Effect Of Thermal Fluctuations Research Articles

A lattice Boltzmann study of particle settling in a fluctuating multicomponent fluid under confinement

We present mesoscale numerical simulations based on the coupling of the fluctuating lattice Boltzmann method for multicomponent systems with a wetted finite-size particle model. This newly coupled methodologies are used to study the motion of a spherical particle driven by a constant body force in a confined channel with a fixed square cross section. The channel is filled with a mixture of two liquids under the effect of thermal fluctuations. After some validations steps in the absence of fluctuations, we study the fluctuations in the particle’s velocity at changing thermal energy, applied force, particle size, and particle wettability. The importance of fluctuations with respect to the mean settling velocity is quantitatively assessed, especially in comparison with unconfined situations. Results show that the expected effects of confinement are very well captured by the numerical simulations, wherein the confinement strongly enhances the importance of velocity fluctuations, which can be one order of magnitude larger than what expected in unconfined domains. The observed findings underscore the versatility of the proposed methodology in highlighting the effects of confinement on the motion of particles in the presence of thermal fluctuations.

Superflow decay in a toroidal Bose gas: The effect of quantum and thermal fluctuations

We theoretically investigate the stochastic decay of persistent currents in a toroidal ultracold atomic superfluid caused by a perturbing barrier. Specifically, we perform detailed three-dimensional simulations to model the experiment of Kumar et al. in [Phys. Rev. A 95 021602 (2017)], which observed a strong temperature dependence in the timescale of superflow decay in an ultracold Bose gas. Our ab initio numerical approach exploits a classical-field framework that includes thermal fluctuations due to interactions between the superfluid and a thermal cloud, as well as the intrinsic quantum fluctuations of the Bose gas. In the low-temperature regime our simulations provide a quantitative description of the experimental decay timescales, improving on previous numerical and analytical approaches. At higher temperatures, our simulations give decay timescales that range over the same orders of magnitude observed in the experiment, however, there are some quantitative discrepancies that are not captured by any of the mechanisms we explore. Our results suggest a need for further experimental and theoretical studies into superflow stability.

BCS-BEC crossover in a two-band superconductor with odd-parity hybridization

We study the crossover from the weak coupling Bardeen-Cooper-Schrieffer (BCS) state to Bose-Einstein condensation (BEC) at strong coupling in a two-band superconductor with orbitals of opposite parity coexisting at a common Fermi surface in the metallic state. In such systems hybridization can play a role similar to spin-orbit interaction in fermionic spinor gases, enhancing interband pairing and opening the possibility for driving the BCS-BEC crossover. Building on a mean-field analysis, we investigate the BCS-BEC crossover induced by a variation of the hybridization strength. We show that the relevant scale for the crossover depends strongly on the ratio of effective masses, with the crossover favored in systems with one dispersive and one flat band. Including the effect of thermal pair fluctuations in a one-loop approximation, we calculate the dependence of the critical temperature on the microscopic parameters in the strong coupling regime.

Lattice Boltzmann simulations of stochastic thin film dewetting.

We study numerically the effect of thermal fluctuations and of variable fluid-substrate interactions on the spontaneous dewetting of thin liquid films. To this aim, we use a recently developed lattice Boltzmann method for thin liquid film flows, equipped with a properly devised stochastic term. While it is known that thermal fluctuations yield shorter rupture times, we show that this is a general feature of hydrophilic substrates, irrespective of the contact angle θ. The ratio between deterministic and stochastic rupture times, though, decreases with θ. Finally, we discuss the case of fluctuating thin film dewetting on chemically patterned substrates and its dependence on the form of the wettability gradients.

Quantum self-bound droplets in Bose-Bose mixtures: Effects of higher-order quantum and thermal fluctuations

We systematically study the effects of higher-order quantum and thermal fluctuations on the stabilization of self-bound droplets in Bose mixtures employing the time-dependent Hartree-Fock-Bogoliubov theory. We calculate the ground-state energy, the droplet equilibrium density, the depletion and anomalous density of the droplets as well as the critical temperature as a function of the relevant parameters. Our findings are compared with previous analytical predictions and diffusion Monte Carlo simulations. We employ our theory together with the local density approximation for quantum and thermal fluctuations to obtain an extended finite-temperature Gross-Pitaevskii equation. The density profiles and breathing modes of the droplet are deeply examined in terms of the interaction strength and the temperature by numerically solving the developed generalized Gross-Pitaevskii equation.

