Fourfold Basal Plane Anisotropy of the Nonlocal Magnetization ofYNi2B2C

Abstract

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...

Quaternary borocarbides: Relatively high T intermetallic superconductors and magnetic superconductors

“…Magnetization measurements in the superconducting state of both LuNi 2 B 2 C [118] and YNi 2 B 2 C [119] show anisotropy in the a-b plane with fourfold symmetry.…”

On The Influence of a Non-Local Electrodynamics in the Irreversible Magnetization of Non-Magnetic Borocarbides

“…As we pointed out in ref. 32 , the large out-of-plane F p anisotropy sharply contrasts with the very small (< 10%) mass anisotropy 24,26,25 . Therefore, explanations based on the anisotropic scaling frequently used in high T c superconductors 39 are ruled out.…”

“…The Y-0 crystal is the same one that was previously investigated in Refs. 23,24,26,27,32 . The normal state magnetization follows a Curie law that indicates the presence of a very dilute distribution of localized magnetic moments.…”

Influence of nonlocal electrodynamics on the anisotropic vortex pinning inYNi2B2C

“…2,11 Electrical resistivity data yielded a residual resistance ratio of 10 and an estimate of the electronic mean free path lϳ300 Å. Magnetic measurements were carried out in two Quantum Design superconducting quantum interference device magnetometers.…”

“…Another potential source of this anisotropic pinning is the small amount of magnetic impurities present in this crystal, 11 about ϳ0.1% of a rare earth substituting for Y, whose moments lie in the basal plane. However, although we cannot totally rule out this possibility, the data suggest that this is not the case.…”