5,722 research outputs found
Low-Energy Charge-Density Excitations in MgB: Striking Interplay between Single-Particle and Collective Behavior for Large Momenta
A sharp feature in the charge-density excitation spectra of single-crystal
MgB, displaying a remarkable cosine-like, periodic energy dispersion with
momentum transfer () along the -axis, has been observed for the first
time by high-resolution non-resonant inelastic x-ray scattering (NIXS).
Time-dependent density-functional theory calculations show that the physics
underlying the NIXS data is strong coupling between single-particle and
collective degrees of freedom, mediated by large crystal local-field effects.
As a result, the small- collective mode residing in the single-particle
excitation gap of the B bands reappears periodically in higher Brillouin
zones. The NIXS data thus embody a novel signature of the layered electronic
structure of MgB.Comment: 5 pages, 4 figures, submitted to PR
Scaling Analysis of Magnetic Filed Tuned Phase Transitions in One-Dimensional Josephson Junction Arrays
We have studied experimentally the magnetic field-induced
superconductor-insulator quantum phase transition in one-dimensional arrays of
small Josephson junctions. The zero bias resistance was found to display a
drastic change upon application of a small magnetic field; this result was
analyzed in context of the superfluid-insulator transition in one dimension. A
scaling analysis suggests a power law dependence of the correlation length
instead of an exponential one. The dynamical exponents were determined to
be close to 1, and the correlation length critical exponents were also found to
be about 0.3 and 0.6 in the two groups of measured samples.Comment: 4 pages, 4 figure
Commensurability oscillations in the rf conductivity of unidirectional lateral superlattices: measurement of anisotropic conductivity by coplanar waveguide
We have measured the rf magnetoconductivity of unidirectional lateral
superlattices (ULSLs) by detecting the attenuation of microwave through a
coplanar waveguide placed on the surface. ULSL samples with the principal axis
of the modulation perpendicular (S_perp) and parallel (S_||) to the microwave
electric field are examined. For low microwave power, we observe expected
anisotropic behavior of the commensurability oscillations (CO), with CO in
samples S_perp and S_|| dominated by the diffusion and the collisional
contributions, respectively. Amplitude modulation of the Shubnikov-de Haas
oscillations is observed to be more prominent in sample S_||. The difference
between the two samples is washed out with the increase of the microwave power,
letting the diffusion contribution govern the CO in both samples. The failure
of the intended directional selectivity in the conductivity measured with high
microwave power is interpreted in terms of large-angle electron-phonon
scattering.Comment: 8 pages, 5 figure
Vertical transport and electroluminescence in InAs/GaSb/InAs structures: GaSb thickness and hydrostatic pressure studies
We have measured the current-voltage (I-V) of type II InAs/GaSb/InAs double
heterojunctions (DHETs) with 'GaAs like' interface bonding and GaSb thickness
between 0-1200 \AA. A negative differential resistance (NDR) is observed for
all DHETs with GaSb thickness 60 \AA below which a dramatic change in the
shape of the I-V and a marked hysteresis is observed. The temperature
dependence of the I-V is found to be very strong below this critical GaSb
thickness. The I-V characteristics of selected DHETs are also presented under
hydrostatic pressures up to 11 kbar. Finally, a mid infra-red
electroluminescence is observed at 1 bar with a threshold at the NDR valley
bias. The band profile calculations presented in the analysis are markedly
different to those given in the literature, and arise due to the positive
charge that it is argued will build up in the GaSb layer under bias. We
conclude that the dominant conduction mechanism in DHETs is most likely to
arise out of an inelastic electron-heavy-hole interaction similar to that
observed in single heterojunctions (SHETs) with 'GaAs like' interface bonding,
and not out of resonant electron-light-hole tunnelling as proposed by Yu et al.
A Zener tunnelling mechanism is shown to contribute to the background current
beyond NDR.Comment: 8 pages 12 fig
A Deformation of Sasakian Structure in the Presence of Torsion and Supergravity Solutions
We discuss a deformation of Sasakian structure in the presence of totally
skew-symmetric torsion by introducing odd dimensional manifolds whose metric
cones are K\"ahler with torsion. It is shown that such a geometry inherits
similar properties to those of Sasakian geometry. As an example of them, we
present an explicit expression of local metrics and see how Sasakian structure
is deformed by the presence of torsion. We also demonstrate that our example of
the metrics admits the existence of hidden symmetries described by non-trivial
odd-rank generalized closed conformal Killing-Yano tensors. Furthermore, using
these metrics as an {\it ansatz}, we construct exact solutions in five
dimensional minimal (un-)gauged supergravity and eleven dimensional
supergravity. Finally, we discuss the global structures of the solutions and
obtain regular metrics on compact manifolds in five dimensions, which give
natural generalizations of Sasaki--Einstein manifolds and
. We also discuss regular metrics on non-compact manifolds in eleven
dimensions.Comment: 38 pages, 1 table, v2: version to appear in Class. Quant. Gra
Hidden conformal symmetry of extreme and non-extreme Einstein-Maxwell-Dilaton-Axion black holes
The hidden conformal symmetry of extreme and non-extreme
Einstein-Maxwell-Dilaton-Axion (EMDA) black holes is addressed in this paper.
For the non-extreme one, employing the wave equation of massless scalars, the
conformal symmetry with left temperature and right
temperature in the near region is
found. The conformal symmetry is spontaneously broken due to the periodicity of
the azimuthal angle. The microscopic entropy is derived by the Cardy formula
and is fully in consistence with the Bekenstein-Hawking area-entropy law. The
absorption cross section in the near region is calculated and exactly equals
that in a 2D CFT. For the extreme case, by redefining the conformal
coordinates, the duality between the solution space and CFT is studied. The
microscopic entropy is found to exactly agree with the area-entropy law.Comment: V3, typos corrected, version to appear in JHE
The Entropy for General Extremal Black Holes
We use the Kerr/CFT correspondence to calculate the entropy for all known
extremal stationary and axisymmetric black holes. This is done with the help of
two ansatzs that are general enough to cover all such known solutions.
Considering only the contribution from the Einstein-Hilbert action to the
central charge(s), we find that the entropy obtained by using Cardy's formula
exactly matches with the Bekenstein-Hawking entropy.Comment: Minor corrections, section 5 refined, references added
Baryonium, tetra-quark state and glue-ball in large N_c QCD
From the large-N_c QCD point of view, baryonia, tetra-quark states, hybrids,
and glueballs are studied. The existence of these states is argued for. They
are constructed from baryons. In N_f=1 large N_c QCD, a baryonium is always
identical to a glueball with N_c valence gluons. The ground state 0^{-+}
glueball has a mass about 2450 MeV. f_0(1710) is identified as the lowest
0^{++} glueball. The lowest four-quark nonet should be f_0(1370), a_0(1450),
K^*_0(1430) and f_0(1500). Combining with the heavy quark effective theory,
spectra of heavy baryonia and heavy tetra-quark states are predicted. 1/N_c
corrections are discussed.Comment: 16 pages, 3 figure
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