Influence of hydrostatic pressure on the phase transition in the complex antifluorites and hexammines: Rigid-sphere model interpretation

Abstract

The pressure coefficient of the phase transition temperature T_c, dT_c/dp = -(11⁺-1) K/GPa, has been determined for Ni (NH₃)₆Cl₂ using a new high pressure and low temperature probe. The relations between T_c and dT_c/dp were determined for antifluorite K₂MCl₆ compounds and hexammines applying the rigid-sphere model.     

Electrical properties and Tc of CaLaBaCu3Ox at pressures up to 9 GPa

Abstract

By means of a four-point resistivity method the critical temperature (T_c) of the tetragonal high temperature super-conductor CaLaBaCu₃O_x was studied for pressures up to 9 GPa. The pressure dependence of T_c is small and negative, dT_c/dp = - 0.77 K/GPa, and agrees with the general trend observed in previous data.     

Electrical resistivity and critical temperature of Bi-based High-TC superconductors to 1 GPa

Abstract

We have measured the electrical resistance R of a sintered, two-phase, high-T_C superconductor with the nominal composition BiSrCaCu₂O_x, as a function of T and p. We find d(lnR)/dp ? -0.06 GPa^?1 at 295 K, while dT_C/dp is 2.5 K/GPa for the phase with T_c ? 76 and 2 K/GPa for that with T_C ? 106 K.     

T1系单相超导材料的高氧压合成及压力对超导性的影响

 系统地研究了在0~3 MPa氧压下T1系超导材料的制备过程及其超导性质。结果表明：0.25~0.90 MPa氧压下所制备的样品为纯2223相，T_c0最高可达125.3 K；1.45~3.00 MPa氧压下样品为纯2212相，T_c0在95~100 K之间；0.90~1.45 MPa氧压下样品为2223及2212两相共存。对两种单相样品的高压研究结果表明，2223相样品比2212相样品有着较强的压力效应，在0~0.52 GPa压力下分别为4.0 K/GPa及2.0 K/GPa。     

High-pressure properties of high-TC superconductor samples produced by hot isostatic pressing

Abstract

The electrical resistance of dense YBa₂Cu₃O_x and YBa₂Cu₄O_y produced by hot isostatic pressing has been measured vs. T and p. At 295 K we find d (ln R)/dp ? -0.12 and -0.09 GPa^?1, respectively, with no systematic dependence on initial density. For 1-2-4, dT_C/dp ? 5.1 K/GPa, which is ten times that of 1-2-3.     

准静水冷压处理后的Bi0.8Pb0.2SrCaCu2Ox超导材料

着重讨论了准静水冷压对Bi系高温超导材料超导电性的影响,并分析了经冷压-退火处理后超导电性的改善情况。准静水冷压处理后,Bi系超导材料晶粒间界连接发生显著退化,晶粒内部产生大量缺陷,由此导致了宏观超导电性的退化。冷压造成了体密度的极大提高和一定的晶粒择优取向。这些现象主要是由于准静水压下剪切应力的作用。退火后,晶界连接得到很大加强,部分缺陷消失或形成堆错使材料各项性能均获改善。J_c(77K,零场)较初始样品有大幅度提高,达约1600A/cm²。此外还考察了静水压 关键词：     

The pressure dependence of the superconducting transition temperature of LaT4P12(T = Fe,Ru, Os)

The superconducting transition temperature, T_c, of the LaT₄P₁₂ compounds with T = Fe, Ru, or Os has been measured under hydrostatic pressure P up to 1.8 GPa. The T = Fe compound exhibits a substantial increase of T_c from T_c (P = 0) = 4.1 K at a rate (dT_c/dP)_P=0= +7.2 x 10^-1 K/GPa. In contrast, the Ru and Os compounds exhibit only weak decreases of T_c from T_c (P = 0) = 7.2 K and 1.8 K with (dT_c/dP)_P=0= -1.6 x 10^-1 K/GPa and -9.5 x 10^-2 K/GPa, respectively. An analysis of this strikingly divergent behavior of T_c(P) in terms of the structural characteristics of the RT₄X₁₂ class of compounds where R = rare earth element, T = Fe, Ru, or Os, and X = P, As, or Sb suggests that T_c(P) for these materials consists of two competing contributions: a depression of T_c due to the compression of the lattice (i.e., decrease in volume), and an enhancement of T_c due to the effect of pressure on La itself.     

Superconducting transition temperature,its pressure dependence and structural transformation in (La,Ce)Ru2 alloys

The superconducting transition temperature and its dependence on hydrostatic pressure to 22 kbar have been determined for a series of (La, Ce)Ru₂ alloys. The variations of these quantities with composition are found to be generally related; however, specific irregularities in the curve of dT_c/dp versus composition are not reflected in the smooth variation of the transition temperature T_c. Some features of dT_c/dp may be correlated with the occurrence of a cubic-tetragonal transformation at low temperatures.     