Greybody factor for regular Bardeen black hole surrounded by PFDM

The main aim of this work is to construct an analytical expression of greybody factor (GBF) for regular Bardeen black hole (BH) surrounded by perfect fluid dark matter (PFDM). To this end, we first analyze the profile of effective potential by transforming the radial part of Klein–Gordon (KG) equation into standard Schrödinger equation. Afterwards, the two asymptotic solutions are found in distinct regions, namely, BH and far-field horizons. To extend the viability of the derived solutions over the whole radial regime, we match them smoothly in an intermediate region. It is found that as the intensity of PFDM decreases around the BH, its attractive behavior decreases (the ability of attracting radiations towards itself decreases). As a result, the less radiations can reach infinity and hence the greybody factor decreases. We also study the effect of thermal fluctuations on thermodynamics of regular BH surrounded by PFDM. For this purpose, we compute corrected entropy near the equilibrium position which implies modification in other thermodynamic potentials. It is concluded that in the presence of quantum corrections, the large BHs becomes stable due to positive range of specific heat.

Effects of thermal fluctuations on the Kerr–Newman–NUT–AdS black hole

This paper is devoted to studying the impact of thermal fluctuations on thermodynamics of rotating as well as charged anti-de Sitter black holes with the Newman–Unti–Tamburino (NUT) parameter. To this end, we derive the analytic expression of thermodynamic variables, namely the Hawking temperature, volume, angular velocity, and entropy within the limits of extended phase space. These variables meet the first law of thermodynamics as well as the Smarr relation in the presence of new NUT charge. To analyze the effects of quantum fluctuations, we derive the exact expression of corrected entropy, which yields modification in other thermodynamical equations of state. The local stability and phase transition of the considered black hole are also examined through specific heat. It is found that the NUT parameter increases the stability of small black holes, while the logarithmic corrections induce instability in the system.

Functional renormalization group study of the Kitaev- Γ model on the honeycomb lattice and emergent incommensurate magnetic correlations

The theoretical inception of the Kitaev honeycomb model has had defining influence on the experimental hunt for quantum spin liquids, bringing unprecedented focus onto the synthesis of materials with bond-directional interactions. Numerous Kitaev materials, which are believed to harbor ground states parametrically close to the Kitaev spin liquid, have been investigated since. Yet, in these materials the Kitaev interaction often comes hand in hand with off-diagonal $\Gamma$ interactions -- with the competition of the two potentially giving rise to a magnetically ordered ground state. In an attempt to aid future material investigations, we study the phase diagram of the spin-1/2 Kitaev-$\Gamma$ model on the honeycomb lattice. Employing a pseudofermion functional renormalization group approach which directly operates in the thermodynamic limit and captures the joint effect of thermal and quantum fluctuations, we unveil the existence of extended parameter regimes with emergent incommensurate magnetic correlations at finite temperature. We supplement our results with additional calculations on a finite cylinder geometry to investigate the impact of periodic boundary conditions on the incommensurate order, thereby providing a perspective on previous numerical studies on finite systems.

Universal gravitational-wave signatures from heavy new physics in the electroweak sector

We calculate the gravitational-wave spectra produced by the electroweak phase transition with TeV-scale Beyond-Standard-Model physics in the early universe. Our study captures the effect of quantum and thermal fluctuations within a non-perturbative framework. We discover a universal relation between the mean bubble separation and the strength parameter of the phase transition, which holds for a wide range of new-physics contributions.The ramifications of this result are three-fold: first, they constrain the gravitational-wave spectra resulting from heavy (TeV-scale) new physics. Second, they contribute to distinguishing heavy from light new physics directly from the gravitational-wave signature. Third, they suggest that a concerted effort of gravitational-wave observations together with collider experiments could be required to distinguish between different models of heavy new physics.

Study of thermal fluctuations in five-dimensional rotating regular black hole

In this paper, we discuss the effects of thermal fluctuations on thermodynamics of rotating regular black hole in five-dimensional spacetime. We first evaluate thermodynamic quantities such as Hawking temperature, entropy, angular velocity and electric potential of the considered model. To discuss the effects of thermal fluctuations, we compute the logarithmic correction terms to entropy around the equilibrium state. We also check the validity of the first law of thermodynamics in the presence of these correction terms. Finally, we determine stability of the system through specific heat and Hessian matrix. It is concluded that logarithmic corrections originated from thermal fluctuations yield a stable model.

Local affection of weak gravitational field from supercondensates

We study the mutual interaction between a superconducting sample and the weak, static Earth’s gravitational field, exploiting the gravito-Maxwell formalism combined with the time-dependent Ginzburg-Landau model. We will also determine the appropriate conditions to enhance the desired gravity/superfluid interplay, analysing the effects of thermal fluctuations and optimizing the superconductor parameters and sample geometry.

