Showing papers on "Phase transition published in 2019"
TL;DR: In this paper, the authors report the fabrication of magic-angle twisted bilayer graphene devices with highly uniform twist angles, which enables the observation of new superconducting domes, orbital magnets and Chern insulating states.
Abstract: Superconductivity can occur under conditions approaching broken-symmetry parent states1. In bilayer graphene, the twisting of one layer with respect to the other at ‘magic’ twist angles of around 1 degree leads to the emergence of ultra-flat moire superlattice minibands. Such bands are a rich and highly tunable source of strong-correlation physics2–5, notably superconductivity, which emerges close to interaction-induced insulating states6,7. Here we report the fabrication of magic-angle twisted bilayer graphene devices with highly uniform twist angles. The reduction in twist-angle disorder reveals the presence of insulating states at all integer occupancies of the fourfold spin–valley degenerate flat conduction and valence bands—that is, at moire band filling factors ν = 0, ±1, ±2, ±3. At ν ≈ −2, superconductivity is observed below critical temperatures of up to 3 kelvin. We also observe three new superconducting domes at much lower temperatures, close to the ν = 0 and ν = ±1 insulating states. Notably, at ν = ± 1 we find states with non-zero Chern numbers. For ν = −1 the insulating state exhibits a sharp hysteretic resistance enhancement when a perpendicular magnetic field greater than 3.6 tesla is applied, which is consistent with a field-driven phase transition. Our study shows that broken-symmetry states, interaction-driven insulators, orbital magnets, states with non-zero Chern numbers and superconducting domes occur frequently across a wide range of moire flat band fillings, including close to charge neutrality. This study provides a more detailed view of the phenomenology of magic-angle twisted bilayer graphene, adding to our evolving understanding of its emergent properties. The fabrication of magic-angle twisted bilayer graphene devices with highly uniform twist angles enables the observation of new superconducting domes, orbital magnets and Chern insulating states.
968 citations
Abstract: A novel lead-free polar dielectric ceramic with linear-like polarization responses was found in (1 − x)(Bi0.5Na0.5)TiO3–xNaNbO3 ((1 − x)BNT–xNN) solid solutions, exhibiting giant energy storage density/efficiency and super stability against temperature and frequency. High-resolution transmission electron microscopy, Raman scattering and Rietveld refinements of X-ray diffraction data suggest that these property characteristics can be derived from temperature and electric field insensitive large permittivity as a result of relaxor antiferroelectricity (AFE) with polar nanoregions. Additionally, this feature intrinsically requires a high driving field for AFE to ferroelectric (FE) phase transitions due to large random fields. Measurements of temperature-dependent permittivity and polarization versus electric field hysteresis loops indicate that the high-temperature AFE P4bm phase in BNT was gradually stabilized close to room temperature, accompanying a phase transition from relaxor rhombohedral FEs to relaxor tetragonal AFEs approximately at x = 0.15–0.2. A record high of recoverable energy-storage density W ∼ 7.02 J cm−3 as well as a high efficiency η ∼ 85% was simultaneously achieved in the x = 0.22 bulk ceramic, which challenges the existing fact that W and η must be seriously compromised. Furthermore, desirable W (>3.5 J cm−3) and η (>88%) with a variation of less than 10% can be accordingly obtained in the temperature range of 25–250 °C and frequency range of 0.1–100 Hz. These excellent energy-storage properties would make BNT-based lead-free AFE ceramic systems a potential candidate for application in pulsed power systems.
359 citations
TL;DR: This study shows that broken-symmetry states, interaction-driven insulators, orbital magnets, states with non-zero Chern numbers and superconducting domes occur frequently across a wide range of moiré flat band fillings, including close to charge neutrality.
