Transport properties of a and c axis oriented (Y1?xCax)Ba2Cu3Oy thin films

Anisotropy and Hall effect measurements have been performed in calcium-doped, i.e., overdoped YBa₂Cu₃O_y ((Y_1−xCa_x)Ba₂Cu₃O_y) thin films witha andc axis orientations. In highly overdoped films (x=0.4), the anisotropy of the normal resistivity decreases and a drastic change in Hall conductivity in the mixed state is observed. The change in Hall conductivity in the overdoped region is consistent with recent experimental results for La_2−xSr_xCuO₄ films and seems to be common in highT _c superconductors.     

Superconductivity, Seebeck coefficient, and band structure transformation in Y1−x CaxBa2Cu3−x CoxOy (x=0–0.3)

The temperature dependences of the resistivity and of the Seebeck coefficient S is studied in three series of Y_1−x Ca_xBa₂Cu_3−x Co_xO_y samples (x=0–0.3) differing in oxygen content. It was found that the critical temperature decreases for y≈7.0, and S(T=300 K) increases with doping, whereas oxygen deficiency results in a nonmonotonic variation of these quantities with increasing x. The band structure parameters have been determined from an analysis of the S(T) relations using a phenomenological theory of electron transport. It was found that an increase in x results in a gradual increase in band asymmetry, which is caused by calcium-induced creation of additional states in the band responsible for conduction in the normal phase. An analysis has shown that high impurity concentrations in oxygen-deficient Y_1−x Ca_xBa₂Cu_3−x Co_xO_y samples bring about an additional ordering of the structure, which may be caused by formation of a cobalt superlattice. It has also been shown that, in the case of Ca and Co codoping, the dependence of critical temperature on effective conduction-band width coincides with the universal correlation relation observed in the YBa₂Cu₃O_y system with single substitutions in various lattice sites. Fiz. Tverd. Tela (St. Petersburg) 41, 1363–1371 (August 1999)     

Determination of the parameters of the normal state in doped yttrium high-temperature superconductors from thermopower coefficients in terms of different models of electron transport

The temperature dependences of the thermopower coefficient for two systems of doped yttrium high-temperature superconductors Y_{1 − x} Pr _x Ba₂Cu₃O _y (x = 0.1−0.6) and Y_{1 − x} Ca _x Ba_1.5La_0.5Cu₃O _y (x = 0.05–0.30) have been analyzed using three models of electron transport, i.e., the Xin two-band model, the two-band model with an additional term linear in temperature, and the narrow-band model. For parameters of different models with a similar physical meaning, identical tendencies have been revealed in their variation with an increase in the dopant content. The mechanisms of influence of the dopants under investigation on the parameters of the band spectrum and the systems of charge carriers have been examined. It has been demonstrated that the revealed features of the dynamics of the Fermi level in the Y_{1 − x} Ca _x Ba_1.5La_0.5Cu₃O _y system are explained by the formation of an additional peak in the density of states upon doping with calcium, and the energy position of this peak is estimated.     

The behavior of thermopower in the YBa2−x LaxCu3Oy system. Correlation between the band parameters in normal state and critical temperature

The thermopower in the YBa_2−x La_xCu₃O_y system (x=0–0.5) has been studied. An analysis made in terms of the narrow-band model revealed trends in the variation of the main band parameters with increasing lanthanum concentration. The dependence of the conduction band width on lanthanum concentration was found to pass through a weak minimum at x=0.05–0.1, which correlates in position with the maximum in the concentration dependences of the critical temperature and degree of sample orthorhombicity. The results obtained and their comparison with Fe, Co→Cu substitution data permit a conclusion that the main factor affecting the properties of the YBa₂Cu₃O_y system under nonisovalent substitution of various cations is the increasing disorder in the chain-oxygen subsystem caused by increasing impurity content. Fiz. Tverd. Tela (St. Petersburg) 40, 17–22 (January 1998)     

Structural studies and T c dependence in La2−x Dy x Ca y Ba2Cu4+y O z type mixed oxide superconductors

A new series of mixed oxide superconductors with the stoichiometric composition La_2−x Dy _x Ca _y Ba₂Cu_4+y O _z (x=0.0 − 0.5, y=2x) has been studied for structural and superconductiong properties. Our earlier studies on La_2−x (Y/Er) _x Ca _y Ba₂Cu_4+y O _z series, show a strong dependence of T _c on hole concentration (p _sh). In the present work, the results of the analysis of the neutron diffraction measurements at room temprerature on x=0.3 and 0.5 samples are reported. It is interesting to know that Ca substitutes for both La and Ba site with concomitant displacement of La onto Ba site. Superconductivity studies show that maximum T _c is obtained for x=0.5, y=1.0 sample (T _c ∼ 75 K), for La_1.5Dy_0.5Ca₁Ba₂Cu₅O _z (La-2125).     

