scispace - formally typeset
Search or ask a question

Showing papers on "Phase transition published in 2008"



Journal ArticleDOI
TL;DR: It is shown that a simple gravitational theory can provide a holographically dual description of a superconductor and evidence is found that the condensate consists of pairs of quasiparticles.
Abstract: We show that a simple gravitational theory can provide a holographically dual description of a superconductor There is a critical temperature, below which a charged condensate forms via a second order phase transition and the (dc) conductivity becomes infinite The frequency dependent conductivity develops a gap determined by the condensate We find evidence that the condensate consists of pairs of quasiparticles

2,158 citations


Journal ArticleDOI
TL;DR: In this article, the disorder and order phases of poly(l-lactide) (PLLA) are formed at low (Tc < 100 °C) and high (tc ≥ 120 °C), respectively, depending on the crystallization temperature.
Abstract: It has been found that, dependent on the crystallization temperature (Tc), the disorder (α‘) and order (α) phases of poly(l-lactide) (PLLA) are formed at low (Tc < 100 °C) and high (Tc ≥ 120 °C) temperatures, respectively. In the DSC curves, the sample with α‘ phase demonstrates a peculiar small exothermal peak around 160 °C just prior to the melting point, while the sample crystallized at temperatures around 120 °C (between 110 and 130 °C) shows a double melting behavior. These distinct thermal behaviors of various PLLA samples were investigated in detail by simultaneous measurements of WAXD and DSC. It is confirmed that the small exothermal peak corresponds to the disorder-to-order (α‘-to-α) phase transition, in which the chain packing of the crystal lattice becomes more compacted. In the process of the α‘-to-α phase transition, the isosbestic points were found in the temperature-dependent WAXD profiles. So far, the α‘-to-α transition was considered to occur apparently continuously as long as the main 2...

729 citations


Book
01 Jan 2008
TL;DR: In this article, a nanoscale view of the liquid-vapor interfacial region is presented, and a macroscopic treatment of the interfacial interface is proposed to evaluate the effect of liquid surface tension on contact angle.
Abstract: Pt. 1. Thermodynamic and mechanical aspects of interfacial phenomena and phase transitions -- 1. Liquid-vapor interfacial region - A nanoscale perspective -- 1.1. Molecular perspective on liquid-vapor transitions -- 1.2. Interfacial region - Molecular theories of capillarity -- 1.3. Nanoscale features of the interfacial region -- 1.4. Molecular dynamics simulation studies of interfacial region thermophysics -- 2. Liquid-vapor interface - a macroscopic treatment -- 2.1. Thermodynamic analysis of interfacial tension effects -- 2.2. Determination of interface shapes at equilibrium -- 2.3. Temperature and surfactant effects on interfacial tension -- 2.4. Surface tension in mixtures -- 2.5. Near critical point behavior -- 2.6. Effects of interfacial tension gradients -- 3. Wetting phenomena and contact angles -- 3.1. Equilibrium contact angles on smooth surfaces -- 3.2. Wettability, cohesion, and adhesion -- 3.3. Effect of liquid surface tension on contact angle -- 3.4. Adsorption --^

625 citations


Journal ArticleDOI
TL;DR: In this paper, the authors report on extensive experimental studies on thin film, single crystal, and ceramics of multiferroic bismuth ferrite BiFeO3 using differential thermal analysis, high-temperature polarized light microscopy, hightemperature x-ray diffraction, dc conductivity, optical absorption and reflectivity, and domain imaging.
Abstract: We report on extensive experimental studies on thin film, single crystal, and ceramics of multiferroic bismuth ferrite BiFeO3 using differential thermal analysis, high-temperature polarized light microscopy, high-temperature and polarized Raman spectroscopy, high-temperature x-ray diffraction, dc conductivity, optical absorption and reflectivity, and domain imaging, and show that epitaxial (001) thin films of BiFeO3 are clearly monoclinic at room temperature, in agreement with recent synchrotron studies but in disagreement with all other earlier reported results. We report an orthorhombic order-disorder beta phase between 820 and 925 (±5) °C, and establish the existence range of the cubic gamma phase between 925 (±5) and 933 (±5) °C, contrary to all recent reports. We also report the refined Bi2O3-Fe2O3 phase diagram. The phase transition sequence rhombohedral-orthorhombic-cubic in bulk [monoclinic-orthorhombic-cubic in (001)BiFeO3 thin film] differs distinctly from that of BaTiO3. The transition to the cubic gamma phase causes an abrupt collapse of the band gap toward zero (insulator-metal transition) at the orthorhombic-cubic beta-gamma transition around 930 °C. Our band structure models, high-temperature dc resistivity, and light absorption and reflectivity measurements are consistent with this metal-insulator transition.

