Showing papers on "Phase transition published in 2006"
TL;DR: Finite-size scaling analysis shows that the finite-temperature QCD transition in the hot early Universe was not a real phase transition, but an analytic crossover (involving a rapid change, as opposed to a jump, as the temperature varied).
Abstract: The standard model of particle physics predicts two phase transitions that are relevant for the evolution of the early Universe. One, the quantum chromodynamics transition, involves the strong force that binds quarks into protons and neutrons. Despite much theoretical effort, the nature of this transition remains ambiguous. Now Aoki et al. report computationally demanding calculations that suggest that there was no true phase transition. Instead, an analytic crossover took place, involving a rapid, continuous change with temperature as opposed to a jump. This means that it will be difficult to find experimental evidence of a transition from astronomical observations. The standard model of particle physics predicts two transitions that are relevant for the evolution of the early Universe. Computationally demanding calculations now reveal that a real phase transition did not occur, but rather an analytic crossover, involving a rapid change (as opposed to a jump) as the temperature varies. Quantum chromodynamics (QCD) is the theory of the strong interaction, explaining (for example) the binding of three almost massless quarks into a much heavier proton or neutron—and thus most of the mass of the visible Universe. The standard model of particle physics predicts a QCD-related transition that is relevant for the evolution of the early Universe. At low temperatures, the dominant degrees of freedom are colourless bound states of hadrons (such as protons and pions). However, QCD is asymptotically free, meaning that at high energies or temperatures the interaction gets weaker and weaker1,2, causing hadrons to break up. This behaviour underlies the predicted cosmological transition between the low-temperature hadronic phase and a high-temperature quark–gluon plasma phase (for simplicity, we use the word ‘phase’ to characterize regions with different dominant degrees of freedom). Despite enormous theoretical effort, the nature of this finite-temperature QCD transition (that is, first-order, second-order or analytic crossover) remains ambiguous. Here we determine the nature of the QCD transition using computationally demanding lattice calculations for physical quark masses. Susceptibilities are extrapolated to vanishing lattice spacing for three physical volumes, the smallest and largest of which differ by a factor of five. This ensures that a true transition should result in a dramatic increase of the susceptibilities. No such behaviour is observed: our finite-size scaling analysis shows that the finite-temperature QCD transition in the hot early Universe was not a real phase transition, but an analytic crossover (involving a rapid change, as opposed to a jump, as the temperature varied). As such, it will be difficult to find experimental evidence of this transition from astronomical observations.
1,606 citations
TL;DR: In this paper, the field dependence of the magnetic entropy change can be expressed as ΔSM∆Hn for soft magnetic amorphous alloys, and a master curve behavior for the temperature dependence of ΔSM measured for different maximum fields is proposed.
Abstract: The field dependence of the magnetic entropy change can be expressed as ΔSM∝Hn For soft magnetic amorphous alloys n=1 well below the Curie temperature (TC), n=2 in the paramagnetic range, and n≈075 for T=TC The first value can be explained with simple arguments, n=2 is a consequence of the Curie-Weiss law, but n(TC) deviates from mean field predictions From the Arrott-Noakes equation of state, a relation between n(TC) and the critical exponents has been obtained, showing remarkable agreement with experimental data (for an example alloy, predicted n=072 versus experimental n=073) A master curve behavior for the temperature dependence of ΔSM measured for different maximum fields is proposed
858 citations
TL;DR: The observation of a BKT-type crossover in a trapped quantum degenerate gas of rubidium atoms is reported, providing direct experimental evidence for the microscopic mechanism underlying the BKT theory, and raising new questions regarding coherence and superfluidity in mesoscopic systems.
