Photoinduced phase transition to a new macroscopic spin-crossover-complex phase

We clarified for the first time that the photoinduced phase is quite different in structure from the thermally induced phase by the resonant Raman spectroscopy in the spin-crossover complex, [Fe(2-pic)(3)]-Cl2EtOH. In the photoinduced phase we observed a number of additional lines assigned to infra-active vibrational modes which are strongly prohibited by selection rules in the thermally induced phase. These results indicate that a dramatic symmetry lowering should take place in the photoinduced phase. The cooperative Jahn-Teller transformation is a plausible candidate for the symmetry lowering in the photoinduced phase.     

On the nature of a new phase transition in α-MnS

Magnetic-susceptibility and X-ray-diffraction data of polycrystalline and oriented single crystal α-MnS reveal a new phase transition at T_tr = 131 K below the Néel temperature T_N = 148 K. The phase transition is characterized by an abrupt inversion of the rhombohedral distortion of the f.c.c. lattice along [111]. At T_tr there is a discontinuous change in the susceptibility of single crystals.     

Hypermagnetic helicity flux in the nuclei of a new phase in the electroweak phase transition

The dynamics of the magnetic helicity in the electroweak phase transition in the early Universe has been analyzed. It has been shown that the interface surface between the symmetric (hypermagnetic) and broken (Maxwellian) phases serves as a membrane at which the helicity is separated. If the number of knots of the hyper-magnetic field is negative, the helicity accumulated in the Maxwellian phase is left-handed.     

Thermal expansion and a new phase transition in pyroelectric LiKSO4

Single crystal macroscopic thermal expansion coefficient measurements have been made on uniaxial lithium potassium sulphate crystal both along and normal to the six fold axis, employing Fizeau’s interferometer method. Measurements were made in the range of −120°C to 500°C. The results show that lithium potassium sulphate exhibits two major anomalies in its expansion coefficients around −95°C and 422°C respectively, the one at −95°C has been observed for the first time. The nature of dimensional changes of the crystal at the upper and lower transition points are opposite in nature. The crystal shows considerable lattice anisotropy. Megaw’s tilt concept has been invoked to explain the relative magnitudes of expansion coefficients alonga andc directions. Structural features responsible for the absence of ferroelectricity in this crystal have been pointed out.     

Ultrasonic attenuation near the phase transition for the magnetic field induced excitonic phase in bismuth

The fluctuation contribution to the ultrasonic attenuation above T_c is shown to have a log T form similar to the log T components in properties of Kondo systems. The recent results by Mase et al. show this behaviour in bismuth.     

Discovery of a new phase transition in gallium nitride at 22 GPa

X-ray data on the volume-pressure dependence V(P) for elementary cells of crystalline GaN are approximated using equations of continuum mechanics. The best approximation of function V(P) is obtained by assuming that a previously unknown continuous phase transition occurs at a pressure of 22 GPa.     

Ultrasonic investigation of the phase transition in flux-grown SrTiO3

Ultrasonic attenuation and velocity have been measured close to and above the 105 K transition in flux-grown SrTiO₃. The data are compared with data from Verneuil-grown SrTiO₃. Larger exponents than expected are observed. The difference between relaxation times in the two samples is interpreted as being due to different impurity contents.     

Neutron diffraction and electrical transport studies on the incommensurate magnetic phase transition in holmium at high pressures

Neutron diffraction and electrical transport measurements have been made on the heavy rare earth metal holmium at high pressures and low temperatures in order to elucidate its transition from a paramagnetic (PM) to a helical antiferromagnetic (AFM) ordered phase as a function of pressure. The electrical resistance measurements show a change in the resistance slope as the temperature is lowered through the antiferromagnetic Néel temperature. The temperature of this antiferromagnetic transition decreases from approximately 122 K at ambient pressure at a rate of -4.9 K GPa(-1) up to a pressure of 9 GPa, whereupon the PM-to-AFM transition vanishes for higher pressures. Neutron diffraction measurements as a function of pressure at 89 and 110 K confirm the incommensurate nature of the phase transition associated with the antiferromagnetic ordering of the magnetic moments in a helical arrangement and that the ordering occurs at similar pressures as determined from the resistance results for these temperatures.     

A new large N phase transition in YM2

Inspired by the interpretation of two-dimensional Yang–Mills theory on a cylinder as a random walk on the gauge group, we point out the existence of a large N transition which is the gauge theory analogue of the cutoff transition in random walks. The transition occurs in the strong coupling region, with the 't Hooft coupling scaling as $\alpha \log N$, at a critical value of α ($\alpha =4$ on the sphere). The two phases below and above the transition are studied in detail. The effective number of degrees of freedom and the free energy are found to be proportional to $N^{2-\frac{\alpha }{2}}$ below the transition and to vanish altogether above it. The expectation value of a Wilson loop is calculated to the leading order and found to coincide in both phases with the strong coupling value.     

Ultrasonic velocity measurements near the 250°K phase transition in BaMnF4

Ultrasonic velocity measurements for longitudinal and shear modes propagating parallel to the crystallographic axes of orthorhombic BaMnF₄ are presented for temperatures between 200°K and 290°K. Sharp anomalies at ～ 250°K are observed for the modes V_bc = V_cb and V_cc.     

