We report the first direct observation of the oxygen-isotope ((16)O/(18)O) effect on the in-plane penetration depth lambda(ab) in a nearly optimally doped YBa(2)Cu(3)O(7-delta) film using the novel low-energy muon-spin rotation technique. Spin-polarized low-energy muons are implanted in the film at a known depth z beneath the surface and process in the local magnetic field B(z). This feature allows us to measure directly the profile B(z) of the magnetic field inside the superconducting film in the Meissner state and to make a straightforward determination of lambda(ab). A substantial isotope shift Delta lambda(ab)/lambda(ab)=2.8(1.0)% at 4 K is observed, implying that the in-plane effective supercarrier mass m*(ab) is oxygen-isotope dependent with Delta m*(ab)/m*(ab)=5.5(2.0)%. These results are in good agreement with magnetization measurements on powder samples.

We discuss the multi-exponential nuclear magnetization recovery which occurs in spin-lattice relaxation when NMR lines are split by quadrupole interaction. We treat the general situation in which both magnetic and quadrupolar fluctuations are present and consider three cases differing in their initial conditions: (1) a short radio-frequency pulse is applied selectively to one of the transitions; (2) all lines are saturated at once; (3) a selected line is saturated by continuous waves or by means of a long comb of pulses. Exact solutions are presented for spin I = 1 and I = 3/2, whereas for spin I = 5/2, exact solutions are given for special cases and approximate solutions for the general case. The spin I = 7/2 case is treated for magnetic fluctuations only. The detailed analysis reveals that the form of the recovery law is surprisingly insensitive to an additional relaxation channel, e.g. quadrupolar fluctuations in the presence of predominantly magnetic fluctuations or vice versa.

We performed high accuracy 63 Cu NQR spin-lattice relaxation and SQUID magnetization measurements on 16 O and 18 O exchanged YBa 2 Cu 4 O 8 to determine the isotope shift of the temperature of the opening of the spin gap, T ‫ء‬ , and the superconducting transition temperature, T c . The corresponding isotope exponents are a T ‫ء‬ 0.061͑8͒ and a Tc 0.056͑12͒ which are the same within the error bars and suggest a common origin for the superconducting and the spin gap. [S0031-9007(98)08085-5] 74.25.Nf, 74.62.Dh One of the central and heavily debated questions in high-temperature superconductivity research concerns the origin of the so-called pseudogap occurring in the normal state of underdoped superconductors [1]. The pseudogap refers to the transfer to higher energy of the density of low-energy excited states. One may ask whether the pseudogap is caused by superconducting correlations, which develop above T c , or whether it is an independent phenomenon. In nuclear magnetic resonance (NMR) and neutron scattering experiments, the pseudogap reveals itself as a spin gap. For instance, the Cu spin-lattice "relaxation rate per temperature unit," ͑T 1 T ͒ 21 , increases with falling temperature and reaches a maximum at T ‫ء‬ , which is a proper scale for the temperature dependence of the spin gap. For YBa 2 Cu 4 O 8 (Y124), the corresponding values are T c 81 K and T ‫ء‬ ഠ 150 K.Recently [2], we detected anomalies in the temperature dependence of several NMR and NQR (nuclear quadrupole resonance) quantities measured in the normal state of Y124, for instance in NQR frequencies, Knight shifts, line widths, and relaxation times. These anomalies, which occur around T y 180 K, are the signature of an electronic crossover which involves enhanced charge fluctuations in planes and chains. Because of the proximity of T y and T ‫ء‬ , we have argued that the spin gap effect in Y124 is caused by a transition due to a charge density wave (CDW) instability [3]. Using the t-J model and including electron-phonon interaction, we could, among others, explain the strong temperature dependence of the magnetic shift of the planar Cu nuclei, which we had measured previously [4], and we predicted a dependence of T ‫ء‬ on the isotope mass. Thus, corresponding measurements allow one to check the consistency of the CDW model.NMR͞NQR are techniques that can determine the pseudogap with a precision allowing one to establish whether an isotope effect is present or not. Y124, because of its well-defined oxygen stoichiometry and its negligible oxygen diffusion, is the ideal compound for such a study that requires experimental results not hampered by reproducibility problems. Therefore, the NQR study of the isotope effect on the spin gap in Y124 and its comparison with the superconducting gap is a unique experiment to explore the relation between the two gaps.In this Letter, we report a high-accuracy NQR study of the planar 63 Cu nuclei, supplemented by susceptibility measurements, on 16 O and 18 O exchanged Y124 samples which revealed the presenc...

We present results of 0 and ' Cu nuclear magnetic resonance (NMR) and nuclear quadrupolar resonance (NQR) studies in the normal and superconducting state of the 82-K superconductor YBazCu408. The various components of the Cu and 0 Knight-shift tensors show strong but similar temperature dependences over the temperature range from 8.5 to 300 K in both the CuOz planes and the chains, supporting the picture that there is only one spin component in the planes and the chains, although with difFerent susceptibilities. The oxygen data obey the Korringa relation. This may be interpreted as Fermi-liquid behavior of the electronic system far away from the antiferromagnetic wave vector. The temperature dependence of both the planar Cu and 0 shift tensors and the planar Cu spin-lattice relaxation rate suggest the opening of a pseudo-spin-gap well above T, The . very different temperature dependence of 1/Ti at the planar 0 and Cu sites points to the reduced role of the antiferromagnetic correlated spin fluctuations at the 0 site. The data favor the conclusion that YBazCu408 is a d-wave superconductor. Evidence is provided by three data sets: the chain Knight shifts, the ratio of the planar copper and oxygen relaxation rates, and the individual low-temperature behavior of these rates.

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