Finite Temperature Behavior Research Articles

Ising fracton spin liquid on the honeycomb lattice

We study a classical Ising model on the honeycomb lattice with local two-body interactions and present strong evidence that at low temperature it realizes a higher-rank Coulomb liquid with fracton excitations. We show that the excitations are (type-I) fractons, appearing at the corners of membranes of spin flips. Because of the threefold rotational symmetry of the honeycomb lattice, these membranes can be locally combined such that no excitations are created, giving rise to a set of ground states described as a liquid of membranes. We devise a cluster Monte Carlo algorithm purposefully designed for this problem that moves pairs of defects, and use it to study the finite-temperature behavior of the model. We show evidence for a first order transition from a high-temperature paramagnet to a low-temperature phase whose correlations precisely match those predicted for a higher-rank Coulomb phase. Published by the American Physical Society 2024

A lattice pairing-field approach to ultracold Fermi gases

We develop a pairing-field formalism for ab initio studies of non-relativistic two-component fermions on a (d+1)-dimensional spacetime lattice. More specifically, we focus on theories where the interaction between the two components can be described by the exchange of a corresponding pairing field. The introduction of a pairing field may indeed be convenient for studies of, e.g., the finite-temperature phase structure and critical behavior of, e.g., ultracold atomic Fermi gases. Moreover, such a formalism allows to directly compute the momentum and frequency dependence of the pair propagator, from which the pair-correlation function can be extracted. For a first illustration of the application of our formalism, we compute the density equation of state and the superfluid order parameter for a gas of unpolarized fermions in (0+1) dimensions by employing the complex Langevin approach to surmount the sign problem.

Dynamics of a Magnetic Polaron in an Antiferromagnet.

The t-J model remains an indispensable construct in high-temperature superconductivity research, bridging the gap between charge dynamics and spin interactions within antiferromagnetic matrices. This study employs the multiple Davydov Ansatz method with thermo-field dynamics to dissect the zero-temperature and finite-temperature behaviors. We uncover the nuanced dependence of hole and spin deviation dynamics on the spin-spin coupling parameter J, revealing a thermally-activated landscape where hole mobilities and spin deviations exhibit a distinct temperature-dependent relationship. This numerically accurate thermal perspective augments our understanding of charge and spin dynamics in an antiferromagnet.

Non-collinear magnetic atomic cluster expansion for iron

The Atomic Cluster Expansion (ACE) provides a formally complete basis for the local atomic environment. ACE is not limited to representing energies as a function of atomic positions and chemical species, but can be generalized to vectorial or tensorial properties and to incorporate further degrees of freedom (DOF). This is crucial for magnetic materials with potential energy surfaces that depend on atomic positions and atomic magnetic moments simultaneously. In this work, we employ the ACE formalism to develop a non-collinear magnetic ACE parametrization for the prototypical magnetic element Fe. The model is trained on a broad range of collinear and non-collinear magnetic structures calculated using spin density functional theory. We demonstrate that the non-collinear magnetic ACE is able to reproduce not only ground state properties of various magnetic phases of Fe but also the magnetic and lattice excitations that are essential for a correct description of finite temperature behavior and properties of crystal defects.

Majorana-fermion mean field theories of Kitaev quantum spin liquids

We determine the phase diagrams of anisotropic Kitaev-Heisenberg models on the honeycomb lattice using parton mean-field theories based on different Majorana fermion representations of the S=1/2 spin operators. First, we use a two-dimensional Jordan-Wigner transformation (JWT) involving a semi-infinite snake string operator. To ensure that the fermionized Hamiltonian remains local, we consider the limit of extreme Ising exchange anisotropy in the Heisenberg sector. Second, we use the conventional Kitaev representation in terms of four Majorana fermions subject to local constraints, which we enforce through Lagrange multipliers. For both representations, we self-consistently decouple the interaction terms in the bond and magnetization channels and determine the phase diagrams as a function of the anisotropy of the Kitaev couplings and the relative strength of the Ising exchange. While both mean-field theories produce identical phase boundaries for the topological phase transition between the gapless and gapped Kitaev quantum spin liquids, the JWT fails to correctly describe the magnetic instability and finite-temperature behavior. Our results show that the magnetic phase transition is first order at low temperatures but becomes continuous above a certain temperature. At this energy scale, we also observe a finite-temperature crossover on the quantum-spin-liquid side, from a fractionalized paramagnet at low temperatures, in which gapped flux excitations are frozen out, to a conventional paramagnet at high temperatures. Published by the American Physical Society 2024

