Strong-randomness phenomena in quantum Ashkin–Teller models

We investigate the non-equilibrium stationary states of the Ising, X Y , and Heisenberg models on a simple cubic lattice using Monte Carlo simulations. The system’s evolution is governed by a competition between two dynamics controlled by a parameter q : a one-spin flip dynamic that drives the system, in contact with a thermal reservoir at temperature T , towards a lower energy state with probability q , and a two-spin flip dynamic that drives it, subjected to an external energy flux, towards a higher energy state with probability 1 − q . We construct the phase diagram of T as a function of q , which exhibits a rich phenomenology of self-organization across the different observed phases and is characterized by continuous phase transitions between them. For all three models, we identify three distinct phases: an antiferromagnetic ordered phase at low q values, a disordered paramagnetic phase at intermediate q , and a ferromagnetic ordered phase at high q values and low T . Furthermore, we calculate the critical exponents for the phase transitions and show that they remain unchanged from their equilibrium values, indicating that the non-equilibrium nature of the dynamics does not alter the universality class of the models. This suggests that the underlying symmetries of the models are the primary determinants of their critical behavior, even in the presence of a persistent energy flux. • We study Ising, XY, and Heisenberg models under competing stochastic dynamics. • Self-organized antiferromagnetic, paramagnetic, and ferromagnetic phases emerge. • Critical exponents match equilibrium values despite detailed balance violation. • Universality classes are preserved in non-equilibrium driven magnetic systems.

We propose generalized variants of the XY model capable of exhibiting an arbitrary number of phase transitions only by varying temperature. They are constructed by supplementing the magnetic coupling with n_{t}-1 nematic terms of exponentially increasing order with the base q=2,3,4, and 5, and increasing interaction strength. It is found that for q=2,3, and 4 with sufficiently large coupling strength of the final term, the models exhibit a number of phase transitions equal to the number of the terms in the generalized Hamiltonian. Starting from the paramagnetic phase, the system transitions through the cascade of n_{t}-1 nematic phases of the orders q^{k},k=n_{t}-1,n_{t}-2,⋯,1, that are characterized by q^{k} preferential spin directions symmetrically disposed around the circle, to the ferromagnetic (FM) phase at the lowest temperatures. Besides the Berezinskii-Kosterlitz-Thouless (BKT) transition from the paramagnetic phase, all the remaining transitions have a non-BKT nature: depending on the value of q they belong to either the Ising (q=2 and 4) or the three-state Potts (q=3) universality class. For q=5, due to the interplay between different terms, the phase transitions between the ordered phases observed for q<5 split into two, and the number of the ordered phases increases to 2n_{t}-1. These phases are characterized by a domain structure with the gradually increasing short-range FM ordering within domains that extends to different kinds of FM ordering in the last two low-temperature phases. The respective transitions do not seem to obey any universality.

Optical properties of NdAg in the ferromagnetic and paramagnetic phases

There is considerable controversy surrounding the nature of the paramagnetic to ferromagnetic phase transition in La0.7Ca0.3MnO3. We have used transmission electron microscopy to perform micromagnetic imaging in order to determine whether the phase change is first or second order. On warming through the transition point, the ferromagnetic phase retreats from the sample surface as it is replaced by the paramagnetic phase. This coexistence of ferromagnetic and paramagnetic phases indicates a primarily first-order transition. However, there is also a continuous loss of magnetization which precedes the phase transition. We compare this with the ferromagnetic transition in nickel which displays a purely continuous phase change. We discuss the accuracy and range of applicability of the micromagnetic imaging techniques of electron holography and Fresnel imaging which were used in this investigation.

The quantum critical detector (QCD), recently introduced for weak signal amplification [L.-P. Yang and Z. Jacob, Opt. Express 27, 10482 (2019)], functions by exploiting high sensitivity near the phase transition point of first-order quantum phase transitions (QPTs). We contrast the behavior of the first-order and the second-order quantum phase transitions in the detector. We find that the giant sensitivity, which can be utilized for quantum amplification, only exists in the first-order QPTs. We define two new magnetic order parameters to quantitatively characterize the first-order QPT of the interacting spins in the detector. We also introduce the Husimi Q-functions as a powerful tool to show the fundamental change in the ground-state wave function of the detector during the QPTs, especially the intrinsic dynamical change within the detector during a quantum critical amplification. We explicitly show the high figures of merit of the QCD via the quantum gain and the signal-to-quantum noise ratio. Specifically, we predict the existence of a universal first-order QPT in the interacting-spin system resulting from two competing ferromagnetic orders. Our results motivate new designs of weak signal detectors by engineering first-order QPTs, which are of fundamental significance in the search for new particles, quantum metrology, and information science.

