Electronic specific heat of overdoped Tl₂Ba₂CuO_6+δ in a magnetic field

Abstract

We have measured the magnetic field dependence of the electronic specific heat γ^el (=C^el/T) of polycrystalline Tl₂Ba₂CuO_6+δ from 8 K to above T_c, for samples with T_c = 24 K to 72 K using a high resolution, differential technique. We find that in applied fields of up to 13 T the position of the specific heat jump does not shift significantly for any T_c. This is in strong disagreement with H_C2(T) inferred from magnetoresistance experiments on the same compound, where the resistive transition is shifted down considerably by similar fields. However, there is a dramatic collapse of the specific heat anomaly height in field — signifying a loss in entropy around T_c — compensated by a corresponding increase in γ^el at lower temperatures. The superconducting anomaly is almost completely suppressed in 13 T for the lowest T_c sample, compared to a reduction of 65 % for the sample with T_c = 72 K. We find simple vertical scaling of γ^el(T) — γ_n suggesting that the field forces a proportion of the superconductor into the normal state, while the remainder in the superconducting state is unaffected.