Superconductivity of YBa2?xNdxCu3Oy Solid Solution

The solubility of Nd at the Ba sites and the superconductivity of YBa_2–x Nd _x Cu₃O _y were investigated by X-ray powder diffraction and measurements of the electrical resistance and ac susceptibility. The single Re123 phase was obtained for x0.30. The onset transition temperature is insensitive to the Nd content x in the region of x0.40. All are higher than 95 K. The zero resistance transition temperatures , however, exhibits two-step variation with the increase of x. For x0.25, are all above 92 K. The highest of 94 K was obtained for x=0.25. For x0.3 drops sharply to about 84 K. Finally falls to 30 K and is below 10 K for x=0.5. The two-step variation of T _c might be an indication of the existence of two trap levels for holes.     

Room-temperature oxidation of La2?xSrxCuO4

A series of oxidized La_2–x Sr _x CuO_4+y compounds has been prepared by treating the starting materials with a solution of Br₂ in NaOH at room temperature. The structural modifications due to the oxidation of the materials have been studied by X-ray diffraction. Some of the observed changes are: (i) a large increase in the long parameter of the unit cell for samples with Sr content and (ii) a slight decrease in a along the whole range ofx. Interesting features have been observed regarding the critical temperatures of these materials: transition temperatures are higher for those containing lower Sr amounts (some of them were even nonsuperconducting before the oxidation treatment) in contrast to materials with x0.15, whoseT _c's do not change very much. The influence of both oxygen and strontium contents on the structural modifications and the superconducting properties of the oxidized materials will be discussed.     

Magnetic Field Influence on the Low Temperature Heat Capacity and Heat Release of (TMTSF)2PF6

We have measured under applied magnetic field the heat capacity c _p and heat release of the quasi-1D conductor (TMTSF)₂PF₆ in its spin density wave (SDW) ground state. The low-temperature heat capacity (T < 0.5 K) is dominated by a Schottky anomaly contribution. A corresponding term was also found for the heat release. In this T-range both properties are strongly sensitive to moderate magnetic fields (i.e., below 0.5 T): their amplitudes have a sharp maximum for H _c=0.2 T. We show that the corresponding density of two-level states N _S undergoes a sharp maximum at H _c, which rules out an extrinsic origin for the two-level states. Instead we suppose an intrinsic origin, such as defects of commensurability of the SDW, as the (slow) excitations related to the domain walls. This effect is found to be independent on the field orientations H a and H a.     

Effect of Short-Time Magnetic Relaxation on Hysteresis Curves in Single-Crystal Bi2Sr2CaCu2Ox

We have studied the role of short relaxation time on magnetic hysteresis in a Bi₂Sr₂CaCu₂O _x single crystal. Effects were examined by comparing isothermal magnetization curves obtained over a range of temperatures as a function of the applied magnetic field, H, for magnetic field ramp rates of 3 and 30 Oe/s. At low temperature, a second magnetization peak, H _p, was produced by differences in magnetic relaxation occurring over a short period during the initial stage of relaxation. The data suggested an absence of pinning crossover for H H _p.     

The Effect of Zn-Substitution on the Anomalous Resistance Behavior of La1−x Ca x Mn1−y Zn y O3 Compounds

We have studied the effect of Zn substitution on the temperature dependence of the resistivity and the Curie temperature T _p of La_1–x Ca _x Mn_1–y Zn _y O₃ system (with x=0, y=0, and x=0.33, 0.00y0.15) during their pass to the paramagnetic conducting phase. All the specimens show a clear metal-insulator transition passing through the Curie temperature T _p . With increasing Zn concentration, T _p decreases for La/Ca substitution and also for Zn/Mn substitution with fixed 33 at.% of Ca. T _p is found to be slightly decreases for low concentrations of Zn (up to y=0.075) followed by a sharp decrease up to y=0.15. A possible explanation is given. Moreover, the activation energy E _a is calculated in the three different temperature regions characterized in the (-T) curve. Decreasing the temperature below T _p can increase the parallel alignment of the Mn spins and, thus decreases the resistivity of the samples. E _a increases with Zn content (up to y=0.075) followed by a sharp decrease. A strong correlation in the behavior of the normal state and residual resistivity as well as T _p and E _a with Zn content is reported.     

