Showing papers on "Phase transition published in 2011"

Direct imaging of the coexistence of ferromagnetism and superconductivity at the LaAlO3/SrTiO3 interface

Abstract: LaAlO{sub 3} and SrTiO{sub 3} are insulating, nonmagnetic oxides, yet the interface between them exhibits a two-dimensional electron system with high electron mobility, superconductivity at low temperatures, and electric-field-tuned metal-insulator and superconductor-insulator phase transitions Bulk magnetization and magnetoresistance measurements also suggest some form of magnetism depending on preparation conditions and suggest a tendency towards nanoscale electronic phase separation Here we use local imaging of the magnetization and magnetic susceptibility to directly observe a landscape of ferromagnetism, paramagnetism, and superconductivity We find submicron patches of ferromagnetism in a uniform background of paramagnetism, with a nonuniform, weak diamagnetic superconducting susceptibility at low temperature These results demonstrate the existence of nanoscale phase separation as suggested by theoretical predictions based on nearly degenerate interface subbands associated with the Ti orbitals The magnitude and temperature dependence of the paramagnetic response suggests that the vast majority of the electrons at the interface are localized, and do not contribute to transport measurements In addition to the implications for magnetism, the existence of a 2D superconductor at an interface with highly broken inversion symmetry and a ferromagnetic landscape in the background suggests the potential for exotic superconducting phenomena

On the phase identity and its thermal evolution of lead free (Bi1/2Na1/2)TiO3-6 mol% BaTiO3

Abstract: Temperature-dependent dielectric permittivity of 0.94(Bi1/2Na1/2)TiO3-0.06BaTiO3 (BNT-6BT) lead-free piezoceramics was studied to disentangle the existing unclear issues over the crystallographic aspects and phase stability of the system. Application of existing phenomenological relaxor models enabled the relaxor contribution to the entire dielectric permittivity spectra to be deconvoluted. The deconvoluted data in comparison with the temperature-dependent dielectric permittivity of a classical perovskite relaxor, La-modified lead zirconate titanate, clearly suggest that BNT-6BT belongs to the same relaxor category, which was also confirmed by a comparative study on the temperature- dependent polarization hysteresis loops of both materials. Based on these results, we propose that the low-temperature dielectric anomaly does not involve any phase transition such as ferroelectric- to-antiferroelectric. Supported by transmission electron microscopy and X-ray diffraction experiments at ambient temperature, we propose that the commonly observed two dielectric anomalies are attributed to thermal evolution of ferroelectric polar nanoregions of R3c and P4bm symmetry, which coexist nearly throughout the entire temperature range and reversibly transform into each other with temperature.

Aqueous Liquid Crystals of Graphene Oxide

Abstract: The formation of liquid crystals (LCs) is the most viable approach to produce macroscopic, periodic self-assembled materials from oriented graphene sheets. Herein, we have discovered that well-soluble and single-layered graphene oxide (GO) sheets can exhibit nematic liquid crystallinity in water and first established their isotropic−nematic solid phase diagram versus mass fraction and salt concentration. The zeta potential of GO dispersion is around −64 mV, and its absolute value decreases with increasing salt concentration, implying that the electrostatic repulsive force between negatively charged GO sheets is the dominant interaction in the system of GOLCs and also explaining the salt-dependent phase behavior. For single-layer GO sheets with average diameter of 2.1 μm and polydispersity index of 83%, the isotropic−nematic phase transition occurs at a mass concentration of ∼0.025%, and a stable nematic phase forms at ∼0.5%. Rheological measurements showed that GO aqueous dispersions performed as typical ...

Phase behavior and properties of the liquid-crystal dimer 1′′,7′′-bis(4-cyanobiphenyl-4′-yl) heptane: A twist-bend nematic liquid crystal

