7,685 research outputs found
Microscopic theory for the glass transition in a system without static correlations
We study the orientational dynamics of infinitely thin hard rods of length L,
with the centers-of-mass fixed on a simple cubic lattice with lattice constant
a.We approximate the influence of the surrounding rods onto dynamics of a pair
of rods by introducing an effective rotational diffusion constant D(l),l=L/a.
We get D(l) ~ [1-v(l)], where v(l) is given through an integral of a
time-dependent torque-torque correlator of an isolated pair of rods. A glass
transition occurs at l_c, if v(l_c)=1. We present a variational and a
numerically exact evaluation of v(l).Close to l_c the diffusion constant
decreases as D(l) ~ (l_c-l)^\gamma, with \gamma=1. Our approach predicts a
glass transition in the absence of any static correlations, in contrast to
present form of mode coupling theory.Comment: 6 pages, 3 figure
Pressure-Induced Superconductivity in Sc to 74 GPa
Using a diamond anvil cell with nearly hydrostatic helium pressure medium we
have significantly extended the superconducting phase diagram Tc(P) of Sc, the
lightest of all transition metals. We find that superconductivity is induced in
Sc under pressure, Tc increasing monotonically to 8.2 K at 74.2 GPa. The Tc(P)
dependences of the trivalent d-electron metals Sc, Y, La, and Lu are compared
and discussed within a simple s-d charge transfer framework.Comment: to be published in Phys. Rev. B (Brief Reports
Microscopic theory of glassy dynamics and glass transition for molecular crystals
We derive a microscopic equation of motion for the dynamical orientational
correlators of molecular crystals. Our approach is based upon mode coupling
theory. Compared to liquids we find four main differences: (i) the memory
kernel contains Umklapp processes, (ii) besides the static two-molecule
orientational correlators one also needs the static one-molecule orientational
density as an input, where the latter is nontrivial, (iii) the static
orientational current density correlator does contribute an anisotropic,
inertia-independent part to the memory kernel, (iv) if the molecules are
assumed to be fixed on a rigid lattice, the tensorial orientational correlators
and the memory kernel have vanishing l,l'=0 components. The resulting mode
coupling equations are solved for hard ellipsoids of revolution on a rigid
sc-lattice. Using the static orientational correlators from Percus-Yevick
theory we find an ideal glass transition generated due to precursors of
orientational order which depend on X and p, the aspect ratio and packing
fraction of the ellipsoids. The glass formation of oblate ellipsoids is
enhanced compared to that for prolate ones. For oblate ellipsoids with X <~ 0.7
and prolate ellipsoids with X >~ 4, the critical diagonal nonergodicity
parameters in reciprocal space exhibit more or less sharp maxima at the zone
center with very small values elsewhere, while for prolate ellipsoids with 2 <~
X <~ 2.5 we have maxima at the zone edge. The off-diagonal nonergodicity
parameters are not restricted to positive values and show similar behavior. For
0.7 <~ X <~ 2, no glass transition is found. In the glass phase, the
nonergodicity parameters show a pronounced q-dependence.Comment: 17 pages, 12 figures, accepted at Phys. Rev. E. v4 is almost
identical to the final paper version. It includes, compared to former
versions v2/v3, no new physical content, but only some corrected formulas in
the appendices and corrected typos in text. In comparison to version v1, in
v2-v4 some new results have been included and text has been change
Asymptotic energy profile of a wavepacket in disordered chains
We investigate the long time behavior of a wavepacket initially localized at
a single site in translationally invariant harmonic and anharmonic chains
with random interactions. In the harmonic case, the energy profile averaged on time and disorder decays for large as a power
law where and 3/2 for
initial displacement and momentum excitations, respectively. The prefactor
depends on the probability distribution of the harmonic coupling constants and
diverges in the limit of weak disorder. As a consequence, the moments of the energy distribution averaged with respect to disorder
diverge in time as for , where
for . Molecular dynamics simulations yield good agreement with
these theoretical predictions. Therefore, in this system, the second moment of
the wavepacket diverges as a function of time despite the wavepacket is not
spreading. Thus, this only criteria often considered earlier as proving the
spreading of a wave packet, cannot be considered as sufficient in any model.
The anharmonic case is investigated numerically. It is found for intermediate
disorder, that the tail of the energy profile becomes very close to those of
the harmonic case. For weak and strong disorder, our results suggest that the
crossover to the harmonic behavior occurs at much larger and larger
time.Comment: To appear in Phys. Rev.
Location- and observation time-dependent quantum-tunneling
We investigate quantum tunneling in a translation invariant chain of
particles. The particles interact harmonically with their nearest neighbors,
except for one bond, which is anharmonic. It is described by a symmetric double
well potential. In the first step, we show how the anharmonic coordinate can be
separated from the normal modes. This yields a Lagrangian which has been used
to study quantum dissipation. Elimination of the normal modes leads to a
nonlocal action of Caldeira-Leggett type. If the anharmonic bond defect is in
the bulk, one arrives at Ohmic damping, i.e. there is a transition of a
delocalized bond state to a localized one if the elastic constant exceeds a
critical value . The latter depends on the masses of the bond defect.
Superohmic damping occurs if the bond defect is in the site at a finite
distance from one of the chain ends. If the observation time is smaller
than a characteristic time , depending on the location M of the
defect, the behavior is similar to the bulk situation. However, for tunneling is never suppressed.Comment: 17 pages, 2 figure
Studies on the Weak Itinerant Ferromagnet SrRuO3 under High Pressure to 34 GPa
The dependence of the Curie temperature Tc on nearly hydrostatic pressure has
been determined to 17.2 GPa for the weak itinerant ferromagnetic SrRuO3 in both
polycrystalline and single-crystalline form. Tc is found to decrease under
pressure from 162 K to 42.7 K at 17.2 GPa in nearly linear fashion at the rate
dTc/dP = -6.8 K/GPa. No superconductivity was found above 4 K in the pressure
range 17 to 34 GPa. Room-temperature X-ray diffraction studies to 25.3 GPa
reveal no structural phase transition but indicate that the average Ru-O-Ru
bond angle passes through a minimum near 15 GPa. The bulk modulus and its
pressure derivative were determined to be B =192(3) GPa and B' = 5.0(3),
respectively. Parallel ac susceptibility studies on polycrystalline CaRuO3 at 6
and 8 GPa pressure found no evidence for either ferromagnetism or
superconductivity above 4 K
Regular-to-chaotic tunneling rates using a fictitious integrable system
We derive a formula predicting dynamical tunneling rates from regular states
to the chaotic sea in systems with a mixed phase space. Our approach is based
on the introduction of a fictitious integrable system that resembles the
regular dynamics within the island. For the standard map and other kicked
systems we find agreement with numerical results for all regular states in a
regime where resonance-assisted tunneling is not relevant.Comment: 4 pages, 4 figure
Beautiful Baryons from Lattice QCD
We perform a lattice study of heavy baryons, containing one () or
two -quarks (). Using the quenched approximation we obtain for the
mass of
The mass splitting between the and the B-meson is found to increase
by about 20\% if the light quark mass is varied from the chiral limit to the
strange quark mass.Comment: 11 pages, Figures obtained upon request from [email protected]
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