Yu.I. Zhdanov

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

Abstract 63 Cu and 17 O NMR measurements in the normal and superconducting states of Tl2Ba2Ca2Cu3O10−δ with different δ are reported. In the overdoped Tl2223 sample with Tc=117 K (Tcopt=123 K) and δ1 17 K are revealed for inequivalent CuO2 layers. For the inner CuO2 layer with the square oxygen coordination of Cu the decrease of 17 K with temperature is more gradual. In going towards the underdoped Tl2223 with Tc=104 K and δ2>δopt the changes of 63,17 K 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 nh 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].

Spin-lattice relaxation, NQR 63Cu and NMR 205Tl in Tl2Ba2CuO6+δ

Abstract We measured the nuclear spin-lattice relaxation rate of 63 Cu and 205 Tl, the magnetic hyperfine shift of 205 Tl, and the NQR of 63 Cu in the raw of Tl 2 Ba 2 CuO 6+δ with different T c ( δ ) in the range 3–400 K, to investigate low-frequency spin dynamics in perovskite CuO 2 layers. The possibility is discussed of explaining the obtained results in terms of the “antiferromagnetic Fermi liquid” model proposed by Monien, Millis and Pines. We found that T c is lowered and growth of the uniform spin susceptibility χ ( q = 0) takes place which is accompanied by a decrease in the intensity of a short-wave AF spin correlation between copper atoms in the CuO 2 plane.

Local distortions in the BiO6-octahedra sublattice of BaPbxBi1−xO3 as seen by 137Ba NMR/NQR

Abstract The 137 Ba NMR and NQR spectra as well as the spin-lattice relaxation rate were measured for the first time in the BaPb x Bi 1− x O 3 samples with x = 0.91 (normal metal), x = 0.75 (superconductor) and x = 0.64 (semiconductor). The strong quadrupole broadening of the NMR lines with lowering temperature was found for all the samples. An evolution of static distortions in the BiO 6 -octahedra sublattice along with asymmetry of the electric field gradient at the Ba-sites is discussed. This asymmetry caused by the nearest non-cubic surrounding of barium is originated from the temperature dependent tilt of BiO 6 -octahedra. Spin-lattice relaxation rate of 137 Ba is monitored by fluctuating part of electric field gradient at the Ba-site and its exponential growth at low temperatures evidences the thermally activated collective rotations of BiO 6 -octahedra around their static tilt. An activation energy and a strength of these dynamic distortions are evaluated and found to increase in going from metallic to semiconducting phase of BaPb x Bi 1− x O 3 .

17O NMR IN HIGH-TC SUPERCONDUCTOR TL2BA2CACU2OY

Abstract 17 O NMR measurements have been performed between 10 and 400 K on an overdoped sample of Tl 2 Ba 2 CaCu 2 O y ( T c = 103 K). The electric field gradient parameters were deduced for all the oxygen sites. The temperature dependence of the magnetic hyperfine shift was measured for all these sites. No thermal variation of the shift was detected down to T c for oxygen in the TlO reservoir plane. As for oxygen in the CuO 2 plane the temperature dependence of the shift is similar to that for 205 Tl NMR line from thallium defects in the Ca layer measured in the same powder sample. This similarity is consistent with the single spin fluid model. The spin contribution to 17 K iso ( T ) and magnetic hyperfine constant 17 A iso were deduced.

NQR and spin-lattice relaxation of 63Cu in radiation disordered YBa2Cu3O6.95

Abstract We report on NQR spectra and the spin-lattice relaxation rate of 63 Cu in YBa 2 Cu 3 O 6.95 ceramic samples, irradiated by a fluence of fast neutrons ( F = 0–2 × 10 19 sm -2 ). The formation of localized magnetic moments at the Cu(2) position was found under disordering. An evaluation of magnetic excitation spectra monitoring the spin lattice relaxation process at Cu atoms of irradiated samples is discussed.

Spin susceptibility and doping dependence in Tl0.5Pb0.5Sr2Ca1−xYxCu2O7−δ a round the metal-insulator transition 63,65Cu, 207Pb and 205Tl NMR and spin-lattice relaxation

Abstract NMR line shifts of 63Cu, 207Pb and 205Tl were measured in the cuprates Tl0.5Pb0.5Sr2Ca1−xCu2O7 (x = 0.0–1.0) in order to investigate the evolution of the spin susceptibility χS around the metal-insulator transition, which occurs when the yttrium content increases up to x ≌ 0.6. The Knight shift was evaluated for 63Cu, 207Pb and 205Tl and the corresponding change of the static spin susceptibility χS (q = 0) was determined from the NMR data in the superconducting state. In the superconducting phase, where Tc has a maximum at x = 0.2, the value of χS (q = 0) increases gradually with hole doping. The 63Cu zero-field NMR and the NMR spectra of 207Pb and 205Tl for Tl0.5Pb0.5Sr2YCu2O7 reported here present the first NMR observation of antiferromagnetic ordering in these thallium-based cuprates.