Thermal quantum correlations in the two-qubit Heisenberg XYZ spin chain with Dzyaloshinskii–Moriya interaction

We investigate the quantum correlations of a two-qubit XYZ Heisenberg spin-1/2 chain model with Dzyaloshinskii–Moriya interaction. The two-qubit system is considered in thermal equilibrium. The variations of logarithmic negativity, uncertainty-induced quantum nonlocality (UIN) and trace distance discord, versus the parameters characterizing the system, are analyzed. The results show that the UIN measure captures quantum correlations that cannot be revealed by entanglement and trace discord. We also show that the Dzyaloshinskii–Moriya interaction enhances the non-classical correlations between the spins and can weaken the undesirable destructive effects of thermal fluctuations. In addition, an entangled–unentangled phase transition can be detected from the behavior of logarithmic negativity.

Exchange-bias and magnetic anisotropy fields in core\u2013shell ferrite nanoparticles

Exchange bias properties of MnFe_2O_4@gamma–Fe_2O_3 core–shell nanoparticles are investigated. The measured field and temperature dependencies of the magnetization point out a well-ordered ferrimagnetic core surrounded by a layer with spin glass-like arrangement. Quasi-static SQUID magnetization measurements are presented along with high-amplitude pulse ones and are cross-analyzed by comparison against ferromagnetic resonance experiments at 9 GHz. These measurements allow one to discern three types of magnetic anisotropies affecting the dynamics of the magnetic moment of the well-ordered ferrimagnetic NP’s core viz. the easy-axis (uniaxial) anisotropy, the unidirectional exchange-bias anisotropy and the rotatable anisotropy. The uniaxial anisotropy originates from the structural core–shell interface. The unidirectional exchange-bias anisotropy is associated with the spin-coupling at the ferrimagnetic/spin glass-like interface; it is observable only at low temperatures after a field-cooling process. The rotatable anisotropy is caused by partially-pinned spins at the core/shell interface; it manifests itself as an intrinsic field always parallel to the external applied magnetic field. The whole set of experimental results is interpreted in the framework of superparamagnetic theory, i.e., essentially taking into account the effect of thermal fluctuations on the magnetic moment of the particle core. In particular, it is found that the rotatable anisotropy of our system is of a uniaxial type.

Thermal buckling and symmetry breaking in thin ribbons under compression

Understanding thin sheets, ranging from the macro to the nanoscale, can allow control of mechanical properties such as deformability. Out-of-plane buckling due to in-plane compression can be a key feature in designing new materials. While thin-plate theory can predict critical buckling thresholds for thin frames and nanoribbons at very low temperatures, a unifying framework to describe the effects of thermal fluctuations on buckling at more elevated temperatures presents subtle difficulties. We develop and test a theoretical approach that includes both an in-plane compression and an out-of-plane perturbing field to describe the mechanics of thermalised ribbons above and below the buckling transition. We show that, once the elastic constants are renormalised to take into account the ribbon’s width (in units of the thermal length scale), we can map the physics onto a mean-field treatment of buckling, provided the length is short compared to a ribbon persistence length. Our theoretical predictions are checked by extensive molecular dynamics simulations of thin thermalised ribbons under axial compression.

Perturbed thermodynamics of charged black hole solution in Rastall theory

We study the thermodynamics of a charged black hole surrounded by a perfect fluid in Rastall theory and investigate three different cases of quintessence, dust and radiation fields. By considering thermal fluctuations, we study the corrected thermodynamics of the system. We investigate the effects of thermal fluctuations on the black hole stability, phase transition and critical points. We show that the thermal fluctuations increase instability of the black hole and may yield to the second order phase transition. We also compare our result with uncharged cases to find the effects of the black hole charge on the thermodynamics quantities. We find that large black holes behave like a Van der Waals fluid, while for the small black hole where thermal fluctuations become important, there is no Van der Waals behaviour. Finally, we briefly discuss about geometric thermodynamics to obtain corrected scalar curvature.

On the thermal nonclassical correlations in a two-spin XYZ Heisenberg model with Dzyaloshinskii–Moriya interaction

The quantum correlations between two neighbouring anisotropic XYZ Heisenberg spins under the effect of Dzyaloshinskii–Moriya interaction at thermal equilibrium is considered. We evaluate and compare the negativity, uncertainty-induced quantum nonlocality and local quantum uncertainty corresponding to involved system. By exploiting the behaviour of each quantifier, in terms of the system parameters, we have shown that the uncertainty-induced quantum nonlocality can reveal certain non-classicality that goes beyond negativity and local quantum uncertainty. Further, we show that the presence of Dzyaloshinskii–Moriya interaction along the z-axis can reduce the undesirable destructive effects of thermal fluctuations, and therefore enhances quantum correlations in the two-spin system. In addition, an entangled–unentangled phase transition can be detected from the behaviour of negativity.

