Spontaneous Phase Transition Research Articles

Surface structure induced by Ar +-bombardment of decagonal AlNiCo

We have investigated the surfaces of decagonal AlNiCo single quasicrystals in UHV by Reflection High Energy Electron Diffraction (RHEED), Atomic Force Microscopy (AFM) and Auger Electron Spectroscopy (AES). The surfaces were prepared by Ar +-sputtering and annealing. We observe well-defined RHEED patterns directly after sputtering at room temperature, revealing a ten-fold symmetry with respect to the azimuthal angle of the incident beam. Two patterns occur, alternating every 18°. This symmetry behaviour is probably caused by a bcc overlayer consisting of multiple domains in different orientations. The RHEED patterns show a strong temperature dependence. A spontaneous phase transition into a quasiperiodic structure is observed upon annealing to about 350 °C. RHEED patterns of a flat, quasicrystalline surface of good structural quality are observed after sputtering at 400 °C. As indicated by AES, the influence of oxygen on the structural behaviour seems to be important.

AF–FRI metamagnetic transition in itinerant Mn 2− xCo xSb system: high-field and high-pressure effects

The magnetization of Mn 2− x Co x Sb system with 0⩽ x⩽0.35 has been measured under high magnetic fields up to 100 T and high pressure up to 12 kbar. Mn 2 Sb is a ferrimagnet and substitution of Co for Mn at 0.18⩽ x⩽0.35 results in the appearance of the spontaneous first-order magnetic phase transition from ferrimagnetic (FRI) to antiferromagnetic (AF) state at T= T t with decreasing temperature at a critical Co concentration. Below T t a first-order field-induced AF–FRI transition is observed at a critical field B c. The critical field B c is found to decrease with increasing pressure applied at T=4.2 K . The AF state which appeared for the compound with the critical concentration x=0.18 can be transformed to the FRI one by the application of the high pressure of about 16 kbar by increasing the amount of FRI phase. Using the experimental data, the magnetic phase diagrams B c– T– x and B c– p– x are determined. The observed magnetic properties and magnetic phase diagrams of Mn 2− x Co x Sb are explained by the itinerant magnetism of d-electrons and by peculiarities of electronic structure. The itinerant nature of AF–FRI transition is confirmed by specific heat measurements in AF- and field-induced FRI states.

Dielectric properties of Pb (Zn1/3Nb2/3)O3-PbZrO3-PbTiO3 solid solution near morphotropic phase boundary

The dielectric properties of Pb(Zn1/3Nb2/3) O3-PbZrO3−PbTiO3 (PZN-PZ-PT) system near the rhombohedral/tetragonal morphotropic phase boundary (MPB) are carefully studied in this paper. It is found that, for all samples, the curves around the temperatures of dielectric permittivity peak show the characteristics of diffuse phase transition. The change in PbZrO3/PbTiO3 ratio has much influence on the dielectric properties of the samples. The extent of diffuse phase transition increases with the increasing Zr/Ti ratio. The samples in rhombohedral region have typical diffuse phase transition in the temperature range measured. However, for the samples with tetragonal symmetry, a spontaneous normal ferroelectric-relaxor phase transition exists at temperature lower than that of permittivity peak. This normal ferroelectric-relaxor phase transition is confirmed by the experiment of thermally driven current. The analysis of TEM reveals that the samples in tetragonal region show a 90% macrodomain structure, while the samples in rhombohedral region have the configuration of microdomain structure.

The Peculiarities of DC and AC-Field Dependent Relaxor/Ferroelectric Properties of Some Disordered Crystals

The detailed study of transition processes between normal and relaxor behaviors under DC and AC external electric fields has been carried out in the stoichiometric PbSc 1/2 Ta 1/2 O 3 (PST) and PbSc 1/2 Nb 1/2 O 3 (PSN) crystals with different degree of ions ordering. The methods used are the measurements of the dielectric permittivity( l ), optical transmission(OT) and small-angle light scattering (SALS) as a function of temperature at different amplitudes of electric field application. The spontaneous phase transition occurring in these crystals is shown to be accompanied by a sharp SALS intensity peak indicating the percolation nature of this transition. Applying an dc electric field reduces the temperature interval of existence of the relaxor state. Two different types of nonlinearities with respect to the amplitude of the ac drive are revealed in intermediate relaxor state between ferroelectric and paraelectric phases.