561 citations


Journal ArticleDOI
01 Feb 2008-Science
TL;DR: Thermodynamic reasoning and direct observations on noncrystallizing nanoconfined water indicate that the glass transition in ambient-pressure water is qualitatively distinct from that found in the usual molecular liquids.
Abstract: The vitrification of pure water is compared with that of molecular solutions rich in water, and gross differences are noted. Thermodynamic reasoning and direct observations on noncrystallizing nanoconfined water indicate that the glass transition in ambient-pressure water is qualitatively distinct from that found in the usual molecular liquids. It belongs instead to the order-disorder class of transition seen in molecular and ionic crystalline materials. The distinctive "folding funnel" energy landscape for this type of system explains the extreme weakness of the glass transition of water as well as the consequent confusion that has characterized its scientific history; it also explains the very small excess entropy at the glass transition temperature. The relation of confined water behavior to that of bulk is discussed, and the "fragile-to-strong" transition for supercooled water is interpreted by adding a "critical point-free" scenario to the two competing scenarios for understanding supercooled bulk water.

488 citations


Journal ArticleDOI
TL;DR: Although structural distortions are robust to changes in particle size, what is affected is the coherency of the distortions, which is decreased in the smaller particles, in contrast to the sharp transition that is found for the bulk sample.
Abstract: We have investigated the paraelectric-to-ferroelectric phase transition of various sizes of nanocrystalline barium titanate (BaTiO3) by using temperature-dependent Raman spectroscopy and powder X-ray diffraction (XRD). Synchrotron X-ray scattering has been used to elucidate the room temperature structures of particles of different sizes by using both Rietveld refinement and pair distribution function (PDF) analysis. We observe the ferroelectric tetragonal phase even for the smallest particles at 26 nm. By using temperature-dependent Raman spectroscopy and XRD, we find that the phase transition is diffuse in temperature for the smaller particles, in contrast to the sharp transition that is found for the bulk sample. However, the actual transition temperature is almost unchanged. Rietveld and PDF analyses suggest increased distortions with decreasing particle size, albeit in conjunction with a tendency to a cubic average structure. These results suggest that although structural distortions are robust to cha...

480 citations


Journal ArticleDOI
16 Oct 2008-Nature
TL;DR: In this article, an experimental and theoretical study of the Bose-Einstein condensation phase transition of a trapped atomic gas is presented, where vortices formed spontaneously during condensation are observed and characterized, and the results provide further understanding of the development of coherence in superfluids.
Abstract: One prominent element of many continuous phase transitions is the spontaneous formation of topological defects as the system passes through the critical point. The microscopic dynamics of defect formation in such transitions are generally difficult to investigate. The authors present an experimental and theoretical study of the Bose-Einstein condensation phase transition of a trapped atomic gas. They observe and statistically characterize the vortices (or defects) formed spontaneously during condensation. The results provide further understanding of the development of coherence in superfluids, and may allow for direct investigation of universal phase transition dynamics. This paper presents an experimental and theoretical study of the Bose–Einstein condensation phase transition of a trapped atomic gas. The vortices formed spontaneously during condensation are observed and characterized, and the results provide further understanding of the development of coherence in superfluids. Phase transitions are ubiquitous in nature, and can be arranged into universality classes such that systems having unrelated microscopic physics show identical scaling behaviour near the critical point. One prominent universal element of many continuous phase transitions is the spontaneous formation of topological defects during a quench through the critical point1,2,3. The microscopic dynamics of defect formation in such transitions are generally difficult to investigate, particularly for superfluids4,5,6,7. However, Bose–Einstein condensates (BECs) offer unique experimental and theoretical opportunities for probing these details. Here we present an experimental and theoretical study of the BEC phase transition of a trapped atomic gas, in which we observe and statistically characterize the spontaneous formation of vortices during condensation8,9. Using microscopic theories10,11,12,13,14,15,16,17 that incorporate atomic interactions and quantum and thermal fluctuations of a finite-temperature Bose gas, we simulate condensation and observe vortex formation in close quantitative agreement with our experimental results. Our studies provide further understanding of the development of coherence in superfluids, and may allow for direct investigation of universal phase transition dynamics.

