Crossover Temperature Research Articles

Effect of heavy impurities on the dynamics of supercooled liquids.

We study the effect of heavy impurities on the dynamics of supercooled liquids. In a supercooled liquid, when we make a small fraction of particles heavier, they exhibit slower dynamics than the original particles and also make the overall system slower. If one looks at the overlap correlation function to quantify dynamics in the system, it has different behaviors for the heavy and the light particles. In particular, at the relaxation time of the overall system, the degree of relaxation achieved by the heavier particles is lesser, on average, than that achieved by the lighter particles. This difference in relaxation, however, goes down drastically as a crossover temperature, T0, is crossed. Below this crossover temperature, particles in the system have similar relaxation times irrespective of their masses. This crossover temperature depends on the fraction of the heavy particles and their masses. Next, we isolate the effect of mass heterogeneity on the dynamics of supercooled liquids and find that its effect increases monotonically with temperature. We also see that the development of dynamical heterogeneity with decreasing temperature is less dramatic for the system with impurities than for the pure system. Finally, the introduction of heavy impurities can be seen as a way of reducing the kinetic fragility of a supercooled liquid.

Frustration effects at finite temperature in the half filled Hubbard model

We investigate the finite temperature properties of the half filled Hubbard model in two dimensions, with onsite interaction (U), in presence of (frustrating) next nearest neighbor hopping (t′) using a semiclassical approximation scheme. We show that introduction of t′ results in a finite temperature pseudogapped (PG) phase that separates the small U Fermi liquid and large U Mott insulator. We map out the PG to normal metal crossover temperature scale (T*) as a function of U and t′. We demonstrate that in the PG phase, the quadratic dependence of resistivity on temperature is violated due to thermally induced spin fluctuations. We conclude with exact diagonalization calculations, that complement our finite temperature results, and indicate the presence of a frustration driven PG state between the Fermi liquid and the Mott insulator at zero temperature as well.

Comparison of four different PC- SAFT versions to predict liquid-liquid equilibria of polymer and asphaltene systems at low temperatures

In this work, a simple temperature- and density-effective diameter expression recently published in the literature was combined with a new set of universal constants for the perturbed chain-statistical association fluid theory equation of state (PC- SAFT EoS). Then, the resulting PC- SAFT version was used to perform thermodynamic calculations of Brown's first- order ideal curves, phase behavior for simple hydrocarbon binary mixtures (critical loci), cloud-points (CP) for ternary polymeric mixtures, and asphaltene onset pressures (AOPs) for ill- and well-defined hydrocarbon mixtures prone to precipitate asphaltenes. In addition, the new PC- SAFT version was used to predict the crossover temperatures of one of the ill-defined petroleum mixtures under the injection of different amounts of CO2. This modified PC- SAFT model predicted both AOPs and CPs lower than those found by using the original universal constants by Gross and Sadowski (GS). The predictive calculations with both GS and Liang- Kontogeorgis (LK) universal constants produced as a result that a temperature- and density-dependent effective diameter significantly modifies the predictions for dense and complex systems, while does not generate an important impact on the calculation of properties of systems made-up by simple molecules under ordinary conditions.

Thermoelectric power of Sachdev-Ye-Kitaev islands: Probing Bekenstein-Hawking entropy in quantum matter experiments

The Sachdev-Ye-Kitaev (SYK) model describes electrons with random and all-to-all interactions, and realizes a many-body state without quasiparticle excitations, and a non-vanishing extensive entropy $S_0$ in the zero temperature limit. Its low energy theory coincides the low energy theory of the near-extremal charged black holes with Bekenstein-Hawking entropy $S_0$. Several mesoscopic experimental configurations realizing SYK quantum dynamics over a significant intermediate temperature scale have been proposed. We investigate quantum thermoelectric transport in such configurations, and describe the low temperature crossovers out of SYK criticality into regimes with either Fermi liquid behavior, a Coulomb blockade, or criticality associated with Schwarzian quantum gravity fluctuations. Our results show that thermopower measurements can serve as a direct probe for $S_0$.

