Triplet superconductivity and interference commensurate oscillations in the 1D molecular organic conductor tetramethyltetraselenafulvalene perchlorate

The molecular organic conductor (TMTSF)2ClO4 exhibits novel electronic phenomena in each of its normal metal, antiferromagnetic insulating and superconducting phases. In the superconducting state, experimental evidence has been building over the last decade for an unconventional "spin triplet" pairing state. However, in spite of a quarter century of efforts, no reliable experiments have been performed on tunneling into this superconductor, until now. We have measured the conductance across a barrier between two branches of a natural bicrystal, and find an energy gap of 2Delta = 4.3 kBTc, 20% higher than the standard BCS value of 3.52 kBTc. This is the first direct measurement of Delta in this material. Of more importance, we have observed a large zero-bias conductance peak, which we interpret as indicative of a mid-gap, Andreev bound state at the interface of two spin triplet superconductors having px symmetry. Finally, we have measured the upper critical magnetic field Hc2 along the a-axis, and use this to extract critical information about the order parameter d-vector in this triplet superconductor. In the normal state, three distinct angular magnetoresistance oscillation (AMRO) effects have been discovered in recent years, one for each principal crystal plane. The third of these was initially thought to be a mere projection of the first two. However, a very recent theoretical analysis suggested a different origin, involving a unique "interference commensurate" effect. We have made the first direct test of this conjecture, and find substantial support in its favor, via detailed measurements of the angle and field dependence of the magnetoresistance of (TMTSF)2ClO4 at high magnetic fields and ultralow temperatures.