The crystal structure of (Pb0.5Cd0.5)Sr2 (Y1−xCax)Cu2O7

The crystal structures of (Pb_0.5Cd_0.5)Sr₂YCu₂O₇, (Pb_0.5Cd_0.5)Sr₂(Y_0.6Ca_0.4)Cu₂O₇ and (Pb_0.5Cd _0.5)Sr₂(Y_0.5Ca_0.5)Cu₂O₇ have been refined by the Rietveld method for the X-ray diffraction data. The refinement results indicate that both Pb and Cd atoms in the (Pb, Cd)O layers and O(3) atoms are displaced from their ideal sites, and that there is a complicated occupation in the (Pb, Cd)O layers, i.e., other cations, such as Sr²⁺, Cu²⁺ and Ca²⁺, can occupy the (Pb, Cd) site for these samples. This may be the reason why the lattice constants do not vary monotanically with the calcium content, and why there exist broad superconducting transitions in the system (Pb_0.5Cd_0.5)Sr₂(Y_1−xCa_x)Cu₂O₇. The refinement results also indicate that, as the calcium content increases, the O(2) atoms move close to the CuO₂ planes and far from the (Pb, Cd)O layers. Thus, the increase of calcium content results in charge transfer from the charge reservoir layer (Pb, Cd)O to the CuO₂ planes. This result is consistent with the bond valence sums of the Cu ions in the CuO₂ planes.     

Hole formation and charge transfer in Y1−xCaxSr2Cu2GaO7, a new oxide superconductor

The technique of transmission electron energy loss spectrometry (EELS) in parallel detection has been utilized to analyze the fine structures associated with the O K and Cu L₃ absorption edges of recently discovered Y_1−xCa_xSr₂Cu₂GaO₇ (0.1 ≤ x ≤ 0.4) series of compounds, which exhibit superconductivity when annealed under high-pressure oxygen atmosphere. We find subtle but significant changes in EELS spectral features of nonsuperconducting and superconducting specimens of these compounds. A broad pre-edge feature is observed below the O K absorption edge, at about 528.2 eV which emerges with Ca-doping alone. Evidence is presented for another O K secondary pre-edge feature, at about 1.1 eV below the first O K pre-edge feature in only those specimens which have undergone high-pressure oxygen treatment (i.e. superconducting specimens). We interpret the O K pre-edge feature as due to formation of holes on oxygen sites and propose that the first broad pre-edge feature (at 528.2 eV) is associated with holes on oxygen sites other than the CuO₂ planes, which are responsible for normal conductivity. The second smaller pre-edge feature (at 527.1 eV) is most probably associated with holes in the CuO₂ planes, which are associated with superconductivity in this system. The presence of a secondary smaller O K pre-edge feature in the superconducting specimens appears to correlate well with a broad shoulder on the high energy side of the Cu L₃ edge. The results are interpreted in terms of oxidation of CuO₂ planes through charge transfer between copper and oxygen in the CuO₂ planes, i.e. covalent mixing of O and Cu orbitals at the Fermi level as a result of high oxygen pressure annealing.     

Quantum critical point, scaling and universality in high Tc(CaxLa1−x)(Ba2−c−xLac+x)Cu3Oy

Using charge transport in sintered ceramic samples it is observed that at all doping, including non superconducting overdoped samples, there exists a temperature in which below it dR/dT < 0. This suggests that either the quantum critical point is not necessarily inside the superconducting dome or that the CuO₂ plane is never overdoped. Data relating experimental Cooper pair density, conductivity and T_c suggest that Homes’ relation might need a more specific definition of the conductivity σ.     

The nuclear contribution to the specific heat of single crystals of YBa2Cu3O7 and Bi2Sr2CaCu2O8

Specific heat data below 1 K of Bi₂Sr₂CaCu₂O₈ and YBa₂Cu₃O₇ are analyzed. For YBa₂Cu₃O₇ the nuclear specific heat, C_N, amounts to 38T⁻² μJ/mol K. C_N for Bi₂Sr₂CaCu₂O₈ exceeds that of YBa₂Cu₃O₇ by a factor of 15. The nuclear quadrupolar specific heat contribution alone is insufficient to explain the data for YBa₂Cu₃O₇, while lack of NQR data does not allow such a comparison in Bi₂Sr₂CaCu₂O₈ to be made. The contribution to C_N from nuclear spins coupled via the contact hyperfine interaction with correlated magnetic spins (in the CuO₂ plane) is derived as a function of the correlation length. This contribution can be treated independently from the quadrupolar term. We show that the excess specific heat in YBa₂Cu₃O₇ likely originates in a few percentage of an impurity (oxygen deficient) phase with a strong hyperfine field on the Cu nuclei.     

