Low temperature singularities of electron density in a two-gap superconductor ZrB12

Abstract

Fine details of crystal structure of ZrB12 two-gap superconductor are studied at low temperatures by precise x-ray diffraction technique. Small static Jahn-Teller distortions of a face centered cubic lattice are observed in the range 30–70 K, leading to emergence of two types singularities of electron density (ED). These are: (i) dynamic charge stripes along selected directions <110> and <112> in the crystal and (ii) triangular lattice of the ED antinodes located in the interstices of the boron lattice in the {111} planes. The second anomaly was detected for the first time, and it is very pronounced for ZrB12, being intensified at low temperatures. The anisotropy of the two-gap superconductivity in ZrB12 is confirmed by measurements of the low temperature heat capacity in a magnetic field directed along three principal axes in the crystal, H || [100], H || [110] and H || [111]. We conclude in favor of the magnetic field-induced anisotropy arising from the interaction of vortex lattice in this superconductor with fluctuating ED (charge stripes) of a two-type filamentary structure.