Thermal Conductivity of Superconducting(TMTSF)2ClO4: Evidence for a Nodeless Gap

We report on the first measurements of thermal conductivity in the superconducting state of $(\mathrm{TMTSF}{)}_2{\mathrm{ClO}}_4$. The electronic contribution to heat transport is found to decrease rapidly below $T_c$, indicating the absence of low-energy electronic excitations. We argue that this result provides strong evidence for a nodeless superconducting gap function, but does not exclude a possible unconventional order parameter.