Optical study of charge dynamics along thec-axis in high-T c superconductors

Comparing the optical spectra withE∥c for typical high-T _c superconducting cuprates, we discuss the charge dynamics along thec-axis. The plasma energy or the mass anisotropic factorm _* ^c //m _* ^b is one of the key parameters determining the spectrum forE∥c. In Bi₂Sr₂CaCu₂O₈ and La_2?x Sr _x CuO₄ with a largem _* ^c /m _* ^b , the plasma energy is smaller than the superconducting gap energy, and thus the supercurrent along thec-axis is a kind of Josephson current flowing through insulating layers such as the BiO layers. On the other hand, in YBa₂Cu₃O₇ with a smallm _* ^c /m _* ^a , it seems that the coherent supercurrent flows along thec-axis. Although the spectrum forE∥c strongly depends on samples, presumably due to the difference in the hole concentration, a clear anisotropy between theab- and thec-directions is observed in the characteristics energy scales such as the plasma energy as well as the reflectivity knee energy.     

Anisotropy in the resistive superconducting transition under magnetic fields in single crystal Pb2Sr2Ho0.5Ca0.5Cu3O8

Anisotropie properties of the single crystal Pb₂Sr₂Ho_0.5Ca_0.5Cu₃O₈ have been investigated by measuring the electrical resistivity in theab-plane _ab (H, ,T), which depends on the angle between theab-plane and the magnetic-field direction, in various constant fieldsH perpendicular to the current direction. All the angle-dependent values of _ab (H, ,T) at a constant temperature are scaled to be on one curve as a function of reduced field. The anisotropic parameter (m _c ^* /m _ab ^* )^1/2 is estimated as 12–13, which is larger than that of YBa₂Cu₃O₇ and much smaller than that of Bi₂Sr₂CaCu₂O₈. It has been concluded that the anisotropy does not always depend on the thickness of the blocking layer but seems to depend on the overlap of the electronic wave functions along thec-axis. Anisotropy in the pinning potential has also been discussed from the resistive tail in the temperature dependence of _ab (H,,T).     

Single-crystal growth and anisotropic electrical properties of (La1− xCa x )2CaCu2O6

Large single crystals of (La_1?x Ca _x )₂CaCu₂O₆ were synthesized by the travelling solvent floating zone (TSFZ) technique. The dimension of the grown boules was typically 4 mmφ×30 mm long with thec-axis perpendicular to the growth direction. After oxygen loading at 1080°C in 400 atm of O₂, they became superconductive with a sharp transition at around 50 K. The resistivity was metallic both along thec-axis and within theab-plane, with an anisotropy ratio ofγ ²≡(ρ _c/ρ _ab ) ～ 50 which is almost independent of temperature. Details of the crystal growth, heat treatment procedures, and the results of electrical and magnetic property measurements are presented.     

Magnetic torque of κ-(BEDT-TTF)2Cu(NCS)2 single crystals

The magnetic torque ofκ-(BEDT-TFF)₂Cu(NCS)₂ was measured as a function of field directionθ with respect to thea ^*-axis under constant magnetic fields,H, up to 8 kOe in the temperature range from 1.3 to 8 K. A sharp cusp, C₁, in the irreversible region was found atθ _c1 near theH∥bc-plane between 1.3 and 7 K. In addition, extra cusps, C₂ and C₃, were observed atθ _c2 andθ _c3, respectively, between 2.5 and 6 K. At each temperature, the perpendicular component ofH giving each cusp is kept constant as $H\cos \theta _{cn} = const \equiv H_{cp_n } (n = 1,2,3),$ i.e., cusps C₁, C₂, and C₃ are ruled by the characteristic field perpendicular to thebc-planeH _cp1,H _cp2, andH _cp3, respectively. These behaviors are almost the same as those we found in the oxide superconductor Bi₂Sr₂CaCu₂O₈. These results suggest that the cusps are intrinsic for irreversible vortex states of these layered superconductors.     

