Charge ordering and lattice dimerization in α′-NaV2O5: One or two phase transitions?

The charge redistribution and the structural change in the α′-NaV₂O₅ quasi-one-dimensional magnet that are initiated at the transition to the low-temperature charge-ordered, structurally dimerized nonmagnetic phase was studied by optical Fourier spectroscopy. Polarized far-infrared transmission spectra obtained in the temperature region T=6–300 K were used to measure the temperature dependences of the charge-ordering-induced variation of the refractive index and of the intensity of the folded phonon mode which forms in the doubling of the lattice period. The charge-ordering and the structural phase-transition temperatures were found to coincide, T _co≈T _c ≈34 K.     

正交和四方YBa2Cu3O7-x的热膨胀系数

用电容法测量了正交和四方YBa₂Cu₃O_7-x的热膨胀系数与温度的关系。结果表明,正交样品在205K附近,四方样品在92—130K之间热膨胀系数有异常。估计前者可能与晶格的不稳定性有关,说明具有较强的电-声子相互作用,后者可能与结构转变有关,因此破坏了高温超导电性。同时也发现正交样品在92K热膨胀系数有微小跳跃。由此估计了压力效应 关键词：     

X-ray diffraction analysis and occurrence of multiple phases in Bi-Sr-Ca-Cu-O superconductors

Starting composition 1112 for Bi-Sr-Ca-Cu-oxide yields multiphase super-conductors with the proportion of constituent phases depending sensitively on the annealing temperature. The R-T curves show zero resistivity and the transition corresponding toT _c = 80 K phase prominently. However, indexing of X-ray diffraction peaks reveals presence of 80 K (lowT _c) as well as 108 K (highT _c) phase. The lowT _c phase thus corresponds to the orthorhombic structure with a unit cell ofa = 5.4Å,b = 27 Å andc = 30.56 Å. This is further understood to be composed of a pseudotetragonal cell ofa =b = 5.41 Å. The highT _c phase similarly pertains to the orthorhombic structure withc = 36 Å.     

Giant thermal hysteresis of sound velocity and internal friction in a La0.8Sr0.2MnO3 single crystal

Temperature dependences of the velocity of longitudinal sound V ₁ and the internal friction Q ^?1 are studied for a La_0.8Sr_0.2MnO₃ single crystal in the temperature range 5–350 K. The latter includes the temperature of the structural phase transition T _s ≈95 K (from the Pnma orthorhombic low-temperature phase to the $R\bar 3c$ rhombohedral high-temperature one) and the Curie point T _c =308 K. Near the temperatures T _s and T _c , the curves V ₁(T) and Q ^?1(T) exhibit pronounced singularities. Outside the vicinities of T _s and T _c , the velocity of sound monotonically decreases with increasing temperature. A thermal hysteresis of giant width is observed in the aforementioned dependences. The hysteresis is attributed to the following mechanism: when the crystal under study is heated starting at temperatures T<T _s , some regions occupied by the Pnma low-temperature phase are retained in the $R\bar 3c$ matrix up to the temperature T=350 K.     

PREPARATION, SUPERCONDUCTIVITY AND STRUCTURE OF THE LaBaCaCu3O7-x SUPERCONDUCTORS WITH Tc HIGHER THAN 80 K

In this paper, the preparation process of the tetragonal LaBaCaCu₃O_7-x (LBCCO) superconductor with T_c=84 K is repored. The superconductivity and structure of the LBCCO superconducting phase are investigated by means of X-ray diffraction (XRD), transmission electron microscope(TEM) and superconductivity measurements, The results indicate that the superconducting phase with T_c=82K in the LBCCO system has a tetragonal structure with a=0.3966nm, c=l.1838nm and space group P4/mmm. About 50% of the Ca and La atoms are located in 1d sites (i.e. Y site in the YBCO structure). It is confirmed that there is in fact no orthorhombic to tetragonal transition in this material. The relationship between the oxygen content and T_c is also presented.     

Low energy electron diffraction beam profiles and phase transition at Si(111)-7 X 7 surface

Angular profiles of low energy electron diffraction (LEED) beams from Si(111)-7 × 7 are measured for various crystal temperatures T near the phase transition with apparent critical temperature T_c ≈ 1140 K. From analyses of the profiles it is concluded that (1) long range superstructure order persists for T up to at least 50 K above T_c and (2) with increasing T the correlation length characterizing the short-range order peaks for T ≈ T_c ? 100 K and decreases rapidly for T >T_c. Conclusion (1) is discussed with reference to a dislocation network model of Si(111)-7 × 7 reconstruction.     

On the theory of localized superconductivity: Fluctuational diamagnetism

The diamagnetic susceptibility of a twinning plane at temperatures slightly higher than the localized superconductivity temperature T_c as well as the heat capacity jump at T = T_c are calculated. The possibility of an appreciable increase of the superconductivity temperature in small particles containing twinning planes is studied.     