To understand the mechanism of first-order magnetic phase transition in ordered FeRh thin films, the magnetic properties and first-order antiferromagnetic (AFM)–ferromagnetic (FM) phase transition behavior of single-crystal FeRh thin film are investigated in detail. The first-order magnetic phase transition is seen at a temperature of around 120 °C during heating and 145 °C cooling processes in perpendicular direction. The M–H loops measured isothermally amidst the AFM–FM transition regime show an opening at high magnetic field, which indicate a reversible AFM–FM transition induced by magnetic field. The clusters of the FM phase nucleate in the AFM matrix heterogeneously and vice versa during the first-order phase transition and the mechanism of nucleation and growth kinetics of the first-order magnetic phase transition in ordered FeRh thin film is quite similar to that of the crystallization of solids described by the Avrami model.

Magnetic, electronic, and thermal transport properties of ${\mathrm{Eu}}_{0.55}{\mathrm{Sr}}_{0.45}\mathrm{Mn}{\mathrm{O}}_{3}$ have been experimentally studied. The compound is found to exhibit a complex magnetic behavior with the change of temperature and magnetic field. Without magnetic field, it stays in a Griffiths-like state in a wide temperature range above $\ensuremath{\sim}100\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, characterized by the presence of ferromagnetic (FM) clusters of the size of $\ensuremath{\sim}8$ Mn ions, and an antiferromagnetic (AFM) state below $\ensuremath{\sim}100\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, evidenced by thermopower and heat conductivity. FM phase emerges and grows in the AFM matrix with applied field, resulting in a series of phase transitions from the paramagnetic (PM) state first to the AFM state, then to the FM and the AFM states upon cooling $(\ensuremath{\sim}0.8\phantom{\rule{0.3em}{0ex}}\mathrm{T}lHl\ensuremath{\sim}1\phantom{\rule{0.3em}{0ex}}\mathrm{T})$, or a simple PM-FM transition $(Hg\ensuremath{\sim}2.3\phantom{\rule{0.3em}{0ex}}\mathrm{T})$. The AFM state is unstable under high fields, and the high- and low-temperature AFM transitions are depressed by the fields above $\ensuremath{\sim}1$ and $\ensuremath{\sim}2.3\phantom{\rule{0.3em}{0ex}}\mathrm{T}$, respectively. The FM transition is incomplete when the field is below $\ensuremath{\sim}1.5\phantom{\rule{0.3em}{0ex}}\mathrm{T}$, leading to a coexistence of the FM and PM (or AFM) phases in the intermediate temperature range. A spin-glass-like behavior is observed in the AFM background below $\ensuremath{\sim}50\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. Significant response of resistance, thermopower, and heat conduction to magnetic transition, either FM or AFM transition, has been observed. Unlike the typical AFM manganites, for which usually a depression of thermal conduction occurs at the AFM state, the AFM transition in ${\mathrm{Eu}}_{0.55}{\mathrm{Sr}}_{0.45}\mathrm{Mn}{\mathrm{O}}_{3}$ enhances the thermal conduction. From the PM phase to the AFM phase and to the FM phase, thermal conductivity increases monotonically. A remarkable result of the present work is the different behaviors of thermopower and resistivity. The former displays a metallic behavior below a distinct temperature that is significantly lower than the metal-to-insulator transition temperature determined by resistivity. Furthermore, thermopower remains metallic while the resistivity shows up an upturn due to the AFM transition in the low-temperature range under the field of $1.5\phantom{\rule{0.3em}{0ex}}\mathrm{T}$. Based on these data, a magnetic phase diagram is proposed.

