A. Knizhnik

Common energy scale for magnetism and superconductivity in underdoped cuprates : A muon spin resonance investigation of (CaxLa1-x) (Ba1.75-xLa0.25+x)Cu3Oy

We characterize the spontaneous magnetic field, and determine the associated temperature T(g), in the superconducting state of (Ca(x)La(1-x)) (Ba(1.75-x)La(0.25+x)) Cu(3)O(y) using zero and longitudinal field muon spin resonance measurements for various values of x and y. Our major findings are (i) T(g) and T(c) are controlled by the same energy scale, (ii) the phase separation between hole poor and hole rich regions is a microscopic one, and (iii) spontaneous magnetic fields appear gradually with no moment size evolution.

Transport measurements in the 1-2-3 system CLBLCO in both the oxygen-underdoped and -overdoped regions

Abstract “CLBLCO” is a family of 1-2-3 superconductors, represented by the chemical formula (Ca x La 1− x )(Ba c − x La 2− c + x )Cu 3 O y . By annealing in oxygen at various pressures, up to 4300 bar, at a temperature of 300°C and above, we have prepared specimens whose oxygen content covers a wide range, from y =6.4 (a totally “underdoped” insulating phase) to a highly overdoped phase with y ≳7.354. In contrast to YBCO, the specimens remain tetragonal for all values of x , c , and y . Measurements of the superconducting transition temperature, the resistivity, and the thermoelectric power, over a range of y values, 6.84≤ y ≤7.30 are presented, for samples with calcium content from x =0.1 to 0.4, and with c =1.75. We also report some transport data for CLBLCO with nickel impurities. Some significant differences were found between the properties of the underdoped and overdoped regimes. In contrast to YBCO, the thermoelectric power characteristics behave like those of most other high- T c cuprates. We found that a proposed universal relation between the thermoelectric power and the ratio between T c and its maximum value T c,max is not truly universal; it holds in some cases, but fails in others. The temperature-dependence of the resistivity shows a departure from linearity in the overdoped region, which is perhaps the signature of the appearance of a normal-state gap. The temperature at which the non-linearity sets in appears to increase as the doping level falls.

Fully oxygenated RBa2Cu3−xCoxOy, (R=Y, Eu, Pr, and 0≤x≤1)— from high‐temperature superconductors to high‐resistivity nonmetals

Reported here are the results of measurements of the resistivity (ρ) up to 300 K and of the thermoelectric power (TEP) up to 400 K of ceramic samples of the title materials. We determined also their room‐temperature lattice parameters and oxygen content as functions of Co concentrations. The metal–nonmetal transition in YBa2Cu3−xCoxOy and in EuBa2Cu3−xCoxOy is marked by the onset of the deviation of the maximum absolute TEP from a value calculated from a simple narrow‐band formula. The results indicate that the effective valency of Pr in PrBa2Cu3−xCoxOy varies from ∼3.5 at x=0 to ∼3 for x=0.5. For x=0 this material is close to the metal–nonmetal transition. In the nonmetallic regime the electrical transport is by activated hopping. In certain ranges of Co content the results are consistent with two‐band hopping conductivity with two branches, a low‐temperature and a high‐temperature branch of variable‐range‐hopping (VRH) conductivity. In the VRH regime there is a remarkable correlation between the two res...

Determination of oxygen stoichiometry in a small mass of cuprate superconductor by iodometric microtitration

The authors have developed a method of determining oxygen stoichiometry (y) of small masses of cuprate oxides, based on iodometric microtitration. It comprises double microtitration. The first microtitration was carried out on a Cu2+ solution of accurately known concentration. A small mass ( approximately 500 mu g) was accurately determined by an absolute calibration of a microbalance to within +or- approximately 0.8 mu g. They obtained y=6.960+or-0.009 from ten microtitrations on small masses extracted from a large ceramic sample of YBa2Cu3Oy. This method can be used to determine oxygen stoichiometry in single crystals and in thin films of cuprate superconductors.

Interrelation of preparation conditions, morphology, chemical reactivity and homogeneity of ceramic YBCO

Abstract YBa2Cu3Oy ceramics having different morphologies: low porosity, composed of well shaped and closely connected large crystal grains, high porosity, composed of badly packed irregularly shaped small grains, and of intermediate microstructure,were investigated with respect to reactions that lead to the deviation of oxygen content from its equilibrium value and loss of homogeneity of oxygen ion distribution. These reactions, which occur during quenching performed after the equilibrium of YBCO with oxygen has been attained, are the oxygen absorption during quenching in liquid oxygen or in common liquid nitrogen (containing 2–3% dissolved oxygen) and the oxygen release during quenching in pure liquid argon. In these reactions and in the reaction with CO2 impurities during attaining equilibrium (which also leads to an inhomogeneous sample), the reactivity of the investigated ceramics varied from high (in high porosity ceramic) to almost inert (in low porosity ceramic), while even an almost inert ceramic attains equilibrium under 0.01–1 atm O2 and 450–920 °C during less than 35 h. Therefore this ceramic is most suitable for preparation of the samples having any oxygen content. The interrelations found make it easier to search for the proper conditions of ceramic sintering, equilibration with oxygen and quenching for preparation of both homogeneous and inhomogeneous samples having the desired morphology and properties.