Abstract: Any state of matter is classified according to its order, and the type of order that a physical system can possess is profoundly affected by its dimensionality. Conventional long-range order, as in a ferromagnet or a crystal, is common in three-dimensional systems at low temperature. However, in two-dimensional systems with a continuous symmetry, true long-range order is destroyed by thermal fluctuations at any finite temperature. Consequently, for the case of identical bosons, a uniform two-dimensional fluid cannot undergo Bose-Einstein condensation, in contrast to the three-dimensional case. However, the two-dimensional system can form a 'quasi-condensate' and become superfluid below a finite critical temperature. The Berezinskii-Kosterlitz-Thouless (BKT) theory associates this phase transition with the emergence of a topological order, resulting from the pairing of vortices with opposite circulation. Above the critical temperature, proliferation of unbound vortices is expected. Here we report the observation of a BKT-type crossover in a trapped quantum degenerate gas of rubidium atoms. Using a matter wave heterodyning technique, we observe both the long-wavelength fluctuations of the quasi-condensate phase and the free vortices. At low temperatures, the gas is quasi-coherent on the length scale set by the system size. As the temperature is increased, the loss of long-range coherence coincides with the onset of proliferation of free vortices. Our results provide direct experimental evidence for the microscopic mechanism underlying the BKT theory, and raise new questions regarding coherence and superfluidity in mesoscopic systems.
758 citations
Book•
01 Jan 2006
TL;DR: In this article, the authors introduce statistical mechanics, including random walks and emergent properties, as well as phase-space dynamics and ergodicity, including free energy and entropy.
Abstract: 1. What is Statistical Mechanics? 2. Random walks and emergent properties 3. Temperature and equilibrium 4. Phase-space dynamics and ergodicity 5. Entropy 6. Free Energies 7. Quantum statistical mechanics 8. Calculation and computation 9. Order parameters, broken symmetry, and topology 10. Correlations, response, and dissipation 11. Abrupt phase transitions 12. Continuous phase transitions APPENDIX: FOURIER METHODS
614 citations
TL;DR: In this article, the mechanism by which LiFePO 4 is transformed into isostructural FePO 4 has been elucidated using electron microscopy on large, hydrothermally grown LiFe PO 4 crystals following chemical delithiation.
Abstract: The mechanism by which LiFePO 4 is transformed into isostructural FePO 4 has been elucidated using electron microscopy on large, hydrothermally grown LiFePO 4 crystals following chemical delithiation. Lithium is extracted at narrow, disordered transition zones on the ac crystal surface as the phase boundary progresses in the direction of the a-axis. The substantial lattice mismatch along a (ca. 5%) causes crack formation in the bc plane. Despite considerable disorder in the transition zone, the general structural arrangement is preserved, leading to good crystallinity in the newly created FePO 4 domains. Implications for improved electrode performance are discussed.
573 citations
TL;DR: A flocking model is constructed that exhibits a continuous transition to the ordered phase, while assuming only short-range interactions and no explicit information about the knowledge of the directions of motion of neighbors.
Abstract: We have recorded the swarming-like collective migration of a large number of keratocytes (tissue cells obtained from the scales of goldfish) using long-term videomicroscopy. By increasing the overall density of the migrating cells, we have been able to demonstrate experimentally a kinetic phase transition from a disordered into an ordered state. Near the critical density a complex picture emerges with interacting clusters of cells moving in groups. Motivated by these experiments we have constructed a flocking model that exhibits a continuous transition to the ordered phase, while assuming only short-range interactions and no explicit information about the knowledge of the directions of motion of neighbors. Placing cells in microfabricated arenas we found spectacular whirling behavior which we could also reproduce in simulations.
464 citations
TL;DR: The holographic dual of a finite-temperature gauge theory with a small number of flavors typically contains D-brane probes in a black hole background and it is shown that this phase transition is always first order.
Abstract: The holographic dual of a finite-temperature gauge theory with a small number of flavors typically contains D-brane probes in a black hole background At low temperature, the branes sit outside the black hole and the meson spectrum is discrete and possesses a mass gap As the temperature increases, the branes approach a critical solution Eventually, they fall into the horizon and a phase transition occurs In the new phase, the meson spectrum is continuous and gapless At large Nc and large 't Hooft coupling, we show that this phase transition is always first order In confining theories with heavy quarks, it occurs above the deconfinement transition for the glue
398 citations
TL;DR: In this article, the results of Monte Carlo simulations on a system of hard ellipsoids of revolution with length-to-breadth ratios a/b = 3, 2·75, 2, 1·25 and b/a = 3.
