Model Of Phase Fluctuations Research Articles

Analysis of coherent laser echo characteristics back scattered from rough Gaussian target

The surface property of rough target can cause the decoherence and depolarization effects, which is among the main limiting factors in heterodyne detection. In the decoherence analyses, the phase fluctuation model of laser echo from rough target is established based on the spectral density method, and the phase fluctuations under typical roughness conditions are obtained by Monte Carlo method. The distribution properties of heterodyne signal amplitudes are analyzed under different surface height fluctuation σ and correlation length T. Then the corresponding experiments are carried out, which show that the phase fluctuations of laser echo satisfy the Gaussian distribution, and the decoherence effect induces the reductions and fluctuations of heterodyne signal amplitudes, especially with larger roughness surface. For typical rough metal targets, the depolarization characteristics of laser echoes from different scattering positions are obtained, which suggest that the depolarization effect increases with the rise of roughness. The positive correlation between the decoherence and depolarization effects provides a reference for the design of heterodyne lidar system, and the statistics can also be adopted for target roughness inference and target type identification.

An electric-field representation of the harmonic XY model

The two-dimensional harmonic XY (HXY) model is a spin model in which the classical spins interact via a piecewise parabolic potential. We argue that the HXY model should be regarded as the canonical classical lattice spin model of phase fluctuations in two-dimensional condensates, as it is the simplest model that guarantees the modular symmetry of the experimental systems. Here we formulate a lattice electric-field representation of the HXY model and contrast this with an analogous representation of the Villain model and the two-dimensional Coulomb gas with a purely rotational auxiliary field. We find that the HXY model is a spin-model analogue of a lattice electric-field model of the Coulomb gas with an auxiliary field, but with a temperature-dependent vacuum (electric) permittivity that encodes the coupling of the spin vortices to their background spin-wave medium. The spin vortices map to the Coulomb charges, while the spin-wave fluctuations correspond to auxiliary-field fluctuations. The coupling explains the striking differences in the high-temperature asymptotes of the specific heats of the HXY model and the Coulomb gas with an auxiliary field. Our results elucidate the propagation of effective long-range interactions throughout the HXY model (whose interactions are purely local) by the lattice electric fields. They also imply that global spin-twist excitations (topological-sector fluctuations) generated by local spin dynamics are ergodically excluded in the low-temperature phase. We discuss the relevance of these results to condensate physics.

Non-conventional superconducting fluctuations in Ba(Fe1−xRhx)2As2 iron-based superconductors

We measured the static uniform spin susceptibility of Ba(Fe1−xRhx)2As2 iron-based superconductors, over a broad range of doping (0.041 ⩽ x ⩽ 0.094) and magnetic fields. At small fields (H ⩽ 1 kOe) we observed, above the transition temperature Tc, the occurrence of precursor diamagnetism, which is not ascribable to the Ginzburg–Landau theory. On the contrary, our data agree with a phase fluctuation model, which has been used to interpret a similar phenomenology occurring in the high-Tc cuprate superconductors. Additionally, in the presence of strong fields, the unconventional fluctuating diamagnetism is suppressed, whereas Ginzburg–Landau fluctuations are found, in agreement with literature.