New experimental evidence of the structural modification in single crystal Bi2Sr2CaCu2Oy by X-ray diffraction observation

The variations of the high angle 00 peak-shape by means of X-ray l scans of the 00l fundamental reflections were investigated in detail for a highly oriented Bi₂Sr₂CaCu₂O_y (Bi2212) crystal with sufficiently small intrinsic mosaicity and the same crystal annealed in air at 250, 300, 400, 600, and 750°C for 20 h in consequence. For the first time, we observed a new additional reflection almost overlapped original 00l fundamental reflection at annealing temperature below 400°C by X-ray diffraction measurement, which shows that there coexisted two sets of lattice periodicity in the c-direction of the annealed crystal. The new additional reflection appeared at 250°C and disappeared at 400°C. Its intensity was increased at 300°C. The measurements of the AC susceptibility, c-axis parameter and full width at half maximum (FWHM) of the 00 peaks showed that the new additional reflection was associated with the oxygen diffusion in CuO₂ planes and the changes of strain field. The results provide the new experimental evidence that the structural distortion is more sensitive to the oxygen diffusion in CuO₂ planes than to that in Bi–O layers.     

Upper bound to the charge-transfer energy on a 2D Wigner lattice in high-Tc cuprates

Assuming that the energy of a 2D Wigner lattice, which has been frequently observed on the CuO₂ plane in layered high-T_c cuprates, is composed of Coulomb energy, kinetic energy, and charge-transfer energy which is necessary to transfer electrons or holes onto the CuO₂ plane (the magnitude of the energy is expected to be of the order of the work function of the compound constituting the plane from which the electrons are transferred, if the CuO₂ plane easily accepts the electrons or holes, as if the CuO₂ plane were crystal surface), we find in the mean-field approximation, that the total energy for a unit lattice, E₀, can be given by a function of the charge-transfer energy E_φ as E₀= (E_φ−E_φ⁰+O((E_φ−E_φ⁰)₂)(>0), where the value of E_φ⁰(1.5 in the ground state, for example) is independent of the lattice configuration. This relation implies that the upper bound to the charge-transfer energy is given by E_φ⁰. The smallness of E_φ⁰, compared with the work function of ordinary metals and alloys (with the work function around 4 eV), is necessary for the realization of the 2D Wigner lattice in many layered high-T_c cuprates.     

Ramans study of Y1−xPrxBa2Cu3O7−δ and YB2Cu3−xZnxO7−δ single crystals

Single crystals with known T_c values of Y_1−xPr_xBa₂Cu₃O_7−δ (Y---Pr1:2:3) and YBa₂Cu_3−xZn_3−xZn_xO_7−δ (Y---Zn1:2:3) systems are studied by Raman measurements. The Raman spectra for (Y---Pr1:2:3) single crystals show that the frequencies of Ba and O_z modes increase as the Pr content increases. The results are consistent with the hole-localization scheme proposed for the suppression of superconductivity in the polycrystalline Y---Pr1:2:3 systems. On the other hand, in the Y---Zn1:2:3 system, all the Raman modes do not change in frequencies. However, the FWHM of the Cu(2) mode increases with the decrease of T_c, indicating strong scattering of charge carriers by the substituted Zn ions in the CuO₂ planes. The induced disorder in the CuO₂ planes may be related with suppression of T_c in the Y---Zn1:2:3 system. Thus, the suppression mechanism in the Y---Zn1:2:3 systems seems to be different from that in the Y---Pr1:2:3 systems.     