Abstract: We present the results of Monte Carlo simulations on a system of hard ellipsoids of revolution with length-to-breadth ratios a/b = 3, 2·75, 2, 1·25 and b/a = 3, 2·75, 2, 1·25. We identify four distinct phases, viz. isotropic fluid, nematic fluid, ordered solid and plastic solid. The coexistence points of all first order phase transitions are located by performing absolute free energy computations for all coexisting phases. We find nematic phases only for a/b ⩾ 2·75 and a/b ⩽ 1>/2·75. A plastic solid is only observed for 1·25 ⩾ a/b ⩾ 0·8. It is found that the phase diagram is surprisingly symmetric under interchange of the major and minor axes of the ellipsoids.
367 citations
TL;DR: The Hubbard model in the large-N limit is shown to have a semimetal-antiferromagnetic insulator quantum critical point in the universality class of the Gross-Neveu model, and its consequences for various physical quantities are examined.
Abstract: The low-energy theory of interacting electrons on graphene's two-dimensional honeycomb lattice is derived and discussed. In particular, the Hubbard model in the large-N limit is shown to have a semimetal-antiferromagnetic insulator quantum critical point in the universality class of the Gross-Neveu model. The same equivalence is conjectured to hold in the physical case N=2, and its consequences for various physical quantities are examined. The effects of the long-range Coulomb interaction and the magnetic field are discussed.
363 citations
TL;DR: In this paper, single-phase insulating Bi1−xLaxFeO3 (BLFOx, x=0.30) ceramics were prepared and an obvious phase transition from rhombohedral to orthorhombic phase was observed near x = 0.30.
Abstract: Single-phase, insulating Bi1−xLaxFeO3 (BLFOx, x=0.05, 0.10, 0.15, 0.20, 0.30, and 0.40) ceramics were prepared. An obvious phase transition from rhombohedral to orthorhombic phase was observed near x=0.30. It is found that the phase transition destructs the spin cycloid of BiFeO3 (BFO), and therefore, releases the locked magnetization and enhances magnetoelectric interaction. As a result, improved multiferroic properties of the BLFO0.30 ceramics with remnant polarization and magnetization (2Pr and 2Mr) of 22.4μC∕cm2 and 0.041emu∕g, respectively, were established.
358 citations
TL;DR: Measurements show that the ferroelectric phase transition temperature (TC) is depressed as the nanowire diameter (dnw) decreases, following a 1/dnw scaling, and density functional theory (DFT) calculations indicate that ferroElectricity in nanowires is stabilized by molecular adsorbates such as OH and carboxylates.
Abstract: We report scanned probe characterizations of the ferroelectric phase transition in individual barium titanate (BaTiO3) nanowires. Variable-temperature electrostatic force microscopy is used to manipulate, image, and evaluate the diameter-dependent stability of ferroelectric polarizations. These measurements show that the ferroelectric phase transition temperature (TC) is depressed as the nanowire diameter (dnw) decreases, following a 1/dnw scaling. The diameter at which TC falls below room temperature is determined to be ∼3 nm, and extrapolation of the data indicates that nanowires with dnw as small as 0.8 nm can support ferroelectricity at lower temperatures. We also present density functional theory (DFT) calculations of bare and molecule-covered BaTiO3 surfaces. These calculations indicate that ferroelectricity in nanowires is stabilized by molecular adsorbates such as OH and carboxylates. These adsorbates are found to passivate polarization charge more effectively than metallic electrodes, explaining ...
TL;DR: The in situ distribution of the density difference between two trapped spin components is obtained using phase-contrast imaging and 3D image reconstruction and the phase transition induces a dramatic change in the density profiles as excess fermions are expelled from the superfluid.
