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The superconducting energy gap and vortex lattice structure in LuNi2B2C

par De Wilde, Yannick ;Iavarone, Maria;Welp, Ulrich W.;Metlushko, Vitali;Koshelev, Alexei E.;Aranson, Igor S.;Crabtree, George W.;Gammel, Peter L.;Bishop, David J.;Canfield, Paul C.
Référence Physica. C, Superconductivity, 282-287, PART 1, page (355-358)
Publication Publié, 1997-08

Article révisé par les pairs

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Résumé :

We present Scanning Tunneling Microscope (STM) observations of the superconducting energy gap and vortex lattice (VL) structure of the (001) face of a LuNi2B2C single crystal. In zero field, the conductance versus voltage characteristics consistently show a peaked structure that can be fitted with a broadened BCS like density of states, giving a gap value of 2.2 meV and 2Δ/kTc = 3.2 ± 0.1, implying weak coupling superconductivity. In applied magnetic fields of 0.375 T and 1.5 T, we make the first STM observation of a square VL, with its unit cell rotated 45° with respect to the crystallographic unit cell. The VL structure is analyzed with Ginzburg-Landau theory, including an additional free energy term with four-fold symmetry arising from the tetragonal crystal structure. The square VL is found to be stable if the four-fold free energy term is large enough to cause the ratio of the in-plane upper critical fields Hc2 (45°)/Hc2 (0) to be less than 0.97. Measurements of the in-plane upper critical field anisotropy by SQUID magnetometry show a ratio of 0.92 at 4.2 K, consistent with the Ginzburg-Landau analysis.