Abstract: Superconductivity often occurs close to broken-symmetry parent states and is especially common in doped magnetic insulators. When twisted close to a magic relative orientation angle near 1 degree, bilayer graphene has flat moire superlattice minibands that have emerged as a rich and highly tunable source of strong correlation physics, notably the appearance of superconductivity close to interaction-induced insulating states. Here we report on the fabrication of bilayer graphene devices with exceptionally uniform twist angles. We show that the reduction in twist angle disorder reveals insulating states at all integer occupancies of the four-fold spin/valley degenerate flat conduction and valence bands, i.e. at moire band filling factors nu = 0, +(-) 1, +(-) 2, +(-) 3, and superconductivity below critical temperatures as high as 3 K close to - 2 filling. We also observe three new superconducting domes at much lower temperatures close to the nu = 0 and nu = +(-) 1 insulating states. Interestingly, at nu = +(-) 1 we find states with non-zero Chern numbers. For nu = - 1 the insulating state exhibits a sharp hysteretic resistance enhancement when a perpendicular magnetic field above 3.6 tesla is applied, consistent with a field driven phase transition. Our study shows that symmetry-broken states, interaction driven insulators, and superconducting domes are common across the entire moire flat bands, including near charge neutrality.
351 citations
TL;DR: The growth of entanglement in a quantum system changes qualitatively when it is observed more frequently than a certain critical rate as discussed by the authors, an important insight for describing quantum systems computationally.
Abstract: The growth of entanglement in a quantum system changes qualitatively when it is observed more frequently than a certain critical rate, an important insight for describing quantum systems computationally.
328 citations
TL;DR: It is shown that the dominant postmerger GW frequency f_{peak} may exhibit a significant deviation from an empirical relation between f_{ peak} and the tidal deformability if a strong first-order phase transition leads to the formation of a gravitationally stable extended quark matter core in the postmergers remnant.
Abstract: We identify an observable imprint of a first-order hadron-quark phase transition at supranuclear densities on the gravitational-wave (GW) emission of neutron-star mergers. Specifically, we show that the dominant postmerger GW frequency fpeak may exhibit a significant deviation from an empirical relation between fpeak and the tidal deformability if a strong first-order phase transition leads to the formation of a gravitationally stable extended quark matter core in the postmerger remnant. A comparison of the GW signatures from a large, representative sample of microphysical, purely hadronic equations of state indicates that this imprint is only observed in those systems which undergo a strong first-order phase transition. Such a shift of the dominant postmerger GW frequency can be revealed by future GW observations, which would provide evidence for the existence of a strong first-order phase transition in the interior of neutron-stars.
318 citations
TL;DR: This work presents the first fully general-relativistic simulations of merging neutron-stars including quarks at finite temperatures that can be switched off consistently in the equation of state and shows that the phase transition leads to a very hot and dense quark core that, when it collapses to a black hole, produces a ringdown signal different from the hadronic one.
Abstract: Merging binaries of neutron-stars are not only strong sources of gravitational waves, but also have the potential of revealing states of matter at densities and temperatures not accessible in laboratories. A crucial and long-standing question in this context is whether quarks are deconfined as a result of the dramatic increase in density and temperature following the merger. We present the first fully general-relativistic simulations of merging neutron-stars including quarks at finite temperatures that can be switched off consistently in the equation of state. Within our approach, we can determine clearly what signatures a quark-hadron phase transition would leave in the gravitational-wave signal. We show that if after the merger the conditions are met for a phase transition to take place at several times nuclear saturation density, they would lead to a postmerger signal considerably different from the one expected from the inspiral, that can only probe the hadronic part of the equations of state, and to an anticipated collapse of the merged object. We also show that the phase transition leads to a very hot and dense quark core that, when it collapses to a black hole, produces a ringdown signal different from the hadronic one. Finally, in analogy with what is done in heavy-ion collisions, we use the evolution of the temperature and density in the merger remnant to illustrate the properties of the phase transition in a QCD phase diagram.
311 citations
284 citations
TL;DR: The state that is reported here, with its strongly broken electronic flavour symmetry and revived Dirac-like electronic character, is important in the physics of magic-angle graphene, forming the parent state out of which the more fragile superconducting and correlated insulating ground states emerge.
