Intrinsic Multiple Andreev Reflections in layered Th-doped Sm$_{1-x}$Th$_x$OFeAs

TL;DR

This study uses intrinsic multiple Andreev reflections spectroscopy to analyze layered Sm$_{1-x}$Th$_x$OFeAs, revealing detailed superconducting gap information and demonstrating the technique's effectiveness in different doping levels.

Contribution

It presents the application of IMARE spectroscopy to layered Th-doped Sm-1111, providing detailed measurements of superconducting gaps and demonstrating improved accuracy in gap determination.

Findings

Observation of subharmonic gap structures up to 4 SGS dips.

Accurate determination of superconducting gaps in optimally doped and underdoped samples.

Validation of IMARE spectroscopy as a precise tool for layered oxypnictide analysis.

Abstract

Layered oxypnictide Sm$_{1-x}$Th$_x$OFeAs (Sm-1111) is an ideal candidate to be probed by intrinsic multiple Andreev reflections effect (IMARE) spectroscopy. Using the classical "break-junction" technique, we formed ballistic Andreev arrays of identical S--n--S-contacts, where $S$ is superconductor, and $n$ is a layer of normal metal. For $T < T_C$, the I(V) curve shows an excess-current and a subharmonic gap structure (SGS): a set of sharp dI(V)/dV-dips at positions which depend on the superconducting gap value, the number of junctions in the array, and the natural subharmonic order, thus manifesting the effect of intrinsic multiple Andreev reflections. Here we present the I(V) and dI(V)/dV with up to 4 SGS dips for Andreev arrays formed in optimally doped Sm-1111 with critical temperatures $T_C \approx 49$\,K, as well as in underdoped samples with $T_C \approx 37$\,K. We show that a number of Andreev subharmonics facilitates the determination of the superconducting gap with a better accuracy.