Magnetic properties of RPdIn (R=Gd–Er) compounds

AC susceptibility and DC magnetization measurements were performed for the RPdIn (R=Gd–Er) compounds both in the paramagnetic and in the ordered state. In opposite to GdPdIn, which is a ferromagnet (T_c=102 K), the other samples show a complex ferrimagnetic behavior with the additional transition at T_t<T_c. In the high-temperature phase (for T_t<T<T_c), a ferromagnetic interaction dominates, while in the low-temperature phase (for T⩽T_t) antiferromagnetic interactions with the magnetocrystalline anisotropy, especially strong for TbPdIn, come into play. The ordering temperatures are T_c=70, 34, 25 and 12.3 K for Tb-, Dy-, Ho- and ErPdIn respectively, while transition temperatures are T_t=6, 14 and 6 K for Tb-, Dy- and HoPdIn respectively. TbPdIn reveals an additional transition at 27 K connected with the intermediate ferrimagnetic phase. The critical fields for the magnetization process of the low-temperature phase are high (52 and 150 kOe for TbPdIn and 32 kOe for DyPdIn at T=4.2 K) yet these values decrease remarkably with increasing temperature. Results of the study are compared with magnetic and neutron diffraction data hitherto available. We state that irreversibility of the zero-field cooled–field cooled magnetization is not connected with the spin-glass phase claimed elsewhere.     

Supraleitung und Temperverhalten abschreckend kondensierter Germanium-Edelmetall-Legierungen

Thin films of Ge-Cu, Ge-Ag and Ge-Au alloys have been condensed onto substrates held at 4 K. An amorphous, metallic phase has been obtained. The superconductivity of this phase is believed to be due to Ge which is forced into a liquidlike structure with a higher coordination number than that of the semiconducting diamond structure. The maximum transition temperatures of the Ge-Cu and Ge-Au films are 3.3 and 3.6 K, respectively, whereas Ge-Ag films show a maximumT _c of 1.2 K. The difference inT _c is explained by the band structure of the noble metals.     

Superconductivity with transition temperature up to 80 K for TbSr2Cu2.7Mo0.3O7+δ

A Tb-123 phase with the composition, TbSr₂Cu_2.7Mo_0.3O_7+δ, has been synthesized in single-phase form by the solid-state reaction route. Its phase purity has been confirmed from neutron powder diffraction studies. The as-synthesized Tb-123 sample does not show superconductivity down to 5 K. On the other hand, an unusually high antiferromagnetic ordering temperature (T_N) of around 7 K is seen for the Tb moments. After 120-atm-O₂ post-annealing, bulk superconductivity is achieved in this compound with a superconducting transition temperature (T_c) of about 30 K, without any significant effect on T_N. To achieve higher oxygen content and higher T_c, the as-synthesized sample was subjected to high-pressure oxygenation, carried out in a closed cell, at 5 GPa and 400 °C in the presence of AgO as an excess-oxygen source. This sample exhibited superconductivity onset at around 80 K with a Meissner fraction larger than 10% at 5 K. Our observation of superconductivity at 80 K is the highest T_c to-date for the Tb-123 type compounds.     

Superconductivity in the intermetallic compound YRhAl

The intermetallic compound, YRhAl, has been prepared and is found to be isomorphic with RRhAl (R=Pr, Nd, Gd, Ho and Tm) compounds crystallizing in the orthorhombic TiNiSi-type structure (space group Pnma). Heat capacity and electrical resistivity measurements in the He-3 temperature range reveal that this compound is superconducting with a transition temperature, T_c, of 0.9 K. The electronic specific heat coefficient, γ, and the Debye temperature are found to be 6.1 mJ/mol K and 197 K, respectively. The specific heat jump at the superconducting transition is found to be consistent with the BCS weak-coupling limit. This combined with the earlier observation of superconductivity in LaRhAl (T_c=2.4 K) having a different structure than that of YRhAl, suggests that the underlying structure is not very crucial for the occurrence of superconductivity in RRhAl series of compounds.     

X-ray study of the low-temperature phase transition in K2CoCl4

Precise lattice parameter measurements and intensity measurements of selected main and satellite reflections of K₂CoCl₄ have been performed in the temperature range 100 to 300 K in the vicinity of the low-temperature phase transition (commensurate-commensurate phase transition, T _c = 142 K). A broadening of the FWHM for the h01 reflections was observed below 142 K which suggests a transition from an orthorhombic phase to a monoclinic phase.     

Crystal structure of low-temperature rubidium monoaluminate modification

Rubidium monoaluminate RbAlO₂ has been studied by powder neutron diffraction and differential scanning calorimetry. A structural phase transition has been found at 1050°C. It is shown that the low-temperature modification RbAlO2 has the orthorhombic structure (Pnma, a = 0.5570(2) nm, b = 1.1189(4) nm, c = 1.5818(6) nm) close to the crystal structures of low-temperature modifications RbGaO₂ and RbFeO₂, not a face-centered cubic structure, as assumed previously.     