Large isotope effect onTcin cuprates despite a small electron-phonon coupling

We calculate the isotope coefficients $\ensuremath{\alpha}$ and ${\ensuremath{\alpha}}^{\ensuremath{\ast}}$ for the superconducting critical temperature ${T}_{c}$ and the pseudogap temperature ${T}^{\ensuremath{\ast}}$ in a mean-field treatment of the $t\text{\ensuremath{-}}J$ model including phonons. The pseudogap phase is identified with the $d$-charge-density wave $(d\text{-CDW})$ phase in this model. Using the small electron-phonon coupling constant ${\ensuremath{\lambda}}_{d}\ensuremath{\sim}0.02$ obtained previously in local-density approximation calculations in ${\text{YBa}}_{2}{\text{Cu}}_{3}{\text{O}}_{7}$, ${\ensuremath{\alpha}}^{\ensuremath{\ast}}$ is negative but negligibly small, whereas $\ensuremath{\alpha}$ increases from about 0.03 at optimal doping to values around 1 at small dopings, in agreement with the general trend observed in many cuprates. Using a simple phase fluctuation model where the $d\text{-CDW}$ has only short-range correlations, it is shown that the large increase of $\ensuremath{\alpha}$ at low dopings is rather universal and does not depend on the existence of sharp peaks in the density of states in the pseudogap state or on specific values of the phonon cutoff. Rather, it is caused by the large depletion of spectral weight at low frequencies by the $d\text{-CDW}$ and thus should also occur in other realizations of the pseudogap.

Phase diffusions due to radio-frequency excitations in one-dimensional arrays of superconductor/ insulator/superconductor junctions

The dc response of one-dimensional (1D) arrays of superconductor/insulator/superconductor (SIS) junctions under radio-frequency (RF) irradiation has been experimentally studied. The zero-bias resistance as a function of RF amplitude in the low-frequency regime shows an oscillatory behaviour, and can be explained using the phase fluctuation model for Cooper-pair tunnelling. It follows that an effective classical detector theory is applicable and the oscillation period can be compared with the superconducting gap. The results suggest a primary and direct RF detection scheme using 1D SIS junction arrays.

Model of phase fluctuations in a latticed-wave superconductor: Application to the Cooper-pair charge-density wave in underdoped cuprates

We introduce and study an $XY$-type model of thermal and quantum phase fluctuations in a two-dimensional correlated lattice $d$-wave superconductor based on the ${\mathrm{QED}}_{3}$ effective theory of high-temperature superconductors. General features of and selected results obtained within this model were reported earlier in an abbreviated format (Z. Te\ifmmode \check{s}\else \v{s}\fi{}anovi\ifmmode \acute{c}\else \'{c}\fi{}, e-print cond-mat/0405235). The model is geared toward describing not only the long distance but also the intermediate length-scale physics of underdoped cuprates. In particular, we elucidate the dynamical origin and investigate specific features of the charge-density wave of Cooper pairs, which we argue is the state behind the periodic charge-density modulation discovered in recent scanning-tunneling-microscopy experiments. We illustrate how Mott-Hubbard correlations near half-filling suppress superfluid density and favor an incompressible state which breaks translational symmetry of the underlying atomic lattice. We show how the formation of the Cooper pair charge-density wave in such a strongly quantum fluctuating superconductor can naturally be understood as an Abrikosov-Hofstadter problem in a type-II dual superconductor, with the role of the dual magnetic field played by the electron density. The resulting Abrikosov lattice of dual vortices translates into a periodic modulation of the Bogoliubov--de Gennes (BdG) gap function and the electronic density. We numerically study the energetics of various Abrikosov-Hofstadter dual vortex arrays and compute their detailed signatures in the single-particle local tunneling density of states. A $4\ifmmode\times\else\texttimes\fi{}4$ checkerboard-type modulation pattern naturally arises as an energetically favored ground state at and near the $x=1∕8$ doping and produces the local density of states in good agreement with experimental observations. The leading-order behavior of nodal BdG fermions remains unaffected.

Superfluid density suppression and quasiparticle interaction inBi2Sr2Ca1−xPrxCu2O8+δ single crystals

The Pr suppression of the superfluid density inBi2Sr2Ca1−xPrxCu2O8+δ single crystals is discussed using the zero-temperature in-plane penetration depth extractedfrom the second peak field in magnetization curves. It is found that the results fit better tothe modified dirty d-wave model than the usual model. The relationship between thesuperfluid density and the critical temperature is also discussed in the underdoped regionand one finds that the phase fluctuation model cannot account for the criticaltemperature in the system, which may be due to the interaction between thethermally excited quasiparticles and the impurity scattering induced quasiparticles.

