From Friedel oscillations and Kondo effect to the pseudogap in cuprates

TL;DR

This paper reviews the historical development of NMR studies on the Kondo effect and pseudogap in cuprates, highlighting the role of impurities and electronic correlations in high-temperature superconductors.

Contribution

It provides an updated synthesis of NMR research on the pseudogap phenomenon and its implications for understanding competing orders in cuprates.

Findings

Pseudogap occurs below temperature T* and is linked to electronic correlations.

Robustness of the pseudogap to impurities suggests a competing order.

NMR is a valuable local probe for studying impurity effects in correlated materials.

Abstract

One of the great achievements performed by Jacques Friedel in France has been to promote experimental activities on the electronic properties in condensed matter physics, which has led him, among others, to be an active promoter of the up-rise of the "Laboratoire de Physique des Solides" (LPS). A selective presentation of some of the work achieved in my Nuclear Magnetic Resonance (NMR) group allows me to underline the importance of the initial choices done by J. Friedel for the LPS. I give here an updated review of the work done on Kondo effect in the 1970's which has allowed me to enlighten the real interest of the NMR probe as a local experimental technique to visualize the perturbation brought by impurities in normal metals. In a second part I summarize the early NMR experiments done in the high Tc cuprates which have given evidence for the existence of strong electronic correlations and established the occurrence of the "pseudogap", on which I have initially had some exchanges with J. Friedel. The pseudogap, occurring below a temperature T*, has during the last twenty years raised endless discussions about the incidence of correlations on superconductivity. Many researchers are still advocating today that the pseudogap is due to the existence of preformed pairs above Tc. Its robustness to impurities and disorder that we evidenced from the early days rather suggested that it results from a competing order of the correlated state. This is apparently better accepted nowadays, although various distinct ordered states detected below T* are not yet understood. I also underline the large contrast between the scientific behaviours which prevailed between these two periods of scientific activity. I suggest that these changes might have a negative incidence, in my opinion, on the research and education system, at least in our field of science.