Influence of the doping level on the charge distribution among the inequivalent CuO2 layers in Tl2Ba2Ca2Cu3O10−δ : a NMR study

and NMR measurements in the normal and superconducting states of Tl₂Ba₂Ca₂Cu₃O_10−δ with different δ are reported. In the overdoped Tl2223 sample with T_c=117 K (T_c^opt=123 K) and δ₁<δ^opt different temperature dependencies of the Knight shift are revealed for inequivalent CuO₂ layers. For the inner CuO₂ layer with the square oxygen coordination of Cu the decrease of with temperature is more gradual. In going towards the underdoped Tl2223 with T_c=104 K and δ₂>δ^opt the changes of with temperature are found to be the same for both types of copper layers. The quadrupole coupling constants for copper and oxygen from different CuO₂ layers were obtained. From the variations with doping of the valence contribution to the electric field gradient at copper sites, we estimate both the hole numbers at Cu and oxygen sites and the real concentration of mobile hole carriers n_h in each of inequivalent CuO₂ layers. In the overdoped Tl2223 sample the charge density in the inner layer differs from the one in the outer plane (with five-fold oxygen coordination for Cu). Our results show that the inhomogeneity of the charge distribution disappears in the underdoped regime. The results are compared with calculations of the charge distribution among the CuO₂ planes in multilayered cuprates reported by Haines and Tallon [E.M. Haines, J.L. Tallon, Phys. Rev. B 45 (1992) 3127].     

Effects of excess oxygen content on the hole-carrying CuO2-layers in Tl2(Ba1−xSrx)2Ca2Cu3Oy superconducting oxides

The effects of oxygen doping on the hole-carrying CuO₂-layers in Tl₂(Ba_1−xSr_x)₂Ca₂Cu₃O_y were studied by combined chemical and valence analysis, T_c measurements and neutron diffraction. The highest T_c is characterized by an optimal excess oxygen content, Δy, dichotomizing the under- and over-doped regions for each Sr concentration. While the average Tl valence is close to 3.0 and independent of Δy, the average Cu valence shows a linear dependence with Δy. An increase of the flatness of the CuO₂ plane, characterized by the O(2)-Cu(2)-O(2) bond angle of ∼176°, was observed at the optimal Δy.     

NMR in highly correlated superconductors

Results of our systematic NMR study in highT _c cuprates are reviewed. The antiferromagnetic spin fluctuations (AFSF) decrease in the order of La_1.85Sr_0.15CuO₄. YBa₂Cu₃O₇ and Tl₂Ba₂CuO_6+y . 1/T ₁ of⁶³Cu in the CuO₂ plane in the normal state follows essentially a Curie-Weiss law at high temperature andT ₁ T = const. law at low temperature. The temperature dependence of 1/T ₁ and the Knight shift together with their impurity effect in the superconducting state strongly suggest d-wave pairing implying the AFSF to be responsible for the occurrence of superconductivity. From the NQR frequency measurement the density of Cu 3d and O 2p holes decreases and increases, respectively, in the order of La, Y and Tl compounds, which is consistent with the change of AFSF. The relation betweenT _c andv _Q, and their pressure dependence suggest that there exists and optimum value of the ratio of Cu 3d and O 2p hole density to give a maximum inT _c.     

205Tl NMR in the Cuprate Superconductor Tl2Ba2CaCu2O8−δ: Evidence for a pseudo spin gap

²⁰⁵Tl-NMR in Tl₂Ba₂Ca_n-1Cu_nO_8?δ for differentn and δ is reviewed. Forn=2,3 the²⁰⁵Tl spectra consist of two lines. The main line arises from Tl(1) in the TlO-layer whereas the defect line belongs to Tl(2) in the Ca-layers between the CuO₂-layers. The temperature dependent Knight shift of the Tl(2) (defect) line is caused by the hyperfine interaction with the CuO₂-layer and is strongly related to the oxygen content of the sample. We find quasipartticle behavior for the T1(2) line similar to the⁸⁹Y Knight shift in the YBa₂Cu₃O_6+δ system. In contrast the T1(1) line shows strong antiferromagnetic correlations with indication of pseudo spin gap opening above the superconducting transition temperatureT _c.     

