Multiband behavior and non-metallic low-temperature state of K$_{0.50}$Na$_{0.24}$Fe$_{1.52}$Se$_{2}$

TL;DR

This study reveals multiband transport and an insulating low-temperature state in a superconducting iron selenide, highlighting a superconductor-insulator transition linked to nanoscale phase separation.

Contribution

It provides evidence for multiband behavior and an insulating normal state in K$_{0.50}$Na$_{0.24}$Fe$_{1.52}$Se$_{2}$, with insights into the superconductor-insulator transition.

Findings

Multiband transport behavior confirmed.

Insulating logarithmic temperature dependence at low temperatures.

Superconductor-insulator transition related to nanoscale phase separation.

Abstract

We report evidence for multiband transport and an insulating low-temperature normal state in superconducting K$_{0.50}$Na$_{0.24}$Fe$_{1.52}$Se$_{2}$ with $T_{c}\approx 20$ K. The temperature-dependent upper critical field, $H_{c2}$, is well described by a two-band BCS model. The normal-state resistance, accessible at low temperatures only in pulsed magnetic fields, shows an insulating logarithmic temperature dependence as $T \rightarrow 0$ after superconductivity is suppressed. This is similar as for high-$T_{c}$ copper oxides and granular type-I superconductors, suggesting that the superconductor-insulator transition observed in high magnetic fields is related to intrinsic nanoscale phase separation.