Na2CO3的高压拉曼光谱研究

碳酸盐是碳在地球内部的重要载体之一,其在地幔高温高压条件下的晶体化学是理解地球深部碳的赋存状态和循环过程的关键,而结构稳定性和相变是晶体化学最基本的研究内容。碳酸钠（Na₂CO₃）是一种常见的碱性碳酸盐矿物,在产自地幔过渡带-下地幔的金刚石中已发现含钠的碳酸盐矿物包裹体,这成为碳酸钠能够俯冲进入地幔深部的直接矿物学证据。前人利用拉曼光谱技术研究了Na₂CO₃在常温常压下的晶格振动模式,但其在高压下的稳定性和结构变化却鲜有报道。利用金刚石压腔装置结合先进的共聚焦拉曼光谱技术,以硅油作为传压介质,在准静水压力条件下,在0.001～27.53 GPa压力区间对Na₂CO₃粉末在600~1 200 cm-1波段的振动特征进行了细致地分析。本次实验重点分析了[CO₃]2-基团振动模式在升压和卸压过程中的行为。结果表明,在0.001～11.88 GPa压力范围内,[CO₃]2-基团对称伸缩振动γ₁（1 088.06和1 070.76 cm-1）、反对称伸缩振动γ₃（865.10和797.50 cm-1）和面内弯曲振动γ₄（720.10和696.71 cm-1）都出现了振动峰的分裂。随着压力增加,所有振动峰都向高频率漂移,半高宽也逐渐增加。在13.40 GPa时,Na₂CO₃发生结构相变,具体表现为690.08 cm-1处出现1条新的拉曼峰,并且随着压力升高该峰的强度逐渐增大。同时反对称伸缩振动峰γ₃以及面内弯曲振动峰γ₄的强度持续减弱,半高宽也继续变大。这些现象表明Na₂CO₃结构相变源于[CO₃]2-内部晶格变化。当压力卸载到4.18 GPa时,[CO₃]2-的振动模式与常温常压下的完全吻合,相变出现的新峰也已经消失,表明该相变是由[CO₃]2-基团畸变引起的并且具有可逆性。继续升压至27.53 GPa,拉曼光谱继续蓝移,Na₂CO₃的拉曼谱线再没有变化,说明高压相在这一压强范围内保持稳定。在整个加压过程中,反对称伸缩振动γ₃和面内弯曲振动γ₄处的拉曼峰出现强度减弱现象。同时也计算了各个峰频率对压力的依赖系数dγ/dP,结果显示[CO₃]2-基团内各个振动模式对压力的响应是不同的,这很可能与Ｃ-Ｏ键的键长有关。最后,对比发现,对称伸缩振动γ₁峰的强度比反对称伸缩振动γ₃和面内弯曲振动γ₄峰的强度大,并且[CO₃]2-基团对称伸缩振动γ₁受压力影响相对较小,可以用来区别不同种类的碳酸盐矿物。     

Giant pressure effect in oxygen deficient YBa2Cu3Ox

The pressure effect on T_c of polycrystalline and single crystalline YBa₂Cu₃O_x investigated as a function of oxygen content x by ac-susceptibility measurements under helium pressure. In the overdoped region x> 6.93 the single crystals show a negative dT_c/d p, as expected from the charge transfer model. For optimally doped samples with x = 6.93 we find dT_c/d P = 0.4 K/GPa which points to pressure effects on T_c aside from charge transfer. In the underdoped region x < 6.93 the dT_c/d p values obtained from the experiment depend strongly on the storage temperature of the sample during the experiment. When the samples are stored at temperatures well below 240 K throughout the entire experiment including pressure application and pressure release, dT_c/d p increases to approx. 7 K/GPa at x = 6.7 but with a further decrease of the oxygen content the dT_c/d p drops to approx. 2 K/GPa at x = 6.4. These effects are intrinsic to the YBa₂Cu₃O_x structure and can be explained by considering the anisotropic structure of YBa₂Cu₃O_x. The decrease of the c-axis lattice parameter results in a charge transfer to the CuO₂-planes mainly [1], whereas the compression of the a- and b-axis lattice parameter is known to produce different pressure effects which are responsible for the peak in dT_c/d p at x = 6.7 [2]. When pressure is changed at room temperature oxygen ordering effects occur which cause a relaxation of T_c to the equilibrium value T_c(p) at this pressure with a time constant depending on the oxygen content x. A decrease x results in a peak effect in dT_c/d p at x = 6.7 again, which is enhanced to approx. 12 K/GPa. If the oxygen content is decreased further, dT_c/d p first drops to 5 K/GPa at x = 6.6, but the increases to values of more than 20 K/GPa for x < 6.42. These giant pressure effects at low oxygen contents are mainly caused by a reversible T_c increase (dT_c/d p)_O due to pressure induced oxygen ordering via oxygen motion between unit cells.     

