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

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

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

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

Finite-Field Ground State of the S=1 Antiferromagnetic-Ferromagnetic Bond-Alternating Chain

We investigate the finite-field ground state of the S=1 antiferromagnetic-ferromagnetic bond-alternating chain described by the Hamiltonian {\calH}=\sum\nolimits_{\ell}\bigl\{\vecS_{2\ell-1}\cdot\vecS_{2\ell} +J\vecS_{2\ell}\cdot\vecS_{2\ell+1}\bigr\} +D\sum\nolimits_{\ell} \bigl(S_{\ell}^z)^2 -H\textstyle\sum\nolimits_\ell S_\ell^z, where \hbox{$J\leq0$} and \hbox{$-\infty<D<\infty$}. We find that two kinds of magnetization plateaux at a half of the saturation magnetization, the 1/2-plateaux, appear in the ground-state magnetization curve; one of them is of the Haldane type and the other is of the large-$D$-type. We determine the 1/2-plateau phase diagram on the $D$ versus $J$ plane, applying the twisted-boundary-condition level spectroscopy methods developed by Kitazawa and Nomura. We also calculate the ground-state magnetization curves and the magnetization phase diagrams by means of the density-matrix renormalization-group method