Well-posedness for solid-liquid phase transitions with a fourth-order nonlinearity

A phase-field system which describes the evolution of both the absolute temperature θ and the phase variable f during first-order transitions in thermal insulators is considered. A thermodynamic approach is developed by regarding the order parameter as a phase field and its evolution equation as a balance law. By virtue of the special form of the internal energy, a third-order nonlinearity appears in the energy balance in place of the (customarily constant) latent heat. As a consequence, the bounds 0≤f≤1 hold whenever θ is positive valued. In addition, a nonlinear Fourier law with conductivity proportional to temperature is assumed. Well-posedness for the resulting initial and boundary value problem are then established in a suitable setting.     

High Ni2+-mobility in the Chevrel phase Ni2Mo6S8: a quasielastic neutron scattering study

A quasielastic neutron scattering study of Ni₂Mo₆S₈ has established relatively fast long-range motion of the intercalated Ni²⁺ ions, with a diffusion constantD=3×10^–9 cm² s^–1. A model with a jump distance of about 2.1 Å and an activation energy of 24 kJ/mol is favoured. A critical consideration of the information contained in fixed window measurements is given.     

On the n-p phase transitions in Mn1-x Cu x Cr2S4

The Seebeck coefficient for polycrystalline samples of: 1. Mn_1-x Cu _x Cr₂S₄ (0.0 ≤ x < 1.00) at 313 K and 2. MnCr₂S₄ between 313–393 K is presented. The electrical resistivity of Mn_1-x Cu _x Cr₂S₄ as a function of x at the room temperature is also presented. The n-p phase transition is observed in two cases: 1. on changing x at constant temperature (313 K) for 0.0 ≤ x ≤ 0.1 and 2. on changing temperature for MnCr₂S₄ at about 326 K. The first case is probably connected with the noncollinear ferromagnetic interaction of the chromium 3+ and 4+ ions. The second case follows from the strong ferrimagnetic interaction of the Mn²⁺ and Cr³⁺ ions in pure MnCr₂S₄.     

Electrical properties of Cu2S

The electrical properties of polycrystalline thick layers and the corresponding thin films of the new semiconducting material Cu₂S are investigated. It is established that this compound is a semiconductor with p-type conductivity and a wide forbidden band (1.8 ± 0.1 eV), that it does not break down under thermal sublimation, and that the properties of thick layers are largely retained in thin films, if the thickness of the latter is greater than 0.7 .     

Rapid heating of a strongly coupled plasma near the solid-liquid phase transition

Between 10(4) and 10(6) 9Be+ ions were trapped in a Penning trap and laser cooled to approximately 1 mK, where they formed a crystalline plasma. We measured the ion temperature as a function of time after turning off the laser cooling and observed a rapid temperature increase as the plasma underwent the solid-liquid phase transition at T approximately 10 mK (Gamma approximately 170). We present evidence that this rapid heating is due to a sudden release of energy from weakly cooled degrees of freedom involving the cyclotron motion of trapped impurity ions. This equilibration of cyclotron motion with motion parallel to the magnetic field is more than 10 orders of magnitude faster than that predicted by currently available theory, which is valid only in the absence of correlations (Gamma<1).     

基于相位差混合处理方法的高分辨力成像技术

为了提高成像系统的分辨能力, 并尽量减小系统的复杂度, 本文将相位差波前探测技术和相位差图像恢复技术结合起来构成相位差混合处理方法, 给出了点目标和扩展目标情况下混合处理方法的数值仿真结果, 并针对点目标情况进行了实验验证. 实验表明, 在像差较大的情况下, 直接用事后处理方法无法得到满意的结果. 在三种湍流强度下, 经混合方法处理后得到光斑的半高宽分别由自适应光学系统校正后的5.1, 5.1和5.0个像素减小到3.3, 3.2和3.0个像素. 可以看出, 利用相位差混合处理方法得到的图像明显优于单独的事后图像处理方法和自适应光学校正, 相位差混合处理方法在高分辨力成像领域有着巨大的应用潜力.     

Investigation of phase transformations in Cu1.8S-Cu2S thin films by kinematic electron diffraction

The phase transformations of the compounds Cu_1.8S-Cu₂S have been investigated. All the changes in the sample due to a thermal treatment up to 300°C were continuously photographed by the kinematic electron diffraction method. It was shown that, depending on experiment conditions, the low-temperature ordered phase of digenite Cu_Z-XS transforms completely or partly into the hexagonal modification of chalcocite Cu₂S, which becomes ordered in its low-temperature orthorhombic modification. At temperatures from room to 100°C the orthorhombic chalcocite coexists with the low-temperature digenite, in a range from 100°C to 300°C the high-temperature hexagonal chalcocite coexists with the low-temperature digenite and at 300°C the sample contains a mixture of the hexagonal chalcocite and the f.c.c.-phase of digenite. Djurleite and other phases with intermediate compositions are not observed in these experiments.     

High magnetic field study of sulphur spinel Cu1/2In1/2Cr2S4

High magnetic field measurements indicate that a first order transition occurs in Cu_1/2In_1/2Cr₂S₄ atT<35 K along aH _c (T) line in a (H, T) phase diagram. This confirms previous calorimetric and neutron diffraction experiments. It is suggested that this set of experimental results may be explained by a transition from long range to spin-glass magnetic ordering related to the frustration existing on the underlying fcc Bravais lattice obtained by grouping four by four ferromagnetically coupled spins on top of a tetrahedron.     