Inflationary perturbations from a potential with a step

We use a numerical code to compute the density perturbations generated during an inflationary epoch which includes a spontaneous symmetry breaking phase transition. A sharp step in the inflaton potential generates k dependent oscillations in the spectrum of primordial density perturbations. The amplitude and extent in wave number of these oscillations depends on both the magnitude and gradient of the step in the inflaton potential. We show that observations of the cosmic microwave background anisotropy place strong constraints on the step parameters.

X-ray study of Nd0.5Sr0.5MnO3 manganite structure above and below the ferromagnetic metal–antiferromagnetic insulator spontaneous phase transition

The crystal structure of the manganite Nd0.5Sr0.5MnO3 is studied at temperature T=300 and 77.3 K by means of an x-ray diffractometer. It is shown that the transition from the ferromagnetic metallic state to the antiferromagnetic insulating charge-ordered state is accompanied by a lowering of the symmetry of the structure from orthorhombic to monoclinic. The space-group symmetry of the orthorhombic and monoclinic phases is identified as Imma and P21/m, respectively. Twinning of the crystal and the formation of a twin domain structure with coherent boundaries in the (00l) crystallographic planes are found.

Magnetic phase transitions and phase diagrams of DyMn2Ge2

The experimental studies of magnetic phase transitions in the layered tetragonal intermetallic compound DyMn2Ge2 are continued. The existence of spontaneous phase transitions is confirmed by the results of measurements of the temperature dependences of lattice parameters and the initial magnetic susceptibility. The measurements in strong (up to 50 T) and ultrastrong (up to 150 T) fields revealed two new field-induced magnetic transitions. The inclusion of the exchange interaction between next-to-nearest layers of manganese and the crystal field effects for the rare-earth subsystem along with the antiferromagnetic exchange interaction of nearest Mn layers has made it possible to describe the magnetic properties of DyMn2Ge2 in a wide range of magnetic fields. The parameters of these interactions are determined from a comparison of the experimental and theoretical magnetization curves and H-T phase diagrams.

Magnetooptical studies of the H–T phase diagram of an Fe/Si multilayered film

The magnetization reversal of a multilayered Fe/Si film having cubic magnetocrystalline anisotropy is investigated in the temperature interval 25–300 K by magnetooptical methods. It is found that the growth of the biquadratic exchange interaction as the temperature is lowered causes a spontaneous second-order phase transition from a collinear antiferromagnetic state to a noncollinear state. The presence of cubic anisotropy in the film gives rise to spontaneous and magnetic-field-induced first-order phase transitions between noncollinear states. Magnetooptical studies permit constructing the H–T magnetic phase diagram of the multilayered Fe/Si film for an orientation of the external field along the hard magnetization axis [110]. A calculation of the H–T phase diagram in the framework of a model taking into account the bilinear exchange and cubic anisotropy, with constants I1 and K that are assumed to be independent of temperature, and also the biquadratic exchange with a linearly temperature-dependent constant I2. Satisfactory agreement is obtained between the experimental and calculated phase diagrams.

The observation of the contribution of longitudinal susceptibility to the frequency of the soft magnetoresonance mode in SmFeO3

The behavior of the spectrum of the soft magnetoresonance mode in samarium orthoferrite was experimentally and theoretically studied in the region of the Γ4↔Γ24 orientation phase transition induced by an external magnetic field. The special features of this behavior can be explained by the relative contributions of interaction between ferrite subsystems and longitudinal susceptibility. It is also shown that the contribution of longitudinal susceptibility to the gap of the soft magnetoresonance mode in samarium orthoferrite can also be substantial in low fields; that is, in the vicinity of spontaneous orientation phase transitions. This is explained by the occurrence of spontaneous orientation phase transitions in samarium orthoferrite at high temperatures, at which longitudinal susceptibility is comparable in magnitude with transverse susceptibility.

