Anomalous behaviour of sound velocity in the highT c superconductor YBa2Cu3O7

The longitudinal acoustic wave velocity in YBa₂Cu₃O₇ has been measured in the range 300 to 85 K using the ultrasonic pulse superposition technique. The sound velocity shows a steep drop in its value at 260 K and decreases continuously till the superconducting transition temperature. BelowT _c the sound velocity increases steeply. A hysteresis is seen when the experiments are performed during a cooling-warming cycle. The results are compared with recent measurements by Ewertet al on the samples with the same chemical composition.     

Study of the superconducting transition in YBa2Cu3O7−δ single crystal by thermopower measurements

We have determined the superconducting transition in YBa₂Cu₃O_7−δ single crystal by measurements of thermopower. Two transitions were observed in both the ab-plane and the c-direction. The high temperature transition may have a close relation to the surface shell of the crystal.     

氧缺位对单相YBa2Cu3O7-x热电势率的影响

研究了氧缺位对单相YBa₂Cu₃O_7-x体系超导电性及热电势率的影响。结果表明,超导转变温度随氧缺位的增加而下降,热电势率随氧缺位的增加而增大。由实验结果推断,单相YBa₂Cu₃O_7-x的多数载流子为空穴。存在较大的声子曳引热电势表明。在YBa₂Cu₃O_7-x超导体中存在较强的电-声子相互作用。 关键词：     

Positron annihilation measurements across the superconducting transition in Y1Ba2Cu3O7−x

Positron lifetime and Doppler broadened annihilation radiation lineshape measurements have been carried out in Y₁Ba₂Cu₃O_7−x as a function of temperature in the range of 300 K to 58 K. The positron lifetime and the peak parameter of the annihilation radiation lineshape are observed to decrease on lowering the temperature without showing any discontinuous change across the superconducting transition temperature of 90 K as determined by susceptibility measurements. The variation of positron annihilation parameters with temperature in the superconducting state is significantly larger than that in the normal state. This is qualitatively explained in terms of the dimerization of oxygen ions in the superconducting state of Y₁Ba₂Cu₃O_7−x .     

Magnetization harmonics for YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7 − <Emphasis Type="Italic">x</Emphasis></Subscript> textured polycrystals above the superconducting transition temperature

The temperature dependences of the third and other higher magnetization harmonics for YBa₂Cu₃O_{7 ? x} textured polycrystals are experimentally investigated in the temperature range 77–120 K. It is revealed that the magnetization of the YBa₂Cu₃O_{7 ? x} textured polycrystals exhibits a nonlinear behavior up to temperatures considerably higher than the superconducting transition temperature. The observed nonlinearity of the magnetization of the YBa₂Cu₃O_{7 ? x} textured polycrystals is attributed to the pseudogap state that appears for this compound at a temperature T ～ 102 K.     

Magnetic-relaxation study in melt-textured ErBa2Cu3O7−δ ceramic material

We have studied the zero-field-cooled magnetic relaxation in the melt-textured ErBa₂Cu₃O_7– ceramic material. The magnetic relaxation rate dM/d (ln t) exhibits strong temperature and field dependence in the temperature range up to 80 K and the field range up to 5.5 T. The magnetic relaxation behavior observed in the melt-textured ErBa₂Cu₃O_7– ceramic sample is similar to that in single crystals. The magnetization to noise ratio is much improved, in particular, for temperatures approaching the superconducting transition temperature and for low magnetic fields. Several theoretical models were explored to fit the experimental data.     

Resistivity,thermopower and single-particle tunneling studies on some zinc-doped yttrium barium copper oxide superconductors

Abstract

Resistance and thermopower measurements have been made on a series of compounds, YBa₂Cu₃- _x Zn _x O_7-y, with x = 0.025, 0.05, 0.1, 0.15 and 0.2. The superconducting transition temperature decreases as the zinc concentration increases. In a range of temperatures below T_M , the mid point of the transition, the resistance shows an exponential temperature dependence fitting the phenomenological formula proposed by Ausloos et al. From the plot of logarithm of resistivity vs. (T_M ? T) ^1/2/T, one deduces a value of the average dimension of the Josephson junction to be a few tens of Å, suggesting the microtwin boundaries to be the location of the junctions. The thermopower shows a peak always just above T_c . This conclusively shows that phonon drag is not the cause of the peak. The temperature dependence of the thermopower appears to resemble closely the earlier observations of Srinivasan et al. on yttrium barium copper oxide. Single-particle tunneling measurements carried out for two concentrations, x = 0 and 0.05, appear to indicate that the energy gap parameter scales with T_c , and 2Δ/kT_c has an approximate value of 5.5.     

