Spatial and temporal hadron correlators below and above the chiral phase transition

Hadronic correlation functions at finite temperature in QCD, with four flavours of dynamical quarks, have been analyzed both above and below the chiral symmetry restoration temperature. We have used both point and extended sources for spatial as well as temporal correlators. The effect of periodic temporal boundary conditions for the valence quarks on the spatial meson correlators has also been investigated. All our results are consistent with the existence of individual quarks at high temperatures. A measurement of the residual interaction between the quarks is presented.     

Local structure changes in V2O3 below and above the metal-insulator transition

According to diffraction measurements of the average structure, V₂O₃ changes on heating from monoclinic to trigonal, accompanied by a 1.4% volume decrease, where some V - V distances decrease by about 0.11Å, favoring the Mott - Hubbard mechanism of the phase transition from insulator to metal. Our x-ray absorption fine structure measurements of the local structure of the single crystal V₂O₃ show the same decrease in volume but no change in local symmetry in the transition, indicating that the phase transition contains a significant order-disorder component, contrary to the purely displacive model based on diffraction results.     

The band structure of Pb1?xGexTe above and below the structural phase transition

Magneto-optical experiments are reported which demonstrate the influence of the structural phase transition in Pb_1–xGe_xTe (x<0.012) on the band structure. Cyclotron resonance has been observed both above and below T_c. Above T_c the band structure is similar to that of PbTe with four equivalent valleys at the L-point in both conduction and valence bands. At T_c a rhombohedral phase transition occurs and the valleys are split into a singly degenerate (T) valley and three equivalent (L) valleys. The effective masses and relative band-edge energies for the different types of valley are deduced.     

Spontaneous local distortions and structural phase transitions

Xray Absorption Fine Structure (XAFS) measurements of the local atomic structure of perovskite crystals undergoing various structural phase transitions are summarized and discussed. The results show that the local structure of crystals undergoing ferroelectric antiferroelectric and antiferrodistortive transitions is distorted in a disordered fashion far above the transition to the high symmetry phase. The size of the distortions is a large fraction of the distortion at temperatures far below T _c. Based on these results we propose a model of ferroelectricity which accounts quantitatively for the temperature dependence of the dielectric function the soft mode frequency, the imaginary part of the dielectric constant and the central peak in PbTiO₃ and KNbO₃.     

Pseudocritical NMR frequency shift above the normal-incommensurate phase transition

The NMR satellite frequencies were measured as a function of temperature in the normal high-temperature phase for ⁸⁷Rb in Rb₂ZnBr₄ and Rb₂ZnCl₄ and for ³⁵Cl in betaine calciumchloride dihydrate. Approaching the respective normal-incommensurate phase transition an anomalous shift of the NMR frequency is observed for the first two cases. This effect is ascribed to the increasing order parameter fluctuations. The experimental data are compared to calculations which relate the observed behaviour of the NMR frequencies to the non-classical critical behaviour of the substances under investigation. Received 6 August 1998     

Domain population in SrTiO3 below the cubic-to-tetragonal phase transition

High-angle double-crystal X-ray diffractometry (HADOX) has been applied to measure the intensity around three reciprocal lattice points: 400, 040 and 004 of SrTiO₃. Each point splits into two in the tetragonal phase according to the appearance of three kinds of domains; the temperature dependence of the intensity of component peaks at these points was measured under identical conditions. The results show that the domain population varies with temperature in a range of about 20 K below the transition. It is found, however, that the total intensities of 400 and 004 of the tetragonal phase are constant; this is observed by summing up the relevant intensity at the three reciprocal lattice points.     

Trace anomaly and dimension two gluon condensate above the phase transition

The dimension two gluon condensate has been used previously within a simple phenomenological model to describe power corrections from available lattice data for the renormalized Polyakov loop and the heavy quark-antiquark free energy in the deconfined phase of QCD [1,2]. The QCD trace anomaly of gluodynamics also shows unequivocal inverse temperature power corrections which may be encoded as dimension two gluon condensate. We analyze lattice data of the trace anomaly and compare with other determinations of the condensate from previous references, yielding roughly similar numerical values.     

Microscopic approach to shear viscosities of unitary Fermi gases above and below the superfluid transition

Recent experiments on the shear viscosity η in a unitary Fermi gas fail to see the theoretically predicted upturn in η at the lower T. In this Letter, we compute η in a fashion which is demonstrably consistent with conservation laws and, in the process, provide an understanding of recent experiments. We show that this disagreement with prior theories cannot be readily attributed to the trap, since (via edge effects) trap-averaged viscosities will be larger than their homogeneous counterparts. The small values of η we find can be simply understood; they reflect the fact that the Goldstone bosons (phonons) do not couple to transverse probes such as η, and fermionic excitations, which determine the viscosity, are necessarily absent in the ground state.     

Self-consistent approximations for itinerant ferromagnets above the phase transition point

The “conserving and self-consistent” approximation scheme of Kadanoff and Baym is generalized to systems of particles of non-zero spin. The additional conservation law of the total spin of the system restricts the possible approximations for the self-energy part, and thus the approximations for the irreducible vertex part occuring in the equation for the correlation function. This guarantees, for instance, the correct behavior of the dynamical susceptibility in the long wave length limit. Two examples are discussed under these aspects: the Hartree-Fock and theT-matrix approximation for the self-energy part and the resulting susceptibility.     

