238 research outputs found
Mode-coupling theory for structural and conformational dynamics of polymer melts
A mode-coupling theory for dense polymeric systems is developed which
unifyingly incorporates the segmental cage effect relevant for structural
slowing down and polymer chain conformational degrees of freedom. An ideal
glass transition of polymer melts is predicted which becomes molecular-weight
independent for large molecules. The theory provides a microscopic
justification for the use of the Rouse theory in polymer melts, and the results
for Rouse-mode correlators and mean-squared displacements are in good agreement
with computer simulation results.Comment: 4 pages, 3 figures, Phys. Rev. Lett. in pres
Theory for Superconducting Properties of the Cuprates: Doping Dependence of the Electronic Excitations and Shadow States
The superconducting phase of the 2D one-band Hubbard model is studied within
the FLEX approximation and by using an Eliashberg theory. We investigate the
doping dependence of , of the gap function and
of the effective pairing interaction. Thus we find that becomes maximal
for doping. In {\it overdoped} systems decreases due to the
weakening of the antiferromagnetic correlations, while in the {\it underdoped}
systems due to the decreasing quasi particle lifetimes. Furthermore, we find
{\it shadow states} below which affect the electronic excitation spectrum
and lead to fine structure in photoemission experiments.Comment: 10 pages (REVTeX) with 5 figures (Postscript
Spin susceptibility in bilayered cuprates: resonant magnetic excitations
We study the momentum and frequency dependence of the dynamical spin
susceptibility in the superconducting state of bilayer cuprate superconductors.
We show that there exists a resonance mode in the odd as well as the even
channel of the spin susceptibility, with the even mode being located at higher
energies than the odd mode. We demonstrate that this energy splitting between
the two modes arises not only from a difference in the interaction, but also
from a difference in the free-fermion susceptibilities of the even and odd
channels. Moreover, we show that the even resonance mode disperses downwards at
deviations from . In addition, we demonstrate that there
exists a second branch of the even resonance, similar to the recently observed
second branch (the -mode) of the odd resonance. Finally, we identify the
origin of the qualitatively different doping dependence of the even and odd
resonance. Our results suggest further experimental test that may finally
resolve the long-standing question regarding the origin of the resonance peak.Comment: 8 pages, 5 figure
Spin Josephson effect in ferromagnet/ferromagnet tunnel junctions
We consider the tunnel spin current between two ferromagnetic metals from a
perspective similar to the one used in superconductor/superconductor tunnel
junctions. We use fundamental arguments to derive a Josephson-like spin tunnel
current . Here the phases are
associated with the planar contribution to the magnetization,
. The crucial step in our
analysis is the fact that the -component of the spin is canonically
conjugate to the phase of the planar contribution: . This is
analogous to the commutation relation in superconductors, where
is the phase associated to the superconducting order parameter and
is the Cooper pair number operator. We briefly discuss the experimental
consequences of our theoretical analysis.Comment: LaTex, seven pages, no figures; version to appear in Europhys. Lett.;
in order to make room for a more extended microscopic analysis, the
phenomenological discussion contained in v2 was remove
Theory for the Doping Dependence of Spin Fluctuation Induced Shadow States in High-T Superconductors
We analyze the doping dependence of the intensity and energetical position of
shadow states in high -T superconductors within the 2D Hubbard model and
using our recently developed numerical method for the self consistent summation
of bubble and ladder diagrams. It is shown that shadow states resulting from
short range antiferromagnetic correlations occur for small but finite
excitation energies which decrease for decreasing doping, reflecting a
dynamically broken symmetry with increasing lifetime. Simultanously, the
intensity of these new states increases, the quasiparticle dispersion is
strongly flattened, and a pseudogap in the density of states occurs. Finally,
we discuss the importance of flat bands at the Fermi level and nesting of the
Fermi surface as general prerequisites for the observability of shadow states.Comment: 9 pages (TeX) with 3 figures (Postscript
Electronic Theory for Bilayer-Effects in High-T_c Superconductors
The normal and the superconducting state of two coupled CuO_2 layers in the
High-T_c superconductors are investigated by using the bilayer Hubbard model,
the FLEX approximation on the real frequency axis and the Eliashberg theory. We
find that the planes are antiferromagnetically correlated which leads to a
strongly enhanced shadow band formation. Furthermore, the inter-layer hopping
is renormalized which causes a blocking of the quasi particle inter-plane
transfer for low doping concentrations. Finally, the superconducting order
parameter is found to have a d_{x^2-y^2} symmetry with significant additional
inter-layer contributions.Comment: 5 pages, Revtex, 4 postscript figure
Electromagnetic Pulse Driven Spin-dependent Currents in Semiconductor Quantum Rings
We investigate the non-equilibrium charge and spin-dependent currents in a
quantum ring with a Rashba spin orbit interaction (SOI) driven by two
asymmetric picosecond electromagnetic pulses. The equilibrium persistent charge
and persistent spin-dependent currents are investigated as well. It is shown
that the dynamical charge and the dynamical spin-dependent currents vary
smoothly with a static external magnetic flux and the SOI provides a SU(2)
effective flux that changes the phases of the dynamic charge and the dynamic
spin-dependent currents. The period of the oscillation of the total charge
current with the delay time between the pulses is larger in a quantum ring with
a larger radius. The parameters of the pulse fields control to a certain extent
the total charge and the total spin-dependent currents. The calculations are
applicable to nano-meter rings fabricated in heterojuctions of III-V and II-VI
semiconductors containing several hundreds electrons.Comment: 15pages, 5 figure
Theory for the Interdependence of High-T Superconductivity and Dynamical Spin Fluctuations
The doping dependence of the superconducting state for the 2D one-band
Hubbard Hamiltonian is determined. By using an Eliashberg-type theory, we find
that the gap function has a symmetry in momentum
space and T becomes maximal for doping. Since we determine the
dynamical excitations directly from real frequency axis calculations, we obtain
new structures in the angular resolved density of states related to the
occurrence of {\it shadow states} below T. Explaining the anomalous
behavior of photoemission and tunneling experiments in the cuprates, we find a
strong interplay between -wave superconductivity and dynamical spin
fluctuations.Comment: 4 pages (REVTeX) with 4 figures (Postscript
Electronic Theory for the Transition from Fermi-Liquid to Non-Fermi-Liquid Behavior in High-T Superconductors
We analyze the breakdown of Fermi-liquid behavior within the 2D Hubbard model
as function of doping using our recently developed numerical method for the
self consistent summation of bubble and ladder diagrams. For larger doping
concentrations the system behaves like a conventional Fermi-liquid and for
intermediate doping similar to a marginal Fermi-liquid. However, for smaller
doping pronounced deviations from both pictures occur which are due to the
increasing importance of the short range antiferromagnetic spin fluctuations.
This is closely related to the experimental observed shadow states in the
normal state of high- superconductors. Furthermore, we discuss the
implications of our results for transport experiments.Comment: 11 pages (REVTeX) with 4 figures (Postscript
Theory for the Ultrafast Structural Response of optically excited small clusters: Time-dependence of the Ionization Potential
Combining an electronic theory with molecular dynamics simulations we present
results for the ultrafast structural changes in small clusters. We determine
the time scale for the change from the linear to a triangular structure after
the photodetachment process Ag. We show that the
time-dependent change of the ionization potential reflects in detail the
internal degrees of freedom, in particular coherent and incoherent motion, and
that it is sensitive to the initial temperature. We compare with experiment and
point out the general significance of our results.Comment: 10 pages, Revtex, 3 postscript figure
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