593 research outputs found
Photoinduced Changes of Reflectivity in Single Crystals of YBa2Cu3O6.5 (Ortho II)
We report measurements of the photoinduced change in reflectivity of an
untwinned single crystal of YBa2Cu3O6.5 in the ortho II structure. The decay
rate of the transient change in reflectivity is found to decrease rapidly with
decreasing temperature and, below Tc, with decreasing laser intensity. We
interpret the decay as a process of thermalization of antinodal quasiparticles,
whose rate is determined by an inelastic scattering rate of quasiparticle
pairs.Comment: 4 pages, 4 figure
Chaotic flow and efficient mixing in a micro-channel with a polymer solution
Microscopic flows are almost universally linear, laminar and stationary
because Reynolds number, , is usually very small. That impedes mixing in
micro-fluidic devices, which sometimes limits their performance. Here we show
that truly chaotic flow can be generated in a smooth micro-channel of a uniform
width at arbitrarily low , if a small amount of flexible polymers is added
to the working liquid. The chaotic flow regime is characterized by randomly
fluctuating three-dimensional velocity field and significant growth of the flow
resistance. Although the size of the polymer molecules extended in the flow may
become comparable with the micro-channel width, the flow behavior is fully
compatible with that in a table-top channel in the regime of elastic
turbulence. The chaotic flow leads to quite efficient mixing, which is almost
diffusion independent. For macromolecules, mixing time in this microscopic flow
can be three to four orders of magnitude shorter than due to molecular
diffusion.Comment: 8 pages,7 figure
Relaxation Dynamics of Photoinduced Changes in the Superfluid Weight of High-Tc Superconductors
In the transient state of d-wave superconductors, we investigate the temporal
variation of photoinduced changes in the superfluid weight. We derive the
formula that relates the nonlinear response function to the nonequilibrium
distribution function. The latter qunatity is obtained by solving the kinetic
equation with the electron-electron and the electron-phonon interaction
included. By numerical calculations, a nonexponential decay is found at low
temperatures in contrast to the usual exponential decay at high temperatures.
The nonexponential decay originates from the nonmonotonous temporal variation
of the nonequilibrium distribution function at low energies. The main physical
process that causes this behavior is not the recombination of quasiparticles as
previous phenomenological studies suggested, but the absorption of phonons.Comment: 18 pages, 12 figures; to be published in J. Phys. Soc. Jpn. Vol. 80,
No.
On the universality of the Discrete Nonlinear Schroedinger Equation
We address the universal applicability of the discrete nonlinear Schroedinger
equation. By employing an original but general top-down/bottom-up procedure
based on symmetry analysis to the case of optical lattices, we derive the most
widely applicable and the simplest possible model, revealing that the discrete
nonlinear Schroedinger equation is ``universally'' fit to describe light
propagation even in discrete tensorial nonlinear systems and in the presence of
nonparaxial and vectorial effects.Comment: 6 Pages, to appear in Phys. Rev.
The possible - mixing in QCD sum rules
We calculate the on-shell - mixing parameter with
the method of QCD sum rule. Our result is MeV. The electromagnetic interaction is not included
Nodal quasiparticle meltdown in ultra-high resolution pump-probe angle-resolved photoemission
High- cuprate superconductors are characterized by a strong
momentum-dependent anisotropy between the low energy excitations along the
Brillouin zone diagonal (nodal direction) and those along the Brillouin zone
face (antinodal direction). Most obvious is the d-wave superconducting gap,
with the largest magnitude found in the antinodal direction and no gap in the
nodal direction. Additionally, while antinodal quasiparticle excitations appear
only below , superconductivity is thought to be indifferent to nodal
excitations as they are regarded robust and insensitive to . Here we
reveal an unexpected tie between nodal quasiparticles and superconductivity
using high resolution time- and angle-resolved photoemission on optimally doped
BiSrCaCuO. We observe a suppression of the nodal
quasiparticle spectral weight following pump laser excitation and measure its
recovery dynamics. This suppression is dramatically enhanced in the
superconducting state. These results reduce the nodal-antinodal dichotomy and
challenge the conventional view of nodal excitation neutrality in
superconductivity.Comment: 7 pages, 3 figure. To be published in Nature Physic
Absolutely stable proton and lowering the gauge unification scale
A unified model is constructed, based on flipped SU(5) in which the proton is absolutely stable. The model requires the existence of new leptons with masses of order the weak scale. The possibility that the unification scale could be extremely low is discussed
Fermion Doubling and a Natural Solution of the Strong CP Problem
We suggest the fermion doubling for all quarks and leptons. It is a
generalization of the neutrino doubling of the seesaw mechanism. The new quarks
and leptons are singlets and carry the electromagnetic charges of their
lighter counterparts. An {\it anomaly free global symmetry} or a
discrete symmetry can be introduced to restrict the Yukawa couplings. The form
of mass matrix is belonging to that of Nelson and Barr even though our model
does not belong to Barr's criterion. The weak CP violation of the
Kobayashi-Maskawa form is obtained through the spontaneous breaking of CP
symmetry at high energy scale. The strong CP solution is through a specific
form of the mass matrix. At low energy, the particle content is the same as in
the standard model. For a model with a global symmetry, in addition there
exists a massless majoron.Comment: SNUTP 93-68, 19 pages 1 TeX figure, ReVTeX 3.
Hemisystems of small flock generalized quadrangles
In this paper, we describe a complete computer classification of the
hemisystems in the two known flock generalized quadrangles of order
and give numerous further examples of hemisystems in all the known flock
generalized quadrangles of order for . By analysing the
computational data, we identify two possible new infinite families of
hemisystems in the classical generalized quadrangle .Comment: slight revisions made following referee's reports, and included raw
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