Crystal chemistry of the Tl2−xBa2Ca n Cu n+1O2n+6 (n=0, 1, 2) high-T c superconductors

We report on a comparative crystal chemical study of the Tl_2–xBa₂Ca _n Cu _n+1O_2n+6(n=0, 1, 2) high-T _c superconductors. Superconducting properties were determined by DC-conductivity and-susceptibility measurements. An X-ray diffraction analysis reveals the nonstoichiometry of the compounds as a link between structural and superconducting behavior. A comparison of the different structures gives evidence that T1 deficiency in the TlO double layers increases with increasingn; this deficiency causes apparent Cu valences larger than 2+in the superconducting phases thus maintaining the charge balance. By extracting appropriate chemical formulas and performing a bond strength calculation we obtained the mixed-valent oxidation state of copper for the superconducting tetragonal phases as being 2.17+(n=0), 2.25+(n=1) and 2.31+(n=2) respectively; for the non-superconducting orthorhombic phase Tl_2–xBa₂CuO_6–y a Cu valence of about 2+is suggested.     

Superconductivity of Bi2 (Sr0.9Ca0.1)2 CuO y single crystals

Bi₂ (Sr_0.9Ca_0.1)₂ CuO _y single crystals were grown by the self-flux method. The temperature dependence of resistance and ac susceptibility measurements showed that the as-grown crystal is semiconductive between 4.2 K and 300 K. However, after annealing in air for more than 20 days the crystal became superconducting with a T _c of 66 K obtained from the R-T curve. The ac susceptibility data also showed a strong superconducting transition at 71 K. X-ray diffraction indicated that the structure of the as-grown and the annealed crystal both are orthorhombic. The T _c of 71 K of the Bi₂ (Sr_0.9Ca_0.1)₂ CuO _y crystal may be due to the increase of oxygen content in the crystal after the long-time annealing in air. It is suggested that the superconductivity of the Ca-doped 2201 phase of the Bi-based system, which up to now was found to have a T _c=7–22 K, is similar to the 2201 phase of the Tl-based system.     

Study of the vortex dynamics in single crystals of the high-T c superconductor Tl2Ba2CaCu2O8 via205Tl NMR

The²⁰⁵Tl linewidth in single crystals of the high-T _c superconductor Tl₂Ba₂CaCu₂O₈ (T _c≈105 K) is analyzed for the normal (N) and impurity (I) Tl-sites. On the I-sites Ca has been replaced by Tl. Measurements were performed in a field of 4.7 T. The normal state linewidth on the N and I sites is shown to be mainly determined by a distribution in Knight shifts. To simulate the final increase below 50 K we have studied the vortex lattice in a single superconducting sheet by a molecular dynamics method for systems up to 2000 vortices. The results compare well with the experimental data. This implies that the temperature dependence of the linewidth in this highly anisotropic material is determined by flux dynamics and only marginally by the temperature dependence of the penetration depth.     

Spin‐freezing in superconducting La2CuO4.03 single crystal

Electron spin‐freezing at T_f=8 K has been detected in superconducting (T_c=12\ K) single crystal La₂CuO_4+y (y\simeq0.03) by ZF‐μSR. According to diffraction data, the crystal is in Bmab phase without any traces of structural phase separation. TF‐μSR experiment has shown that no Abrikosov flux line lattice is formed below T_c. The data allow us to assume that the magnetic and superconducting regions in the crystal are space separated on the microscopic scales ~ 10² Å. The presence of large field induced broadening of the Knight shift distribution k_\sigma > 1000 ppm indicates that the crystal contains micro‐regions possessing enhanced magnetic susceptibility.     

Crystal structures of high-T c oxides

A comprehensive review of structure work on high-T _c oxides as reported during the years 1987 and 1988 is given. Thirteen structures are refined from X-ray single-crystal and/or neutron powder diffraction data:I. (Ba_1–x ,K _x )BiO₃ (T _c =30 K),II. (La_2–x , Sr _x )CuO₄ (T _c =40 K),III. (Nd, Ce, Sr)₂CuO₄ (T _c =28 K),IV. (Nd_2–x , Ce _x )CuO₄ (T _c =24 K),V. YBa₂Cu₃O₇ (T _c =90 K),VI. YBa₂Cu₄O₈ (T _c =80 K),VII. Y₂Ba₄Cu₇O₁₄ (T _c =40 K),VIII. Pb₂Sr₂NdCu₃O₈ (T _c =70 K),IX. TlBa₂CaCu₂O₇ (T _c =103 K),X. TlBa₂Ca₂Cu₃O₉ (T _c =120 K),XI. Tl₂Ba₂CuO₆ (T _c =90 K),XII. Tl₂Ba₂CaCu₂O₈ (T _c =112 K),XIII. Tl₂Ba₂Ca₂Cu₃O₁₀ (T _c =125 K). Except forI (perovskite type),II (K₂NiF₄ type) andIV (Nd₂CuO₄ type) they represent new structure types. Structure data, bond distances, structure drawings and calculated X-ray powder diffraction patterns are given for each compound. Structural features and correlations with superconductivity are discussed. The review contains 301 citations.     