Abstract: We have observed phase separation between the superfluid and the normal component in a strongly interacting Fermi gas with imbalanced spin populations. The in situ distribution of the density difference between two trapped spin components is obtained using phase-contrast imaging and 3D image reconstruction. A shell structure is clearly identified where the superfluid region of equal densities is surrounded by a normal gas of unequal densities. The phase transition induces a dramatic change in the density profiles as excess fermions are expelled from the superfluid.
TL;DR: In this paper, a unified framework for rare region effects at weakly disordered classical, quantum, and nonequilibrium phase transitions based on the effective dimensionality of the rare regions is presented.
Abstract: Rare regions, i.e., rare large spatial disorder fluctuations, can dramatically change the properties of a phase transition in a quenched disordered system. In generic classical equilibrium systems, they lead to an essential singularity, the so-called Griffiths singularity, of the free energy in the vicinity of the phase transition. Stronger effects can be observed at zero-temperature quantum phase transitions, at nonequilibrium phase transitions, and in systems with correlated disorder. In some cases, rare regions can actually completely destroy the sharp phase transition by smearing.
This topical review presents a unifying framework for rare region effects at weakly disordered classical, quantum, and nonequilibrium phase transitions based on the effective dimensionality of the rare regions. Explicit examples include disordered classical Ising and Heisenberg models, insulating and metallic random quantum magnets, and the disordered contact process.
TL;DR: In this paper, the specific behavior of the different solid phase transitions (effect of heating rate) and the melting region, and emphasizes the liquid state (T > 2000 K) was analyzed.
Abstract: Ti-6Al-4V (TA6V) titanium alloy is widely used in industrial applications such as aeronautic and aerospace due to its good mechanical properties at high temperatures Experiments on two different resistive pulse heating devices (CEA Valduc and TU-Graz) have been carried out in order to study thermophysical properties (such as electrical resistivity, volume expansion, heat of fusion, heat capacity, normal spectral emissivity, thermal diffusivity, and thermal conductivity) of both solid and liquid Ti-6Al-4V Fast time-resolved measurements of current, voltage, and surface radiation and shadowgraphs of the volume have been undertaken At TU-Graz, a fast laser polarimeter has been used for determining the emissivity of liquid Ti-6Al-4V at 6845 nm and a differential scanning calorimeter (DSC) for measuring the heat capacity of solid Ti-6Al-4V This study deals with the specific behavior of the different solid phase transitions (effect of heating rate) and the melting region, and emphasizes the liquid state (T > 2000 K)
TL;DR: In this paper, a unified framework for rare region effects at weakly disordered classical, quantum and nonequilibrium phase transitions based on the effective dimensionality of the rare regions is presented.
Abstract: Rare regions, i.e., rare large spatial disorder fluctuations, can dramatically change the properties of a phase transition in a quenched disordered system. In generic classical equilibrium systems, they lead to an essential singularity, the so-called Griffiths singularity, of the free energy in the vicinity of the phase transition. Stronger effects can be observed at zero-temperature quantum phase transitions, at nonequilibrium phase transitions and in systems with correlated disorder. In some cases, rare regions can actually completely destroy the sharp phase transition by smearing. This topical review presents a unifying framework for rare region effects at weakly disordered classical, quantum and nonequilibrium phase transitions based on the effective dimensionality of the rare regions. Explicit examples include disordered classical Ising and Heisenberg models, insulating and metallic random quantum magnets, and the disordered contact process.
TL;DR: Using ab initio calculations, the results show that the metastable structure consists of special repeated units possessing rocksalt symmetry, whereas the so-called vacancy positions are highly ordered and layered and just result from the cubic symmetry.
