631 research outputs found
Macroscopic Entanglement and Phase Transitions
This paper summarises the results of our research on macroscopic entanglement
in spin systems and free Bosonic gases. We explain how entanglement can be
observed using entanglement witnesses which are themselves constructed within
the framework of thermodynamics and thus macroscopic observables. These
thermodynamical entanglement witnesses result in bounds on macroscopic
parameters of the system, such as the temperature, the energy or the
susceptibility, below which entanglement must be present. The derived bounds
indicate a relationship between the occurrence of entanglement and the
establishment of order, possibly resulting in phase transition phenomena. We
give a short overview over the concepts developed in condensed matter physics
to capture the characteristics of phase transitions in particular in terms of
order and correlation functions. Finally we want to ask and speculate whether
entanglement could be a generalised order concept by itself, relevant in
(quantum induced) phase transitions such as BEC, and that taking this view may
help us to understand the underlying process of high-T superconductivity.Comment: 9 pages, 7 figures (color), Submitted to special OSID issue,
Proceedings of the 38th Symposium on Mathematical Physics - Quantum
Entanglement & Geometry, Torun (Poland), June 200
Enhancement of superconductive critical temperatures in almost empty or full bands in two dimensions: possible relevance to beta-HfNCl, C60 and MgB2
We examine possibility of enhancement of superconductive critical temperature
in two-dimensions. The weak coupling BCS theory is applied, especially when the
Fermi level is near the edges of the electronic bands. The attractive
interaction depends on due to screening. The density of states(DOS)
does not have a peak near the bottom of the band, but -dependent
contribution to DOS (electron density on the Fermi surface) has a diverging
peak at the bottom or top. These features lead to significant enhancement of
the critical temperatures. The results are qualitatively consistent with the
superconductive behaviors of HfNCl (\Tc \le 25K) and ZrNCl(\Tc \le 15K),
C with a field-effect transistor configuration (\Tc = 52K), and
MgB (\Tc \approx 40K) which have the unexpectedly high superconductive
critical transition temperatures.Comment: 5 pages,4 figure
Noninvasiveness and time symmetry of weak measurements
Measurements in classical and quantum physics are described in fundamentally
different ways. Nevertheless, one can formally define similar measurement
procedures with respect to the disturbance they cause. Obviously, strong
measurements, both classical and quantum, are invasive -- they disturb the
measured system. We show that it is possible to define general weak
measurements, which are noninvasive: the disturbance becomes negligible as the
measurement strength goes to zero. Classical intuition suggests that
noninvasive measurements should be time symmetric (if the system dynamics is
reversible) and we confirm that correlations are time-reversal symmetric in the
classical case. However, quantum weak measurements -- defined analogously to
their classical counterparts -- can be noninvasive but not time symmetric. We
present a simple example of measurements on a two-level system which violates
time symmetry and propose an experiment with quantum dots to measure the
time-symmetry violation in a third-order current correlation function.Comment: 19 pages, 5 figures, more information at
http://www.fuw.edu.pl/~abednorz/tasym
Proposal for a cumulant-based Bell test for mesoscopic junctions
The creation and detection of entanglement in solid state electronics is of
fundamental importance for quantum information processing. We prove that
second-order quantum correlations can be always interpreted classically and
propose a general test of entanglement based on the violation of a classically
derived inequality for continuous variables by fourth-order quantum correlation
functions. Our scheme provides a way to prove the existence of entanglement in
a mesoscopic transport setup by measuring higher order cumulants without
requiring the additional assumption of a single charge detectionComment: 6 pages, 1 figure, detailed proof of weak positivity and Bell-type
inequalit
Quasiprobabilistic Interpretation of Weak measurements in Mesoscopic Junctions
The impossibility of measuring noncommuting quantum mechanical observables is
one of the most fascinating consequences of the quantum mechanical postulates.