474 citations


Journal ArticleDOI
TL;DR: In this article, a spin S quantum Heisenberg model on the Fe lattice of the rare-earth oxypnictide superconductors was studied, and it was shown that this model exhibits a sequence of two phase transitions: from a high-temperature symmetric pha
Abstract: We study a spin S quantum Heisenberg model on the Fe lattice of the rare-earth oxypnictide superconductors. Using both large S and large N methods, we show that this model exhibits a sequence of two phase transitions: from a high-temperature symmetric pha

450 citations


Journal ArticleDOI
TL;DR: Critical behavior in giant plasma membrane vesicles that are isolated directly from living cells suggest that the compositions of mammalian plasma membranes are tuned to reside near a miscibility critical point and that heterogeneity corresponding to < 50 nm-sized compositional fluctuations are present in GPMV membranes at physiological temperatures.
Abstract: We demonstrate critical behavior in giant plasma membrane vesicles (GPMVs) that are isolated directly from living cells. GPMVs contain two liquid phases at low temperatures and one liquid phase at high temperatures and exhibit transition temperatures in the range of 15 to 25 degrees C. In the two-phase region, line tensions linearly approach zero as temperature is increased to the transition. In the one-phase region, micrometer-scale composition fluctuations occur and become increasingly large and long-lived as temperature is decreased to the transition. These results indicate proximity to a critical point and are quantitatively consistent with established theory. Our observations of robust critical fluctuations suggest that the compositions of mammalian plasma membranes are tuned to reside near a miscibility critical point and that heterogeneity corresponding to < 50 nm-sized compositional fluctuations are present in GPMV membranes at physiological temperatures. Our results provide new insights for plasma membrane heterogeneity that may be related to functional lipid raft domains in live cells.

447 citations


Journal ArticleDOI
TL;DR: Evidence is presented that this is a classical paraelectric to antiferroelectric phase transition that is driven by ordering of the hydrogen atoms, which is unprecedented in hybrid frameworks and opens up an exciting new direction in rational synthetic strategies to create extended hybrid networks for applications in ferroic-related fields.
Abstract: [(CH3)2NH2]Zn(HCOO)3, 1, adopts a structure that is analogous to that of a traditional perovskite, ABX3, with A = [(CH3)2NH2], B = Zn, and X = HCOO. The hydrogen atoms of the dimethyl ammonium cation, which hydrogen bond to oxygen atoms of the formate framework, are disordered at room temperature. X-ray powder diffraction, dielectric constant, and specific heat data show that 1 undergoes an order−disorder phase transition on cooling below ∼156 K. We present evidence that this is a classical paraelectric to antiferroelectric phase transition that is driven by ordering of the hydrogen atoms. This sort of electrical ordering associated with order−disorder phase transition is unprecedented in hybrid frameworks and opens up an exciting new direction in rational synthetic strategies to create extended hybrid networks for applications in ferroic-related fields.

Journal ArticleDOI
TL;DR: In this article, the phase-field approach is applied to inhomogeneous systems containing domain structures and it is demonstrated that using a set of independently measured thermodynamic parameters for the corresponding bulk single crystals, the phase field approach is able to quantitatively predict not only the strain effect on phase transition temperatures but also the correct ferroelectric domain structures for a given strain and temperature.
Abstract: This article briefly reviews recent applications of phase-field method to ferroelectric phase transitions and domain structures in thin films. It starts with a brief introduction to the thermodynamics of coupled electromechanical systems and the Landau description of ferroelectric transitions in homogeneous ferroelectric single crystals. The thermodynamic potentials of a homogeneous crystal under different mechanical boundary conditions are presented, including the thin-film boundary conditions. The phase-field approach to inhomogeneous systems containing domain structures is then outlined. It describes a domain structure using the spatial distribution of spontaneous polarization. The evolution of a domain structure towards equilibrium is driven by the reduction in the total-free energy of an inhomogeneous domain structure including the chemical driving force, domain wall energy, electrostatic energy as well as elastic energy. A number of examples are discussed, including phase transitions and domain stability in ferroelectric thin films and superlattices. It is demonstrated that using a set of independently measured thermodynamic parameters for the corresponding bulk single crystals, the phase-field approach is able to quantitatively predict not only the strain effect on phase transition temperatures but also the correct ferroelectric domain structures for a given strain and temperature.

