Phase Transition Point Research Articles

Time- and frequency-domain two-particle correlations of a driven dissipative Bose-Hubbard model

We investigate theoretically the time- and frequency-domain two-particle correlations of a driven-dissipative Bose-Hubbard model at and near a dissipative phase transition (DPT). We compute the Hanbury Brown--Twiss (HBT)-type two-particle temporal correlation function ${g}^{2}(\ensuremath{\tau})$ which, as a function of time delay $\ensuremath{\tau}$, exhibits oscillations with frequencies determined by the imaginary part of the Liouvillian gap. As the gap closes near a transition point, the oscillations at that point die down. For parameters slightly away from the transition point, the HBT correlations show oscillations from superbunching to antibunching regimes. We show that the Fourier transform of HBT correlations into the frequency domain provide information about DPT and Liouvillian dynamics. We numerically solve the many-body Lindblad master equation and calculate the Wigner distribution of the system in the steady state to ascertain the DPT. Below a certain drive strength, the Fourier transform shows a two-peak structure, while above that strength it exhibits either a Lorentzian-like single-peak structure or a structure with two dips. The width of the single-peak structure is minimal at the phase-transition point and the peak of this structure always lies at zero frequency. The positions of the two symmetrical peaks in case of a two-peak structure are given by the imaginary parts of the Liouvillian gap while their half width at half maximum is given by the real part of the gap. The positions and widths of the two dips are also related to low-lying eigenvalues of the Liouvillian operator. We discuss quantum statistical properties of the model in terms of the HBT correlation function and its Fourier transform.

Weak mixed phase in the mutator model.

We consider the mutator model with unidirected transitions from the wild type to the mutator type, with different fitness functions for the wild types and mutator types. We calculate both the fraction of mutator types in the population and the surpluses, i.e., the mean number of mutations in the regular part of genomes for the wild type and mutator type, which have never been derived exactly. We identify the phase structure. Beside the mixed (ordinary evolution phase with finite fraction of wild types at large genome length) and the mutator phase (the absolute majority is mutators), we find another new phase as well-it has the mean fitness of the mixed phase but an exponentially small (in genome length) fraction of wild types. We identify the phase transition point and discuss its implications.

Evolution and universality of two-stage Kondo effect in single manganese phthalocyanine molecule transistors

The Kondo effect offers an important paradigm to understand strongly correlated many-body physics. Although under intensive study, some of the important properties of the Kondo effect, in systems where both itinerant coupling and localized coupling play significant roles, are still elusive. Here we report the evolution and universality of the two-stage Kondo effect, the simplest form where both couplings are important using single molecule transistor devices incorporating Manganese phthalocyanine molecules. The Kondo temperature T* of the two-stage Kondo effect evolves linearly against effective interaction of involved two spins. Observed Kondo resonance shows universal quadratic dependence with all adjustable parameters: temperature, magnetic field and biased voltages. The difference in nonequilibrium conductance of two-stage Kondo effect to spin 1/2 Kondo effect is also identified. Messages learned in this study fill in directive experimental evidence of the evolution of two-stage Kondo resonance near a quantum phase transition point, and help in understanding sophisticated molecular electron spectroscopy in a strong correlation regime.

An Instant Change of Elastic Lattice Strain during Cu2Se Phase Transition: Origin of Abnormal Thermoelectric Properties

AbstractThe superionic conductor Cu2Se is a promising thermoelectric material due to its low thermal conductivity. An abnormal but clear change in the thermoelectric parameters has been observed during the phase transformation from the ordered and non‐cubic α‐Cu2Se to the disordered and cubic β‐Cu2Se. However, the microstructural origin of the abnormal change and its implications for thermoelectric applications remain largely unknown. Herein, by mimicking the real working conditions of thermoelectrics, the phase transition from α‐ to β‐Cu2Se induced by the rising temperature has been carefully investigated by in situ transmission electron microscopy. It is observed that an abrupt and anisotropic volume‐change in the Se‐sublattice occurs when the temperature is raised to the phase transition point. The abnormal change in the crystalline volume versus temperature, which is caused by the local migration of Cu‐ions, induces an instant and uncommon strain‐field, which reduces the carrier's mobility and increases the electrical resistance. Local migration of Cu‐ions is responsible for a quite low thermal conductivity. Such effects exist only at the instance of the phase transition. Observing the thermoelectric response of the structure during the phase transition may provide insights into the development of high performance thermoelectric materials, which fall beyond the traditional approaches.

