3,296 research outputs found
Optical evidence of surface state suppression in Bi based topological insulators
A key challenge in condensed matter research is the optimization of
topological insulator (TI) compounds for the study and future application of
their unique surface states. Truly insulating bulk states would allow the
exploitation of predicted surface state properties, such as protection from
backscattering, dissipationless spin-polarized currents, and the emergence of
novel particles. Towards this end, major progress was recently made with the
introduction of highly resistive BiTeSe, in which surface state
conductance and quantum oscillations are observed at low temperatures.
Nevertheless, an unresolved and pivotal question remains: while room
temperature ARPES studies reveal clear evidence of TI surface states, their
observation in transport experiments is limited to low temperatures. A better
understanding of this surface state suppression at elevated temperatures is of
fundamental interest, and crucial for pushing the boundary of device
applications towards room-temperature operation. In this work, we
simultaneously measure TI bulk and surface states via temperature dependent
optical spectroscopy, in conjunction with transport and ARPES measurements. We
find evidence of coherent surface state transport at low temperatures, and
propose that phonon mediated coupling between bulk and surface states
suppresses surface conductance as temperature rises.Comment: 13 pages, 10 figure
Giant magnetoresistance of multiwall carbon nanotubes: modeling the tube/ferromagnetic-electrode burying contact
We report on the giant magnetoresistance (GMR) of multiwall carbon nanotubes
with ultra small diameters. In particular, we consider the effect of the
inter-wall interactions and the lead/nanotube coupling. Comparative studies
have been performed to show that in the case when all walls are well coupled to
the electrodes, the so-called inverse GMR can appear. The tendency towards a
negative GMR depends on the inter-wall interaction and on the nanotube le ngth.
If, however, the inner nanotubes are out of contact with one of the electrodes,
the GMR remains positive even for relatively strong inter-wall interactions
regardless of the outer nanotube length. These results shed additional light on
recently reported experimental data, where an inverse GMR was found in some
multiwall carbon nanotube samples.Comment: 5 pages, 5 figure
Out-of-plane instability and electron-phonon contribution to s- and d-wave pairing in high-temperature superconductors; LDA linear-response calculation for doped CaCuO2 and a generic tight-binding model
The equilibrium structure, energy bands, phonon dispersions, and s- and
d-channel electron-phonon interactions (EPIs) are calculated for the
infinite-layer superconductor CaCuO2 doped with 0.24 holes per CuO2. The LDA
and the linear-response full-potential LMTO method were used. In the
equilibrium structure, oxygen is found to buckle slightly out of the plane and,
as a result, the characters of the energy bands near EF are found to be similar
to those of other optimally doped HTSCs. For the EPI we find lambda(s)=0.4, in
accord with previous LDA calculations for YBa2Cu3O7. This supports the common
belief that the EPI mechanism alone is insufficient to explain HTSC.
Lambda(x^2-y^2) is found to be positive and nearly as large as lambda(s). This
is surprising and indicates that the EPI could enhance some other d-wave
pairing mechanism. Like in YBa2Cu3O7, the buckling modes contribute
significantly to the EPI, although these contributions are proportional to the
static buckling and would vanish for flat planes. These numerical results can
be understood from a generic tight-binding model originally derived from the
LDA bands of YBa2Cu3O7. In the future, the role of anharmonicity of the
buckling-modes and the influence of the spin-fluctuations should be
investigated.Comment: 19 pages, 9 Postscript figures, Late
Generic Rotation in a Collective SD Nucleon-Pair Subspace
Low-lying collective states involving many nucleons interacting by a random
ensemble of two-body interactions (TBRE) are investigated in a collective
SD-pair subspace, with the collective pairs defined dynamically from the
two-nucleon system. It is found that in this truncated pair subspace collective
vibrations arise naturally for a general TBRE hamiltonian whereas collective
rotations do not. A hamiltonian restricted to include only a few randomly
generated separable terms is able to produce collective rotational behavior, as
long as it includes a reasonably strong quadrupole-quadrupole component.
Similar results arise in the full shell model space. These results suggest that
the structure of the hamiltonian is key to producing generic collective
rotation.Comment: 11 pages, 5 figure
Amplitude Analysis of the Decays and
Based on a sample of events collected with the
BESIII detector, an amplitude analysis of the isospin-violating decays
and is performed. A significant -wave contribution from
is observed for the first time
in . The branching fraction is
determined to be , where the
first uncertainty is statistical, the second systematic, and the third model
dependent. In addition to the nonresonant -wave component, there is a
significant meson component. The branching fractions of the combined
-wave components are determined to be and , respectively. The latter one is consistent with previous BESIII
measurements.Comment: 7 pages, 3 figure
Mean-field analysis of interacting boson models with random interactions
We investigate the origin of the regular features observed in numerical
studies of the interacting boson model with random interactions, in particular
the dominance of L=0 ground states and the occurrence of vibrational and
rotational band structures. It is shown that all of these properties can be
interpreted and explained in terms of a Hartree-Bose mean-field analysis, in
which different regions of the parameter space are associated with geometric
shapes. The same conclusions hold for the vibron model.Comment: 8 pages, 4 figures, 2 tables. Physical Review C, in pres
Photoswitchable diacylglycerols enable optical control of protein kinase C.
Increased levels of the second messenger lipid diacylglycerol (DAG) induce downstream signaling events including the translocation of C1-domain-containing proteins toward the plasma membrane. Here, we introduce three light-sensitive DAGs, termed PhoDAGs, which feature a photoswitchable acyl chain. The PhoDAGs are inactive in the dark and promote the translocation of proteins that feature C1 domains toward the plasma membrane upon a flash of UV-A light. This effect is quickly reversed after the termination of photostimulation or by irradiation with blue light, permitting the generation of oscillation patterns. Both protein kinase C and Munc13 can thus be put under optical control. PhoDAGs control vesicle release in excitable cells, such as mouse pancreatic islets and hippocampal neurons, and modulate synaptic transmission in Caenorhabditis elegans. As such, the PhoDAGs afford an unprecedented degree of spatiotemporal control and are broadly applicable tools to study DAG signaling
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