Crossover Temperature Research Articles

Thermal properties of light mesons from holography

The thermal properties of light mesons, including the temperature dependence of their masses (both screening and pole masses) and thermal widths, are studied in a two-flavor (Nf = 2) soft-wall AdS/QCD model. By solving the spatial correlation functions, we extract the screening masses (mscr) from their poles. The screening masses of pseudo-scalar (π) and axial-vector (a1) mesons increase almost monotonously with the increase of temperature. The screening masses of scalar (σ) and vector (ρ) mesons decrease at low temperature and increase at high temperature. The pole masses (mpole) and the thermal widths (Γ) are extracted from the temporal correlation functions and the corresponding spectral functions. The results indicate that the pole masses have local minima at low temperature and increase at high temperature. The thermal widths increase rapidly above the chiral crossover temperature Tcp, indicating the dissociations of mesons at high temperature. Furthermore, the degeneration of the chiral partners (π and σ, ρ and a1) above Tcp is observed from the screening and pole masses, revealing the chiral symmetry restoration at the hadronic spectrum level. Finally, we numerically verify that the spectral functions in the temporal regime are strongly related to the quasi-normal modes with complex frequencies ω0 = mpole− iΓ/2.

Josephson Junctions as Single Microwave Photon Counters: Simulation and Characterization

Detection of light dark matter, such as axion-like particles, puts stringent requirements on the efficiency and dark-count rates of microwave-photon detectors. The possibility of operating a current-biased Josephson junction as a single-microwave photon-detector was investigated through numerical simulations, and through an initial characterization of two Al junctions fabricated by shadow mask evaporation, done in a dilution refrigerator by measuring escape currents at different temperatures, from 40 mK up to the Al transition temperature. The escape dynamics of the junctions were reproduced in the simulation, including the dissipative effects. Inhibition of thermal activation was observed, leaving the macroscopic quantum tunneling as the dominant effect well beyond the crossover temperature.

Electrochemical studies of iron(II) complexeswith 4‐amino‐1,2,4‐triazole possessing spin crossover

AbstractThe electrochemical properties of a number of coordination compounds of iron(II) with 4‐amino‐1,2,4‐triazole (NH2trz), which have a sharp thermally induced spin crossover with hysteresis on the temperature curves of the effective magnetic moment dependence, were studied using method cyclic voltammetry (CV) at 298 K. The complexes were mixed with carbon powder and silicon oil and included in the paste electrode. The measurements were made in solutions of 0.10 M KNO3. It was found that the area 0.5–0.6 V and 0.3–0.4 V contains the anode and cathode current peaks of freshly prepared paste electrodes. In all cases, the redox process in the paste is quasi‐reversible. At the same molar concentration of complexes in the paste (0.1 mol kg−1), a number of current peaks depends on the nature of the anion. The peak current value correlates with the calculated fraction of the high spin form in the solid phase of the complex and the temperature of the direct spin crossover (when heated, Tc↑). The magnetic properties of Fe(NH2trz)3(NO3)2 complex, its mixture with carbon powder and the paste were measured by Faraday method in a temperature range 80–370 K. The effective magnetic moment was depended on changes in variations in the environment of the molecules of the complex in the studied samples.

Entanglement Effect and Angular Momentum Conservation in a Nonseparable Tunneling Treatment.

The important, and often dominant, role of tunneling in low temperature kinetics has resulted in numerous theoretical explorations into the methodology for predicting it. Nevertheless, there are still key aspects of the derivations that are lacking, particularly for nonseparable systems in the low temperature regime, and further explorations of the physical factors affecting the tunneling rate are warranted. In this work we obtain a closed-form rate expression for the tunneling rate constant that is a direct analog of the rigid-rotor-harmonic-oscillator expression. This expression introduces a novel "entanglement factor" that modulates the reaction rate. Furthermore, we are able to extend this expression, which is valid for nonseparable systems at low temperatures, to properly account for the conservation of angular momentum. In contrast, previous calculations have considered only vibrational transverse modes and so effectively employ a decoupled rotational partition function for the orientational modes. We also suggest a simple theoretical model to describe the tunneling effects in the vicinity of the crossover temperature (the temperature where tunneling becomes the dominating mechanism). This model allows one to naturally classify, interpret, and predict experimental data. Among other things, it quantitatively explains in simple terms the so-called "quantum bobsled" effect, also known as the negative centrifugal effect, which is related to curvature of the reaction path. Taken together, the expressions obtained here allow one to predict the thermal and E-resolved rate constants over broad ranges of temperatures and energies.