Abstract: The liquid-crystal dimer 1'',7''-bis(4-cyanobiphenyl-4'-yl)heptane (CB7CB) exhibits two liquid-crystalline mesophases on cooling from the isotropic phase. The high-temperature phase is nematic; the identification and characterization of the other liquid-crystal phase is reported in this paper. It is concluded that the low-temperature mesophase of CB7CB is a new type of uniaxial nematic phase having a nonuniform director distribution composed of twist-bend deformations. The techniques of small-angle x-ray scattering, modulated differential scanning calorimetry, and dielectric spectroscopy have been applied to establish the nature of the nematic-nematic phase transition and the structural features of the twist-bend nematic phase. In addition, magnetic resonance studies (electron-spin resonance and (2)H nuclear magnetic resonance) have been used to investigate the orientational order and director distribution in the liquid-crystalline phases of CB7CB. The synthesis of a specifically deuterated sample of CB7CB is reported, and measurements showed a bifurcation of the quadrupolar splitting on entering the low-temperature mesophase from the high-temperature nematic phase. This splitting could be interpreted in terms of the chirality of the twist-bend structure of the director. Calculations using an atomistic model and the surface interaction potential with Monte Carlo sampling have been carried out to determine the conformational distribution and predict dielectric and elastic properties in the nematic phase. The former are in agreement with experimental measurements, while the latter are consistent with the formation of a twist-bend nematic phase.

Two-Step Melting in Two Dimensions: First-Order Liquid-Hexatic Transition

Abstract: Melting in two spatial dimensions, as realized in thin films or at interfaces, represents one of the most fascinating phase transitions in nature, but it remains poorly understood. Even for the fundamental hard-disk model, the melting mechanism has not been agreed upon after 50 years of studies. A recent Monte Carlo algorithm allows us to thermalize systems large enough to access the thermodynamic regime. We show that melting in hard disks proceeds in two steps with a liquid phase, a hexatic phase, and a solid. The hexatic-solid transition is continuous while, surprisingly, the liquid-hexatic transition is of first order. This melting scenario solves one of the fundamental statistical-physics models, which is at the root of a large body of theoretical, computational, and experimental research.

Superconductor-insulator transition in La2 - xSrxCuO4 at the pair quantum resistance.

Abstract: High-temperature superconductivity in copper oxides arises when a parent insulator compound is 'doped' by adding or removing valence electrons, usually by inserting atoms into the lattice structure. This would be better achieved by tuning the carrier density using the electric field effect, as it removes ambiguity about whether the electronic properties change because of alterations in the crystal structure or in the electronic structure. Such tuning is difficult to achieve because it requires perfect, ultrathin films and a huge local field. Bollinger et al. report the synthesis of one-cell-thick epitaxial films of La2xSrxCuO4, and the use of the films to make double-layer transistors. The transistors have very large fields, and by changing the surface carrier density, the critical temperature can be shifted by up to 30 K. The resistance varies as predicted for a two-dimensional superconductor–insulator transition. High-temperature superconductivity in copper oxides arises when a parent insulator compound is doped beyond some critical concentration; what exactly happens at this superconductor–insulator transition is a key open question1. The cleanest approach is to tune the carrier density using the electric field effect2,3,4,5,6,7; for example, it was learned in this way5 that weak electron localization transforms superconducting SrTiO3 into a Fermi-glass insulator. But in the copper oxides this has been a long-standing technical challenge3, because perfect ultrathin films and huge local fields (>109 V m−1) are needed. Recently, such fields have been obtained using electrolytes or ionic liquids in the electric double-layer transistor configuration8,9,10. Here we report synthesis of epitaxial films of La2− xSr x CuO4 that are one unit cell thick, and fabrication of double-layer transistors. Very large fields and induced changes in surface carrier density enable shifts in the critical temperature by up to 30 K. Hundreds of resistance versus temperature and carrier density curves were recorded and shown to collapse onto a single function, as predicted for a two-dimensional superconductor–insulator transition11,12,13,14. The observed critical resistance is precisely the quantum resistance for pairs, RQ = h/(2e)2 = 6.45 kΩ, suggestive of a phase transition driven by quantum phase fluctuations, and Cooper pair (de)localization.

Light-induced spin-crossover magnet

Abstract: The light-induced phase transition between the low-spin (LS) and high-spin (HS) states of some transition-metal ions has been extensively studied in the fields of chemistry and materials science. In a crystalline extended system, magnetically ordering the HS sites of such transition-metal ions by irradiation should lead to spontaneous magnetization. Previous examples of light-induced ordering have typically occurred by means of an intermetallic charge transfer mechanism, inducing a change of valence of the metal centres. Here, we describe the long-range magnetic ordering of the extended Fe(II)(HS) sites in a metal-organic framework caused instead by a light-induced excited spin-state trapping effect. The Fe-Nb-based material behaves as a spin-crossover magnet, in which a strong superexchange interaction (magnetic coupling through non-magnetic elements) between photo-produced Fe(II)(HS) and neighbouring Nb(IV) atoms operates through CN bridges. The magnetic phase transition is observed at 20 K with a coercive field of 240 Oe.

