Finite Temperature Effects Research Articles

On the Origin of Energetic Disorder in Mixed Halides Lead Perovskites

AbstractMixed halide perovskites are emerging as promising candidates for wide‐bandgap components in tandem solar cells and color‐tunable light‐emitting diodes. Yet, halide mixing poses a fundamental question of whether the inhomogeneous halide distribution impacts the intrinsic electronic disorder in these materials. To address this point, density functional theory (DFT)‐based molecular dynamics (MD) simulations are performed for pure and mixed halide perovskites, accounting for disorder stemming from inhomogeneous chemical composition associated with the halide component and from finite temperature effects. For pure halide perovskites, finite‐temperature band gap fluctuations from the MD simulations are in good agreement with the broadening measured using photoluminescence. Furthermore, these calculations confirm the natively modest inhomogeneous disorder in the electronic structure of these materials. Most notably, such a low degree of electronic disorder is preserved in models mimicking finely intermixed Br/I solid‐state solutions. In contrast, models featuring halide segregation show comparably wider band gap fluctuations, with a sizable contribution from inhomogeneous (static) broadening, which is associated, at least in part, with structural distortions stemming from lattice mismatch.

Influence of stellar compactness on finite-temperature effects in neutron star merger simulations

Influence of stellar compactness on finite-temperature effects in neutron star merger simulations

Accurate energy barriers for catalytic reaction pathways: an automatic training protocol for machine learning force fields

We introduce a training protocol for developing machine learning force fields (MLFFs), capable of accurately determining energy barriers in catalytic reaction pathways. The protocol is validated on the extensively explored hydrogenation of carbon dioxide to methanol over indium oxide. With the help of active learning, the final force field obtains energy barriers within 0.05 eV of Density Functional Theory. Thanks to the computational speedup, not only do we reduce the cost of routine in-silico catalytic tasks, but also find an alternative path for the previously established rate-limiting step, with a 40% reduction in activation energy. Furthermore, we illustrate the importance of finite temperature effects and compute free energy barriers. The transferability of the protocol is demonstrated on the experimentally relevant, yet unexplored, top-layer reduced indium oxide surface. The ability of MLFFs to enhance our understanding of extensively studied catalysts underscores the need for fast and accurate alternatives to direct ab-initio simulations.

Axion–neutrino couplings, late-time phase transitions and the far infrared physics

The far infrared physics is a fascinating topic for theoretical physics, since the foundation of quantum field theory and neutrinos seem to be strongly related with the far infrared physics of our Universe. In this work we shall explore the possibility of a late-time thermal phase transition caused by the axion–neutrino interactions. The axion is assumed to be the misalignment axion which is coupled primordially to a chiral symmetric neutrino. The chiral symmetry is supposed to be broken either spontaneously or explicitly, and two distinct phenomenological models of axion–neutrinos are constructed. The axion behaves as cold dark matter during all its evolution eras, however if we assume that the axion and the neutrino fields interact coherently in a classical way as fields, or as ensembles, then we consider thermal effects in the axion sector, due to the values of operators ϕ for the axion and ν̄ν due to the neutrinos. The thermal equilibrium between the two has no effect to the axion effective potential for a wide temperature range. As we show, contrary to the existing literature, the axion never becomes destabilized due to the finite temperature effects, however if axion-Higgs higher order non-renormalizable operators are present in the Lagrangian, the axion potential is destabilized in the temperature range T∼0.1MeV down to T∼0.01eV and a first order phase transition takes place. The initial axion vacuum decays to the energetically more favorable axion vacuum, and the latter decays to the Higgs vacuum which is more preferable. This late-time phase transition might take place in the redshift range z∼385−37 and thus it may cause density fluctuations in the post-recombination era. This might be the source of large scale matter structure at high redshifts z≥9. Following the literature, we qualitatively discuss the implications of such a late-time phase transition at the astrophysical and cosmological level.

WIMPs, FIMPs, and Inflaton phenomenology via reheating, CMB and ∆Neff

In this paper, we extensively analyzed the reheating dynamics after inflation and looked into its possible implication on dark matter (DM) and inflaton phenomenology. We studied the reheating through various possible channels of inflaton going into massless scalars (bosonic reheating) and fermions (fermionic reheating) via non-gravitational and gravity-mediated decay processes. We further include the finite temperature effect on the decay process. Along with their precise roles in governing the dynamics, we compared the relative importance of different temperature-corrected decay channels in the gradual process of reheating depending on the reheating equation of state (EoS), which is directly related to inflaton potential. Particularly, the universal gravitational decay of inflaton is observed to play a very crucial role in the reheating process for a large range of inflaton decay parameters. For our study, we consider typical α-attractor inflationary models. We further establish the intriguing connection among those different inflaton decay channels and the CMB power spectrum that can have profound implications in building up a unified model of inflation, reheating, and DM. We analyze both fermion and scalar DM with different physical processes being involved, such as gravitational scattering, thermal bath scattering, and direct inflaton decay. Gravitational decay can again be observed to play a crucial role in setting the maximum limit on DM mass, especially in the FIMP scenario, which has already been observed earlier in the literature [52]. Depending on the coupling strength, we have analyzed in detail the production of both FIMP and WIMP-like DM during reheating and their detailed phenomenological implications from the perspective of various cosmological and laboratory experiments.

