Suppression of the superconducting transition temperature Tc around x∼0.115 in La2-xSrxCuO4 (original) (raw)
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Suppression of superconducting transition temperature in orthorhombic La2-xCaxCuO4
Physica C: Superconductivity, 1993
The calcium solid-solubility limit in La2_xCaxCuO4 has been extended from x~0.10 for air-synthesized material to x=0.2 using synthesis at an oxygen pressure of 600 arm and 1110°C. The tetragonal-to-orthorhombic transition occurs at higher temperatures, or larger doping levels, than for the isostructural Sr-substituted material. The superconducting transition temperature increases with doping to 34 K at x= 0.15 and is suppressed for x> 0.15, similar to the Sr-substituted material. Comparison of structural and superconducting properties for Ca-and Sr-substituted materials indicated that: (1) both orthorhombic and tetragonal structures are superconducting; (2) at a fixed doping level, the optimum superconducting properties (the highest To) are found for a perfectly fiat and square CuO2 plane; (3) T= is suppressed by the distortions of the CuO2 plane or by overdoping resulting in a maximum of Tc found at x= 0.15 under ambient conditions.
Physica C: Superconductivity, 1998
The insertion of an excess of oxygen within the structure of La2−xCaxCuO4 (x≤0.12) by means of room temperature chemical oxidation modifies the physical properties and the crystal structure of these cuprates. The superconducting features of the starting La2−xCaxCuO4 samples improve considerably upon oxidation. The oxidized Ca-doped materials with x≤0.08 show an almost constant Tc of ∼38 K, close to that corresponding to the optimum hole-doping in La2−xMxCuO4; however, the oxidized samples with higher Ca contents present slightly lower Tcs. This decrease of Tc is connected with the ability of these compounds to incorporate extra oxygen, which decreases as the Ca-doping increases and is controlled by a structural limit. The behavior of the La2−xCaxCuO4 materials under the oxidation process and the changes induced by the interstitial oxygen are strongly conditioned by the smaller size of Ca2+ with respect to La3+. Indeed, this peculiarity clearly differentiates the oxidized La2−xCaxCuO4+y series from the Sr2+ and the Ba2+ substituted systems, where the size of the dopant is larger than that of La3+. In the present work the analogies and the differences concerning the insertion of oxygen and the modifications induced in La2−xMxCuO4+y (M=Ca, Sr, Ba) are presented and discussed.
Defect structure and superconducting properties ofLa1.8SrxCa1.2−xCu2O6−δ
Physical Review B, 1993
We have studied the relationship between structural defects and superconductivity in La& 8Sr Ca& 2 Cu&06 z (O~x~0.8). The samples were prepared by synthesis under different oxygen pressures, P(02) = 50, 250, and 400 atm. Six of the seven samples were found to be superconducting with 22~T,~58 K. The structural properties were determined by neutron powder diffraction. The inter-Cu02-plane spacing, dc"c", increases as Ca on the M(1) site between these planes is replaced by the larger Sr and La ions. The metal-site ordering is inAuenced by the oxygen pressure during synthesis. We have used the inter-Cu02-plane spacing, neutron-diffraction measurements of the scattering from the metal sites, and chemical constraints to determine the occupancies of La, Sr, and Ca at the M(1) site. For the same overall composition, higher oxygen pressure leads to a larger fraction of La on the M(1) site. Oxygen occupancy of the vacant O(3) site in the M(1) plane increases sharply when dc"c"exceeds 0 3.5 A. The superconducting transition temperature T, decreases systematically as the occupancy of O(3) increases for samples that would otherwise be expected to be superconducting. (T, = 108 K) and Bi2CaSr2Cu20& (T, =92 K), the building blocks are separated by T10 and BiO double layers. ' The first superconducting "twolayered" oxide with these building blocks and without separating planes or chains, with composition La, 6Sro 4CaCuz06 & and with T, =60 K, was discovered in 1990. A transition temperature of 60 K is considerably lower than T, =83-108 K, which had already been found in the other two-layered cuprates. This motivated numerous studies of the di6'erent metal and oxygen compositions in this cuprate in a search of T, higher than 60 K. The regions of the metal-composition diagram in which these cuprates were successfully synthesized are shown in Fig. 4e Cũ 2a M(1) La, Sr, Ca 4e M(2) La, Sr, Ca Q 8g O(l) 4e O(2) ', 'i 2b O(3) dCu-Cu FIG. 1. Crystal structure of La2~Sr Ca&+~, Cu&06 &. The Wyckoff site designation (Ref. 2) is given. The O(3) site (dashed circles) is normally vacant. The inter-CuOz-plane spacing d«« is also shown.