Journal ArticleDOI
19 Sep 2008-Science
TL;DR: In this article, the authors used femtosecond time and angle-resolved photoelectron spectroscopy (trARPES) to optically pump and probe TbTe3, an excellent model system with which to study microscopic cooperative effects.
Abstract: Obtaining insight into microscopic cooperative effects is a fascinating topic in condensed matter research because, through self-coordination and collectivity, they can lead to instabilities with macroscopic impacts like phase transitions. We used femtosecond time- and angle-resolved photoelectron spectroscopy (trARPES) to optically pump and probe TbTe3, an excellent model system with which to study these effects. We drove a transient charge density wave melting, excited collective vibrations in TbTe3, and observed them through their time-, frequency-, and momentum-dependent influence on the electronic structure. We were able to identify the role of the observed collective vibration in the transition and to document the transition in real time. The information that we demonstrate as being accessible with trARPES will greatly enhance the understanding of all materials exhibiting collective phenomena.

Journal ArticleDOI
TL;DR: In this article, it was shown that the rhombohedral to tetragonal phase transition in Na0.5Bi 0.5TiO3 (NBT) is a two-step process.
Abstract: This study shows for the first time that the rhombohedral to tetragonal phase transition in Na0.5Bi0.5TiO3 (NBT) is a two step phase transition. The transformation begins by a first order phase transition involving the reconstructive transformation of the rhombohedral phase into an orthorhombic one, through the formation of an intermediate modulated phase. This phase transition begins slightly over 200 °C by the disappearance of the ferroelectric−ferroelastic domains. The intermediate modulated phase is then formed from 230 to 300 °C, the temperature at which it disappears. The modulated phase corresponds to an intergrowth of rhombohedral perovskite blocks in which Pnma orthorhombic sheets are formed by a microtwinning process of the rhombohedral phase. The intermediate orthorhombic phase is then formed at 300 °C and immediately turns to the tetragonal one. A model is presented explaining the formation of the modulated phase and the origin of the antiferroelectric and relaxor behaviors of NBT.

Journal ArticleDOI
TL;DR: In this paper, the authors extracted the bulk viscosity of hot quark-gluon matter in the presence of light quarks from the recent lattice data on the QCD equation of state.

Journal ArticleDOI
TL;DR: In this article, the relationship between phase diagrams and the electrical properties of (Bi1/2Na 1/2)TiO3 (BNT)-based solid solutions was demonstrated.
Abstract: In this study, we demonstrated the relationship between the phase diagrams and the electrical properties of (Bi1/2Na1/2)TiO3 (BNT)-based solid solutions. In this study, (1−x)(Bi1/2Na1/2)TiO3–xNaNbO3 and (1−x)(Bi1/2Na1/2)TiO3–xKNbO3 (abbreviated to BNT-NN100x and BNT-KN100x) ceramics were prepared by a conventional ceramic fabrication process, and (1−x)(Bi1/2Na1/2)TiO3–x(Bi1/2K1/2)TiO3 (abbreviated to BNKT100x) ceramic was prepared for comparison. We revealed the phase transition temperatures, such as the depolarization temperature Td, rhombohedral-tetragonal phase transition temperature TR-T, and the temperature Tm of the maximum dielectric constant, from the temperature dependence of dielectric properties using poled and unpoled specimens. As a result, it was shown that the BNT-based solid solutions form three types of phase diagrams. In addition, we clarified the relationship between the phase diagrams and the electrical properties of BNT-NN100x, BNT-KN100x, and BNKT100x. The piezoelectric properties we...