Stability of the low-temperature tetragonal phase in La2?xBaxCuO4 and related compounds

In La_2–x Ba _x CuO₄ (LBCO), the structural transition to a low-temperature tetragonal phase below 60 K and suppression of superconductivity are observed when the carrier density isp 1 /8 per copper. The replacement by divalent ions smaller than Ba²⁺ suppresses the static deformation of the lattice. We have found that the variationsT _d2 and superconducting transition temperatureT _c are quantitatively characterized by the averaged ionic radius at the La site or lattice parameters. This aspect of substitution could be regarded as the effect of chemical pressure, since similar variations have been reported on applying hydrostatic pressure. In La_2–x–y Nd _y (Ba, Sr) _x CuO₄,T _d2 increases with increasingy in a wide range ofp whileT _c is suppressed only at p l /8. The structural transition atT _d2 here should be ascribed mainly to the crystallochemical origin.     

How the Surface Resistance Rs of Patterned High-Tc Superconducting Thin Film Is Affected by the Film''s Edge

This paper defines an effective microwave surface resistance for the nonuniform distribution of microwave surface resistance R _s in the strip of a microstrip. It is proved that is equivalent to the expression of R _s used in experiments, and that the is dominated by the edge part, i.e., the area of width ²/2t from the strip edge, where is the magnetic penetration depth and t is the film thickness. Under the assumption that where is the component of rf magnetic field along the film thickness and n is an integer, the ratio of the contributions of the edge part and the rest of the strip to is calculated by using an approximate analytical expression of the surface current density distribution J _s in the strip and calculated by the London equation. The effect of film's edge on R _s was studied using a microstrip resonator. It is found that the perfectness of the edge could affect the magnitude of the power dependence of R _s significantly, which agreed with our analysis.     

How “Musical” Are High-Tc Superconductors and What Can Empirical Rules Tell About Their Origin?

It is shown that parameters such as optimal T _c for cuprate superconductors or details of their doping curves can be organized on phenomenological rules. Accordingly, T _c in a range between a kink and the optimum scale linearly with the number of effective holes h _e, according to T _e = h _e T _c ^e, with T _c ^e = 600 K. Effective holes are composed of the difference between holes in the Cu—O bonds in the CuO₂ planes, h _p, and holes in the Cu—O bonds with the c axis or apical O, h _e, according to h _e = h _p– h_c = fh _p. The deleterious effect of the apical O manifests itself in three levels, depending on the basic modes of its coordination of the CuO₂ planes in zero, one, or two sheets (according to factor f = 1, 2/3, ¹/₂). The values of h _p at T _c optimum tend to rational fractions, ranging from 1/6 to 1/3, and are determined by lattice pressure. This musical or harmonic T _c matrix, originating from two structurally determined factors, groups optimal T _c into families. Knight shift data, establishing h _p, bear out the general assumptions. Some flexibility in the range within families is observed. This flexibility indicates the operation of more complex influences from structural detail, such as the varying distance of Cu to the apical O. The existence of ranges within optimal T _c families indicate a somewhat tunable rather than a strict musical T _c-level scheme with measured intervals. The details of the doping curves are similarly organized. These phenomenological rules suggest the operation of bond ordering effects. Arguments for the actual nature of the bond orderings are presented in terms of local pairs of doped bonds in trijugate positions. These quantitative concepts can be expanded to other characteristic features in the doping curves of cuprates and other high-T _c materials such as C or B containing systems, providing a universal frame for explaining high-T _c superconductivity in bond ordering terms.     

Theab-plane optical conductivity of high-Tc superconductors

There is anisotropy in theab-plane optical properties of the high-temperature superconductors, both in the normal state and in the superconducting state. In both states, two components appear in the optical conductivity: a free carrier part and a midinfrared component. BelowT _c , the free carriers form the superconducting condensate. In YBa₂Cu₃O_7–, the anisotropy of the penetration depth shows that the chains contribute strongly to this superfluid. In Bi₂Sr₂CaCu₂O₈, where chains are absent, there is stillab plane anisotropy. BelowT _c a finite absorption parallelb remains at frequencies as small as 20 meV. This anisotropy could be due to anisotropy either of the superconducting gap or the midinfrared component.     