Phase Transitions and Hysteresis for a Simple Model Liquid Crystal by Replica-Exchange Monte Carlo Simulations.

In this work, the advantages of applying the temperature and pressure replica-exchange method to investigate the phase transitions and the hysteresis for liquid-crystal fluids were demonstrated. In applying this method to the commonly used Hess–Su liquid-crystal model, heat capacity peaks and points of phase co-existence were observed. The absence of a smectic phase at higher densities and a narrow range of the nematic phase were reported. The identity of the crystalline phase of this system was found to a hexagonal close-packed solid. Since the nematic-solid phase transition is strongly first order, care must be taken when using this model not to inadvertently simulate meta-stable nematic states at higher densities. In further analysis, the Weighted Histogram Analysis Method was applied to verify the precise locations of the phase transition points.

Square-root non-Bloch topological insulators in non-Hermitian ring resonators.

We investigate the topological skin effect in a ring resonator array which can be mapped into the square root of a Su-Schrieffer-Heeger (SSH) model with non-Hermitian asymmetric coupling. The asymmetric coupling is realized by integrating the same amount of gain and loss into the two half perimeters of linking rings that effectively couple two adjacent site rings. Such a square-root topological insulator inherits the properties from its parent Hamiltonian, which has the same phase transition points and exhibits non-Bloch features as well. We show the band closing points for open chain are different from that of periodic chain as a result of the skin effect. Moreover, the square-root insulator supports multiple topological edge modes as the number of band gaps is doubled compared to the original Hamiltonian. The full-wave simulations agree well with the theoretical analyses based on a tight-binding model. The study provides a promising approach to investigate the skin effect by utilizing ring resonators and may find potential applications in light trapping, lasers, and filters.

Self-organized criticality in neural networks from activity-based rewiring.

Neural systems process information in a dynamical regime between silence and chaotic dynamics. This has lead to the criticality hypothesis, which suggests that neural systems reach such a state by self-organizing toward the critical point of a dynamical phase transition. Here, we study a minimal neural network model that exhibits self-organized criticality in the presence of stochastic noise using a rewiring rule which only utilizes local information. For network evolution, incoming links are added to a node or deleted, depending on the node's average activity. Based on this rewiring-rule only, the network evolves toward a critical state, showing typical power-law-distributed avalanche statistics. The observed exponents are in accord with criticality as predicted by dynamical scaling theory, as well as with the observed exponents of neural avalanches. The critical state of the model is reached autonomously without the need for parameter tuning, is independent of initial conditions, is robust under stochastic noise, and independent of details of the implementation as different variants of the model indicate. We argue that this supports the hypothesis that real neural systems may utilize such a mechanism to self-organize toward criticality, especially during early developmental stages.

String order parameters for symmetry fractionalization in an enriched toric code

We study a simple model of symmetry-enriched topological order obtained by decorating a toric code model with lower-dimensional symmetry-protected topological states. We show that the symmetry fractionalization in this model can be characterized by string order parameters, and that these signatures are robust under the effects of external fields and interactions, up to the phase transition point. This extends the recent proposal of [New Journal of Physics 21, 113016 (2019)] beyond the setting of fixed-point tensor network states, and solidifies string order parameters as a useful tool to characterize and detect symmetry fractionalization. In addition to this, we observe how the condensation of an anyon that fractionalizes a symmetry forces that symmetry to spontaneously break, and we give a proof of this in the framework of projected entangled pair states. This phenomenon leads to a notable change in the phase diagram of the toric code in parallel magnetic fields.

High temperature record for lithium-ion battery with PCM: design and simulation validation

Under abuse conditions, the temperature of lithium-ion battery increases rapidly and brings high safety risk. To monitor the high temperature incident of battery with a simple and reliable method, a novel design scheme was presented and validated by thermal numerical calculation, in which phase change material (PCM) block is bonded with the battery shell and gets melt down when the battery temperature exceeds its phase-transition point. The melting process is irrecoverable and can be perceived by easy visual observation. Computation results show that the temperature rising of the PCM block gets slower when it meets the melting point, but it generally increases synchronously with the shell of the battery and can be used to reflect the temperature rising of the inner cell. Thus the design scheme is valid, and may possess a good applied perspective in the after-sales maintenance and second-hand transactions of lithium-ion battery.