Evolving morphotropic phase boundary in lead-free (Bi1/2Na1/2)TiO3–BaTiO3 piezoceramics

Abstract: The correlation between structure and electrical properties of lead-free (1−x)(Bi1/2Na1/2)TiO3–xBaTiO3 (BNT-100xBT) polycrystalline piezoceramics was investigated systematically by in situ synchrotron diffraction technique, combined with electrical property characterization It was found that the morphotropic phase boundary (MPB) between a rhombohedral and a tetragonal phase evolved into a morphotropic phase region with electric field In the unpoled material, the MPB was positioned at the transition from space group R3m to P4mm (BNT-11BT) with optimized permittivity throughout a broad single-phase R3m composition regime Upon poling, a range of compositions from BNT-6BT to BNT-11BT became two-phase mixture, and maximum piezoelectric coefficient was observed in BNT-7BT It was shown that optimized electrical properties are related primarily to the capacity for domain texturing and not to phase coexistence

New route for stabilization of 1T-WS2 and MoS2 phases

Abstract: The phenomenon of a partial 2H\rightarrow1T phase transition within multiwalled WS2 nanotubes under substitutional Rhenium doping is discovered by means of high-resolution transmission electron microscopy. Using density-functional calculations for the related MoS2 compound we consider a possible origin of this phase transition, which was known formerly only for WS2 and MoS2 intercalated by alkali metals. An interplay between the stability of layered or nanotubular forms of 2H and 1T allotropes is found to be intimately related with their electronic structures and electro-donating ability of an impurity.

New Route for Stabilization of 1T-WS2 and MoS2 Phases

Abstract: The phenomenon of a partial 2H→1T phase transition within multiwalled WS2 nanotubes under substitutional rhenium doping is discovered by means of high-resolution transmission electron microscopy. Using density-functional tight-binding calculations for the related MoS2 compound, we consider a possible origin of this phase transition, which was known formerly only for WS2 and MoS2 intercalated by alkali metals. An interplay between the stability of layered or nanotubular forms of 2H and 1T allotropes is found to be intimately related with their electronic structures and electro-donating ability of an impurity.

The Antiferroelectric ↔ Ferroelectric Phase Transition in Lead-Containing and Lead-Free Perovskite Ceramics

Abstract: A comprehensive review on the latest development of the antiferroelectric ↔ ferroelectric phase transition is presented. The abrupt volume expansion and sudden development of polarization at the phase transition has been extensively investigated in PbZrO3-based perovskite ceramics. New research developments in these compositions, including the incommensurate domain structure, the auxetic behavior under electric fields in the induced ferroelectric phase, the ferroelastic behavior of the multicell cubic phase, the impact of radial compression, the unexpected electric field-induced ferroelectric-to-antiferroelectric transition, and the phase transition mechanical toughening effect have been summarized. Due to their significance to leadfree piezoelectric ceramics, compounds with antiferroelectric phases, including NaNbO3, AgNbO3, and (Bi1/2Na1/2)TiO3, are also critically reviewed. Focus has been placed on the (Bi1/2Na1/2)TiO3–BaTiO3 solid solution where the electric field-induced ferroelectric phase remains even after the applied field is removed at room temperature. Therefore, the electric field-induced antiferroelectric-to-ferroelectric phase transition is a key to the poling process to develop piezoelectricity in morphotropic phase boundary (MPB) compositions. The competing phase transition and domain switching processes in 0.93 (Bi1/2Na1/2)TiO3–0.07BaTiO3 are directly imaged with nanometer resolution using the unique in situ transmission electron microscopy (TEM) technique.

Dimensionality Control of Electronic Phase Transitions in Nickel-Oxide Superlattices

Abstract: The competition between collective quantum phases in materials with strongly correlated electrons depends sensitively on the dimensionality of the electron system, which is difficult to control by standard solid-state chemistry. We have fabricated superlattices of the paramagnetic metal lanthanum nickelate (LaNiO 3 ) and the wide-gap insulator lanthanum aluminate (LaAlO 3 ) with atomically precise layer sequences. We used optical ellipsometry and low-energy muon spin rotation to show that superlattices with LaNiO 3 as thin as two unit cells undergo a sequence of collective metal-insulator and antiferromagnetic transitions as a function of decreasing temperature, whereas samples with thicker LaNiO 3 layers remain metallic and paramagnetic at all temperatures. Metal-oxide superlattices thus allow control of the dimensionality and collective phase behavior of correlated-electron systems.