NdBa2Cu3O7-δ/YBa2Cu3O7-δ多层膜体系的外延结构和磁通钉扎的研究

采用脉冲激光沉积技术,在单晶SrTiO₃基底上外延生长了一系列名义结构为p×(NdBa₂Cu₃O_7-δ(m)/YBa₂Cu₃O_7-δ(n))的多层膜和准多层膜(单元层NdBa₂Cu₃O_7-δ较厚而YBa₂Cu₃O_7-δ呈非连贯的岛状分布,m,n为激光脉冲数,p为重复周期).样品的超导转变温度在87—91 K范围,具体大小取决于不同的调制结构,多层膜的重复周期越大,层状界面越多,超导转变温度就越低.磁传输测量表明,准多层的样品不仅具有较高的超导转变温度,而且具有较强的磁通钉扎性能,77K零场下的临界电流密度高达4×10⁶ A/cm²,显示出良好的应用前景. 关键词： 2Cu₃O_7-δ')" href="#">NdBa₂Cu₃O_7-δ 多层膜 磁通钉扎 临界电流密度     

Setup for studying nonlinear magnetic properties of high-temperature superconductors with the aid of magnetization harmonics

A setup for measuring cophasal and quadrature components of higher harmonics of an electromotive-force signal of the response of a high-temperature superconductor makes it possible to study nonlinear magnetic properties of superconductors in variable magnetic fields of up to 1 kOe and constant magnetic fields of up to 10 T in the temperature range of 5?C300 K. This setup was used to measure the temperature dependences of the absolute values of the real and imaginary parts of the first and third harmonics of the magnetization of textured Yba₂Cu₃O_{7 ? x} polycrystalline samples in the temperature range of 77?C220 K at various values of variable and constant magnetic fields. An analysis of resulting data made it possible to reveal the presence of different dynamical modes of the magnetic flux in YBa₂Cu₃O_{7 ? x} that were dominant in different temperature ranges. The nonlinearity of the magnetization of YBa₂Cu₃O_{7 ? x} (the appearance of higher harmonics) was observed up to temperatures in the range of T = 103?C112 K, which were substantially higher than the temperature of the transition of this compound to a superconducting state. The observed feature in the magnetization of YBa₂Cu₃O_{7 ? x} was associated with the emergence of a pseudogap state in this compound.     

First-order phase transition characteristic of the high temperature metal–semiconductor transition in [Ca<Subscript>2</Subscript>CoO<Subscript>3</Subscript>]<Subscript>0.62</Subscript>[CoO<Subscript>2</Subscript>]

Transportation and thermodynamic properties of misfit-layered polycrystalline [Ca₂CoO₃]_0.62[CoO₂] were measured in order to clarify the nature of metal– semiconductor transition (MST) at T _MS≈400 K, above which the simultaneous decrease of resistivity and increase of thermopower with temperature give rise to a great enhancement of thermoelectric power factor up to 1000 K. A first-order phase transition characteristic around T _MS was revealed by anomalies of resistivity, differential scanning calorimetry, and thermal expansion. The first-order characteristic of the MST can be rationalized from the Virial theorem at an itinerant to localized electron transition in the narrow e ^T band within the [CoO₂] plane. Above T _MS, the reduction of the retained delocalized states within the matrix of localized states and the enhancement of charge carrier effective mass with increasing temperature might account for the considerable enhancement of the thermopower.     

Influence of oxygen deficiency on the thermoelectric power of Y1Ba2Cu3O7-x

In this paper we report measurements of the thermoelectric power on a series of Y₁Ba₂Cu₃O_7-x specimens with varying amounts of oxygen deficiency obtained by changing the cooling rate of the sintered specimens. The specimens have been characterized by X-ray diffraction measurements, electric resistivity and oxygen contents. The temperature variation of the thermopower reveals a peak just before the onset of superconducting transition. We examine possible theoretical explanations of this anomaly. In particular we argue that this anomaly is associated with the pairing fluctuations in the normal state close toT _c . We present some theoretical results in support of this conclusion.     