Electronic transport under high electric fields in Sr2IrO4

We report measurements of conductivity under high electric fields of polycrystalline Sr2IrO4 at various temperatures, motivated by an earlier report on negative differential resistivity of a single crystal of this material. While our dc I‐V characteristics are strongly nonlinear, the pulsed characteristics show almost negligible nonlinearity up to fields of ∼500V∕cm. We report also on low-field electronic transport measurements (resistivity and thermopower) as function of temperature, which are in qualitative agreement with recently published results.

Dependencies of Tc on the composition of 1:2:3 superconductors (CaxLa1−x)(Bac−xLa2−c+x)Cu3Oy and their interpretation by the cluster component method

Abstract The temperature of superconductive transitions T c was measured for a variety of compositions ( x , c ) of the 1:2:3 tetragonal superconductors (Ca x La 1− x )(Ba c− x La 2−c+ x )Cu 3 O y (so called CLBLCO or CLBCO or Ca–La–Ba–Cu–O) both in oxygen underdoped and overdoped region. The maximal values of T c ( T c,max ) were calculated directly from these T c vs y graphs. We found that instead of the 1:2:3 superconductors without calcium where substitution of Ba for La effectively decreases T c,max , Ca rich compositions of CLBLCO do not change T c,max over the wide range of this substitution. This fact and also the previously found increasing of T c,max with the Ca-content ( x ) are explained by the cluster component method; we found that the compositions with unit cells containing La on both the Y and Ba-sites, are nonsuperconductive, while all compositions containing unit cells with Ca on the Y-site have the highest T c,max =81 K . All the experimentally found T c,max ( x , c ) may be described by the varying of the ratios of these unit cells in the sample. The role of Ca in stabilizing of T c,max at substitution of Ba for La, and also in the increasing of T c,max with increasing the Ca-content is because of the much higher ability of Ca to bind La (from the Ba site) than the ability of La on the Y site for the same binding. This prevents the formation of La(LaBa)Cu 3 O y unit cells, the presence of which in CLBLCO decreases T c,max .

Internal charge transfer in tetragonal superconductor (CaxLa1−x)(Ba1.75−xLa0.25+x) Cu3Oy

Abstract We report (Ca x La 1− x ) (Ba 1.75− x La 0.25+ x ) Cu 3 O y , that is a tetragonal superconductor family within the 1:2:3 system existing for 0≤ x ≤0.5. With constant Q (= 7.25, Q denoting the charge on the non copper cations) and constant y ( y ≅7.1) the average and mobile hole densities do not change with x . Nevertheless, there is a big increase in T c with x . Besides, T c is very sensitive to small changes in Q or in y . In addition, the resistivity of low x compositions increases by an order of magnitude and changes from metallic to semiconductorlike behaviour upon small decrease in y . We attribute this unusual behaviour to La that substitutes for Ba. It attracts more oxygens in the plane of the “chains” that induce the orthorhombic to tetragonal phase transition. These oxygens then promote additional internal electron transfer from the planes to the “chains”.

Variable range hopping in A2MnReO6 (A=Ca, Sr, Ba)

We report on transport measurements on polycrystalline samples of A2MnReO6 (A=Ca, Sr, Ba), CMRO, SMRO, and BMRO, in the Ohmic and non-Ohmic regimes. The resistivity follows, over wide ranges of temperatures, the relation derived for variable range hopping of carriers localized in a parabolic gap, ρ=ρo exp(To/T)1/2. The values of To are unusually high for the Efros Shklovskii Coulomb gap interpretation. The thermopower is large and positive for SMRO and BMRO and even larger and negative for CMRO. The magnitude of the thermopower and its temperature dependence indicate that it may be unrelated to the hopping conductivity. The theoretical relations for hopping in the intermediate electric field regime provide reasonable approximations for the measured nonlinear conductivity of most samples (and for one sample also for the high-field regime). However, localization lengths derived from the non-Ohmic regime are much larger than those estimated from To. Clues to settle these inconsistencies are suggested.

Equilibrium and non-equilibrium states of the YBCO?O2 system

The conditions (temperature, pressure and time) for attaining equilibrium with oxygen for different YBCO materials are reported. The equilibrium oxygen content was used in order to determine more exact values for the enthalpies and entropies of interaction of YBa2Cu3Oy with oxygen. Internal equilibrium in YBCO is proposed and is able to explain the effects of ageing and low temperature annealing on Tc. Homogeneity and transitions to superconductivity of ceramics having equilibrium and non-equilibrium oxygen contents are discussed.