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Dynamical quantum phase transitions (DQPTs) induced by flux-quench in an extended transversed XY spin-chain have been investigated in this paper. We discussed the conditions for the appearance of DQPTs, and the different regions of the flux quench restricted by strength of transverse field were given. The Loschmidt echo, rate function, geometric phase, as well as dynamical topological order parameter (DTOP) have been calculated, which consistently verified the emergence of DQPTs. [ABSTRACT FROM AUTHOR]

The wetting transition in the one-dimensional cluster Ising model with opposite boundary fields is studied. Tuning one boundary field while fixing another leads to the occurrence of phase transitions, where the transition points depend on the cluster coupling. Furthermore, the phase diagram is divided into three regions with different phase transition. For weak and strong cluster coupling, the phase transition is continuous and belongs to the same universality of transverse Ising model with boundary fields. For intermediate cluster coupling, the phase transition is first order. In the strong cluster coupling region, the critical region becomes exponentially small and the asymptotic behavior is absent even for lattice size up to 1 0 4 . A numerical method to solve the energy gap and the correlation length is proposed on an infinite long spin chain. With this method, one can get the critical behavior as close to the critical point as possible provided that the numerical accuracy is high enough. In the light of this method, we clearly show that there is a preasymptotic regime in which the apparent critical exponents depend on the cluster coupling. Moreover, we obtain the accurate energy gap exponent z ν and the correlation length exponent ν in the asymptotic critical region. • The phase diagram is given. • A numerical method for solving the energy gap and the correlation length on an infinite long spin chain is proposed. With it the asymptotic critical behavior can be studied. • It is found that in the strong cluster term region, there exist wide preasymptotic regions where the apparent critical exponents depend on the cluster term. The real critical exponents are obtained in the asymptotic region where is extremely close to the critical point. [ABSTRACT FROM AUTHOR]

We study the quantum phase transitions occurring in the ferrimagnetic mixed spin- (1 / 2 , 1) chain with single-ion anisotropy under the influence of an external magnetic field. Using quantum information theory concepts, we unravel these transitions without prior knowledge of system symmetries or order parameters. It is observed that a more comprehensive understanding of quantum phase transitions can be obtained using a deep analysis of multipartite negativity. The entanglement characteristics of the mixed system are explored by employing numerical diagonalization through the Lanczos algorithm. Fidelity susceptibility reveals quantum phase transitions, notably at the degeneracy lifting point where magnetic effects cancel. A low-temperature phase diagram emerges, delineating four distinct phases: quantum spin-liquid, Lieb-Mattis ferrimagnetic, Luttinger spin-liquid, and fully polarized. We observe a direct second-order transition from Lieb-Mattis ferrimagnetic to a fully polarized phase under specific conditions. Results are extrapolated to the thermodynamic limit, providing insights into macroscopic behavior. [ABSTRACT FROM AUTHOR]

This article presents a design for the two-dimensional heterostructure systems (2DHS) consisting of graphene quantum dot (QD) array in graphane (GQD/Graphane) and silicene QD array in silicane (SiQD/Silicane). First-principles method was used to evaluate the effect of deformation on the magnetism and electronic properties of these 2DHS. For the band engineering, the tuning range of the energy gap of GQD/Graphane and SiQD/Silicane 2DHS can be up to 1.20 and 1.35 eV, respectively, through strains ranging from −7% to 10 %. A strain-sharing effect is found, wherein the hydrogenated region shares a portion of strain within the QD array. This effect is stronger in SiQD/silicane than in GQD/graphane. Strain sharing enhances band coupling between the QDs and their hydrogenated counterpart in the low-energy region. This alters the electronic properties of the 2DHS and the magnetic properties of triangular and parallelogram SiQD/Silicane arrays under large compressive strain. Strain modulates the band gap of 2DHS, triggers a phase transition from semiconductor to metal in SiQD/Silicane systems under homogeneous strain, and removes the magnetism of triangular and parallelogram SiQD arrays under compressive strain. These findings suggest that the 2DHS could be promising for designing nanoelectronic devices and binary logic based on nanoscale magnetism. [Display omitted] • Band engineering through strains for the 2D heterostructure (2DHS) systems is revealed. • The turning range of the energy gaps is up to 1.20 and 1.35 eV for the GQD/Graphane and SiQD/Silicane 2DHS, respectively. • A strain-sharing effect was observed in the 2DHS systems, where its hydrogenated counterpart partially shares the strain on the QD array. • Under strains, a magnetic-to-nonmagnetic transition and semiconductor-to-metal transition were observed in the SiQD/Silicane 2DHS system. [ABSTRACT FROM AUTHOR]

Quantum renormalization group (QRG) is a tractable method for studying the criticalities of one-dimensional (1D) and two-dimensional (2D) many-body systems. By employing the QRG method, we first derive the effective Hamiltonian and QRG equations of a 2D XXZ model with Dzyaloshinskii–Moriya (DM) interaction analytically. The linear-entropy-based uncertainty, the quantum discord (QD), and the multipartite quantum coherence based on the square root of the quantum Jensen–Shannon divergence of the 2D XXZ model are then studied as the indicators of quantum phase transitions (QPTs). The nonanalytic and scaling behaviors of the uncertainty, QD and quantum coherence are also analyzed through numerical calculations. Moreover, we investigate the effect of the easy-axis anisotropy parameter and DM interaction on the QPT. We find that the uncertainty, QD, and quantum coherence can all be utilized to detect QPTs. Our findings could shed new light on the observable of the QPT of the many-body system with the uncertainty and quantum coherence, and enrich the application of QRG method to Heisenberg spin models. • New quantum renormalization group equations and effective Hamiltonian are derived. • Uncertainty relation and quantum coherence characterize quantum phase transitions. • Non-analytic phenomena and scaling behaviors near critical points are explored. • Utilization of 3-site renormalization technique saves memory and computing time. [ABSTRACT FROM AUTHOR]