Surface phase transitions and crystal habits of ice in the atmosphere.

With climate modeling predicting a raise of at least 2°C by year 2100, the fate of ice has become a serious concern, but we still do not understand how ice grows (or melts). In the atmosphere, crystal growth rates of basal and prism facets exhibit an enigmatic temperature dependence and crossover up to three times in a range between 0° and -40°. Here, we use large-scale computer simulations to characterize the ice surface and identify a sequence of previously unidentified phase transitions on the main facets of ice crystallites. Unexpectedly, we find that as temperature is increased, the crystal surface transforms from a disordered phase with proliferation of steps to a smooth phase with small step density. This causes the anomalous increase of step free energies and provides the long sought explanation for the enigmatic crossover of snow crystal growth rates found in the atmosphere.

Dynamical Transitions of Supercooled Water in Graphene Oxide Nanopores: Influence of Surface Hydrophilicity.

Molecular dynamics simulations are carried out to explore the dynamical crossover phenomenon in strongly confined and mildly supercooled water in graphene oxide nanopores. Since the extent of hydrophilicity can be varied on graphene oxide surfaces, they offer energetically heterogeneous environments that can potentially modulate the rotational and translational relaxation dynamics of confined water. The influence of the physicochemical nature of the graphene oxide surface on the dynamical transitions is investigated by varying the extent of hydrophobicity on the confining surfaces placed at an intersurface separation of 10 Å. Water forms two distinct layers in contact with the graphene oxide surface at this separation. All dynamical quantities show a typical slowing down as the temperature is lowered from 298 to 200 K; however, the nature of the transition is a distinct function of the surface type. Water confined between surfaces consisting of alternating hydrophilic and hydrophobic regions exhibit a strong-to-strong dynamical transition in the diffusion coefficients and rotational relaxation times at a crossover temperature of 237 K and show a fragile-to-strong transition in the α-relaxation time at 238 K. The observed crossover temperature is higher than the freezing point of the SPC/E water model used in this study, indicating that these dynamical transitions can occur with mild supercooling under strong confinement in the absence of bulk-like water. In contrast, water confined in a hydrophilic nanopore shows a single Arrhenius energy barrier over the entire temperature range. Our results indicate that in addition to confinement, the nature of the surface can play a critical role in determining the dynamical transitions for water upon supercooling.

Characteristics of reactivity controlled combustion with n-heptane low temperature reforming products

This paper presents a comprehensive study of the effects of n-heptane low temperature reforming (LTR) products on n-heptane/LTR products ignition characteristics by two-dimensional direct numerical simulation (2-D DNS) and zero-dimensional (0-D) reactor under advanced compression ignition engine-like conditions. N-heptane/LTR products reactivity controlled compression ignition is a concept based on “single-fuel” reactivity controlled compression ignition. Two reforming gas compositions are obtained through a reforming-cooling combined process, and the parent fuel (n-heptane) conversion rates are 66.0% and 85.6%, respectively. LTR products are found to retard or promote the ignition depending on the initial mixture composition and temperature in both 2-D DNS and 0-D reactor. Particularly, 0-D results show that LTR products will suppress the ignition event at low initial temperature, while with more n-heptane addition, LTR products will help to shorten the ignition delay time. Moreover, the Negative Temperature Coefficient (NTC) behavior is weakened with LTR products, and this phenomenon is more obvious with higher degree LTR products. Finally, based on the chemical reaction pathway analysis, it is found that the onset of low temperature ignition is advanced in the presence of LTR products, but it may delay the high temperature ignition (HTI). The basic reason of LTR products effect on HTI attributes to heat accumulation in the early phase. This heat accumulation depends on not only the production of active radicals before the crossover temperature around 1000 K, but also the competition of active radicals between LTR products and n-heptane.

