Coherence-incoherence crossover in the normal state of iron-oxypnictides and importance of the Hund's rule coupling

TL;DR

This paper investigates the anomalous normal state of iron-oxypnictide superconductors, highlighting the role of Hund's rule coupling in the coherence-incoherence crossover and its implications for high-temperature superconductivity.

Contribution

It demonstrates the significance of Hund's rule coupling in the normal state properties and estimates its strength, providing insight into the multiorbital electronic structure of iron-oxypnictides.

Findings

Normal state shows Curie Weiss susceptibility and linear resistivity at high T

Crossover temperature T* depends strongly on Hund's coupling J_Hund

Estimated J_Hund is 0.35-0.4 eV, influencing magnetic exchange ratios

Abstract

A new class of high temperature superconductors based on iron and arsenic was recently discovered, with superconducting transition temperature as high as 55 K. Here we show, using microscopic theory, that the normal state of the iron pnictides at high temperatures is highly anomalous, displaying a Curie Weiss susceptibility and a linear temperature dependence of the resistivity. Below a coherence scale T*, the resistivity sharply drops and susceptibility crosses over to Pauli-like temperature dependence. Remarkably, the coherence-incoherence crossover temperature is a very strong function of the strength of the Hund's rule coupling J_Hund. On the basis of the normal state properties, we estimate J_Hund to be 0.35-0.4 eV. In the atomic limit, this value of J_Hund leads to the critical ratio of the exchange constants J_1/J_2~2. While normal state incoherence is in common to all strongly correlated superconductors, the mechanism for emergence of the incoherent state in iron-oxypnictides, is unique due to its multiorbital electronic structure.