Perturbed thermodynamics of charged black hole solution in Rastall theory

We study the thermodynamics of a charged black hole surrounded by a perfect fluid in Rastall theory and investigate three different cases of quintessence, dust and radiation fields. By considering thermal fluctuations, we study the corrected thermodynamics of the system. We investigate the effects of thermal fluctuations on the black hole stability, phase transition and critical points. We show that the thermal fluctuations increase instability of the black hole and may yield to the second order phase transition. We also compare our result with uncharged cases to find the effects of the black hole charge on the thermodynamics quantities. We find that large black holes behave like a Van der Waals fluid, while for the small black hole where thermal fluctuations become important, there is no Van der Waals behaviour. Finally, we briefly discuss about geometric thermodynamics to obtain corrected scalar curvature.

Phase transitions in a non-Hermitian Aubry-André-Harper model

The Aubry-Andr\'e-Harper model provides a paradigmatic example of aperiodic order in a one-dimensional lattice displaying a delocalization-localization phase transition at a finite critical value ${V}_{c}$ of the quasiperiodic potential amplitude $V$. In terms of the dynamical behavior of the system, the phase transition is discontinuous when one measures the quantum diffusion exponent $\ensuremath{\delta}$ of wave-packet spreading, with $\ensuremath{\delta}=1$ in the delocalized phase $V<{V}_{c}$ (ballistic transport), $\ensuremath{\delta}\ensuremath{\simeq}1/2$ at the critical point $V={V}_{c}$ (diffusive transport), and $\ensuremath{\delta}=0$ in the localized phase $V>{V}_{c}$ (dynamical localization). However, the phase transition turns out to be smooth when one measures, as a dynamical variable, the speed $v(V)$ of excitation transport in the lattice, which is a continuous function of potential amplitude $V$ and vanishes as the localized phase is approached. Here we consider a non-Hermitian extension of the Aubry-Andr\'e-Harper model, in which hopping along the lattice is asymmetric, and show that the dynamical localization-delocalization transition is discontinuous, not only in the diffusion exponent $\ensuremath{\delta}$, but also in the speed $v$ of ballistic transport. This means that even very close to the spectral phase transition point, rather counterintuitively, ballistic transport with a finite speed is allowed in the lattice. Also, we show that the ballistic velocity can increase as $V$ is increased above zero, i.e., surprisingly, disorder in the lattice can result in an enhancement of transport.

Thermally conductive silicone composites modified by graphene-oxide aerogel beads loaded with phase change materials as efficient heat sinks

Electrically insulating phase change composite materials (PCCMs) with lightweight, good thermal management performance, high stability and high mechanical strength are strongly desired for lithium-ion battery (LIB) thermal management. This research study presents a group of PCCMs based on the integration of reduced graphene-oxide aerogel beads (rGOAB), phase change materials (PCMs) and thermally conductive silicone rubbers (SRs). To demonstrate the thermal management performance, a 3 × 3 square 18650 LIB module was inserted into a SR pack. The temperature reduction (temperature reduction of the battery cells at the hottest point in the SR pack compared with in air) reached 8.6 °C, 13.0 °C and 14.7 °C at 1C, 2C and 3C discharge rate, respectively, at a concentration of 20 wt% PCM beads in SR with thermal conductivity of 0.5 W/mK. The specific energy reduction of above-mentioned pack is only 15.8%, substantially lower than the commercial level (above 30% to 40%). The lowest specific energy reduction in SR pack with thermal conductivity of 0.3 W/mK goes down to 13.4%, indicating its remarkable lightweight and high energy density features. The effects of PCM-encapsulant type, SR matrix thermal conductivity and bead size on LIB temperature reduction were also discussed. The heat transfer mechanism between the matrixes and rGOAB/PCM was thoroughly explained. Mechanical properties and battery thermal management performance of the composites were systematically examined. The rGOAB/PCM@SR composites exhibited more than 10 times of impact strength compared with expanded graphite-based PCCMs and higher compressive toughness above the phase transition point.

Faster State Preparation across Quantum Phase Transition Assisted by Reinforcement Learning.