Journal ArticleDOI
TL;DR: In this article, the temperature dependence of line tension between liquid domains and of fluctuation correlation lengths in lipid membranes was quantitatively evaluated to obtain a critical exponent, nu = 1.2 +/- 0.2.

Journal ArticleDOI
TL;DR: The polar Kerr effect in the high-T_(c) superconductor YBa2Cu3O6+x was measured with high precision using a cyogenic Sagnac fiber interferometer and anomalous magnetic behavior in magnetic-field training of the effect suggests that time reversal symmetry is already broken above room temperature.
Abstract: The polar Kerr effect in the high-Tc superconductor YBa2Cu3O6+x was measured at zero magnetic field with high precision using a cyogenic Sagnac fiber interferometer. We observed nonzero Kerr rotations of order ~1 µrad appearing near the pseudogap temperature T* and marking what appears to be a true phase transition. Anomalous magnetic behavior in magnetic-field training of the effect suggests that time reversal symmetry is already broken above room temperature.

Journal ArticleDOI
Pengju Pan1, Bo Zhu1, Weihua Kai1, Tungalag Dong1, Yoshio Inoue1 
TL;DR: In this article, the effects of annealing conditions and molecular weight on the crystalline phase transition in poly(l-lactide) (PLLA) were studied by wide-angle X-ray diffraction (WAXD), Fourier transform infrared (FTIR) spectroscopy, and differential scanning calorimetry (DSC).
Abstract: Effects of annealing conditions and molecular weight (MW) on the crystalline phase transition in poly(l-lactide) (PLLA) were studied by wide-angle X-ray diffraction (WAXD), Fourier transform infrared (FTIR) spectroscopy, and differential scanning calorimetry (DSC) The disordered crystal (α′-form) of PLLA was found to transform into the α one during annealing process at elevated temperatures The α′-to-α transition is quite dependent on the annealing period (ta: 0−1440 min) and annealing temperature (Ta: 120−160 °C) With increasing Ta, the polymorphic transition progresses much more rapidly The α′-to-α transition is mainly involved by the slight rearrangement of the chain conformation (especially related to the side groups) and packing manner in the unit cell to the more energy-favorable state, corresponding to the reduction of unit cell dimension Besides, it was proposed that the α′-to-α transformation mainly proceeds by the direct solid−solid transition mechanism Moreover, it was found that MW affec

Journal ArticleDOI
TL;DR: In this article, a hyperbolic two-phase flow model involving five partial differential equations is constructed for liquid-gas interface modelling, which is able to deal with interfaces of simple contact where normal velocity and pressure are continuous as well as transition fronts where heat and mass transfer occur, involving pressure and velocity jumps.
Abstract: A hyperbolic two-phase flow model involving five partial differential equations is constructed for liquid-gas interface modelling. The model is able to deal with interfaces of simple contact where normal velocity and pressure are continuous as well as transition fronts where heat and mass transfer occur, involving pressure and velocity jumps. These fronts correspond to extra waves in the system. The model involves two temperatures and entropies but a single pressure and a single velocity. The closure is achieved by two equations of state that reproduce the phase diagram when equilibrium is reached. Relaxation toward equilibrium is achieved by temperature and chemical potential relaxation terms whose kinetics is considered infinitely fast at specific locations only, typically at evaporation fronts. Thus, metastable states are involved for locations far from these fronts. Computational results are compared to the experimental ones. Computed and measured front speeds are of the same order of magnitude and the same tendency of increasing front speed with initial temperature is reported. Moreover, the limit case of evaporation fronts propagating in highly metastable liquids with the Chapman-Jouguet speed is recovered as an expansion wave of the present model in the limit of stiff thermal and chemical relaxation.