Phase transitions in ferroelectric silicon doped hafnium oxide

Abstract: We investigated phase transitions in ferroelectric silicon doped hafnium oxide (FE-Si:HfO2) by temperature dependent polarization and x-ray diffraction measurements. If heated under mechanical confinement, the orthorhombic ferroelectric phase reversibly transforms into a phase with antiferroelectric behavior. Without confinement, a transformation into a monoclinic/tetragonal phase mixture is observed during cooling. These results suggest the existence of a common higher symmetry parent phase to the orthorhombic and monoclinic phases, while transformation between these phases appears to be inhibited by an energy barrier.

Precursor Phenomena at the Magnetic Ordering of the Cubic Helimagnet FeGe

Abstract: We report on detailed magnetic measurements on the cubic helimagnet FeGe in external magnetic fields and temperatures near the onset of long-range magnetic order at ${T}_{C}=278.2(3)\text{ }\text{ }\mathrm{K}$. Precursor phenomena display a complex succession of temperature-driven crossovers and phase transitions in the vicinity of ${T}_{C}$. The $A$-phase region, present below ${T}_{C}$ and fields $Hl0.5\text{ }\text{ }\mathrm{kOe}$, is split in several pockets. The complexity of the magnetic phase diagram is theoretically explained by the confinement of solitonic kinklike or Skyrmionic units that develop an attractive and oscillatory intersoliton coupling owing to the longitudinal inhomogeneity of the magnetization.

Pressure-induced superconductivity in topological parent compound Bi2Te3

Abstract: We report a successful observation of pressure-induced superconductivity in a topological compound Bi2Te3 with Tc of ∼3 K between 3 to 6 GPa. The combined high-pressure structure investigations with synchrotron radiation indicated that the superconductivity occurred at the ambient phase without crystal structure phase transition. The Hall effects measurements indicated the hole-type carrier in the pressure-induced superconducting Bi2Te3 single crystal. Consequently, the first-principles calculations based on the structural data obtained by the Rietveld refinement of X-ray diffraction patterns at high pressure showed that the electronic structure under pressure remained topologically nontrivial. The results suggested that topological superconductivity can be realized in Bi2Te3 due to the proximity effect between superconducting bulk states and Dirac-type surface states. We also discuss the possibility that the bulk state could be a topological superconductor.

From a single-band metal to a high-temperature superconductor via two thermal phase transitions.

Abstract: The nature of the pseudogap phase of cuprate high-temperature superconductors is one of the most important unsolved problems in condensed matter physics. We studied the commencement of the pseudogap state at temperature T* using three different techniques (angle-resolved photoemission spectroscopy, polar Kerr effect, and time-resolved reflectivity) on the same optimally-doped Bi2201 crystals. We observe the coincident onset at T* of a particle-hole asymmetric antinodal gap, a non-zero Kerr rotation, and a change in the relaxational dynamics, consistent with a phase transition. Upon further cooling, spectroscopic signatures of superconductivity begin to grow close to the superconducting transition temperature (T{sub c}), entangled in an energy-momentum dependent fashion with the pre-existing pseudogap features.