An energy gap develops near quantum critical point of quantum phase transition in a finite many-body (MB) system, facilitating the ground state transformation by adiabatic parameter change. In real application scenarios, however, the efficacy for such a protocol is compromised by the need to balance finite system lifetime with adiabaticity, as exemplified in a recent experiment that prepares three-mode balanced Dicke state near deterministically [Y.-Q. Zou et al., Proc. Natl. Acad. Sci. U.S.A. 115, 6381 (2018)PNASA60027-842410.1073/pnas.1715105115]. Instead of tracking the instantaneous ground state as unanimously required for most adiabatic crossing, this work reports a faster sweeping policy taking advantage of excited level dynamics. It is obtained based on deep reinforcement learning (DRL) from a multistep training scheme we develop. In the absence of loss, a fidelity ≥99% between prepared and the target Dicke state is achieved over a small fraction of the adiabatically required time. When loss is included, training is carried out according to an operational benchmark, the interferometric sensitivity of the prepared state instead of fidelity, leading to better sensitivity in about half of the previously reported time. Implemented in a Bose-Einstein condensate of ∼10^{4} ^{87}Rb atoms, the balanced three-mode Dicke state exhibiting an improved number squeezing of 13.02±0.20 dB is observed within 766ms, highlighting the potential of DRL for quantum dynamics control and quantum state preparation in interacting MB systems.

Exponentially charged dilaton black holes in rainbow gravity

The explicit form of the field equations has been obtained by varying the action of the four-dimensional Einstein-dilaton gravity in the presence of the scalar-coupled exponential nonlinear electrodynamics. The exact black hole solutions of the field equations have been obtained in an energy-dependent spherically symmetric geometry. It has been found that the solution of the scalar field equation can be obtained by combining two Liouville potentials. We have introduced three types of exponentially charged dilatonic black holes and their thermodynamic properties have been studied noting the effects of rainbow functions. The impacts of rainbow functions on the conserved and thermodynamic quantities of the new black hole solutions have been explored. Through the Smarr mass formula, it has been demonstrated that the first law of black hole thermodynamics remains valid for either of the new rainbow black hole solutions. The quantum gravitational effects on the thermodynamic phase transition or thermal stability of the black holes have been studied by use of the canonical ensemble method. By calculating the black hole heat capacity, the type-1 and type-2 phase transition points and the conditions under which the exponentially charged rainbow black holes are locally stable have been identified.

System size dependence of baryon-strangeness correlation in relativistic heavy ion collisions from a multiphase transport model

The system size dependence of baryon-strangeness (BS) correlations ($C_{BS}$) are investigated with a multiphase transport (AMPT) model for various collision systems from $\mathrm{^{10}B+^{10}B}$, $\mathrm{^{12}C+^{12}C}$, $\mathrm{^{16}O+^{16}O}$, $\mathrm{^{20}Ne+^{20}Ne}$, $\mathrm{^{40}Ca+^{40}Ca}$, $\mathrm{^{96}Zr+^{96}Zr}$, and $\mathrm{^{197}Au+^{197}Au}$ at RHIC energies $\sqrt{s_{NN}}$ of 200, 39, 27, 20, and 7.7 GeV. Both effects of hadron rescattering and a combination of different hadrons play a leading role for baryon-strangeness correlations. When the kinetic window is limited to absolute rapidity $|y|>3$, these correlations tend to be constant after the final-state interaction whatever kind of hadrons subset we chose based on the AMPT framework. The correlation is found to smoothly increase with baryon chemical potential $\mu_B$, corresponding to the collision system or energy from the quark-gluon-plasma-like phase to the hadron-gas-like phase. Besides, the influence of initial nuclear geometrical structures of $\alpha$-clustered nuclear collision systems of $\mathrm{^{12}C+^{12}C}$ as well as $\mathrm{^{16}O+^{16}O}$ collisions is discussed but the effect is found negligible. The current model studies provide baselines for searching for the signals of Quantum Chromodynamics (QCD) phase transition and critical point in heavy-ion collisions through the BS correlation.

