Universal Phase Diagram and Reentrant Superconductivity of $Bi$layer Hydrated Na$_x$CoO$_2\cdot y$H$_2$O

TL;DR

This study investigates the time-dependent superconductivity and magnetic properties of hydrated Na$_x$CoO$_2$·$y$H$_2$O, revealing a reentrant superconducting behavior and a universal phase diagram with coexisting magnetic order.

Contribution

It introduces a universal phase diagram for hydrated Na$_x$CoO$_2$·$y$H$_2$O, showing reentrant superconductivity and magnetic phases classified by NQR frequency, with new insights into magnetic coexistence.

Findings

Reentrant superconductivity observed with time dependence.

Universal phase diagram with two superconducting and one magnetic phase.

Magnetic critical slowing down at 5 K and coexistence of magnetic order with superconductivity.

Abstract

We report duration dependence of superconductivity in a 43 \% humidity atmosphere and $^{59}$Co nuclear quadrupole resonance (NQR) of polycrystalline samples of $bi$layer hydrated Na$_x$CoO$_2 \cdot y$H$_2$O. We found a reentrant behavior of superconductivity with respect to the time duration. We obtained the universal phase diagram where two superconducting phases and an in-between magnetic phase are classified by $^{59}$Co NQR frequency. Zero field $^{59}$Co nuclear spin-lattice relaxation rates $1/T_1$ indicated a magnetic critical slowing down at 5 K in the magnetic phase, excluding a charge density wave ordering, and an enhancement at and just above $T_\mathrm{c}$ in a new superconducting phase, suggesting a coexistence of magnetic ordering.