Onsite density matrix description of the extended Falicov-Kimball model at finite temperatures

We propose a single-site mean-field description, an analogue of Weiss mean-field theory, suitable for narrow-band systems with correlation-induced hybridisation at finite temperatures. Presently this approach, based on the notion of a fluctuating on-site density matrix (OSDM), is developed for the case of extended Falicov-Kimball model (EFKM). In an EFKM, an excitonic insulator phase can be stabilised at zero temperature. With increasing temperature, the excitonic order parameter (interaction-induced hybridisation on-site, characterised by the absolute value and phase) eventually becomes disordered, which involves fluctuations of both its phase and (at higher T) its absolute value. In order to build an adequate finite-temperature description, it is important to clarify the nature of degrees of freedom associated with the phase and absolute value of the induced hybridisation, and correctly account for the corresponding phase-space volume. We show that the OSDM-based treatment of the local fluctuations indeed provides an intuitive and concise description (including the phase-space integration measure). This allows to describe both the lower-temperature regime where phase fluctuations destroy the long-range order, and the higher temperature crossover corresponding to a decrease of the absolute value of hybridisation. In spite of the rapid progress in the studies of excitonic insulators, a unified picture of this kind has not been available to date. We briefly discuss recent experiments on ${\rm Ta_2 Ni Se_5 }$ and also address the amplitude mode of collective excitations in relation to the measurements reported for 1T--${\rm TiSe_2}$. Both the overall scenario and the theoretical framework are also expected to be relevant in other contexts, including the Kondo lattice model.

Radius of convergence in lattice QCD at finite μB with rooted staggered fermions

In typical statistical mechanical systems the grand canonical partition function at finite volume is proportional to a polynomial of the fugacity $e^{\mu/T}$. The zero of this Lee-Yang polynomial closest to the origin determines the radius of convergence of the Taylor expansion of the pressure around $\mu=0$. The computationally cheapest formulation of lattice QCD, rooted staggered fermions, with the usual definition of the rooted determinant, does not admit such a Lee-Yang polynomial. We argue that the radius of convergence is then bounded by the spectral gap of the reduced matrix of the unrooted staggered operator. This is a cutoff effect that potentially affects all estimates of the radius of convergence with the standard staggered rooting. We suggest a new definition of the rooted staggered determinant at finite chemical potential that allows for a definition of a Lee-Yang polynomial, and, therefore of the numerical study of Lee-Yang zeros. We also describe an algorithm to determine the Lee-Yang zeros and apply it to configurations generated with the 2-stout improved staggered action at $N_t = 4$. We perform a finite-volume scaling study of the leading Lee-Yang zeros and estimate the radius of convergence of the Taylor expansion extrapolated to an infinite volume. We show that the limiting singularity is not on the real line, thus giving a lower bound on the location of any possible phase transitions at this lattice spacing. In the vicinity of the crossover temperature at zero chemical potential, the radius of convergence turns out to be $\mu_B/T \approx 2$ and roughly temperature independent. Our simulations are performed at strange quark chemical potential $\mu_s=0$, but the method can be straightforwardly extended to strangeness chemical potential $\mu_S=0$ or strangeness neutrality.

Near-limit H2-O2-N2 combustion in nonpremixed counterflow mixing layers

Numerical computations employing the relevant 9-step detailed chemistry are used to characterize the different combustion modes emerging in mixing layers separating nitrogen-diluted counterflowing planar streams of hydrogen and oxygen. Attention is focused on high degrees of dilution, resulting in near-limit flames, with peak temperatures close to the crossover temperature. A bifurcation diagram is presented in a plane, having the stoichiometric mixture fraction and normalized strain rate as coordinates, that identifies six different combustion regimes involving four different flame types, namely, diffusion-flame sheets, advancing and retreating edge flames, multiple flame tubes, and single isolated flame tubes. Multiple-tube flame configurations vary from small, round, widely separated flame strings at high strain rates to wide, flat, densely packed flame strips, with narrow flame-free gaps between them, at lower strain rates, and they are steady and stable in various arrays over a continuum of tube-separation distances. The observed flame behavior exhibits hysteresis in a certain range of parameters, with the structure that is established depending on the ignition mechanism, as it also does at high strain rates, and a continuum of different stable steady-state flame configurations exists, each accessed from a different initial condition.