Journal ArticleDOI
07 Feb 2008-Nature
TL;DR: The phase diagram of a spin-polarized Fermi gas of 6Li atoms at unitarity is presented, experimentally mapping out the superfluid phases versus temperature and density imbalance and the implementation of an in situ ideal gas thermometer provides quantitative tests of theoretical calculations on the stability of resonant superfluidity.
Abstract: A major controversy has surrounded the stability of superfluidity in spin-polarized Fermi gas systems with resonant interactions when the 'up' and 'down' spin components are imbalanced. This problem is explored for a Fermi gas of 6Li atoms, using tomographic techniques to map out the superfluid phases as the temperature and density imbalance are varied. Evidence is found for various types of phase transitions, enabling quantitative tests of theoretical calculations on the stability of resonant superfluidity. The pairing of fermions lies at the heart of superconductivity and superfluidity. The stability of these pairs determines the robustness of the superfluid state, and the quest for superconductors with high critical temperature equates to a search for systems with strong pairing mechanisms. Ultracold atomic Fermi gases present a highly controllable model system for studying strongly interacting fermions1. Tunable interactions (through Feshbach collisional resonances) and the control of population or mass imbalance among the spin components provide unique opportunities to investigate the stability of pairing2,3,4—and possibly to search for exotic forms of superfluidity5,6. A major controversy has surrounded the stability of superfluidity against an imbalance between the two spin components when the fermions interact resonantly (that is, at unitarity). Here we present the phase diagram of a spin-polarized Fermi gas of 6Li atoms at unitarity, experimentally mapping out the superfluid phases versus temperature and density imbalance. Using tomographic techniques, we reveal spatial discontinuities in the spin polarization; this is the signature of a first-order superfluid-to-normal phase transition, and disappears at a tricritical point where the nature of the phase transition changes from first-order to second-order. At zero temperature, there is a quantum phase transition from a fully paired superfluid to a partially polarized normal gas. These observations and the implementation of an in situ ideal gas thermometer provide quantitative tests of theoretical calculations on the stability of resonant superfluidity.

Journal ArticleDOI
TL;DR: In this paper, a complete phase diagram of barium zirconate titanate, Ba(Zr x Ti 1-x )O 3 system with compositions 0.00≤x≤1.00 has been developed based on their dielectric behavior.
Abstract: In the course of searching environmental friendly lead-free relaxor ferroelectrics a complete phase diagram of barium zirconate titanate, Ba(Zr x Ti 1-x )O 3 system with compositions 0.00≤x≤1.00 has been developed based on their dielectric behavior. It has been shown that BaZr x Ti 1-x O 3 system depending on the composition, successively depicts the properties extending from simple dielectric (pure BaZr0 3 ) to polar cluster dielectric, relaxor ferroelectric, second order like diffuse phase transition, ferroelectric with pinched phase transitions and then to a proper ferroelectric (pure BaTiO 3 ). A comprehensive structure-property correlation of BaZr x Ti 1-x O 3 ceramics has been studied to understand the various ferroelectric phenomena in the whole phase diagram.

Journal ArticleDOI
TL;DR: In this paper, a detailed experimental investigation of LaFeAsO, the parent material in the series of ''FeAs'' based oxypnictide superconductors, is presented.
Abstract: We present results from a detailed experimental investigation of LaFeAsO, the parent material in the series of ``FeAs'' based oxypnictide superconductors. Upon cooling, this material undergoes a tetragonal-orthorhombic crystallographic phase transition at $\ensuremath{\sim}160\text{ }\text{K}$ followed closely by an antiferromagnetic ordering near 145 K. Analysis of these phase transitions using temperature dependent powder x-ray and neutron-diffraction measurements is presented. A magnetic moment of $\ensuremath{\sim}0.35{\ensuremath{\mu}}_{B}$ per iron is derived from M\"ossbauer spectra in the low-temperature phase. Evidence of the structural transition is observed at temperatures well above the transition temperature (up to near 200 K) in the diffraction data as well as the polycrystalline elastic moduli probed by resonant ultrasound spectroscopy measurements. The effects of the two phase transitions on the transport properties (resistivity, thermal conductivity, Seebeck coefficient, and Hall coefficient), heat capacity, and magnetization of LaFeAsO are also reported, including a dramatic increase in the magnitude of the Hall coefficient below 160 K. The results suggest that the structural distortion leads to a localization of carriers on Fe, producing small local magnetic moments which subsequently order antiferromagnetically upon further cooling. Evidence of strong electron-phonon interactions in the high-temperature tetragonal phase is also observed.

