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.

No abstract

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.