Influence of thermal oxidative aging on temperature induced self-healing transition of polymer modified bitumens

In this study, the fatigue-healing-fatigue (FHF) test is conducted to investigate influence pattern of temperature and thermal oxidative aging on the self-healing ability of styrene-butadiene-styrene (SBS) modified bitumen (SBS-MB) and polyethylene (PE) modified bitumen (PE-MB). Results display that the optimum healing temperature (OHT) of each bitumen (wherever unaged or aged) is in the phase transition (PT) zone, and is lower than its softening point, followed by its Newtonian flow temperature. Two types of healing crossover temperature points are found in the healing-temperature curves of polymer modified bitumen (PMB) samples with various aging degrees, indicating the complexity of self-healing transition phenomenon in the aged PMB samples. Hence, the evaluation of aging effects on the self-healing ability of PMBs should be performed in a wide temperature range. Furthermore, the self-healing ability of PMBs is gradually weakened with the deepening of aging extents. Notably, the thermal oxidative aging imparts totally different effects to the self-healing ability of SBS-MB and PE-MB at lower temperatures. Moreover, the results of softening point test, fluorescence microscopy test, frequency sweep test and differential scanning calorimeter test convey that the altered phase structure and polymer distribution of PMBs during aging elicit the healing-temperature curves shift and self-healing transition.

One-dimensional few-electron effective Wigner crystal in quantum and classical regimes

A system of confined charged electrons interacting via the long-range Coulomb force can form a Wigner crystal due to their mutual repulsion. This happens when the potential energy of the system dominates over its kinetic energy, i.e., at low temperatures for a classical system and at low densities for a quantum one. At $T=0$, the system is governed by quantum mechanics, and hence, the spatial density peaks associated with crystalline charge localization are sharpened for a lower average density. Conversely, in the classical limit of high temperatures, the crystalline spatial density peaks are suppressed (recovered) at a lower (higher) average density. In this paper, we study those two limits separately using an exact diagonalization of small one-dimensional (1D) systems containing few ($<10$) electrons and propose an approximate method to connect them into a unified effective phase diagram for Wigner few-electron crystallization. The result is a qualitative quantum-classical crossover phase diagram of an effective 1D Wigner crystal. We show that the spatial density peaks associated with the quasi-crystallization should be experimentally observable in a few-electron 1D system. We find that the effective crystalline structure slowly disappears with both the crossover average density and crossover temperature for crystallization decreasing with increasing particle number, consistent with the absence of any true long-range 1D order. In fact, one peculiar aspect of the effective finite-size nature of 1D Wigner crystallization we find is that even a short-range interaction would lead to a finite-size 1D crystal, except that the crystalline order vanishes much faster with increasing system size in the short-range interacting system compared with the long-range interacting one.

Electron irradiation and annealing effects on the pseudogap in optimally doped YBCO single crystals

The effect of room-temperature annealing on the basal-plane conductivity of YBCO single crystals is investigated after their irradiation with fast electrons. It is revealed that the excess conductivity [Formula: see text] of the samples obeys an exponential temperature dependence in a broad temperature range [Formula: see text]. Here, [Formula: see text] is the crossover temperature from the pseudogap to the fluctuation conductivity regime. The description of the excess conductivity with the expression [Formula: see text] can be interpreted in terms of the mean-field theory where [Formula: see text] corresponds to the mean-field transition temperature to the pseudogap state and the dependence [Formula: see text] is described within the framework of the BCS-BEC crossover theory.

