Influence Of Thermal Fluctuations Research Articles

Features of vortex pinning and magnetic flux creep in epitaxial thin films of high-Tc superconductor YBa2Cu3O7−δ near the critical temperature

The temperature dependence of the dynamic relaxation rate in YBa2Cu3O7−δ epitaxial thin films is investigated in the temperature range from 77K to the critical temperature Tc with and without an applied dc magnetic field, and the dependence on the dc field at 77K is measured at fields up to 45mT. It is shown that the experimental results are in good agreement with the vortex lattice pinning model proposed previously which considers the main pinning centers in thin films to be threading dislocations on a network of low-angle grain boundaries. From the results of the experiment it is concluded that the influence of thermal fluctuations on the vortex pinning by threading dislocations and on the magnetic flux creep near Tc is not so crucial as in the case of pinning by extended linear defects in thick films or single crystals. Estimates are made which show that this fact can be explained by a transformation of the Abrikosov vortices into Pearl vortices when the magnetic field penetration depth becomes greater than the film thickness as the temperature approaches a critical value. Because of this, the mechanisms of pinning and flux creep in thin films are of a substantially collective character even in extremely weak magnetic fields and at temperatures very close to Tc.

Influence of Thermal Fluctuations on Interfacial Electron Transfer in Functionalized TiO2 Semiconductors

The influence of thermal fluctuations on the dynamics of interfacial electron transfer in sensitized TiO2-anatase semiconductors is investigated by combining ab initio DFT molecular dynamics simulations and quantum dynamics propagation of transient electronic excitations. It is shown that thermal nuclear fluctuations speed up the underlying interfacial electron transfer dynamics by introducing nonadiabatic transitions between electron acceptor states, localized in the vicinity of the photoexcited adsorbate, and delocalized states extended throughout the semiconductor material, creating additional relaxation pathways for carrier diffusion. Furthermore, it is shown that room-temperature thermal fluctuations reduce the anisotropic character of charge diffusion along different directions in the anatase crystal and make similar the rates for electron injection from adsorbate states of different character. The reported results are particularly relevant to the understanding of temperature effects on surface charge separation mechanisms in molecular-based photo-optic devices.

Noise Induced Timing Jitter: A General Restriction for High Speed RSFQ Devices

All complex devices in rapid single flux quantum (RSFQ) technique work at much lower clock rates than possible for simple cells. New data driven self timed or asynchronous concepts can reduce this gap, but even in this case still a discrepancy between simulation and experiment is shown. It has been pointed out in previous studies, that the influence of thermal fluctuations in RSFQ circuits is divided into static and dynamic switching bit errors as well as timing jitter induced failures. The bit error rate depends exponentially on the temperature and is not the main issue in the low temperature technique. Our last results show for the first time, that the variance of the switching time is much slower decreased by reducing the temperature and is still important at 4.2 K. We describe a general method for calculating the switching time distribution for RSFQ cells. Furthermore, we present detailed results for a timing analysis of a dc/SFQ-converter. The delay between output signal and the input current ramp shows a variation over 1 ps which is more than 10% of the switching time itself. This new understanding of the background of timing jitter enables a goal-oriented improvement in the design process of RSFQ circuits.

Temperature dependence of vesicle adhesion

The influence of thermal fluctuations on the adhesion behavior of fluid vesicles is investigated with the help of Monte Carlo simulations. The adhesion area A(ad) of a fluid vesicle adhering to a smooth attractive substrate is studied systematically for different values of temperature, adhesion strength, and potential range. For low temperatures T , the ratio A(ad) /A between the adhesion area and the total area A of the vesicle is a linear function of T/kappa , where kappa is the bending rigidity. Linear fits of the simulation data allow an extrapolation to T=0 which corresponds well with data obtained from a simplified analytic model. A new ansatz for A(ad) (T) which is based on the eigenmodes of the adhering vesicle explains the linear behavior of A(ad) (T) for low T and helps to define a fit function which reproduces the linear behavior of the obtained simulation data. This fit function may be used in order to determine the bending rigidity and the adhesion strength from the observed adhesion geometry.