Abstract: Twisted bilayer graphene near the magic angle exhibits remarkably rich electron correlation physics, displaying insulating, magnetic, and superconducting phases. Here, using measurements of the local electronic compressibility, we reveal that these phases originate from a high-energy state with an unusual sequence of band populations. As carriers are added to the system, rather than filling all the four spin and valley flavors equally, we find that the population occurs through a sequence of sharp phase transitions, which appear as strong asymmetric jumps of the electronic compressibility near integer fillings of the moire lattice. At each transition, a single spin/valley flavor takes all the carriers from its partially filled peers, "resetting" them back to the vicinity of the charge neutrality point. As a result, the Dirac-like character observed near the charge neutrality reappears after each integer filling. Measurement of the in-plane magnetic field dependence of the chemical potential near filling factor one reveals a large spontaneous magnetization, further substantiating this picture of a cascade of symmetry breakings. The sequence of phase transitions and Dirac revivals is observed at temperatures well above the onset of the superconducting and correlated insulating states. This indicates that the state we reveal here, with its strongly broken electronic flavor symmetry and revived Dirac-like electronic character, is a key player in the physics of magic angle graphene, forming the parent state out of which the more fragile superconducting and correlated insulating ground states emerge.
278 citations
TL;DR: In this paper, the authors studied the conditions for successful bubble percolation and completion of the electroweak phase transition in theories beyond the Standard Model featuring polynominal potentials, and showed that these conditions set a lower bound on the temperature of the transition.
Abstract: What is the maximum possible strength of a first-order electroweak phase transition and the resulting gravitational wave (GW) signal? While naively one might expect that supercooling could increase the strength of the transition to very high values, for strong supercooling the Universe is no longer radiation-dominated and the vacuum energy of the unstable minimum of the potential dominates the expansion, which can jeopardize the successful completion of the phase transition. After providing a general treatment for the nucleation, growth and percolation of broken phase bubbles during a first-order phase transition that encompasses the case of significant supercooling, we study the conditions for successful bubble percolation and completion of the electroweak phase transition in theories beyond the Standard Model featuring polynominal potentials. For such theories, these conditions set a lower bound on the temperature of the transition. Since the plasma cannot be significantly diluted, the resulting GW signal originates mostly from sound waves and turbulence in the plasma, rather than bubble collisions. We find the peak frequency of the GW signal from the phase transition to be generically $f \gtrsim 10^{-4}$ Hz. We also study the condition for GW production by sound waves to be long-lasting (GW source active for approximately a Hubble time), showing it is generally not fulfilled in concrete scenarios. Because of this the sound wave GW signal could be weakened, with turbulence setting in earlier, resulting in a smaller overall GW signal as compared to current literature predictions.
267 citations
TL;DR: In this paper, an ultrafast phase transition into a hidden ferroelectric phase can be dynamically induced in quantum paraelectric strontium titanate (SrTiO3) using terahertz electric field excitation.
Abstract: "Hidden phases" are metastable collective states of matter that are typically not accessible on equilibrium phase diagrams. These phases can host exotic properties in otherwise conventional materials and hence may enable novel functionality and applications, but their discovery and access are still in early stages. Using intense terahertz electric field excitation, we found that an ultrafast phase transition into a hidden ferroelectric phase can be dynamically induced in quantum paraelectric strontium titanate (SrTiO3). The induced lowering in crystal symmetry yields substantial changes in the phonon excitation spectra. Our results demonstrate collective coherent control over material structure, in which a single-cycle field drives ions along the microscopic pathway leading directly to their locations in a new crystalline phase on an ultrafast time scale.
254 citations
TL;DR: In this paper, it was shown that optical excitation of lattice vibrations can induce polar order in Strontium titanate (SrTiO3) and this metastable polar phase, observed up to temperatures exceeding 290 kelvin, persists for hours after the optical pump is interrupted.
Abstract: Fluctuating orders in solids are generally considered high-temperature precursors of broken symmetry phases. However, in some cases, these fluctuations persist to zero temperature and prevent the emergence of long-range order. Strontium titanate (SrTiO3) is a quantum paraelectric in which dipolar fluctuations grow upon cooling, although a long-range ferroelectric order never sets in. Here, we show that optical excitation of lattice vibrations can induce polar order. This metastable polar phase, observed up to temperatures exceeding 290 kelvin, persists for hours after the optical pump is interrupted. Furthermore, hardening of a low-frequency vibration points to a photoinduced ferroelectric phase transition, with a spatial domain distribution suggestive of a photoflexoelectric coupling.
