Current-induced voltage oscillations in superconducting Y<Subscript>1</Subscript>Ba<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7-δ</Subscript>

We investigated influence of bidirectional square wave current with long periods and dc current on the evolution of the voltage-time (V - t) curves in superconducting polycrystalline bulk Y₁Ba₂Cu₃O_7-δ (YBCO) material at the temperatures near the critical temperature. In a well-defined range of amplitudes and periods of driving current, and temperatures, novel type of non-linear dynamic responses was observed by means of the V-t curves. It was seen that such a non-linear response to bidirectional square wave current sometimes reflects itself as regular sinusoidal-like voltage oscillations. The sinusoidal-like and non-sinusoidal oscillations were discussed mainly in terms of the dynamic competition between pinning and depinning and significant relaxation effects which appear in this competing process. The density fluctuations associated with the current induced self-magnetic flux (SMF) lines and semi-elastic coupling of SMF lines with the pinning centers were also considered as possible physical mechanisms in the interpretation of the experimental results.     

Superconductivity in Sr<Subscript>0.75</Subscript>Y<Subscript>0.75</Subscript>Ba<Subscript>1.5</Subscript>Cu<Subscript>3</Subscript> O<Subscript>7−<Emphasis Type="Italic">d</Emphasis></Subscript> compounds

The effect of simultaneous Sr substitution at the Ba and Y sites has been studied in the Sr_0.75Y_0.75Ba_1.5Cu₃O_7–dsystem. Attempts to replace 25% Y and 25% Ba have been successful and superconductivity was observed above 78 K for Sr_0.75Y_0.75Ba_1.5Cu₃O_7–d compound with high oxygen content, i.e., O_6.76, and having orthorhombic crystal symmetry. The compound was treated in argon gas at 800°C to reduce the oxygen content and to induce some structural changes. The Sr_0.75Y_0.75Ba_1.5Cu₃O_6.1 compound thus obtained has tetragonal symmetry and low oxygen content, O_6.1. It has also shown superconductivity at 28 K. The paper presents a careful comparison of the structural and electrical properties and infrared absorbance spectra of the two compounds with the same metallic composition, Sr_0.75Y_0.75Ba_1.5Cu₃, but with different oxygen content and crystal symmetry. The study clearly establishes the occurrence of superconductivity in tetragonal Sr-substituted (both at Y and Ba sites in) YBCO.     

Temperature dependence of the critical current of YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7−δ</Subscript> films

The temperature dependence of the critical current of YBa₂Cu₃O_7?δ films is studied experimentally. The performed analysis allows separating two components of the critical current owing to pinning of vortices both on defects in the volume of the superconductor and on oxygen vacancies in the CuO₂ planes. The established temperature dependences of components make it possible to correctly describe the behavior of the total critical current in the studied temperature range from 4.2 K to the irreversibility temperature.     

Normal state conduction in Gd(Ba<Subscript>2–x</Subscript>Pr<Subscript>x</Subscript>)Cu<Subscript>3</Subscript>O<Subscript><Emphasis Type="Bold">7+δ</Emphasis></Subscript>

The normal state resistivity of single phase polycrystalline Gd(Ba_2–xPr_x)Cu₃O₇₊ samples with 0.0x0.50 have been investigated. There is a distinct metal-insulator transition at and a superconductor-insulator transition at with the increase of x. The two-dimensional variable range hopping is dominant in the normal state resistivity of the samples. The localization length, hopping range, and hopping energy of carriers show that Pr doping strongly localizes the carriers in normal state, and finally causes the suppression of superconductivity.     

Subsolidus phase relation and crystal structure in the Pr<Subscript>1+x-y</Subscript>Ba<Subscript>2-x-z</Subscript>Ca<Subscript>y+z</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7±δ</Subscript> system