We use the Hilbert space formulation of classical mechanics, known as the Koopman–von Neumann formalism, to study adiabatic driving, geometric phases, and the geometric tensor for classical states. In close relation to what happens to a quantum state, a classical Koopman–von Neumann eigenstate will acquire a geometric phase factor e x p i Φ after a closed variation of the parameters λ in its associated Hamiltonian. The explicit form of Φ is then derived for integrable systems, and its relation with the Hannay angle is shown. Additionally, we use quantum formulas to write an adiabatic gauge potential that generates adiabatic unitary flow between classical eigenstates, and we explicitly show the relationship between the potential and the classical geometric phase. We also define a classical analog of the geometric tensor, thus defining a Fubini–Study metric for classical states, and we use the singularities of the tensor to link the transition from Arnold–Liouville integrability to chaos with some of the mathematical formalism of quantum phase transitions. While the formulas and definitions we use originate in quantum mechanics, all the results found are purely classical, no classical or semiclassical limit is ever taken. • Information geometry approach the transition from classical integrability to chaos. • Definition of a Fubini–Study metric for Koopman–von Neumann states. • Holonomy of Liouville eigenfunctions and Hannay angles. [ABSTRACT FROM AUTHOR]

Physicists are actively debating the nature of the quantum critical phase transition that determines the low-temperature properties of metals with heavy fermions. Important experimental observations of their transport properties incisively probe the nature of the quantum critical phase transition. In our short review, we consider the transport properties of strongly correlated Fermi systems like heavy fermion metals and high—Tc superconductors. Their transport properties are defined by strong inter-particle interactions, forming flat bands in these compounds. These properties do not coincide with those of conventional metals. Indeed, in contrast to the behavior of the transport properties of conventional metals, the strongly correlated compounds exhibit linear temperature resistivity ρ(T)∝T. We analyze the magnetoresistance and show that under the application of the magnetic field, it becomes negative. It is shown that near a quantum phase transition, when the density of the electronic states diverges, semiclassical physics remains applicable to describe the resistivity ρ of strongly correlated metals due to the presence of a transverse zero-sound collective mode, representing the phonon mode in solids. We demonstrate that when T exceeds the extremely low Debye temperature TD, the resistivity ρ(T) changes linearly with T since the mechanism of formation of the T-dependence ρ(T) is a similar electron-phonon mechanism, which predominates at high temperatures in ordinary metals. Thus, in the region of T-linear resistance, electron-phonon scattering leads to a lifetime of τ quasiparticles practically independent of the material, which is expressed as the ratio of the Planck constant ℏ to the Boltzmann constant kB, Tτ∼ℏ/kB. We explain that due to the non-Fermi-liquid behavior, the real part of the frequency-dependent optical conductivity σoptR(ω) exhibits a scaling behavior and demonstrates the unusual power law behavior σoptR(ω)∝ω−1, rather than the well-known one shown by conventional metals, σoptR(ω)∝ω−2. All our theoretical considerations are illustrated and compared with the corresponding experimental facts. Our results are in a good agreement with experimental observations.

We deal here with an exactly solvable N-nucleon system that has been used to mimic typical features of quantum many-body systems. There is in the literature some controversy regarding the possible existence of a quantum phase transition in the model. We show here that an appeal to a suitable statistical quantifier called thermal efficiency puts an end to the controversy.

Electrostatic confinement in semiconductors provides a flexible platform for the emulation of interacting electrons in a two-dimensional lattice, including in the presence of gauge fields. This combination offers the potential to realize a wide host of quantum phases. Capacitance spectroscopy provides a technique that allows one to directly probe the density of states of such two-dimensional electron systems. Here, we present a measurement and fabrication scheme that builds on capacitance spectroscopy and allows for the independent control of density and periodic potential strength imposed on a two-dimensional electron gas. We characterize disorder levels and (in)homogeneity and develop and optimize different gating strategies at length scales where interactions are expected to be strong. A continuation of these ideas might see to fruition the emulation of interaction-driven Mott transitions or Hofstadter butterfly physics. [ABSTRACT FROM AUTHOR]

Quantum fluids exhibit quantum mechanical effects at the macroscopic level, which contrast strongly with classical fluids. Gain-dissipative solid-state exciton-polaritons systems are promising emulation platforms for complex quantum fluid studies at elevated temperatures. Recently, halide perovskite polariton systems have emerged as materials with distinctive advantages over other room-temperature systems for future studies of topological physics, non-Abelian gauge fields, and spin-orbit interactions. However, the demonstration of nonlinear quantum hydrodynamics, such as superfluidity and Čerenkov flow, which is a consequence of the renormalized elementary excitation spectrum, remains elusive in halide perovskites. Here, using homogenous halide perovskites single crystals, we report, in both one- and two-dimensional cases, the complete set of quantum fluid phase transitions from normal classical fluids to scatterless polariton superfluids and supersonic fluids—all at room temperature, clear consequences of the Landau criterion. Specifically, the supersonic Čerenkov wave pattern was observed at room temperature. The experimental results are also in quantitative agreement with theoretical predictions from the dissipative Gross-Pitaevskii equation. Our results set the stage for exploring the rich non-equilibrium quantum fluid many-body physics at room temperature and also pave the way for important polaritonic device applications. Lead halide perovskites have recently emerged as a promising platform for the study of polariton superfluidity at room temperature. Here the authors report a complete set of quantum fluid phase transitions in both 1D and 2D homogeneous single crystals of CsPbBr3. [ABSTRACT FROM AUTHOR]

