Magnetic and thermodynamic properties of CuxTiSe2 single crystals

We present a detailed study of the phase diagram of copper-intercalated TiSe2 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 CuxTiSe2 samples from three different sources with various copper content x and superconducting critical temperatures Tc 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, Hc1. We then show that the temperature dependence of the superfluid density deduced from magnetic measurements (both Hc1 and TDO techniques) clearly suggests the existence of a small energy gap in the system, with a coupling strength 2Δs∼[2.4-2.8]kBTc, regardless of the copper content, in puzzling contradiction with specific-heat measurements which can be well described by one single large gap 2Δl∼[3.7-3.9]kBTc. Finally, our measurements reveal a nontrivial doping dependence of the condensation energy, which remains to be understood.