A phase-field simulation on the influence of thermal fluctuation on secondary branch growth in undercooled melt

The dendritic growth in an undercooled melt of pure substance is simulated by a phase_field model. The relationship between the dendritic growth and the parameters used in phase_field method (including undercooling, anisotropic coefficient, magnitude of thermal fluctuation) has been studied quantitatively. By employing the thermal fluctuations the birth of secondary branches has been successfully simulated. The spacing and amplitude of the secondary branches are obtained, which are in reasonable agreement with those by using the Wentzel_Kramers_Brillouin approach. Simulations indicate that the undercooling and the anisotropic coefficient have remarkable influence on the growth of the secondary branches, in which secondary branch spacing decreases with the increase of undercooling, but increases with the increase of anisotropic coefficient. The amplitude of the secondary branch decreases with the increase of anisotropic coefficient. Thermal fluctuations show strong influence on the amplitude of the secondary branch, however show less influence on the secondary branch spacing.

The Influence of Thermal Fluctuations on the Electronic Transport of Alkeno-Thiolates

In this paper, we investigate the influence of molecular vibrations on the tunneling of electrons through alkeno-thiolates of varying lengths sandwiched in between two gold contacts. The study is confined to the elastic scattering. The vibrational modes are treated quantum-mechanically and the tunneling current is computed as an ensemble average over the distribution of the atomic configurations, obtained by a suitable approximation of the density matrix for the normal mode oscillators. The quantum-mechanical treatment is necessary in order to correctly include the zero-point fluctuations. The calculations show no temperature dependence for the tunneling current in the regime between 270-350 K.

Influence of thermal fluctuations on quantum phase transitions in one-dimensional disordered systems: Charge density waves and Luttinger liquids

The low-temperature phase diagram of one-dimensional weakly disordered quantum systems such as charge or spin density waves and Luttinger liquids is studied by a full finite temperature renormalization-group (RG) calculation. For vanishing quantum fluctuations this approach is amended by an exact solution in the case of strong disorder and by a mapping onto the Burgers equation with noise in the case of weak disorder, respectively. At zero temperature we reproduce the quantum phase transition between a pinned (localized) and an unpinned (delocalized) phase for weak and strong quantum fluctuations, respectively, as found previously by Fukuyama or Giamarchi and Schulz. At finite temperatures the localization transition is suppressed: the random potential is wiped out by thermal fluctuations on length scales larger than the thermal de Broglie wavelength of the phason excitations. The existence of a zero-temperature transition is reflected in a rich crossover phase diagram of the correlation functions. In particular we find four different scaling regions: a classical disordered, a quantum disordered, a quantum critical, and a thermal region. The results can be transferred directly to the discussion of the influence of disorder in superfluids. Finally we extend the RG calculation to the treatment of a commensurate lattice potential. Applications to related systems are discussed as well.

Fast switching of magnetic nanoparticles: simulation of thermal noise effects using the Langevin dynamics

The most straightforward method to simulate fast switching in magnetic systems is the solution of stochastic equations of motion for magnetic moments (Langevin dynamics), where thermal fluctuations are taken into account by the thermal (random) field H/sup fl/. In this paper, we address first an important methodical problem of this formalism: the choice of the stochastic calculus (Ito or Stratonovich). We prove that both Ito and Stratonovich stochastic integrals give identical results, despite the multiplicative noise present in the stochastic Landau-Lifshitz-Gilbert equation. Discussing correlation properties of H/sup fl/ (which is usually assumed to be /spl delta/ correlated both in space and time), we point out that finite correlation time and radius of this field can be due not only to physical reasons (heat-bath correlations), but can also arise from the finite-element representation of the continuous problem. Afterwards, we present simulation results concerning the influence of thermal fluctuations on the fast switching of magnetic nanoelements. We consider three typical situations: (1) thermal noise influence on the switching which would happen also in the absence of thermal fluctuations (thermally assisted switching); (2) thermally induced switching of the metastable states; and (3) changing of the switching mode as the consequence of thermal fluctuations.