Pressure-induced structural phase transition of iodine

The structural phase transition of iodine was observed at about 210 kbar and at room temperature by the high-pressure x-ray diffraction technique using a diamond-anvil cell and a position-sensitive detector. It was found to occur reversibly in both processes of increasing and decreasing pressure.     

Pressure-induced structural phase transition in RbAu

Using the first principle method based on density functional theory, the structural and elastic properties calculations of RbAu have been performed. The results demonstrate that RbAu is stable in the CsCl structure (B2) at ambient pressure, which is in well agreement with the experimental results. And there exists a structural phase transition from CsCl-type structure (B2) to NaTi-type structure (B32) at the transition pressure of approximate 6 GPa. The pressure effects on the elastic properties are discussed and the elastic property calculation indicates elastic instability maybe provide phase transition driving force according to the variations relation of the elastic constant versus pressure.     

Dynamic correlations below structural phase transitions in weakly anisotropic systems

Below the critical temperature of structural phase transitions dynamic behavior is studied along the longitudinal excitation branch in slightly anisotropic systems with an =3 component order parameter field. Exploiting the consequences of small deviations from isotropy and of the relatively high frequencies involved, it is found that a proportionality between the longitudinal excitation frequency and the order parameter can be expected to hold also beyond the static mean field temperature region. The applicability of the theory to certain perovskites is briefly discussed.A short account of this work was reported at the Eighth MECO-Seminar on Phase Transitions and Critical Phenomena, March 23–25, 1981, Saarbrücken, FRG and at the Seminar on Nonlinear Phenomena at Phase Transitions and Instabilities, March 29-April 9, 1981, Geilo, Norway     

Quadratic crystal field tensors and spin Hamiltonian tensors of Fe3+ in Li2Ge7O15 above and below the phase transition temperature

Dopant Fe³⁺ ions in tetrahedral and octahedral positions of Ge⁴⁺ in the crystal Li₂Ge₇O₁₅ were studied using EPR. Fe³⁺ substitutes for Ge⁴⁺ with a local charge compensation. The octahedral site and the tetrahedral sites significantly differ by the value of the invariant sumS(B ₄) of theB ₄ tensor of the spin Hamiltonian of Fe³⁺. The irreducible tensor products {V ₄ ? V ₄}₄ and {V ₄?V ₄}₂ theV ₄ tensor of the crystal field calculated using the point-charge model for octahedral and tetrahedral complexes provide the predominant contribution of the crystal field to theB ₄ andB ₂ tensors of the spin Hamiltonian of Fe³⁺, respectively. A comparison of the fourth-rank tensorsB ₄ of the spin Hamiltonian and {V ₄?V ₄}₄ of the crystal field determined at 300 K with those determined at 77 K supports the conclusion that the phase transition is accompanied by combined rotation of the [GeO₄] tetrahedra with the [Ge(1)O₆] octahedron almost unaltered. The spectrum lines are narrow and the variety of point defects in the vicinity of the paramagnetic impurity ions Fe³⁺, Cr³⁺ and Cu²⁺ is not detected. These facts are inconsistent with the statistically distributed model for the Li(2) atom. In specific cases at 300 K, when the wings of the two spectrum lines of theM→M+1 and theM+2 →M+3 transitions of Fe³⁺ ions belonging to one system of translationally equivalent positions overlap an extra line appears in the center between these lines. It is suggested that this effect is due to the soft phonon mode above the phase transition temperature.     

Universal structural phase transition in network glasses

Raman and Mössbauer spectroscopy provide evidence for a transition from a molecular cluster network at x=0 to a continuous network at x=0.35 in Ge_1?xSn_xSe₂ alloy glasses. The nature of this morphological transition involves a reformation of molecular cluster surfaces in the heterogeneous phase to yield a homogeneous phase. The transition is believed to be a universal property of the easy glass formers and can be effected in one of several ways.     

Unusual structural phase transition in nanocrystalline zirconia

The present work is the first example demonstrating that a hydrous zirconia formed by precipitation can yield a nearly pure nanocrystalline monoclinic zirconia at a temperature as low as 320 °C. The X-ray diffraction pattern of the hydrous zirconia heated to 310 °C shows that diffraction peaks begin to emerge and reveals a just crystallized mixture of predominantly monoclinic zirconia (70%) with some tetragonal zirconia(30%). In other words, the hydrous zirconia formed in the present work yields the predominantly monoclinic structure coexisting with the tetragonal one as soon as crystallization starts at low temperature (310 °C). This is an important exception to the general principle that amorphous zirconia precursors first convert to the tetragonal structure of zirconia with increasing calcination temperature and then transform to the monoclinic one at a higher temperature (∼600 °C). At the crystallization temperature (310 °C), the monoclinic crystallite size is about 17 nm and the tetragonal one 28 nm. The monoclinic crystallite is much smaller than the tetragonal one with which it co-exists. This result is also not consistent with the traditional view that a critical particle size effect is responsible for the stability of the tetragonal and monoclinic structures. When the temperature (310 °C) is slightly raised to 320 °C, the XRD pattern shows a nearly pure monoclinic zirconia. The crystallite size of the monoclinic zirconia is around 15 nm, and it does not change appreciably as calcination temperature is increased from 320 to or above 400 °C. The unusual structural phase transition has been investigated by several complementary experimental tools: X-raydiffraction and surface analyses, and infrared and Raman spectroscopies. PACS 81.07.-b; 64.70.Nd; 82.80.-d; 78.67.-n; 81.05.Je