Effect of Sr and Sr-Ca substitution on the superconductivity of (Er0.76Ca0.24)Ba2(Cu2.76Co0.24)Oz

The structural and superconducting properties of (Er_1-xCa_x)Ba_2-ySr_y(Cu_2.76Co_0.24)O_z are investigated using X-ray diffraction, resistivity, ac susceptibility, and oxygen content measurements. Increasing the Sr content in (Er_0.76Ca_0.24)Ba_2-ySr_y(Cu_2.76Co_0.24)O_z lowers the oxygen content and decreases theT _c, which is attributed to the rearrangement of oxygen atoms in the basal plane. This suppression ofT _c at_{y =0.6} cannot be improved by appropriate hole doping with Ca in (Er_1-xCa_x)Ba_1.4Sr_0.6(Cu_2.76Co_0.24)O_z forx = 0.24-0.48.     

Doping dependence of the critical field Hc2 and the transition temperature in Zn doped YBa2(Cu1−xZnx)3O7−δ

We present data on the doping dependence of T_c in Zn doped YBa₂(Cu_1−xZn_x)₃O_6.94, for x up to 10.6%. We find re-entrant behaviour in the range between 9.0 and 10.6% Zn. Data on the magneto-resistance of a sample with extremely low T_c (9% Zn, zero field resistive onset near 3.5 K) show linear normal resistance in all fields down to 1.8 K in 17.5 T. A resistively defined critical field H^*, related to H_c2, shows divergent behaviour. The temperature dependence of H^* is well represented by expressions derived for the Bose condensation of pre-formed pairs.     

Transport properties of Nd1.85Ce0.15 δ single crystals: The narrow-band model

The anisotropy of the resistivity and thermoelectric power (TEP)S of Nd_1.85Ce_0.15CuO_4?δ single crystal (T _c =17 K) has been investigated. In the temperature rangeT _c <T<300 K the ratioρ _c/ρ_ab≈10⁴ and the dependencesρ _ab (T) andρ _c (T) change from quadratic to linear atT～200 K. The dependencesS _ab (T) andS _c (T) reach a maximum atT>T _c and then decrease almost linearly with increasing temperature, changing sign from positive to negative nearT～ 150 K. The features of the resistivity and TEP temperature dependences (the lawρ∝T ² changing toρ∝T, the change in the sign of S with temperature, and the low TEP anisotropy at largeρ anisotropy) have been interpreted in the framework of the narrow-band model.     

Temperature Dependence of the Intergranular Critical Current Density in Uniaxially Pressed Bi1.65Pb0.35Sr2Ca2Cu3O10+δ Samples

We have studied the normal and superconducting transport properties of Bi_1.65Pb_0.35Sr₂Ca₂Cu₃O_10+δ (Bi-2223) ceramic samples. Four samples, from the same batch, were prepared by the solid-state reaction method and pressed uniaxially at different compacting pressures, ranging from 90 to 250 MPa before the last heat treatment. From the temperature dependence of the electrical resistivity, combined with current conduction models for cuprates, we were able to separate contributions arising from both the grain misalignment and microstructural defects. The behavior of the critical current density as a function of temperature at zero applied magnetic field, J _c (T), was fitted to the relationship J _c (T)∝(1?T/T _c ) ⁿ , with n≈ 2 in all samples. We have also investigated the behavior of the product J _c ρ _sr , where ρ _sr is the specific resistance of the grain-boundary. The results were interpreted by considering the relation between these parameters and the grain-boundary angle, θ, with increasing the uniaxial compacting pressure. We have found that the above type of mechanical deformation improves the alignment of the grains. Consequently the samples exhibit an enhance in the intergranular properties, resulting in a decrease of the specific resistance of the grain-boundary and an increase in the critical current density.     