Pressure effects on the superconductivity of ternary rare earth palladium Heusler alloys

The application of hydrostatic pressure linearly depresses the superconducting transition temperature with a typical rate of dT_c/dP ～10^-5 K/bar for the Heusler alloys RPd₂Z, where R=Sc, Y, Tm, Yb, Lu and Z=Sn, Pb. This depression can be explained by a stiffening of the primitive cubic palladium sublattice with increasing pressure.     

Phase relations in the Fe-P system at high pressures and temperatures from ab initio computations

ABSTRACT

Based on the first-principles calculations within the density functional theory and crystal structure prediction algorithms iron phosphide phases stable under pressure of the Earth’s core and temperatures up to 4000?K were determined. A new low-temperature modification FeP-P2₁/c stable above ～75?GPa was predicted. Fe₂P with the allabogdanite structure has been established to be stable in the low-temperature region at ambient conditions. At 750?K it transforms into the barringerite structure. The transition from Fe₃P with schreibersite structure to Fe₃P-Cmcm was observed at 27?GPa, and the phase transition boundary is nearly isobaric. Fe₂P and FeP are thermodynamically stable at the Earth’s inner core pressures and 0?K according to the obtained results, whereas Fe₃P stabilizes with respect to decomposition to Fe?+?Fe₂P at high temperatures above ～3200?K.     

高温超导体低温、高压下的物理性质的研究

 在3~20 GPa压力范围内，测量了含氧量较低的YBa₂Cu₃O_7-δ（δ=0.46）单晶压力增强效应（dT_c/dp=4.9KGPa^-1）；YBa₂Cu₃O₇（T_c0=90 K）单晶在压力下临界电流密度随压力变化；外磁场H=30 kO_e时，T_c与磁场、压力关系；压力达16.5 GPa下，Bi₂Sr₂CaCu₂O_x单晶T_c(p)关系（dT_c/dp=-0.4 KGPa^-1）。发现Y系高温超导体的温度压力导数dT_c/dp与T_c0中间呈dT_c/dp=b-mT_c0线性关系（b、m为常数）。结合压力下Y系超导体结构相变和含氧量对T_c影响，分析这类超导体T_c有很强的正压力效应的原因。把实验结果同几种超导电性微观理论模型进行了分析和比较。     

High pressure μSR studies on single crystalline gadolinium

Muon spin rotation data on a single crystalline gadolinium sample have been obtained as function of temperature and hydrostatic external pressure up to 0.6 GPa. In the ferromagnetic state the application of pressure has a strong influence on the whole spin‐turning‐process of the spontaneous magnetization: The onset of the spin‐turning is shifted towards lower temperatures with a rate of approximately dT_st/ dp\approx -50 K/GPa. Higher values of the turning angle up to \vartheta_ext=90^\circ can be reached and stabilized over a wider temperature range. At low temperatures the magnetization turns back again. In the paramagnetic regime the data show a similar behaviour as under ambient pressure: both the Knight‐shift and the muon spin relaxation show deviations from their high temperature behaviour in the critical regime below (T-T_C) < 10 K. So there is no indication of pressure induced effects in this temperature range. This revised version was published online in July 2006 with corrections to the Cover Date.     

Superconducting transition temperature in hafnium under pressures up to 64 GPa

The superconducting transition temperature T _c of hafnium is measured as a function of pressure up to 64 GPa. The character of the pressure dependence of T _c observed at α–ω–β transitions in Hf is found to be similar to that observed for Zr. In the regions of α and β phases, T _c increases with pressure with the slopes dT _c /dP=0.05 and 0.16 K/GPa, respectively. At the α–ω transition, T _c (P) exhibits a tendency to a decrease, while at the ω–β transition, T _c increases stepwise from 5.8 to 8.0 K. The α–ω transition occurs at pressures between 31.2 and 35.9 GPa, and the ω–β transition, at a pressure of 62±2 GPa.     

Gd1-xCaxBa2Cu3O7-y高温超导体压力效应的研究

研究了Ｇｄ_1-xＣａ_xＢａ₂Ｃｕ₃Ｏ_7-y（０．０≤Ｘ≤０．２０）高温超导体在常压和高压下的超导电性在１－３００Ｋ温度范围内，利用Ｂｒｉｄｇｍａｎ对顶砧获得压力达９．０ＧＰａ，测量了（Ｘ＝０．１０，０．１５，０．２０）样品的ｄＴ_c／ｄｐ分别为７．６８，７．８和４．４６Ｋ／ＧＰａ。发现Ｔ_c的压力导数随着ｃａ²⁺含量的增加而下降，分析了氧含量对Ｔ_c和ｄＴ_c／ｄＰ的影响．利用常压下晶格参数精修值和阳离子与氧离子间距随压力的改变，说明ＣｕＯ₂面在超导电性上的作用，用ＣｕＯ₂面之间耦合解释Ｔ_c（Ｐ）曲线的非线性关系。 关键词：     