Analysis of the possible reasons for the suppression of superconductivity in the Y1−x PrxBa2Cu3Oy system on the basis of thermoelectric power data

The temperature dependence of the thermoelectric power in samples of Y_1−x Pr_xBa₂Cu₃O_y (x=0−0.6) is investigated. The principal parameters of the system of charge carriers are determined on the basis of an analysis of this dependence within a narrow-conduction-band model, and their variation with increasing praseodymium content is analyzed. It is found that an increase in x leads to a very weak increase in the electron filling and considerable broadening of the conduction band accompanied by strong localization of the charge carriers. It is concluded on the basis of a comparative analysis of the results obtained and data for the YBa₂Cu₃O_y system in the case of aliovalent substitutions in the barium and copper sites that the valence of praseodymium in Y_1−x Pr_xBa₂Cu₃O_y is very close to 3+. It is shown that there is a universal correlation between the effective width of the conduction band and the critical temperature following various cation substitutions in the Y-Ba-Cu-O system. It is concluded that the main reason for suppression of the superconducting properties of Y_1−x Pr_xBa₂Cu₃O_y is the strong modification of the band spectrum caused by praseodymium. Fiz. Tverd. Tela (St. Petersburg) 39, 1520–1525 (September 1997)     

Dynamics and distribution of doped holes in the CuO2 plane of slightly doped antiferromagnetic YBa2(Cu1 ? zLiz)3O6 + x (x < 0.1) studied by Cu(1) NQR

Incidence of doped holes in the CuO₂ plane on the AF state was studied by Cu(1) nuclear quadrupole resonance (NQR) in slightly doped YBa₂(Cu_{1 − z} Li _z )₃O_{6 + x} compounds. Inhomogeneous distribution of doped holes in the plane was detected in the low temperature measurements of transverse (1/T ₂) and longitudinal (1/T ₁) relaxation rates. We establish that at lower T the holes motion slows down and we estimate that the holes localize finally in restricted regions (∼3 lattice constants) in the Coulomb potential of the Li+ ions. Also we compared the hole behavior in slightly doped YBa₂(Cu_{1 − z} Li _z )₃O_{6 + x} samples with that in slightly doped Y_{1 − y} Ca _y Ba₂Cu₃O₆. A stronger trapping potential of the in-plane Li⁺ impurities was concluded as compared to slightly doped Y_{1 − y} Ca _y Ba₂Cu₃O₆ compound with out-of-plane Ca²⁺ impurities.     

Relation between highT c superconductivity and structure: A study of Ba3+x Cu6O14+δ and YBa2Cu3O7−δ systems

Superconductivity is found in tetragonal La_3−x Ba_3+x Cu₆O_14+δ and La, Ba)_6−x Sr _x Cu₆O_14+δ even though they do not possess Cu-O chains or the K₂NiF₄ structure. Resistivity measurements confirm the occurrence of a transformation from chain-superconductivity to sheet-superconductivity in YBa₂Cu₃O_7−δ as δ is varied in the range 0.0–0.5. Contribution No. 481 from the Solid State and Structural Chemistry Unit     

Specific features of praseodymium-doping induced changes in the critical temperature and energy spectrum parameters of YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript><Emphasis Type="Italic">y</Emphasis></Subscript> in the presence of calcium ions in the lattice

The results of the experimental study of the specific features of changes in the temperature and concentration dependences of the thermopower coefficient and the critical temperature in the Y_{0.85 − x} Ca_0.15Pr _x Ba₂Cu₃O _y system as the praseodymium content increases are presented. The results obtained have been analyzed based on the narrow band model, the energy spectrum and charge carrier system parameters have been determined, and their behavior with an increase in the doping level has been analyzed. It has been found that both superconducting properties and parameters of the normal state of Y_{0.85 − x} Ca_0.15Pr _x Ba₂Cu₃O _y vary differently in various doping ranges. Based on a comparison of the results obtained with the available data for the case of single doping of YBa₂Cu₃O _y with praseodymium, conclusions have been drawn regarding the mechanism of the energy spectrum modification in the studied compound. The Fermi-level pinning effect has been revealed in the region of a local peak of the density-of-states function, and the energy position of this peak has been determined. It has been shown that the consideration of the Fermi level dynamics caused by the specific features of the structure and transformation of the Y_{0.85 − x} Ca_0.15Pr _x Ba₂Cu₃O _y energy spectrum upon doping makes it possible to explain the observed dependence of the critical temperature on the praseodymium content.     

