137 research outputs found
Phase diagram of S=1/2 two-leg XXZ spin ladder systems
We investigate the ground state phase diagram of the S=1/2 two-leg spin
ladder system with an isotropic interchain coupling. In this model, there is
the Berezinskii-Kosterlitz-Thouless transition which occurs at the XY-Haldane
and the XY-rung singlet phase boundaries. It was difficult to determine the
transition line using traditional methods. We overcome this difficulty using
the level spectroscopy method combined with the twisted boundary condition
method, and we check the consistency. We find out that the phase boundary
between XY phase and Haldane phase lies on the line. And we show
that there exist two different XY phases, which we can distinguish
investigating a correlation function
Finite temperature analysis of a quasi2D dipolar gas
We present finite temperature analysis of a quasi2D dipolar gas. To do this,
we use the Hartree Fock Bogoliubov method within the Popov approximation. This
formalism is a set of non-local equations containing the dipole-dipole
interaction and the condensate and thermal correlation functions, which are
solved self-consistently. We detail the numerical method used to implement the
scheme. We present density profiles for a finite temperature dipolar gas in
quasi2D, and compare these results to a gas with zero-range interactions.
Additionally, we analyze the excitation spectrum and study the impact of the
thermal exchange
Dynamic Kosterlitz-Thouless transition in 2D Bose mixtures of ultra-cold atoms
We propose a realistic experiment to demonstrate a dynamic
Kosterlitz-Thouless transition in ultra-cold atomic gases in two dimensions.
With a numerical implementation of the Truncated Wigner Approximation we
simulate the time evolution of several correlation functions, which can be
measured via matter wave interference. We demonstrate that the relaxational
dynamics is well-described by a real-time renormalization group approach, and
argue that these experiments can guide the development of a theoretical
framework for the understanding of critical dynamics.Comment: 5 pages, 6 figure
Quantum phase transitions of the asymmetric three-leg spin tube
We investigate quantum phase transitions of the S=1/2 three-leg
antiferromagnetic spin tube with asymmetric inter-chain (rung) exchange
interactions. On the basis of the electron tube system, we propose a useful
effective theory to give the global phase diagram of the asymmetric spin tube.
In addition, using other effective theories we raise the reliability of the
phase diagram. The density-matrix renormalization-group and the numerical
diagonalization analyses show that the finite spin gap appears in a narrow
region around the rung-symmetric line, in contrast to a recent paper by
Nishimoto and Arikawa [Phys. Rev. B78, 054421 (2008)]. The numerical
calculations indicate that this global phase diagram obtained by use of the
effective theories is qualitatively correct. In the gapless phase on the phase
diagram, the numerical data are fitted by a finite-size scaling in the c=1
conformal field theory. We argue that all the phase transitions between the
gapful and gapless phases belong to the Berezinskii-Kosterlitz-Thouless
universality class.Comment: 12 pages, 7 figures, 2 column, final versio
Phase transition of clock models on hyperbolic lattice studied by corner transfer matrix renormalization group method
Two-dimensional ferromagnetic N-state clock models are studied on a
hyperbolic lattice represented by tessellation of pentagons. The lattice lies
on the hyperbolic plane with a constant negative scalar curvature. We observe
the spontaneous magnetization, the internal energy, and the specific heat at
the center of sufficiently large systems, where the fixed boundary conditions
are imposed, for the cases N>=3 up to N=30. The model with N=3, which is
equivalent to the 3-state Potts model on the hyperbolic lattice, exhibits the
first order phase transition. A mean-field like phase transition of the second
order is observed for the cases N>=4. When N>=5 we observe the Schottky type
specific heat below the transition temperature, where its peak hight at low
temperatures scales as N^{-2}. From these facts we conclude that the phase
transition of classical XY-model deep inside the hyperbolic lattices is not of
the Berezinskii-Kosterlitz-Thouless type.Comment: REVTeX style, 4 pages, 6 figures, submitted to Phys. Rev.
