Theory of Point Contact Spectroscopy in Correlated Materials

TL;DR

This paper develops a microscopic theory for point-contact conductance in strongly correlated materials, linking conductance to the spectral function and explaining zero-bias peaks observed in experiments.

Contribution

It introduces a non-equilibrium microscopic model that relates conductance to the spectral function and clarifies conditions for zero-bias peaks in non-Fermi liquids.

Findings

Conductance proportional to the integrated spectral function.

Zero-bias conductance peaks can occur in non-Fermi liquids.

The theory explains experimental observations in iron-based superconductors.

Abstract

We develop a microscopic theory for the point-contact conductance between a metalic electrode and a strongly correlated material using the non-equilibrium Schwinger-Kadanoff-Baym-Keldysh formalism. We explicitly show that in the classical limit, contact size shorter than the scattering length of the system, the microscopic model can be reduced to an effective model with transfer matrix elements that conserve in-plane momentum. We find that the conductance $dI/dV$ is proportional to the {\it effective density of states}, that is, the integrated single-particle spectral function $A(\omega=eV)$ over the whole Brillouin zone. From this conclusion, we are able to establish the conditions under which a non-Fermi liquid metal exhibits a zero-bias peak in the conductance. This finding is discussed in the context of recent point-contact spectroscopy on the iron pnictides and chalcogenides which has exhibited a zero-bias conductance peak.