TL;DR: In this paper, the authors address the question of whether entanglement can sustain repeated measurements in a toy hybrid-circuit model, where qubits undergo both unitary time evolution and sporadic local measurements.
Abstract: Quantum information tends to spread out in interacting systems, leading to thermalization as characterized by the volume-law entropy of entanglement, while local measurements that extract classical information of the state tend to interrupt the propagation of information and reduce the entanglement entropy. Can entanglement sustain repeated measurements? The authors address this question in a toy hybrid-circuit model, where qubits undergo both unitary time evolution and sporadic local measurements. The phase diagram features a stable volume-law phase of entanglement for weak measurements and an area-law phase for strong measurements. The two domains in the diagram are separated by a single second-order phase transition. Characterization of both phases as well as of the critical point are presented.
TL;DR: The concepts of parity-time symmetry need not be restricted to wave dynamics and may offer opportunities to manipulate heat and mass transport, and systems described by diffusive dynamics are considered.
Abstract: Various concepts related to parity-time symmetry, including anti-parity-time symmetry, have found broad applications in wave physics. Wave systems are fundamentally described by Hermitian operators, whereas their unusual properties are introduced by incorporation of gain and loss. We propose that the related physics need not be restricted to wave dynamics, and we consider systems described by diffusive dynamics. We study the heat transfer in two countermoving media and show that this system exhibits anti-parity-time symmetry. The spontaneous symmetry breaking results in a phase transition from motionless temperature profiles, despite the mechanical motion of the background, to moving temperature profiles. Our results extend the concepts of parity-time symmetry beyond wave physics and may offer opportunities to manipulate heat and mass transport.
TL;DR: The experimental and molecular dynamics results indicate that a theory beyond classical nucleation theory is needed to describe early-stage nucleation at the atomic scale, and it is anticipated that the reported approach will open the door to the study of many fundamental problems in materials science, nanoscience, condensed matter physics and chemistry.
Abstract: Nucleation plays a critical role in many physical and biological phenomena that range from crystallization, melting and evaporation to the formation of clouds and the initiation of neurodegenerative diseases1-3. However, nucleation is a challenging process to study experimentally, especially in its early stages, when several atoms or molecules start to form a new phase from a parent phase. A number of experimental and computational methods have been used to investigate nucleation processes4-17, but experimental determination of the three-dimensional atomic structure and the dynamics of early-stage nuclei has been unachievable. Here we use atomic electron tomography to study early-stage nucleation in four dimensions (that is, including time) at atomic resolution. Using FePt nanoparticles as a model system, we find that early-stage nuclei are irregularly shaped, each has a core of one to a few atoms with the maximum order parameter, and the order parameter gradient points from the core to the boundary of the nucleus. We capture the structure and dynamics of the same nuclei undergoing growth, fluctuation, dissolution, merging and/or division, which are regulated by the order parameter distribution and its gradient. These experimental observations are corroborated by molecular dynamics simulations of heterogeneous and homogeneous nucleation in liquid-solid phase transitions of Pt. Our experimental and molecular dynamics results indicate that a theory beyond classical nucleation theory1,2,18 is needed to describe early-stage nucleation at the atomic scale. We anticipate that the reported approach will open the door to the study of many fundamental problems in materials science, nanoscience, condensed matter physics and chemistry, such as phase transition, atomic diffusion, grain boundary dynamics, interface motion, defect dynamics and surface reconstruction with four-dimensional atomic resolution.
11 Sep 2019
TL;DR: In this paper, a bulk probing method based on the Lyapunov exponent calculation of a quantum walker on the lattice was proposed to reveal non-Bloch phase transitions, the non-Hermitian skin effect and breakdown of the bulk-boundary correspondence.