Dynamic hysteresis at a noisy saddle node shows power-law scaling but nonuniversal exponent.

Dynamic hysteresis, viz., delay in switching of a bistable system on account of the finite sweep rate of the drive, has been extensively studied in dynamical and thermodynamic systems. Dynamic hysteresis results from slowing of the response around a saddle-node bifurcation. As a consequence, the hysteresis area increases with the sweep rate. Mean-field theory, relevant for noise-free situations, predicts power-law scaling with the area scaling exponent of 2/3. We have experimentally investigated the dynamic hysteresis for a thermally driven metal-insulator transition in a high-quality NdNiO_{3} thin film and found the scaling exponent to be about 1/3, far less than the mean-field value. To understand this, we have numerically studied Langevin dynamics of the order parameter and found that noise, which can be thought to parallel finite temperature effects, influences the character of dynamic hysteresis by systematically lowering the dynamical exponent to as small as 0.2. The power-law scaling character, on the other hand, is unaffected in the range of chosen parameters. This work rationalizes the ubiquitous power-law scaling of the dynamic hysteresis as well as the wide variation in the scaling exponent between 0.66 and 0.2 observed in different systems over the last 30 years.

Thermal Effects in Binary Neutron Star Mergers

We study the impact of finite-temperature effects in numerical-relativity simulations of binary neutron star mergers with microphysical equations of state and neutrino transport in which we vary the effective nucleon masses in a controlled way. We find that, as the specific heat is increased, the merger remnants become colder and more compact due to the reduced thermal pressure support. Using a full Bayesian analysis, we demonstrate that this effect will be measurable in the postmerger gravitational wave signal with next-generation observatories at signal-to-noise ratios of 15.

Effects of finite temperature on the magnetized equation of state in neutron stars composed of a Bose-Einstein condensate

Effects of finite temperature on the magnetized equation of state in neutron stars composed of a Bose-Einstein condensate

Correlating Stokes Shift with Huang‐Rhys Parameter for Diatomic Molecules: Effects of Finite Temperature, Anharmonicity and Breakdown of Franck‐Condon Approximation

AbstractIn order to compute optical absorption and fluorescence spectra, the harmonic oscillator model for molecular vibrations under Franck‐Condon approximation for optical transition at absolute zero temperature is often considered for ease of numerical simulation, which never truly reproduce the experimental spectra. In this article, we show how systematic introduction of realistic situation, i. e., anharmonicity of vibrations, thermalization and non‐adiabatic transition leading to failure of Franck‐Condon approximation plays a colossal role in determining the spectral shape of a model diatomic molecule. In particular, we investigate how the Huang‐Rhys parameter, describing the relative positioning of the ground and excited electronic surfaces, is correlated with the Stokes shift, considering both right‐ and left‐shifted potential wells, which is crucial to estimate changes in molecular length‐scale from experimentally determined quantities. Further, the result for iodine is shown to have excellent agreement with experimental absorption spectrum. These findings are promised to bring insights into origin of spectral shapes in complex polyatomic molecules.

Suppression of intervalley scattering and enhanced phonon anharmonic interactions in 2D Bi2TeSe2: Crystal-field symmetry and band convergence

The electron-phonon (el-ph) and phonon-phonon interactions play a key role in determining electronic and thermal transport properties, respectively, in promising two-dimensional (2D) semiconductor devices. In this study, we investigated el-ph interactions using Wannier-Fourier interpolation method and renormalized phonon scattering considering finite-temperature effects in Bi2TeSe2 monolayer. The results show that the optical phonon modes dominate the carrier scattering, where level repulsion induced by crystal field splitting and spin-orbit coupling (SOC) effect effectively suppresses intervalley scattering, leading to high hole mobility. Moreover, the strong anharmonicity in Bi2TeSe2 monolayer results in the temperature-dependent softening of its optical phonons, along with a corresponding variation in interatomic force constants (IFCs). As a result, the lattice thermal conductivity is remarkably low and exhibits weak temperature dependence. Finally, the predicted dimensionless thermoelectric figure of merit exceeds unity in the range of 200–800 K, indicating the potential of Bi2TeSe2 monolayer for thermoelectric applications. This work provides new insights into the elimination of intervalley scattering by crystal field splitting and SOC effects, and paves the way for the evaluation of thermoelectric properties of materials with complex scattering mechanisms and strong anharmonicity.

