Thermal Crossover Research Articles

Thermal crossover from a Chern insulator to a fractional Chern insulator in pentalayer graphene

Thermal crossover from a Chern insulator to a fractional Chern insulator in pentalayer graphene

Intelligent bulk cargo terminal scheduling based on a novel chaotic-optimal thermodynamic evolutionary algorithm

This paper presents a chaotic optimal thermodynamic evolutionary algorithm (COTEA) designed to address the integrated scheduling problems of berth allocation, ship unloader scheduling, and yard allocation at bulk cargo terminals. Our proposed COTEA introduces a thermal transition crossover method that effectively circumvents local optima in the scheduling solution process. Additionally, the method innovatively combines a good point set with chaotic dynamics within an integrated initialization framework, thereby cultivating a robust and exploratory initial population for the optimization algorithm. To further enhance the selection process, our paper proposes a refined parental selection protocol that employs a quantified hypervolume contribution metric to discern superior candidate solutions. Postevolution, our algorithm employs a Cauchy inverse cumulative distribution-based neighborhood search to effectively explore and enhance the solution spaces, significantly accelerating the convergence speed during the scheduling solution process. The proposed method is adept at achieving multiobjective optimization, simultaneously improving the service level and reducing costs for bulk cargo terminals, which in turn boosts their competitiveness. The effectiveness of our COTEA is demonstrated through extensive numerical simulations.

STFTrack: Spatio-Temporal-Focused Siamese Network for Infrared UAV Tracking

The rapid popularity of UAVs has encouraged the development of Anti-UAV technology. Infrared-detector-based visual tracking for UAVs provides an encouraging solution for Anti-UAVs. However, it still faces the problem of tracking instability caused by environmental thermal crossover and similar distractors. To address these issues, we propose a spatio-temporal-focused Siamese network for infrared UAV tracking, called STFTrack. This method employs a two-level target focusing strategy from global to local. First, a feature pyramid-based Siamese backbone is constructed to enhance the feature expression of infrared UAVs through cross-scale feature fusion. By combining template and motion features, we guide prior anchor boxes towards the suspicious region to enable adaptive search region selection, thus effectively suppressing background interference and generating high-quality candidates. Furthermore, we propose an instance-discriminative RCNN based on metric learning to focus on the target UAV among candidates. By measuring calculating the feature distance between the candidates and the template, it assists in discriminating the optimal target from the candidates, thus improving the discrimination of the proposed method to infrared UAV. Extensive experiments on the Anti-UAV dataset demonstrate that the proposed method achieves outstanding performance for infrared tracking, with 91.2% precision, 66.6% success rate, and 67.7% average overlap accuracy, and it exceeded the baseline algorithm by 2.3%, 2.7%, and 3.5%, respectively. The attribute-based evaluation demonstrates that the proposed method achieves robust tracking effects on challenging scenes such as fast motion, thermal crossover, and similar distractors. Evaluation on the LSOTB-TIR dataset shows that the proposed method reaches a precision of 77.2% and a success rate of 63.4%, outperforming other advanced trackers.

Luminescence Temperature Quenching in Mn2+ Phosphors

AbstractNarrower band red and green emission in phosphor‐converted white light‐emitting diodes (wLEDs) can improve the efficacy and color gamut in lighting and display applications. A promising luminescent ion is Mn2+ that can have both narrowband green (tetrahedral coordination) and red (octahedral coordination) emission. Unlike in earlier lighting applications of Mn2+ phosphors, temperature quenching is important in wLEDs. Insight into the thermal quenching behavior of Mn2+ luminescence is lacking. Here systematic research is reported for a variety of Mn2+‐doped phosphors; a huge variation in the luminescence quenching temperature T50, ranging from 50 K for Mn2+ in ZnTe to 1200 K in MgAl2O4, is revealed. The value T50 shows a positive correlation with the bandgap of the host, but no correlation with the full width half maximum (FWHM) of the emission band, indicating that thermally activated photoionization, not thermal crossover, is the operative quenching mechanism. This is confirmed by thermally stimulated luminescence (TSL) measurements that show a rise in TSL signal following photoexcitation at temperatures around T50 providing evidence that quenching is correlated with generation of free charge carriers. Based on these findings, as a design rule is obtained that for temperature‐stable Mn2+ luminescence in (high power) LEDs a wide‐bandgap host material is required.

