2,786 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
Theoretical study of X-ray absorption of three-dimensional topological insulator
X-ray absorption edge singularity which is usually relevant for metals is
studied for the prototype topological insulator .
The generalized integral equation of Nozi\`eres and Dominicis type for X-ray
edge singularity is derived and solved. The spin texture of surfaces states
causes a component of singularity dependent on the helicity of the spin
texture. It also yields another component for which the singularity from
excitonic processes is absent.Comment: RevTeX 4.1. 4 pages, no figur
Aharonov-Bohm oscillations in the local density of states
The scattering of electrons with inhomogeneities produces modulations in the
local density of states of a metal. We show that electron interference
contributions to these modulations are affected by the magnetic field via the
Aharonov-Bohm effect. This can be exploited in a simple STM setup that serves
as an Aharonov-Bohm interferometer at the nanometer scale.Comment: 4 pages, 2 figures. v2 added reference
Quantum Phase Transitions
We give a general introduction to quantum phase transitions in
strongly-correlated electron systems. These transitions which occur at zero
temperature when a non-thermal parameter like pressure, chemical
composition or magnetic field is tuned to a critical value are characterized by
a dynamic exponent related to the energy and length scales and
. Simple arguments based on an expansion to first order in the effective
interaction allow to define an upper-critical dimension (where
and is the spatial dimension) below which mean-field description is
no longer valid. We emphasize the role of pertubative renormalization group
(RG) approaches and self-consistent renormalized spin fluctuation (SCR-SF)
theories to understand the quantum-classical crossover in the vicinity of the
quantum critical point with generalization to the Kondo effect in heavy-fermion
systems. Finally we quote some recent inelastic neutron scattering experiments
performed on heavy-fermions which lead to unusual scaling law in
for the dynamical spin susceptibility revealing critical local modes beyond the
itinerant magnetism scheme and mention new attempts to describe this local
quantum critical point.Comment: 13 pages, 4 figure
The possibility of measuring intrinsic electronic correlations in graphene using a d-wave contact Josephson junction
While not widely recognized, electronic correlations might play an important
role in graphene. Indeed, Pauling's resonance valence bond (RVB) theory for the
pp-bonded planar organic molecules, of which graphene is the infinite
extension, already established the importance of the nearest neighbor
spin-singlet bond (SB) state in these materials. However, despite the recent
growth of interest in graphene, there is still no quantitative estimate of the
effects of Coulomb repulsion in either undoped or doped graphene. Here we use a
tight-binding Bogoliubov-de Gennes (TB BdG) formalism to show that in
unconventional d-wave contact graphene Josephson junctions the intrinsic SB
correlations are strongly enhanced. We show on a striking effect of the SB
correlations in both proximity effect and Josephson current as well as
establishing a 1/(T-T_c) functional dependence for the superconducting decay
length. Here T_c is the superconducting transition temperature for the
intrinsic SB correlations, which depends on both the effects of Coulomb
repulsion and the doping level. We therefore propose that d-wave contact
graphene Josephson junctions will provide a promising experimental system for
the measurement of the effective strength of intrinsic SB correlations in
graphene.Comment: 4 pages, 4 figure
The effect of nearest neighbor spin-singlet correlations in conventional graphene SNS Josephson junctions
Using the self-consistent tight-binding Bogoliubov-de Gennes formalism we
have studied the effect of nearest neighbor spin-singlet bond (SB) correlations
on Josephson coupling and proximity effect in graphene SNS Josephson junctions
with conventional s-wave superconducting contacts. Despite the s-wave
superconducting state in the contacts, the SB pairing state inside the junction
has d-wave symmetry and clean, sharp interface junctions resemble a
'bulk-meets-bulk' situation with very little interaction between the two
different superconducting states. In fact, due to a finite-size suppression of
the superconducting state, a stronger SB coupling constant than in the bulk is
needed in order to achieve SB pairing in a junction. For both short clean
zigzag and armchair junctions a d-wave state that has a zero Josephson coupling
to the s-wave state is chosen and therefore the Josephson current decreases
when a SB pairing state develops in these junctions. In more realistic
junctions, with smoother doping profiles and atomic scale disorder at the
interfaces, it is possible to achieve some coupling between the contact s-wave
state and the SB d-wave states. In addition, by breaking the appropriate
lattice symmetry at the interface in order to induce another d-wave state, a
non-zero Josephson coupling can be achieved which leads to a substantial
increase in the Josephson current. We also report on the LDOS of the junctions
and on a lack of zero energy states at interfaces despite the unconventional
order parameters, which we attribute to the near degeneracy of the two d-wave
solutions and their mixing at a general interface.Comment: 13 pages, 9 figures. Typos correcte
Effective action for the Kondo lattice model. New approach for S=1/2
In the partition function of the Kondo lattice, spin matrices are exactly
replaced by bilinear combinations of Fermi operators with the purely imaginary
chemical potential lambda=-i.pi.T/2 (Popov representation). This new
representation of spin operators allows one to introduce new Green's functions
with Matsubara frequencies 2.pi.T(n+1/4) for S=1/2. A simple temperature
diagram technique is constructed with the path integral method. This technique
is standard and does not contain the complicated combinatoric rules
characteristic of most of the known variants of the diagram techniques for spin
systems. The effective action for the almost antiferromagnetic Kondo lattice is
derived.Comment: 7 pages, Proceedings of SCES98/Paris; one reference adde
Competition between Kondo screening and indirect magnetic exchange in a quantum box
Nanoscale systems of metal atoms antiferromagnetically exchange coupled to
several magnetic impurities are shown to exhibit an unconventional re-entrant
competition between Kondo screening and indirect magnetic exchange interaction.
Depending on the atomic positions of the magnetic moments, the total
ground-state spin deviates from predictions of standard
Ruderman-Kittel-Kasuya-Yosida perturbation theory. The effect shows up on an
energy scale larger than the level width induced by the coupling to the
environment and is experimentally verifiable by studying magnetic field
dependencies.Comment: 5 pages, 2 figures, v3 with minor change
- …