Abstract: Phase change materials based on chalcogenide alloys play an important role in optical and electrical memory devices. Both applications rely on the reversible phase transition of these alloys between amorphous and metastable cubic states. However, their atomic arrangements are not yet clear, which results in the unknown phase change mechanism of the utilization. Here using ab initio calculations we have determined the atomic arrangements. The results show that the metastable structure consists of special repeated units possessing rocksalt symmetry, whereas the so-called vacancy positions are highly ordered and layered and just result from the cubic symmetry. Finally, the fast and reversible phase change comes from the intrinsic similarity in the structures of the amorphous and metastable states.
TL;DR: This work reports the direct observation of the superfluid phase transition in a strongly interacting gas of 6Li fermions, through sudden changes in the shape of the clouds—in complete analogy to the case of weakly interacting Bose gases.
Abstract: Phase transitions are dramatic phenomena: water freezes into ice, atomic spins spontaneously align in a magnet, and liquid helium becomes superfluid. Sometimes, such a drastic change in behaviour is accompanied by a visible change in appearance. The hallmark of Bose-Einstein condensation and superfluidity in trapped, weakly interacting Bose gases is the sudden formation of a dense central core inside a thermal cloud. However, in strongly interacting gases--such as the recently observed fermionic superfluids--there is no longer a clear separation between the superfluid and the normal parts of the cloud. The detection of fermion pair condensates has required magnetic field sweeps into the weakly interacting regime, and the quantitative description of these sweeps presents a major theoretical challenge. Here we report the direct observation of the superfluid phase transition in a strongly interacting gas of 6Li fermions, through sudden changes in the shape of the clouds--in complete analogy to the case of weakly interacting Bose gases. By preparing unequal mixtures of the two spin components involved in the pairing, we greatly enhance the contrast between the superfluid core and the normal component. Furthermore, the distribution of non-interacting excess atoms serves as a direct and reliable thermometer. Even in the normal state, strong interactions significantly deform the density profile of the majority spin component. We show that it is these interactions that drive the normal-to-superfluid transition at the critical population imbalance of 70 +/- 5 per cent (ref. 12).
TL;DR: In this paper, a first-order hysteretic discontinuities in strain within the same unipolar electric field cycle for PZN-5PT and PMN-30.5PT were observed.
Abstract: Electric-field-induced phase transitions have been evidenced by macroscopic strain measurements at temperatures between 25 degrees C and 100 degrees C in [001](C)-poled (1-x)Pb(Mg1/3Nb2/3)O-3-xPbTiO(3) [(PMN-xPT);x=0.25,0.305,0.31] and (1-x)Pb(Zn1/3Nb2/3)O-3-xPbTiO(3) [(PZN-xPT);x=0.05,0.065,0.085] single crystals. Such measurements provide a convenient way of ascertaining thermal and electrical phase stabilities over a range of compositions and give direct evidence for first-order phase transitions. A pseudorhombohedral (M-A)-pseudo-orthorhombic (M-C)-tetragonal (T) polarization rotation path is evidenced by two first-order-like, hysteretic discontinuities in strain within the same unipolar electric field cycle for PZN-5PT, PMN-30.5PT, and PMN-31PT whereas, in PMN-25PT, a single first-order-like M-A-T transition is observed. This agrees well with in situ structural studies reported elsewhere. Electric-field-temperature (E-T) phase diagrams are constructed showing general trends for M-A, M-C, and T phase stabilities for varying temperatures and electric fields in poled samples over the given range of compositions. The complex question of whether the M-A and M-C states constitute true phases, or rather piezoelectrically distorted versions of their rhombohedral (R) and orthorhombic (O) parents, is discussed. Finally, stress-induced phase transitions are evidenced in [001](C)-poled PZN-4.5PT by application of a moderate compressive stress (< 100 MPa) both along and perpendicularly to the poling direction (longitudinal and transverse modes, respectively). The rotation path is likely R-M-B-O, via a first-order, hysteretic rotation within the M-B monoclinic plane. The results are presented alongside a thorough review of previously reported electric-field-induced and stress-induced phase transitions in PMN-xPT and PZN-xPT.
TL;DR: A giant transfer of spectral weight with distinct features that require an explanation which goes beyond the Peierls transition model as well as the standard single-band Hubbard model is observed.
