Добірка наукової літератури з теми "Berezinskii-Kosterlitz-Thouless transition temperature"

AbstractIdeal two-dimensional (2D) Heisenberg magnets lack long range magnetic order. However, the XY model with spins confined to a plane shows a topological phase transition at a finite temperature corresponding to binding and unbinding of vortices. Experimental evidence for such Berezinskii-Kosterlitz-Thouless (BKT) transitions has been difficult to obtain in condensed matter systems, where, even a weak interlayer coupling that is invariably present leads to long-range order, pre-empting the BKT transition. The BKT signatures are still discernible above the long-range ordering temperature, however, in the characteristic exponential temperature dependence of the coherence length of the fluctuations. In this work we report that an applied magnetic field can induce such BKT correlations not only in quasi 2-dimensional systems but also in nominally 3-dimensional manganites undergoing antiferromagnetic transitions. We arrive at this unexpected conclusion based on our studies of temperature dependence of electron paramagnetic resonance (EPR) linewidth ΔH(T) of Cr3+ doped bismuth strontium manganite Bi0.5Sr0.5Mn1-xCrxO3 (x= 0.04, 0.1) (BSMCO04 and BSMCO10).

We obtain nonperturbative results on the sine-Gordon model using the lattice field technique. In particular, we employ the Fourier accelerated hybrid Monte Carlo algorithm for our studies. We find the critical temperature of the theory based autocorrelation time, as well as the finite size scaling of the “thickness” observable used in an earlier lattice study by Hasenbusch et al.

We have performed a Monte Carlo (MC) study of the classical XY-model on a Sierpiński carpet, which is a planar fractal structure with infinite order of ramification and fractal dimension 1.8928. We employed the Wolff cluster algorithm in our simulations and our results, in particular those for the susceptibility and the helicity modulus, indicate the absence of finite-temperature Berezinskii–Kosterlitz–Thouless (BKT) transition in this system.

The electrical conductance and I–V characteristics of ultrathin Pd films prepared by low temperature deposition have been measured in situ. An Arrhenius behavior was found for the electrical conductance. A crossover between regimes with different activation energies was observed at a temperature TK. Below TK, the I–V characteristics of the film were found to be nonlinear. These observations are explained using a charge Kosterlitz-Thouless-Berezinskii transition picture which involves the ionization of charge-anticharge pairs.

We have performed detailed transport measurements on a 3 nm thick (as-grown) Al film on GaAs prepared by molecular beam epitaxy (MBE). Such an epitaxial film grown on a GaAs substrate shows the Berezinskii-Kosterlitz-Thouless (BKT) transition, a topological transition in two dimensions. Our experimental data shows that the MBE-grown Al nanofilm is an ideal system for probing interesting physical phenomena such as the BKT transition and superconductivity. The increased superconductor transition temperature (~2.4 K) compared to that of bulk Al (1.2 K), together with the ultrathin film quality, may be advantageous for future superconductor-based quantum devices and quantum information technology.

We investigate the effects of higher-order quantum fluctuations on the bulk properties of self-bound droplets in three-, two- and one-dimensional binary Bose mixtures using the Hartree–Fock–Bogoliubov theory. We calculate higher-order corrections to the equation of state of the droplet at both zero and finite temperatures. We show that our results for the ground-state energy are in a good agreement with recent quantum Monte Carlo simulations in any dimension. Our study extends to the finite temperature case where it is found that thermal fluctuations may destabilize the droplet state and eventually destroy it. In two dimensions, we reveal that the droplet occurs at temperatures well below the Berezinskii–Kosterlitz–Thouless transition temperature.

Path integral molecular dynamics (PIMD) has been successfully applied to perform simulations of large bosonic systems in a recent study [Hirshberg et al., Proc. Natl. Acad. Sci. U. S. A. 116, 21445 (2019)]. In this work, we extend PIMD techniques to study Green’s function for bosonic systems. We demonstrate that the development of the original PIMD method enables us to calculate Green’s function and extract momentum distribution from our simulations. We also apply our method to systems of identical interacting bosons to study Berezinskii–Kosterlitz–Thouless transition around its critical temperature.

