronic properties of
WTe, a material that was predicted to be a type-II Weyl semimetal. The Weyl
fermion states in WTe2 were proposed to emerge at the crossing points of
electron and hole pockets; and Fermi arcs connecting electron and hole pockets
would be visible in the spectral function on (001) surface. Here we report the
observation of such Fermi arcs in WTe2 confirming the theoretical predictions.
This provides strong evidence for type-II Weyl semimetallic states in WTe2.Comment: 5 pages, 4 figure
Temperature induced Lifshitz transition in WTe2
We use ultra-high resolution, tunable, VUV laser-based, angle-resolved
photoemission spectroscopy (ARPES) and temperature and field dependent
resistivity and thermoelectric power (TEP) measurements to study the electronic
properties of WTe2, a compound that manifests exceptionally large, temperature
dependent magnetoresistance. The temperature dependence of the TEP shows a
change of slope at T=175 K and the Kohler rule breaks down above 70-140 K
range. The Fermi surface consists of two electron pockets and two pairs of hole
pockets along the X-Gamma-X direction. Upon increase of temperature from 40K,
the hole pockets gradually sink below the chemical potential. Like BaFe2As2,
WTe2 has clear and substantial changes in its Fermi surface driven by modest
changes in temperature. In WTe2, this leads to a rare example of temperature
induced Lifshitz transition, associated with the complete disappearance of the
hole pockets. These dramatic changes of the electronic structure naturally
explain unusual features of the transport data.Comment: 5 pages, 3 figure
Fragility of Fermi arcs in Dirac semimetals
We use tunable, vacuum ultraviolet laser-based angle-resolved photoemission
spectroscopy and density functional theory calculations to study the electronic
properties of Dirac semimetal candidate cubic PtBi. In addition to bulk
electronic states we also find surface states in PtBi which is expected
as PtBi was theoretical predicated to be a candidate Dirac semimetal.
The surface states are also well reproduced from DFT band calculations.
Interestingly, the topological surface states form Fermi contours rather than
double Fermi arcs that were observed in NaBi. The surface bands forming the
Fermi contours merge with bulk bands in proximity of the Dirac points
projections, as expected. Our data confirms existence of Dirac states in
PtBi and reveals the fragility of the Fermi arcs in Dirac semimetals.
Because the Fermi arcs are not topologically protected in general, they can be
deformed into Fermi contours, as proposed by [Kargarian {\it et al.}, PNAS
\textbf{113}, 8648 (2016)]. Our results demonstrate validity of this theory in
PtBi.Comment: 6 pages, 4 figure
Single pair of Weyl fermions in the half-metallic semimetal EuCd2As2
Materials with the ideal case of a single pair of Weyl points (WPs) are highly desirable for elucidating the unique properties of Weyl fermions. EuC d 2 A s 2 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 EuC d 2 A s 2 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 EuC d 2 A s 2 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
Phonon-Induced Topological Transition to a Type-II Weyl Semimetal
Given the importance of crystal symmetry for the emergence of topological
quantum states, we have studied, as exemplified in NbNiTe2, the interplay of
crystal symmetry, atomic displacements (lattice vibration), band degeneracy,
and band topology. For NbNiTe2 structure in space group 53 (Pmna) - having an
inversion center arising from two glide planes and one mirror plane with a
2-fold rotation and screw axis - a full gap opening exists between two band
manifolds near the Fermi energy. Upon atomic displacements by optical phonons,
the symmetry lowers to space group 28 (Pma2), eliminating one glide plane along
c, the associated rotation and screw axis, and the inversion center. As a
result, twenty Weyl points emerge, including four type-II Weyl points in the
G-X direction at the boundary between a pair of adjacent electron and hole
bands. Thus, optical phonons may offer control of the transition to a Weyl
fermion state
Anisotropic physical properties and pressure dependent magnetic ordering of CrAuTe4
Systematic measurements of temperature-dependent magnetization, resistivity, and angle-resolved photoemission spectroscopy (ARPES) at ambient pressure as well as resistivity under pressures up to 5.25 GPa were conducted on single crystals of CrAuTe4. Magnetization data suggest that magnetic moments are aligned antiferromagnetically along the crystallographic c axis below TN=255 K. ARPES measurements show band reconstruction due to the magnetic ordering. Magnetoresistance data show clear anisotropy, and, at high fields, quantum oscillations. The NĂ©el temperature decreases monotonically under pressure, decreasing to TN=236 K at 5.22 GPa. The pressure dependencies of (i) TN, (ii) the residual resistivity ratio, and (iii) the size and power-law behavior of the low-temperature magnetoresistance all show anomalies near 2 GPa suggesting that there may be a phase transition (structural, magnetic, and/or electronic) induced by pressure. For pressures higher than 2 GPa a significantly different quantum oscillation frequency emerges, consistent with a pressure induced change in the electronic states
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