5,060 research outputs found
Spin-spin Correlation lengths of Bilayer Antiferromagnets
The spin-spin correlation length and the static structure factor for bilayer
antiferromagnets, such as YBaCuO, are calculated using field
theoretical and numerical methods. It is shown that these quantities can be
directly measured in neutron scattering experiments using energy integrated
two-axis scan despite the strong intensity modulation perpendicular to the
layers. Our calculations show that the correlation length of the bilayer
antiferromagnet diverges considerably more rapidly, as the temperature tends to
zero, than the correlation length of the corresponding single layer
antiferromagnet typified by LaCuO. This rapid divergence may have
important consequences with respect to magnetic fluctuations of the doped
superconductors.Comment: This paper supersedes cond-mat/9703138 and contains numerical
simulation results to compare against analytical results. 6 pages, 2
postscript figures (embedded), uses EuroPhys.sty and EuroMac
Superconductivity in hole-doped C60 from electronic correlations
We derive a model for the highest occupied molecular orbital band of a C60
crystal which includes on-site electron-electron interactions. The form of the
interactions are based on the icosahedral symmetry of the C60 molecule together
with a perturbative treatment of an isolated C60 molecule. Using this model we
do a mean-field calculation in two dimensions on the [100] surface of the
crystal. Due to the multi-band nature we find that electron-electron
interactions can have a profound effect on the density of states as a function
of doping. The doping dependence of the transition temperature can then be
qualitatively different from that expected from simple BCS theory based on the
density of states from band structure calculations
Floating phase in a dissipative Josephson junction array
We consider dissipative quantum phase transitions in Josephson junction
arrays and show that the disordered phase in this extended system can be viewed
as an unusual floating phase in which the states of local -dimensional
elements (single Josephson junctions) can slide past each other despite
arbitrary range spatial couplings among them. The unusual character of the
metal-superconductor quantum critical point can be tested by measurements of
the current voltage characteristic. This may be the simplest and most natural
example of a floating phase.Comment: 4 pages, RevTex4. The revised version contains higher order
renormalization group equations and the corresponding phase diagra
Quantum-Critical Behavior in a Two-Layer Antiferromagnet
We analyze quantum Monte Carlo data in the vicinity of the quantum transition
between a Neel state and a quantum paramagnet in a two-layer, square lattice
spin 1/2 Heisenberg antiferromagnet. The real-space correlation function and
the universal amplitude ratio of the structure factor and the dynamic
susceptibility show clear evidence of quantum critical behavior at low
temperatures. The numerical results are in good quantitative agreement with
calculations for the non-linear sigma model. A discrepancy,
reported earlier, between the critical properties of the antiferromagnet and
the sigma model is resolved. We also discuss the values of prefactors of the
dynamic susceptibility and the structure factor in a single layer
antiferromagnet at low .Comment: 11 pages, REVtex file, 5 figures in a uuencoded, gziped file. One
citation added
Quantum dynamics of an Ising spin-chain in a random transverse field
We consider an Ising spin-chain in a random transverse magnetic field and
compute the zero temperature wave vector and frequency dependent dynamic
structure factor numerically by using Jordan-Wigner transformation. Two types
of distributions of magnetic fields are introduced. For a rectangular
distribution, a dispersing branch is observed, and disorder tends to broaden
the dispersion peak and close the excitation gap. For a binary distribution, a
non-dispersing branch at almost zero energy is recovered. We discuss the
relationship of our work to the neutron scattering measurement in
.Comment: 4 pages and 6 eps figures; minor clarifications were made; the text
was shortened to add an additional figur
Correlation Lengths in Quantum Spin Ladders
Analytic expressions for the correlation length temperature dependences are
given for antiferromagnetic spin-1/2 Heisenberg ladders using a finite-size
non-linear sigma-model approach. These calculations rely on identifying three
successive crossover regimes as a function of temperature. In each of these
regimes, precise and controlled approximations are formulated. The analytical
results are found to be in excellent agreement with Monte Carlo simulations for
the Heisenberg Hamiltonian.Comment: 5 pages LaTeX using RevTeX, 3 encapsulated postscript figure
Bound states in d-density-wave phases
We investigate the quasiparticle spectrum near surfaces in a two-dimensional
system with d-density-wave order within a mean-field theory. For Fermi surfaces
with perfect nesting for the ordering wave vector of the d-density-wave, a zero
energy bound state occurs at [110] surfaces, in close analogy with the known
effect in d-wave superconducting states or graphite. When the shape of the
Fermi surface is changed by doping, the bound state energy moves away from the
Fermi level. Furthermore, away from half-filling we find inhomogeneous phases
with domain walls of the d-density-wave order parameter. The domain walls also
support low energy bound states. These phenomena might provide an experimental
test for hidden d-density-wave order in the high-Tc cuprates.Comment: 6 pages, 5 figure
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