Dielectric properties for the ordered-perovskite cuprate Sr2Cu(Re0.69Ca0.31)O6

Dielectric properties are reported on polycrystalline cubic ordered-perovskite cuprate Sr₂Cu(Re_0.69Ca_0.31)O₆ in the frequency range 10 Hz-100 kHz at temperature from 300 to 500 K. Both the dielectric permittivity and dielectric loss factor are found to be frequency and temperature dependent. The enhanced value of the low frequency dielectric permittivity is associated to ionic polarization and interfacial phenomena. The material is found to possess significantly high dielectric permittivity. The calculated ac conductivity suggests semiconducting behaviour for the Sr₂Cu(Re_0.69Ca_0.31)O₆.     

Critical opalescence in SrTiO3

We have measured the temperature and angular dependence of the total scattered light intensity near the structural phase transition of SrTiO₃ and interpreted it in terms of the theory of Ornstein and Zernike. It is concluded that in light scattering the direct coupling to the phonon density fluctuations or central mode in SrTiO₃ prevails over the indirect coupling via the soft mode.     

Critical behaviour of Mn1.11Al0.89 alloys

The diffraction maxima (001) and (100) of the Mn_1.11Al_0.89 alloy have been measured by the neutron powder diffraction method in the temperature range from 23 to 427 C. The Curie temperature is found to be (404 ±2)°C. Temperature dependence of the magnetic structure factor gave the critical exponent β = 0.31±0.02.The disorder parameter has been found to decrease with the temperature. A convenient expression describing this behaviour has been proposed and discussed.     

Critical phenomena of amorphous Nd4(Fe0.75Cr0.25)77.5B18.5 alloys

The magnetic behavior of amorphous Nd₄(Fe_0.75Cr_0.25)_77.5B_18.5 alloys was investigated in the critical region. The Curie temperature T_C and critical exponents β, γ and δ are found to be 141 K, 0.45±0.02, 1.64±0.08 and 4.66±0.10, respectively. The data are fitted to a magnetic equation of state characteristic of a second-order phase transition over a rather wide range of temperatures both above and below T_C. It is noted that the values of the exponents are in disagreement with those derived for a three-dimensional Heisenberg ferromagnet and show an enhancement. This anomalous critical behavior may originate from magnetic inhomogeneity.     

Critical point wetting: The effect of hydrodynamical and steric interfacial interactions

It is shown for the first time that the capillary waves and hydrodynamic correlations in an adsorbed film of a simple fluid provide incomplete wetting in some vicinity of the critical point T_c. On approaching T_c the film thickness grows infinitely and the wetting angle tends to zero due to the steric repulsion between the film-bulk fluid interface and the substrate.     

Critical currents of LaMo6S8 thin films

The thin films of LaMo₆S₈ were prepared by d.c. getter sputtering method. Critical current have been measured vs applied magnetic field B_⊥ up to 8 T at the temperature ranged from 1.7 to 4.2 K. The highest critical current density reached was J_c = 1.7 × 10⁷ Am^?2 at B = 0 and T = 1.7 K. Based on the measurements performed, it was possible to indicate that depending on the heat treatment applied, the pinning forces created in the LaMo₆S₈ thin films obey the scaling law.     

0.6-eV bandgap In0.69Ga0.31As thermophotovoltaic devices with compositionally undulating step-graded InAsyP1-y buffers

Single-junction,lattice-mismatched In0.69Ga0.31As thermophotovoltaic(TPV) devices each with a bandgap of 0.6 eV are grown on InP substrate by metal-organic chemical vapour deposition(MOCVD).Compositionally undulating stepgraded InAsyP1-y buffer layers with a lattice mismatch of ～1.2% are used to mitigate the effect of lattice mismatch between the device layers and the InP substrate.With an optimized buffer thickness,the In0.69Ga0.31As active layers grown on the buffer display a high crystal quality with no measurable tetragonal distortion.High-performance single-junction devices are demonstrated,with an open-circuit voltage of 0.215 V and a photovoltaic conversion efficiency of 6.9% at a short-circuit current density of 47.6 mA/cm2,which are measured under the standard solar simulator of air mass 1.5-global(AM 1.5 G).     

