787 research outputs found
Phase diagram of a semiflexible polymer chain in a solvent: application to protein folding
We consider a lattice model of a semiflexible homopolymer chain in a bad
solvent. Beside the temperature , this model is described by (i) a curvature
energy , representing the stiffness of the chain (ii) a
nearest-neighbour attractive energy , representing the solvent
(iii) the monomer density , where and
denote respectively the number of monomers and the number of lattice sites.
This model is a simplified view of the protein folding problem, which
encompasses the geometrical competition between secondary structures (the
curvature term modelling helix formation) and the global compactness (modeled
here by the attractive energy), but contains no side chain information...Comment: 17 pages, plain tex, 2 figures available upon reques
Ring Exchange and Phase Separation in the Two-dimensional Boson Hubbard model
We present Quantum Monte Carlo simulations of the soft-core bosonic Hubbard
model with a ring exchange term K. For values of K which exceed roughly half
the on-site repulsion U, the density is a non-monotonic function of the
chemical potential, indicating that the system has a tendency to phase
separate. This behavior is confirmed by an examination of the density-density
structure factor and real space images of the boson configurations. Adding a
near-neighbor repulsion can compete with phase separation, but still does not
give rise to a stable normal Bose liquid.Comment: 12 pages, 23 figure
Exact analytic Gorkov-Ginzburg-Landau theory of type-II superconductivity in the magneto-quantum oscillations limit
A new Green's function representation is employed in a microscopic derivation
of a Ginzburg-Landau theory of strongly type superconductivity at high magnetic
fields. An exact analytical, physically transparent expression for the quartic
term in the corresponding order parameter expansion is presented. The resulting
expression reveals singular non-local contributions to the superconducting (SC)
free energy, associated with highly coherent cyclotron motions of the paired
electrons near the Fermi surface, which are strongly coupled to the vortex
lattice. A major part of these contributions arises from incoherent scattering
by the spatially averaged pair-potential, which is purely harmonic in the de
Haas van Alphen frequency. However, coherent scatterings by the ordered vortex
lattice generate, at low temperatures, large erratically oscillating (i.e.
paramagnetic-diamagnetic) contribution to the SC free energy as a function of
the magnetic field. Vortex lattice disorder, which tends to suppress this
oscillatory component, is found to preserve the singular harmonic part of the
SC free energy
Nonequilibrium steady states of driven magnetic flux lines in disordered type-II superconductors
We investigate driven magnetic flux lines in layered type-II superconductors
subject to various configurations of strong point or columnar pinning centers
by means of a three-dimensional elastic line model and Metropolis Monte Carlo
simulations. We characterize the resulting nonequilibrium steady states by
means of the force-velocity / current-voltage curve, static structure factor,
mean vortex radius of gyration, number of double-kink and half-loop
excitations, and velocity / voltage noise spectrum. We compare the results for
the above observables for randomly distributed point and columnar defects, and
demonstrate that the three-dimensional flux line structures and their
fluctuations lead to a remarkable variety of complex phenomena in the
steady-state transport properties of bulk superconductors.Comment: 23 pages, IOP style, 18 figures include
Linear response theory of interacting topological insulators
Chiral surface states in topological insulators are robust against
interactions, non-magnetic disorder and localization, yet topology does not
yield protection in transport. This work presents a theory of interacting
topological insulators in an external electric field, starting from the quantum
Liouville equation for the many-body density matrix. Out of equilibrium,
topological insulators acquire a current-induced spin polarization.
