# Single-gap superconductivity and dome of superfluid density in Nb-doped SrTiO3

## TL;DR

This study investigates the superconducting properties of Nb-doped SrTiO$_3$ across its doping range, revealing a single-gap superconducting state and a dome-shaped superfluid density, with detailed microwave measurements down to very low temperatures.

## Contribution

It provides the first detailed microwave optical response analysis of Nb-doped SrTiO$_3$ across its superconducting dome, demonstrating single-gap behavior and the dome-shaped evolution of key superconducting parameters.

## Key findings

- Single-gap electrodynamics despite multiple bands.
- Gap homogenization over the Fermi surface.
- Superfluid density exhibits a dome shape with doping.

## Abstract

SrTiO$_3$ exhibits a superconducting dome upon doping with Nb, with a maximum critical temperature \mbox{$T_\mathrm{c} \approx 0.4$~K}. Using microwave stripline resonators at frequencies from 2 to 23~GHz and temperatures down to 0.02~K, we probe the low-energy optical response of superconducting SrTiO$_3$ with charge carrier concentration from 0.3 to $2.2\times 10^{20}$~cm$^{-3}$, covering the majority of the superconducting dome. We find single-gap electrodynamics even though several electronic bands are superconducting. This is explained by a single energy gap $2\Delta$ due to gap homogenization over the Fermi surface consistent with the low level of defect scattering in Nb-doped SrTiO$_3$. Furthermore, we determine $T_\mathrm{c}$, $2\Delta$, and the superfluid density as a function of charge carrier concentration, and all three quantities exhibit the characteristic dome shape.