Fluctuation-induced conductivity in melt-textured Pr-doped YBa2Cu3O7-δ composite superconductor

In this study, the effects of thermal fluctuations on the electrical conductivity in melt-textured YBa₂Cu₃O_7-δ, Y_0.95Pr_0.05Ba₂Cu₃O_7-δ and (YBa₂Cu₃O_7-δ)_0.95–(PrBa₂Cu₃O_7-δ)_0.05 composite superconductor were considered. The composite superconductor samples were prepared through the top seeding method using melt-textured NdBa₂Cu₃O_7-d seeds. The resistivity measurements were performed with a low-frequency, low-current AC technique in order to extract the temperature derivative and analyze the influence of the praseodymium ion on the normal superconductor transition and consequently on the fluctuation regimes. The results show that the resistive transition is a two-step process. In the normal phase, above the critical temperature, Gaussian and critical fluctuation regimes were identified, while below the critical temperature, in the regime near the approach to the zero-resistance state, the fluctuation conductivity diverges as expected in a paracoherent-coherent transition.     

Low concentration of the superconducting phase and thermoelectric power in 90 K superconductors

The thermoelectrical power (TEP) and the electrical resistivity behaviour of three 90 K superconductors viz, Y₁Ba₂Cu₃O_7−x , Sm₁Ba₂Cu₃O_7−x and Gd₁Ba₂Cu₃O_7−x , after the specimens were quenched from the sintering temperature (920°C) to 77 K, are reported. Interestingly the Y₁₂₃ specimen, which has the presence of trace amount of the orthorhombic phase in an otherwise tetragonal phase and does not show a superconducting transition down to 77 K shows zero TEP around 82 K, theT _c for the well oxygenated specimen. The Sm and Gd specimens on the other hand show completely tetragonal structure, semiconducting behaviour in resistivity and no zero TEP up to 77 K. It is argued that the critical concentration of the superconducting phase necessary to make the TEP zero is much smaller than that required for zero resistivity.     

Permittivity of BaTiO3 epitaxial films grown on the YBa2Cu3O7−δ(001) surface

The epitaxial heterostructures YBa₂Cu₃O_7?δ and YBa₂Cu₃O_7?δ/(5 nm)SrTiO₃/BaTiO₃/(5 nm)SrTiO₃/YBa₂Cu₃O_7?δ are grown by the laser evaporation method on an LaAlO₃(100) substrate. The permittivity of a BaTiO₃ layer is approximately doubled (T=300 K) when a SrTiO₃ thin layer is inserted between a ferroelectric layer and superconducting cuprate electrodes. A maximum in the temperature dependence of the permittivity for a barium titanate layer in the YBa₂Cu₃O_7?δ/(5 nm)SrTiO₃/BaTiO₃/(5 nm)SrTiO₃/YBa₂Cu₃O_7?δ heterostructure is shifted by 70–80 K toward the low-temperature range with respect to its location in the corresponding dependence for the BaTiO₃ bulk single crystal. The bias voltage dependence of the permittivity for the BaTiO₃ grown layers exhibits a clearly pronounced hysteresis (T=300 K). The superconducting transition temperature for the lower YBa₂Cu₃O_7?δ electrode in a superconductor/ferroelectric/superconductor heterostructure considerably depends on the rate of its cooling after the completion of the formation process.     

四方和正交YBa2Cu3O7-x体系的输运性质

测量了一系列在不同淬火温度下制得的单相YBa₂Cu₃O_7-x样品的电阻和热电势率。热电势率与温度的关系表明,在正交结构的超导体中,存在声子曳引峰,而在四方结构的样品中,声子曳引峰消失,说明电-声子相互作用对超导电性有影响。根据热电势率数据,估算了样品的费密能和载流子浓度。 关键词：     

Analysis of nonlinearity of magnetization of YBa2Cu3O7 − x by the magnetic field modulation method

Temperature dependences of the magnetization harmonics of YBa₂Cu₃O_{7 ? x} monodomain samples are studied experimentally at temperatures of 77–120 K. It is found that nonlinearity of magnetization of YBa₂Cu₃O_{7 ? x} (higher harmonics generation) is observed up to temperatures T = 103–112 K, which are much higher than the superconducting transition temperature of this compound. At the same temperatures, the temperature dependence of resistivity begins to deviate from linearity. The observed singularity of the magnetization of YBa₂Cu₃O_{7 ? x} is associated with the emergence of a pseudogap state in this compound.     

