6,859 research outputs found
Microscopic theory of the pseudogap and Peierls transition in quasi-one-dimensional materials
The problem of deriving from microscopic theory a Ginzburg-Landau free energy
functional to describe the Peierls or charge-density-wave transition in
quasi-one-dimensional materials is considered. Particular attention is given to
how the thermal lattice motion affects the electronic states. Near the
transition temperature the thermal lattice motion produces a pseudogap in the
density of states at the Fermi level. Perturbation theory diverges and the
traditional quasi-particle or Fermi liquid picture breaks down. The pseudogap
causes a significant modification of the coefficients in the Ginzburg-Landau
functional from their values in the rigid lattice approximation, which neglects
the effect of the thermal lattice motion. To appear in Physical Review B.Comment: 21 pages, RevTeX, 5 figures in uuencoded compressed tar fil
Mott Transition, Compressibility Divergence and P-T Phase Diagram of Layered Organic Superconductors: An Ultrasonic Investigation
The phase diagram of the organic superconductor
-(BEDT-TTF)Cu[N(CN)Cl has been investigated by ultrasonic
velocity measurements under helium gas pressure. Different phase transitions
were identified trough several elastic anomalies characterized from isobaric
and isothermal sweeps. Our data reveal two crossover lines that end on the
critical point terminating the first-order Mott transition line. When the
critical point is approached along these lines, we observe a dramatic softening
of the velocity which is consistent with a diverging compressibility of the
electronic degrees of freedom.Comment: 4 pages, 5 figure
Density Matrix Renormalization Group Study of the S=1/2 Anisotropic Antiferromagnetic Heisenberg Chains with Quasiperiodic Exchange Modulation
The low energy behavior of the S=1/2 antiferromagnetic XY-like XXZ chains
with precious mean quasiperiodic exchange modulation is studied by the density
matrix renormalization group method. It is found that the energy gap of the
chain with length N scales as with nonuniversal exponent
if the Ising component of the exhange coupling is antiferromagnetic.
This behavior is expected to be the characteristic feature of the quantum spin
chains with relevant aperiodicity. This is in contrast to the XY chain for
which the precious mean exchange modulation is marginal and the gap scales as
. On the contrary, it is also verified that the energy gap scales as
if the Ising component of the exhange coupling is ferromagnetic. Our
results are not only consistent with the recent bosonization analysis of Vidal,
Mouhanna and Giamarchi but also clarify the nature of the strong coupling
regime which is inaccesssible by the bosonization approach.Comment: 8 pages, 15 figures, 1 table; Proceedings of the workshop 'Frontiers
in Magnetism', Kyoto, Oct. 199
Another short-burst host galaxy with an optically obscured high star formation rate: The case of GRB 071227
We report on radio continuum observations of the host galaxy of the short
gamma-ray burst 071227 (z=0.381) with the Australia Telescope Compact Array
(ATCA). We detect the galaxy in the 5.5 GHz band with an integrated flux
density of Fnu = 43 +/- 11 microJy, corresponding to an unobscured
star-formation rate (SFR) of about 24 Msun/yr, forty times higher than what was
found from optical emission lines. Among the ~30 well-identified and studied
host galaxies of short bursts this is the third case where the host is found to
undergo an episode of intense star formation. This suggests that a fraction of
all short-burst progenitors hosted in star-forming galaxies could be physically
related to recent star formation activity, implying a relatively short merger
time scale.Comment: 6 pages, ApJ, accepted for publicatio
Magnetoresistance and magnetic breakdown in the quasi-two-dimensional conductors (BEDT-TTF)MHg(SCN)[M=K,Rb,Tl]
The magnetic field dependence of the resistance of
(BEDT-TTF)MHg(SCN)[M=K,Rb,Tl] in the density-wave phase is explained in
terms of a simple model involving magnetic breakdown and a reconstructed Fermi
surface. The theory is compared to measurements in pulsed magnetic fields up to
51 T. The value implied for the scattering time is consistent with independent
determinations. The energy gap associated with the density-wave phase is
deduced from the magnetic breakdown field. Our results have important
implications for the phase diagram.Comment: 5 pages, RevTeX + epsf, 3 figures. To appear in Physical Review B,
Rapid Communications, September 15, 199
Distributions of gaps and end-to-end correlations in random transverse-field Ising spin chains
A previously introduced real space renormalization-group treatment of the
random transverse-field Ising spin chain is extended to provide detailed
information on the distribution of the energy gap and the end-to-end
correlation function for long chains with free boundary conditions. Numerical
data, using the mapping of the problem to free fermions, are found to be in
good agreement with the analytic finite size scaling predictions.Comment: 12 pages revtex, 10 figures, submitted to Phys. Rev.
Quantum entanglement between electronic and vibrational degrees of freedom in molecules
We consider the quantum entanglement of the electronic and vibrational
degrees of freedom in molecules with a tendency towards double welled
potentials using model coupled harmonic diabatic potential-energy surfaces. The
von Neumann entropy of the reduced density matrix is used to quantify the
electron-vibration entanglement for the lowest two vibronic wavefunctions in
such a bipartite system. Significant entanglement is found only in the region
in which the ground vibronic state contains a density profile that is bimodal
(i.e., contains two separate local minima). However, in this region two
distinct types of entanglement are found: (1) entanglement that arises purely
from the degeneracy of energy levels in the two potential wells and which is
destroyed by slight asymmetry, and (2) entanglement that involves strongly
interacting states in each well that is relatively insensitive to asymmetry.
These two distinct regions are termed fragile degeneracy-induced entanglement
and persistent entanglement, respectively. Six classic molecular systems
describable by two diabatic states are considered: ammonia, benzene,
semibullvalene, pyridine excited triplet states, the Creutz-Taube ion, and the
radical cation of the "special pair" of chlorophylls involved in
photosynthesis. These chemically diverse systems are all treated using the same
general formalism and the nature of the entanglement that they embody is
elucidated
Exact results for quantum phase transitions in random XY spin chains
The effect of disorder on the quantum phase transitions induced by a
transverse field, anisotropy, and dimerization in XY spin chains is
investigated. The low-energy behavior near the critical point is described by a
Dirac-type equation with a random mass for which an exact analytic treatment is
possible. Results obtained for the dynamical critical exponent, the specific
heat, and transverse susceptibility agree with results recently obtained using
a real space renormalization group decimation technique, supporting Fisher's
claim that it is exact. A non-zero transverse field changes the universality
class of the anisotropy transition.Comment: 5 pages, RevTeX + epsf, 2 figures
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