Low-energy excitations and transport functions of the one-dimensional Kondo insulator

Using variational matrix product states, we analyze the finite temperature behavior of a half-filled periodic Anderson model in one dimension, a prototypical model of a Kondo insulator. We present an extensive analysis of single-particle Green’s functions, two-particle Green’s functions, and transport functions creating a broad picture of the low-temperature properties. We confirm the existence of energetically low-lying spin excitations in this model and study their energy-momentum dispersion and temperature dependence. We demonstrate that charge-charge correlations at the Fermi energy exhibit a different temperature dependence than spin-spin correlations. While energetically low-lying spin excitations emerge approximately at the Kondo temperature, which exponentially depends on the interaction strength, charge correlations vanish already at high temperatures. Furthermore, we analyze the charge and thermal conductivity at finite temperatures by calculating the time-dependent current-current correlation functions. While both charge and thermal conductivity can be fitted for all interaction strengths by gapped systems with a renormalized band gap, the gap in the system describing the thermal conductivity is generally smaller than the system describing the charge conductivity. Thus, two-particle correlations affect the charge and heat conductivities in a different way resulting in a temperature region where the charge conductivity of this one-dimensional Kondo insulator is already decreasing while the heat conductivity is still increasing.

Pion dynamics in a soft-wall AdS-QCD model

Pseudo-Goldstone modes appear in many physical systems and display robust universal features. First, their mass m obeys the so-called Gell-Mann-Oakes-Renner (GMOR) relation f2m2 = Hoverline{sigma} , with f the Goldstone stiffness, H the explicit breaking scale and overline{sigma} the spontaneous condensate. More recently, it has been shown that their damping Ω is constrained to follow the relation Ω = m2Dφ, where Dφ is the Goldstone diffusivity in the purely spontaneous phase. Pions are the most paradigmatic example of pseudo-Goldstone modes and they are related to chiral symmetry breaking in QCD. In this work, we consider a bottom-up soft-wall AdS-QCD model with broken SU(2)L × SU(2)R symmetry and we study the nature of the associated pseudo-Goldstone modes — the pions. In particular, we perform a detailed investigation of their dispersion relation in presence of dissipation, of the role of the explicit breaking induced by the quark masses and of the dynamics near the critical point. Taking advantage of the microscopic information provided by the holographic model, we give quantitative predictions for all the coefficients appearing in the effective description. In particular, we estimate the finite temperature behavior of the kinetic parameter mathfrak{r} 2 defined as the ratio between the Goldstone diffusivity Dφ and the pion attenuation constant DA. Interestingly, we observe important deviations from the value mathfrak{r} 2 = 3/4 computed in chiral perturbation theory in the limit of zero temperature.

Studying the impact of diagonal-doping on thermal stability of main-group metal clusters via Born Oppenheimer molecular dynamics

Doping with a heteroatom is considered to be an effective tool in tuning the physico-chemical properties of metal clusters. Over a period of time, several experimental and theoretical investigations have been carried out to understand the effect of doping on structure, electronic properties and reactivity of main group clusters. However, issues pertaining to thermal stabilities of metal clusters, particularly in the context of heteroatom doping remain unexplored. Filling this lacuna, this report focuses on evaluating the finite temperature behaviour of the main group metal clusters which are doped with diagonally adjacent dopant atoms. Born-Oppenheimer Molecular Dynamical (BOMD) simulations are performed on diagonally doped LiMg, BeAl and BSi () clusters (doped diagonal pairs). The thermal response of these doped diagonal pairs is also compared with the corresponding pristine clusters such as Li, Mg, Be, Al, B and Si () clusters. The simulation trajectories and ionic motions are analysed and root mean square bond length fluctuations are calculated as a function of temperature. The results indicate that the diagonally doped BSi () clusters are thermally more stable (1200 K) as compared to the other two doped clusters (LiMg, BeAl). The enhanced thermal stability is attributed to the higher binding energies noted for BSi () clusters. Moreover, it is found that for the B-Si cluster systems, diagonal doping always enhances the thermal stability, whereas both diagonal doping and the size of the cluster affect the thermal stabilities of the LiMg, and BeAl cluster systems. An in-depth analysis of thermal-structural-electronic (frontier molecular orbitals) properties revealed that the strength of dopant-to-parent cluster interactions are playing a significant role in influencing the thermal stabilities of diagonally doped clusters. Overall this study also highlights how the heuristic chemical principle of ‘diagonal relationship’ widely used in main-group chemistry may be extended to cluster science to draw new insights regarding thermal stabilities of metal clusters.