Superconductivity in the cuprates as a consequence of antiferromagnetism and a large hole density of states

We briefly review a theory for the cuprates that has been recently proposed based on the movement and interaction of holes in antiferromagnetic (AF) backgrounds. A robust peak in the hole density of states (DOS) is crucial to produce a large critical temperature once a source of hole attraction is identified. The predictions of this scenario are compared with experiments. The stability of the calculations after modifying some of the original assumptions is addressed. We find that if the dispersion is changed from an antiferromagnetic band at half-filling to a tight-binding cosk _x+cosk _y narrow band at n=0.87, the main conclusions of the approach remain basically the same i.e., superconductivity appears in the -channel andT _c is enhanced by a large DOS. The main features distinguishing these ideas from more standard theories based on antiferromagnetic correlations are here discussed.     

Structural instabilities and superconductivity in La-214 compounds

In La_2–x Ba _x CuO₄ (LBCO) the transition to a low-temperature tetragonal phase and the suppression of superconductivity occur at the carrier concentration p 1/8 per copper. We will discuss the roles of various material parameters that control this instability. An unusual lattice softening has been found by ultrasonic measurement on La_2–x Sr _x CuO₄ (LSCO). This softening is present only in an in-plane shearing mode and is ascribed to the growth of structural fluctuations in the normal state.These phenomena are closely related because both the structural change in LBCO and the applied strain in LSCO lift the degeneracy of in-plane oxygen sites. They clarify the importance of strong coupling between the normal-state electronic system and the lattice by a Peierls-type mechanism.     

Optical Properties, Electronic Structure and Magnetism of α′-NaxV2O5

The optical properties of sodium-deficient -Na _x V₂O₅ (0.85 x 1.00) single crystals are analyzed using ellipsometry, and infrared reflectivity techniques. In sodium deficient samples, the optical absorption peak associated to the fundamental electronic gap develops in the middle of the pure -NaV₂O₅ gap at 0.44 eV, and the material remains insulating up to the maximal achieved hole concentration of about 15%. Nonmetallic behavior under hole doping provoked reinterpretation of the -NaV₂O₅ optical spectra. We argue that the absorption peak at about 0.9 eV corresponds to the photoionization energy of a large polaron.     

Superconductivity in a Two-Component Model with Local Electron Pairs

Superconductivity in the two-component model of coexisting local electron pairs (hard-core charged bosons) and itinerant fermions coupled via charge exchange mechanism is discussed. The cases of isotropic s-wave and anisotropic pairing of extended s-wave and symmetries are analyzed for a 2D square lattice within the BCS-mean-field approximation (MFA) and the Kosterlitz–Thouless theory. The phase diagrams and superconducting characteristics of this induced pairing model as a function of the position of the local pair (LP) level and the total carrier concentration are reviewed. The model exhibits crossovers between the BCS-like behavior and that of LPs. In addition, the Uemura plots are obtained for extended s and pairing symmetries. Finally, we analyze the pairing fluctuation effects (in 3D) within a generalized T-matrix approach. Some of our results are discussed in connection with a two-component scenario of preformed pairs and unpaired electrons for high-temperature superconductors.     

Normal state and superconductivity of ErBa2(Cu1?xCrx)O7?δ(x=0–0.1)

The effects of Cr in ErBa₂(Cu_1–x Cr _x )O_7– (x=0–0.1) superconductor have been investigated. The critical temperature, which was determined by DC electrical resistance measurements, showed no suppression of the onset temperature (T _{c onset}) within the substitution range. The transition width (T _c ) broadened as the Cr content is increased. The normal state changes from the metal-like to semimetal/semiconductor-like for x0.03. Micrographs from the scanning electron microscope, X-ray diffraction pattern, and energy dispersive X-ray analysis results are used to describe the superconducting properties of these materials. The orthorhombic structure was preserved throughout the substitution range. Some possible roles of Cr in the system are discussed.     

Superconductivity and Magnetism in RuSr2GdCu2O8

The ruthenocuprate RuSr₂GdCu₂O₈ orders magnetically at 135 K and then becomes superconducting at 45 K. These transitions have been observed by several groups, but the intrinsically complex nature of the compound, as well as the unexpected coexistence of magnetism with high-temperature superconductivity, makes it uncertain what the magnetic, electronic, and structural character of its ground state is. We find that density functional-based evaluations give a magnetic and structural character that is consistent with the latest data: the RuO₆ octahedra rotate (by 7° around the c^ axis) causing a doubling of the cell, and an antiferromagnetic structure that has this same cell doubling is favored over a ferromagnetic alignment of Ru spins. The minority Ru d _xy states are partially occupied, leaving a metallic RuO₂ layer that dopes the CuO₂ bilayer and may lead to canting of the Ru spins.     

