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We investigated the energy gap associated with the hidden order (HO) phase and the Gruneisen ratio in the URu_2-xRe_xSi₂system using a combination of thermal expansion coefficient and specific heat measurements. As the HO phase transition is suppressed to lower temperature, the ratio between the energy gap and the HO transition temperature decreases by three-fold. This rapid suppression of the energy gap potentially leads to a scenario of a "gapless" HO state, in which the energy gap of the HO phase vanishes before the HO transition temperature is suppressed to 0 K. We also investigated the Gruneisen ratio in the vicinity of the Re substituent composition where the HO is suppressed. The Gruneisen ratio shows divergent behavior at x = 0.12 and 0.15, providing evidence for the existence of a quantum critical point, which is consistent with our hypothesis of a "gapless" HO state.
Research at UCSD was supported by DOE under Grant No. DEFG02-04-ER46105, NSF under Grant No. DMR 1206553, and NNSA through DOE under Grant No. DE-NA0002909.