Specifics of Determination of Orientation Magnetic Phase Transition Characteristics in Hematite Using Magnetoacoustic Methods

The measurement results of the velocity of magnetoelastic waves in hematite corresponding to their quasi-phonon branch have been presented. The dependences of this velocity (V) on the magnetic field (H) at constant uniaxial pressure P, (VP(H)), and on pressure P at constant H, (VH(P)), have been studied. In accordance with the existing theory of magnetoacoustic phenomena, dependences VP(H) and VH(P) should exhibit minima at the orientation phase transition points. These minima were found, but their coordinates on the plane (P, H) did not coincide. The reasons of non-coincidence and requirements to the measurement accuracy are discussed, which yield possibilities of its use to determine characteristics of these transitions.

Influence of pressure and temperature on the attenuation of ultrasound in the nematic phase of the liquid crystal working medium of technical devices in the vicinity of phase transition point

The dependence of the attenuation coefficient of ultrasound in the nematic phase on pressure and temperature, which characterizes the parameters of electronic devices with a liquid-crystal working medium in the vicinity of phase transition point, is studied. Liquid crystals and their mixtures with a wide temperature range of the nematic phase were researched. The analysis is performed taking into account the normal relaxation process associated with conformational transitions of the end chains, and the critical relaxation process due to the nature of this phase transition. The nature of the dependence on the pressure and temperature of the normal, critical and classical components of the attenuation coefficient of ultrasound is established.

Jumping transition in the liquid's heat capacity revealed by the scanning transitiometry

AbstractThis manuscript reports the direct observations of the clear jump transition of the specific orthobaric heat capacity to the specific isochoric heat capacities during the continuous heating of liquid samples in a closed vessel as well as the respective theory explaining such effects. The measurements have been performed with n‐heptane and toluene and the results have been thoroughly analyzed with the derived equations and compared with the values from the known literature data. The detection and accurate tracking of a jump in the heat capacity far from the standard points of phase transitions are not only of fundamental thermodynamic interest but also demonstrates future research targets and now attainable data which can be achieved by modern calorimetric technology. It has been proven that by using high‐pressure scanning calorimeter (transitiometry technology) it is possible to directly measure the specific orthobaric heat capacities and specific isochoric heat capacities accurately.

Statistical complexity is maximized close to criticality in cortical dynamics.

Complex systems are typically characterized as an intermediate situation between a complete regular structure and a random system. Brain signals can be studied as a striking example of such systems: cortical states can range from highly synchronous and ordered neuronal activity (with higher spiking variability) to desynchronized and disordered regimes (with lower spiking variability). It has been recently shown, by testing independent signatures of criticality, that a phase transition occurs in a cortical state of intermediate spiking variability. Here we use a symbolic information approach to show that, despite the monotonical increase of the Shannon entropy between ordered and disordered regimes, we can determine an intermediate state of maximum complexity based on the Jensen disequilibrium measure. More specifically, we show that statistical complexity is maximized close to criticality for cortical spiking data of urethane-anesthetized rats, as well as for a network model of excitable elements that presents a critical point of a nonequilibrium phase transition.

Electronic structure, elastic, optical and thermodynamic properties of cubic perovskite NaBaF3 with pressure effects: First-principles calculations

The electronic structure, elastic, optical and thermodynamic properties of cubic perovskite NaBaF3 under high pressure were investigated using the ab initio calculations. The calculated lattice constant (a0) and bulk modulus (B0) at 0 GPa were determined to be 4.278 Å and 53.555 GPa, respectively, which are consistent with previous reports. The elastic constants and bulk modulus at high pressure were calculated. According to Pugh’s crystal mechanical stability conditions, the phase transition point of NaBaF3 is found to be 68.8 GPa, which has not been reported by any previous paper. The band structure calculations reveal that cubic perovskite NaBaF3 is a direct band-gap material in the pressure range from 0 GPa to 60 GPa. The dielectric function ɛ(ω), absorption coefficient α(ω), reflectivity R(ω), transmittance T(ω), absorbance A(ω) and complex refractive index N(ω) at 0 GPa, 20 GPa, 40 GPa and 60 GPa were studied. It is found that the band gap increases with the increasing pressure, and the absorption spectra show blue shift. The Debye temperature at 300 K and the values of the relative volume, heat capacity, bulk modulus, thermal expansion coefficient, heat capacities, and relative Debye temperature with pressures and temperatures effect were investigated. Our results will provide theoretical guidance for the experimental investigations and industrial applications of NaBaF3 in future.