Quantum Depinning of a Magnetic Skyrmion.

We investigate the quantum depinning of a weakly driven skyrmion out of an impurity potential in a mesoscopic magnetic insulator. For small barrier height, the Magnus force dynamics dominates over the inertial term, and the problem is reduced to a massless charged particle in a strong magnetic field. The universal form of the WKB exponent, the rate of tunneling, and the crossover temperature between thermal and quantum tunneling are provided, independently of the detailed form of the pinning potential. The results are discussed in terms of macroscopic parameters of the insulator Cu_{2}OSeO_{3} and various skyrmion radii. We demonstrate that small enough magnetic skyrmions, with a radius of ∼10 lattice sites, consisting of some thousands of spins, can behave as quantum objects at low temperatures in the millikelvin regime.

Mean-field interaction-induced dimensional crossover from two to three dimensions in a weakly interacting Bose gas

We study an interaction-induced dimensional crossover from two to three dimensions in a weakly interacting $^{87}\mathrm{Rb}$ Bose-Einstein condensate (BEC). A highly oblate harmonic optical dipole trap was used where we independently controlled the chemical potential and the temperature of the gas. The dimensional crossover was studied from the thermal state occupancy of atoms. In particular, the growth rate of the thermal component was measured, which contradicted the expected behavior in a fixed dimension. Crossover chemical potential and temperature were obtained at the crossover point. We further validate our observation by numerical calculations based on mean-field Hartree-Fock theory and local density approximation, which not only provided a qualitative picture of the underlying mechanism for the dimensional crossover, but also matched well with the experimentally measured crossover parameter.

Approaching itinerant magnetic quantum criticality through a Hund's coupling induced electronic crossover in the YFe2Ge2 superconductor

Here, by conducting a systematic $^{89}$Y NMR study, we explore the nature of the magnetic ground state in a newly discovered iron-based superconductor YFe$_2$Ge$_2$. An incoherent-to-coherent crossover due to the Hund's coupling induced electronic correlation is revealed below the crossover temperature $T^*\sim 75\pm15\,\mathrm{K}$. During the electronic crossover, both the Knight shift ($K$) and the bulk magnetic susceptibility ($\chi$) exhibit a similar nonmonotonic temperature dependence, and a so-called Knight shift anomaly is also revealed by a careful $K$-$\chi$ analysis. Such an electronic crossover has been also observed in heavily hole-doped pnictide superconductors \emph{A}Fe$_2$As$_2$ (\emph{A} = K, Rb, and Cs), which is ascribed to the Hund's coupling induced electronic correlation. Below $T^*$, the spin-lattice relaxation rate divided by temperature $(1/T_1T)$ shows a similar suppression as the Knight shift, suggesting the absence of critical spin fluctuations. This seems to be in conflict with a predicted magnetic quantum critical point (QCP) near this system. However, considering a $\mathbf{q}$-dependent "filter" effect on the transferred hyperfine field, a predominant spin fluctuation with A-type correlation would be perfectly filtered out at $^{89}$Y sites, which is consistent with the recent inelastic neutron scattering results. Therefore, our results confirm that, through a Hund's coupling induced electronic crossover, the magnetic ground state of YFe$_2$Ge$_2$ becomes close to an itinerant magnetic QCP with A-type spin fluctuations. In addition, the possible superconducting pairing due to spin fluctuations is also discussed.

Strange Metal in Magic-Angle Graphene with near Planckian Dissipation.

