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

ABSTRACT

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.

KEYWORDS:

• Hidden order
• URu${}_{2-x}$Re${}_x$Si${}_2$
• gapless
• quantum critical point
• Grüneisen ratio

Acknowledgments

The authors acknowledge Quantum Design, Inc. for the development of the dilatometer option which was used for the thermal expansion measurements above 1.8 K. Identification of commercial equipment does not imply endorsement by NIST.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

Research at UCSD was supported by the US Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering, under Grant No. DEFG02-04-ER46105 (single crystal growth and characterization) and the National Nuclear Security Administration under the Stewardship Science Academic Alliance Program through the US Department of Energy (DOE) under Grant No. DE-NA0002909 (specific heat and thermal expansion measurements). The work at Occidental College was supported by the National Science Foundation under Grant No. DMR-1408598 (low-temperature thermal expansion measurements). The research at KSU was supported by the National Science Foundation under Grant No. DMR-1505826 (low-temperature specific heat measurements).