Mechanism responsible for the modification of the band spectrum and superconducting properties in the Tl2Ba2Ca1 ? x Y x Cu2 ? y Co y O z system

The modification of the temperature dependences of the thermopower coefficient for the Tl₂Ba₂Ca_{1 − x} Y _x Cu_{2 − y} Co _y O _z system is investigated as a function of the dopant content. The model of an asymmetric narrow band is shown to be applicable to high-temperature superconductors in the thallium system. The parameters of the band spectrum and charge carrier system for samples of a close-to-optimum composition are determined by analyzing the temperature dependences of the thermopower coefficient within the narrow band model. The character and mechanism of their variation in the course of the transition to an underdoped regime are analyzed together with the dynamics of changes in the superconducting properties. The critical temperature T _c is found to correlate with the effective conduction-band width W _D . It was shown that the dependence T _c (W _D ) for Tl₂Ba₂Ca_{1 − x} Y _x Cu_{2 − y} Co _y O _z has a close-to-universal character, but the suppression of T _c upon the broadening of the band occurs faster than in the case of the yttrium system. Original Russian Text ? O.A. Martynova, V.é. Gasumyants, 2007, published in Fizika Tverdogo Tela, 2007, Vol. 49, No. 9, pp. 1537–1542.     

Mössbauer study of tin-doped 1-2-3 and 2-1-4 highT c superconductors

Samples of YBa₂(Cu_0.85Sn_0.15)₃O_7–y (1-2-3), La₂(Cu_0.95Sn_0.05)O4_4–x, and La_1.85Ba_0.15(Cu_0.95Sn_0.05)O_4–x (2-1-4), with different oxygen stoichiometry, have been studied by Mössbauer spectroscopy. These measurements reveal the existence of two inequivalent sites for Sn in the (1-2-3) compounds. The (1-2-3) spectra display two quadrupole doublets which we associate with Sn in Cu(1) and Cu(2) sites, respectively. La compounds show a single quadrupole doublet.     

The possibility of introducing additional states in the conduction band of YBa2Cu3Oy by Ca doping

Measurements of the temperature dependences of the resistivity and Seebeck coefficient S, as well as of the Hall coefficient R _H at T=300 K are reported for two series of ceramic Y_1−x Ca_xBa₂Cu₃O_y samples with oxygen contents in the initial sample close to stoichiometric and then lowered by annealing. It has been found that an increase in calcium content in the first series results in a drop of the critical temperature T _c , a weak variation of S (300 K), and an increase of R _H (300 K), whereas in the second series T _c increases, S (300 K) decreases, and R _H (300 K) remains practically constant. The S (T) relations acquire additional features with increasing doping level, which are not typical of the YBa₂Cu₃O_y system for other types of substitution. An analysis of the data obtained made in terms of the phenomenological band model permits a conclusion that calcium is capable of introducing additional states in the conduction band. Based on this assumption, we have succeeded in providing a qualitative explanation for the unusual features in the behavior of the transport coefficients and critical temperature in Y_1−x Ca_xBa₂Cu₃O_y. Fiz. Tverd. Tela (St. Petersburg) 40, 2145–2152 (December 1998)     

Correlations betweenT c andn s/m * (carrier density/ effective mass) in high-T c and organic superconductors

We update our muon spin relaxation studies of the magnetic field penetration depth which show the correlations betweenT _c and the relaxation rate σ αn _s/m ^* (carrier density/effective mass) of hole-doped high-T _c cuprate superconductors (La₂, Sr₃)CuO₄, YBa₂Cu₃O₇ (Y_1−xPr_x)Ba₂Cu₃O₇, and other double and triple layer systems. These studies are extended to the organic superconductor (BEDT-TTF)₂Cu(NCS)₂.     

The Nernst-Ettingshausen coefficient in the normal phase of doped HTSCs of the YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>y</Subscript> system

The temperature dependence of the Nernst-Ettingshausen coefficient Q(T) in the normal phase of doped HTSCs of the yttrium system was studied. The main features characterizing the behavior of this coefficient were revealed, and the character and mechanism of the effect that various nonisovalent substituents exert on the Q(T) dependence were analyzed. It is shown that the narrow-band model permits one not only to describe all the specific features observed in the Q(T) curves but also to perform a simultaneous quantitative analysis of the temperature dependences of four kinetic coefficients (the electrical resistivity and the Seebeck, Hall, and Nernst-Ettingshausen coefficients) with the use of a common set of model parameters characterizing the band structure and carrier system in the normal phase of an HTSC. This approach was employed to determine the carrier mobilities and the asymmetry of the dispersion curve in the systems studied (YBa₂Cu₃O_y, y = 6.37–6.91; YBa₂Cu_3?xCo_xO_y, x = 0–0.3; Y_1?xCa_xBa₂Cu₃O_y, x = 0–0.25; Y_1?xCa_xBa_2?xLa_xCu₃O_y, x = 0–0.5) and to analyze the effect of the substitutions involved on the variation of these parameters.     

