Probing hidden Mott gap and incommensurate charge modulation on the polar surfaces of PdCrO$_2$

TL;DR

This study combines STM experiments and DMFT calculations to reveal a hidden Mott gap and incommensurate charge modulation on PdCrO$_2$ surfaces, advancing understanding of its correlated electronic properties.

Contribution

It provides the first direct observation of a hidden Mott gap and incommensurate charge modulation on PdCrO$_2$ surfaces, linking surface phenomena with strong electron correlations.

Findings

Detection of a gap-like feature on CrO$_2$-terminated surface due to strong correlations.

Observation of incommensurate charge modulation on well-ordered Pd surface.

Identification of nano-scale non-periodic domains on reconstructed Pd surface.

Abstract

Here we report a combined study of low-temperature scanning tunneling microscopy (STM) and dynamical mean-field theory (DMFT) on PdCrO$_2$, a delafossite metal with an antiferromagnetic order below ~37.5 K. First, on the CrO$_2$-terminated polar surface we detect a gap-like feature both below and above the N\'eel temperature. The DMFT calculations indicate that this gap is opened due to the strong correlations of Cr-3d electrons, suggesting the hidden Mott nature of the gap. Then, we observe two kinds of Pd-terminated polar surfaces. One is a well-ordered Pd surface with the Fermi-surface-nesting-induced incommensurate charge modulation, while the other one is a reconstructed Pd surface with the individual nano-scale non-periodic domain structures. On the well-ordered Pd surface, the interference between the incommensurate charge modulation and the atomic lattice forms the periodic moir\'e pattern. Our results provide important microscopic information for fully understanding the correlated electronic properties of this class of materials.