Abstract: We present a detailed study of the valence and conduction bands of VO2 across the metal-insulator transition using bulk-sensitive photoelectron and O K x-ray absorption spectroscopies. We observe a giant transfer of spectral weight with distinct features that require an explanation which goes beyond the Peierls transition model as well as the standard single-band Hubbard model. Analysis of the symmetry and energies of the bands reveals the decisive role of the V 3d orbital degrees of freedom. Comparison to recent realistic many body calculations shows that much of the k dependence of the self-energy correction can be cast within a dimer model.
TL;DR: In this article, the authors have processed high-quality films of VO2 by pulsed laser deposition, which were grown epitaxially on (0001) sapphire substrate via domain matching epitaxy, involving matching of integral multiples of lattice planes between the film of monoclinic structure and the substrate.
Abstract: Vanadium oxide (VO2) exhibits a very interesting semiconductor to metal transition as the crystal structure changes from tetragonal or rutile to monoclinic upon cooling close to 68°C The characteristics of this transition are very interesting scientifically and are of immense technological importance due to a variety of sensor- and memory-type applications We have processed high-quality films of VO2 by pulsed laser deposition, which were grown epitaxially on (0001) sapphire substrate via domain matching epitaxy, involving matching of integral multiples of lattice planes between the film of monoclinic structure and the sapphire substrate These films exhibit a sharp transition near 68°C, large amplitude, and very small hysteresis, similar to bulk single crystal of VO2 The sharpness and amplitude of the transition and the hysteresis upon heating and cooling are found to be a strong function of crystal structure and microstructure (grain size, characteristics of grain boundaries, and defect content) Here
TL;DR: The thermal response of a spin 1/2 fermion at the BCS-BEC crossover should be classified as that of a new type of superfluid.
Abstract: We study, in a fully nonperturbative calculation, a dilute system of spin 1/2 interacting fermions, characterized by an infinite scattering length at finite temperatures. Various thermodynamic properties and the condensate fraction are calculated and we also determine the critical temperature for the superfluid-normal phase transition in this regime. The thermodynamic behavior appears as a rather surprising and unexpected melange of fermionic and bosonic features. The thermal response of a spin 1/2 fermion at the BCS-BEC crossover should be classified as that of a new type of superfluid.
TL;DR: In this paper, Steinbach et al. presented a multi phase field model for the investigation of the effect of transformation strain on the transformation kinetics, morphology and thermodynamic stability in multi phase materials.
TL;DR: In this article, the dimension of the order parameter decides upon whether the ferroelectric phase transition is destroyed (e.g. in cubic PbMg1/3Nb2/3O3, PMN) or modified towards RF Ising model behavior, and it is shown that below T c ≈ 350 K RF pinning of the walls of frozen-in nanodomains gives rise to non-Debye dielectric response.
Abstract: Substitutional charge disorder giving rise to quenched electric random-fields (RFs) is probably at the origin of the peculiar behavior of relaxor ferroelectrics, which are primarily characterized by their strong frequency dispersion of the dielectric response and by an apparent lack of macroscopic symmetry breaking at the phase transition. Spatial fluctuations of the RFs correlate the dipolar fluctuations and give rise to polar nanoregions in the paraelectric regime as has been evidenced by piezoresponse force microscopy (PFM) at the nanoscale. The dimension of the order parameter decides upon whether the ferroelectric phase transition is destroyed (e.g. in cubic PbMg1/3Nb2/3O3, PMN) or modified towards RF Ising model behavior (e.g. in tetragonal Sr1−x BaxNb2O6, SBN, x ≈ 0.4). Frustrated interaction between the polar nanoregions in cubic relaxors gives rise to cluster glass states as evidenced by strong pressure dependence, typical dipolar slowing-down and theoretically treated within a spherical random bond-RF model. On the other hand, freezing into a domain state takes place in uniaxial relaxors. While at T c non-classical critical behavior with critical exponents ρ ≈ 1.8, β ≈ 0.1 and α ≈ 0 is encountered in accordance with the RF Ising model, below T c ≈ 350 K RF pinning of the walls of frozen-in nanodomains gives rise to non-Debye dielectric response. It is relaxation- and creep-like at radio and very low frequencies, respectively.