Abstract: This paper uncovers a bulk probing method to catch physical effects hindered in topological non-Hermitian crystals. The method is based on Lyapunov exponent calculation of a quantum walker on the lattice and can reveal non-Bloch phase transitions, the non-Hermitian skin effect and breakdown of the bulk-boundary correspondence.
TL;DR: A novel strategy to suppress the octahedral tilting of [PbI6 ]4- units in cubic CsPb I3 by systematically controlling the steric hindrance of surface organic terminal groups is provided and contributes to a stable power conversion efficiency of 13.2% for the inverted planar perovskite solar cells (PSCs), which is the highest efficiency achieved by the inverted-structure inorganic PSCs.
Abstract: All-inorganic cesium lead iodide perovskites (CsPbI3 ) are promising wide-bandgap materials for use in the perovskite/silicon tandem solar cells, but they easily undergo a phase transition from a cubic black phase to an orthorhombic yellow phase under ambient conditions. It is shown that this phase transition is triggered by moisture that causes distortion of the corner-sharing octahedral framework ([PbI6 ]4- ). Here, a novel strategy to suppress the octahedral tilting of [PbI6 ]4- units in cubic CsPbI3 by systematically controlling the steric hindrance of surface organic terminal groups is provided. This steric hindrance effectively prevents the lattice distortion and thus increases the energy barrier for phase transition. This mechanism is verified by X-ray diffraction measurements and density functional theory calculations. Meanwhile, the formation of an organic capping layer can also passivate the surface electronic trap states of perovskite absorber. These modifications contribute to a stable power conversion efficiency (PCE) of 13.2% for the inverted planar perovskite solar cells (PSCs), which is the highest efficiency achieved by the inverted-structure inorganic PSCs. More importantly, the optimized devices retained 85% of their initial PCE after aging under ambient conditions for 30 days.
TL;DR: A comprehensive list of phase change solvents reported in the recent literature, including those subject to chemically or thermally triggered phase changes, non-aqueous or aqueous systems, and those forming either a CO2-enriched solid or a liquid phase are provided and their physiochemical properties.
01 Dec 2019
TL;DR: Independent signatures of criticality are tested and it is shown that a phase transition occurs in an intermediate value of spiking variability, in both anesthetized and freely moving animals.
Abstract: Since the first measurements of neuronal avalanches, the critical brain hypothesis has gained traction. However, if the brain is critical, what is the phase transition? For several decades, it has been known that the cerebral cortex operates in a diversity of regimes, ranging from highly synchronous states (with higher spiking variability) to desynchronized states (with lower spiking variability). Here, using both new and publicly available data, we test independent signatures of criticality and show that a phase transition occurs in an intermediate value of spiking variability, in both anesthetized and freely moving animals. The critical exponents point to a universality class different from mean-field directed percolation. Importantly, as the cortex hovers around this critical point, the avalanche exponents follow a linear relation that encompasses previous experimental results from different setups and is reproduced by a model.
TL;DR: It is demonstrated that transport under steady-state illumination is considerably altered at the structural phase transition in the MA compounds, providing advanced insights into the evolution of the crystal structure with decreasing temperature that are essential to interpret the growing interest in investigating the electronic, optical, and photonic properties of lead-halide perovskite materials.
Abstract: The extraordinary properties of lead-halide perovskite materials have spurred intense research, as they have a realistic perspective to play an important role in future photovoltaic devices. It is known that these materials undergo a number of structural phase transitions as a function of temperature that markedly alter their optical and electronic properties. The precise phase transition temperature and exact crystal structure in each phase, however, are controversially discussed in the literature. The linear thermal expansion of single crystals of APbX3 (A = methylammonium (MA), formamidinium (FA); X = I, Br) below room temperature is measured using a high-resolution capacitive dilatometer to determine the phase transition temperatures. For δ-FAPbI3 , two wide regions of negative thermal expansion below 173 and 54 K, and a cascade of sharp transitions for FAPbBr3 that have not previously been reported are uncovered. Their respective crystal phases are identified via powder X-ray diffraction. Moreover, it is demonstrated that transport under steady-state illumination is considerably altered at the structural phase transition in the MA compounds. The results provide advanced insights into the evolution of the crystal structure with decreasing temperature that are essential to interpret the growing interest in investigating the electronic, optical, and photonic properties of lead-halide perovskite materials.