Phase transformation study of Cu2S

The phase transformations of crystalline Cu₂S in the temperature range 50?C to 600?C have been studied through electric quadrupole interactions of¹¹¹Cd impurities observed by time-differential perturbed angular correlations. The Cd is subject to no discernable quadrupolar interaction in the cubic phase above 430?C. In the hexagonal phase between approximately 103?C and 430?C, a single-site, weakly temperature dependent, axially symmetric quadrupolar interaction is found. At lower temperature, the structure is complex with a number of different sites. These data were taken with a microcomputer-controlled TDPAC spectrometer with logical steering that allows a standard 4-detector spectrometer to collect simultaneously 12 channels of data.     

Pressure-induced phase transitions in single-crystalline Cu4Bi4S9 nanoribbons

In situ angle dispersive synchrotron X-ray diffraction and Raman scattering measurements under pressure are em- ployed to study the structural evolution of Cu4Bi4S9 nanoribbons, which are fabricated by using a facile solvothermal method. Both experiments show that a structural phase transition occurs near 14.5 GPa, and there is a pressure-induced re- versible amorphization at about 25.6 GPa. The electrical transport property of a single Cu4Bi4S9 nanoribbon under different pressures is also investigated.     

Compounds in mixed phase CeBa2Cu3Oy and TbBa2Cu3Oy systems

An analysis of the composition of Ce and Tb barium copper oxide samples with the nominal composition CeBa₂Cu₃O_y and TbBa₂Cu₃O_y, prepared with the same solid state reaction method used for the other superconducting rare earth (R) barium copper oxide RBa₂Cu₃O_7- compounds, is described. The analysis indicates that these samples are multiphase and consist mostly of 1) the perovskite phases BaCeO₃ or BaTbO₃, 2) CuO, and 3) BaCuO₂. No trace of the oxygen deficient orthorhombic perovskite type structure of YBa₂Cu₃O_7- could be found in the X-ray diffraction patterns. Consequently, the compounds do not display superconductivity. The magnetization measurements indicate that BaCeO₃ is weakly paramagnetic which is consistent with tetravalent Ce. The BaTbO₃ compound exhibits an antiferromagnetic transition at a Néel temperature T_N-35 K, and its magnetic susceptibility above T_N can be described by a Curie-Weiss law with an effective moment that corresponds to tetravalent Tb.     

Structure and phase transitions in Cu2P2O7

X-ray diffraction studies of the temperature variation of the Cu₂P₂O₇ crystal structure were performed in the vicinity of the transition from the α-phase (C2/c space group) to the β-phase (C2/m space group). These results as well as those of Raman spectroscopy studies and temperature variation of the electric capacitance point to the existence of an intermediate phase in the temperature range 347–363?K.     

Facile Fabrication of a Cu2O Thin Film with a High Seebeck Coefficient

JETP Letters - Thermoelectric materials can directly convert waste heat into electricity with all solid state devices. Here, we report the thermoelectric properties of a Cu2O thin film with a high...     

Disorder in superconductors—a study on Cu2Mo6S8

Some of the recent work on disorder-induced changes inT _c is reviewed. Shock-pressures induce a disorder uncomplicated by antisite disorder typical of particle irradiation, and have generated interest because of the shock-synthesis of A-15 Nb₃Si. In this paper we present our results on laser-induced shock-damage, and compare it with the results on V₃Si and the results on particle irradiation of Chevrel phase superconductors.     

Pressure-induced hydrogen bonding: structure of D2S phase I'

The full structure of the high-pressure cubic phase I' of hydrogen sulfide has been solved using neutron diffraction data. The molecules are partially rotationally disordered about the <111> axes, as in phase II at ambient pressure but with markedly greater nonuniformity of the toroidal D distribution. The changes in structure at the II-->I' transition signal the onset of significant pressure-induced hydrogen bonding.     

Hadron-quark phase coexistence in a hybrid MIT-Bag model

The phase transition of hadronic to quark matter and the boundaries of the hadron-quark coexistence phase are studied within the two Equation of State (EoS) models. The relativistic effective mean-field approach with constant and density-dependent meson-nucleon couplings is used to describe hadronic matter, and the MIT-Bag model is adopted to describe quark matter. The boundaries of the mixed phase for different Bag constants are obtained solving the Gibbs equations. We notice that the dependence on the Bag parameter of the critical temperatures (at zero chemical potential) can be well reproduced by a fermion ultrarelativistic quark gas model, without contribution from the hadron part. At variance, the critical chemical potentials (at zero temperature) are very sensitive to the EoS of the hadron sector. Hence, the contribution of the hadronic interaction is much more relevant for the determination of the transition to the quark-gluon plasma at finite baryon density and low T . Moreover, in the low-temperature and finite chemical potential region no solutions of the Gibbs conditions are existing for small Bag-constant values, B < (135 MeV)⁴ . Isospin effects in asymmetric matter appear important in the high chemical-potential regions at lower temperatures, of interest for the inner-core properties of neutron stars and for heavy-ion collisions at intermediate energies.     

Electronic phase separation in TmBa2Cu4O8

The NQR spectra of Cu(2) in the superconductor TmBa₂Cu₄O₈ are studied at temperatures from 300 to 4.2 K. In analyzing the spectra it is assumed that the NQR line of each isotope contains two Gaussian components — narrow (n) and broad (b). It is discovered that the NQR frequencies have a minimum at the temperature T*=150 K. The frequencies of the components of the spectrum are close at temperatures from T* to 4.2 K and differ substantially at temperatures T>T*. Both components are broadened as the temperature decreases, but this broadening occurs especially rapidly at temperatures T<T*. The relative intensity of the narrow component I _n/(I _n+I _b) equals 1/6 for T=225−160 K, increases abruptly at T=T*, and remains constant (1/3) at temperatures T from 125 to 4.2 K. Analysis of the experimental data showed that the anomalous temperature dependences of the Cu(2) NQR spectra could be due to electronic phase separation (stratification) in the CuO₂ planes at temperatures T⩽T*. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 3, 214–219 (10 February 1996)