Quantum critical metamagnetism of Sr₃Ru₂O₇ under hydrostatic pressure

Using ac susceptibility, we have determined the pressure dependence of the metamagnetic critical endpoint temperature T* for a field applied in the ab plane in the itinerant metamagnet Sr3Ru2O7. We find that T* falls monotonically to zero as pressure increases, producing a quantum critical endpoint (QCEP) at P-c similar to 13.6 +/- 0.2 kbar. New features are observed near the QCEP-the slope of T* versus pressure changes at similar to 12.8 kbar, and weak subsidiary maxima appear on either side of the main susceptibility peak at pressures near P-c-indicating that some new physics comes into play near the QCEP. Clear signatures of a nematic phase, however, that were seen in field-angle tuning of T* are not observed. As T* is suppressed by pressure, the metamagnetic peak in the susceptibility remains sharp as a function of an applied magnetic field. As a function of temperature, however, the peak becomes broad with only a very weak maximum, suggesting that, near the QCEP, the uniform magnetization density is not the order parameter for the metamagnetic transition.