TL;DR: A dynamically tunable reflectarray metasurface that continuously modulates the phase of reflected light in the near-infrared wavelength range under active electrical control of the phase transition from semiconducting to semimetallic states is reported.
Abstract: We report a dynamically tunable reflectarray metasurface that continuously modulates the phase of reflected light in the near-infrared wavelength range under active electrical control of the phase transition from semiconducting to semimetallic states. We integrate a vanadium dioxide (VO2) active layer into the dielectric gap of antenna elements in a reflectarray metasurface, which undergoes an insulator-to-metal transition upon resistive heating of the metallic patch antenna. The induced phase transition in the VO2 film strongly perturbs the magnetic dipole resonance supported by the metasurface. By carefully controlling the volume fractions of coexisting metallic and dielectric regions of the VO2 film, we observe a continuous shift of the phase of the reflected light, with a maximal achievable phase shift as high as 250°. We also observe a reflectance modulation of 23.5% as well as a spectral shift of the resonance position by 175 nm. The metasurface phase modulation is fairly broadband, yielding large phase shifts at multiple operation wavelengths.
TL;DR: In this article, electron tunneling through few-layer crystals of the layered antiferromagnetic insulator CrCl3 was used to probe its magnetic order, finding a tenfold enhancement in the interlayer exchange compared to bulk crystals.
Abstract: Following the recent isolation of monolayer CrI3, there has been a surge of new two-dimensional van der Waals magnetic materials, whose incorporation in van der Waals heterostructures offers a new platform for spintronics, proximity magnetism, and quantum spin liquids. A primary question in this burgeoning field is how exfoliating crystals to the few-layer limit influences their magnetism. Studies on CrI3 have shown a different magnetic ground state for ultrathin exfoliated films but the origin is not yet understood. Here, we use electron tunneling through few-layer crystals of the layered antiferromagnetic insulator CrCl3 to probe its magnetic order, finding a ten-fold enhancement in the interlayer exchange compared to bulk crystals. Moreover, temperature- and polarization-dependent Raman spectroscopy reveal that the crystallographic phase transition of bulk crystals does not occur in exfoliated films. This results in a different low temperature stacking order and, we hypothesize, increased interlayer exchange. Our study provides new insight into the connection between stacking order and interlayer interactions in novel two-dimensional magnets, which may be relevant for correlating stacking faults and mechanical deformations with the magnetic ground states of other more exotic layered magnets, such as RuCl3.
TL;DR: Dense (1-x)(0.775NBT-0.225BSN)-xBZ ternary ceramics were fabricated by solid state methods and their dielectric and ferroelectric properties were systematically investigated.
TL;DR: In this paper, it was shown that, for mixed initial states, a balanced competition between measurements and entangling interactions within the system can result in a dynamical purification phase transition between (i) a phase that locally purifies at a constant system-size independent rate, and (ii) a "mixed" phase where the purification time diverges exponentially in the system size.
Abstract: Continuously monitoring the environment of a quantum many-body system reduces the entropy of (purifies) the reduced density matrix of the system, conditional on the outcomes of the measurements. We show that, for mixed initial states, a balanced competition between measurements and entangling interactions within the system can result in a dynamical purification phase transition between (i) a phase that locally purifies at a constant system-size-independent rate, and (ii) a "mixed" phase where the purification time diverges exponentially in the system size. The residual entropy density in the mixed phase implies the existence of a quantum error-protected subspace where quantum information is reliably encoded against the future non-unitary evolution of the system. We show that these codes are of potential relevance to fault-tolerant quantum computation as they are often highly degenerate and satisfy optimal tradeoffs between encoded information densities and error thresholds. In spatially local models in 1+1 dimensions, this phase transition for mixed initial states occurs concurrently with a recently identified class of entanglement phase transitions for pure initial states. The mutual information of an initially completely-mixed state in 1+1 dimensions grows sublinearly in time due to the formation of the error protected subspace. The purification transition studied here also generalizes to systems with long-range interactions, where conventional notions of entanglement transitions have to be reformulated. Purification dynamics is likely a more robust probe of the transition in experiments, where imperfections generically reduce entanglement and drive the system towards mixed states. We describe the motivations for studying this novel class of non-equilibrium quantum dynamics in the context of advanced quantum computing platforms and fault-tolerant quantum computation.
