Thermodynamic signature of a magnetic-field-driven phase transition within the superconducting state of an underdoped cuprate

Abstract

More than a quarter century after the discovery of the high-temperature superconductor (HTS) YBa 2 Cu 3 O 6+(YBCO; ref.), studies continue to uncover complexity in its phase diagram. In addition to HTS and the pseudogap, there is growing evidence for multiple phases with boundaries which are functions of temperature (T), doping (p) and magnetic field. Here we report the low-temperature electronic specific heat (C elec) of YBa 2 Cu 3 O 6.43 and YBa 2 Cu 3 O 6.47 (p = 0.076 and 0.084) up to a magnetic field (H) of 34.5 T, a poorly understood region of the underdoped H-T-p phase space. We observe two regimes in the low-temperature limit: below a characteristic magnetic field H′ â ‰ 12-15 T, C elec /T obeys an expected H 1/2 behaviour; however, near H′ there is a sharp inflection followed by a linear-in-H behaviour. H′ rests deep within the superconducting phase and, thus, the linear-in-H behaviour is observed in the zero-resistance regime. In the limit of zero temperature, C elec /T is proportional to the zero-energy electronic density of states. At one of our dopings, the inflection is sharp only at lowest temperatures, and we thus conclude that this inflection is evidence of a magnetic-field-driven quantum phase transition.