Laser-excited ultrahigh-resolution photoemission spectroscopy of NaxCoO2.yH2O:Evidence for pseudogap formation

TL;DR

This study uses laser-excited ultrahigh-resolution photoemission spectroscopy to reveal pseudogap formation in Na0.35CoO2-based layered cobaltate superconductors, indicating complex electronic behavior beyond simple superconductivity.

Contribution

It provides direct spectroscopic evidence of pseudogap formation in Na0.35CoO2 compounds, highlighting a larger energy scale than the superconducting gap and suggesting competing orders.

Findings

Pseudogap with ~20 meV energy scale observed

Pseudogap absent in Na0.7CoO2

Implications for competing order parameters in superconductivity

Abstract

We have studied the temperature-dependent electronic structure near the Fermi level (EF) of the layered cobaltate superconductor, Na0.35CoO2.1.3H2O, and related materials, using laser-excited ultrahigh-resolution photoemission spectroscopy. We observe the formation of a pseudogap with an energy scale of ~ 20 meV in Na0.35CoO2.1.3H2O and Na0.35CoO2.0.7H2O, which is clearly absent in Na0.7CoO2. The energy scale of the pseudogap is larger than the expected value for the superconducting gap, suggesting an additional competing order parameter at low temperatures. We discuss implications of the pseudogap in relation to available transport and magnetic susceptibility results.