2,015 research outputs found
Anomalous Phase Shift of Quantum Oscillations in 3D Topological Semimetals
Berry phase physics is closely related to a number of topological states of
matter. Recently discovered topological semimetals are believed to host a
nontrivial Berry phase to induce a phase shift of in the
quantum oscillation ( for hole and for electron carriers). We
theoretically study the Shubnikov-de Haas oscillation of Weyl and Dirac
semimetals, taking into account their topological nature and inter-Landau band
scattering. For a Weyl semimetal with broken time-reversal symmetry, the phase
shift is found to change nonmonotonically and go beyond known values of and . For a Dirac semimetal or paramagnetic Weyl semimetal,
time-reversal symmetry leads to a discrete phase shift of or , as a function of the Fermi energy. Different from the previous works, we
find that the topological band inversion can lead to beating patterns in the
absence of Zeeman splitting. We also find the resistivity peaks should be
assigned integers in the Landau index plot. Our findings may account for recent
experiments in CdAs and should be helpful for exploring the Berry phase
in various 3D systems.Comment: 5 pages, 3 figures, with Supplemental Materia
Intervalley Scattering and Localization Behaviors of Spin-Valley Coupled Dirac Fermions
We study the quantum diffusive transport of multivalley massive Dirac cones,
where time-reversal symmetry requires opposite spin orientations in
inequivalent valleys. We show that the intervalley scattering and intravalley
scattering can be distinguished from the quantum conductivity that corrects the
semiclassical Drude conductivity, due to their distinct symmetries and
localization trends. In immediate practice, it allows transport measurements to
estimate the intervalley scattering rate in hole-doped monolayers of group-VI
transition metal dichalcogenides (e.g., molybdenum dichalcogenides and tungsten
dichalcogenides), an ideal class of materials for valleytronics applications.
The results can be generalized to a large class of multivalley massive Dirac
systems with spin-valley coupling and time-reversal symmetry.Comment: 5 pages+4 pages of supplemental materials, 4 figure
Revealing A Head-on Major Merger in the Nearby NGC 6338 Group with Chandra and VLA observations
By analyzing the Chandra archival data of the nearby NGC 6338 galaxy group,
we identify two X-ray bright clumps (N-clump and S-clump) within the central
100 kpc, and detect an arc-like X-ray brightness discontinuity at the south
boundary of the N-clump, which is defined as a cold front with a gas flow Mach
number of M<0.8. Furthermore, at the north-east boundary of the S-clump
(dominated by galaxy NGC 6338) another X-ray edge is detected that corresponds
to a weaker cold front. Therefore, the two clumps are approaching each other
approximately from opposite directions, and the group is undergoing a head-on
collision that is in a stage of pre-core passage. This merger scenario is also
supported by the study of the line-of-sight velocity distribution of the group
member galaxies. The merger mass ratio is about 1:1.8 as estimated from the
central gas temperature of the two clumps, which suggests the merger is most
likely to be a major merger. We also analyze the VLA 1.4 and 4.9 GHz radio
data, but we do not detect any extended radio emission that is associated with
the merger.Comment: Accepted by Ap
Vortex images on Ba{1-x}KxFe2As2 observed directly by the magnetic force microscopy
The vortex states on optimally doped Ba0.6K0.4Fe2As2 and underdoped
Ba0.77K0.23Fe2As2 single crystals are imaged by magnetic force microscopy at
various magnetic fields below 100 Oe. Local triangular vortex clusters are
observed in optimally doped samples. The vortices are more ordered than those
in Ba(Fe{1-x}Co{x})2As2, and the calculated pinning force per unit length is
about 1 order of magnitude weaker than that in optimally Co-doped 122 at the
same magnetic field, indicating that the Co doping at the Fe sites induces
stronger pinning. The proportion of six-neighbored vortices to the total amount
increases quickly with increasing magnetic field, and the estimated value
reaches 100% at several tesla. Vortex chains are also found in some local
regions, which enhance the pinning force as well as the critical current
density. Lines of vortex chains are observed in underdoped samples, and they
may have originated from the strong pinning near the twin boundaries arising
from the structural transition.Comment: 7 pages, 8 figure
Massive Dirac fermions and spin physics in an ultrathin film of topological insulator
We study transport and optical properties of the surface states which lie in
the bulk energy gap of a thin-film topological insulator. When the film
thickness is comparable with the surface state decay length into the bulk, the
tunneling between the top and bottom surfaces opens an energy gap and form two
degenerate massive Dirac hyperbolas. Spin dependent physics emerges in the
surface bands which are vastly different from the bulk behavior. These include
the surface spin Hall effects, spin dependent orbital magnetic moment, and spin
dependent optical transition selection rule which allows optical spin
injection. We show a topological quantum phase transition where the Chern
number of the surface bands changes when varying the thickness of the thin
film.Comment: 7 pages, 5 figure
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