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.     

The thermopower of amorphous CuTi-alloys as a function of concentration and temperature

We report on thermopower measurements of several Cu_100–x Ti _x -alloys (x=30, 35, 40, 45, 50, 55, 60 and 65) in the temperature range from 4.4 to 320 K. In all cases the thermopower is positive. It shows a strong variation with concentration. An analysis of the temperature dependence is presented at low and high temperatures. The correlation of the thermopower with the pressure dependence of the electrical resistivity and the Hall-coefficient is discussed and compared with theoretical predictions.We wish to dedicate this article to the memory of A.B. Bhatia who died on September 27, 1984     

Epitaxial combination of NdBa2Cu3O7−δ /SrTiO3: growth characteristics, structure, and parameters

We have studied the growth characteristics, structure, and parameters of the epitaxial heterostructures (001)NdBa₂Cu₃O_7−δ /(100)SrTiO₃/(001)NdBa₂Cu₃O_7−δ grown by laser ablation on a (100)LaAlO₃ substrate with a thin (∼2 nm) YBa₂Cu₃O_7−δ intermediate layer. The use of an YBa₂Cu₃O_7−δ intermediate layer promotes layered growth of the (200 nm) NdBa₂Cu₃O_7−δ layer, whose free-surface roughness is 4–5 nm. The resistance of the NdBa₂Cu₃O_7−δ layers began to fall off abruptly at T=92 K, and at T≈87 K it vanished completely. The critical current density in the NdBa₂Cu₃O_7−δ layers at T=76 K exceeded 10⁶ cm² A/cm². The dielectric constant of the (400 nm) SrTiO₃ layer sandwiched between the NdBa₂Cu₃O_7−δ epitaxial layers grew by roughly threefold as the temperature was lowered in the interval 300–4.2 K. When a bias voltage of ±2.5V was applied to the NdBa₂Cu₃O_7−δ electrodes, the relative dielectric constant of the (400 nm) SrTiO₃ intermediate layer fell from 1150 to 400 (T=32 K, f=100 kHz). The conductivity of the SrTiO₃ intermediate layer in the direction perpendicular to the substrate plane increased with temperature and the electric field strength. Fiz. Tverd. Tela (St. Petersburg) 41, 395–403 (March 1999)     

Thermopower in quasi-two-dimensional (BEDT-TTF) m X n organic conductors

Thermopower of (BEDT-TTF) _m X _n organic conductors has been studied using a dedicated measurement technique in the temperature range of 4.2 to 300 K. It turned out that some features of the thermopower in quasi-two-dimensional metals, namely the presence of a peak in the thermopower of α-(BEDT-TTF)₂MHg(SCN)₄ and a plateau in κ-(BEDT-TTF)₂Cu(NCS)₂ in the temperature interval between 10 and 50 K, are probably due to the phonon drag effect. Similar temperature dependences of the Seebeck coefficient can be satisfactorily interpreted in terms of a simple model taking into account the real experimental curve of the phonon heat capacity versus temperature, C ∝ T ², which is not described by the Debye formula. One feature distinguishing organic superconductors from magnetically ordered metals is a stronger temperature dependence of the characteristic electron-phonon scattering time τ _e-ph(T). Phonon drag effects also determine the behavior of the thermopower in the (BEDT-TTF)₃Cl₂·2H₂O organic conductor, which is characterized by a metal-insulator transition at T∼150 K. An analysis of measurements of the conductivity and thermopower vs. temperature taken together indicates that the transition in this compound has a complex nature: first (at T∼150 K) a metal-insulator transition occurs, which produces an energy gap in the band spectrum, then at a lower temperature (T∼20 K) a transition to a charge-density wave state takes place. Zh. éksp. Teor. Fiz. 113, 323–338 (January 1998)