The Pr_1+xBa_2-xCu₃O_7± solid solution was investigated by means of X-ray powder diffraction combined with Rietveld analysis. A Pr123 single phase could be synthesized under Pr-rich conditions by sintering at 950 °C in air. The solubility range of Pr_1+xBa_2-xCu₃O_7± solid solution is 0.08x0.80. The structure of Pr_1+xBa_2-xCu₃O_7± for 0.08x<0.30 is orthorhombic. The structure transforms into tetragonal for 0.30x0.80. To form the Pr123 single phase, the Ba sites in the Pr123 structure must have partial Pr ions, and the least amount is x=0.08. Ba ions cannot occupy the sites of Pr ions. In the Pr123 structure, Ca ions can replace Pr ions; the highest value is x=0.4 in the PrBa_2-xCa_xCu₃O_7± system under our experimental conditions. However, Ca ions cannot replace Ba ions. The ionic radius plays a more important role than the chemical properties in the substitution between Pr, Ba and Ca ions in the Pr123 structure. PACS 61.50.Ah; 64.70.Kb; 61.72.Ff     

Effect of hydrostatic pressure on the superconductivity in fluorinated Hg<Subscript>0.8</Subscript>Ba<Subscript>2</Subscript>Ca<Subscript>2</Subscript>Cu<Subscript>3.2</Subscript>O<Subscript>8+δ</Subscript>

The effect of hydrostatic pressure on the superconducting transition temperature was measured for the Hg-1223 phase of a fluorinated mercury cuprate high-temperature superconductor with T_c(optim)=38 K. The value of the T_c derivative with respect to pressure was found to be rather high (11.0 K/GPa); at P=1.5 GPa, T_c=153.5 K. The results obtained are discussed in connection with works on the synthesis of such samples.     

Peculiarities in galvanomagnetic properties of granular YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7−δ</Subscript> high-temperature superconductors in very weak magnetic fields

The character of the evolution of a system of weak links in granular high-temperature superconductors under the action of an external magnetic field H _ext has been studied by measuring the current-voltage characteristics E(j)_{Hext = const}E{(j)_{{H_{ext}} = const}} of YBa₂Cu₃O_{7 − δ} (δ ≈ 0.05) ceramic samples. The measurements have been performed at T = 77.3 K in a range of very weak magnetic fields 0 < H _ext ≲ 0.5H _c2J, where H _c2J is the upper critical field of the Josephson weak links. The results have been used to construct the field dependences of the magnetoresistance Δρ(H _ext) of the superconducting ceramics. It has been established that the parameters of the power equation E = A(j − j _cJ)^ν and the magnetoresistance Δρ are nonmonotonic functions of the external magnetic field. The presence of extrema in the curves A(H _ext), j _cJ(H _ext), ν(H _ext), and Δρ(H _ext) indicates that different systems of weak links between grain boundaries, which are capable of forming extended Josephson contacts, undergo sequential transitions to a resistive state with an increase in H _ext.     

Surface of underdoped YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7-</Subscript><Subscript>δ</Subscript> as revealed by STM/STS

We performed scanning tunneling microscopy and spectroscopy on untwinned crystals of underdoped YBa₂Cu₃O_{7- δ} at δ = 0.4. A comprehensive statistical analysis of our topographic data indicates a doping dependent cleaving behavior of this material. We find in particular that at δ = 0.4 the material primarily cleaves in multiples of one unit cell along the c-axis with a high corrugation of the topmost layer. Our data suggest that the low temperature cleaving mainly results in a disruption of the CuO chain layers involving a redistribution of the layer atoms onto the two cleaving planes. In a few instances, fractional step heights (in terms of the c-axis lattice constant) are observed as well. Scanning tunneling spectroscopy reveals that such fractional steps connect surfaces which differ significantly in their tunneling conductance.     

High-Temperature Heat Capacity of Zn<Subscript>2</Subscript>V<Subscript>2</Subscript>O<Subscript>7</Subscript>–Cu<Subscript>2</Subscript>V<Subscript>2</Subscript>O<Subscript>7</Subscript> Solid Solutions

Differential scanning calorimetry has been used to study the influence of temperature on the heat capacity of synthesized vanadates Zn₂V₂O₇, (Cu_0.56Zn_1.44)V₂O₇, and (Cu_1.0Zn_1.0)V₂O₇. It is found that dependences C_p = f(T) have extremes. The thermodynamic properties of Zn₂V₂O₇ have been determined.     

Current dependent fluctuations in a Bi<Subscript>2</Subscript>Sr<Subscript>2</Subscript>CuO<Subscript>6+δ</Subscript> thin film

The current dependence of the excess conductivity is measured up to ≃3T_c for a Bi₂Sr₂CuO_6+δ thin film, as a function of doping. It is found to be anomalously sensitive to the transport current and to behave as a universal function of T/T_c in the whole doping range. We discuss these results in the perspective of a granular superconductor with a gapless-like behavior.     