Change of quasiparticle dispersion in crossingTcin the underdoped cuprates

One of the most remarkable properties of the high-temperature superconductors is a pseudogap regime appearing in the underdoped cuprates above the superconducting transition temperature T_c. The pseudogap continously develops out of the superconducting gap. In this paper, we demonstrate by means of a detailed comparison between theory and experiment that the characteristic change of quasiparticle dispersion in crossing T_c in the underdoped cuprates can be understood as being due to phase fluctuations of the superconducting order parameter. In particular, we show that within a phase fluctuation model the characteristic back-turning BCS bands disappear above T_c whereas the gap remains open. Furthermore, the pseudogap rather has a U-shape instead of the characteristic V-shape of a d_{x^2-y^2}-wave pairing symmetry and starts closing from the nodal k=(pi/2,pi/2) directions, whereas it rather fills in at the anti-nodal k=(pi,0) regions, yielding further support to the phase fluctuation scenario.

Algebraic Fermi liquid from phase fluctuations: "topological" fermions, vortex "berryons," and QED3 theory of cuprate superconductors.

Within the phase fluctuation model for the pseudogap state of cuprate superconductors we identify a novel statistical "Berry phase" interaction between the nodal quasiparticles and fluctuating vortex-antivortex excitations. The effective action describing this model assumes the form of an anisotropic Euclidean quantum electrodynamics in (2+1) dimensions (QED (3)) and naturally generates non-Fermi liquid behavior for its fermionic excitations. The doping axis in the x -T phase diagram emerges as a quantum critical line which regulates the low energy fermiology.

Gapless state in low- and high- Tc superconductors: evidence for thermal phase fluctuations

We have carried out a comparative study of the low-temperature dependence of the magnetic penetration depth λ in (1) superconducting NbN films grown on R-plane sapphire and MgO substrates and (2) Bi2212 single crystals with different doping levels ranging from under- to over-doping. The NbN films display textured or epitaxial microstructure depending on growth conditions. The thermally activated dependence of λ( T) characteristic of conventional s-wave superconductors was found only in the epitaxial or thin textured NbN films. All thick textured films exhibit below 4 K a linear dependence Δλ∼ T characteristic of a gapless superconducting state. This is quantitatively explained by a model of granularity-induced thermal phase fluctuations of the order parameter. As for Bi2212, we have found in all crystals a linear low-temperature dependence Δλ ab ∼ T in the ab-plane. The slope Δλ ab / ΔT displays a pronounced minimum ∼8 Å/K at optimum doping. This value is in good agreement with the d-wave model proposed for cuprates. However, this model fails to account for the large slope values >40 Å/K observed at both under- and over-doping. These values are quantitatively explained by the same model of phase fluctuations adopted for the textured NbN films, in spite of the difference between these two superconducting systems.

Preparation and analysis of electronic-transport properties of BSCCO-2223 platelets

High-quality single-phase thin platelets of BSCCO-2223 have been produced by a novel method on thermal-gradient-enhanced KCl flux which allows the growth of the 2223-phase in an extremely short time interval compared to those reported in the literature. In-plane electrical resistivity measurements in zero applied magnetic field have been performed. Experimental data have been analysed and interpreted within the Aslamazov-Larkin theory of amplitude fluctuations and the Ginzburg-Landau Coulomb gas model of phase fluctuations. These theories seem to fit in a very reliable way the experimental R(T) curves.

Stochastic models of stable frequency and time sources and their relationship

Different stochastic models of a stable frequency and time sources showing white phase noise, white frequency noise, and random walk of frequency noise are considered. A continuous time model of phase fluctuations is associated with the power law model of relative frequency fluctuations. The ARIMA (0,2,2) and the Kalman models of the sampled phase derivations are derived from the continuous model. Equations relating the characteristic parameters of these three representations of the source behavior are given. The Allan variance of relative frequency fluctuations is expressed as a function of the characteristic parameters. The approximation inherent to the simplified Kalman model is discussed, and the limit of validity of this model is stated.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>