Neutron scattering studies of crystal structure and crystalline electric field in high-Tc ErBa2Cu3Ox disordered by fast neutron irradiation

The crystal structure and the crystalline-electric-field splitting of the Er³⁺ 4f-shell in the high-T_c superconductor ErBa₂Cu₃O_x (x = 6.95, 6.18) disordered with fast neutron irradiation has been investigated by elastic and inelastic neutron scattering techniques, respectively. The results yield evidence for the absence of a charge transfer between the planes and chains under disorder and point to the atomic displacements being the main radiation-induced defects in the CuO₂ planes.     

X-ray single crystal analysis of HgBa2Ca2Cu3O8+δ and influence of oxygen stoichiometry on the superconducting properties

X-ray diffraction analyses of good-quality single crystals of HgBa₂Ca₂Cu₃O_8+δ, synthetized by a closed-vessel technique at low pressure, have lead to more definite results with respect to previous structural studies. Samples with T_c = 135 K showed a Cu occupancy of 16.1% on the Hg site at the origin of the unit cell. Excess oxygen appears to be present only in the basal plane at the interstitial site , 0 with an occupancy δ = 0.190 ± 0.015. This compound has the shortest copper oxygen apical distance within the mercury family (2.696(3) Å) together with an almost complete planarity of the CuO₂ planes. Variations in the excess oxygen content upon different heat treatments were analyzed by thermogravimetry and magnetic-susceptibility measurements. A correlation between δ and T_c could be derived, showing in particular that superconductivity exists down to a very low interstitial oxygen content.     

Peculiarity of charge dynamics in High-Tc cuprates

Discussion is focused on the peculiarity of the out-of-plane and the in-plane charge dynamics related to the two-dimensionality in high-T_c cuprates. Recent observations of a double Josephson plasmon and a intra-bilayer Josephson effect remind us that each CuO₂-plane is in fact an independent superconducting layer, suggesting that the superconducting coherence length in the c-direction is shorter than the interlayer distance. The origin of this extremely short c-axis coherence length as well as the long c-axis penetration depth is discussed in relation with the incoherent c-axis transport in the normal state. Another peculiarity is a strong itinerant nature of charges within the planes. As an example, it is demonstrated that a static charge order is inhibited even in the “stripe” phase in the cuprates.     

Superconductivity due to charge transfer to the CuO2 sheet from the (Tl,Pb) O layer in Tl0.5Pb0.5Sr2−xNdxCuO5−δ

The superconductivity of Tl_0.5Pb_0.5Sr_2−xNd_xCuO_5−δ was observed for x ≥ 0.3, although the calculated Cu valence assuming Tl³⁺ and Pb⁴⁺ is slightly above or below 2.0. The binding energy of the Tl and Pb 4f core levels measured by XPS shifted to higher energy than those of Tl³⁺ and Pb⁴⁺ in the reference materials, showing that the Tl and Pb valences are lower than 3 + and 4 +, respectively. These observations strongly suggest that holes in the CuO₂ sheet are created by charge transfer from the (Tl,Pb)O layer, similar to the double-layered Tl---Ba cuprates.

In contrast, the temperature variation of electrical conductivity of Tl_0.5Pb_0.5Sr₂CuO_5−δ changed from metallic to semiconducting with increasing oxygen deficiency, δ, and no superconductivity was observed through any control of δ. Although XPS measurement also suggested that Tl and Pb valences lowered with increasing oxygen deficiency, δ, the reason why the system did not show superconductivity can be understood by the depletion of oxygen from the CuO₂ plane during deoxygenation.     

掺Pr的YBCO体系的EXAFS研究

通过对掺Pr的YBCO体系的EXAFS数据计算机分析发现:体系中的O的位置随Pr含量改变发生了变化。CuO₂平面曲折度增加,Cu-O链上的O更趋近于CuO₂面,Ba-O3间距增大。所有这些变化导致载流子浓度降低和载流子严重局域化,引起超导性发生变化。     

NMR study of local hole distribution, spin fluctuation and superconductivity in Tl2Ba2Ca2Cu3O10