The experimental advances in the study of time-dependent phenomena has opened a new path to investigating the complex electronic structure of strongly correlated and topological materials. Yu-Shiba-Rusinov (YSR) states induced by magnetic impurities in s-wave superconductors provide an ideal candidate system to study the response of a system to time-dependent manipulations of the magnetic environment. Here, we show that by imposing a time-dependent change in the magnetic exchange coupling, by changing the relative alignment of magnetic moments in an impurity dimer, or through a periodic drive of the impurity moment, one can tune the system through a time-dependent quantum phase transition, in which the system undergoes a transition from a singlet to a doublet ground state. We show that the electronic response of the system to external perturbations can be imaged through the time-dependent differential conductance, dI(t)/dV, which, in analogy to the equilibrium case, is proportional to a non-equilibrium local density of states. Our results open the path to visualizing the response of complex quantum systems to time-dependent external perturbations. The problem of manipulating quantum phenomena, such as quantum phase transitions, has a longstanding history in condensed matter research. This paper studies the non-equilibrium emergence and manipulation of Yu-Shiba-Rusinov states in response to external perturbations of their magnetic environment. [ABSTRACT FROM AUTHOR]

Sp2-hybridized monolayer semiconductors (e.g., planar group III-V and IV-IV binary compounds) with inversion symmetry breaking (ISB) display piezoelectricity governed by their σ- and π-bond electrons. Here, we studied their bond-orbital-resolved electronic piezoelectricity (i.e., the σ- and π-piezoelectricity). We formulated a tight-binding piezoelectric model to reveal the different variations of σ- and π-piezoelectricity with the ISB strength (Δ). As Δ varied from positive to negative, the former decreased continuously, but the latter increased piecewise and jumped at Δ = 0 due to the criticality of the π-electrons' ground-state geometry near this quantum phase-transition point. This led to a piezoelectricity predominated by the π-electrons for a small | Δ | . By constructing an analytical model, we clarified the microscopic mechanisms underlying the anomalous π-piezoelectricity and its subtle relations with the valley Hall effect. The validation of our models was justified by applying them to the typical sp2 monolayers including hexagonal silicon carbide, Boron-X (X = N, P, As, Ab), and a BN-doped graphene superlattice. [ABSTRACT FROM AUTHOR]

Quantum melting is the phenomenon of cold (zero-temperature) melting of a pressure-ionized substance which represents a lattice of bare ions immersed in the background of free electrons, i.e., the so-called one-component plasma (OCP). It occurs when the compression of the substance corresponds to the zero-point fluctuations of its ions being so large that the ionic ordered state can no longer exist. Quantum melting corresponds to the classical melting curve reaching a turnaround point beyond which it starts going down and eventually terminates, when zero temperature is reached, at some critical density. This phenomenon, as well as the opposite phenomenon of quantum crystallization, may occur in dense stellar objects such as white dwarfs, and may play an important role in their evolution that requires a reliable thermoelasticity model for proper physical description. Here we suggest a modification of our unified analytic melt-shear thermoelasticity model in the region of quantum melting, and derive the corresponding Grüneisen parameters. We demonstrate how the new functional form for the cold shear modulus can be combined with a known equation of state. One of the constituents of the new model is the melting curve of OCP crystal which we also present. The inclusion of quantum melting implies that the modified model becomes applicable in the entire density range of the existence of the solid state, up to the critical density of quantum melting above which the solid state does not exist. Our approach can be generalized to model melting curves and cold shear moduli of different solid phases of a multi-phase material over the corresponding ranges of mechanical stability. [ABSTRACT FROM AUTHOR]

It is becoming increasingly clear that breakthrough in quantum applications necessitates materials innovation. In high demand are conductors with robust topological states that can be manipulated at will. This is what we demonstrate in the present work. We discover that the pronounced topological response of a strongly correlated "Weyl-Kondo" semimetal can be genuinely manipulated—and ultimately fully suppressed—by magnetic fields. We understand this behavior as a Zeeman-driven motion of Weyl nodes in momentum space, up to the point where the nodes meet and annihilate in a topological quantum phase transition. The topologically trivial but correlated background remains unaffected across this transition, as is shown by our investigations up to much larger fields. Our work lays the ground for systematic explorations of electronic topology, and boosts the prospect for topological quantum devices. Manipulation of topology of the electronic structure is highly desirable for practical applications of topological materials. Here the authors demonstrate tuning and annihilation of Weyl nodes in momentum space by means of the Zeeman effect in a strongly correlated topological semimetal Ce3Bi4Pd3. [ABSTRACT FROM AUTHOR]

The hadron-quark phase transition in quantum chromodynamics has been suggested as an alternative explosion mechanism for core-collapse supernovae. We study the impact of three different hadron-quark equations of state (EoS) with first-order (DD2F_SF, STOS-B145) and second-order (CMF) phase transitions on supernova dynamics by performing 97 simulations for solar- and zero-metallicity progenitors in the range of |$14\tt {-}100\, \text{M}_\odot$|⁠. We find explosions only for two low-compactness models (14 and |$16\, \text{M}_\odot$|⁠) with the DD2F_SF EoS, both with low explosion energies of |${\sim }10^{50}\, \mathrm{erg}$|⁠. These weak explosions are characterized by a neutrino signal with several minibursts in the explosion phase due to complex reverse shock dynamics, in addition to the typical second neutrino burst for phase-transition-driven explosions. The nucleosynthesis shows significant overproduction of nuclei such as 90Zr for the |$14\hbox{-} \text{M}_\odot$| zero-metallicity model and 94Zr for the |$16\hbox{-}\text{M}_\odot$| solar-metallicity model, but the overproduction factors are not large enough to place constraints on the occurrence of such explosions. Several other low-compactness models using the DD2F_SF EoS and two high-compactness models using the STOS EoS end up as failed explosions and emit a second neutrino burst. For the CMF EoS, the phase transition never leads to a second bounce and explosion. For all three EoS, inverted convection occurs deep in the core of the protocompact star due to anomalous behaviour of thermodynamic derivatives in the mixed phase, which heats the core to entropies up to 4 k B/baryon and may have a distinctive gravitational-wave signature, also for a second-order phase transition. [ABSTRACT FROM AUTHOR]

