Direct observation of the d-wave contribution to the low-temperature specific heat of the high-temperature superconductor YBa2Cu3O7

The direct observation of the d-wave contribution to the low-temperature sp ecific heat of YBa2Cu3O7 has been obscured up to now by the presence of ext rinsic magnetic Schottky terms. For a pure 18 mg single crystal grown in Ba ZrO3, these terms are almost negligible, so that the presence of line nodes in the gap is directly seen in the difference C(B, T)-C(0, T) as a functio n of the magnetic field, as expected from the Volovik effect. (C) 2000 Else vier Science B.V. All rights reserved.