Critical behaviour of the two-dimensional ising antiferromagnets K2CoF4 and Rb2CoF4

The studies have been performed by means of neutron scattering. We report here the first experimental verification of the theoretical predictions concerning temperature dependence of the long range order parameter of two-dimensional Ising systems, giving the values B = 1·16±0·03 and β = 0·119±0·008 for the critical parameters. Also the parameters for the inverse correlation length and the staggered susceptibility were determined, with exponents v = 0·89±0·1 and γ = 1·34±0·22. It was discovered that frozen-in states of stacking of the two-dimensional planes appeared upon quick cooling through the ordering temperature.     

Critical relaxation spectra in EuS above T0

Using a bridge method, we have measured the high frequency magnetic susceptibility of EuS in the critical region and obtained its critical relaxation spectra. The appearance of double peaks in the frequency dependence curve of χ″ has been found for the first time.     

Critical behaviour of the linear birefringence in 3d-Heisenberg antiferromagnets : RbMnF3, KNiF3 and KCoF3

The linear birefringence of stress induced single antiferromagnetic T-domains of RbMnF₃, KNiF₃ and KCoF₃ has been measured in the vicinity of T_N. We deduce the critical behaviour in such 3d-Heisenberg systems, showing a symmetry breaking at the transition involving a unique wave vector of the order parameter in the Brillouin zone. The critical exponent $\text{β}$ of the linear birefringence has been found to be larger than 2β, in agreement with the theoretical predictions of G. Gehring.     

Free energy of (PO4)2Pb3 around the transition point

The structures of α and β(PO₄)₂Pb₃ show that there is a condensation of an Au type soft mode at the ferroelastic transition point (180°C). Since only the C₁₁ elastic constant has a strong discontinuity at the transition point, the interaction term in the free energy is ΔW₃=2b?₁₁Q². The calculated discontinuities of the spontaneous deformations are in good accordance with the experimental results.     

Critical temperature and density of states in amorphous Zr70Cu30

By means of thermal heat treatment of amorphous Zr₇₀Cu₃₀ it is shown that the induced decrease in critical temperature is followed by a reduction in the electronic density of states of the same magnitude as that obtained by changing the Cu concentration. This result indicates that the density of states is the fundamental microscopic parameter determining the superconducting behavior.     

Electronic and annealing properties of the E0.31 defect introduced during Ar plasma etching of germanium

Low energy (±80 eV) Ar plasma etching has been successfully used to etch several semiconductors, including GaAs, GaP, and InP. We have studied the only prominent defect, E_0.31, introduced in n-type Sb-doped Ge during this process by deep level transient spectroscopy (DLTS). The E_0.31 defect has an energy level at 0.31 eV below the conduction band and an apparent capture cross-section of 1.4×10⁻¹⁴ cm². The fact that no V-Sb defects and no interstitial-related defects were observed implies that the etch process did not introduce single vacancies or single interstitials. Instead it appears that higher order vacancy or interstitial clusters are introduced due to the large amount of energy deposited per unit length along the path of the Ar ions in the Ge. The E_0.31 defect may therefore be related to one of these defects. DLTS depth profiling revealed the E_0.31 concentration had a maximum (6×10¹³ cm⁻³) close to the Ge surface and then it decreased more or less exponentially into the Ge. Finally, annealing at 250 °C reduced the E_0.31 concentration to below the DLTS detection limit.     