Thermal Effects on Pull-Off Force in the Johnson\u2013Kendall\u2013Roberts Model

In this study, we investigate the effects of thermal fluctuations on the generalized Johnson–Kendall–Roberts (JKR) model. We show that the distribution of pull-off forces in this model is similar to that of the Bradley model, and is also consistent with the experiment result observed in Wierez-Kien et al. (Nanotechnology 29(15):155704 2018). Increasing temperature leads to a broadening of the distribution, while leads to a reduction of the pull-off force. Additionally, the pull-off force, which is separated into an athermal term and a thermal-induced reduction term, is measured by using spring velocity ranging over 5 orders of magnitude. We show that for compliant spring, the pull-off force is significantly enhanced with increasing velocity, which is mainly attributed to the contribution of the thermal-induced reduction term, while the athermal term is barely sensitive to changes in velocity.

Magnetic Lovelock black holes with a cloud of strings and quintessence

We consider a model of exponential electrodynamics as a source of gravitational field and worked for the solution describing magnetic black hole surrounded by a quintessence and cloud of strings. In this set up, a new family of Lovelock black hole solutions of the gravitational field equations are constructed. The thermodynamics of these black holes is also studied and the mass as well as the other associated thermodynamic quantities such as Hawking temperature and heat capacity in terms of horizon radius are computed. These quantities are dependent on the parameters of exponential electrodynamics, cloud of strings and quintessential dark energy, which have been used for the identification of the regions corresponding to thermal phase transitions and thermodynamic stability of black holes. Furthermore, the effects of thermal fluctuations are also discussed and modified thermodynamic quantities associated to logarithmic correction in entropy are calculated. The Hawking radiations corresponding to these black holes are briefly discussed as well.

Effect of the surface magnetic anisotropy of neodymium atoms on the coercivity in neodymium permanent magnets

The Nd permanent magnet (${\mathrm{Nd}}_{2}{\mathrm{Fe}}_{14}\mathrm{B}$) is an indispensable material used in modern energy conversion devices. The realization of high coercivity at finite temperatures is an important issue. One of the important ingredients for controlling the coercive force is the surface property of magnetic grains. It has been found by first-principles studies that the Nd atoms in the first (001) surface layer facing the vacuum have in-plane anisotropy perpendicular to the $c$ axis, which may decrease the coercivity. Focusing on the surface anisotropy effect on the coercivity, we examine the coercivity at zero and finite temperatures using an atomistic model reflecting the lattice structure of the Nd magnet with a stochastic Landau-Lifshitz-Gilbert equation method. We study three general cases, in which the Nd atoms in surface layers have (1) no anisotropy, (2) in-plane anisotropy, and (3) reinforced anisotropy for two types of surface, (001) and (100) surfaces. We find that in contrast to the zero-temperature case, owing to the thermal fluctuation effect, the modification of only the first surface layer has little effect on the coercivity at finite temperatures. However, the modification of a few layers results in significant effects. We discuss the details of the dependences of the coercivity on temperature, the type of surface, and modified layer depth, and also the features of domain growth in magnetization reversal.

Sensitivity analysis of thermophysical properties on PCM selection under steady and fluctuating heat sources: A comparative study

Previous studies on phase change material (PCM) selection for latent thermal energy storage (LTES) mainly focused on steady heat source conditions without considering the effects of thermal fluctuation of real heat sources, and how fluctuating heat sources will affect the charging performance of LTES and material selection is still unclear. This study aims to compare the difference in material selection for a shell-and-tube LTES under steady and fluctuating heat sources, which comprehensively considers the effects of PCM thermophysical properties including the melting temperature, density, specific heat capacity, thermal conductivity and latent heat, as well as their interaction effects. By taking heat storage capacity and charging rate as objectives, orthogonal experiment design and stepwise regression analysis have been conducted to specify the significant factors among these parameters under steady and fluctuating heat source conditions. To further investigate the difference in the ranking of candidate PCMs under two conditions, fourteen pre-screened PCMs are ranked under steady and fluctuating heat sources. The results show that the order of prominent factors for heat storage capacity is ρ·Cp, followed by ρ·L under both conditions. However, when considering the charging rate, temperature fluctuation will weaken the effect of melting temperature, and strengthen the effect of specific heat capacity and latent heat. The order of prominent factors for charging rate is λ·Cp and Cp·L under fluctuating heat source. According to the ranking results, LiNO3-NaNO2 within the melting temperature of 100–200 °C has an excellent comprehensive charging performance under both conditions.