Polarized X-ray absorption study of Bi2Sr2CuO6 and Bi2Sr2CaCu2O8

Polarization dependent X-ray absorption measurements on single crystal Bi₂Sr₂CuO₆ (T _c =9 K) and Bi₂Sr₂CaCu₂O₈ (T _c =80 K) with one and two CuO₂ layers, respectively, show no energy shift of the Cu 2p main peak, and no relation between the amount of 3d(m=0, ±1) character and the critical temperatureT _c . At grazing incidence a structure in Bi₂Sr₂CaCu₂O₈ is found at 937 eV, which can be ascribed to a composite state of Cu 3d(z ²), Cu 4s ando–2p(z).     

Features of the spin fluctuations and superconductivity of Tl2Ba2CaCu2O8−δ according to 63Cu and 17O NMR data

Data on the NMR line shifts, the spin-lattice relaxation rate of ⁶³Cu and ¹⁷O nuclei, and the spin-spin relaxation rate of ⁶³Cu are obtained for Tl₂Ba₂CaCu₂O_8−δ (T _c =112 K) in the normal and superconducting states. The hyperfine constants at the copper and oxygen atoms in a CuO₂ plane are estimated from an analysis of the temperature dependence of the Knight shift. The temperature-dependent behavior of the long-and short-wavelength parts of the dynamic spin susceptibility is discussed by modeling an antiferromagnetic Fermi liquid. The possible relation between the characteristics of the spin-fluctuation spectrum and the superconducting transition temperature is analyzed for the oxide investigated. Zh. éksp. Teor. Fiz. 115, 991–1001 (March 1999)     

Coexistence of superconductivity and magnetic order in YBa2Cu4O8

⁵⁷Fe Mössbauer spectroscopy, magnetic susceptibility and powder x-ray-diffraction measurements were used to study superconductivity and magnetic order in YBa₂(Cu_1?xFe_x)₄O_8+δ. T_c is decreasing with x, disappearing for x>x_c≈0.04. For xc iron substitutes Cu, predominantly in the Cu(1) site exhibiting a single quadrupole Mössbauer spectrum at 90 K. For x>x_c magnetic order is observed in the Cu(2) site, T_N=380 (5) K for x=0.1 and H_eff (Cu(2), 4.2 K)=510(2) kOe. However, the most surprising discovery is that for x=0.025, for which T_c=27(2) K, the Fe in the Cu(1) site orders magnetically at T_N=30(2) K and H_eff (Cu(1), 4.2 K)=461(2) kOe. The coexistence and competition between superconductivity and magnetic order in the Cu(1) and Cu(2) sites in YBa₂Cu₄O₈ are discussed in terms of the previously observed phase diagrams for Y_1?xPr_xBa₂(Cu_1?yFe_y)₃O_z.     

Si doped Cu0.5Tl0.5Ba2Ca2Cu3O10−δ superconductors

The Si doped (Cu_0.5Tl_0.5)Ba₂Ca₂Cu_3−ySi_yO_10−δ (y=0, 0.25, 0.5, 0.75, 1.0, 1.25) superconductor samples have been synthesized to investigate the effect of mobile carriers on the critical temperature and magnitude of diamagnetism of this family of cuprates. The Si doped samples have tetragonal structure as observed from the X-ray diffraction spectra. The c-axis length of the unit cell of (Cu_0.5Tl_0.5)Ba₂Ca₂Cu_3−ySi_yO_10−δ was increased after the increase of Si concentration, whereas the critical temperature and the magnitude of diamagnetism have been decreased. The decrease in T_c(0) and magnitude of diamagnetism is possibly due to the deficiency of carriers in CuO₂/SiO₂ planes caused by the +4 state of Si atoms. However, the post-annealing of these samples in oxygen atmosphere has increased the critical temperature and the magnitude of diamagnetism. The FTIR absorption measurements of (Cu_0.5Tl_0.5)Ba₂Ca₂Cu_3−ySi_yO_10−δ samples have shown a softening of the apical oxygen mode of the type Cu(1)O_ACu(2) and TlO_ACu(2) with increased concentration of Si in the unit cell; the softening of this mode is directly linked with the increase in the c-axis lattice parameter of the (Cu_0.5Tl_0.5)Ba₂Ca₂Cu_3−ySi_yO_10−δ superconductor.     