Influence of long-range and strong correlation effects on the electron-phonon coupling in high-Tc oxides

Calculations of the Eliashberg function ²F and the corresponding transport function _tr ² F for high-T _c oxides are presented using a screened ionic model (rigidly shifted ionic potentials screened by charge carriers in the CuO₂ planes within the RPA) for the electron-phonon coupling. It is shown that this model yields a large difference between the transport and the superconducting electron-phonon interaction due to imperfect screening and contributions beyond nearest-neighbor interactions. Using these results, the electron and the lattice heat conductivities are calculated both in the normal and the superconducting state and compared with experiment. Finally, effects due to a strong on-site electron-electron repulsion are included in leading order in an 1/N expansion, whereN is the number of spin degrees of freedom. In particular, it is shown for the infiniteU, one-band Hubbard model that correlations tend to suppress _tr ² F strongly and ² F somewhat.The author thanks M. Kulic for discussions and cooperation on the topic discussed in Section 4.     

Scaling Behavior and Estimation of Activation Energy of Polycrystalline RuSr2GdCu2O8 Superconductor from AC Susceptibility Measurements

In this study, the measured curves of AC susceptibility (ACS) components, (T) and (T), of polycrystalline RuSr₂GdCu₂O₈ (Ru-1212) superconductor were scaled onto a single curve using the peak temperature of its imaginary part (T _p) as the scaling parameter for various AC field amplitudes from 0.5 to 24 G. The dependence of the AC magnetic field amplitude on T _p is scaled as: H _ac (1–T _p/T _c)^2.25. Similarly, the current density J _c, extracted from the AC field amplitude is also scaled as: J _c (1 – T _p/T _c)^2.25. The dependencies of T _p on frequency and AC field amplitude are also investigated and the time parameter t ₀ of the order of 10^–8 s is estimated from the dependence of T _p on frequency. The dependencies of activation energy on temperature, T, and the field amplitude, H _ac, are obtained from the Arrhenius-like semilog plot of frequency () and T _p. Such dependencies on temperature and field amplitude can be described by a scaling law of the form: U(H _ac,T) = U ₀[1 – T/T _p]H _ac ^–0.17.     

Mean polarizabilities and second and third virial coefficients of the gases C2H4, C2H6, and SF6

Mean polarizabilities ₀(₀, T) as well as second and third virial coefficients B(T) and C(T) of the equation of state of the gases C₂H₄, C₂H₆, and SF₆ have been determined from refractive index measurements with a Michelson interferometer for wavelength ₀=632.99 nm in the overall ranges 250 KT 340 K and 0p3 MPa of temperature T and pressure p. Some negative C(T) values at low temperatures have been obtained. The C(T) data could be fitted satisfactorily to the simple three-parameter function C(T)= a(T–T ₀) exp[ –b(T–T ₀)], with the maximum near the critical temperature T _c. A possible correlation between C(T) and the vapor pressure p _v(T) is discussed.     

Microwave properties of highly oriented YBa2Cu3O7-δ superconducting thin films

We have performed millimeter-wave frequency (94 GHz) measurements on high-quality YBa₂Cu₃O₇- superconducting films on yttrium-stabilized (100) ZrO₂ and MgO substrates. The 0.2m thin films fabricated by magnetron sputteringin situ with the YBa₂Cu₃O₇- powders as target exhibit superconducting transition temperatures up to 88 K. The critical current density of 6×10⁵ A/cm² at 77 K and the X-ray diffraction spectrum as well as scanning electron microscope photographs indicate these thin films are fullyc-axis oriented, extremely high in density, and universally homogeneous. Millimeter-wave surface resistances have been measured on a hemisphere open resonator in the temperature range of 20 K toT _c and beyond. The surface resistance at 94 GHz and 77 K for these films is found to be about 30 m, nearly 1/4 that for copper, and a drop of two orders in the surface resistance within 4 K is observed, which indicates that these films are good materials for applications in the millimeter-wave range, especially for fabricating microwave devices. We observed such low surface resistance in these thin films due to the near absence of grain and phase boundaries coupled with a high degree of crystalline orientation.