Underdoped superconducting cuprates as topological superconductors

Superconductivity in copper oxide (cuprate) high-transition-temperature superconductors follows from the chemical doping of an antiferromagnetic insulating state. The consensus that the wavefunction of the superconducting carrier, the Cooper pair, has d(x2-y2) symmetry(1,2) has long been reached. This pairing symmetry implies the existence of nodes in the superconducting energy gap. Recently, a series of angle-resolved photoemission spectroscopy experiments(3-9) have revealed that deeply underdoped cuprates exhibit a particle-hole symmetric(3) superconducting-like energy gap at the momentum-space locations where the d(x2-y2) gap nodes are expected. Here we discuss the possibility that this phenomenon is caused by a fully gapped topological superconducting state that coexists with the antiferromagnetic order. If experimentally confirmed, this result will completely change our view of how exactly the high-temperature superconductivity state evolves from the insulating antiferromagnet.