The Kibble-Zurek mechanism (KZM) captures the essential physics of nonequilibrium quantum phase transitions with symmetry breaking. KZM predicts a universal scaling power law for the defect density which is fully determined by the system's critical exponents at equilibrium and the quenching rate. We experimentally tested the KZM for the simplest quantum case, a single qubit under the Landau-Zener evolution, on an open access IBM quantum computer (IBM-Q). We find that for this simple one-qubit model, experimental data validates the central KZM assumption of the adiabatic-impulse approximation for a well isolated qubit. Furthermore, we report on extensive IBM-Q experiments on individual qubits embedded in different circuit environments and topologies, separately elucidating the role of crosstalk between qubits and the increasing decoherence effects associated with the quantum circuit depth on the KZM predictions. Our results strongly suggest that increasing circuit depth acts as a decoherence source, producing a rapid deviation of experimental data from theoretical unitary predictions. [ABSTRACT FROM AUTHOR]

In the problem of classification of the parameter-controlled quantum phase transitions, attention is turned from the conventional manipulations with the energy-level mergers at exceptional points to the control of mergers of the exceptional points themselves. What is obtained is an exhaustive classification which characterizes every phase transition by the algebraic and geometric multiplicity of the underlying confluent exceptional point. Typical qualitative characteristics of non-equivalent phase transitions are illustrated via a few elementary toy models. [ABSTRACT FROM AUTHOR]

Recent numerical studies indicate that the antiferromagnetic Kitaev honeycomb lattice model undergoes a magnetic-field-induced quantum phase transition into a new spin-liquid phase. This intermediate-field phase has been previously characterized as a gapless spin liquid. By implementing a recently developed variational approach based on the exact fractionalized excitations of the zero-field model, we demonstrate that the field-induced spin liquid is gapped and belongs to Kitaev's 16-fold way. Specifically, the low-field non-Abelian liquid with Chern number C = ±1 transitions into an Abelian liquid with C = ±4. The critical field and the field-dependent behaviors of key physical quantities are in good quantitative agreement with published numerical results. Furthermore, we derive an effective field theory for the field-induced critical point which readily explains the ostensibly gapless nature of the intermediate-field spin liquid. Previous numerical work has predicted a new spin liquid phase in the antiferromagnetic Kitaev model at intermediate magnetic fields; however, its nature is not easily discernible by numerical approaches. Here, by using a variational approach, the authors show that this phase is a gapped Abelian spin liquid [ABSTRACT FROM AUTHOR]

Facile stoichiometric and phase selective synthesis of doped/alloyed metal chalcogenide colloidal quantum dots has been an important pursuit because of the opportunity for tunable photoconductivity. Herein, the structural and optical properties of relatively monodispersed copper indium sulphide incorporated with Ag+ ions, i.e., AgxCuyInS2 (Ag:CIS) quantum dots (average diameter of 4.9 ± 0.6 nm) synthesized via a hot-injection colloidal method are investigated. The Ag:CIS quantum dots exhibit a degree of wurtzite to chalcopyrite phase change with increasing Ag+ concentration (1.1–6.8%) under primarily kinetic synthetic conditions at 180 °C for 10 minutes using copper(II) hexafluoroacetylacetonate hydrate, indium(III) and silver(I) diethyldithiocarbamate precursors. The indium-rich and copper-deficient quantum dots are close to the CuInS2 stoichiometry with tunable bandgaps between 1.60 and 1.81 eV influenced by Ag+ concentration, intrinsic defects, minimal quantum confinement and structural permutations. They exhibit broad photoluminescence emission via a dual radiative pathway with long decay lifetimes, τ1 (0.68–2.11 ± 0.02 μs) and τ2 (3.37–7.38 ± 0.20 μs), implicating donor–acceptor pair transitions of indium interstitials, and/or indium–copper antisites, to copper vacancies, for low Ag+ concentrations but primarily silver interstitials, to for higher Ag+ concentrations. Importantly, this study is the first involving Ag+ ion-dependent wurtzite to chalcopyrite phase transformation of CIS quantum dots and with their long radiative emission lifetimes are potentially effective photo-absorbers in quantum dot solar cells. [ABSTRACT FROM AUTHOR]

The recent growth in popularity of the non-Hermitian quantum Hamiltonians H (λ) with real spectra is strongly motivated by the phenomenologically innovative possibility of an access to the non-Hermitian degeneracies called exceptional points (EPs). What is actually presented in the present paper is a perturbation-theory-based demonstration of a fine-tuned nature of this access. This result is complemented by a toy-model-based analysis of the related details of quantum dynamics in the almost degenerate regime with λ ≈ λ (E P) . In similar studies, naturally, one of the decisive obstacles is the highly nontrivial form of the underlying mathematics. Here, many of these obstacles are circumvented via several drastic simplifications of our toy models—i.a., our N by N matrices H (λ) = H (N) (λ) are assumed real, tridiagonal and PT -symmetric, and our H (N) (λ) is assumed to be split into its Hermitian and non-Hermitian components staying in interaction. This is shown to lead to several remarkable spectral features of the model. Up to N = 8 , their description is even shown tractable non-numerically. In particular, it is shown that under generic perturbation, the "unfolding" removal of the spontaneous breakdown of PT -symmetry proceeds via intervals of λ with complex energy spectra. [ABSTRACT FROM AUTHOR]