The possibility of increasingT c in oxide high-T c superconductor films: Interaction with elastic fields of defects

The influence onT _c of dilatation centers (DC) as elastic defects near the surface or in thin films of oxide high-T_c superconductors is considered. The distribution function ofT _c in this case was obtained. This is the Lorentzian with center shifted to largerT _c values. The half width of theT _c distribution βn (n is the concentration of DC) is larger than its center shift and corresponds to the increase ofT _c fluctuations near the surface. As a result, a continuum percolation behavior with increasing critical temperature percolation levelT ^(c) near the surface appears. The inequalityβn>T ^(c)>T _c ^initial is fulfilled. ForT>T ^(c) the quasi-2D Josephson media takes place where a finite superconductive domain withT _c ^local >T ^(c) exists. The influence of DC considerably increases for strong DC such as off-center impurity ions.     

Anisotropic uniaxial pressure effects in YBa2Cu3O7−δ

We present measurements of the uniaxial pressure dependence ofT _c of untwinned YBa₂Cu₃O_7?δ crystals with various oxygen stoichiometries. For all samples investigated,T _c decreases for pressure alonga, increases for pressure alongb, and, in oxygen deficient samples, increases strongly for pressure alongc. These results are compared to the behavior found in the La_2?x Sr _x CuO₄ and YBa₂Cu₄O₈ systems. Neither the model of pressure-induced charge transfer nor coupling to orthorhombic distortions can explain all the data. However, the presence of singularities in the electronic density of states close to the Fermi energy is a possible origin of the observed behavior. Our preliminary data on the pressure dependence of thec-axis and in-plane resistivities in twinned crystals are consistent with this view.     

Microwave surface resistance of epitaxial YBa2Cu3O7 thin films at 18.7 GHz measured by a dielectric resonator technique

We used a dielectric resonator technique for highly sensitive measurements of the temperature dependence of the microwave surface resistanceR _s of 1×1 cm² superconducting films at 18.7 GHz. It consists of a sapphire disc positioned on the film under investigation within a copper cavity which is acting as a radiation shield. In the TE_01δ oscillation mode the highly reproducible quality factor of about 10⁵ results in a sensitivity of ±50μΩ forR _s measurements. The temperature dependence ofR _s can be measured up to values as high as 1 Ω. We have investigated several YBa₂Cu₃O₇ thin films prepared by high oxygen pressure d.c. sputtering on LaAlO₃ and NdGaO₃. Our best films exhibit a pronounced nonlinear behavior of the d.c. resistivityρ(T) withρ(300K)/ρ(100K) values of about 3.7. Those films show, besides the initial fall-off just belowT _c , a further strong decrease ofR _s at low temperatures. This was observed both at 18.7 GHz and 87 GHz, as measured by a conventional cavity end plate replacement technique. ForT?T_c/2 these films exhibit an exp (?αT _c/T) dependence ofR _s withα-values around 0.4. These observations may be explained by a superconducting energy gap with 2Δ/kT _c≈0.8 for charge carriers localized in the CuO chains for YBa₂Cu₃O₇.     

Vortex motion in coupled DyBa2Cu3O7/(Y0.6Pr0.4)Ba2Cu3O7 heterostructures

Multilayer structures containing 24 Å thick DyBa₂Cu₃O₇ layers, separated by 96 Å of an (Y_0.6Pr_0.4)Ba₂Cu₃O₇ alloy, are studied to investigate the effect of coupling on vortex dynamics. With the magnetic field perpendicular to theab plane, and as a function of the number of superconducting layers in the structure, we find that the activation energy for flux motion increases, first linearly, and then saturates. This linear increase is taken as evidence that pancake vortices belonging to different DyBa₂Cu₃O₇ layers are stacked and have a coupled motion. Above a characteristic number of superconducting layers,N _c , shear of the vortex structures becomes important and the thermally activated process only displaces a stack ofN _c pancake vortices, meaning that the vortex lattice is turning three dimensional. In these structures we findN _c to be 2 to 3.     

