The low-field phase of the organic superconductor $(\mathrm{TMTSF}{)}_2{\mathrm{ClO}}_4$ is studied by muon-spin rotation. The zero temperature limit of the magnetic penetration depth within the TMTSF layers is obtained to be ${\lambda }_{ab}(0)=0.86(2)\mu \mathrm{m}$. Temperature dependence of the muon-spin relaxation shows no indication of gap nodes on the Fermi surface nor of any spontaneous fields due to time-reversal-symmetry breaking. The weight of evidence suggests that the symmetry of this low-field phase is odd-frequency $p$-wave singlet, a novel example of odd-frequency pairing in a bulk superconductor.

• Received 4 October 2012

DOI:https://doi.org/10.1103/PhysRevLett.110.107005