Detection of YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7−x</Subscript> magnetization nonlinearity in a pseudogap region

The temperature dependences of magnetization higher harmonics were studied experimentally in single-domain YBa₂Cu₃O_7?x samples above T _c . YBa₂Cu₃O_7?x magnetization nonlinearity was found in different samples up to temperatures T = 103–112 K, which is much higher than the temperature of transition into the superconducting state of the given compound. The observed specific feature of YBa₂Cu₃O_7?x magnetization is associated with the occurrence of pseudogap state in this compound.     

Hysteresis of the magnetoresistance of granular HTSC YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7 − δ</Subscript> in weak fields

Hysteresis of the magnetoresistance of ceramic YBa₂Cu₃O_～6.95 HTSC samples is studied at T = 77.3 K in an external magnetic field H _ext changing in 0 → H _max → 0 cycles, where H _max is the maximum magnitude of H _ext. Information is obtained about the dependences of the critical fields of Josephson weak links H _c2J , the lower critical fields of superconducting grains H _c1A , and the critical fields H _BG-VG of the Bragg glass-vortex glass phase transition in the vortex matter on transport current I, magnetic field, and the mutual orientation of I and H _ext. It is found that the magnetoresistance δρ⁺/ρ_{273 K} measured with increasing H _ext is significantly higher than Δρ^?/ρ_{273 K} and that H _c2J ⁺ < H _c2J ^? , H _c1A ⁺ < H _c1A ^? , and H _BG-VG ⁺ < H _BG-VG ^? .     

Penetration of a magnetic field into the YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7−δ</Subscript> high-temperature superconductor: Magnetoresistance in weak magnetic fields

The penetration of a magnetic field into superconducting grains and weak links of YBa₂Cu₃O_7?δ ceramic high-temperature superconductors is investigated using measurements of the transverse and longitudinal magnetoresistances at T=77.3 K and 0≤H≤～500 Oe as a function of the transport current in the range ～0.01≤I/I _c ≤～0.99. The effects associated with the complete penetration of Josephson vortices into weak links of the high-temperature superconductor in magnetic fields H_c2J, the onset of penetration of Abrikosov vortices into superconducting grains in magnetic fields H_c1A, and the first-order transition from the Bragg glass phase to the vortex glass phase in fields H_BG-VG are revealed and interpreted. The I-H phase diagrams YBa₂Cu₃O_7?δ high-temperature superconductors are constructed for I ⊥ H and I ‖ H.     

Features of the Terahertz Photoconductivity in YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7–δ</Subscript> near the Superconducting Transition Temperature

Positive terahertz photoconductivity has been observed in underdoped high-temperature superconductors YBa₂Cu₃O_7–δ at temperatures somewhat higher than the midpoint of the superconducting transition. The amplitude of the effect is almost independent of the temperature and the power of the incident radiation if the latter exceeds a certain threshold value. The mechanisms responsible for the appearance of the effect are discussed.     

Contribution of superconducting grains and grain boundaries to the magnetoresistance of ceramic YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7 − δ</Subscript> high-temperature superconductors in weak magnetic fields

The current-voltage characteristics of granular YBa₂Cu₃O_6.95 high-temperature superconductor samples have been measured at a temperature of 77.3 K in external transverse magnetic fields H _ext with a strength of up to H _ext ≈ 500 Oe for low transport current densities (0.1 A/cm² ≤ j ≤ 0.6 A/cm²). The current-voltage characteristics obtained have been used to construct dependences of the magnetoresistance ρ on the quantities j (ρ(j) _Hext=const) and H _ext(ρ(H _ext) _{j = const}). It has been revealed that the current and field dependences of the magnetoresistance exhibit anomalies at H _ext ≥ H _c1g , where H _c1g is the lower critical field of superconducting grains. A comparative analysis of the dependences ρ(j)H _{ext = const} and ρ(H _ext) _{j = const} has made it possible to develop concepts regarding the influence of the processes of redistribution of the magnetic field between grain boundaries and superconducting grains on the transport and galvanomagnetic properties of granular high-temperature superconductors. It has been established that the field dependences of the magnetoresistance exhibit specific features associated with the beginning of penetration of Josephson vortices into grain boundaries in the magnetic field H _c1J and with the breaking of a continuous chain of Josephson junctions in the magnetic field H _c2J .     