Phase Transition of Josephson Vortices Under High Magnetic Fields up to 30 T in Heavily Overdoped YBa2Cu3O7−δ Single Crystals

We have measured the temperature and the magnetic-field dependences of the resistivity, ρ(T) and ρ(H), under magnetic fields parallel to the ab planes up to 30 T for fully oxidized YBa₂Cu₃O_7?δ single crystals. Above 11 T, ρ(T) showed a kink structure and successively a gradual transition toward the zero-resistivity. Below 9 T, the resistive transition was continuous. These results indicate the appearance of a slush regime.     

d-Hole localization and the suppression of superconductivity in YBa2(Cu1-xZnx)3O7−y

The temperature and Zn concentration dependence of the electrical resistivity, specific heat, magnetic susceptibility, and electron paramagnetic resonance (EPR) spectra of YBa₂(Cu_1?x Zn _x )₃O_7?y withy～0.1 has been measured forx≤0.16. In addition, the temperature and field dependence of the magnetization has been measured for 2<T<300K and 0<H<9.0T, along with the temperature and quasihydrostatic pressure dependence of the electrical resistivity for selected samples for 0<P<13 GPa. The substitution of Zn for Cu in YBa₂Cu₃O_7?y causes a rapid and nearly linear depression of the superconducting transition temperature,T _c , withT _c going to 0 K forx≥ 0.10. YBa₂(Cu_1?x Zn _x )₃O_7?y retains the YBa₂Cu₃O_7-y orthorhombic structure forx≤0.16 for both the superconducting and nonsuperconducting samples. Initially, the unit cell volume increases nearly linearly with Zn content; however, an abrupt change occurs in the vicinityx=0.8–0.10. Forx<0.10, the temperature dependence of the electrical resistivity,ρ(T), is metallic-like (dρ/dT>0) andρ increases gradually with increasing Zn content. However, forx≥ 0.10,ρ(T) becomes semiconductor-like, with a very rapid increase of the resistivity with increasingx. The electrical resistivity, magnetic susceptibility, EPR spectra, and specific heat all indicate that thed-holes associated with the Cu ions become localized in the nonsuperconducting phase,x>-0.10.     

Transport properties of oxygen-deficient YBa2Cu3O7−x thin films

We report on measurements of the in-plane resistivityρ and Hall coefficientR _H (B∥c) of various oxygen-deficient epitaxial films of YBa₂Cu₃O_7?x in the normal state. The superconducting transition temperaturesT _c of the samples vary from 14 to 90 K. Both the resistivity and the Hall coefficient exhibit a strong dependence on the oxygen content and the temperature. Asx increases,T _c decreases continuously, whileρ andR _H gradually increase in magnitude. Furthermore, also the characteristic linear dependences ofραT andR _Hα T ^?1 of the highly doped compounds changes to a nonlinear behavior for the samples withT _c lower than 60 K. The unusual doping and temperature dependence ofR _H will be compared to the predictions of our calculations, based on a two-dimensional tight-binding model using the relaxation-time approximation. The model considers also the next-nearest-neighbor hopping, which strongly influences the predicted Hall coefficient. Additionally, the cotangent of the Hall angle cot(Θ _H ) is discussed in the framework of the two-dimensional Luttinger liquid theory.     

Optical study of charge dynamics along thec-axis in high-Tc superconductors

Comparing the optical spectra withEc for typical high-T _c superconducting cuprates, we discuss the charge dynamics along thec-axis. The plasma energy or the mass anisotropic factorm _* ^c //m _* ^b is one of the key parameters determining the spectrum forEc. In Bi₂Sr₂CaCu₂O₈ and La_2–x Sr _x CuO₄ with a largem _* ^c /m _* ^b , the plasma energy is smaller than the superconducting gap energy, and thus the supercurrent along thec-axis is a kind of Josephson current flowing through insulating layers such as the BiO layers. On the other hand, in YBa₂Cu₃O₇ with a smallm _* ^c /m _* ^a , it seems that the coherent supercurrent flows along thec-axis. Although the spectrum forEc strongly depends on samples, presumably due to the difference in the hole concentration, a clear anisotropy between theab- and thec-directions is observed in the characteristics energy scales such as the plasma energy as well as the reflectivity knee energy.     