Heat-bath approach to anomalous thermal transport: Effects of inelastic scattering

We present results for the entire set of anomalous charge and heat transport coefficients for metallic systems in the presence of a finite-temperature heat bath. In realistic physical systems this necessitates the inclusion of inelastic dissipation mechanisms; relatively little is known theoretically about their effects on anomalous transport. Here we demonstrate how these dissipative processes are strongly intertwined with Berry-curvature physics. Our calculations are made possible by the introduction of a Caldeira-Leggett reservoir which allows us to avoid the sometimes-problematic device of the pseudogravitational potential. Using our formulas, we focus on the finite-temperature behavior of the important anomalous Wiedemann-Franz ratio. Despite previous expectations, this ratio is found to be non-universal as it can exhibit either an upturn or a downturn as temperature increases away from zero. We emphasize that this derives from a \textit{competition} between Berry curvatures having different signs in different regions of the Brillouin zone. We point to experimental support for these observations and for the behavior of an alternative ratio involving a thermoelectric response which, by contrast, appears to be more universal at low temperatures. Our work paves the way for future theory and experiment, demonstrating how inelastic scattering at non-zero temperature affects the behavior of all anomalous transport coefficients.

Mapping the Finite-Temperature Behavior of Conformations to Their Potential Energy Barriers: Case Studies on Si6B and Si5B Clusters.

Dynamical simulations of molecules and materials have been the route to understand the rearrangement of atoms within them at different temperatures. Born–Oppenheimer molecular dynamical simulations have further helped to comprehend the reaction dynamics at various finite temperatures. We take a case study of Si6B and Si5B clusters and demonstrate that their finite-temperature behavior is rather mapped to the potential energy surface. The study further brings forth the fact that an accurate description of the dynamics is rather coupled with the accuracy of the method in defining the potential energy surface. A more precise potential energy surface generated through the coupled cluster method is finally used to identify the most accurate description of the potential energy surface and the interconnected finite-temperature behavior.

Finite-temperature critical behavior of long-range quantum Ising models

We study the phase diagram and critical properties of quantum Ising chains with long-range ferromagnetic interactions decaying in a power-law fashion with exponent \alphaα, in regimes of direct interest for current trapped ion experiments. Using large-scale path integral Monte Carlo simulations, we investigate both the ground-state and the nonzero-temperature regimes. We identify the phase boundary of the ferromagnetic phase and obtain accurate estimates for the ferromagnetic-paramagnetic transition temperatures. We further determine the critical exponents of the respective transitions. Our results are in agreement with existing predictions for interaction exponents \alpha<1α>1 up to small deviations in some critical exponents. We also address the elusive regime \alpha < 1α<1, where we find that the universality class of both the ground-state and nonzero-temperature transition is consistent with the mean-field limit at \alpha = 0α=0. Our work not only contributes to the understanding of the equilibrium properties of long-range interacting quantum Ising models, but can also be important for addressing fundamental dynamical aspects, such as issues concerning the open question of thermalization in such models.

Lattice Model of Multilayer Adsorption of Particles with Orientation Dependent Interactions at Solid Surfaces.

