Studies of single crystal YNi 2 B 2 C have revealed a fourfold anisotropy of the equilibrium magnetization in the square crystallographic basal plane. This p͞2 periodicity occurs deep in the superconductive mixed state. In this crystal symmetry, an ordinary superconductive mass anisotropy (as in usual London theory) allows only a constant, isotropic response. In contrast, the experimental results are well described by generalized London theory incorporating nonlocal electrodynamics, as needed for this clean, intermediate-k superconductor. PACS numbers: 74.25.Ha, 74.25.Bt, 74.70.Dd Borocarbide superconductors have received considerable recent attention, due in part to the interaction between magnetism and superconductivity. A rich superconducting phase diagram, including transitions between hexagonal, rhombohedral, and square vortex lattices, has been observed [1][2][3][4]. The existence of vortex lattices with nonhexagonal symmetry has been attributed to nonlocality effects on the superconducting electrodynamics [4,5], which arise from the large electronic mean free path ᐉ of these clean superconductors. Geometrically, a vortex directed along the tetragonal c axis has squarelike current contours [6]. It has been shown [7] in the nonmagnetic borocarbide YNi 2 B 2 C that the deviations from the standard (local) London magnetic field dependence of the equilibrium magnetization M eq~l n͑H͒ can be quantitatively accounted for by introducing nonlocal electrodynamics into the London model [8]. Traditionally, it was widely thought that nonlocality effects should be significant only in materials with a Ginzburg-Landau parameter k l͞j ϳ 1, where l is the London penetration depth and j is the superconducting coherence length [9]. Those materials, e.g., Nb, were clean enough to have ᐉ ¿ j, but the large vortex cores with j ഠ l make theoretical analysis very difficult. With the development of clean intermetallics and compounds, e.g., YNi 2 B 2 C where k ഠ 10 15, core effects are much smaller. Thus a more tractable nonlocal London formalism has been recently developed [8] for understanding these intermediate-to-high k materials.In the local London model of superconducting vortices, the material anisotropy is introduced via a second rank mass tensor m ij . In tetragonal materials such as YNi 2 B 2 C or LuNi 2 B 2 C, the masses in both principal directions in the square basal plane are the same, m a m b ; thus the superconducting properties are isotropic in the a-b plane. In contrast, nonlocal corrections are expected to introduce [10] a fourfold anisotropy as a function of the magnetic field orientation within the a-b plane. A temperaturedependent in-plane anisotropy of the upper critical field H c2 has been observed [11] in the nonmagnetic borocarbide LuNi 2 B 2 C and described within a Ginzburg-Landau framework incorporating nonlocal effects. However, a direct observation of the in-plane anisotropy deep in the superconducting phase, where the nonlocal London model applies and unusual vortex lattices are observed, has not b...

The equilibrium magnetization M of single crystal YNi 2 B 2 C superconductor has been studied with magnetic field Hʈc axis. The material is clean, with low electrical resistivity and weak magnetic hysteresis ͑for H ϭ10 kG, the critical current density is less than 10 Ϫ6 of the depairing current density͒. The magnetization M (H) deviates from conventional London predictions, but is well described by recent nonlocal London theory, with well-behaved superconductive parameters. The temperature dependence of the London penetration depth (T) and other parameters are deduced. ͓S0163-1829͑99͒50810-2͔ RAPID COMMUNICATIONS R6622PRB 59 K. J. SONG et al.

Bulk polycrystalline Tl2Ba2CaCu2Ox materials were irradiated with 0.8 GeV protons to introduce randomly oriented columnar defects. Proton fluences up to 8.7×1020 m-2 were used to create defect arrays corresponding to a `matching field' of 1.5 T. Studies were conducted on the superconductive transition temperature, the Meissner fraction, the intragrain persistent current density against the magnetic field and temperature, and the equilibrium magnetization. The magnetization was modelled using London theory with the addition of vortex-defect interactions, yielding physically reasonable parameters.

The domain structure of the fenoelastic BiVOa single crystal has been investigated using the nuclear magnetic resonance (NMR) of "V and by the electron panmagnetic resonance (WR) of the Mn2+ ions contained in the crystal as an impurity. The 14 resonance lines of "V ( I = f ) in a BiVOd cryswl with the twin domain were measured in the crystallographic U& (or c-b) and e-a (or 0 -c ) planes with an FT NMR spectrometer. Two sets of Mn2; EPR signals were also obtained in the C(I plane. These two Sets of NMR and EPR signals originate from the twin-domain StNCtUre. From these two sets of experimentd data from ''V NMR and MnZ+ EPR. it is confirmed that the BiV04 single crystal has the prominent (W-plane) domain wall reported previously. The investigated domain S t N C t U E is found to be stable with time in COntmSt with a previous report by Bamn el al. The previous model of the twinning mechanism derived fmm NMR and x-ray diffraction data has been improved by employing the FPR results. The observed W plane of the domain wall in BiVOa can be explained by the ferroelastic Species 4Immm F 2 / m instead of 41" F 2 / m .

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