Evidence for an inflationary phase transition from the LSS and CMB anisotropy data

In the light of the recent Boomerang and Maxima observations of the CMB which show an anomalously low second acoustic peak, we reexamine the prediction by Adams et al (1997) that this would be the consequence of a ‘step’ in the primordial spectrum induced by a spontaneous symmetry breaking phase transition during primordial inflation. We demonstrate that a deviation from scale-invariance around k ∼ 0.1 h Mpc −1 can simultaneously explain both the feature identified earlier in the APM galaxy power spectrum as well the recent CMB anisotropy data, with a baryon density consistent with the BBN value. Such a break also allows a good fit to the data on cluster abundances even for a critical density matter-dominated universe with zero cosmological constant.

Ordering and electric-field effects on the ferroelectric phase transition of some relaxors

Optical transmission and small-angle light scattering (SALS) of PbSc½Ta½O3 (PST) and PbSc½Nb½O3(PSN) crystals with different degree of ions ordering, as well as PbMg⅓Nb⅔O3 (PMN) and PbZn⅓Nb⅔O3 (PZN) crystals were investigated under various dc-field. The first direct experimental evidence for the percolation nature of a spontaneous ferroelectric phase transition accompanied by an anomalous peak of SALS intensity has been received both in ordered and disordered stoichiometric PST and PSN crystals without electric field application. The application of electric field above some critical value is required to induce percolation phase transition in PMN crystal. Phase diagrams, with respect to dc field and temperature, were proposed for PST, PSN and PMN crystals.

The phase transition of (PMN)0.67(PT)0.33 single crystal

The dielectric properties and mechanical properties of (PMN)0.67(PT)0.33 single crystal were studied. A kink of dielectric permittivity was found around 70°C, which was induced by the rhombohedral to tetragonal phase transition. A sharp increase of dielectric permittivity and a modulus softening were observed around 130°C, which correspond to the spontaneous ferroelectric-relaxor phase transition. The dielectric properties in ferroelectric phase can be strongly influenced by the cooling rate during the phase transition, which can be attributed to the size effect of subdomains.

SPONTANEOUS AND FIELD-INDUCED MAGNETIC PHASE TRANSITIONS IN THE INTERMETALLIC COMPOUND DyMn2Ge2

The temperature dependence of lattice constants a and c of intermetallic compund DyMn2Ge2 is measured in the temperature range 10-800K by using the X-ray method. The magnetoelastic anomaly of lattice constants a is observed at spontaneous magnetic phase transition from paramagnetic state to antiferromagnetic state. The ac magnetic susceptibility of DyMn2Ge2 is measured in the temperature range 4.2-200K. Low temperature magnetic phase transitions in DyMn2Ge2 are discussed in the molecular field approximation by taking into account of next nearest Mn-Mn interlayer exchange interaction. The temperature dependence of magnetization and magnetic curves at different temperatures of DyMn2Ge2 single crystal are calculated. Theoretical calculation shows that the observed field-induced magnetic phase transition in DyMn2Ge2 single crystal at temperature lower than 33K is the transition from ferrimagnetic state into intermediate state.

Molecular Dynamics Simulation of Protein Translocation across a Membrane

Biological membranes, their spontaneous formation or phase transitions and the molecular dynamics therein, have long been studied intensively. On the other hand, the structure and function of the membrane proteins are central problems in molecular biology and are attracting tremendous interest. However, the cooperative process of lipids and proteins occurring within the membrane is still very difficult to understand. Recent advances in computer technology have proved the molecular simulation approach to be very promising in various fields of research, and here we demonstrate a possible application of the simulation method to the molecular process of protein translocation across the membrane.

Optical study of the spontaneous ferroelectric phase transition in lead scandoniobate single crystals

A study is reported of the optical transmission and small-angle light scattering (SAS) in ordered and disordered stoichiometric single crystals of lead scandoniobate (PSN) with a zero and a dc electric field applied. It is shown that the spontaneous phase transition occurring in both crystals is accompanied by a sharp peak in SAS intensity, which implies the percolation nature of this transition. A field-temperature phase diagram has been constructed for the PSN single crystals studied in the work.