Critical behavior of Fe15Ni65B18Si2 ferromagnetic glass

AC susceptibility, magnetization and electrical resistivity around the Curie temperature (T_c) were measured for Fe₁₅Ni₆₅B₁₈Si₂ glass. The results yield T_c = (307.6±0.1) K and the following critical exponents γ = 1.50±0.03, β = 0.375±0.01, δ = 5.1±0.1 andα = -0.29±0.05. These values were obtained in the reduced temperature interval $\text{1×10}{}^{-3}\text{?}\text{|T?T}{}_{\mathrm{c}}\text{|}\text{T}{}_{\mathrm{c}}\text{?5 ×10}{}^{-2}$. In spite of the fact that these values for the critical exponents were obtained from different measurements they obey the equality relations γ = β(δ?1) and γ+2β+α = 2. Reduced magnetisation and field follow a magnetic equation of state derived for a second-order phase transition over a wide temperature range. This set of critical exponents is compared with those derived from the Heisenberg model as well as with the usual ones for a pure crystalline ferromagnets. The comparison shows that the values of |α| and γ, for our alloy, are considerably larger than those from the model and the usual crystalline ones. A similar difference is also observed in some other amorphous and dilute crystalline ferromagnets and is probably due to magnetic inhomogeneities.     

Study of jump models for the diffusion of hydrogen in NbH0.02 by means of the quasielastic scattering of slow neutrons

The quasielastic scattering of slow neutrons in a NbH_0.02 single crystal has been measured for a wavevector transfer Q between 0.5 ≤ |Q| ≤ 2.5 Å^?1 at 293, 431 and 581 K for several sample orientations. The data have been analysed in terms of several jump models for the diffusion of hydrogen by comparing the spectral lineshape with the incoherent scattering law S_inc(Q, ω) of the models. The model of jumps to nearest-neighbor sites becomes progressively worse with increasing temperature. The discrepancy cannot be remedied by taking into account jumps to the second-neighbor sites. On the other hand the measured spectral lineshape can be well reproduced in the whole |Q| range by a double-jump model and a two-state model. For comparison, the method of fitting one Lorentzian to theory and experiment has also been used.     

EPR study of Mn2+ around the ferroelastic transition point of Pb3(PO4)2

The spin Hamiltonian parameters of Mn²⁺ have been measured above and below the transition point (180°C) of the lead phosphate. They show that Mn²⁺ substitutes a Pb_I ion. Between 175 and 180°C the principal axis OX of the fine tensor is parallel to the wave vector of the soft mode which condensates at the transition point. An exaltation of the linewidth is observed. The linewidth remains constant within 5°C of T_c; in this temperature range, the “static regime” is achieved, and the correlation time of the fluctuations is less than 10^?8 sec.     

Ionic conductivity and point defects in pure and doped MnF2 and MgF2 single crystals

Conductivity, σ, of MnF₂ and MgF₂ single crystals, pure and doped (with Li⁺, Na⁺, Y³⁺, Gd³⁺), has been measured, from room temperature to 500°C. Further, some crystals were contaminated with O^2? as an additional impurity. These tetragonal (rutile structure) crystals both behave like typical ionic conductors. Of particular interest is the existence of a large anisotropy, σ being largest when measured parallel to the c-axis. Study of the conductivity isotherms and anisotropy as functions of impurity concentration allows identification of the conduction mechanism in terms of the migration of two mobile defects: the fiuorine-ion vacancy, V_F, and interstitial, F_i. A value of 1.44 eV was obtained for the enthalpy of formation of the intrinsic anion Frenkel defect, 0.80 eV for the migration enthalpy of a V_F and 0.88 eV for an F₁ in MnF₂ parallel to the c-axis. Similar values were obtained for MgF₂. This work shows that more information about point defects can be obtained from conductivity measurements in non-cubic cyrstals than in cubic ionic crystals, because of the additional information from conductivity anisotropy.     

Enhancement of Critical Current Density and Flux Pinning in Acetone and La2O3 Codoped MgB2 Tapes

MgB2 tape samples with simultaneous additions of acetone and La2O3 were prepared by an in？situ processed powder-in-tube method. Compared to the pure and single doped tapes, both transport Jc and fluxing pinning are greatly improved by acetone and La2O3 codoping. Acetone supplies carbon into the MgB2 crystal lattice and increases the upper critical field, while the La2O3 reacts with B to form LaB6 nanoparticles as effective flux pining centers. The improvement of the superconducting properties in codoped tapes can be attributed to the combined effects of improvement in Hc2 and flux pinning.