Low dimensional fermionic quantum systems are exceptionally interesting because they reveal distinctive physical phenomena, including among others, topologically protected excitations, edge states, frustration, and fractionalization. Our aim was to confine 3He on a suspended carbon nanotube to form 2-dimensional Fermi-system. Here we report our measurements of the mechanical resonance of the nanotube with adsorbed sub-monolayer down to 10 mK. At intermediate coverages we have observed the famous 1/3 commensurate solid. However, at larger monolayer densities we have observed a quantum phase transition from 1/3 solid to an unknown, soft, and mobile solid phase. We interpret this mobile solid phase as a bosonic commensurate crystal consisting of helium dimers with topologically-induced zero-point vacancies which are delocalized at low temperatures. We thus demonstrate that 3He on a nanotube merges both fermionic and bosonic phenomena, with a quantum phase transition between fermionic solid 1/3 phase and the observed bosonic dimer solid. Probing fundamental quantum systems and their phase change is interesting. Here the authors demonstrate the existence of mobile quantum solid phase composed of dimerized 3He atoms and topology-induced vacancies using 3He adsorbed on carbon nanotube. [ABSTRACT FROM AUTHOR]

We report on the nonlinear optical signatures of quantum phase transitions in the high-temperature superconductor YBCO, observed through high harmonic generation. While the linear optical response of the material is largely unchanged when cooling across the phase transitions, the nonlinear optical response sensitively imprints two critical points, one at the critical temperature of the cuprate with the exponential growth of the surface harmonic yield in the superconducting phase and another critical point, which marks the transition from strange metal to pseudogap phase. To reveal the underlying microscopic quantum dynamics, a strong-field quasi-Hubbard model was developed, which describes the measured optical response dependent on the formation of Cooper pairs. Further, the theory provides insight into the carrier scattering dynamics and allows us to differentiate between the superconducting, pseudogap, and strange metal phases. Thedirect connection between nonlinear optical response andmicroscopic dynamics provides a powerful methodology to study quantum phase transitions in correlated materials. Further implications are light wave control over intricate quantum phases, light–matter hybrids, and application for optical quantum computing. [ABSTRACT FROM AUTHOR]

The present paper investigates the spin-1/2 XX model with three-spin interaction (TSI) from a view point of bipartite entanglement. The three-spin system is initially chosen by well-known W entangled state. By analyzing the time-dependency of the concurrence between the nearest and the next-nearest neighbor pair of spins, we show that where the quantum phase transition (QPT) of the infinite-size system might happen and can be observed, even from the short-time dynamics of such a finite spin system. [ABSTRACT FROM AUTHOR]

Harnessing the fascinating properties of correlated oxides requires precise control of their carrier density. Compared to other methods, oxygen doping provides an effective and more direct way to tune the electronic properties of correlated oxides. Although several approaches, such as thermal annealing and oxygen migration, have been introduced to change the oxygen content, a continuous and reversible solution that can be integrated with modern electronic technology is much in demand. Here, we report a novel ionic field-effect transistor using solid Gd-doped CeO2 as the gate dielectric, which shows a remarkable carrier-density-tuning ability via electric-field-controlled oxygen concentration at room temperature. In Bi2Sr2CaCu2O8+δ (Bi-2212) thin flakes, we achieve a reversible superconductor–insulator transition by driving oxygen ions in and out of the samples with electric fields, and map out the phase diagram all the way from the insulating regime to the over-doped superconducting regime by continuously changing the oxygen doping level. Scaling analysis indicates that the reversible superconductor–insulator transition for the Bi-2212 thin flakes follows the theoretical description of a two-dimensional quantum phase transition. Our work provides a route for realizing electric-field control of phase transition in correlated oxides. Moreover, the configuration of this type of transistor makes heterostructure/interface engineering possible, thus having the potential to serve as the next-generation all-solid-state field-effect transistor. [ABSTRACT FROM AUTHOR]

Based on the boson realization of the Euclidean algebras, it is found that the E(n) dynamical symmetry (DS) may emerge at the critical point of the U(n)-SO( n + 1 ) quantum phase transition. To justify this finding, we provide a detailed analysis of the transitional Hamiltonian in the U( n + 1 ) vibron model in both quantal and classical ways. It is further shown that the low-lying structure of 82 Kr can serve as an excellent empirical realization of the E(5) DS, which provides a specific example of the Euclidean DS in experiments. [ABSTRACT FROM AUTHOR]

Using several independent methods, we find that the metal-insulator transition occurs in the strongly-interacting two-valley two-dimensional electron system in ultra-high mobility SiGe/Si/SiGe quantum wells in zero magnetic field. The transition survives in this system in parallel magnetic fields strong enough to completely polarize the electrons' spins, thus making the electron system "spinless." In both cases, the resistivity on the metallic side near the transition increases with decreasing temperature, reaches a maximum at a temperature Tmax, and then decreases. The decrease reaches more than an order of magnitude in zero magnetic field. The value of Tmax in zero magnetic field is found to be close to the renormalized Fermi temperature. However, rather than increasing along with the Fermi temperature, the value Tmax decreases appreciably for spinless electrons in spin-polarizing magnetic fields. The observed behavior of Tmax cannot be described by existing theories. The results indicate the spin-related origin of the effect. At the same time, the low-temperature resistivity drop in both spin-unpolarized and spinless electron systems is described quantitatively by the dynamical mean-field theory. [ABSTRACT FROM AUTHOR]

The phase diagram of lattice hard core bosons with nearest-neighbor interactions allowed to vary independently, from repulsive to attractive, along different crystallographic directions, is studied by quantum Monte Carlo simulations. We observe a superfluid phase, as well as two crystalline phases at half filling, either checkerboard or striped. Just like in the case of isotropic interactions, no supersolid phase is observed. [ABSTRACT FROM AUTHOR]