Structure and Properties of Liquid Crystals

Abstract: Foreword - Introduction - Literature Part I STRUCTURE OF LIQUID CRYSTALS Chapter 1 Symmetry11 Point group symmetry12 Translational symmetry Chapter 2 Mesogenic Molecules and Orientational Order21 Molecular shape and properties22 Intermolecular interactions 23 Orientational distribution functions for molecules24 Principal orientational order parameter (microscopic approach)25 Macroscopic definition of the orientational order parameter25 Apparent order parameters for flexible chains Chapter 3 Liquid Crystal Phases31 Polymorphism studies32 Main calamitic phases33 Discotic and bowl-type phases34 Role of polymerization35 Lyotropic phases36 General remarks on the role of chirality 37 Cholesterics38 Blue phases39 Chiral smectic C* phase310 Chiral smectic A*311 Spontaneous break of mirror symmetry Chapter 4 Principles of Structure Analysis and X-ray Diffraction 41 Diffraction studies and X-ray experiment41 X-ray scattering43 Diffraction on a periodic structure44 Fourier transforms and diffraction45 X-ray Diffraction by Crystals46 Diffraction by the isotropic and nematic phase47 Diffraction by smectic phases Chapter 5 Phase Transitions51 Landau approach52 Isotropic liquid -- nematic transition53 Nematic-Smectic A transition54 Smectic A -- Smectic C transition55 Dynamics of order parameter56 Molecular Statistic Approach to Phase Transitions57 Nematic-isotropic transition (molecular approach)58 Phase transitions in magn & el fields Part II PHYSICAL PROPERTIES Chapter 6 Magnetic, Electric and Transport Properties61 Magnetic phenomena62 Dielectric properties63 Transport propertiesChapter 7 Elasticity and Defects71 Tensor of elasticity72 Elasticity of nematics and cholesterics73 Variational problem and elastic torque74 Defects in nematics and cholesterics75 Smectic phases Chapter 8 Elements of Hydrodynamics81 Hydrodynamic variables82 Hydrodynamics of isotropic liquids83 Viscosity of nematics 83 Flow in cholesterics and smecticsChapter 9 Liquid Crystal -- Solid Interface 91 General properties of interfaces92 Surface energy and anchoring of nematics93 Liquid crystal alignment Part III ELECTRO-OPTICS Chapter 10 Optics and Field Effects in Nematic and Smectic A Liquid Crystals101 Optical properties of uniaxial phases102 Fredericks transition103 Flexoelectricity104 Electrohydrodynamic instability Chapter 11 Electro-Optical Effects in Cholesteric Phase111 Cholesteric as one-dimensional photonic crystal112 Field instabilities of cholesterics113 Bistability and memory114 Flexoelectricity in cholestericsChapter 12 Ferroelectricity and anti-ferroelectricity121 Crystalline ferroelectrics122 Ferroelectric cells with a non-ferroelectric liquid crystal?123 Phase transition SmA*-SmC* 124 Electro-optical effects in ferroelectric cells125 Anti-ferroelectrics Subject index

Energy‐Storage Properties of 0.89Bi0.5Na0.5TiO3–0.06BaTiO3–0.05K0.5Na0.5NbO3 Lead‐Free Anti‐ferroelectric Ceramics

Abstract: Energy-storage properties of 0.89Bi0.5Na0.5TiO3–0.06BaTiO3–0.05K0.5Na0.5NbO3 (0.89BNT–0.06BT–0.05KNN) lead-free ceramics were first investigated. Measurements of dielectric properties together with switching current curves indicate a rather diffuse ferroelectric (FE) to anti-ferroelectric (AFE) phase transition when heating from 20°C to 90°C. The energy density (W), which was calculated from P–E hysteresis loops, increases linearly with the external electric field when E exceeds AFE-FE transition field. W is independent of temperature and frequency, and maintains around 0.59 J/cm3 under 5.6 kV/mm in the stable AFE phase region. These properties indicate that 0.89BNT–0.06BT–0.05KNN ceramics might be a promising lead-free AFE material for energy-storage capacitor application.

Supramolecular bola-like ferroelectric: 4-methoxyanilinium tetrafluoroborate-18-crown-6.

Abstract: Molecular motion is one of the structural foundations for the development of functional molecular materials such as artificial motors and molecular ferroelectrics. Herein, we show that pendulum-like motion of the terminal group of a molecule causes a ferroelectric phase transition. Complex 4-methoxyanilinium tetrafluoroborate-18-crown-6 ([C7H10NO(18-crown-6)]+[BF4]−, 1) shows a second-order ferroelectric phase transition at 127 K, together with an abrupt dielectric anomaly, Debye-type relaxation behavior, and the symmetry breaking confirmed by temperature dependence of second harmonic generation effect. The origin of the polarization is due to the order–disorder transition of the pendulum-like motions of the terminal para-methyl group of the 4-methoxyanilinium guest cation; that is, the freezing of pendulum motion at low temperature forces significant orientational motions of the guest molecules and thus induces the formation of the ferroelectric phase. The supramolecular bola-like ferroelectric is distin...