Oscillations of the electron energy loss rate in two-dimensional transition-metal dichalcogenides in the presence of a quantizing magnetic field

We study the hot electron energy-loss rate (ELR) induced by acoustic ${P}^{\mathrm{ac}}$ and optical ${P}^{\mathrm{op}}$ phonons, in two-dimensional transition-metal dichalcogenides (TMDCs), in presence of quantizing magnetic field $B$, including the hot-phonon effect. At the low-temperature regime, the ELR is found to display a quantum oscillations with their amplitude being found to increase with the increase of the magnetic field. The unscreened TA phonon due to deformation potential (DP) coupling is dominant over the other screened acoustic phonon contributions. In the extreme Bloch-Gr\"uneisen (BG) region, the ELR displays a pronounced ${P}^{\mathrm{ac}}\ensuremath{\propto}{T}^{4}\phantom{\rule{0.16em}{0ex}}({T}^{6})$ temperature dependence for unscreened TA-DP phonons (other screened mechanisms). When the temperature increases, ${T}_{e}>{T}_{\mathrm{BG}}$, the ELR shows a linear ${P}^{\mathrm{ac}}\ensuremath{\propto}T$ behavior in the equipartition region. At higher temperatures, there is a crossover from ${P}^{\mathrm{ac}}$ dominated ELR to ${P}^{\mathrm{op}}$ by zeroth-order DP coupling dominated ELR with the cross-over temperature being about ${T}_{e}\ensuremath{\sim}50\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. The hot phonon effect is demonstrated to reduce ELR significantly. The effect of the magnetic field is found to enhance the ELR significantly, making $B$ as its another tuning knob. Among the four TMDC materials, ${\mathrm{MoS}}_{2}$ (${\mathrm{MoSe}}_{2}$) displays the biggest ELR due to acoustic (optical) phonon scattering while ${\mathrm{WSe}}_{2}$ (${\mathrm{WS}}_{2}$) shows the smallest ELR. Our results for the ${\mathrm{MoS}}_{2}$ material are compared with those of the zero-field ELR.

Relationship between Rheological Indices and Cracking Performance of Virgin, Recycled, and Rejuvenated Asphalt Binders and Mixtures

Long-term cracking performance of asphalt mixtures is heavily influenced by asphalt binder rheology, and changes in binder stiffness, ductility, and aging condition significantly affect the cracking resistance of the corresponding asphalt mixture. This study evaluated the effectiveness of several common and recently developed binder rheological indices in capturing the effects of binder performance grade (PG) and source, recycled binder content, recycling agent (rejuvenator) dose, and long-term aging. These binder rheological indices included the Superpave intermediate-temperature PG (PGI), the Glover-Rowe (G-R) parameter, the crossover temperature (Tδ = 45°), the rheological index (R-value), and ΔTc. This study also directly compared the binder rheological indices with the cracking performance of corresponding asphalt mixtures to explore possible correlations and their robustness. Asphalt mixture cracking performance was evaluated using the Illinois Flexibility Index Test (I-FIT) for intermediate-temperature cracking, and the disk-shaped compact tension (DCT) test and the uniaxial thermal stress and strain test (UTSST) for low-temperature cracking. Results indicated that all the binder rheological indices (except PGI) consistently captured the effects of binder blend composition and proportions and aging condition, with a few exceptions. Results also showed that the G-R parameter, the crossover temperature (Tδ = 45°), and ΔTc had the best correlation to asphalt mixture and field core cracking performance as compared with other rheological indices (PGI and R-value), with ΔTc demonstrating the overall best correlation to mixture cracking performance.

Effective magnetic Hamiltonian at finite temperatures for rare-earth chalcogenides

Alkali metal rare-earth chalcogenide $ARECh2$ (A=alkali or monovalent metal, RE=rare earth, Ch=O, S, Se, Te), is a large family of quantum spin liquid (QSL) candidates we discovered recently. Unlike $YbMgGaO4$, most members in the family except for the oxide ones, have relatively small crystalline electric-field (CEF) excitation levels, particularly the first ones. This makes the conventional Curie-Weiss analysis at finite temperatures inapplicable and CEF excitations may play an essential role in understanding the low-energy spin physics. Here we considered an effective magnetic Hamiltonian incorporating CEF excitations and spin-spin interactions, to accurately describe thermodynamics in such a system. By taking $NaYbSe2$ as an example, we were able to analyze magnetic susceptibility, magnetization under pulsed high fields and heat capacity in a systematic and comprehensive way. The analysis allows us to produce accurate anisotropic exchange coupling energies and unambiguously determine a crossover temperature ($\sim$25 K in the case of $NaYbSe2$), below which CEF effects fade away and pure spin-spin interactions stand out. We further validated the effective picture by successfully explaining the anomalous temperature dependence of electron spin resonance (ESR) spectral width. The effective scenario in principle can be generalized to other rare-earth spin systems with small CEF excitations.

