The anisotropy gamma of the superconducting state of high quality single crystals of MgB2 was determined, using torque magnetometry with two different methods. The anisotropy of the upper critical field was found to be temperature dependent, decreasing from gamma approximately 6 at 15 K to 2.8 at 35 K. Reversible torque data near T(c) reveal a field dependent anisotropy, increasing nearly linearly from gamma approximately equal to 2 in zero field to 3.7 in 10 kOe. The unusual temperature dependence is a true bulk property and can be explained by nonlocal effects of anisotropic pairing and/or the k--> dependence of the effective mass tensor.

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.

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Single crystals of the oxypnictide superconductor SmFeAsO 0.8 F 0.2 with T c ≃ 45(1) K were investigated by torque magnetometry. The crystals of mass ≤ 0.1 µg were grown by a high-pressure cubic anvil technique. The use of a high-sensitive piezoresistive torque sensor made it possible to study the anisotropic magnetic properties of these tiny crystals. The anisotropy parameter γ was found to be field independent, but varies strongly with temperature ranging from γ ≃ 8 at T T c to γ ≃ 23 at T ≃ 0.4T c . This unusual behavior of γ signals unconventional superconductivity.

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.

In this paper we reinterpret the magnetic-susceptibility data and present and discuss specificheat data on MEM-(TCNQ)2 in terms of a spin-Peierls transition theory. We find that the data can be described reasonably well by a mean-field spin-Peierls transition theory which suggests that at low temperatures the TCNQ chain should be tetramerized. The magnetic susceptibility above the transition temperature is shown. to behave like a one-dimensiona) Heisenberg antiferromagnet. The consequences of this behavior on the relative magnitude of the on-site Coulomb interaction are discussed.

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...

Single crystals of MgB 2 with a size up to 1.5x0.9x0.2 mm 3 have been grown with a high pressure cubic anvil technique. The crystal growth process is very peculiar and involves an intermediate nitride, namely MgNB 9 . Single crystals of BN and MgB 2 grow simultaneously by idth of ~0.5 K. The high quality of the crystals allowed the accurate determination of magnetic, transport (electric and heat) and optical properties as well as scanning tunnelling spectroscopy (STS) and decoration studies. Investigations of crystals with torque magnetometry show that H c2 //c for high quality crystals is very low (24 kOe at 15 K) and saturates with decreasing temperature, while H c2 //ab increases up to 140 kOe at 15 K. The upper critical field anisotropy = H c2 //ab / H c2 //c was found to be temperature dependent (decreasing from ≅ 6 at 15 K to 2.8 at 35 K). The effective anisotropy γ eff , as calculated from reversible torque data near T c , is field dea peritectic decomposition of MgNB 9 . Magnetic measurements with SQUID magnetometry in fields of 1-5 Oe show sharp transitions to the superconducting state at 37-38.6 K with wpendent (increasing roughly linearly from eff ≅ 2 in zero field to 3.7 in 10 kOe). The temperature and field dependence of the anisotropy can be related to the double gap structure of MgB 2 with a large two-dimensional gap and small three-dimensional gap, the latter of which is rapidly suppressed in a magnetic field. Torque magnetometry investigations also show a pronounced peak effect, which indicates an order-disorder phase transition of vortex matter. Decoration experiments and STS visualise a hexagonal vortex lattice. STS spectra in zero field evidence two gaps 3 meV and 6 meV with a weight depending on the tunnelling direction. Magneto-optic investigations in the far infrared region with H//c show a clear signature of the smaller of the two superconducting gaps, completely disappearing only in fields higher than H c2 //c .