Investigation on step overcharge to self-heating behavior and mechanism analysis of lithium ion batteries

To obtain intrinsic overcharge boundary and investigate overcharge mechanism, here we propose an innovative method, the step overcharge test, to reduce the thermal crossover and distinguish the overcharge thermal behavior, including 5% state of charge (SOC) with small current overcharge and resting until the temperature equilibrium under adiabatic conditions. The intrinsic thermal response and the self-excitation behaviour are analysed through temperature and voltage changes during the step overcharge period. Experimental results show that the deintercalated state of the cathode is highly correlated to self-heating parasitic reactions. Before reaching the upper limit of Negative/Positive (N/P) ratio, the temperature changes little, the heat generation is significantly induced by the reversible heat (endothermic) and ohmic heat, which could balance each other. Following that the lithium metal is gradually deposited on the surface of the anode and reacts with electrolyte upon overcharge, inducing self-heating side reaction. However, this spontaneous thermal reaction could be “self-extinguished”. When the lithium in cathode is completely deintercalated, the boundary point of overcharge is about 4.7 V (∼148% SOC, >40 °C), and from this point, the self-heating behaviour could be continuously triggered until thermal runaway (TR) without additional overcharge. The whole static and spontaneous process lasts for 115 h and the side reaction heat is beyond 320,000 J. The continuous self-excitation behavior inside the battery is attributed to the interaction between the highly oxidized cathode and the solvent, which leads to the dissolution of metal ions. The dissolved metal ions destroy the SEI (solid electrolyte interphase) film on the surface of the deposited Li of anode, which induces the thermal reaction between lithium metal and the solvent. The interaction between cathode, the deposited Li of anode, and solvent promotes the temperature of the battery to rise slowly. When the temperature of the battery reaches more than 60 °C, the reaction between lithium metal and solvent is accelerated. After the temperature rises rapidly to the melting point of the separator, it triggers the thermal runaway of the battery due to the short circuit of the battery.

Attribute-Based Progressive Fusion Network for RGBT Tracking

RGBT tracking usually suffers from various challenge factors, such as fast motion, scale variation, illumination variation, thermal crossover and occlusion, to name a few. Existing works often study fusion models to solve all challenges simultaneously, and it requires fusion models complex enough and training data large enough, which are usually difficult to be constructed in real-world scenarios. In this work, we disentangle the fusion process via the challenge attributes, and thus propose a novel Attribute-based Progressive Fusion Network (APFNet) to increase the fusion capacity with a small number of parameters while reducing the dependence on large-scale training data. In particular, we design five attribute-specific fusion branches to integrate RGB and thermal features under the challenges of thermal crossover, illumination variation, scale variation, occlusion and fast motion respectively. By disentangling the fusion process, we can use a small number of parameters for each branch to achieve robust fusion of different modalities and train each branch using the small training subset with the corresponding attribute annotation. Then, to adaptive fuse features of all branches, we design an aggregation fusion module based on SKNet. Finally, we also design an enhancement fusion transformer to strengthen the aggregated feature and modality-specific features. Experimental results on benchmark datasets demonstrate the effectiveness of our APFNet against other state-of-the-art methods.

A constructive theory of the numerically accessible many-body localized to thermal crossover

The many-body localised (MBL) to thermal crossover observed in exact diagonalisation studies remains poorly understood as the accessible system sizes are too small to be in an asymptotic scaling regime. We develop a model of the crossover in short 1D chains in which the MBL phase is destabilised by the formation of many-body resonances. The model reproduces several properties of the numerically observed crossover, including an apparent correlation length exponent \nu=1ν=1, exponential growth of the Thouless time with disorder strength, linear drift of the critical disorder strength with system size, scale-free resonances, apparent 1/\omega1/ω dependence of disorder-averaged spectral functions, and sub-thermal entanglement entropy of small subsystems. In the crossover, resonances induced by a local perturbation are rare at numerically accessible system sizes LL which are smaller than a \lambdaλ. For L \gg \sqrt{\lambda}L≫λ (in lattice units), resonances typically overlap, and this model does not describe the asymptotic transition. The model further reproduces controversial numerical observations which Refs. claimed to be inconsistent with MBL. We thus argue that the numerics to date is consistent with a MBL phase in the thermodynamic limit.