The cluster slave-spin method is implemented to research the ground state properties of the honeycomb lattice Hubbard model with doping $\delta$ and coupling $U$ being its parameters. At half-filling, a single direct and continuous phase transition between the semi-metal and antiferromagnetic (AFM) insulator is found at $U_{\text{AFM}}=2.43t$ that is in the Gross-Neveu-Yukawa universality class, where a relation between the staggered magnetization $M$ and the AFM energy gap $\Delta_{\text{AFM}}$ is established as $M \propto \Delta_{\text{AFM}}$, compared to $M \propto \Delta_{\text{AFM}} ( \ln{\Delta_{\text{AFM}}})^2$ in the square lattice case. A first-order semi-metal to the underlying paramagnetic (PM) insulator Mott transition is corroborated at $U_{\text{Mott}}=8.36t$, which is responsible for a broad crossover around $U_{c} = 5.4t$ between the weak- and strong-coupling regimes in the AFM state that increases with $\delta$, in contrast to the square lattice case. In the doped system, the compressibility $\kappa$ near the van Hove singularity at $\delta=1/4$ is suppressed substantially by the interaction before the semi-metal to AFM transition occurs, whereas $\kappa$ near the Dirac points is very close to the noninteracting one, indicating that the Dirac cone structure of the energy dispersion is rather robust. An overall phase diagram in the $U$-$\delta$ plane is presented, consisting of four regimes: the AFM insulator at $\delta=0$ for $U> U_{\text{AFM}}$, the AFM metal with compressibility $\kappa>0$ or $\kappa<0$, and the PM semi-metal, and the AFM metal with $\kappa<0$ only exists in an extremely small area near the phase boundary between the AFM and PM state.

Self- and impurity diffusivities in body-centered-cubic (bcc) iron have been calculated within the formalisms of harmonic transition-state theory and the Le Claire nine-frequency model for vacancy-mediated diffusion. The approach combines first-principles calculations of vacancy formation, migration, and solute-binding enthalpies and entropies in the ferromagnetic phase, with an empirical relationship for the effect of magnetic disorder on diffusion activation energies. Calculated Fe self-diffusion and Mo and W impurity-diffusion coefficients are shown to agree within a factor of five with the most recent experimental measurements in both the ferromagnetic and paramagnetic phases. Calculated diffusion coefficients for Mo and W impurities are comparable to or larger than that for Fe self-diffusion at all temperatures below the α–γ phase transition. Calculated activation energies for Ta and Hf impurities suggest that these solutes should also display impurity-diffusion coefficients larger than that for self-diffusion in body-centered cubic Fe.

Inspired by the recent discovery of superconductivity in the nickelate Nd$_{1-x}$Sr$_x$NiO$_2$, we study a generalized $t-J$ model to investigate the correlated phases induced by doping spin-one Ni$^{2+}$ into a spin $1/2$ Mott insulator formed by Ni$^{1+}$. Based on a three-fermion parton mean field analysis, we identify a robust fractional Fermi liquid (FL*) phase for almost every doping level. The FL* state is characterized by a small Fermi pocket on top of a spin liquid, which violates the Luttinger theorem of a conventional Fermi liquid and is an example of a symmetric pseudogap metal. Furthermore, we verify our theory in one dimension through density matrix renormalization group (DMRG) simulations on both the generalized $t-J$ model and a two-orbital Hubbard model. The fractional Fermi liquid reduces to fractional Luttinger liquid (LL*) in one dimension, which is connected to the conventional Luttinger liquid through a continuous quantum phase transition by tuning interaction strength. Our findings offer new insights into correlated electron phenomena in nickelate superconductors and other multi-orbital transition metal oxide with spin-triplet $d^8$ state.

We address the phenomenon of critical Kondo destruction in pseudogap Bose-Fermi Anderson and Kondo quantum impurity models. These models describe a localized level coupled both to a fermionic bath having a density of states that vanishes like |\epsilon|^r at the Fermi energy (\epsilon=0) and, via one component of the impurity spin, to a bosonic bath having a sub-Ohmic spectral density proportional to |\omega|^s. Each bath is capable by itself of suppressing the Kondo effect at a continuous quantum phase transition. We study the interplay between these two mechanisms for Kondo destruction using continuous-time quantum Monte Carlo for the pseudogap Bose-Fermi Anderson model with 0<r<1/2 and 1/2<s<1, and applying the numerical renormalization-group to the corresponding Kondo model. At particle-hole symmetry, the models exhibit a quantum critical point between a Kondo (fermionic strong-coupling) phase and a localized (Kondo-destroyed) phase. The two solution methods, which are in good agreement in their domain of overlap, provide access to the many-body spectrum, as well as to correlation functions including, in particular, the single-particle Green's function and the static and dynamical local spin susceptibilities. The quantum-critical regime exhibits the hyperscaling of critical exponents and \omega/T scaling in the dynamics that characterize an interacting critical point. The (r,s) plane can be divided into three regions: one each in which the calculated critical properties are dominated by the bosonic bath alone or by the fermionic bath alone, and between these two regions, a third in which the bosonic bath governs the critical spin response but both baths influence the renormalization-group flow near the quantum critical point.