Driving a quantum system across quantum critical points leads to non-adiabatic excitations in the system. This in turn may adversely affect the functioning of a quantum machine which uses a quantum critical substance as its working medium. Here we propose a bath-engineered quantum engine (BEQE), in which we use the Kibble–Zurek mechanism and critical scaling laws to formulate a protocol for enhancing the performance of finite-time quantum engines operating close to quantum phase transitions. In the case of free fermionic systems, BEQE enables finite-time engines to outperform engines operating in the presence of shortcuts to adiabaticity, and even infinite-time engines under suitable conditions, thus showing the remarkable advantages offered by this technique. Open questions remain regarding the use of BEQE based on non-integrable models. [ABSTRACT FROM AUTHOR]

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The past two decades have witnessed a surge of interest in exploring correlation and coherence measures to investigate quantum phase transitions (QPTs). Here, motivated by the continued push along this direction, we propose a measure which is built upon the so-called degree of coherence, and advocate using the susceptibility of our measure to detect QPTs. We show that our measure can capture both the notions of coherence and correlations exhibited in bipartite states and therefore represents a hybrid of these two notions. Through examining the XXZ model and the Ritaev honeycomb model, we demonstrate that our measure is favorable for detecting QPTs in comparison to many previous proposals. [ABSTRACT FROM AUTHOR]

An unambiguous observation of the Bose-Einstein condensation (BEC) of excitons in a photoexcited bulk semiconductor and elucidation of its inherent nature have been longstanding problems in condensed matter physics. Here, we observe the quantum phase transition and a Bose-Einstein condensate appearing in a trapped gas of 1s paraexcitons in bulk Cu2O below 400 mK, by directly visualizing the exciton cloud in real space using mid-infrared induced absorption imaging that we realized in a dilution refrigerator. Our study shows that the paraexciton condensate is undetectable by conventional luminescence spectroscopy. We find an unconventionally small condensate fraction of 0.016 with the spatial profile of the condensate well described by mean-field theory. Our discovery of this new type of BEC in the purely matter-like exciton system interacting with a cold phonon bath could pave the way for the classification of its long-range order, and for essential understanding of quantum statistical mechanics of non-equilibrium open systems. Bose-Einstein condensate of excitons is expected in photo-excited bulk semiconductors, but a direct experimental evidence has been lacking. Here the authors report the observation of a condensate of 1s paraexcitons in Cu2O using real-space mid-infrared absorption imaging realized in a dilution refrigerator. [ABSTRACT FROM AUTHOR]

The dynamical quantum phase transitions (DQPTs) after a sudden quench in periodic quantum Ising chains (QICs) are studied. We obtain the formulas of the Loschmidt echo and the Fisher zeros of the Loschmidt amplitude in the periodic QIC. It is found that for the quench across the quantum phase transitions (QPTs), the periodic QICs have richer DQPTs than that in the homogeneous QIC, and the number of critical times of the DQPTs are dependent on the specifical parameter of the pre- and post-quench Hamiltonian. For instance, in the period-two QIC, there is one critical time for the quench from the FM phase to the PM phase, and three critical times for the quench from the PM phase to the FM phase. In the period-three QIC, there may have three or four critical times for the quench from FM phase to the PM phase, but may have two or three critical times for the quench from PM to the FM phase. The reason is that the periodic QICs have multiple quasiparticle excitation spectra, and the Fisher zeros of the periodic systems consist of several separated branches, which is different from that in the homogeneous QIC. For different quenches across the QPTs, different branches will intersect with the imaginary axis, which correspond to different critical times. Our conclusion also provides insight in the property of the DQPT in the inhomogeneous systems. [ABSTRACT FROM AUTHOR]

Definition: Recently, great attention has been devoted to the problem of the undecidability of specific questions in quantum mechanics. In this context, it has been shown that the problem of the existence of a spectral gap, i.e., energy difference between the ground state and the first excited state, is algorithmically undecidable. Using this result herein proves that the existence of a quantum phase transition, as inferred from specific microscopic approaches, is an undecidable problem, too. Indeed, some methods, usually adopted to study quantum phase transitions, rely on the existence of a spectral gap. Since there exists no algorithm to determine whether an arbitrary quantum model is gapped or gapless, and there exist models for which the presence or absence of a spectral gap is independent of the axioms of mathematics, it infers that the existence of quantum phase transitions is an undecidable problem. [ABSTRACT FROM AUTHOR]

Multipartite nonlocality is a measure of multipartite quantum correlations. In this paper, we investigate multipartite nonlocality in a spin- 1 2 XXZ model on a one-dimensional (1D) infinite-size zigzag lattice. In the ground states, the model can undergo topological-type quantum phase transitions (QPTs) between a singlet dimer (SD) phase and an even-parity dimer (ED) phase. Two nonlocality measures S o (defined on the odd-bond subchains) and S e (defined on the even-bond subchains) are used to characterize these topological-type QPTs. Both measures show some kinds of singularity (i.e., a discontinuity of the measure or the divergence of its derivative) in the QPTs. Furthermore, in the SD phase and in the vicinity of critical regions, S o is relatively large, and in most regions of the ED phase, S o is nearly zero. Thus, similar to order parameters in traditional phase transitions, S o is an effective physical quantity to characterize these topological-type QPTs. Scaling behavior of the nonlocality measure is also discussed. [ABSTRACT FROM AUTHOR]

Entanglement entropy (EE) of a state is a measure of correlation or entanglement between two parts of a composite system and it may show appreciable change when the ground state (GS) undergoes a qualitative change in a quantum phase transition (QPT). Therefore, the EE has been extensively used to characterise the QPT in various correlated Hamiltonians. Similarly fidelity also shows sharp changes at a QPT. We characterized the QPT of frustrated antiferromagnetic Heisenberg spin-1/2 systems on 3/4, 3/5 and 5/7 skewed ladders using the EE and fidelity analysis. It is noted that all the non-magnetic to magnetic QPT boundary in these systems can be accurately determined using the EE and fidelity, and the EE exhibits a discontinuous change, whereas fidelity shows a sharp dip at the transition points. It is also noted that in case of the degenerate GS, the unsymmetrized calculations show wild fluctuations in the EE and fidelity even without actual phase transition, however, this problem is resolved by calculating the EE and the fidelity in the lowest energy state of the symmetry subspaces, to which the degenerate states belong. [ABSTRACT FROM AUTHOR]