Brownian motion of coupled vortices in Type-II superconductor: response to external forces as a interacting bosons system

Brownian motion of coupled pancake vortices in Type-II superconductor is investigated. When the pancake vortices are under the influence of thermal fluctuation, they behave as interacting bosons in zero temperature. Using the Green's function method for boson, we obtain the effective Fokker-Planck equation in the low density limit. And the I–V characteristic is investigated by this equation

Hysteresis and avalanches in disordered systems

Rate-independent hysteresis is studied in magnetic systems driven by an external field for which the influence of thermal fluctuations is negligible. In such systems, the hysteresis cycles are not continuous, but rather are composed of a sequence of magnetisation jumps or avalanches between metastable states; the so-called Barkhausen noise. The study of the statistical distribution of such avalanches provides an alternative description to the more common procedure of measuring properties of the loop shape. We focus on four different zero-temperature 3d lattice models: the random field Ising model, the random bond Ising model, the site-diluted Ising model and the random anisotropy Ising model. By defining appropriate local dynamics, we have studied the metastable evolution by numerical simulations. We analyse the avalanche size distribution as a function of the degree of quenched disorder in these systems. For specific amounts of disorder, the distributions exhibit critical behaviour that can be characterised by universal exponents.

Influence of thermal fluctuations on Cherenkov radiation from fluxons in dissipative Josephson systems

The nonlinear dynamics of fluxons in Josephson systems with dispersion and thermal fluctuations is analyzed using the ``quasiparticle'' approach to investigate the influence of noise on the Cherenkov radiation effect. Analytical expressions for the stationary amplitude of the emitted radiation and its spectral distribution have been obtained in an annular geometry. It is demonstrated that noise reduces the amplitude of the radiated wave and broadens its spectrum. The effect of the radiated wave on the fluxon dynamics leads to a considerably smaller linewidth than observed in the usual flux flow oscillator. A resonant behavior of both the mean amplitude and the linewidth as functions of bias current is found. The obtained results enable an optimization of the main parameters (power, tunability, and linewidth) of practical mm- and sub-mm wave Cherenkov flux flow oscillators.

Miniaturization of sensors and influence of their thermal noise on measurement accuracy

The influence of thermal fluctuations in the mechanical and electrical elements of microsensors on their measurement accuracy is considered. The limits of the minimum attainable measurement uncertainty as a function of the miniaturization degree of the sensor are evaluated. Possibilities of improvement of the sensor performance through optimal fitting of the sensor and ADC parameters are examined. Recommendations for an optimal choice of parameters of the analog part of the sensor as well of the ADC resolution are given. The analysis is based on the information-energetic theory of measurements, and on general results of statistical physics.

Depinning transition and thermal fluctuations in the random-field Ising model.

We analyze the depinning transition of a driven interface in the three-dimensional (3D) random field Ising model (RFIM) with quenched disorder by means of Monte Carlo simulations. The interface initially built into the system is perpendicular to the [111] direction of a simple cubic lattice. We introduce an algorithm which is capable of simulating such an interface independent of the considered dimension and time scale. This algorithm is applied to the 3D RFIM to study both the depinning transition and the influence of thermal fluctuations on this transition. It turns out that in the RFIM characteristics of the depinning transition depend crucially on the existence of overhangs. Our analysis yields critical exponents of the interface velocity, the correlation length, and the thermal rounding of the transition. We find numerical evidence for a scaling relation for these exponents and the dimension d of the system.

Thermally activated transformation of magnetization-reversal modes in ultrathin nanoparticles

The influence of thermal fluctuations on magnetization-reversal processes in ultrathin magnetic particles is investigated on the basis of a numerical solution of the Landau-Lifshitz-Gilbert equations taking account the thermal-activation fluctuation field. It is shown that for nanoparticles there exists a region of magnetic and geometric parameters where a strong jump-like instability of the critical field for magnetization reversal arises. This instability is due to the thermally activated transformation of magnetization configurations far from the switching threshold. The thermal-instability mechanism described is important for particles of much larger sizes than for the single-mode Neel-Brown instability.