Mechanics and chemical thermodynamics of phase transition in temperature-sensitive hydrogels

Abstract: This paper uses the thermodynamic data of aqueous solutions of uncrosslinked poly(N-isopropylacrylamide) (PNIPAM) to study the phase transition of PNIPAM hydrogels. At a low temperature, uncrosslinked PNIPAM can be dissolved in water and form a homogenous liquid solution. When the temperature is increased, the solution separates into two liquid phases with different concentrations of the polymer. Covalently crosslinked PNIPAM, however, does not dissolve in water, but can imbibe water and form a hydrogel. When the temperature is changed, the hydrogel undergoes a phase transition: the amount of water in the hydrogel in equilibrium changes with temperature discontinuously. While the aqueous solution is a liquid and cannot sustain any nonhydrostatic stress in equilibrium, the hydrogel is a solid and can sustain nonhydrostatic stress in equilibrium. The nonhydrostatic stress can markedly affect various aspects of the phase transition in the hydrogel. We adopt the Flory–Rehner model, and show that the interaction parameter as a function of temperature and concentration obtained from the PNIPAM–water solution can be used to analyze diverse phenomena associated with the phase transition of the PNIPAM hydrogel. We analyze free swelling, uniaxially and biaxially constrained swelling of a hydrogel, swelling of a core–shell structure, and coexistent phases in a rod. The analysis is related to available experimental observations. Also outlined is a general theory of coexistent phases undergoing inhomogeneous deformation.

Impedance and Raman spectroscopic studies of (Na0.5Bi0.5)TiO3

Abstract: Polycrystalline powder of (Na0.5Bi0.5)TiO3 (NBT) was prepared by a high-temperature solid-state reaction route. Preliminary x-ray diffraction analysis carried out at room temperature showed the formation of a single phase compound with a rhombohedral crystal system. Scanning electron micrograph reveals the polycrystalline nature of the material with size anisotropy. Dielectric study showed an existence of diffuse phase transition around 300 °C. The ac conductivity spectrum obeyed the Jonscher power law. The temperature dependent pre-exponential factor (A) shows peak and frequency exponent (n) possesses a minimum at transition temperature. The bulk conductivity of the material indicates an Arrhenius type of thermally activated process with three different conduction mechanisms as different activation energies are observed. The hopping charge carriers dominate at low temperature, small polaron and oxygen vacancy dominates at intermediate temperature and ionic conduction at higher temperatures. Studies of impedance spectroscopy indicate that the dielectric relaxation is of non-Debye type. In situ high-temperature Raman spectroscopy shows discontinuous changes in the phonon frequencies across the rhombohedral–tetragonal transition. In addition, anomalous changes in the intensity and the linewidth of a lattice mode are found around 350 °C.

Quantized conductance at the Majorana phase transition in a disordered superconducting wire.

Abstract: Superconducting wires without time-reversal and spin-rotation symmetries can be driven into a topological phase that supports Majorana bound states. Direct detection of these zero-energy states is complicated by the proliferation of low-lying excitations in a disordered multimode wire. We show that the phase transition itself is signaled by a quantized thermal conductance and electrical shot noise power, irrespective of the degree of disorder. In a ring geometry, the phase transition is signaled by a period doubling of the magnetoconductance oscillations. These signatures directly follow from the identification of the sign of the determinant of the reflection matrix as a topological quantum number.

Observation of scale invariance and universality in two-dimensional Bose gases.

Abstract: The collective behaviour of a many-body system near a continuous phase transition is insensitive to the details of its microscopic physics; for example, thermodynamic observables follow generalized scaling laws near the phase transition. The Berezinskii-Kosterlitz-Thouless (BKT) phase transition in two-dimensional Bose gases presents a particularly interesting case because the marginal dimensionality and intrinsic scaling symmetry result in a broad fluctuation regime and an extended range of universal scaling behaviour. Studies of the BKT transition in cold atoms have stimulated great interest in recent years, but a clear demonstration of critical behaviour near the phase transition has remained elusive. Here we report in situ density and density-fluctuation measurements of two-dimensional Bose gases of caesium at different temperatures and interaction strengths, observing scale-invariant, universal behaviours. The extracted thermodynamic functions confirm the existence of a wide universal region near the BKT phase transition, and provide a sensitive test of the universality predicted by classical-field theory and quantum Monte Carlo calculations. Our experimental results provide evidence for growing density-density correlations in the fluctuation region, and call for further explorations of universal phenomena in classical and quantum critical physics.