Lorentz-violating extension of scalar QED at finite temperature

In this work, we calculate the one-loop self-energy corrections to the gauge field in scalar electrodynamics modified by Lorentz-violating terms within the framework of the standard model extension (SME). We focus on both CPT-even and CPT-odd contributions. The kinetic part of the scalar sector contains a CPT-even symmetric Lorentz-breaking tensor, and the interaction terms include a vector contracted with the usual covariant derivative in a gauge-invariant manner. We computed the one-loop radiative corrections using dimensional regularization for both the CPT-even and CPT-odd cases. Additionally, we employed the Matsubara formalism to account for finite temperature effects.

Finite temperature effects on the structural stability of Si-doped HfO2 using first-principles calculations

The structural stabilities of the monoclinic and tetragonal phases of Si-doped HfO2 at finite temperatures were analyzed using a computational scheme to assess the effects of impurity doping. We proposed a method that the finite temperature effects, i.e., lattice vibration and impurity configuration effects, are considered. The results show that 6% Si doping stabilizes the tetragonal phase at room temperature, although a higher concentration of Si is required to stabilize the tetragonal phase at zero temperature. These data indicate that lattice vibration and impurity configuration effects are important factors determining structural stability at finite temperatures.

How thermal fluctuations influence the function of the FeMo cofactor in nitrogenase enzymes

The catalytic mechanism of N2 fixation by nitrogenase remains unresolved in how the strong N≡N bond is activated and why the reductive elimination of H2 is required. Here, we use density functional theory and physiologically relevant thermal simulations to elucidate the mechanism of the complete nitrogenase catalytic cycle. Over the accumulation of four reducing equivalents, we find that protons and electrons transfer to the FeMo cofactor to weaken and break its bridge Fe–S bond, leading to temporary H2S formation that exposes the Fe sites to weakly bind N2. Remarkably, we find that subsequent H2 formation is responsible for chemical activation to an N=N double bond accompanied by a low barrier for H2 release. We emphasize that finite temperature effects smooth out mechanistic differences between DFT functionals observed at 0 K, thus leading to a consistent understanding as to why H formation is an obligatory step in N2 adsorption and activation.

Thermal fluctuations of the extended Bose-Hubbard model at finite temperature

We consider the finite-temperature effects of the extended Bose-Hubbard model, especially in the Haldane insulator regime. We show the quantum-to-thermal (normal) crossover temperatures reveal signatures of the zero-temperature quantum phase transition. By studying the heat capacity of the system, we find a qualitative difference between the Mott insulator regimes and the other two regimes for small system sizes. We attribute these effects to the thermal energy fluctuations and find characteristic behaviors of this fluctuation in the Haldane insulator regime. We hope these results will provide insights into further research in this system.

Probe for bound states of SU(3) fermions and colour deconfinement

Fermionic artificial matter realized with cold atoms grants access to an unprecedented degree of control on sophisticated many-body effects with an enhanced flexibility of the operating conditions. Here, we consider three-component fermions with attractive interactions to study the formation of complex bound states, whose nature goes beyond the standard fermion pairing occurring in quantum materials. Such systems display clear analogies with quark matter. We address the nature of the bound states of a three-component fermionic system in a ring-shaped trap through the persistent current. In this way, we demonstrate that we can distinguish between color superfluid and trionic bound states. By analyzing finite temperature effects, we show how finite temperature can lead to the deconfinement of bound states. For weak interactions, the deconfinement occurs because of scattering states. In this regime, the deconfinement depends on the trade-off between interactions and thermal fluctuations. For strong interactions the features of the persistent current result from the properties of a suitable gas of bound states.

Frequencies of f - and p -oscillation modes in cold and hot compact stars

A large collection of equations of state (EOSs) built within the covariant density functional theory of hadronic matter and allowing for density dependent couplings is employed to study polar $f$- and $p$-oscillations of cold and hot compact stars. Correlations between oscillation frequencies of cold purely nucleonic neutron stars (NSs), their global parameters as well as properties of nuclear matter (NM) are investigated by considering a set of models from Beznogov and Raduta, [Phys. Rev. C 107, 045803 (2023)], where a number of constraints on the saturation properties of NM, pure neutron matter and the lower bound of the maximal NS mass were imposed within a Bayesian framework. The roles of finite temperature and exotic particle degrees of freedom, e.g., hyperons, $\mathrm{\ensuremath{\Delta}}$-resonances, antikaon condensates or a hadron to quark phase transition, are addressed by employing a family of models publicly available on compose and assuming idealized profiles of temperature or entropy per baryon and charge fraction. We find that finite temperature effects reduce the oscillation frequencies of nucleonic stars while the opposite effect is obtained for stars with exotic particle degrees of freedom. When the $\mathrm{\ensuremath{\Gamma}}$-law is employed to build finite temperature EOSs, errors in estimating oscillation modes frequencies are of the order of 10% to 30%, depending on the mass. Throughout this work the Cowling approximation is used.