Hence, to date the investigation of quantum measurement and projection is a
fundamentally interesting topic. We propose to test the concept of weak
measurement of noncommuting observables in mesoscopic transport experiments,
using a quasiprobablistic description. We derive an inequality for current
correlators, which is satisfied by every classical probability but violated by
high-frequency fourth-order cumulants in the quantum regime for experimentally
feasible parameters.Comment: 4 pages, published versio
Bipolarons in the Extended Holstein Hubbard Model
We numerically and analytically calculate the properties of the bipolaron in
an extended Hubbard Holstein model, which has a longer range electron-phonon
coupling like the Fr\" ohlich model. In the strong coupling regime, the
effective mass of the bipolaron in the extended model is much smaller than the
Holstein bipolaron mass. In contrast to the Holstein bipolaron, the bipolaron
in the extended model has a lower binding energy and remains bound with
substantial binding energy even in the large-U limit. In comparison with the
Holstein model where only a singlet bipolaron is bound, in the extended
Holstein model a triplet bipolaron can also form a bound state. We discuss the
possibility of phase separation in the case of finite electron doping.Comment: 5 pages, 3 figure
3D wedge filling and 2D random-bond wetting
Fluids adsorbed in 3D wedges are shown to exhibit two types of continuous
interfacial unbinding corresponding to critical and tricritical filling
respectively. Analytic solution of an effective interfacial model based on the
transfer-matrix formalism allows us to obtain the asymptotic probability
distribution functions for the interfacial height when criticality and
tricriticality are approached. Generalised random walk arguments show that, for
systems with short-ranged forces, the critical singularities at these
transitions are related to 2D complete and critical wetting with random bond
disorder respectively.Comment: 7 pages, 3 figures, accepted for publication in Europhysics Letter
Chiral Plaquette Polaron Theory of Cuprate Superconductivity
Ab-initio density functional calculations on explicitly doped
La(2-x)Sr(x)CuO4 find doping creates localized holes in out-of-plane orbitals.
A model for superconductivity is developed based on the assumption that doping
leads to the formation of holes on a four-site Cu plaquette composed of the
out-of-plane A1 orbitals apical O pz, planar Cu dz2, and planar O psigma. This
is in contrast to the assumption of hole doping into planar Cu dx2-y2 and O
psigma orbitals as in the t-J model. Interaction of holes with the d9 spin
background leads to chiral polarons with either a clockwise or anti-clockwise
charge current. When the polaron plaquettes percolate through the crystal at
x~0.05 for LaSrCuO, a Cu dx2-y2 and planar O psigma band is formed. Spin
exchange Coulomb repulsion with chiral polarons leads to D-wave
superconductivity. The equivalent of the Debye energy in phonon
superconductivity is the maximum energy separation between a chiral polaron and
its time-reversed partner. An additive skew-scattering contribution to the Hall
effect is induced by chiral polarons and leads to a temperature dependent Hall
effect that fits the measured values for LaSrCuO. The integrated imaginary
susceptibility satisfies omega/T scaling due to chirality and spin-flip
scattering of polarons along with a uniform distribution of polaron energy
splittings. The derived functional form is compatible with experiments. The
static spin structure factor is computed and is incommensurate with a
separation distance from (pi,pi) given by ~(2pi)x. Coulomb scattering of the
x2-y2 band with polarons leads to linear resistivity. Coupling of the x2-y2
band to the undoped Cu d9 spins leads to the ARPES pseudogap and its doping and
temperature dependence.Comment: 32 pages, 17 figure
Dynamical two electron states in a Hubbard-Davydov model
We study a model in which a Hubbard Hamiltonian is coupled to the dispersive
phonons in a classical nonlinear lattice. Our calculations are restricted to
the case where we have only two quasi-particles of opposite spins, and we
investigate the dynamics when the second quasi-particle is added to a state
corresponding to a minimal energy single quasi-particle state. Depending on the
parameter values, we find a number of interesting regimes. In many of these,
discrete breathers (DBs) play a prominent role with a localized lattice mode
coupled to the quasiparticles. Simulations with a purely harmonic lattice show
much weaker localization effects. Our results support the possibility that DBs
are important in HTSC.Comment: 14 pages, 12 fig
Two phase transitions in (s+id)-wave Bardeen-Cooper-Schrieffer superconductivity
We establish universal behavior in temperature dependencies of some
observables in -wave BCS superconductivity in the presence of a weak
wave. There also could appear a second second-order phase transition. As
temperature is lowered past the usual critical temperature , a less
ordered superconducting phase is created in wave, which changes to a more
ordered phase in wave at (). The presence of two phase
transitions manifest in two jumps in specific heat at and . The
temperature dependencies of susceptibility, penetration depth, and thermal
conductivity also confirm the new phase transition.Comment: 6 pages, 5 post-script figures
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