Specific heat of URu 2 Si 2 in fields up to 42 T: Clues to the hidden order

The large $\ensuremath{\Delta}C$ observed at 17.5 K in ${\mathrm{URu}}_{2}{\mathrm{Si}}_{2}$ is inconsistent with the small, $0.04{\ensuremath{\mu}}_{B}$ moment measured for the antiferromagnetism observed starting (perhaps coincidentally) at the same temperature. We report measurements of this specific-heat transition, thought to be due to some hidden order, in magnetic fields between 24 and 42 T, i.e., through the field-region where three metamagnetic transitions are known to occur at 35.8, 37.3, and 39.4 T. The response of $\ensuremath{\Delta}C$ in single crystal ${\mathrm{URu}}_{2}{\mathrm{Si}}_{2}$ to magnetic field, which includes a change to $\ensuremath{\Delta}C$ being possibly associated with a first-order phase transition for high fields, is analyzed to shed further light on the possible explanations of this unknown ordering process. At fields above 35 T, a new high-field phase comes into being; the connection between this high-field phase revealed by the specific heat and earlier magnetization data is discussed.