Cluster spin glass and superparamagnetism in RuSr<Subscript>2</Subscript>Eu<Subscript>1.5</Subscript>Ce<Subscript>0.5</Subscript>Cu<Subscript>2</Subscript>O<Subscript>10-</Subscript><Subscript>δ</Subscript>

We investigate in detail the dc magnetization and nonlinear ac susceptibility behavior of the superconducting ferromagnet RuSr₂Eu_1.5Ce_0.5Cu₂O_{10- δ} (Ru1222) to develop a comprehensive understanding of the spin glass and superparamagnetism in this material. The structural properties of the system result in the formation of magnetic (ferromagnetic) clusters of different sizes, shapes and properties. The magnetic clustering of the system leads to observation of various features in dc magnetization and ac susceptibility consistent with superparamagnetism and cluster spin glass states, which can coexist or stand alone, depending on the temperature range considered. Experimental results of magnetic measurements in combination with their analysis have enabled us to explain and distinguish these phenomena, as well as to propose a temperature dependent scenario of the system behavior.     

Nanostructural inhomogeneity of EuBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>6 + δ</Subscript> superconductors

Samples of high-temperature superconducting oxide EuBa₂Cu₃O_{6 + δ} (Eu-123) with total cationic composition Eu: Ba: Cu = 1: 2: 3 are investigated by means of local X-ray microanalysis and high-resolution transmission electron microscopy. The cationic nonstoichiometry of Eu-123 oxide is revealed. The particles of the studied samples are inhomogeneous in structure on the nanoscale, with two types of inhomogeneities: one with typical sizes of one to several nanometers, and one with typical sizes of 10 to 20 nm, respectively.     

Meissner effect in the underdoped YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>6+<Emphasis Type="Italic">δ</Emphasis></Subscript> HTSC phase

The dependence of the superconducting (Meissner) phase volume V _m on the YBa₂Cu₃O_6+δ doping level was studied. It was found that V _m monotonically decreases as the doping level is lowered and vanishes at the same value of δ ～ 0.3 as T_c does. It was concluded that the T_c decrease and the increase in the pseudogap formation temperature T* as the doping level is lowered are caused by a decrease in the average size of superconducting clusters. This conclusion suggests an extraordinary superconductivity mechanism in HTSC.     

Magnetoresistance of granular YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7 − δ</Subscript> HTSC in weak magnetic field: Angular dependence

The magnetoresistance of ceramic YBa₂Cu₃O_～6.5 HTSC samples is studied as a function of the mutual orientation of the current I and external magnetic field H _ext at T = 77.3 K in magnetic fields of up to ～500 Oe. It is found that, if the demagnetization factor D is taken into account, the effective critical field of complete penetration of Josephson vortices into weak links H _c2J ^eff does not depend on the mutual orientation of I and H _ext. The lower critical field H _c1A ^eff associated with the beginning of penetration of Abrikosov vortices into superconducting grains increases substantially with the angle between I and H _ext. The strongest variation with the mutual orientation of I and H _ext is exhibited by the critical field of the Bragg glass-vortex glass first-order phase transition H _BG-VG ^eff and by the magnetoresistance jump at this phase transition.     

Anomalous temperature dependence of magnetic relaxation in YBa<Subscript>2</Subscript>Cu<Subscript>2</Subscript>O<Subscript>7 − δ</Subscript> oxygen-deficient single crystals

The dependence of the magnetization relaxation rate S = ?d lnM/dlnt on temperature T is measured in YBa₂Cu₃O_{7 ? δ} samples with various oxygen concentrations. It is found that the S(T) curve changes qualitatively when oxygen deficit δ exceeds the threshold value δ_th = 0.37. For δ < δ_th (T _c > 60 K, where T _c is the superconducting transition temperature), function S(T) has the well-known peak at T/T _c = 0.4. For δ > δ_th (at T _c < 51 K), this peak transforms into a plateau and a new sharp peak appears at T/T _c = 0.1. The threshold value δ_th of the oxygen deficit corresponds to the transition of the sample from the disordered state into the ordered state of oxygen vacancies. We consider the change in the shape of the S(T) curve as a macroscopic manifestation of this transition.