Rapid suppression of the energy gap and the possibility of a gapless hidden order state in URu_2−xRe_xSi₂

We investigated the energy gap associated with the hidden order (HO) phase and the Grüneisen ratio in the URu _2-x Re _x Si ₂ system 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 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 Grüneisen ratio in the vicinity of the Re substituent composition where the HO is suppressed. The Grüneisen ratio shows divergent behaviour 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.