TL;DR: In this paper, the shifts of ferroelectric phase transition temperatures and domain stabilities of BaTiO3 thin films as a function of strain and temperature were studied using phase-field simulations.
Abstract: The shifts of ferroelectric phase transition temperatures and domain stabilities of BaTiO3 thin films as a function of strain and temperature were studied using phase-field simulations. A new Landau-Devonshire thermodynamic potential based on an eighth-order polynomial was employed for describing the bulk free energy of BaTiO3 single crystals, which allows the exploration of domain stability in the full range of experimentally accessible compressive and tensile strains. Based on the simulation results, a phase diagram was constructed, which displays the stability of various ferroelectric phases and domain structures as a function of temperature and strain.
TL;DR: In this article, the authors discuss the physical principles of the two parameters and show that the methods developed and used for induced magnetization cannot be used also for temperature dependence of magnetic susceptibility because there is no physical justification to do so.
Abstract: [1] In many rock magnetic studies, information on magnetic mineralogy is of crucial importance. Besides standard analytical methods, such as X-ray spectroscopy, more sensitive thermomagnetic analyses are often used. Temperature dependence of magnetic parameters can serve as basis for determination of magnetic second-order phase transition temperatures. Although limited by several drawbacks, the most serious being thermally induced transformations of the original minerals, this method provides useful information not only about the presence of magnetic minerals, but also additional knowledge on, e.g., the prevailing grain size distribution or degree of substitution. In thermomagnetic analysis, temperature dependence of two parameters, induced magnetization and magnetic susceptibility, is mostly used. However, let us say because of historical reasons, the same approach for the Curie point determination has been often used in analyzing the two parameters. In our contribution, we discuss the physical principles of the two parameters, showing that the methods developed and used for induced magnetization cannot be used also for temperature dependence of magnetic susceptibility because there is no physical justification to do so. Otherwise, the error in determining the Curie point can be some few degrees but can reach also several tens of degrees. Such an error has serious consequences for further interpretation of the data, e.g., in terms of degree of Ti substitution in Ti magnetite.
TL;DR: The results show that the observed static magnetic configuration seen below the glass-transition temperature arises from the cooperative freezing of the first-order antiferromagnetic (charge ordered) to ferromagnetic transition, and suggest that accommodation strain is important in the kinetics of the phase transition.
Abstract: Spin glasses are founded in the frustration and randomness of microscopic magnetic interactions. They are non-ergodic systems where replica symmetry is broken. Although magnetic glassy behaviour has been observed in many colossal magnetoresistive manganites, there is no consensus that they are spin glasses. Here, an intriguing glass transition in (La,Pr,Ca)MnO3 is imaged using a variable-temperature magnetic force microscope. In contrast to the speculated spin-glass picture, our results show that the observed static magnetic configuration seen below the glass-transition temperature arises from the cooperative freezing of the first-order antiferromagnetic (charge ordered) to ferromagnetic transition. Our data also suggest that accommodation strain is important in the kinetics of the phase transition. This cooperative freezing idea has been applied to structural glasses including window glasses and supercooled liquids, and may be applicable across many systems to any first-order phase transition occurring on a complex free-energy landscape.
TL;DR: In this paper, the phase transition temperatures of BNBK2:1(x) ceramics were investigated using electrical measurements and the determination of the depolarization temperature, Td, and defined the Td for (Bi1/2Na 1/2)TiO3 (BNT)-based solid solutions.