A new mechanism of surface polariton formation at a ferroelastic phase transition

Conditions are determined at which the combined effect of the lattice and the spatial dispersion gives rise to the formation of a surface polariton with the participation of a soft mode in the vicinity of a dipole ferroelastic phase transition.     

A new manganese-based single-molecule magnet with a record-high antiferromagnetic phase transition temperature

We perform both dc and ac magnetic measurements on the single crystal of Mn30（Et-sao）3（C104）（MeOH）3 single- molecule magnet （SMM） when the sample is preserved in air for different durations. We find that, during the oxidation process, the sample develops into another SMM with a smaller anisotropy energy barrier and a stronger antiferromagnetic intermolecular exchange interaction. The antiferromagnetic transition temperature observed at 6.65 K in the new SMM is record-high for the antiferromagnetic phase transition in all the known SMMs. Compared to the original SMM, the only apparent change for the new SMM is that each molecule has lost three methyl groups as revealed by four-circle x-ray diffraction （XRD）, which is thought to be the origin of the stronger antiferromagnetic intermolecular exchange interaction.     

Ultrasonic characterization of Cu-Al-Ni single crystals lattice stability in the vicinity of the phase transition

Measurements of elastic constants of the austenite phase when approaching the phase transformation either upon cooling or stressing is of the crucial interest for the shape memory alloy field. Acoustic properties (wave velocity and also attenuation changes) of the Cu-Al-Ni single crystal were investigated in situ during stress-induced martensitic transformation at constant (room) temperature. The parent austenite cubic lattice of the Cu-Al-Ni exhibits very high elastic anisotropy (anisotropy factor A approximately 12). The measurements were made using nine combinations of (i) applied uniaxial compression in a given crystal direction, (ii) the wave propagation and (iii) polarization vectors. The chosen configurations are sufficient for evaluation of all independent third order elastic constants (TOEC). The longitudinal modes were also measured by the immersion technique, using the transducer pair in a water tank installed on the testing machine. The device works as "a ultrasonic extensometer" measuring a transverse strain of the specimen. The dependencies of both natural and initial wave velocities on the applied stress may be evaluated. Three elastic constants of the stress-induced martensite were determined. The elastic properties were found to vary with the increasing stress above the Ms transformation temperature, which is interpreted as a precursor for the martensitic transformation. The onset of the transformation was additionally identified from the acoustic emission measurement.     

Evidence for a new,stress-induced phase transition in KMnF3

A new, stress-induced phase of KMnF₃ is found in emission, absorption and a.c.-susceptibility measurements. The phase transition is of first order and it occurs under uniaxial stress parallel to the [100] and to the [110] pseudocubic axes at 20.1 and 27 kp/mm² at 1.7 K. The experimental results are explained by a change in the amount of the tetragonal distortion in the new, stress-induced phase.     

On a third-order phase transition

The asymptotic behaviour of random variables of the general form $$\ln \sum\limits_{i = 1}^{\kappa ^N } {\exp (N^{1/p} \beta \zeta _i )} $$ with independent identically distributed random variables ζ _i is studied. This generalizes the random energy model of Derrida. In the limitN→∞, there occurs a particular kind of phase transition, which does not incorporate a bifurcation phenomenon or symmetry breaking. The hypergeometric character of the problem (see definitions of Sect. 4), its Φ-function, and its entropy function are discussed.     

First-order phase transition and phase coexistence in a spin-glass model

We study the mean-field static solution of the Blume-Emery-Griffiths-Capel model with quenched disorder, an Ising-spin lattice gas with random magnetic interaction. The thermodynamics is worked out in the full replica symmetry breaking scheme. The model exhibits a high temperature/low density paramagnetic phase. As temperature decreases or density increases, a phase transition to a full replica symmetry breaking spin-glass phase occurs. The nature of the transition can be either of the second order or, at temperature below a given critical value, of the first order in the Ehrenfest sense, with a discontinuous jump of the order parameter, a latent heat, and coexistence of phases.     

Self-consistent treatment of a phase transition

A self-consistent treatment of a phase transition with a scalar order parameter in the ordered and disordered state is described. The factorization of the correlation functions in the disordered phase leads to a shift of the transition temperature, a linear divergence (=1) for the correlation length, a quadratic divergence (=2) for the susceptibility, and a finite value (=–1) for the specific heat. In the ordered phase the factorization of the correlation functions leads to no divergences in the correlation length and susceptibility. A study of the free energy shows that order persists above the transition temperature found by assuming disorder. The requirement of thermodynamic stability induces a first-order transition at a temperature which lies between the bare transition temperature and the shifted one.Supported in part by NSF grant No-GP-17560.This work is in partial fulfillment of Ph.D. requirements at Brandeis University.     

Dynamics of a quantum phase transition

We present two approaches to the dynamics of a quench-induced phase transition in the quantum Ising model. One follows the standard treatment of thermodynamic second order phase transitions but applies it to the quantum phase transitions. The other approach is quantum, and uses Landau-Zener formula for transition probabilities in avoided level crossings. We show that predictions of the two approaches of how the density of defects scales with the quench rate are compatible, and discuss the ensuing insights into the dynamics of quantum phase transitions.