Multiple superconducting gaps in MgB2 single crystals from magnetic torque

We have measured the magnetic torque of an MgB2, single crystal in the various different fields below 10 kG by using a torque magnetometer and a 4 K closed cycle refrigerator. The MgB2 single crystal was synthesized by the vapor transport method. The torque can be measured as an off-balance signal of the Wheatstone bridge of the four piezoresistors on a Si cantilever. The torque curves are analyzed by the Kogan model. The superconducting anisotropy gamma is rather independent of temperature in 5 and 10 kG, but is dependent on field up to 60 kG. We consider that the field dependence of gamma comes from the nature of the multiple superconducting gaps. The experimental results show that the Ti-band superconducting gaps have been deteriorated gradually up to a crossover field H-* (pi) similar to 20 kG at 10 K when the magnetic field increases. (C) 2004 Elsevier B.V. All rights reserved.