The evolution of octupole shapes and shape phase transitions in neutron rich actinides is studied within the covariant density functional framework. Octupole constrained energy surfaces, and spectroscopic observables of four isotopic chains of: Cm, Cf, Fm and No with neutron numbers 186 ≤ N ≤ 200 are analysed using a collective quadrupole–octupole Hamiltonian (QOCH). The parameters of the Hamiltonian are determined by axially reflection-asymmetric relativistic Hartree–Bogoliubov calculations based on the energy density functional DD-PC1, and a finite-range pairing interaction. The results suggest quantum phase transitions from non-octupole to octupole deformed shapes and to octupole vibrations with increasing neutron number. 288 Cm is possibly close to the critical point of a simultaneous phase transition from spherical to prolate deformed and from non-octupole to stable octupole deformed configurations. [ABSTRACT FROM AUTHOR]

We review our study of critical phenomena in superfluid 4 He confined in nanoporous glasses. 4 He in nanoporous media is an ideal ground to survey the quantum phase transition of bosons. In the present work, critical phenomena were examined using a newly developed hydrodynamic mechanical resonator. The critical exponent of superfluid density ζ was found to be 1.0, in contrast to 0.67 in bulk 4 He. We also demonstrate that the superfluid density is proportional to | P - P c | ζ p with ζ p = 1 at any finite temperatures. These are the decisive pieces of evidence for the 4D XY criticality, which should have been observed only at 0 K, at finite temperatures. We propose a mechanism of the quantum criticality at finite temperatures in terms of phase alignment among the nanoscale localized Bose condensates (LBECs) in nanopores. The proposed mechanism is discussed in the consideration of the correlation length compared with the quantum effect. [ABSTRACT FROM AUTHOR]

We introduce a quantum geometric tensor in a curved space with a parameter-dependent metric, which contains the quantum metric tensor as the symmetric part and the Berry curvature corresponding to the antisymmetric part. This parameter-dependent metric modifies the usual inner product, which induces modifications in the quantum metric tensor and Berry curvature by adding terms proportional to the derivatives with respect to the parameters of the determinant of the metric. The quantum metric tensor is obtained in two ways: By using the definition of the infinitesimal distance between two states in the parameter-dependent curved space and via the fidelity susceptibility approach. The usual Berry connection acquires an additional term with which the curved inner product converts the Berry connection into an object that transforms as a connection and density of weight one. Finally, we provide three examples in one dimension with a nontrivial metric: an anharmonic oscillator, a Morse-like potential, and a generalized anharmonic oscillator; and one in two dimensions: the coupled anharmonic oscillator in a curved space. [ABSTRACT FROM AUTHOR]

We study the quantum phase diagram and the onset of quantum critical phenomena in a generalized Dicke model that includes collective qubit–qubit interactions. By employing semiclassical techniques, we analyze the corresponding classical energy surfaces, fixed points, and the smooth Density of States as a function of the Hamiltonian parameters to determine quantum phase transitions in either the ground (QPT) or excited states (ESQPT). We unveil a rich phase diagram, the presence of new phases, and new transitions that result from varying the strength of the qubits interactions in independent canonical directions. We also find a correspondence between the phases emerging due to qubit interactions and those in their absence but with varying the strength of the non-resonant terms in the light–matter coupling. We expect our work to pave the way and stimulate the exploration of quantum criticality in systems combining matter–matter and light–matter interactions. [ABSTRACT FROM AUTHOR]

This review considers the topological fermion condensation quantum phase transition (FCQPT) that leads to flat bands and allows the elucidation of the special behavior of heavy-fermion (HF) metals that is not exhibited by common metals described within the framework of the Landau Fermi liquid (LFL) theory. We bring together theoretical consideration within the framework of the fermion condensation theory based on the FCQPT with experimental data collected on HF metals. We show that very different HF metals demonstrate universal behavior induced by the FCQPT and demonstrate that Fermi systems near the FCQPT are controlled by the Fermi quasiparticles with the effective mass M * strongly depending on temperature T, magnetic field B, pressure P, etc. Within the framework of our analysis, the experimental data regarding the thermodynamic, transport and relaxation properties of HF metal are naturally described. Based on the theory, we explain a number of experimental data and show that the considered HF metals exhibit peculiar properties such as: (1) the universal T / B scaling behavior; (2) the linear dependence of the resistivity on T, ρ (T) ∝ A 1 T (with A 1 is a temperature-independent coefficient), and the negative magnetoresistance; (3) asymmetrical dependence of the tunneling differential conductivity (resistivity) on the bias voltage; (4) in the case of a flat band, the superconducting critical temperature T c ∝ g with g being the coupling constant, while the M * becomes finite; (5) we show that the so called Planckian limit exhibited by HF metals with ρ (T) ∝ T is defined by the presence of flat bands. [ABSTRACT FROM AUTHOR]

We present the analytic forms of the principal Hugoniots of actinium (Ac) and the lanthanide promethium (Pm), which have both never been measured or calculated before, as well as those of terbium (Tb), thulium (Tm), and lutetium (Lu), the three least studied of the remaining lanthanides. They are based on our new analytic model of principal Hugoniot. A comparison of the five Hugoniots to our own independent theoretical calculations demonstrates very good agreement in every case, but each of the Hugoniots of Tb, Tm, and Ac from the TEFIS database, which ours are also compared to, appear to violate Johnson’s theoretical constraint 4<ηmax<7 for the maximum compression ratio ηmax, which corresponds to the Hugoniot turnaround point. Possible reason for this behavior of the TEFIS Hugoniots is briefly discussed.

