Abstract
We present a detailed study of the phase diagram of copper-intercalated single crystals, combining local Hall-probe magnetometry, tunnel diode oscillator technique (TDO), and specific-heat and angle-resolved photoemission spectroscopy measurements. A series of the samples from three different sources with various copper content and superconducting critical temperatures have been investigated. We first show that the vortex penetration mechanism is dominated by geometrical barriers enabling a precise determination of the lower critical field, . We then show that the temperature dependence of the superfluid density deduced from magnetic measurements (both and TDO techniques) clearly suggests the existence of a small energy gap in the system, with a coupling strength , regardless of the copper content, in puzzling contradiction with specific-heat measurements which can be well described by one single large gap . Finally, our measurements reveal a nontrivial doping dependence of the condensation energy, which remains to be understood.
- Received 30 January 2017
- Revised 28 March 2017
DOI:https://doi.org/10.1103/PhysRevB.95.174512
©2017 American Physical Society