Crystal growth in deeply undercooled Ni50Al50: Signature of the ordering sequence at the interface.

Crystal growth of the intermetallic alloy, Ni50Al50, is investigated by molecular dynamics simulations with two different interatomic potentials. The calculated growth rate can be captured by the Wilson-Frenkel or Broughton-Gilmer-Jackson model at small undercoolings but deviates from the theory at deep undercoolings. Failure of the theory is found to be correlated with the dynamic processes that emerged at the interface, but not apparently with the static interface structure. The chemical segregation of Ni and Al atoms occurs before the geometrical ordering upon crystallization at small undercoolings. In contrast, the geometrical ordering precedes the chemical one at deep undercoolings. These two ordering processes show a collapsed time evolution at the crossover temperature consistent with the onset of the theoretical deviation. We rationalize the delayed chemical segregation behavior by the collective atomic motion, which is characterized by the super-Arrhenius transition of the temperature-dependent diffusivity and structural relaxation time at the crossover point.

Jump-precursor state emerges below the crossover temperature in supercooled o-terphenyl.

In a supercooled liquid, the crossover temperature T_{c} separates a high-temperature region of diffusive dynamics from a low-temperature region of activated dynamics. A molecular-dynamics simulation of all-atom, flexible o-terphenyl [Eastwood et al., J. Phys. Chem. B 117, 12898 (2013)10.1021/jp402102w] is analyzed with advanced statistical methods to reveal the molecular features associated with this crossover. The simulations extend to an α-relaxation time of 14 μs (272.5 K), two orders of magnitude slower than at T_{c} (290 K). At T_{c} and below, a distinct state emerges that immediately precedes an orientational jump. Compared to the initial, tightly caged state, this jump-precursor state has a looser cage, with solid-angular excursions of 0.054-0.0125 × 4π sr. At T_{c} (290 K), rate heterogeneity is already the dominant cause of stretched relaxation. Exchange within the distribution of rates is faster than α relaxation at T_{c}, but becomes equal to it at the lowest temperature simulated (272.5 K). The results trend toward a recent experimental observation near the glass transition (243 K) [Kaur et al., Phys. Rev. E 98, 040603(R) (2018)10.1103/PhysRevE.98.040603], which saw exchange substantially slower than α relaxation. Overall, the dynamic crossover comprises multiple phenomena: the development of heterogeneity, an increasing jump size, an emerging jump-precursor state, and a lengthening exchange time. The crossover is neither sharp, nor a simple superposition of the high- and low-temperature regimes; it is a broad region that contains unique and complex phenomena.

How attractive and repulsive interactions affect structure ordering and dynamics of glass-forming liquids.

The theory developed in our previous papers [Phys. Rev. E 99, 030101(R) (2019)10.1103/PhysRevE.99.030101; Phys. Rev. E 103, 032611 (2021)10.1103/PhysRevE.103.032611] is applied in this paper to investigate the dependence of slowing down of dynamics of glass-forming liquids on the attractive and repulsive parts of intermolecular interactions. Through an extensive comparison of the behavior of a Lennard-Jones glass-forming liquid and that of its WCA reduction to a model with truncated pair potential without attractive tail, we demonstrate why the two systems exhibit very different dynamics despite having nearly identical pair correlation functions. In particular, we show that local structures characterized by the number of mobile and immobile particles around a central particle markedly differ in the two systems at densities and temperatures where their dynamics show large difference and nearly identical where dynamics nearly overlap. We also show how the parameter ψ(T) that measures the role of fluctuations embedded in the system on size of the cooperatively reorganizing cluster (CRC) and the crossover temperature T_{a} depend on the intermolecular interactions. These parameters stemming from the intermolecular interactions characterize the temperature and density dependence of structural relaxation time τ_{α}. The quantitative and qualitative agreements found with simulation results for the two systems suggest that our theory brings out the underlying features that determine the dynamics of glass-forming liquids.