Physica C: Superconductivity, 2002
Two samples of non-stoichiometric La 2 CuO 4 were synthesized, one with La=Cu < 2, and the other with 10% Sn substituting Cu. They were investigated by X-ray diffraction, M€ o ossbauer spectroscopy, and microwave-absorption techniques. The microwave-absorption data indicated that they were both superconducting, with the transition temperatures T c of 40.5 and 41.5 K, the one doped with Sn possessing the higher T c . The M€ o ossbauer spectra revealed that there exist two kinds of Sn(IV) atoms disordered with Cu. Their isomer shift, d ¼ À0:244(4) mm/s, is in agreement with Sn(IV) coordinated by oxygen. One site was characterized by a single M€ o ossbauer line, being associated with a weakly distorted environment, wherein the Sn, coordinated more symmetrically, is surrounded by four Cu 2þ ions. On the other hand, the other site, characterized by a M€ o ossbauer doublet exhibited a quadrupole splitting D ¼ 1:07(2) mm/s, being associated with a highly distorted coordination, explained to be due to Sn occupying two adjacent cationic sites. To our knowledge, such a substitution for copper ions not resulting in a decrease of T c has not been reported previously. Ó
La2−xSrxCuO4−δ superconducting samples prepared by the wet-chemical method
Physica B: Condensed Matter, 2009
In this work, we report on the physical properties of good-quality polycrystalline superconducting samples of La 2Àx Sr x Cu 1Ày Zn y O 4Àd (y ¼ 0, 0.02) prepared by a wet-chemical method, focusing on the temperature dependence of the critical current. Using the wet-chemical method, we were able to produce samples with improved homogeneity compared to the solid-state method. A complete set of samples with several carrier concentrations, ranging from the underdoped (strontium concentration xE0.05) to the highly overdoped (xE0.25) region, were prepared and investigated. The X-ray diffraction analysis, zero-field cooling magnetization and electrical resistivity measurements were reported on earlier. The structural parameters of the prepared samples seem to be slightly modified by the preparation method and their critical temperatures were lower than reported in the literature. The temperature dependence of the critical current was explained by a theoretical model which took the granular structure of the samples into account.
Synthesis and bulk properties of oxychloride superconductor Ca 2-x Na x CuO 2 Cl 2
Journal of Physics: Conference Series, 2008
A series of polycrystalline samples and submillimeter size single crystals of a cuprate oxychloride Ca 2-x Na x CuO 2 Cl 2 (Na-CCOC) with values of Na content ranging from underdoped to optimally doped regions were synthesized at pressure of 30-55 kbar and temperature of 1250-1700 ºC. A systematic variation of the transition temperature T c with a maximum value of 29 K for x≈0.20 has been found as a function of Na content. In order to check the role of the apical oxygen for high-temperature superconductivity, we performed muon-spin rotation and magnetization studies of the in-plane magnetic penetration depth λ ab for Ca 2-x Na x CuO 2 Cl 2 samples with x≈0.11, 0.12, 0.15, 0.18, and 0.19. The absolute value of the in-plane magnetic penetration depth at T=0 was found to increase with decreasing doping from λ ab (0)=316(19) nm for the x≈0.19 sample to λ ab (0)=430(26) nm for the x≈0.11 one. Based on a comparison of the present Na-CCOC data with the data of La 2-x Sr x CuO 4 cuprate superconductors, it is concluded that replacing of apical oxygen by chlorine decreases the coupling between the superconducting CuO 2 planes, leading to an enhancement of the two-dimensional properties of Na-CCOC. The torque studies implies that the anisotropy coefficient γ=84 of Ca 1.82 Na 0.18 CuO 2 Cl 2 single crystals is much more enhanced compared to the structurally related La 1.82 Sr 0.18 CuO 4 where for same doping γ is much lower, i.e. γ≈11.
Effect of Cu addition on the superconducting behavior of La2−xSrxCuOy
Solid State Communications, 1995
The superconducting behavior of stoichiometric samples of La2_xSrxCuOy, prepared with an accurate control of cationic composition for X = 0.15, 0.16, 0.17, 0.18, 0.19 and 0.20, was studied by ac susceptibility and magnetization measurements. Only the samples with X = 0.15 and 0.16 showed a sharp transition at Tco-37 K. For higher values of X the superconducting transition became broader with a tail beginning at the same Tco. Samples prepared starting from the same batch of ceramic powders for X > 0.17, but with addition of small excess of CuO, showed the formation of large superconducting grains. These grains have the same Tco= 37 K as the La2_xSrxCuOy stoichiometric samples, and significantly sharper transitions. Our results are discussed in terms of the miscibility gap which has been proposed to exist for X > 0.16 by previous authors, and the much faster kinetics of evolution towards equilibrium caused by the existence of a Cu excess.
Superconductivity and phase separation in La2CuO4+x single crystals
Physica C: Superconductivity, 1994
We report a phase separation study on high-quality single crystals of La2CuO4+x for two concentrations x= 0.03 and x= 0.04. The excess oxygen was loaded into the crystals at high oxygen pressure p= 3 kbar and temperature T= 650-700°C. The X-ray diffraction study of these crystals shows the presence of a phase separation process for the x= 0.04 sample over the temperature range 230< T< 320 K and the absence of such a decomposition for the x=0.03 sample within the same temperature region. The superconducting transition is observed for both crystals, though the detected Tc~ 12 K value for the x=0.03 sample is much lower than usually reported in the La2CuO4+x system ( ~ 30 K), which is additional evidence that here superconductivity is not connected with the new phase formation. Possible reasons for such a behavior and phase diagram of La2CuO4+x are discussed.