Possibility of <i>d + is</i> superconductivity in <i>t-J</i> model: Implications for cuprate high-T_c superconductors

Early experiments on high-T_c cuprate superconducors found that the superconducting state in these systems do not break time reversal symmetry, and, they have Cooper pairs with predominantly d-wave pairing symmetry. With the improved experimental techniques, the applicability of these findings over the entire superconducting domain have come to be questioned. Recent ARPES studies provide substantial evidence for node-less superconductivity in cuprates at very low doping. At the same time, convincing evidence for Kerr rotation coinciding with the onset of charge density wave ordering has been provided by Karapetyan et al. Here, we present results demonstrating that, within the slave-boson formulation of the t-J model, the d-wave superconductor (d-SC) is unstable at low doping to its anti-symmetric phase mode fluctuations when the effect of fluctuations is treated self-consistently. We then show that this instability gives way to a time reversal symmetry broken d + is-SC in the underdoped region. Finally, we discuss how this physics might be relevant for understanding the recent experiments on cuprates described above.