A simple lattice model has been used to study the formation of multilayer films by fluids with orientation-dependent interactions on solid surfaces. The particles, composed of two halves (A and B) were allowed to take on one of six different orientations. The interaction between a pair of differently oriented neighboring particles was assumed to depend on the degrees to which their A and B parts overlap. Here, we have assumed that the AA interaction was strongly attractive, the AB interaction was set to zero, while the BB interaction was varied between 0 and . The ground state properties of the model have been determined for the systems being in contact with non-selective and selective walls over the entire range of BB interaction energies between 0 and . It has been demonstrated that the structure of multilayer films depends on the strengths of surface potential felt by differently oriented particles and the interaction between the B halves of fluid particles. Finite temperature behavior has been studied by Monte Carlo simulation methods. It has been shown that the bulk phase phase diagram is qualitatively independent of the BB interaction energy, and has the swan neck shape, since the high stability of the dense ordered phase suppresses the possibility of the formation of disordered liquid-like phase. Only one class of non-uniform systems with the BB interaction set to zero has been considered. The results have been found to be consistent with the predictions stemming form the ground state considerations. In particular, we have found that a complete wetting occurs at any temperature, down to zero. Furthermore, the sequences of layering transitions, and the structure of multilayer films, have been found to be the same as observed in the ground state.

Nonconformality, subregion complexity, and meson binding

We study holographically the zero and finite temperature behavior of the potential energy and holographic subregion complexity corresponding to a probe meson in a non-conformal model. We observe that in zero and low temperature non-conformality has a decreasing effect on the dimensionless meson potential energy. However, non-conformal corrections increase absolute value of the dimensionless holographic subregion complexity in both zero and finite temperature which means the non-conformal state needs less information to be specified. In other words, considering the effect of non-conformality, the less bounded meson state needs less information to be specified. In low temperature limits, thermal corrections decrease meson potential energy and do not have a specific effect on holographic subregion complexity. We find that in the vicinity of the phase transition, the zero temperature meson state is more favorable than the finite temperature state, from the holographic subregion complexity point of view.

Frustrated quantum spins at finite temperature: Pseudo-Majorana functional renormalization group approach

The pseudofermion functional renormalization group (PFFRG) method has proven to be a powerful numerical approach to treat frustrated quantum spin systems. In its usual implementation, however, the complex fermionic representation of spin operators introduces unphysical Hilbert-space sectors which render an application at finite temperatures inaccurate. In this work we formulate a general functional renormalization group approach based on Majorana fermions to overcome these difficulties. We, particularly, implement spin operators via an $\text{SO}(3)$ symmetric Majorana representation which does not introduce any unphysical states and, hence, remains applicable to quantum spin models at finite temperatures. We apply this scheme, dubbed pseudo-Majorana functional renormalization group (PMFRG) method, to frustrated Heisenberg models on small spin clusters as well as square and triangular lattices. Computing the finite-temperature behavior of spin correlations and thermodynamic quantities such as free energy and heat capacity, we find good agreement with exact diagonalization and the high-temperature series expansion down to moderate temperatures. We observe a significantly enhanced accuracy of the PMFRG compared to the PFFRG at finite temperatures. More generally, we conclude that the development of functional renormalization group approaches with Majorana fermions considerably extends the scope of applicability of such methods.

Ab initio calculations of the phase behavior and subsequent magnetostriction of Fe1−xGax within the disordered local moment picture

A holistic approach for studying both the nature of atomic order and finite-temperature magnetostrictive behavior in the binary alloy Galfenol (${\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Ga}}_{x}, 0\ensuremath{\le}x\ensuremath{\le}0.25$) is presented. The phase behavior is studied via atomistic modeling with inputs from ab initio calculations, and the ordered phases of interest at nonstoichiometric concentrations are verified to exhibit $B2$- and ${D0}_{3}$-like order. The finite-temperature magnetoelasticity of these phases, in particular the magnetoelastic constant ${B}_{1}$, is obtained within the same ab initio framework using disordered local moment theory. Our results provide an explanation for the origin of the experimentally observed peak and subsequent fall in the material's magnetostriction at $x\ensuremath{\sim}0.19$, which has been disputed. In addition, we show that it is possible to enhance the magnetostriction of ${D0}_{3}\ensuremath{-}{\mathrm{Fe}}_{3}\mathrm{Ga}$ by removing a small fraction of electrons from the system, suggesting that a Fe-Ga-Cu or Fe-Ga-Zn alloy could exhibit greater magnetostrictive properties than Galfenol.