High-T c superconductors (HTSs) have a layered structure, which strongly affects their physics. Bulk crystals exhibit the two-dimensional (2D) behaviour typical for superconducting films of atomic thickness. Fluctuation contribution to conductivity above mean-field critical temperature T c0 is characteristic to 2D superconductors. Finite resistivity in layered HTSs appears above Berezinskii-Kosterlitz-Thouless (BKT) transition T c , which is by several degrees smaller than T c0 . Below T c power-law I-V curves are observed. The transition and its main manifestations are described as a result of thermal creation of 2D-vortices with cores residing inside single superconducting layers. Magnetoresistive effects near the BKT transition are discussed briefly.

In this paper, we use the Replica-Exchange Wang–Landau (REWL) technique to study the behavior of the two-point correlation function of the site diluted classical anisotropic Heisenberg (AH) model in two dimensions. The model has a topological Berezinskii–Kosterlitz–Thouless (BKT) transition for any dilution down to the percolation threshold, [Formula: see text]. Contrary to earlier results that predict the exponent of the correlation function, [Formula: see text], to decay linearly to zero as a function of temperature, we found that it decreases with temperature to a finite value [Formula: see text] at [Formula: see text] for any dilution [Formula: see text].

Thesis (Ph. D.)--Ohio State University, 2005.
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The large variety of phenomena exhibited by the interface between two band-insulators LaAlO3 and SrTiO3 has attracted intense research activity in recent years.The interplay of different symmetries, viz. point group symmetry, charge inversion symmetry, U(1) gauge symmetry and spin rotation symmetry in heterostructures of these complex perovskite oxides [1] lead to the co-existence of a host of intriguing properties - ferroelasticity, ferroelectricity, superconductivity and ferromagnetism [2, 3]. In this thesis, we attempt to address the physics of some of these these phases by probing the resistance and resistance fluctuations in LaAlO3/SrTiO3 heterostructures. In a two dimensional (2D) superconductor, the vortices induced by thermal fluctuations appear as a bound pairs below a characteristic temperature: the Berezinskii-Kosterlitz-Thouless transition temperature (TBKT ). Conventional techniques that probe for signatures of BKT transition such as the measurement of discontinuity in the superfluid density near the transition [4, 5] cannot be applied in the case of heterostructures where the charge carriers are buried at an interface. In this thesis, we use a fluctuation based kinetic probe namely electrical resistance fluctuation to study the BKT transition in 2D superconductors. LaAlO3/SrTiO3 heterostructure exhibits superconductivity at low temperatures with a charge carrier density dependent TBKT [6, 7]. We have experimentally studied low frequency resistance fluctuations (noise) and its higher order statistics near the superconducting transition region by varying the temperature, gate voltage and magnetic field. From the analysis of the higher order statistics of resistance fluctuations, we find large non-Gaussian components (NGC) in resistance fluctuations near TBKT , which signifies strong correlations among interacting vortices in the system. The NGC are found to be completely absent above mean field temperature TC. Theoretical simulations indicate that the large non-Gaussian resistance fluctuations are the manifestation of a percolative transition of a Josephson-coupled superconducting network. LaAlO3/SrTiO3 interface exhibits coexistence of superconductivity and ferromagnetism. In general, superconductivity and ferromagnetism are antagonistic to each other. So, the appearance of two co-existing phase at the interface has opened up a new direction of research in condensed matter physics. In this thesis, we address this physics by analyzing the results of the measurements of perpendicular field magnetotransport above superconducting transition temperature (T=TBKT 2 ). From our experiments, we identify a gate voltage tunable Lifshitz transition between single band to multiband transport. We find that ferromagnetism is strongest below Lifshitz transition. We observed a novel transient superconducting state (TSS) in the presence of a magnetic field applied perpendicular to the interface. We show that the TSS is a consequence of the interplay between ferromagnetism, superconductivity and the finite relaxation time of the in-plane magnetization in this system. We also studied the electrical noise above TBKT to look for possible signatures of effect of multiband transport. In this thesis, we present results of the measurements of noise at T 2TBKT as the chemical potential is tuned across the Lifshitz transition. We demonstrate that the noise have strikingly different features on either side of the Lifshitz transition. Below the Lifshitz transition, noise is dominated by carrier density fluctuations arising from trapping-detrapping of charge carriers from defects in the underlying SrTiO3 substrate. Above the Lifshitz transition, we propose that the noise presumably originate from the scattering of carriers from different available conduction channels.