The role of annealing in determining the yielding behavior of glasses under cyclic shear deformation

Yielding behavior in amorphous solids has been investigated in computer simulations using uniform and cyclic shear deformation. Recent results characterize yielding as a discontinuous transition, with the degree of annealing of glasses being a significant parameter. Under uniform shear, discontinuous changes in stresses at yielding occur in the high annealing regime, separated from the poor annealing regime in which yielding is gradual. In cyclic shear simulations, relatively poorly annealed glasses become progressively better annealed as the yielding point is approached, with a relatively modest but clear discontinuous change at yielding. To understand better the role of annealing on yielding characteristics, we perform athermal quasistatic cyclic shear simulations of glasses prepared with a wide range of annealing in two qualitatively different systems-a model of silica (a network glass) and an atomic binary mixture glass. Two strikingly different regimes of behavior emerge. Energies of poorly annealed samples evolve toward a unique threshold energy as the strain amplitude increases, before yielding takes place. Well-annealed samples, in contrast, show no significant energy change with strain amplitude until they yield, accompanied by discontinuous energy changes that increase with the degree of annealing. Significantly, the threshold energy for both systems corresponds to dynamical cross-over temperatures associated with changes in the character of the energy landscape sampled by glass-forming liquids.

Homogenization and sodium hydrogen phosphate induced effect on physical and rheological properties of ultrafilterd concentrated milk.

The online version contains supplementary material available at 10.1007/s13197-021-05097-2.

Macroscopic Quantum Tunneling in PdO Nanoparticles

We studied the physical behavior of PdO nanoparticles at low temperatures, which presents an unusual behavior clearly related to macroscopic quantum tunneling. The samples show a tetragonal single phase with P42/mmc space group. Most importantly, the particle size was estimated at about 5.07 ± 1.97 nm. Appropriate techniques were used to determine the characteristic of these nanoparticles. The most important aspect of this study is the magnetic characterization performed at low temperatures. It shows a peak at 50 K in zero field cooling (ZFC) mode that corresponds to the blocking temperature (TB). These measurements in ZFC and field cooling (FC) indicate that the peak behavior is due to different relaxation times of the asymmetrical barriers when the electron changes from a metastable state to another. Below TB in FC mode, the magnetization decreases with temperature until 36 K; this temperature is the crossover temperature TCr related to the anisotropy of the barriers, indicative of macroscopic quantum tunneling.

Impact of scale, nuclear PDF and temperature variations on the interpretation of medium-modified jet production data from the LHC

In this paper we present a study of in-medium jet modifications performed with JEWEL and PYTHIA 6.4, focusing on the uncertainties related to variations of the perturbative scales and nuclear parton distribution functions (PDFs) and on the impact of the initial and crossover temperature variations of the medium. The simulations are compared to LHC data for the jet spectrum and the nuclear modification factor. We assess the interplay between the choice of nuclear PDFs and different medium parameters and study the impact of nuclear PDFs and the medium on the jet structure via the Lund plane.

Study on excess conductivity in YBCO + xAg composites

We study the effect of addition of Ag on normal, paracoherent and superconducting coherent state of YBa2Cu3O7-δ (YBCO). We find that both room temperature resistivity and residual resistivity decrease with the addition of Ag in YBCO. In absence of magnetic field, the addition of Ag in YBCO shift various crossover temperatures of paracoherent state and 3D fluctuations dominate the mean-field region. However, the paracoherent state remains nearly unaffected in presence of 12 T magnetic field and 2D superconducting fluctuations dominate the mean-field region. The coherent state of YBCO improves by the addition of Ag in absence as well as in presence of 12 T magnetic field.

Effects of water removal on the structure and spin-crossover in an anilato-based compound

The crucial role played by a crystallization water molecule in the spin crossover (SCO) temperature and its hysteresis is described and discussed in compound [NBu4][Fe(bpp)2][Cr(C6O4Br2)3]⋅2.5H2O (1), where bpp = 2,6-bis(pyrazol-3-yl)pyridine and (C6O4Br2)2− = dianion of the 3,6-dibromo-2,5-dihydroxy-1,4-benzoquinone. The compound has isolated [Fe(bpp)2]2+ cations surrounded by chiral [Cr(C6O4Br2)3]3− anions, NBu4+ cations, and a water molecule H-bonded to one of the non-coordinated N–H groups of one bpp ligand. This complex shows a gradual almost complete two-step spin transition centered at ca. 180 and 100 K with no hysteresis. The loss of the water molecules results in a phase transition from a P21/n phase with only one independent [Fe(bpp)2]2+ cation to a chiral Pn phase with two independent [Fe(bpp)2]2+ cations. Besides, there is an increase in the SCO temperature to 195/202 K with a hysteresis of ca. 7 K. In the dehydrated phase, only one of the two independent [Fe(bpp)2]2+ cations shows the SCO, whereas the second one remains in a high spin configuration at any temperature. In addition, compound 1 exhibits the LIESST (light-induced excited spin-state trapping) effect with a TLIESST of ca. 70 K.