Universality of dilute solutions of ring polymers in the thermal crossover region between θ and athermal solvents

Due to their unique topology of having no chain ends, dilute solutions of ring polymers exhibit behavior distinct from their linear chain counterparts. The universality of their static and dynamic properties, as a function of solvent quality z in the thermal crossover regime between θ and athermal solvents, is studied here using Brownian dynamics simulations. The universal ratio URD of the radius of gyration Rg to the hydrodynamic radius RH is determined, and a comparative study of the swelling ratio αg of the radius of gyration, the swelling ratio αH of the hydrodynamic radius, and the swelling ratio αX of the mean polymer stretch X along the x-axis, for linear and ring polymers, is carried out. The ratio URD for dilute ring polymer solutions is found to converge asymptotically to a constant value as z→∞, which is a major difference from the behavior of solutions of linear chains, where no such asymptotic limit exists. Additionally, the ratio of the mean stretch along the x-axis to the hydrodynamic radius, (X/RH), is found to be independent of z for polymeric rings, unlike in the case for linear polymers. These results indicate a fundamental difference in the scaling of static and dynamic properties of rings and linear chains in the thermal crossover regime.

Phase diagram of QCD in a magnetic background

We provide numerical evidence that the thermal QCD crossover turns into a first order transition in the presence of large enough magnetic background fields. The critical endpoint is found to be located between $eB = 4$ GeV$^2$ (where the pseudocritical temperature is $T_c = (98 \pm 3)$ MeV) and $eB = 9$ GeV$^2$ (where the critical temperature is $T_c = (63 \pm 5)$ MeV). Results are based on the analysis of quark condensates and number susceptibilities, determined by lattice simulations of $N_f = 2+1$ QCD at the physical point, discretized with three different lattice spacings, $a = 0.114, 0.086$ and $0.057$ fm, via rooted stout staggered fermions and a Symanzik tree level improved pure gauge action. We also present preliminary results regarding the confining properties of the thermal theory, suggesting that they could change drastically going across the phase transition

Thermal critical phenomena and crossover between critical regimes in ferromagnets with long-range interactions

Abstract In this brief review, an attempt has been made to bring out clearly the role of long-range interactions (such as dipole – dipole interactions or isotropic exchange interactions decaying with interspin distance ‘r’ as J(r) ~ r –(d+σ) (0 < σ < 2), where ‘d’ is the space dimensionality) (i) in determining the thermodynamic properties of ferromagnets (in which the electrons responsible for magnetism are either localized or itinerant) in the asymptotic critical region, (ii) in causing a crossover from one critical regime to the other, and (iii) in stabilizing long-range magnetic order in site-diluted and/or bond-diluted ferromagnets with composition just above the percolation threshold. In the process, a number of long-standing/puzzling issues concerning the nature of magnetic order, static critical phenomena and crossover scenario in a wide variety of ferromagnetic systems have been resolved.

Thermal and field-induced transitions in ferroquadrupolar Kondo systems

Recent experiments have examined the impact of a magnetic field on ferroquadrupolar orders in the intermetallic Kondo material PrTi$_2$Al$_{20}$. Motivated by this, we use extensive Monte Carlo simulations to study a diamond lattice XY model of non-Kramers pseudospin-$1/2$ Pr$^{3+}$ moments which crucially incorporates three-spin interactions. This model supports a thermal $Z_3$ Potts ordering transition upon cooling from the paramagnetic phase into the ferroquadrupolar phase. An applied magnetic field along the [110] direction leads to a thermal Ising transition out of the quadrupolar ordered phase. A magnetic field along the [001] direction leads to only thermal crossovers, but supports a spinodal transition out of metastable domains which could be strongly pinned by coupling to elastic lattice deformations. We propose noise measurements as a potential probe to "hear" the spinodal transition. Our work highlights the importance of multispin interactions in Kondo materials near the small-to-large Fermi surface transition.

Electric-field control of spin transitions in molecular compounds

We present a theoretical model of spin transitions in stacks of molecular layers. Our model captures the already established physics of these systems (thermal hysteretic transitions and crossovers) and suggests a way towards in situ control of this physics by means of an external electric field. Our results pave the way toward both temperature and voltage controllable organic memory.