Structural properties and Mössbauer spectroscopy of the high TcYBa2Cu3−xFexOy compounds

New results on the Mossbauer spectroscopy of the YBa₂(Cu_1−xFe_x)₃O_y system are reported and the availible reliable experimental data are briefly summarized.     

Effect of Pr on electron band spectrum of YBa2Cu3Oy: study by thermopower data analysis

The temperature dependences of the thermopower, S(T) of Y_1−xPr_xBa₂Cu₃O_y (0≤x≤0.6) have been measured and analyzed within a phenomenological model of electron transport in the case of a narrow conductive band. The band spectrum parameters have been estimated by a quantitative comparison of the experimental and calculated S(T) curves. It was revealed that an increase of the Pr content results in remarkable broadening of the conductive band and a strong localization of the states. This can be considered as the main reason for T_c depression. The effect of Pr on the band filling is very slight that makes it possible to conclude a valence of Pr to be close to 3+. The Pr influence on superconducting properties of YBa₂Cu₃O_y cannot be described in the terms of hole-filling effect.     

X-ray diffraction study of spontaneous strain in Fe-pnictide superconductor, NdFeAsO0.89F0.11

The lattice parameters of a Nd 1111 Fe-pnictide superconductor, NdFeAsO_0.89F_0.11, were accurately measured using X-ray diffraction between 20 K and 280 K. A very small change in the lattice parameter could be detected in the superconducting phase. This change can be attributed to a spontaneous strain generated in the superconducting phase by the coupling between a superconducting order parameter and the strain. The present results are compared with the thermal expansion coefficients of Ba (Fe_1−x Co _x )₂As₂, in addition to the previous lattice parameter measurements of YBa₂Cu₃O_6.5, MgB₂, La_1.85Sr_0.15CuO₄, and Ba_0.6K_0.4BiO₃.     

The superconducting properties of YBa2(Cu1−xMx)3O7 for M=Zn and Ni

Transverse and zero-field μSR measurements were made on YBa₂(Cu_1−xNi_x)₃O_7−y withx=0.1 and 0.2, and YBa₂(Cu_1−x Zn _x )₃O_7−y withx=0.03, 0.06, 0.1, and 0.16, wherey≈0.1. Since doping may lead to magnetic ordering this was searched for with both zero and transverse field μSR, but no evidence was found over the temperature range studied: 10–100 K. However, depolarization rates as functions of temperature were obtained, and the low temperature values of these are σ=3.2 μs⁻¹.1.6μs⁻¹, and 1 μs⁻¹ forx=0.01, and 0.2 Ni, respectively, and σ=0.8 μs⁻¹, 0.75 μs⁻¹, 0.65 μs⁻¹, and 0.4 μs⁻¹ forx=0.03, 0.06, 0.1, and 0.16 Zn, respectively. Estimates for the effect of decreasing electron concentration for Zn are made, but these alone do not account for the drop in σ. Estimates for the effect of scattering on λ and hence σ are made. The reduction in σ for Ni dopant is in surprisingly good agreement with these estimates. For Zn the order of magnitude is correct, but the relative lack of further change in σ after the effect of the first 0.03 addition seems to imply a saturation of the effect of scattering.     

Effect of substrate temperature on electrical and magnetic properties of epitaxial La1−x Pb x MnO3 films

Epitaxial La_1−x Pb _x MnO₃ (LPMO) thin films, grown on (100) SrTiO₃ substrates by laser ablation technique at different temperatures between 600 and 850°C, have been characterized for electrical and magnetic properties. The temperature dependence of resistivity showed that the metal-insulator transition temperature (T _MI) decreases with increasing substrate temperature, which has been attributed to decrease in Pb content in the filsm. The YBa₂Cu₃O _x /La_1−x MnO₃ heterostructures, exhibiting both superconductivity and ferromagnetism, have been fabricated.     

μ− SR studies of oxide-superconductorsSR studies of oxide-superconductors

The negative muon spin rotation method (μ⁻ SR) has been applied to studies of electronic states at oxygen sites of oxide superconductors YBa₂Cu₃O₇, Nd_2−x Ce _x CuO_4−δ (x=0.15, oxygen reduced), LiTi₂O₄ and related oxide-insulators La₂CuO_4−δ, CuO, Cu₂O. The paramagnetic shifts of μ⁻ trapped at oxygen nuclei in these polycrystalline powder samples have been measured at 300 K. All the measured shifts are positive. In copper-oxides the paramagnetic shifts are of the order 10⁻³, while in LiTi₂O₄ is very small (8.4±3.34×10⁻⁵). In YBa₂Cu₃O₇, a fast μ⁻ spin relaxation timeT ₂ ^* (∼ 200 ns) has been observed; the reason for this is unknown and further studies are now in progress.