Superconductivity vs. Charge density wave in CuxTiSe2 single-crystals

    We present heat capacity measurements on a series of superconducting CuxTiSe2 single crystals with different Cu content down to 600 mK and up to 1 T performed by ac microcalorimetry. The samples cover a large portion of the phase diagram from an underdoped to a slightly overdoped region with an increasing superconducting critical temperature and the charge density wave (CDW) order gradually suppressed. The superconducting electronic heat capacity as a function of normalized temperature T/Tc shows no difference regardless of the concentration of copper, i.e., regardless of how much the CDW order is developed in the samples. The data analysis reveals consistently a single s-wave gap with an intermediate coupling strength 2Δ/kBTc ≈ 3.7 for all samples.

J. Kačmarčík, Z. Pribulová, V. Paľuchová, P. Szabó, P. Husaníková, G. Karapetrov, and P. Samuely:
Heat capacity of single-crystal CuxTiSe2 superconductors;
Physical Review B 88, 020507(R) (2013).

Figure 1: Superconducting electronic heat capacity of sample A (open symbols) and corresponding single-gap α-model fit (thick line), compared to that of NbS2 (solid symbols) and α-model fit corresponding to the presence of two energy gaps or one anisotropic gap (thin line). Upper inset: Angular dependence of Tc at 0.15 T (symbols) and a Ginzburgh-Landau fit for a single-gap superconductor (line). Lower inset: Deviation function—(Tc2/Tc2GL)2 for sample A (open symbols) and Hc2/Hc2GL)2 for NbS2 solid symbols); the line is a guide to the eye.

Figure 2: The temperature dependence ΔC/T of samples A, B, C, and D in a temperature scale normalized to the Tc of each sample. Inset: The phase diagram as proposed by Morosan et al. and the critical temperature of our samples with respect to their copper content (large symbols).