Journal ArticleDOI
TL;DR: In this article, the authors examined the crystal structures, magnetic properties and conductivity of undoped (normal conductor) and 14.5% F-doped LaFeAsO (Tc = 20?K) by synchrotron x-ray diffraction (XRD), DC magnetic measurements, and ab?initio calculations demonstrated that the anomaly is associated with a phase transition from tetragonal (P4/nmm) to orthorhombic (Cmma) phases at ~160?K as well as an antiferromagnetic spin ordering transition at
Abstract: Undoped LaFeAsO, the parent compound of the newly found high-Tc superconductor, exhibits a sharp decrease in the temperature-dependent resistivity at ~160?K. The anomaly can be suppressed by F doping with simultaneous appearance of superconductivity appears correspondingly, suggesting a close association of the anomaly with the superconductivity. We examined the crystal structures, magnetic properties and conductivity of undoped (normal conductor) and 14?at.% F-doped LaFeAsO (Tc = 20?K) by synchrotron x-ray diffraction (XRD), DC magnetic measurements, and ab?initio calculations demonstrated that the anomaly is associated with a phase transition from tetragonal (P4/nmm) to orthorhombic (Cmma) phases at ~160?K as well as an antiferromagnetic spin ordering transition at ~140?K. These transitions can be explained by spin configuration-dependent potential energy surfaces derived from the ab?initio calculations. The suppression of the transitions is ascribed to interrelated effects of geometric and electronic structural changes due to doping by F? ions.

Journal ArticleDOI
TL;DR: In this article, the authors investigated magnetic and structural properties at the surface of epitaxial Ni2MnGa(110) Heusler films using x-ray absorption spectroscopy and magnetic circular dichroism both in transmission and total electron yield mode.
Abstract: We investigated magnetic and structural properties at the surface of epitaxial Ni2MnGa(110) Heusler films using x-ray absorption spectroscopy and x-ray magnetic circular dichroism both in transmission and total electron yield mode. The magnetic shape memory films were prepared by dc sputtering from a stoichiometric target onto sapphire substrates at an optimized substrate temperature of 773K. X-ray diffraction confirms a (110) oriented growth on Al2O3(112¯0) and an austenite to martensite transition at 270–280K. At the surface the martensitic phase transition and the magnetization are strongly suppressed. The deviation in the surface properties is caused by a Mn deficiency near the surface.

Journal ArticleDOI
18 Sep 2008-Nature
TL;DR: P polarized neutron diffraction is used to demonstrate for the model superconductor HgBa2CuO4+δ (Hg1201) that the characteristic temperature T* marks the onset of an unusual magnetic order, a demonstration of the universal existence of such a state.
Abstract: The pseudogap region of the phase diagram is an important unsolved puzzle in the field of high transition-temperature superconductivity. Li et al. report polarized neutron diffraction data that demonstrate that the characteristic temperature, T*, marks the onset of an unusual magnetic order, and hence a novel state of matter with broken time-reversal symmetry, for the model superconductor HgBa2CuO4+δ (Hg1201). The findings appear to rule out theories that regard T* as a crossover temperature rather than a phase transition temperature. Instead, they are consistent with the notion that many of the unusual properties arise from the presence of a quantum-critical point. The pseudogap region of the phase diagram is an important unsolved puzzle in the field of high-transition-temperature (high-Tc) superconductivity, characterized by anomalous physical properties1,2. There are open questions about the number of distinct phases and the possible presence of a quantum-critical point underneath the superconducting dome3,4,5. The picture has remained unclear because there has not been conclusive evidence for a new type of order. Neutron scattering measurements for YBa2Cu3O6+δ (YBCO) resulted in contradictory claims of no6,7 and weak8,9 magnetic order, and the interpretation of muon spin relaxation measurements on YBCO10,11 and of circularly polarized photoemission experiments on Bi2Sr2CaCu2O8+δ(refs 12, 13) has been controversial. Here we use polarized neutron diffraction to demonstrate for the model superconductor HgBa2CuO4+δ (Hg1201) that the characteristic temperature T* marks the onset of an unusual magnetic order. Together with recent results for YBCO14,15, this observation constitutes a demonstration of the universal existence of such a state. The findings appear to rule out theories that regard T* as a crossover temperature16,17,18 rather than a phase transition temperature19,20,21. Instead, they are consistent with a variant of previously proposed charge-current-loop order19,20 that involves apical oxygen orbitals22, and with the notion that many of the unusual properties arise from the presence of a quantum-critical point3,4,5,19.