Finite-temperature effects on the x-ray absorption spectra of crystalline alumina from first principles

By including phonon-assisted transitions within plane-wave density functional theory methods for calculating the x-ray absorption spectrum (XAS), we obtain the Al K-edge XAS at 300 K for two crystalline Al2O3 phases. The 300 K XAS reproduces the pre-edge peak for α-Al2O3, which is not visible at the static lattice level of approximation. Configurations from Monte Carlo sampling of the γ-Al2O3 phase space at the 300 K XAS correctly describe two out of the three experimental peaks. We show that the second peak arises from 1s to mixed s-p transitions and is absent in the 0 K XAS. This letter serves as an insight into the electronic origins of the characteristic peaks in the Al K-edge XAS for alumina crystals.

Ab initio study of the effect of interstitial alloying on the intrinsic stacking fault energy of paramagnetic γ-Fe and austenitic stainless steel

Intrinsic stacking fault energy (SFE) values of γ-Fe and AISI 304 austenitic stainless steels were determined as a function of carbon and nitrogen content using ab initio calculations. In contrast to previous investigations, the analysis was conducted incorporating the paramagnetic state to account for the magnetic constitution of real austenitic stainless steels. The effect of finite temperature was partially accounted for by performing ab initio calculations at the experimental volumes at room temperature. Including paramagnetism in γ-Fe increases the SFE of non-magneticγ-Fe by ∼385 mJ.m–2. Interstitial alloying of non-magneticγ-Fe causes a linear increase in intrinsic stacking fault energy with interstitial content. In comparison, interstitial alloying of paramagneticγ-Fe increases the SFE at only about half the rate. The SFE of paramagnetic interstitial-free AISI 304 is within the range of -12 to 0 mJ.m–2 and only deviates slightly from the SFE of paramagneticγ-Fe. It follows a similar, albeit flatter linear dependency on the interstitial content compared to γ-Fe. Both γ-Fe and γ-AISI 304 were found to be metastable in their interstitial-free condition and are stabilized by interstitial alloying. The possible effect of short range ordering between interstitials and Cr on the SFE was discussed. The calculated threshold nitrogen content necessary to stabilize austenite in AISI 304 is in good agreement with experimental investigations of deformation microstructures in dependence of the nitrogen content. Finally, the calculated negative SFE values of AISI 304 were reconciled with experimentally determined positive SFE values using a recent method that accounts for the kinetics of stacking fault formation.

Temperature Variation of Gilbert Damping Constant in 3d Transition Metals from First Principles

A theoretical investigation of temperature-dependent Gilbert damping is presented. We evaluate the Gilbert damping constant α (and the corresponding Landau frequency λ) at a finite temperature for hcp-Co, fcc-Ni, and Cu-doped fcc-Ni from first principles by combining the torque correlation model and the disordered local moment scheme, in which the finite-temperature effect is included as the transverse spin fluctuation. The calculated λ has, from the viewpoint of the material dependence, well reproduced the experimental results, but quantitatively, these values are more underestimated than the experimental values. A comparison with the lifetime approximation results reveals that a change in the electronic structure due to the spin fluctuation affects λ for Co and Ni but not for Fe. On the basis of the variation of the local magnetic moment of Co and Ni at finite temperature, we conclude that these differences in λ are due to the difference in the degree of itinerancy of spins.

Finite size effects in the phase transition patterns of coupled scalar field systems

It is considered in this work the phase transition patterns for a coupled two-scalar field system model under the combined effects of finite sizes and temperature. The scalar fields are taken as propagating in a $D=4$ Euclidean space with the usual periodic compactification in the Euclidean time direction (with dimension given by the inverse of the temperature) and also under a compact dimension in the space direction, which is restricted to size $L$. In the latter case, a Dirichlet boundary condition is considered. Finite-size variation of the critical temperature for the cases of symmetry restoration and inverse symmetry breaking are studied. At fixed finite-temperature values, the variation of the inverse correlation lengths with the size $L$ might display a behavior analogous to reentrant phase transitions. Possible applications of our results to physical systems of interest are also discussed.