The in-plane thermal conductivity $\kappa$ of the layered superconductor ${\mathrm{Cu}}_x{\mathrm{TiSe}}_2$ was measured down to temperatures as low as $T_c/40$, at $x=0.06$ near where the charge-density-wave order vanishes. The absence of a residual linear term at $T\to 0$ is strong evidence for conventional $s$-wave superconductivity in this system. This is further supported by the slow magnetic field dependence, also consistent with a single gap, of uniform magnitude across the Fermi surface. Comparison with the closely related material ${\mathrm{NbSe}}_2$, where the superconducting gap is 3 times larger on the Nb $4d$ band than on the Se $4p$ band, suggests that in ${\mathrm{Cu}}_{0.06}{\mathrm{TiSe}}_2$ the Se $4p$ band is below the Fermi level and Cu doping into the Ti $3d$ band is responsible for the superconductivity.

• Received 26 January 2007

DOI:https://doi.org/10.1103/PhysRevLett.99.107001