Holographic entanglement entropy and generalized entanglement temperature

In this work we study the flow of holographic entanglement entropy in dimensions $d \geq 3$ in the gauge/gravity duality set up. We observe that a generalized entanglement temperature $T_g$ can be defined which gives the Hawking temperature $T_H$ in the infrared region and leads to a generalized thermodynamics like law $E= \left(\frac{d-1}{d}\right)T_g~S_{REE}$, which becomes an exact relation in the entire region of the subsystem size $l$, including both the infrared ($l\rightarrow\infty$) as well as the ultraviolet ($l\rightarrow 0$) regions. Furthermore, in the IR limit, $T_g$ produces the Hawking temperature $T_H$ along with some correction terms which bears the signature of short distance correlations along the entangling surface. Moreover, for $d\geq 3$, the IR limit of the renormalized holographic entanglement entropy gives the thermal entropy of the black hole as the leading term, however, does not have a logarithmic correction to the leading term unlike the BTZ black hole ($d=2$) case. The generalized entanglement temperature $T_g$ also firmly captures the quantum mechanical to thermal crossover in the dual field theory at a critical value $l_c$ of the subsystem size in the boundary which we graphically represent for $AdS_{3+1}$ and $AdS_{4+1}$ black holes. We observe that this critical value $l_c$ where the crossover takes place decreases with increase in the dimension of the spacetime.

Spin transport in an electrically driven magnon gas near Bose-Einstein condensation: Hartree-Fock-Keldysh theory

An easy-plane ferromagnetic insulator in a uniform external magnetic field and in contact with a phonon bath and a normal metal bath is studied theoretically in the presence of dc spin current injection via the spin Hall effect in the metal. The Keldysh path integral formalism is used to model the magnon gas driven into a nonequilibrium steady state by mismatched bath temperatures and/or electrical injection, and we analyze the magnon system in the normal (uncondensed) state, but close to the instability to Bose-Einstein condensation (BEC), within the self-consistent Hartree-Fock approximation. We find that the steady state magnon distribution function generally has a non-thermal form that cannot be described by a single effective chemical potential and effective temperature. We also show that the BEC instability in the electrically-driven magnon system is signaled by a sign change in the imaginary part of the poles for long-wavelength magnon modes and by the divergence of the nonequilibrium magnon distribution function. In the presence of two bath temperatures, we find that the correlation length of the superfluid order parameter fluctuations exhibits nontrivial finite temperature crossover behaviors that are richer than the standard crossover behaviors obtained for the vacuum-superfluid transition in an equilibrium dilute Bose gas. We study the consequences of these thermal crossovers on the magnon spin conductivity and obtain an inverse square-root divergence in the spin conductivity in the vicinity of the electrically-induced BEC instability. A spintronics device capable of testing our spin transport predictions is discussed.

Thermal crossover, transition, and coexistence in Fermi polaronic spectroscopies

We investigate thermal evolution of radio-frequency (RF) spectra of a spin-imbalanced Fermi gas near a Feshbach resonance in which degenerate Fermi-polaron and classical Boltzmann-gas regimes emerge in the low-temperature and high-temperature limits, respectively. By using a self-consistent framework of strong-coupling diagrammatic approaches, both of the ejection and reserve RF spectra available in cold-atom experiments are analyzed. We find a variety of transfers from Fermi polarons to Boltzmann gas such that a thermal crossover expected in the weak-coupling regime is shifted to a sharp transition near unitarity and to double-peak coexistence of attractive and repulsive branches in the strong-coupling regime. Our theory provides semiquantitative descriptions for a recent experiment on the ejection RF spectroscopy at unitarity [Z. Yan {\it et al}., arXiv:1811.00481v1] and demonstrate that the RF spectroscopy is an essential probe to examine effects of strong correlations and finite temperature.

Online Non-Negative Multi-Modality Feature Template Learning for RGB-Assisted Infrared Tracking

Infrared sensors have been deployed in many video surveillance systems because of the insensibility of their imaging procedure to some extreme conditions (e.g. low illumination condition, dim environment). To reduce human labor in video monitoring and perform intelligent infrared video understanding, an important issue we need to consider is how to locate the object of interest in consecutive video frames accurately. Therefore, developing a robust object tracking algorithm for infrared videos is necessary. However, the infrared information may not be reliable (e.g. thermal crossover), and appearance modeling with only the infrared modality may not be able to achieve good results. To address these issues, with the wide deployment of RGB-infrared camera systems, this paper proposes an infrared tracking framework in which information from RGB-modality will be exploited to assist the infrared object tracking. Specifically, within the tracking framework, in order to deal with the contaminated features caused by large appearance variations, an online non-negative feature template learning model is designed. The non-negative constraint enables the model to capture the local part-based characteristic of the target appearance. To ensure more important modality contribute more in appearance representation, an adaptive modality importance weight learning scheme is also incorporated in the proposed feature learning model. To guarantee the model optimality, an iterative optimization algorithm is derived. The experimental results on various RGB-infrared videos show the effectiveness of the proposed method.