Modern quantum machine learning (QML) methods involve variationally optimizing a parameterized quantum circuit on a training data set, and subsequently making predictions on a testing data set (i.e., generalizing). In this work, we provide a comprehensive study of generalization performance in QML after training on a limited number N of training data points. We show that the generalization error of a quantum machine learning model with T trainable gates scales at worst as T / N . When only K ≪ T gates have undergone substantial change in the optimization process, we prove that the generalization error improves to K / N . Our results imply that the compiling of unitaries into a polynomial number of native gates, a crucial application for the quantum computing industry that typically uses exponential-size training data, can be sped up significantly. We also show that classification of quantum states across a phase transition with a quantum convolutional neural network requires only a very small training data set. Other potential applications include learning quantum error correcting codes or quantum dynamical simulation. Our work injects new hope into the field of QML, as good generalization is guaranteed from few training data. The power of quantum machine learning algorithms based on parametrised quantum circuits are still not fully understood. Here, the authors report rigorous bounds on the generalisation error in variational QML, confirming how known implementable models generalize well from an efficient amount of training data. [ABSTRACT FROM AUTHOR]

Quantum critical fluctuations may prove to play an instrumental role in the formation of unconventional superconductivity. Here, we show that the characteristic scaling of a marginal Fermi liquid is present in inelastic light scattering data of an Fe-based superconductor tuned through a quantum critical point (QCP) by chemical substitution or doping. From the doping dependence of the imaginary time dynamics we are able to distinguish regions dominated by quantum critical behavior from those having classical critical responses. This dichotomy reveals a connection between the marginal Fermi liquid behavior and quantum criticality. In particular, the overlap between regions of high superconducting transition temperatures and quantum critical scaling suggests a contribution from quantum fluctuations to the formation of superconductivity. Quantum phase transitions, occurring at zero temperature for a given system, can be induced by the application of physical or chemical pressure, and can help elucidate the underlying mechanisms of unconventional superconductivity. Here, using Raman spectroscopy, the authors report scaling properties indicative of a marginal Fermi liquid for an Fe-based superconductor tuned through a quantum critical point by chemical substitution. [ABSTRACT FROM AUTHOR]

The XY spin-chain in transverse fields under the modulation of δ -function periodic kickings has been studied in this paper. We have obtained the exact expression of the Floquet effective Hamiltonian, and analyzed the corresponding properties of the criticality. We also considered the central spin system’s Loschmidt echo coupling to this spin-chain, and gave the relation between the decoherence factor and the periodic kicking parameters. Our results show that, periodic kickings on the spin-chain can modulate the critical points and the phase diagram of the spin chain, which can be detected by the decoherence of the coupling central spin system. [ABSTRACT FROM AUTHOR]

We report on the dynamics of a magnetic-field-driven antiferromagnetic-to-paramagnetic quantum phase transition in monocrystalline Ce3Al via transverse-field muon spin rotation (TF-µSR) experiments down to low temperature of ∼ 80 mK. The quantum phase transition is of a spin-flip type and takes place on the Ce–Al magnetic chains as a result of competition between the indirect exchange and the Zeeman interaction of the Ce moments with the external field, applied along the chain direction (also the direction of the antiferromagnetic axis). The Ce moments are not static at T → 0, but fluctuate in their direction due to the Heisenberg uncertainty principle. Upon applying the magnetic field sweep, the fluctuations exhibit the largest amplitude at the quantum critical point, manifested in a maximum of the muon transverse relaxation rate at the critical field. The quantum nature of fluctuations observed in the TF-µSR experiments is reflected in the temperature independence of the average local magnetic field component along the external magnetic field at the muon stopping site(s) and the muon transverse relaxation rate within the investigated temperature range 1.5 K–80 mK. Quantum fluctuations are fast on the muon Larmor frequency scale, τ 0 < 10–10 s. [ABSTRACT FROM AUTHOR]

We analyze the possible quantum phase transition patterns occurring within the O(N) × ℤ2 scalar multi-field model at vanishing temperatures in (1 + 1)-dimensions. The physical masses associated with the two coupled scalar sectors are evaluated using the loop approximation up to second order. We observe that in the strong coupling regime, the breaking O(N) × ℤ2→ O(N), which is allowed by the Mermin-Wagner-Hohenberg-Coleman theorem, can take place through a second-order phase transition. In order to satisfy this no-go theorem, the O(N) sector must have a finite mass gap for all coupling values, such that conformality is never attained, in opposition to what happens in the simpler ℤ2 version. Our evaluations also show that the sign of the interaction between the two different fields alters the transition pattern in a significant way. These results may be relevant to describe the quantum phase transitions taking place in cold linear systems with competing order parameters. At the same time the super-renormalizable model proposed here can turn out to be useful as a prototype to test resummation techniques as well as non-perturbative methods. [ABSTRACT FROM AUTHOR]

To this end, let us consider the Hamilton-Jacobi (HJ) equation associated with (3), HT ht Graph (4)where W is Hamilton's characteristic function. Using (24) as a phase transformation [where the phase is W SB 0 sb ( I Q i )], we see that if solves (22), then I i ( I Q i ; I q i ) obeys HT ht Graph (25)which is the phase-transformed constraint. [Extracted from the article]

The XY spin-chain in transverse fields under the modulation of periodic quenching has been investigated in this paper. We have obtained the exact expression of the Floquet effective Hamiltonian and analyzed the phase diagram, the corresponding critical properties, as well as geometric phase of the system. Our results show that the critical properties and phase diagram of the spin-chain can be modulated by the periodic quenching. We also considered the central spin system's Loschmidt echo coupling to such spin-chain and showed that the Loschmidt echo can be used to detect the new critical points of the effective system. [ABSTRACT FROM AUTHOR]