⁶³Cu, ¹⁷O and ²⁰⁵Tl NMR have been performed in the high-T_c superconductor Tl₂Ba₂Ca₂Cu₃O₁₀ whose T_c(max) is 127 K. The hole densities at Cu and oxygen sites in the CuO₂ plane have been extracted from the nuclear quadrupole frequency ν_Q. The striking feature is that the Cu holes are significantly transferred to oxygen site due to strong hybridization between Cu and oxygen. From an analysis of T₁ and T_2G, it has been found that the spectral weight of the spin fluctuation is transferred to higher energy compared to YBa₂Cu₃O₇, while the magnetic correlation length ξ does not differ much. Thus, it is suggested that the higher T_c is due to higher characteristic energy of spin fluctuations, i.e. the superconductivity is spin fluctuation mediated. The superconducting properties are consistently explained by a d-wave superconductivity model with a finite density of states (DOS) at the Fermi level. We show that the disorder of the Ca/TlO layer caused by the partial inter-substitution of Tl and Ca is responsible for the potential scattering to produce such a DOS. It is found that if such a potential scattering were absent, T_c would go up to 132 K which is quite close to the record T_c realized in the Hg based compound.     

Superconductivity at 30 K in the Nd2CuO4-type cuprate Tm1.83Ca0.17CuO4

The Nd₂CuO₄-type Ca-doped rare-earth cuprates R_1.83Ca_0.17CuO₄ (R=Gd, Dy or Tm) have been synthesized under high pressure. Only that with the small ion Tm³⁺ and thus the short Cu---O bonds (1.916 Å) was superconducting (at about 30 K). This was considered to be the first example of p-type superconductivity in the purely square-planar CuO₂ layers without apical oxygens.     

Hole distribution and madelung energy in the high-Tc oxide superconductors

Evidence is presented that in high-T_c superconductors the electrostatic interaction as expressed by the Madelung energy is the key factor in determining the charge distribution and in particular the distribution of the holes between the CuO₂-planes and other structural subunits. Specifically, this explains the phase diagram (T_c as a function of oxygen content and oxidizing power) of cation-substituted YBa₂Cu₃O_6+y and the oxygen ordering in YBa₂Cu₃O_6.5. Moreover, we argue that the dependence of T_c on strontium content in La_2−xSr_xCuO₄ is similarly due to the fact that holes go into the (La,Sr)O-sheets as well as into the CuO₂-planes.     

高压下Zn2GeO4带隙变化的第一性原理研究

采用基于密度泛函理论的第一性原理计算研究了Zn₂GeO₄晶体在高压下的电子结构和带隙变化行为. 研究结果发现, 随着压强的增加, Zn₂GeO₄ 能带间隙先变大, 在压强为9.7 GPa时达到最大值, 然后减小. 通过电子态密度、电荷布居数和电子差分密度分布图的研究分析可知:在低压区域(0< P< 9.7 GPa), 带隙的变大主要是由于原子间距离的减小造成的共价性增强和Ge原子随压强的变大局域性增强引起的; 在高压区域(P>9.7 GPa), 则是出现了离域现象, 诱发了离域电子的产生, 从而使带隙减小.     

Ba2(Cu0.7Cd0.3)O3: a new double-center perovskite structure

A new phase of Ba₂(Cu_0.7Cd_0.3)O₃ has been discovered. The composition of the phase was examined by energy dispersive X-ray spectrometry. The phase exhibits a tetragonal structure with lattice constants a=4.077 Å and c=7.970 Å, and its space group is P4/mmm. We propose a double-center-perovskite structure model, which is similar to that of (Ba, Sr)₂CuO₂(CO₃) [1, 2]. The simulated XRD is close to the observed one. The analysis results of elemental analyzer and Fourier transform infrared spectrometer reveal that the phase does not contain CO . The cuprate is an insulator, but its new cell structure and the large size of the CuO₂ planes seem to suggest a good reason for researching new superconductors. The roles of CdO in the solid state reaction and the formation of the new phase are discussed.     

Double exchange model for RuSr2(Eu,Gd)Cu2O8

We propose a double exchange model to describe the RuO₂ planes of RuSr₂(Eu,Gd)Cu₂O₈. The Ru⁺⁵ ions are described by localized spins, and additional electrons provided by the superconducting CuO₂ planes are coupled ferromagnetically to them by Hund rules coupling. We calculate the spin structure factor, magnetic susceptibility and magnetization as a function of magnetic field and temperature, using a Monte Carlo algorithm in which the Ru⁺⁵ spins are treated as classical. Several experiments which seemed in contradiction with one another are explained by the theory.