Abstract: The phase transition temperatures of x(Bi1/2Na1/2)TiO3–y(Bi1/2K1/2)TiO3–zBaTiO3) [x+y+z=1, y:z=2:1] [abbreviate to BNBK2:1(x)] ceramics were investigated using electrical measurements. We discussed the determination of the depolarization temperature, Td, and defined the Td for (Bi1/2Na1/2)TiO3 (BNT)-based solid solutions. We also determined the rhombohedral–tetragonal phase transition temperatures, TR–T, for BNBK2:1(x), and verified them using dielectric and piezoelectric measurements. It was demonstrated that TR–T corresponded with Td at x=0.94. The existence of an intermediate phase with ferroelectric and antiferroelectric properties at temperatures higher than the Td around the morphotropic phase boundary (MPB) was also revealed.
TL;DR: In this paper, a new concept exploiting electron-electron correlation is introduced for pulse characterization and determination of t=0 to within 100fs as well as for spatial manipulation of the electron beam.
Abstract: Femtosecond electron diffraction (FED) has the potential to directly observe transition state processes. The relevant motions for this barrier-crossing event occur on the hundred femtosecond time-scale. Recent advances in the development of high-flux electron pulse sources with the required time resolution and sensitivity to capture barrier-crossing processes are described in the context of attaining atomic level details of such structural dynamics-seeing chemical events as they occur. Initial work focused on the ordered-to-disordered phase transition of Al under strong driving conditions for which melting takes on nm or molecular scale dimensions. This work has been extended to Au, which clearly shows a separation in time-scales for lattice heating and melting. It also demonstrates that superheated face-centred cubic (FCC) metals melt through thermal mechanisms involving homogeneous nucleation to propagate the disordering process. A new concept exploiting electron-electron correlation is introduced for pulse characterization and determination of t=0 to within 100fs as well as for spatial manipulation of the electron beam. Laser-based methods are shown to provide further improvements in time resolution with respect to pulse characterization, absolute t=0 determination, and the potential for electron acceleration to energies optimal for time-resolved diffraction.
TL;DR: In this article, the effects of hydrogen bonds on stress-, heat-, and solvent-induced changes in macroscopic properties are discussed, including deformation and shrinkage behavior of polymers.
Abstract: Some salient results in nylon research are reviewed to identify the fundamental principles that are applicable to other strongly interacting or hydrogen-bonded polymers, including proteins. The effects of hydrogen bonds on stress-, heat-, and solvent-induced changes in macroscopic properties are discussed. These data provide a window into the chain mobility and linkages between the crystalline and amorphous domains, both of which are important for any predictive model. The changes in the characteristics of the amorphous phase with the crystallinity and orientation require that it be modeled with at least two components: a rigid/immobile/anisotropic component and a soft/mobile/isotropic component. The deformation and shrinkage behavior of these polymers are discussed in terms of the relative contributions of the amorphous and crystalline domains and of the interactions between them. The premelting crystalline transition is accompanied by the merging of intersheet and intrasheet diffraction peaks in some nylons, as observed by Brill, and not in others even though the underlying mechanism that gives rise to these transitions, the onset of volume-increasing librational motion of the crystalline stems, is the same. Because the effects of the temperature, deformation, and solvent have a common origin associated with mobility, a fictive temperature can be associated with a given solvent activity or stress level. The magnitude of this fictive temperature is the amount by which the glass or Brill transition temperature is reduced in the presence of solvents (∼50 °C) or stress or by which the annealing temperature can be reduced in the presence of a solvent (or active stress) to achieve the same structural state as that of a dry (or static) polymer. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 1763–1782, 2006
TL;DR: Magnetic phase transitions in multiferroic bismuth ferrite (BiFeO3) induced by magnetic field, epitaxial strain, and composition modification are considered in this article.
Abstract: Magnetic phase transitions in multiferroic bismuth ferrite (BiFeO3) induced by magnetic field, epitaxial strain, and composition modification are considered These transitions from a spatially modulated spin spiral state to a homogenous antiferromagnetic one are accompanied by tghe release of latent magnetization and a linear magnetoelectric effect that makes BiFeO3-based materials efficient room-temperature single phase multiferroics