Effect of high pressure on the crystal and magnetic structure and on the raman spectra in Pr<Subscript>0.7</Subscript>Ba<Subscript>0.3</Subscript>MnO<Subscript>3</Subscript> manganite

The crystal and magnetic structure and the Raman spectra in Pr_0.7Ba_0.3MnO₃ manganite have been studied by the neutron diffraction technique at pressures up to 5 GPa as well as by the X-ray diffraction and Raman spectroscopy at pressures up to 30 GPa. The pressure dependence is determined for the lattice parameters, unit cell volume, Mn-O bond lengths in the orthorhombic structure of the Imma symmetry, and bending and stretching vibration modes for oxygen octahedra. In the low-temperature range at pressure P = 1.9 GPa, the magnetic transition from the initial ferromagnetic (FM) ground state (T _C = 197 K) to the A-type antiferromagnetic (AFM) state (T _N = 153 K) has been revealed. The FM and AFM phases coexist at pressures up to 5.1 GPa and exhibit negative and positive values of the pressure coefficient for the Curie and Néel temperature, respectively (dT _C/dP = −2.3 K/GPa and dT _N/dP = 8 K/GPa). The pressure dependence of the Curie temperature in Pr_0.7Ba_0.3MnO₃ differs drastically from that observed in other manganites of nearly the same composition with the orthorhombic Pnma and rhombohedral R[`3]cR\bar 3c structures, where the FM phase is characterized by the positive values of dT _C/dP. The structural mechanisms of these phenomena are discussed.     

The excitonic absorption of CdS under pressure

Abstract

We have investigated the pressure dependence of the absorption edge of tlie E_A-E_B- and the E_C-excitons of 1pi thick CtlS(hex) crystal slabs. We obtaiurd dE_Θ/dP =dE_c/dP = 47±3 meV/GPa for T=300K and dE_A/dP= dE_B/dP = 43±3 meV/GPa for T=78K. With tliese values we were able to determine the crystal-field splitting (68f4 nieV) aid the spin-orbit splitting (26±2 meV). The resulting pressure shifts of tlie rxcitonic energies were in agreement witli a κ · p-calculation of tlie transition energies using tlie perturbation niatrix of G.E. Pikus for Δ_so ? Δ_c. The refractive Index n‖c was iiieasured for T=300K up to 2.2 GPa and described by Marples Model.     

Physical properties of the homologous series Y2Ba4Cu6+nO14+n (n=0,1,2) at high pressure

Abstract

We have measured resistivity and magnetic susceptibility of Y₂Ba₄Cu_6+nO_14+n (n=0,1,2) on bulk material and single crystals as well. We have found a pressure dependence of dT_c/dp within the range of 0.05 to 0.55 K/kbar depending on the symmetry.     

Effect of pressure and magnetic field on bilayer La1.25Sr1.75Mn2O7 single crystal

We report the temperature dependence of susceptibility for various pressures, magnetic fields and constant magnetic field of 5 T with various pressures on La_2−2xSr_1+2xMn₂O₇ single crystal to understand the effectiveness of pressure and magnetic field in altering the magnetic properties. We find that the Curie temperature, T_c, increases under pressure (dT_c/dP=10.9 K/GPa) and it indicates the enhancement of ferromagnetic phase under pressure up to 2 GPa. The magnetic field dependence of T_c is about 26 K for 3 T. The combined effect of pressure and constant magnetic field (5 T) shows dT_c/dP=11.3 K/GPa and the peak structure is suppressed and broadened. The application of magnetic field of 5 T realizes 3D spin ordered state below T_c at atmospheric pressure. Both peak structure in χ_c and 3D spin ordered state are suppressed, and changes to 2D-like spin ordered state by increase of pressure. These results reveal that the pressure and the magnetic field are more competitive in altering the magnetic properties of bilayer manganite La_1.25Sr_1.75Mn₂O₇ single crystal.     

Phase Transition and vibration properties of MnCO3 at high pressure and high-temperature by Raman spectroscopy

The high pressure and high-temperature behavior of MnCO₃ was investigated up to 55?GPa at ambient temperature and up to 573?K at ambient pressure by Raman spectroscopy, respectively. Some new modes were detected at ～16 and ～32?GPa, which were assigned to MnCO₃-I below 16?GPa and to MnCO₃-II above 32?GPa, and to a coexisting phase of them in between. The high pressure vibration properties of all Raman modes, especially high frequency modes, were systematically reported. The coexisting phase of MnCO₃-I and MnCO₃-II had much easier compressibility than the MnCO₃-II phase. The thermal stability of MnCO₃ was at least to 573?K and its thermal expansion along the c axis was easier than a and b axes.