216,520 research outputs found
Cancellations Amongst Kloosterman Sums
We obtain several estimates for bilinear form with Kloosterman sums. Such
results can be interpreted as a measure of cancellations amongst with
parameters from short intervals. In particular, for certain ranges of
parameters we improve some recent results of Blomer, Fouvry, Kowalski, Michel,
and Mili\'cevi\'c (2014) and Fouvry, Kowalski and Michel (2014).Comment: 10 page
SO(5) Superconductors in a Zeeman Magnetic Field
The generic symmetry of a system under a uniform Zeeman magnetic field is
U(1) x U(1). However, we show that SO(5) models in the presence of a finite
chemical potential and a finite Zeeman magnetic field can have a exact SU(2) x
U(1) symmetry. This principle can be used to test SO(5) symmetry at any doping
level
Different steady states for spin currents in noncollinear multilayers
We find there are at least two different steady states for transport across
noncollinear magnetic multilayers. In the conventional one there is a
discontinuity in the spin current across the interfaces which has been
identified as the source of current induced magnetic reversal; in the one
advocated herein the spin torque arises from the spin accumulation transverse
to the magnetization of a magnetic layer. These two states have quite different
attributes which should be discerned by current experiments.Comment: 8 pages, no figure. Accepted for publication in Journal of Physics:
Condensed Matte
Massive uncharged and charged particles' tunneling from the Horowitz-Strominger Dilaton black hole
Originally, Parikh and Wilczek's work is only suitable for the massless
particles' tunneling. But their work has been further extended to the cases of
massive uncharged and charged particles' tunneling recently. In this paper, as
a particular black hole solution, we apply this extended method to reconsider
the tunneling effect of the H.S Dilaton black hole. We investigate the behavior
of both massive uncharged and charged particles, and respectively calculate the
emission rate at the event horizon. Our result shows that their emission rates
are also consistent with the unitary theory. Moreover, comparing with the case
of massless particles' tunneling, we find that this conclusion is independent
of the kind of particles. And it is probably caused by the underlying
relationship between this method and the laws of black hole thermodynamics.Comment: 6 pages, no figure, revtex 4, accepted by Int. J. Mod. Phys
Kilohertz QPO Frequency and Flux Decrease in AQL X-1 and Effect of Soft X-ray Spectral Components
We report on an RXTE/PCA observation of Aql X-1 during its outburst in March
1997 in which, immediately following a Type-I burst, the broad-band 2-10 keV
flux decreased by about 10% and the kilohertz QPO frequency decreased from
813+-3 Hz to 776+-4 Hz. This change in kHz QPO frequency is much larger than
expected from a simple extrapolation of a frequency-flux correlation
established using data before the burst. Meanwhile a very low frequency noise
(VLFN) component in the broad-band FFT power spectra with a fractional
root-mean-square (rms) amplitude of 1.2% before the burst ceased to exist after
the burst. All these changes were accompanied by a change in the energy
spectral shape. If we characterize the energy spectra with a model composed of
two blackbody (BB) components and a power law component, almost all the
decrease in flux was in the two BB components. We attribute the two BB
components to the contributions from a region very near the neutron star or
even the neutron star itself and from the accretion disk, respectively.Comment: 12 pages with 4 figures, accepted for publication in ApJ Letters,
typos corrected and references update
Exact results for the extreme Thouless effect in a model of network dynamics
If a system undergoing phase transitions exhibits some characteristics of
both first and second order, it is said to be of 'mixed order' or to display
the Thouless effect. Such a transition is present in a simple model of a
dynamic social network, in which extreme introverts/extroverts always
cut/add random links. In particular, simulations showed that , the average fraction of cross-links between the two groups
(which serves as an 'order parameter' here), jumps dramatically when crosses the 'critical point' , as in typical
first order transitions. Yet, at criticality, there is no phase co-existence,
but the fluctuations of are much larger than in typical second order
transitions. Indeed, it was conjectured that, in the thermodynamic limit, both
the jump and the fluctuations become maximal, so that the system is said to
display an 'extreme Thouless effect.' While earlier theories are partially
successful, we provide a mean-field like approach that accounts for all known
simulation data and validates the conjecture. Moreover, for the critical system
, an analytic expression for the mesa-like stationary
distribution, , shows that it is essentially flat in a range
, with . Numerical evaluations of
provides excellent agreement with simulation data for .
For large , we find ,
though this behavior begins to set in only for . For accessible
values of , we provide a transcendental equation for an approximate
which is better than 1% down to . We conjecture how this approach
might be used to attack other systems displaying an extreme Thouless effect.Comment: 6 pages, 4 figure
Topological insulator and the Dirac equation
We present a general description of topological insulators from the point of
view of Dirac equations. The Z_{2} index for the Dirac equation is always zero,
and thus the Dirac equation is topologically trivial. After the quadratic B
term in momentum is introduced to correct the mass term m or the band gap of
the Dirac equation, the Z_{2} index is modified as 1 for mB>0 and 0 for mB<0.
For a fixed B there exists a topological quantum phase transition from a
topologically trivial system to a non-trivial one system when the sign of mass
m changes. A series of solutions near the boundary in the modified Dirac
equation are obtained, which is characteristic of topological insulator. From
the solutions of the bound states and the Z_{2} index we establish a relation
between the Dirac equation and topological insulators.Comment: 9 pages, published versio
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