Anomalous magnetization process in frustrated spin ladders
We study, at T=0, the anomalies in the magnetization curve of the S=1 two-leg
ladder with frustrated interactions. We focus mainly on the existence of the
M=\Ms/2 plateau, where \Ms is the saturation magnetization. We use
analytical methods (degenerate perturbation theory and non-Abelian
bosonization) as well as numerical methods (level spectroscopy and density
matrix renormalization group), which lead to the consistent conclusion with
each other. We also touch on the M=\Ms/4 and M=(3/4)\Ms plateaux and cusps.Comment: 4 pages, 7 figures (embedded), Conference paper (Highly Frustrated
Magnetism 2003, 26-30th August 2003, Grenoble, France
Origin of intrinsic dark count in superconducting nanowire single-photon detectors
The origin of the decoherence in superconducting nanowire single-photon
detectors, the so-called dark count, was investigated. We measured the
direct-current characteristics and bias-current dependencies of the dark count
rate in a wide range of temperatures from 0.5 K to 4 K, and analyzed the
results by theoretical models of thermal fluctuations of vortices. Our results
indicate that the current-assisted unbinding of vortex-antivortex pairs is the
dominant origin of the dark count.Comment: 10 pages, 2 figure
Quasi-long range order in glass states of impure liquid crystals, magnets, and superconductors
In this review we consider glass states of several disordered systems:
vortices in impure superconductors, amorphous magnets, and nematic liquid
crystals in random porous media. All these systems can be described by the
random-field or random-anisotropy O(N) model. Even arbitrarily weak disorder
destroys long range order in the O(N) model. We demonstrate that at weak
disorder and low temperatures quasi-long range order emerges. In
quasi-long-range-ordered phases the correlation length is infinite and
correlation functions obey power dependencies on the distance. In pure systems
quasi-long range order is possible only in the lower critical dimension and
only in the case of Abelian symmetry. In the presence of disorder this type of
ordering turns out to be more common. It exists in a range of dimensions and is
not prohibited by non-Abelian symmetries.Comment: 32 page
Current Assisted, Thermally Activated Flux Liberation in Ultrathin Nanopatterned NbN Superconducting Meander Structures
We present results from an extensive study of fluctuation phenomena in
superconducting nanowires made from sputtered NbN. Nanoscale wires were
fabricated in form of a meander and operated at a constant temperature
T~0.4Tc(0). The superconducting state is driven close to the electronic phase
transition by a high bias current near the critical one. Fluctuations of
sufficient strength temporarily drive a section of the meander structure into
the normal conducting state, which can be registered as a voltage pulse of
nanosecond duration. We considered three different models (vortex-antivortex
pairs, vortex edge barriers and phase slip centers) to explain the experimental
data. Only thermally excited vortices, either via unbinding of
vortex-antivortex pairs or vortices overcoming the edge barrier, lead to a
satisfactory and consistent description for all measurements.Comment: 41 Pages, 5 Chapters, 7 Figures, 2 Tables, 30 Equations, 68
References; Selected for the January 15, 2010 Issue of the Virtual Journal of
Applications of Superconductivit
Entanglement Perturbation Theory for Infinite Quasi-1D Quantum Systems
We develop Entanglement Perturbation Theory (EPT) for infinite Quasi-1D
quantum systems. The spin 1/2 Heisenberg chain with ferromagnetic nearest
neighbor (NN) and antiferromagnetic next nearest neighbor (NNN) interactions
with an easy-plane anisotropy is studied as a prototypical system. The obtained
accurate phase diagram is compared with a recent prediction
[Phys.Rev.B,81,094430(2010)] that dimer and Neel orders appear alternately as
the XXZ anisotropy Delta approaches the isotropic limit Delta=1. The first and
second transitions (across dimer, Neel, and dimer phases) are detected with
improved accuracy at Delta\approx 0.722 and 0.930. The third transition (from
dimer to Neel phases), previously predicted to be at Delta\approx 0.98, is not
detected at this Delta in our method, raising the possibility that the second
Neel phase is absent.Comment: 5 pages, 5 figure
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