Journal ArticleDOI
TL;DR: In this article, the temperature dependence of the dielectric and ferroelectric properties of lead-free piezoceramics of the composition (1−x−y)Bi0.5Na 0.5NbO3 was investigated.
Abstract: The temperature dependence of the dielectric and ferroelectric properties of lead-free piezoceramics of the composition (1−x−y)Bi0.5Na0.5TiO3–xBaTiO3–yK0.5Na0.5NbO3 (0.05⩽x⩽0.07, 0.01⩽y⩽0.03) was investigated. Measurements of the polarization and strain hystereses indicate a transition to predominantly antiferroelectric order when heating from room temperature to 150°C, while for 150

Journal ArticleDOI
TL;DR: In this article, a comprehensive numerical model for chalcogenide glasses is presented, coupling a physically based electrical model able to reproduce the threshold switching with a local nucleation and growth algorithm to account for the phase transition dynamics.
Abstract: A comprehensive numerical model for chalcogenide glasses is presented, coupling a physically based electrical model able to reproduce the threshold switching with a local nucleation and growth algorithm to account for the phase transition dynamics. The main ingredients of the chalcogenide physics are reviewed and analyzed through simplified analytical models, providing a deeper insight on the origin of the threshold switching mechanism in chalcogenide glasses. A semiconductorlike three-dimensional full-coupled numerical implementation of the proposed model is finally presented and its capabilities to quantitatively reproduce the key elements of the Ge2Sb2Te5 chalcogenide physics are demonstrated in the framework of phase change memory device simulations.

Journal ArticleDOI
TL;DR: In this article, the electromechanical behavior of (1−x)Bi0.5Na 0.5TiO3-xK0.4NbO3 (BNT-KNN) lead free piezoelectric ceramics is investigated for 0⩽x⩾0.12 to gain insight into the antiferroelectric-ferroelectric (AFE-FE) phase transition on the basis of the giant strain recently observed in BNT-based systems.
Abstract: The electromechanical behavior of (1−x)Bi0.5Na0.5TiO3–xK0.5Na0.5NbO3 (BNT-KNN) lead free piezoelectric ceramics is investigated for 0⩽x⩽0.12 to gain insight into the antiferroelectric-ferroelectric (AFE-FE) phase transition on the basis of the giant strain recently observed in BNT-based systems. At x≈0.07, a morphotropic phase boundary (MPB) between a rhombohedral FE phase and a tetragonal AFE phase is found. While the piezoelectric coefficient is largest at this MPB, the total strain further increases with increasing KNN content, indicating the field-induced AFE-FE transition as the main reason for the large strain.

Journal ArticleDOI
TL;DR: In this article, the one-dimensional spin-1/2 Heisenberg chain with competing ferromagnetic nearest-neighbor and antiferromagnetic next-nearest neighbor was studied.
Abstract: We study the one-dimensional spin-1/2 Heisenberg chain with competing ferromagnetic nearest-neighbor ${J}_{1}$ and antiferromagnetic next-nearest-neighbor ${J}_{2}$ exchange couplings in the presence of magnetic field. We use both numerical approaches (the density-matrix renormalization-group method and exact diagonalization) and effective-field-theory approach and obtain the ground-state phase diagram for wide parameter range of the coupling ratio ${J}_{1}/{J}_{2}$. The phase diagram is rich and has a variety of phases, including the vector chiral phase, the nematic phase, and other multipolar phases. In the vector chiral phase, which appears in relatively weak magnetic field, the ground state exhibits long-range order (LRO) of vector chirality which spontaneously breaks a parity symmetry. The nematic phase shows a quasi-LRO of antiferronematic spin correlation and arises as a result of formation of two-magnon bound states in high magnetic fields. Similarly, the higher multipolar phases, such as triatic $(p=3)$ and quartic $(p=4)$ phases, are formed through binding of $p$ magnons near the saturation fields, showing quasi-LRO of antiferromultipolar spin correlations. The multipolar phases cross over to spin-density-wave phases as the magnetic field is decreased before encountering a phase transition to the vector chiral phase at a lower field. The implications of our results to quasi-one-dimensional frustrated magnets (e.g., ${\text{LiCuVO}}_{4}$) are discussed.