Spin susceptibility of neutron-irradiation-disordered YBa2Cu3O6.9 in the normal and superconducting states

The spin-spin relaxation rate 63T2−1 of 63Cu nuclei in CuO2 layers is measured in the normal and superconducting states of the compound YBa2Cu3O6.9 (Tconset=94 K) subjected to radiation-induced disordering by a fast-neutron flux Φ to Tconset=68 K (Φ=7×1018 cm−2) and Tconset<4 K (Φ=12×1018 cm−2). It is found that as the structural disorder increases, the contribution of the indirect spin-spin interaction 63T2G−1, which is related to the value of the spin susceptibility at the boundary of the Brillouin zone of the copper planes χs(q={π/a; π/a}), decreases slightly at the transition to the superconducting state for the initial sample and remains unchanged for the weakly disordered sample. This behavior of the short-wavelength contribution to the spin susceptibility attests to the stability of the x2−y2 symmetry of the energy gap against structural disorder, in accordance with proposed theoretical models of Cooper pairing for high-Tc cuprates.

63Cu and 89Y NMR and spin susceptibility of YBa2Cu3O6.9 disordered by neutron irradiation

Abstract The 63 Cu and 89 Y Knight shift 63(89) K, spin-lattice 63(89) T 1 −1 and spin-spin 63 T 2 −1 relaxation rates were measured in YBa 2 Cu 3 O 6.9 ( T c,ons = 68 K ; T c . A sharp decrease of K being proportional to the uniform part of spin susceptibility χ s (q=0) is found with increase of disorder. According to the relaxation data of copper a short wave contribution. to χ s (q) around q AF = { π , π }/a if found to be nearly independent of temperature above T c up the room temperature in irradiated material. This evidences that magnetic correlation length ζ of the AF spin fluctuations ceases its change with temperature in disordered YBaCuO. The contribution of indirect spin-spin coupling of Cu in CuO 2 layer 63 T 2g −1 ∝ χ s (q AF )/ ζ holds its magnitude in going through T c both for the unirradiated ( T c,ons =94 K ) and irradiated ( T c,ons =94K cuprates. The behavior of ζ s (q RMAF ) below T c is considered as evidence of stable x 2 -y 2 symmetry of the superconducting energy gap in respect to additionally induced structural disorder.

Electric field gradient and hole concentration in inequivalent CuO2 layers of Tl2Ba2Ca2Cu3O10−δ with different Tc

Abstract The 63 Cu and 17 O NMR were measured in oriented ceramic samples of Tl 2 Ba 2 Ca 2 Cu 3 O 10−δ with various Tc values in the normal and superconducting states. The quadrupole coupling constants for copper and oxygen from different CuO 2 layers were obtained. The doping variation of the valence contribution to the electric field gradient at the Cu site was used for estimation of the hole concentration. In the overdoped Tl2223 sample with T c ( δ 1 ) c opt ( δ opt > δ 1 ) the charge density in the inner layer (with four-fold oxygen coordination for Cu) differs from one in the outer plane (with five-fold oxygen coordination for Cu). Our results have shown that the inhomogeneity of the charge distribution disappears in the underdoping regime.

NMR and spin-lattice relaxation rate of89Y,63Cu in radiation disordered YBa2Cu3O6.9

NMR line shifts and spin-lattice relaxation rates of89Y and63Cu in YBa2Cu3O6.9 were measured in disordered samples irradiated by fast neutrons with a fluence of: ϕ=5·1018 cm−2 (Tcons=70 K), ϕ=1.2·1019 cm−2 (Tcons=20 K), in order to investigate the evolution of spin susceptibility χS under irradiation. According to the data obtained, χS decreases markedly as structural disorder accumulates in the samples. The variation of χS is related closely to that observed in oxygen-depleted YBa2Cu3O7−δ compounds with nearly the sameTc value. The data for yttrium display some features that are absent in oxygen-depleted samples. The different temperature dependence of the89Y and63Cu Knight shifts and the increase of the89Y spin-lattice relaxation rate in irradiated samples are discussed in terms of the electron localization effects arising in the CuO2 planes due to the radiation-induced structural disorder.