A new superconducting oxide GdBaCuO with Tc (ρ=0) above 92K

A new superconducting oxide Gd_xBa_1−xCuO_3−y with T_c (ρ=0) above 92K has been successfully synthesized by the ceramic technology. The samples with composition of x=0.5, 0.625 and 0.9 are all observed with high T_c superconductivity. The highest T_c for samples measured is observed in Gd_0.5Ba_0.5CuO_3−y which has a resistive midpoint of 95K with a width of 1.5K, and a zero-resistance temperature of 92.5K. The results of a.c. magnetic susceptibility measurement are also reported.     

Reduced anti-ferromagnetism promoted by Zn 3d substitution at CuO2 planar sites of Cu0.5Tl0.5Ba2Ca3Cu4O12−δ superconductors

The role of charge carriers in ZnO₂/CuO₂ planes of Cu_0.5Tl_0.5Ba₂Ca₃Cu_4−yZn_yO_12−δ material in bringing about superconductivity has been explained. Due to suppression of anti-ferromagnetic order with Zn 3d¹⁰ (S=0) substitution at Cu 3d⁹ sites in the inner CuO₂ planes of Cu_0.5Tl_0.5Ba₂Ca₃Cu₄O_12−δ superconductor, the distribution of charge carriers becomes homogeneous and optimum, which is evident from the enhanced superconductivity parameters. The decreased c-axis length with the increase of Zn doping improves interlayer coupling and hence the three dimensional (3D) conductivity in the unit cell is enhanced. Also the softening of phonon modes with the increased Zn doping indicates that the electron–phonon interaction has an essential role in the mechanism of high-T_c superconductivity in these compounds.     

Electron Raman scattering in Tl2Ba2CuO6+x crystals

It is inferred from the position of the 2Δ peak in the electron light scattering spectra that in a sample with high T _c ≈90 K the ratio 2Δ/T _c is approximately equal to 6 and 7 for different polarizations, while in overdoped samples with T _c ≈40 and 35 K the 2Δ peak is observed only in x′y′ polarization with a substantially lower ratio 2Δ/T _c ≈3.2. With decreasing T _c in Tl₂Ba₂CuO_6+x crystals, a transition occurs from strong to weak coupling; the superconducting gap remains anisotropic at different doping levels. Pis’ma Zh. éksp. Teor. Fiz. 64, No. 11, 760–765 (10 December 1996)     

Superconductivity above 100 K in multi-phase Y−Ba−Cu−O

We report the synthesis, characterization and superconducting temperature of high-temperature superconducting Y_0.3Ba_0.7CuO₃, Y_0.4Ba_0.6CuO₃ and Y_1.2Ba_0.8CuO₄. The volume fraction of the superconducting phase is estimated to be of the order of 10%.HighT _c superconductivity withT _c -onsets up to 125 K (midpoint 102 K, zero resistance: 90 K) is observed in multi-phase Y–Ba–Cu–O oxygen defect compounds with the nominal composition of Y_1.2Ba_0.8CuO_4–y (0y0.4).     

Superconductivity in Tl2Ba2−xSrxCaCu2O8 solid solutions: Tc≈44 K for a composition with x=2.0

Solid solutions in the Tl₂Ba_2−xSr_xCaCu₂O₈ system were prepared and characterized by X-ray powder diffraction, resistivity and magnetic susceptibility measurements. Systematic changes in T_c and lattice parameters with x are reported. A sample with nominal composition of Tl₂Sr₂CaCu₂O₈ (x=2.0) is superconducting with T_c≈44 K.     

Synthesis and superconductivity of a new 1222-type T1-based layered cuprate (T1, Nb) Sr2(Nd,Ce)2Cu2O z

Synthesis and superconductivity of a new 1222-type layered cuprate (Tl_1–x Nb _x ) Sr₂(Nd_1–y Ce _y )₂Cu₂O _z have been studied. The structure of this cuprate is directly related to that of Nb-1222 NbSr₂(Nd, Ce)₂Cu₂O _z with tetragonal body-center lattice. Partial substitution of Tl for Nb in Nb-1222 phase improves its superconductivity. (Tl_1–x Nb _x ) Sr₂(Nd_0.75Ce_0.25)₂Cu₂O _z samples prepared by the typical procedure exhibit superconductivity withT _c of 30–40 K. Effects of Tl and Nb on superconductivity of this cuprate are briefly discussed.     

205Tl NMR observation of vortex lattice formation in the high temperature superconductor Tl2Ba2CuO6

We report on the asymmetric field distribution of the triangular vortex lattice in the superconductor Tl₂Ba₂CuO₆ as observed by²⁰⁵Tl NMR atB _a =4.26 T. A penetration depth of =1700±100 Å is obtained.