Phase boundary for the chiral transition in ( 2 + 1 )-flavor QCD at small values of the chemical potential

Abstract: We determine the chiral phase transition line in (2+1)-flavor QCD for small values of the light quark chemical potential. We show that for small values of the chemical potential the curvature of the phase transition line can be deduced from an analysis of scaling properties of the chiral condensate and its susceptibilities. To do so we extend earlier studies of the magnetic equation of state in (2+1)-flavor QCD to finer lattice spacings, aT = 1/8. We use these universal scaling properties of the chiral order parameter to extract the curvature of the transition line at two values of the cut-off, aT = 1/4 and 1/8. We find that cut-off effects are small for the curvature parameter and determine the transition line in the chiral limit to leading order in the light quark chemical potential. We obtain Tc(µq)/Tc(0) = 1 0.059(2)(4)(µq/T) 2 + O(µ 4).

Observation of transient structural-transformation dynamics in a Cu2S nanorod.

Abstract: The study of first-order structural transformations has been of great interest to scientists in many disciplines. Expectations from phase-transition theory are that the system fluctuates between two equilibrium structures near the transition point and that the region of transition broadens in small crystals. We report the direct observation of structural fluctuations within a single nanocrystal using transmission electron microscopy. We observed trajectories of structural transformations in individual nanocrystals with atomic resolution, which reveal details of the fluctuation dynamics, including nucleation, phase propagation, and pinning of structural domains by defects. Such observations provide crucial insight for the understanding of microscopic pathways of phase transitions.

Interaction-Driven Spectrum Reconstruction in Bilayer Graphene

Abstract: The nematic phase transition in electronic liquids, driven by Coulomb interactions, represents a new class of strongly correlated electronic ground states. We studied suspended samples of bilayer graphene, annealed so that it achieves very high quasiparticle mobilities (greater than 10(6) square centimers per volt-second). Bilayer graphene is a truly two-dimensional material with complex chiral electronic spectra, and the high quality of our samples allowed us to observe strong spectrum reconstructions and electron topological transitions that can be attributed to a nematic phase transition and a decrease in rotational symmetry. These results are especially surprising because no interaction effects have been observed so far in bilayer graphene in the absence of an applied magnetic field.

Rotational symmetry breaking in the hidden-order phase of URu2Si2.

Abstract: A second-order phase transition is characterized by spontaneous symmetry breaking. The nature of the broken symmetry in the so-called “hidden-order” phase transition in the heavy-fermion compound URu2Si2, at transition temperature Th = 17.5 K, has posed a long-standing mystery. We report the emergence of an in-plane anisotropy of the magnetic susceptibility below Th, which breaks the four-fold rotational symmetry of the tetragonal URu2Si2. Two-fold oscillations in the magnetic torque under in-plane field rotation were sensitively detected in small pure crystals. Our findings suggest that the hidden-order phase is an electronic “nematic” phase, a translationally invariant metallic phase with spontaneous breaking of rotational symmetry.

Low‐Symmetry Monoclinic Phases and Polarization Rotation Path Mediated by Epitaxial Strain in Multiferroic BiFeO3 Thin Films

Abstract: A morphotropic phase boundary driven by epitaxial strain has been observed in lead-free multiferroic BiFeO3 thin films and the strain-driven phase transitions have been widely reported as iso-symmetric Cc-Cc by recent works. In this paper, it is suggested that the tetragonal-like BiFeO3 phase identified in epitaxial films on (001) LaAlO3 single crystal substrates is monoclinic MC. This MC phase is different from the MA type monoclinic phase reported in BiFeO3 films grown on low mismatch substrates, such as SrTiO3. This is confirmed not only by synchrotron X-ray studies but also by piezoresponse force microscopy measurements. The polarization vectors of the tetragonal-like phase lie in the (100) plane, not the (110) plane as previously reported. A phenomenological analysis is proposed to explain the formation of MC Phase. Such a low-symmetry MC phase, with its linkage to MA phase and the multiphase coexistence open an avenue for large piezoelectric response in BiFeO3 films and shed light on a complete understanding of possible polarization rotation paths and enhanced multiferroicity in BiFeO3 films mediated by epitaxial strain. This work may also aid the understanding of developing new lead-free strain-driven morphotropic phase boundary in other ferroic systems.

A holographic critical point

Abstract: We numerically construct a family of five-dimensional black holes exhibiting a line of first-order phase transitions terminating at a critical point at finite chemical potential and temperature. These black holes are constructed so that the equation of state and baryon susceptibilities approximately match QCD lattice data at vanishing chemical potential. The critical end point in the particular model we consider has temperature 143 MeV and chemical potential 783 MeV. Critical exponents are calculated, with results that are consistent with mean-field scaling relations.