Cu nuclear spin-spin relaxation in YBa2Cu3O6.98 and YBa2Cu4O8

We have measured the temperature (T) dependence of the transverse relaxation rate (T _G ^–1 ) of the Cu(1) nuclear spin in YBa₂Cu₃O_6.98 (T _c=92 K) and YBa₂Cu₄O₈ (T _c=82 K). From the scaling ratio ofT _G ^–2 (Cu1) toT _G ^–2 (Cu2), we have estimated the strength of a covalent bonding between the CuO₂ plane and the CuO chain to be B0.38×A _zz. The experimentalT _G ^–1 (Cu1) in YBa₂Cu₄O₈ was of the same order of magnitude as the estimated one fromT _G ^–1 (Cu2). These results appear to indicate that the electrons in the CuO₂ plane fairly spread out of the plane in both compounds.     

Calculation of nuclear spin-lattice relaxation rate due to two-magnon scattering in YBa2Cu3O6

YBa₂Cu₃O₆ is known to be an antiferromagnetic insulator with high Neel temperature (T_N≈400≈500 K). The nuclear spin-lattice relaxation rate 1/T₁ due to two-magnon scattering is calculated, using values of ecchange integrals and anisotropy gap published in literature. We found approximately quadratic dependence of relaxation rate 1/T₁ on temperature for temperatures above 100 K. The value of 1/T₁, determined by low-energy magnons, is found to be inversely proportional to magnitude of gap in magnon spectrum.     

Ac-Josephson effect in YBa2Cu3O7/YBa2Cu3O7 point contact junctions

The ac-Josephson effect could be demonstrated up to dc voltages of 20 mV on the I–V characteristic of adjustable YBa₂Cu₃O₇/YBa₂Cu₃O₇ point contacts at 4.2 K. A detailed analysis of the rf power dependence of the size of microwave-induced constant voltage steps is given for microwave frequencies of 70 and 90 GHz. The results prove that electron pairing is responsible for the superconductivity in YBa₂Cu₃O₇.     

Anomalies of the electronic spin-lattice relaxation of Gd3+ in YBa2Cu4O8 and YBa2Cu3O6+x near 200 K

A sharp kink in the temperature dependence of the electronic spin-lattice relaxation rate near 200 K was found in the high-temperature superconductors YBa₂Cu₄O₈ and YBa₂Cu₃O_6+x . The effect is correlated with opening of a spin gap and microscopic phase separation. Pis’ma Zh. éksp. Teor. Fiz. 69, No. 8, 567–572 (25 April 1999)     

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

NMR line shifts and spin-lattice relaxation rates of⁸⁹Y and⁶³Cu in YBa₂Cu₃O_6.9 were measured in disordered samples irradiated by fast neutrons with a fluence of: ?=5·10¹⁸ cm^?2 (T _c ^ons =70 K), ?=1.2·10¹⁹ cm^?2 (T _c ^ons =20 K), in order to investigate the evolution of spin susceptibility χ_S under irradiation. According to the data obtained, χ_S decreases markedly as structural disorder accumulates in the samples. The variation of χ_S is related closely to that observed in oxygen-depleted YBa₂Cu₃O_7?δ compounds with nearly the sameT _c 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.