Measurement of the Heat Capacity of Cuprate Superconductors in High Magnetic Fields

The following presents the details of the study of a class of materials known as cuprate superconductors, primarily by measurement of their specific heat capacities, across two materials at very low temperatures and in very high values of applied magnetic fields. The materials are YBa2Cu3O6+δ and HgBa2CuO4+δ where δ<1 indicates a chemical doping by shifting the molecular oxygen content away from stoichiometric values. The particular temperatures for this study range from 0.5 to 12 kelvin. The magnetic field range is 0 to 35 teslas. The data were produced by a unique system for heat capacity measurement primarily designed and built at Florida State University and the National High Magnetic Field Laboratory. All measurements utilizing magnetic fields above 15 teslas were carried out through the user program of the National High Magnetic Field Laboratory. Such measurements include the actual heat capacity measurements, as well as the separate instrumentation efforts which produced the thermometer calibrations enabling the accurate measurement of temperature up to 35 teslas. The final results of the experimental effort are high-field heat capacity data of unprecedented accuracy for cuprates. The results of the study are both consistent with previous work on cuprate specific heat capacity in magnetic fields, but also show new results. For one particular doping of YBa2Cu3O6+δ, one new result is evidence of a phase transition brought about by the application of sufficiently high magnetic field. Below 10 teslas, the portion of the heat capacity attributed to the electronic excitations of the system follow a predicted functional form which is square-root-of-field and gives way abruptly to a linear dependence on magnetic field above 10 teslas. The change from square-root to linear-in-field heat capacity is a new discovery for cuprates, and, in fact, no similar linear-in-field heat capacity has ever been observed in these materials. For a particular composition of HgBa2CuO4+δ with superconducting transition temperature 72 kelvin, the electronic heat capacity increases with magnetic field in a generic way, but saturates to a constant value above 30 teslas. This saturation is expected for cuprates at sufficiently high magnetic fields, strong enough to induce a transition from a superconductor to a normal conductor. This study implies that 30 teslas is sufficient field to bring about such a transition for the particular composition. The presentation of detailed descriptions of experimental methodology, results, and conclusions follows.