Enhancement of thermoelectric power in layered Bi2Sr2Co2−x Ir x O y single crystals

We report on the structural, electrical, and thermal transport properties of the single crystals of Bi₂Sr₂Co_2?x Ir _x O _y (0 ≤ x ≤ 0.2). Large-sized (centimeter-level) and good-quality single crystals were grown by a modified flux method. The substitution of Ir ions for Co ones makes the in-plane resistivity ρ _ab increase monotonically, whereas, the in-plane thermopower S _ab initially increases and then decreases as x is larger than 0.1. Therefore, among all samples, the in-plane power factor P _ab (=S _ab ² /ρ _ab) of Bi₂Sr₂Co_1.9Ir_0.1O _y would reach a maximum value of 116.4 μWm^?1K^?2 at 300 K and shows a significant improvement of about 20 % compared to that of the parent sample. Our results indicate that an appropriate doping of Ir ions at Co-sites may provide an effective way to enhance the thermoelectric performance of Bi₂Sr₂Co₂O _y system.     

Heat capacity and electrophysical properties of GdMeFe2O5(Me — Li, Na, K, Cs)-type ferrites

The heat capacity of GdMeFe₂O₅(Me — Li, Na, K, Cs) is investigated within the temperature range of 298.15–673 K by a calorimetric method. λ-points are revealed in the C _p ^○ (T) curves at 448 K and 598 K in GdLiFe₂O₅, at 473 K and 573 K in GdNaFe₂O₅, at 448 K and 598 K in GdKFe₂O₅, and at 448 in GdCsFe₂O₅ related to a phase transition of the second kind. The temperature dependencies are calculated for the thermodynamic functions C _p ^○ (T), H ^○(T)-H ^○(298.15), S ^○(T), and Φ**(T). The electrophysical characteristics of the synthesized ferrites are studied within the temperature range of 303–493 K: Effects similar to those found in the C _p ^○ (T) curves are also revealed in the log?(T) and logR (T) curves. The compositions are shown to exhibit a semiconductor-type electrical conductivity. At the T _cond points of the ferrites, the semiconductor-type conductivity changes the metal type one and both the capacities and permittivities vary sharply, and that may be related to a ferroelectric phase transition (the Curie and the Neel points). These effects provide a certain possibility to clarify the nature of the λ-points in the C _p ^○ (T) curves.     

Studies on (Hg0.7Mo0.3)Sr2(Sr1−xLax)Cu2Oz with a Wide Range of Doping State

The single-phased series of Sr-bearing Hg-1212 superconducting cuprate,(Hg_0.7Mo_0.3)Sr₂(Sr_1?x La _x )Cu₂O _z has been prepared. X-ray diffraction showed that, the obtained samples belong to the 1212-structure with tetragonal space group P4/mmm, similar to that of (Hg, Mo)Sr₂(Ca, Y)Cu₂O_z, and stabilized in a wide compositional range of 0.25≤x≤0.75. Refinements of the structure are carried out in which the oxygen atoms at the (Hg, Mo) layer is shifted from high-symmetry position (0.5, 0.5, 0) to (x, x, 0). Magnetization and electrical resistivity measurements show that the as-prepared samples exhibit evidence for superconductivity and their superconducting properties were improved after O₂ annealing with T ^onset _c as high as 92 K.     

Effect of Ca Substitution on the Superconductivity of La2.5Y0.5CaBa3(Cu0.88Fe0.12)7O z

The structural and superconducting properties of single-phase La_2.5?y Y_0.5Ca_1+y Ba₃ (Cu_0.88Fe_0.12)₇O _z (LYCaBCuFe) (y= 0.0?1.0) compounds with triple perovskite structure are investigated using X-ray diffraction, resistivity, a.c. susceptibility, and oxygen content measurements. Increasing Ca substitution for La resulted in a decrease in unit cell axes and volume. T _c ^R=0 shows a marginal increase from 31 K to 37 K for y = 0.0?0.21 and thereafter it decreases with increasing y leading to zero T _c ^R=0 at y ≥ 0.84. This shows that the suppression of T _c from 80 K to 31 K by Fe doping at x = 0.12 La_2.5Y_0.5CaBa₃(Cu_1?x Fe _x )₇O _z cannot be compensated by appropriate hole doping with Ca in LaYCaBCuFe.