Recent experiments on magic-angle twisted bilayer graphene have discovered correlated insulating behavior and superconductivity at a fractional filling of an isolated narrow band. Here we show that magic-angle bilayer graphene exhibits another hallmark of strongly correlated systems-a broad regime of T-linear resistivity above a small density-dependent crossover temperature-for a range of fillings near the correlated insulator. This behavior is reminiscent of similar behavior in other strongly correlated systems, often denoted "strange metals," such as cuprates, iron pnictides, ruthenates, and cobaltates, where the observations are at odds with expectations in a weakly interacting Fermi liquid. We also extract a transport "scattering rate," which satisfies a near Planckian form that is universally related to the ratio of (k_{B}T/ℏ). Our results establish magic-angle bilayer graphene as a highly tunable platform to investigate strange metal behavior, which could shed light on this mysterious ubiquitous phase of correlated matter.

MoRe/YBCO Josephson junctions and π-loops

We have developed Josephson junctions between the d-wave superconductor YBa2Cu3O7−x (YBCO) and the s-wave Mo0.6Re0.4 (MoRe) alloy superconductor (ds-JJs). Such ds Josephson junctions are of interest for superconducting electronics making use of incorporated π-phase shifts. The I(V)-characteristics of the ds-JJs demonstrate a twice larger critical current along the [100] axis of the YBCO film compared to similarly-oriented ds-JJs made with a Nb top electrode. The characteristic voltage IcRn of the YBCO–Au–MoRe ds-JJs is 750 μV at 4.2 K. The ds-JJs that are oriented along the [100] or [010] axes of the YBCO film exhibit a 200 times higher critical current than similar ds-JJs oriented along the [110] axis of the same YBCO film. A critical current density Jc = 20 kA cm−2 at 4.2 K was achieved. Different layouts of π-loops based on the novel ds-JJs were arranged in various mutual coupling configurations. Spontaneous persistent currents in the π-loops were investigated using scanning SQUID microscopy. Magnetic states of the π-loops were manipulated by currents in integrated bias lines. Higher flux states up to ±2.5Φ0 were induced and stabilized in the π-loops. Crossover temperatures between thermally activated and quantum tunneling switching processes in the ds-JJs were estimated. The demonstrated ability to stabilise and manipulate states of π-loops paves the way towards new computing concepts such as quantum annealing computing.

The diffusion, structural relaxation, and fragility of [VIO2+][Tf2N-]2 ionic liquid.

Viologen-based ionic liquids can form various microscopic structures at different temperatures. In this work, the dynamics of dimethyl-viologen bis-(tetrafluoroborate) ([VIO2+][Tf2N-]2) ionic liquid within 400-800K has been exploited by molecular dynamics simulations. [VIO2+][Tf2N-]2 exhibits a supercooled liquid analogous diffusion and structural relaxation even at temperature T = 400K, and behaves more like simple liquids as temperature increases. The variation of the diffusion constant and structural relaxation time with temperature follows a super-Arrhenius law; both can be fitted by two Arrhenius laws with a crossover temperature or fitted by a VFT law. [VIO2+][Tf2N-]2 behaves as a fragile glass former. The decoupling of diffusion and relaxation is observed in [VIO2+] but not in [Tf2N-]. The variation of dynamics with temperature is attributed to the time differences of the persistence in ion cage and exchange out of ion cage.

Energy-level quantization and single-photon control of phase slips in YBa2Cu3O7\u2013x nanowires

Significant progress has been made in superconducting quantum circuits. However new quantum devices that have longer decoherence times at higher temperatures are urgently required for quantum technologies. Superconducting nanowires with quantum phase slips are promising candidates for use in novel quantum devices. Here, we demonstrate YBa2Cu3O7-x nanowires with phase-slip dynamics and study their switching-current statistics at temperatures below 20 K. We apply theoretical models developed for Josephson junctions and show that our results provide strong evidence for energy-level quantization in the nanowires. The crossover temperature to the quantum regime of 12–13 K and the lifetime in the excited state exceeding 20 ms at 5.4 K are superior to those in conventional Josephson junctions. We also show how the absorption of a single photon changes the phase-slip and quantum state of a nanowire, which is important for the development of single-photon detectors with high operating temperature and superior temporal resolution.