Finite temperature behavior of carbon atom-doped silicon clusters: depressed thermal stabilities, coexisting isomers, reversible dynamical pathways and fragmentation channels

BOMD simulations revealed a multifarious thermo-stimuli response (from “solid-state” to reversible dynamics to fragmentation) of experimentally identified SiC mixed clusters at finite temperature.

Quantum critical scaling for finite-temperature Mott-like metal-insulator crossover in few-layered MoS2

The possibility of the strong electron-electron interaction driven insulating phase from the metallic phase in two-dimensions has been suggested for clean systems without intentional disorder, but its rigorous demonstration is still lacking. Here, we examine the finite-temperature transport behavior of a few layered-MoS$_2$ material in the vicinity of the density-driven metal-insulator transition (MIT), revealing previously overlooked universal features characteristic of strongly correlated electron systems. Our scaling analysis, based on the Wigner-Mott theoretical viewpoint, conclusively demonstrates that the transition is driven by strong electron-electron interactions and not disorder, in striking resemblance to what is seen in other Mott systems. Our results provide compelling evidence that transition-metal dichalcogenides provide an ideal testing ground for the study of strong correlation physics, which should open an exciting avenue for future research, making a parallel with recent advances in twisted bilayer graphene

Radio-Frequency Response and Contact of Impurities in a Quantum Gas.

We investigate the radio-frequency spectroscopy of impurities interacting with a quantum gas at finite temperature. In the limit of a single impurity, we show using Fermi's golden rule that introducing (or injecting) an impurity into the medium is equivalent to ejecting an impurity that is initially interacting with the medium, since the "injection" and "ejection" spectral responses are simply related to each other by an exponential function of frequency. Thus, the full spectral information for the quantum impurity is contained in the injection spectral response, which can be determined using a range of theoretical methods, including variational approaches. We use this property to compute the finite-temperature equation of state and Tan contact of the Fermi polaron. Our results for the contact of a mobile impurity are in excellent agreement with recent experiments and we find that the finite-temperature behavior is qualitatively different compared to the case of infinite impurity mass.

A two-dimensional liquid-like phase on Ga-rich GaN (0001) surfaces evidenced by first principles molecular dynamics

First principles molecular dynamics simulations have been used as a tool to investigate at an atomistic level the finite temperature behavior of a typical GaN growing surface. At 1300 K, Ga adatoms show a high degree of destabilization and a departure of the diffusivity from an ordinary Arrhenius trend. Two signatures, a drastic change in the radial distribution function and a sudden increase of the diffusion constant, allow to quantify such a diffusion process. Such a high diffusivity results in the arising of a pronounced peak at about 2.6 Å, typical of Ga–Ga bonds, indicating a tendency to cluster together of the Ga adatoms. Along with the remarkable diffusivity observed at 1300 K, this result indicates rather clearly that at the typical experimental temperature, the epitaxial growth occurs not on a rigid surface in which Ga adatoms are tightly bound to specific sites, but rather on a two-dimensional liquid state.

Non-Heisenberg magnetism in a quaternary spin-gapless semiconductor

Understanding of spin-gapless semiconductors with fully spin-polarized charge carriers is critically important because of their promise for spintronic applications. Here, we report non-collinear spin structures, magnetic ground state, and effective exchange interactions of the spin-gapless semiconductor CoFeCrAl investigated with noncollinear density functional calculations. The ground state of CoFeCrAl is ferrimagnetic and has a spin configuration with ↓ Fe, ↑ Co and ↑ Cr spins. In our constrained calculations, the magnetizations of the Fe, Co, and Cr sublattices are rotated by various angles θ, which give rise to three sets of noncollinear spin structures. For all three elements, the magnetic energy increases with the angle, which reconfirms the ferrimagnetic spin structure. During rotation, the magnitudes of the Co and Cr spins remain almost unchanged, whereas that of Fe strongly decreases as a function of the angle θ. This indicates that the finite-temperature behavior of CoFeCrAl is characterized by a pronounced non-Heisenberg behavior of the ↓ Fe moments, whereas the ↑ Co and ↑ Cr moments are Heisenberg-like. We discuss how this feature affects the finite-temperature behavior of the alloy beyond the commonly considered intersublattice Heisenberg exchange.