Direct Numerical Simulation of hydrogen combustion at auto-ignitive conditions: Ignition, stability and turbulent reaction-front velocity

Direct Numerical Simulations (DNS) are performed to investigate the process of spontaneous ignition of hydrogen flames at laminar, turbulent, adiabatic and non-adiabatic conditions. Mixtures of hydrogen and vitiated air at temperatures representing gas-turbine reheat combustion are considered. Adiabatic spontaneous ignition processes are investigated first, providing a quantitative characterization of stable and unstable flames. Results indicate that, in hydrogen reheat combustion, compressibility effects play a key role in flame stability and that unstable ignition and combustion are consistently encountered for reactant temperatures close to the mixture’s characteristic crossover temperature. Furthermore, it is also found that the characterization of the adiabatic processes is also valid in the presence of non-adiabaticity due to wall heat-loss. Finally, a quantitative characterization of the instantaneous fuel consumption rate within the reaction front is obtained and of its ability, at auto-ignitive conditions, to advance against the approaching turbulent flow of the reactants, for a range of different turbulence intensities, temperatures and pressure levels.

Direct observation of kink evolution due to Hund\u2019s coupling on approach to metal-insulator transition in NiS2\u2212xSex

Understanding characteristic energy scales is a fundamentally important issue in the study of strongly correlated systems. In multiband systems, an energy scale is affected not only by the effective Coulomb interaction but also by the Hund’s coupling. Direct observation of such energy scale has been elusive so far in spite of extensive studies. Here, we report the observation of a kink structure in the low energy dispersion of NiS2−xSex and its characteristic evolution with x, by using angle resolved photoemission spectroscopy. Dynamical mean field theory calculation combined with density functional theory confirms that this kink originates from Hund’s coupling. We find that the abrupt deviation from the Fermi liquid behavior in the electron self-energy results in the kink feature at low energy scale and that the kink is directly related to the coherence-incoherence crossover temperature scale. Our results mark the direct observation of the evolution of the characteristic temperature scale via kink features in the spectral function, which is the hallmark of Hund’s physics in the multiorbital system.

Stabilizing or Destabilizing: Simulations of Chymotrypsin Inhibitor 2 under Crowding Reveal Existence of a Crossover Temperature.

The effect of macromolecular crowding on the stability of proteins can change with temperature. This dependence might reveal a delicate balance between two factors: the entropic excluded volume and the stability-modulating quinary interactions. Here we computationally investigate the thermal stability of the native state of chymotrypsin inhibitor 2 (CI2), which was previously shown by experiments to be destabilized by protein crowders at room temperature. Mimicking experimental conditions, our enhanced-sampling atomistic simulations of CI2 surrounded by lysozyme and bovine serum albumin reproduce this destabilization but also provide evidence of a crossover temperature above which lysozyme is found to become stabilizing, as previously predicted by analysis of thermodynamic data. We relate this crossover to the different CI2-crowder interactions and the local packing experienced by CI2. In fact, we clearly show that the pronounced stabilization induced by lysozyme at high temperatures stems from the tight local packing created around CI2 by this smaller crowder.

Hund's superconductor Li(Fe,Co)As

We combine transport, angle-resolved photoemission spectroscopy, and scanning tunneling spectroscopy to investigate several low-energy manifestations of the Hund's coupling in a canonical FeSC family Li(Fe,Co)As. We determine the doping dependence of the coherent-incoherent crossover temperature and the quasiparticle effective mass enhancement in the normal state. Our tunneling spectroscopy result in the superconducting state supports the idea that superconductivity emerging from Hund's metal state displays a universal maximal superconducting gap vs transition temperature ($2{\mathrm{\ensuremath{\Delta}}}_{\mathrm{max}}/{k}_{B}{T}_{c}$) value, which is independent of doping level and ${T}_{c}$.