Spontaneous and intentional dynamics of bimanual coordination in Parkinson's disease

The aim of this study was to investigate the coordination dynamics of individuals with Parkinson's disease (Pd) during the production of rhythmic pronation and supination movements of the upper limb in symmetric (inphase) and asymmetric (antiphase) patterns, and dynamics associated with intentional switching between patterns. Participants with Pd ( n=4) and age-matched controls ( n=3) exhibited spontaneous phase transitions from antiphase to inphase patterns at an individually specific movement frequency (STF). In metronome-paced trials subjects produced antiphase or inphase patterns at five frequencies ranging from STF −0.5 to STF +0.5 Hz. In a second session the same participants produced intentional transitions (inphase to antiphase, antiphase to inphase) at the four highest frequencies. Participants with Pd exhibited spontaneous transitions at a lower frequency than age-matched controls. The Pd group tended to deviate from intended relative phase relations to a greater extent than controls, resulting in significantly higher asynchrony between hands for the Pd group. Normalised to STF there was equivalence between patients and controls in terms of uniformity (stability) of relative phase, despite the asynchrony. Although no clear differences between the Pd and control participants were found in the transition duration for intentional switching, the time between the signal to switch and the transition onset was longer for the Pd participants than the controls. This finding supports the suggestion that individuals with Pd are impaired in the production of movements that require intentional specification.

Magnetic and crystal structure phase transitions in R1-xBaxCoO3-y (R = Nd, Gd)

Perovskites of the R1-xBaxCoO3-y type where R is Nd or Gd have been prepared and characterized. The substitution of Nd by Ba ions up to x = 0.4 leads to a nonhomogeneous ferromagnetic state appearance similar to that found for the La1-xSrxCoO3 system studied earlier. Structural refinement of Nd0.5Ba0.5CoO3-y reveals the chemical ordering of Nd and Ba ions. The magnetic properties of Nd0.5Ba0.5CoO3-y depend drastically on oxygen content. Reduced samples (y>0.2) exhibit magnetic phase transitions at around Ti~200 K and TC280 K as well as a crystal structure transition at TM350 K which leads to a drop in the electrical conductivity. The transitions at Ti and TM are more pronounced in the Gd1-xBaxCoO3-y series. The decrease of Ba content below x = 0.5 leads to decreasing Ti whereas the increase of Ba ions above x = 0.5 leads to a suppression of both spontaneous magnetization and phase transition at TM. It is supposed that the transition at TM results from the oxygen vacancy ordering whereas at Ti the antiferromagnetic state (low temperature phase) transforms into a weak ferromagnetic one with spontaneous magnetization around 0.2 µB per cobalt ion.

Spontaneous phase transition from relaxor to macrodomain ferroelectric state in single-crystal PbSc0.5Nb0.5O3-BaSc0.5Nb0.5O3 solid solutions

The cooling of Pb1−xBaxSc0.5Nb0.5O3 solid solutions with x≤0.04 leads to a spontaneous transition from a relaxor to a macrodomain ferroelectric state, accompanied by anomalous variation of the dielectric and optical properties of the material. As the barium content in the system increases, the relaxor state becomes more stable and eventually “freezes” at x≈0.05. The crystals with x=0.06 exhibited the appearance of a macrodomain ferroelectric phase induced both by an external electric field with a strength of 1.5 kV/cm and by an internal electric field formed in the course of dielectric aging.

Study of the spontaneous relaxor to normal ferroelectric phase transition in la-doped lead titanate

Highly lanthanum doped lead titanate ceramics exhibit relaxor properties and undergo on cooling a subsequent spontaneous macroscopic phase transition. The dielectric response is strongly affected by chemical (copper) doping and is found to become independent of the applied ac field for temperatures lower than the phase transition one.

SPONTANEOUS MAGNETIC PHASE TRANSITION AND MAGNETOELASTIC ANOMALIES AT TRANSITION S IN INTERMETALLIC COMPOUNDS RMn2Ge2 (R=La,Pr,Nd,Sm,Gd,Tb, Y)

The temperature dependence of lattice constants a and c of intermetallic compounds RMn2Ge2 (R=La,Pr,Nd,Sm,Gd,Tb and Y)is measured in the t emperature range 10—800K using the X-ray method. The magnetoelastic anomalies o f a(T) and c(T) are found at different spontaneous magnetic phase transitions. I t is shown that the magnetoelastic anomalies are mainly determined by the Mn-sub system. Mn-Mn exchange interactions depend not only on the lattice constant a bu t also on constant c. Low-temperature ferromagnetic-antiferromagnetic transition s are discussed in the framework of Kittle's exchange inversion model.