4,682 research outputs found
Multiple Fermi pockets revealed by Shubnikov-de Haas oscillations in WTe2
We use magneto-transport measurements to investigate the electronic structure
of WTe2 single crystals. A non-saturating and parabolic magnetoresistance is
observed in the temperature range between 2.5 to 200 K and magnetic fields up
to 8 T. Shubnikov - de Haas oscillations with beating patterns are observed.
The fast Fourier transform of the SdH oscillations reveals three oscillation
frequencies, corresponding to three pairs of Fermi pockets with comparable
effective masses , m* ~ 0.31 me. By fitting the Hall resistivity, we infer the
presence of one pair of electron pockets and two pairs of hole pockets,
together with nearly perfect compensation of the electron-hole carrier
concentration. These magnetotransport measurements reveal the complex
electronic structure in WTe2, explaining the nonsaturating magnetoresistance.Comment: Submitted to journal on 1 April, 2015, 4 Figure
Observation of topological transition of Fermi surface from a spindle-torus to a torus in large bulk Rashba spin-split BiTeCl
The recently observed large Rashba-type spin splitting in the BiTeX (X = I,
Br, Cl) bulk states due to the absence of inversion asymmetry and large charge
polarity enables observation of the transition in Fermi surface topology from
spindle-torus to torus with varying the carrier density. These BiTeX systems
with high spin-orbit energy scales offer an ideal platform for achieving
practical spintronic applications and realizing non-trivial phenomena such as
topological superconductivity and Majorana fermions. Here we use Shubnikov-de
Haas oscillations to investigate the electronic structure of the bulk
conduction band of BiTeCl single crystals with different carrier densities. We
observe the topological transition of the Fermi surface (FS) from a
spindle-torus to a torus. The Landau level fan diagram reveals the expected
non-trivial {\pi} Berry phase for both the inner and outer FSs. Angle-dependent
oscillation measurements reveal three-dimensional FS topology when the Fermi
level lies in the vicinity of the Dirac point. All the observations are
consistent with large Rashba spin-orbit splitting in the bulk conduction band.Comment: 28 pages, supplementary informatio
Mid-infrared variability of changing-look AGN
It is known that some active galactic nuclei (AGNs) transited from type 1 to
type 2 or vice versa. There are two explanations for the so-called changing
look AGNs: one is the dramatic change of the obscuration along the
line-of-sight, the other is the variation of accretion rate. In this paper, we
report the detection of large amplitude variations in the mid-infrared
luminosity during the transitions in 10 changing look AGNs using WISE and newly
released NEOWISE-R data. The mid-infrared light curves of 10 objects echoes the
variability in the optical band with a time lag expected for dust reprocessing.
The large variability amplitude is inconsistent with the scenario of varying
obscuration, rather supports the scheme of dramatic change in the accretion
rate.Comment: Published by ApjL, 7 pages, 3 figures, 2 table
Single-layer behavior and slow carrier density dynamic of twisted graphene bilayer
We report scanning tunneling microscopy (STM) and spectroscopy (STS) of
twisted graphene bilayer on SiC substrate. For twist angle ~ 4.5o the Dirac
point ED is located about 0.40 eV below the Fermi level EF due to the electron
doping at the graphene/SiC interface. We observed an unexpected result that the
local Dirac point around a nanoscaled defect shifts towards the Fermi energy
during the STS measurements (with a time scale about 100 seconds). This
behavior was attributed to the decoupling between the twisted graphene and the
substrate during the measurements, which lowers the carrier density of graphene
simultaneously
Discovery of a Mid-infrared Echo from the TDE candidate in the nucleus of ULIRG F01004-2237
We present the mid-infrared (MIR) light curves (LCs) of a tidal disruption
event (TDE) candidate in the center of a nearby ultraluminous infrared galaxy
(ULIRG) F01004-2237 using archival {\it WISE} and {\it NEOWISE} data from 2010
to 2016. At the peak of the optical flare, F01004-2237 was IR quiescent. About
three years later, its MIR fluxes have shown a steady increase, rising by 1.34
and 1.04 mag in and m up to the end of 2016. The host-subtracted
MIR peak luminosity is \,erg\,s. We interpret the MIR
LCs as an infrared echo, i.e. dust reprocessed emission of the optical flare.
Fitting the MIR LCs using our dust model, we infer a dust torus of the size of
a few parsecs at some inclined angle. The derived dust temperatures range from
\,K, and the warm dust mass is . Such a large mass
implies that the dust cannot be newly formed. We also derive the UV luminosity
of \,erg\,s. The inferred total IR energy is
\,erg, suggesting a large dust covering factor. Finally, our
dust model suggests that the long tail of the optical flare could be due to
dust scattering
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