Electron-electron interactions renormalize the non-equilibrium spin
polarization and charge conductivity, and disorder in turn enhances this
renormalization by a factor of two. Topological insulator phenomenology remains
intact in the presence of interactions out of equilibrium, and an exact
correspondence exists between the mathematical frameworks necessary for the
understanding of the interacting and non-interacting problems.Comment: 9 pages, 1 figur
Nonequilibrium quantum-impurities: from entropy production to information theory
Nonequilibrium steady-state currents, unlike their equilibrium counterparts,
continuously dissipate energy into their physical surroundings leading to
entropy production and time-reversal symmetry breaking. This letter discusses
these issues in the context of quantum impurity models driven out of
equilibrium by attaching the impurity to leads at different chemical potentials
and temperatures. We start by pointing out that entropy production is often
hidden in traditional treatments of quantum-impurity models. We then use simple
thermodynamic arguments to define the rate of entropy production. Using the
scattering framework recently developed by the authors we show that the rate of
entropy production has a simple information theoretic interpretation in terms
of the Shannon entropy and Kullback-Leibler divergence of nonequilibrium
distribution function. This allows us to show that the entropy production is
strictly positive for any nonequilibrium steady-state. We conclude by applying
these ideas to the Resonance Level Model and the Kondo model.Comment: 5 pages, 1 figure new version with minor clarification
The Atomic Limit of the Boson-Fermion Model
The Boson-Fermion model, describing a mixture of hybridized localized Bosons
and itinerant Fermions on a lattice, is known to exhibit spectral properties
for the Fermions which upon lowering the temperature develop into a three pole
structure in the vicinity of the Fermi level. These spectral features go hand
in hand with the opening of a pseudogap in the density of states upon
approaching the critical temperature Tc when superconductivity sets in. In the
present work we study this model, in the atomic limit where the three pole
structure arises naturally from the local bonding, anti-bonding and non-bonding
states between the Bosons and Fermions.Comment: revtex, 9 pages and 6 eps figures. Submitted to Europhysics Letter
Field theoretic approach to the counting problem of Hamiltonian cycles of graphs
A Hamiltonian cycle of a graph is a closed path that visits each site once
and only once. I study a field theoretic representation for the number of
Hamiltonian cycles for arbitrary graphs. By integrating out quadratic
fluctuations around the saddle point, one obtains an estimate for the number
which reflects characteristics of graphs well. The accuracy of the estimate is
verified by applying it to 2d square lattices with various boundary conditions.
This is the first example of extracting meaningful information from the
quadratic approximation to the field theory representation.Comment: 5 pages, 3 figures, uses epsf.sty. Estimates for the site entropy and
the gamma exponent indicated explicitl
Fractionalized Fermi liquids
In spatial dimensions d >= 2, Kondo lattice models of conduction and local
moment electrons can exhibit a fractionalized, non-magnetic state (FL*) with a
Fermi surface of sharp electron-like quasiparticles, enclosing a volume
quantized by (\rho_a-1)(mod 2), with \rho_a the mean number of all electrons
per unit cell of the ground state. Such states have fractionalized excitations
linked to the deconfined phase of a gauge theory. Confinement leads to a
conventional Fermi liquid state, with a Fermi volume quantized by \rho_a (mod
2), and an intermediate superconducting state for the Z_2 gauge case. The FL*
state permits a second order metamagnetic transition in an applied magnetic
field.Comment: 4 pages, 1 figure; (v2) changed title and terminology, but content
largely unchanged; (v3) updated version to appear in PR
Flow equations for the one-dimensional Kondo lattice model: Static and dynamic ground state properties
The one-dimensional Kondo lattice model is investigated by means of Wegner's
flow equation method. The renormalization procedure leads to an effective
Hamiltonian which describes a free one-dimensional electron gas and a
Heisenberg chain. The localised spins of the effective model are coupled by the
well-known RKKY interaction. They are treated within a Schwinger boson mean
field theory which permits the calculation of static and dynamic correlation
functions. In the regime of small interaction strength static expectation
values agree well with the expected Luttinger liquid behaviour. The parameter
K_rho of the Luttinger liquid theory is estimated and compared to recent
results from density matrix renormalization group studies.Comment: 14 pages, 15 figures, final version, typo corrections, changes in
fig. 6 and fig.
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