TL;DR: In this paper, a series of W-doped LiNiO2 cathodes were synthesized to systematically investigate the stabilization effect of W doping and showed that W doping prolonged the deleterious phase transition to the extent that the two-phase reaction (H2 → H3) merged into a single phase; thus, the phase transition proceeded through a solid-solution-like reaction.
Abstract: A series of W-doped (1.0, 1.5, and 2.0 mol%) LiNiO2 cathodes was synthesized to systematically investigate the stabilization effect of W doping. The 2 mol% W-LiNiO2 cathode delivered 195.6 mA h g−1 even after 100 cycles at 0.5C, which was 95.5% of its initial capacity. The capacity retention of LiNiO2 cycled under the same conditions was 73.7%. In situ X-ray diffraction analysis of the cathodes during charging showed that the W doping protracted the deleterious phase transition to the extent that the two-phase reaction (H2 → H3) merged into a single phase; thus, the phase transition proceeded through a solid-solution-like reaction. The significantly enhanced cycling stability due to W doping largely originated from the reduction of the structural stress associated with the repetitive phase transition caused by the reduction of the abrupt lattice collapse/expansion. The effect of the reduced lattice distortion together with the W-rich surface phase and cation ordering greatly stabilized the LiNiO2 structure during cycling, making W-doped LiNiO2 a candidate material for practical high-energy density cathodes.
TL;DR: In this article, quantum critical behavior at the many-body localization transition in a disordered Bose-Hubbard system of bosonic rubidium atoms in an optical lattice is observed, connecting the macroscopic phenomenology of the transition to the system's microscopic quantum correlations.
Abstract: Phase transitions are driven by collective fluctuations of a system’s constituents that emerge at a critical point1. This mechanism has been extensively explored for classical and quantum systems in equilibrium, whose critical behaviour is described by the general theory of phase transitions. Recently, however, fundamentally distinct phase transitions have been discovered for out-of-equilibrium quantum systems, which can exhibit critical behaviour that defies this description and is not well understood1. A paradigmatic example is the many-body localization (MBL) transition, which marks the breakdown of thermalization in an isolated quantum many-body system as its disorder increases beyond a critical value2–11. Characterizing quantum critical behaviour in an MBL system requires probing its entanglement over space and time4,5,7, which has proved experimentally challenging owing to stringent requirements on quantum state preparation and system isolation. Here we observe quantum critical behaviour at the MBL transition in a disordered Bose–Hubbard system and characterize its entanglement via its multi-point quantum correlations. We observe the emergence of strong correlations, accompanied by the onset of anomalous diffusive transport throughout the system, and verify their critical nature by measuring their dependence on the system size. The correlations extend to high orders in the quantum critical regime and appear to form via a sparse network of many-body resonances that spans the entire system12,13. Our results connect the macroscopic phenomenology of the transition to the system’s microscopic structure of quantum correlations, and they provide an essential step towards understanding criticality and universality in non-equilibrium systems1,7,13. Quantum critical behaviour at the many-body localization transition in a disordered Bose–Hubbard system of bosonic rubidium atoms in an optical lattice is observed, connecting the macroscopic phenomenology of the transition to the system’s microscopic quantum correlations.
TL;DR: In this paper, the authors review different types of phase transitions that can appear in our cosmic history, and their applications and experimental signatures in particular in the context of exciting gravitational waves, which could be potentially be constrained by LIGO/VIRGO, Kagra, LISA, and Decigo.
