1,317 research outputs found
The phase diagram of 2D polar condensates in a magnetic field
Spin one condensates in the polar (antiferromagnetic) phase in two dimensions
are shown to undergo a transition of the Ising type, in addition to the
expected Kosterlitz--Thouless (KT) transition of half vortices, due to the
quadratic Zeeman effect. We establish the phase diagram in terms of temperature
and the strength of the Zeeman effect using Monte Carlo simulations. When the
Zeeman effect is sufficiently strong the Ising and KT transitions merge. For
very strong Zeeman field the remaining transition is of the familiar integer KT
type.Comment: 4 pages, 7 figure
A cluster algorithm for resistively shunted Josephson junctions
We present a cluster algorithm for resistively shunted Josephson junctions
and similar physical systems, which dramatically improves sampling efficiency.
The algorithm combines local updates in Fourier space with rejection-free
cluster updates which exploit the symmetries of the Josephson coupling energy.
As an application, we consider the localization transition of a single junction
at intermediate Josephson coupling and determine the temperature dependence of
the zero bias resistance as a function of dissipation strength.Comment: 4 page
Sequence of phase transitions induced in an array of Josephson junctions by their crossover to pi-state
We show that the transition of Josephson junctions between the conventional
and pi states caused by the decrease in temperature induces in a regular
two-dimensional array of such junctions not just a single phase transition
between two phases with different ordering but a sequence of two, three or four
phase transitions. The corresponding phase diagrams are constructed for the
cases of bipartite (square or honeycomb) and triangular lattices.Comment: 5 pages, v2: as published in EP
Disproportionation and electronic phase separation in parent manganite LaMnO_3
Nominally pure undoped parent manganite LaMnO_3 exhibits a puzzling behavior
inconsistent with a simple picture of an A-type antiferromagnetic insulator
(A-AFI) with a cooperative Jahn-Teller ordering. We do assign its anomalous
properties to charge transfer instabilities and competition between insulating
A-AFI phase and metallic-like dynamically disproportionated phase formally
separated by a first-order phase transition at T_{disp}=T_{JT}\approx 750 K.
The unconventional high-temperature phase is addressed to be a specific
electron-hole Bose liquid (EHBL) rather than a simple "chemically"
disproportionated R(Mn^{2+}Mn^{4+})O_3 phase. New phase does nucleate as a
result of the charge transfer (CT) instability and evolves from the
self-trapped CT excitons, or specific EH-dimers, which seem to be a precursor
of both insulating and metallic-like ferromagnetic phases observed in
manganites. We arrive at highly frustrated system of triplet (e_g^2)^3A_{2g}
bosons moving in a lattice formed by hole Mn^{4+} centers. Starting with
different experimental data we have reproduced a typical temperature dependence
of the volume fraction of high-temperature mixed-valent EHBL phase. We argue
that a slight nonisovalent substitution, photo-irradiation, external pressure
or magnetic field gives rise to an electronic phase separation with a
nucleation or an overgrowth of EH-droplets. Such a scenario provides a
comprehensive explanation of numerous puzzling properties observed in parent
and nonisovalently doped manganite LaMnO_3 including an intriguing
manifestation of superconducting fluctuations.Comment: 20 pages, 8 figure
Simulation results for an interacting pair of resistively shunted Josephson junctions
Using a new cluster Monte Carlo algorithm, we study the phase diagram and
critical properties of an interacting pair of resistively shunted Josephson
junctions. This system models tunneling between two electrodes through a small
superconducting grain, and is described by a double sine-Gordon model. In
accordance with theoretical predictions, we observe three different phases and
crossover effects arising from an intermediate coupling fixed point. On the
superconductor-to-metal phase boundary, the observed critical behavior is
within error-bars the same as in a single junction, with identical values of
the critical resistance and a correlation function exponent which depends only
on the strength of the Josephson coupling. We explain these critical properties
on the basis of a renormalization group (RG) calculation. In addition, we
propose an alternative new mean-field theory for this transition, which
correctly predicts the location of the phase boundary at intermediate Josephson
coupling strength.Comment: 21 pages, some figures best viewed in colo
Electron Transport in Granular Metals
We consider thermodynamic and transport properties of a long granular array
with strongly connected grains (inter-grain conductance g>>1.) We find that the
system exhibits activated behavior of conductance and thermodynamic density of
states ~exp(-T*/T) where the gap, T*, is parametrically larger than the energy
at which conventional perturbation theory breaks down. The scale T* represents
energy needed to create a long single-electron charge soliton propagating
through the array.Comment: 4 pages, 1 figur
Evolution of superconductivity in Fe-based systems with doping
We study the symmetry and the structure of the gap in Fe-based
superconductors by decomposing the pairing interaction obtained in the RPA into
s- and d-wave components and into contributions from scattering between
different Fermi surfaces. We show that each interaction is well approximated by
the lowest angular harmonics and use this simplification to analyze the origin
of the attraction in the two channels, the competition between s- and d-wave
solutions, and the origin of superconductivity in heavily doped systems, when
only electron or only hole pockets are present.Comment: 4pp, 2 figures, 2 table
Coulomb Blockade with Dispersive Interfaces
What quantity controls the Coulomb blockade oscillations if the dot--lead
conductance is essentially frequency--dependent ? We argue that it is the ac
dissipative conductance at the frequency given by the effective charging
energy. The latter may be very different from the bare charging energy due to
the interface--induced capacitance (or inductance). These observations are
supported by a number of examples, considered from the weak and strong coupling
(perturbation theory vs. instanton calculus) perspectives.Comment: 4 page
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