Universality of the collapse transition of sticky polymers

The universality of the swelling of the radius of gyration of a homopolymer relative to its value in the θ state, independent of polymer-solvent chemistry, in the crossover regime between θ and athermal solvent conditions, is well known. Here we study, by Brownian dynamics, a polymer model where a subset of monomers is labelled as "stickers". The mutual interaction of the stickers is more attractive than those of the other ("backbone") monomers, and has an additional important characteristic of "functionality" φ, i.e., the maximum number of stickers that can locally bind to a given sticker. A saturated bond formed in this manner remains bound until it breaks due to thermal fluctuations, a requirement which can be viewed as an additional Boolean degree of freedom that describes the bonding. This, in turn, makes the question of the order of the collapse transition a non-trivial one. Nevertheless, for the parameters that we have studied (in particular, φ = 1), we find a standard second-order θ collapse, using a renormalised solvent quality parameter that takes into account the increased average attraction due to the presence of stickers. We examine the swelling of the radius of gyration of such a sticky polymer relative to its value in the altered θ state, using a novel potential to model the various excluded volume interactions that occur between the monomers on the chain. We find that the swelling of such sticky polymers is identical to the universal swelling of homopolymers in the thermal crossover regime. Additionally, for our model, the Kuhn segment length under θ conditions is found to be the same for chains with and without stickers.

Stokes-Einstein relation of the liquid metal rubidium and its relationship to changes in the microscopic dynamics with increasing temperature.

For liquid rubidium the Stokes-Einstein (SE) relation is well fulfilled near the melting point with an effective hydrodynamic diameter, which agrees well with a value from structural investigations. A wealth of thermodynamic and microscopic data exists for a wide range of temperatures for liquid rubidium and hence it represents a good test bed to challenge the SE relation with rising temperature from an experimental point of view. We performed classical molecular dynamics simulations to complement the existing experimental data using a pseudopotential, which describes perfectly the structure and dynamics of liquid rubidium. The derived SE relation from combining experimental shear viscosity data with simulated diffusion coefficients reveals a weak violation at about 1.3T_{melting}≈400 K. The microscopic relaxation dynamics on nearest neighbor distances from neutron spectroscopy demonstrate distinct changes in the amplitude with rising temperature. The derived average relaxation time for density fluctuations on this length scale shows a non-Arrhenius behavior, with a slope change around 1.5T_{melting}≈450 K. Combining the simulated macroscopic self-diffusion coefficient with that microscopic average relaxation time, a distinct violation of the SE relation in the same temperature range can be demonstrated. One can conclude that the changes in the collective dynamics, a mirror of the correlated movements of the particles, are at the origin for the violation of the SE relation. The changes in the dynamics can be understood as a transition from a more viscous liquid metal to a more fluid-like liquid above the crossover temperature range of 1.3-1.5 T_{melting}. The decay of the amplitude of density fluctuations in liquid aluminium, lead, and rubidium demonstrates a remarkable agreement and points to a universal thermal crossover in the dynamics of liquid metals.

Ultrafast non-thermal laser excitation of gigahertz longitudinal and shear acoustic waves in spin-crossover molecular crystals [Fe(PM–AzA)2(NCS)2

We report GHz longitudinal as well as shear acoustic phonon photoexcitation and photodetection using femtosecond laser pulses in a spin-crossover molecular crystal. From our experimental observation of time domain Brillouin scattering triggered by the photoexcitation of acoustic waves across the low-spin (LS) to high-spin (HS) thermal crossover, we reveal a link between the molecular spin state and photoexcitation of coherent GHz acoustic phonons. In particular, we experimentally evidence a non-thermal pathway for the laser excitation of GHz phonons. We also provide experimental insights into the optical and mechanical parameters evolving across the LS/HS spin crossover temperature range.

Static and dynamic properties of two-dimensional Coulomb clusters.

We study the temperature dependence of static and dynamic responses of Coulomb interacting particles in two-dimensional confinements across the crossover from solid- to liquid-like behaviors. While static correlations that investigate the translational and bond orientational order in the confinements show the footprints of hexatic-like phase at low temperatures, dynamics of the particles slow down considerably in this phase, reminiscent of a supercooled liquid. Using density correlations, we probe long-lived heterogeneities arising from the interplay of the irregularity in the confinement and long-range Coulomb interactions. The relaxation at multiple time scales show stretched-exponential decay of spatial correlations in irregular traps. Temperature dependence of characteristic time scales, depicting the structural relaxation of the system, show striking similarities with those observed for the glassy systems, indicating that some of the key signatures of supercooled liquids emerge in confinements with lower spatial symmetries.