Abstract: The study of cosmic phase transitions are of central interest in modern cosmology. In the standard model of cosmology the Universe begins in a very hot state, right after at the end of inflation via the process of reheating/preheating, and cools to its present temperature as the Universe expands. Both new and existing physics at any scale can be responsible for catalyzing either first, second or cross over phase transition, which could be either thermal or non-thermal with a potential observable imprints. Thus this field prompts a rich dialogue between gravity, particle physics and cosmology. It is all but certain that at least two cosmic phase transitions have occurred-the electroweak and the QCD phase transitions. The focus of this review will be primarily on phase transitions above such scales, We review different types of phase transitions that can appear in our cosmic history, and their applications and experimental signatures in particular in the context of exciting gravitational waves, which could be potentially be constrained by LIGO/VIRGO, Kagra, LISA, and Decigo.
01 Feb 2019
TL;DR: In this article, the critical properties of the super-radiant phase transition and the distinction between equilibrium and none-quilibrium conditions are reviewed, as well as some aspects of real-time dynamics, including superconducting qubits, trapped ions, and using spin-orbit coupling for cold atoms.
Abstract: The Dicke model describes the coupling between a quantized cavity field and a large ensemble of two-level atoms. When the number of atoms tends to infinity, this model can undergo a transition to a superradiant phase, belonging to the mean-field Ising universality class. The superradiant transition was first predicted for atoms in thermal equilibrium and was recently realized with a quantum simulator made of atoms in an optical cavity, subject to both dissipation and driving. In this Progress Report, we offer an introduction to some theoretical concepts relevant to the Dicke model, reviewing the critical properties of the superradiant phase transition, and the distinction between equilibrium and nonequilibrium conditions. In addition, we explain the fundamental difference between the superradiant phase transition and the more common lasing transition. Our report mostly focuses on the steady states of atoms in single-mode optical cavities, but we also mention some aspects of real-time dynamics, as well as other quantum simulators, including superconducting qubits, trapped ions, and using spin-orbit coupling for cold atoms. These realizations differ in regard to whether they describe equilibrium or non-equilibrium systems.
TL;DR: In this article, the authors investigate the light-induced melting of a unidirectional charge density wave (CDW) in LaTe and find that a fast recovery of the CDW amplitude is followed by a slower re-establishment of phase coherence.
Abstract: Upon excitation with an intense laser pulse, a symmetry-broken ground state can undergo a non-equilibrium phase transition through pathways different from those in thermal equilibrium. The mechanism underlying these photoinduced phase transitions has long been researched in the study of condensed matter systems1, but many details in this ultrafast, non-adiabatic regime still remain to be clarified. To this end, we investigate the light-induced melting of a unidirectional charge density wave (CDW) in LaTe3. Using a suite of time-resolved probes, we independently track the amplitude and phase dynamics of the CDW. We find that a fast (approximately 1 picosecond) recovery of the CDW amplitude is followed by a slower re-establishment of phase coherence. This longer timescale is dictated by the presence of topological defects: long-range order is inhibited and is only restored when the defects annihilate. Our results provide a framework for understanding other photoinduced phase transitions by identifying the generation of defects as a governing mechanism. Three different ultrafast probes investigate a non-adiabatic phase transition and find substantial evidence of topological defects inhibiting the reformation of the equilibrium phase.
TL;DR: It is shown that interlayer electronic ordering can play an important role in electronic phase transitions in layered materials, including the pressure- and doping-induced transitions and the hysteresis behavior.
Abstract: Using density functional theory calculations, we investigate the origin of the insulating phase and metal-insulator transition (MIT) in octahedral tantalum disulfide (1T-TaS_{2}), a layered van der Waals material with a prominent two-dimensional (2D) charge density wave (CDW) order. We show that the MIT is driven not by the 2D order itself, but by the vertical ordering of the 2D CDWs or the 3D CDW order. We identify two exceptionally stable 3D CDW configurations; one is insulating and the other is metallic. The competition and mixing of the two CDW configurations account for many mysterious features of the MIT in 1T-TaS_{2}, including the pressure- and doping-induced transitions and the hysteresis behavior. The present results emphasize that interlayer electronic ordering can play an important role in electronic phase transitions in layered materials.