We report on resistivity measurements in La{sub 0.67}Ca{sub 0.33}MnO{sub 3} and Nd{sub 0.7}Sr{sub 0.3}MnO{sub 3} thin films in order to elucidate the underlying mechanism for the colossal magnetoresistance behavior. The experimental results are analyzed in terms of quantum phase-transition ideas to study the nature of the metal-insulator transition in manganese oxides. Resistivity curves as functions of magnetization for various temperatures show the absence of scaling behavior expected in a continuous quantum phase transition, which leads us to conclude that the observed metal-insulator transition is most likely a finite temperature crossover phenomenon. (c) 2000 The American Physical Society.

We present evidence of a direct, continuous quantum phase transition between a Bose superfluid and the ν=1/2 fractional Chern insulator in a microscopic lattice model. In the process, we develop a detailed field theoretic description of this transition in terms of the low energy vortex dynamics. The theory explicitly accounts for the structure of lattice symmetries and predicts a Landau forbidden transition that is protected by inversion. That the transition is continuous enables the quasiadiabatic preparation of the fractional Chern insulator in nonequilibrium, quantum optical systems.

We define the characteristic temperature T C of a material by using the behaviour of the specific heat in the vicinity of this temperature: C p,N (T ) = f(1 − (T/T C ),p,N ) for T < T C and C p,N (T ) = g((T/T C ) − 1,p,N ) for T > T C where p is the external pressure and N is the number of particles. The characteristic temperature T C = T C ( p, N ) can e.g. be identified with the transition temperature in the case of continuous phase transitions or with one of the two characteristic temperatures (on heating) and (on cooling) in the case of discontinuous phase transitions. Using the idea of the characteristic temperature, we present a new method on how to calculate the Gibbs potential G = G(T, p, N) in the vicinity of the characteristic temperature, without it being a necessity to know the explicite expressions for f and g. This general and uniform approach allows to derive many thermodynamical relations connecting measurable quantities at the characterisic point as e.g. Clausius–Clapeyron equation and its analogs, valid for discontinuous phase transitions and 15 relations valid for both continuous and discontinuous phase transitions, 6 of them are new. As a special case of these latter relations, we find six relations valid for continuous phase transitions (Ehrenfest equation and its analogs). The derived relations result exclusively from the definition of the characteristic point and are independent from the underlying microscopical mechanism leading to phase transitions (as e.g. solid–liquid, liquid–vapour, magnetic, ferroelectric, structural and superconducting transitions) and therefore, they are generally valid. These relations can be very important for material science. Using some of them it is possible to predict (without performing additional experiments) whether the transition or characteristic temperature T C of a given material sample will increase or decrease under doping (injection) or applied pressure. Similar 24 thermodynamical relations can also be found with ease for systems described by the Helmholtz potential F = F(T,V, N ).

We employ the microcanonical inflection-point analysis method, developed for the systematic identification and classification of phase transitions in systems of any size, to study the two-dimensional Ising model at various lattice sizes and in the thermodynamic limit. Exact results for the density of states, which were obtained by exact algorithmic computation, provide evidence for higher-order transitions in addition to the well-studied second-order ferromagnetic-paramagnetic phase transition. An independent third-order phase transition is identified in the ferromagnetic phase, whereas another third-order transition resides in the paramagnetic phase. The latter is a dependent transition, i.e., it is inevitably associated with the critical transition, but it remains separate from the critical point in the thermodynamic limit. For a deeper insight into the nature of these additional transitions, a detailed analysis of spin clusters is performed.