Theory of spin inelastic tunneling spectroscopy for superconductor-superconductor and superconductor-metal junctions

TL;DR

This paper develops a theoretical framework for understanding spin inelastic tunneling spectroscopy in superconductor-superconductor and superconductor-metal junctions, explaining experimental observations and predicting effects of various parameters.

Contribution

It introduces a comprehensive theory linking Cooper pair correlations to local spin anisotropy and conductance spectra, with detailed analysis of different junction types and external influences.

Findings

Finite uniaxial anisotropy field affects local spin in superconducting junctions.

Additional conductance peaks arise from excitations between higher-energy spin states.

Temperature and magnetic field influence in-gap transitions and spin excitation spectra.

Abstract

We address the tunneling conductance and spin inelastic tunneling spectroscopy of localized paramagnetic moments in a superconducting environment, pertaining to recent measurements on Fe-octaethylporphyrin-chloride using superconducting scanning tunneling microscopy. We demonstrate that the Cooper pair correlations in the tip and substrate generate a finite uniaxial anisotropy field acting on the local spin moment, and we argue that this field may be a source for the observed changes in the conductance spectrum for decreasing distance between the scanning tunneling tip and the local magnetic moment. We make a side-by-side comparison between the superconductor-superconductor junction and normal-metal--superconductor junction, and find qualitative agreement between the two setups while quantitative differences become explicit. When simulating the effects of electron pumping, we obtain additional peaks in the conductance spectrum that can be attributed to excitations between higher-energy spin states. The transverse anisotropy field couples basis states of the local spin which opens for transitions between spin states that are otherwise forbidden by conservation of angular momentum. Finally, we explore the influences of temperature, which tend to enable in-gap transitions, and an external magnetic field, which enables deeper studies of the spin excitation spectrum. We especially notice the appearance of a low and high excitation peak on each side of the main coherence peak as an imprint of transitions between the Zeeman split ground states.