Relationship between the crystal structure and the reentrant superconducting properties of (Sn1-xErx)Er4Rh6Sn18

Single crystals of (Sn_1-xEr_x)Er₄Rh₆Sn₁₈ are superconducting (T_c = 1.3K) for x 0, are reetrant superconducting (T_c = 1.24K and T_M = 0.34K) for x ～.30 and undergo a singel magnetic transition (T_M=0.68K) for x～.75. Since the occupancy of the [Sn(1)_1-xEr(1)_x]Er(2)₄ sublattice is responsible for the variation of the low-temperature properties, one can make predictions as to new reentrant superconductors in the MRh_xSn_y series (M=RE). This appears to be the first system of reentrant superconductors where stoichiometry within a sublattice controls both magnetic ordering and superconductivity.     

Superconductivity and structural transformation in HfRuAs

HfRuAs exists in two structural modifications. The high-temperature phase (ZrRuSi-type structure) exhibits superconductivity at 4.93-4.37 K, whereas the low-temperature phase (TiFeSi-type superstructure) remains normal at 1 K. The lattice distortion giving rise to the superstructure indicates zig-zag chains of metal atoms are crucial to superconductivity in this class of compounds.     

Structural,magnetic and superconducting phase transitions in CaFe2As2 under ambient and applied pressure

At ambient pressure CaFe₂As₂ has been found to undergo a first order phase transition from a high temperature, tetragonal phase to a low-temperature orthorhombic/antiferromagnetic phase upon cooling through T ～ 170 K. With the application of pressure this phase transition is rapidly suppressed and by ～0.35 GPa it is replaced by a first order phase transition to a low-temperature collapsed tetragonal, non-magnetic phase. Further application of pressure leads to an increase of the tetragonal to collapsed tetragonal phase transition temperature, with it crossing room temperature by ～1.7 GPa. Given the exceptionally large and anisotropic change in unit cell dimensions associated with the collapsed tetragonal phase, the state of the pressure medium (liquid or solid) at the transition temperature has profound effects on the low-temperature state of the sample. For He-gas cells the pressure is as close to hydrostatic as possible and the transitions are sharp and the sample appears to be single phase at low temperatures. For liquid media cells at temperatures below media freezing, the CaFe₂As₂ transforms when it is encased by a frozen media and enters into a low-temperature multi-crystallographic-phase state, leading to what appears to be a strain stabilized superconducting state at low temperatures.     

YBa2-xSrxCu3O7-δ系统的超导电性与结构的关联

本文系统地研究了YBa_2-xSr_xCu₃O_7-δ中超导电性与物相以及结构变化之间的关系。发现当0c并不随Sr的含量而单调变化。在x=1.0处,T_c出现一峰值。同时,超导临界温度与样品的晶体结构之间呈现出强烈的相关性:随着正交性的增强(即a,b间差值的增大),T_c单调地上升。本文还就S 关键词：     

Die Supraleitung von Zinn und Blei unter sehr hohem Druck

The dependence of the critical temperatureT _c upon pressureP is measured in the pressure range up to 160 kbar. The experimental technique developed for very high pressure-low temperature experiments (preceding article) is improved by introducing a double-sample electrical resistance cell. An internal pressure calibration is therefore possible at some well-established room temperature pressure reference points commonly used. Both metals, tetragonal white tin and fcc-lead, show a monotonic decrease ofT _c vs.P with upward curvature. The results recommend the use of Pb as a secondary standard for very high pressure experiments at Helium temperatures. In addition, high pressure polymorphic modifications of Sn and Pb are found to show superconductivity withT _c =(5.30±0.10) ?K for Sn III atP=113 kbar andT _c =(3.55±0.10) ?K for Pb II atP=160 kbar.     

Superconducting Properties of Indium Nanostructured in Pores of Thin Films of SiO<Subscript>2</Subscript> Microspheres

Samples of a superconducting indium nanocomposite based on a thin-film porous dielectric matrix prepared by the Langmuir–Blodgett method are obtained for the first time, and their low-temperature electrophysical and magnetic properties are studied. Films with thickness b ≤ 5 μm were made from silicon dioxide spheres with diameter D = 200 and 250 nm; indium was introduced into the pores of the films from the melt at a pressure of P ≤ 5 kbar. Thus, a three-dimensional weakly ordered structure of indium nanogranules was created in the pores, forming a continuous current-conducting grid. Measurements of the temperature and magnetic field dependences of the resistance and magnetic moment of the samples showed an increase in the critical parameters of the superconductivity state of nanostructured indium (critical temperature T_c ≤ 3.62 K and critical magnetic field H_c at T = 0 K H_c(0) ≤ 1700 Oe) with respect to the massive material (T_c = 3.41 K, H_c(0) = 280 Oe). In the dependence of the resistance on temperature and the magnetic field, a step transition to the superconductivity state associated with the nanocomposite structure was observed. A pronounced hysteresis M(H) is observed in the dependence of the magnetic moment M of the nanocomposite on the magnetic field at T < T_c, caused by the multiply connected structure of the current-conducting indium grid. The results obtained are interpreted taking into account the dimensional dependence of the superconducting characteristics of the nanocomposite.