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We investigate the effects of density fluctuations on the near-ordering phase of a flock by studying the Malthusian Toner–Tu theory. Because of the birth/death process, characteristic of this Malthusian model, density fluctuations are partially suppressed. We show that unlike its incompressible counterpart, where the absence of the density fluctuations renders the ordering phase transition similar to a second-order phase transition, in the Malthusian theory density fluctuations may turn the phase from continuous to first-order. We study the model using a perturbative renormalization group approach. At one loop, we find that the renormalization group flow drives the system in an unstable region, suggesting a fluctuation-induced first-order phase transition.

Ultrafast first-order phase transitions exhibit distinct transition pathways and dynamical properties that are not accessible during quasi-equilibrium transitions. Phenomena arising at the ultrafast timescale are important for understanding the transition mechanisms and in applications using the fast switching of electronic properties or magnetism. These transitions are accompanied by nanoscale structural dynamics that have been challenging to explore by optical or electronic transport probes. Here, X-ray nanodiffraction imaging shows that the nanoscale structural dynamics arising in ultrafast phase transitions differ dramatically from the transitions under slowly varying parameters. The solid-solid phase transitions in a FeRh thin film involve concurrent structural and magnetic changes and can be sensitively probed by monitoring their diffraction signatures following femtosecond optical excitation. Time-dependent nanodiffraction maps with 100-ps temporal and 25-nm spatial resolutions reveal that the preexisting nanoscale variation in phase composition results in spatially inhomogeneous changes of phase fraction after ultrafast optical excitation. The spatial inhomogeneity leads to nanoscale temperature variations and subsequent in-plane heat transport, which are responsible for spatially distinct relaxation pathways on nanometer length scales. The spatial gradients of the phase composition and elastic strain increase upon excitation rather than exhibiting the decrease previously reported in quasi-equilibrium transformations. Long-range elastic interactions thus do not play significant roles in the ultrafast phase transition. These microscopic insights into first-order phase transitions provide routes to manipulate nanoscopic phases in functional materials on ultrafast time scales by engineering initial nanoscale phase distributions.

The pulsar timing array (PTA) collaborations have recently reported compelling evidence for a stochastic signal consistent with a gravitational-wave background. In this paper, we combine the latest data sets from NANOGrav, PPTA, and EPTA to explore cosmological interpretations for the PTA signal from first-order phase transitions, domain walls, and cosmic strings, respectively. We find that the domain wall model is strongly disfavored with the Bayes factors compared with the first-order phase transitions and cosmic strings being 1/90 and 1/189, respectively, breaking the degeneracy among these models in individual data set. We also find that: (1) a strong phase transition at temperatures below the electroweak scale is favored, and the bubble collisions make the dominant contribution to the energy density spectrum; (2) a small reconnection probability p < 1.55 × 10 −1 allowed by strings in (super)string theory is favored at the 95% confidence level, and ground-based detectors can further constrain the parameter space.

Understanding the dynamics of phase-transitions, interpretations of their experimental observations and their agreement with theoretical predictions continue to be a long-standing research interest. Here, we present detailed phase-transition dynamics of rare earth nickelates associated with its first-order metal–insulator transition. The thermal hysteresis shows absence of training effect and defies the Preisach model. A large phase-coexistence in insulating state during cooling suggests kinetically arrested glassy dynamics of the phase-transition. Experimentally derived hysteresis scaling exponent is much larger than the mean-field predicted universal value of 2/3. In the phase-coexistence region, the quench and hold measurement depicts higher stability of the metallic state compare to that of the insulating one; highlighting the manifestation of phase-coexistence via asymmetric spinodal decomposition. All these observations for nickelates are in stark contrast to the phase-transition dynamics of canonically similar vanadates but are closer to those of glasses, alloys. A substantial disagreement between the experiment and theory emphasizes the necessity to incorporate system-dependent details for the accurate interpretation of the experimental results.

In this paper, we study the nonequilibrium dynamics of the Bose-Hubbard model with the nearest-neighbor repulsion by using time-dependent Gutzwiller (GW) methods. In particular, we vary the hopping parameters in the Hamiltonian as a function of time, and investigate the dynamics of the system from the density wave (DW) to the superfluid (SF) crossing a first-order phase transition and vice versa. From the DW to SF, we find scaling laws for the correlation length and vortex density with respect to the quench time. This is a reminiscence of the Kibble-Zurek scaling for continuous phase transitions and contradicts the common expectation. We give a possible explanation for this observation. On the other hand from SF to DW, the system evolution depends on the initial SF state. When the initial state is the ground state obtained by the static GW methods, a coexisting state of the SF and DW domains forms after passing through the critical point. Coherence of the SF order parameter is lost as the system evolves. This is a phenomenon similar to the glass transition in classical systems. When the state starts from the SF with small local phase fluctuations, the system obtains a large size DW domain structure with thin domain walls.

Recently, much attention has been focused on the false vacuum islands that are flooded by an expanding ocean of true-vacuum bubbles slightly later than most of the other parts of the world. These delayed decay regions will accumulate locally larger vacuum energy density by staying in the false vacuum longer than those already transited into the true vacuum. A false vacuum island with thus acquired density contrast of a super-horizon size will evolve locally from radiation dominance to vacuum dominance, creating a local baby Universe that can be regarded effectively as a local closed Universe. If such density contrasts of super-horizon sizes can ever grow large enough to exceed the threshold of gravitational collapse, primordial black holes will form similar to those collapsing curvature perturbations on super-horizon scales induced by small-scale enhancements during inflation. If not, such density contrasts can still induce curvature perturbations potentially observable today. In this paper, we revisit and elaborate on the generations of primordial black holes and curvature perturbations from delayed-decayed false vacuum islands during asynchronous first-order phase transitions with fitting formulas convenient for future model-independent studies.

The recent NANOGrav evidence of a common-source stochastic background provides a hint to gravitational waves (GW) radiation from the Early Universe. We show that this result can be interpreted as a GW spectrum produced from first order phase transitions (FOPTs) around a temperature in the keV-MeV window. Such a class of FOPTs at temperatures much below the electroweak scale can be naturally envisaged in several warm dark matter models such as Majoron dark matter.

Here, we report the results of qualitative and quantitative investigations of the first-order phase transition in the ionic liquid 1-ethylpyridinium triflate exhibiting a high variability of temperature ranges, within which the freezing and melting occur. By two methods, the direct fast quenching/annealing and the slow temperature-controlled differential scanning calorimeter, it is revealed that despite the almost constant absolute enthalpies of phase transition, the freezing occurs faster with the larger temperature contrast (cooling rate) between the initially hotter sample and the colder surrounding. This feature is a clear exhibition of the Mpemba effect. The regularity in the change of the melting point is analyzed as well.

The magnetically frustrated manganese nitride antiperovskite family displays significant changes of entropy under changes in hydrostatic pressure near a first-order antiferromagnetic to paramagnetic phase transition that can be useful for the emerging field of solid-state barocaloric cooling. In previous studies, the transition hysteresis has significantly reduced the reversible barocaloric effects (BCE). Here we show that the transition hysteresis can be tailored through quaternary alloying in the Mn _3 Cu $_{1-x}$ Sn $_{x}$ N system. We find the magnitude of hysteresis is minimised when Cu and Sn are equiatomic ( x  = 0.5) reaching values far less than previously found for Mn _3 A N ( $A = $ Pd, Ni, Ga, Zn), whilst retaining entropy changes of the same order of magnitude. These results demonstrate that reversible BCE are achievable for p  < 100 MPa in the Mn _3 ( A , B )N family and suggest routes to modify the transition properties in compounds of the same family.