AC losses in high-temperature superconductors: revisiting the fundamentals of the loss modelling

The fundamentals of the electromagnetic modelling of high-temperature superconductors are discussed. Special attention is paid to intrinsic features of high-temperature superconductors different to those of low-temperature superconductors. Examples of those features are strong thermal fluctuations, which results in enhanced flux creep and slanted E( J)-characteristics, anisotropy of critical current density and material resistivity, and the granularity of the material. Having established the fundamental principles for the loss modelling, the influence of thermal fluctuations, anisotropy and granularity on the AC losses are considered.

Fluctuation of the delay time of nonhysteretic Josephson junctions during a linear current rise

The influence of thermal fluctuations on the delay time of nonhysteretic Josephson junctions during a linear rise in the current through them is investigated in two cases: a) during a slow rise, and b) at a high rate of increase.

Localized excitations in discrete nonlinear Schrödinger systems: Effects of nonlocal dispersive interactions and noise

A one-dimensional discrete nonlinear Schrödinger (DNLS) model with the power dependence, r − s on the distance r, of dispersive interactions is proposed. The stationary states of the system are studied both analytically and numerically. Two kinds of trial functions, exp-like and sech-like are exploited and the results of both approaches are compared. Both on-site and inter-site stationary states are investigated. It is shown that for s sufficiently large all features of the model are qualitatively the same as in the DNLS model with nearest-neighbor interaction. For s less than some critical value, s cr, there is an interval of bistability where two stable stationary states exist at each excitation number. The bistability of on-site solitons may occur for dipole-dipole dispersive interaction ( s = 3), while s cr for inter-site solitions is close to 2.1. In the framework of the DNLS equation with nearest-neighbor coupling we discuss the stability of highly localized, “breather-like”, excitations under the influence of thermal fluctuations. Numerical analysis shows that the lifetime of the breather is always finite and in a large parameter region inversely proportional to the noise variance for fixed damping and nonlinearity. We also find that the decay rate of the breather decreases with increasing nonlinearity and with increasing damping.

Adsorption isotherms and thermal fluctuations.

The influence of thermal fluctuations on adsorption isotherms is calculated within the context of a self-consistent mean-field theory, and it is found that such fluctuations cannot be neglected in the analysis of adsorption data. This result arises from our observation that substrate-induced hindrance of thermal fluctuations can significantly alter the form of an adsorption isotherm, particularly in the thin-film regime (\ensuremath{\approx}0-5 nm) which is most commonly probed by adsorption experiments. Previous experiments involving room-temperature adsorption on flat surfaces have been reanalyzed, and the reported discrepancies with the Lifshitz theory of van der Waals forces are found to be much reduced when thermal fluctuations of the thickness of the adsorbed layer are taken into account. Recently published data for nitrogen adsorption on rough substrates have also been reanalyzed, and the thermal fluctuations are found to be more important for thin adsorbed layers than undulations of the film induced by the roughness of the substrate. The analysis reveals that, as long suspected, the scaling regime of asymptotic divergence has not yet been reached for film thicknesses remaining below 5 nm.

On the influence of thermal fluctuations on the current—phase relation in superconducting point contacts

The effect of thermal fluctuations on the current—phase relation (CPR) in superconducting weak links is studied. We consider the case of a superconducting ring with sufficiently small inductance, interrupted by a ballistic point contact or a tunnel junction. The fluctuation effected CPR is calculated and compared with the CPR without fluctuations. The effect of the fluctuations are observable in the measured CPR by a reduction of the maximum value of the current, as well as in the shape of the CPR curve. It is shown that the value of the phase corresponding to the maximum in the current is shifted to smaller values in accordance with recent experimental data.

Radiation emission by a polaron in a molecular chain

We study the stability of the soliton-like polaron states in a molecular chain by means of a perturbation approach. We demonstrate that, under the influence of thermal fluctuations in the host lattice, the soliton gradually decays into delocalized ('excitonic') states. The soliton lifetime depends strongly on temperature and the coupling strength.