Imaging the Fano Lattice to 'Hidden Order' Transition in URu2Si2

TL;DR

This study uses spectroscopic imaging to visualize the electronic structure of URu2Si2, revealing how the 'hidden order' transition involves changes in hybridization and heavy fermion states, not conventional density waves.

Contribution

It provides direct imaging evidence linking the 'hidden order' transition to hybridization changes and heavy fermion band formation in URu2Si2.

Findings

Revealed the Fano lattice structure above T_o

Observed a partial energy gap below T_o

Identified rapid splitting into heavy fermion bands

Abstract

Within a Kondo lattice, the strong hybridization between electrons localized in real space (r-space) and those delocalized in momentum-space (k-space) generates exotic electronic states called 'heavy fermions'. In URu2Si2 these effects begin at temperatures around 55K but they are suddenly altered by an unidentified electronic phase transition at To = 17.5 K. Whether this is conventional ordering of the k-space states, or a change in the hybridization of the r-space states at each U atom, is unknown. Here we use spectroscopic imaging scanning tunnelling microscopy (SI-STM) to image the evolution of URuSi2 electronic structure simultaneously in r-space and k-space. Above To, the 'Fano lattice' electronic structure predicted for Kondo screening of a magnetic lattice is revealed. Below To, a partial energy gap without any associated density-wave signatures emerges from this Fano lattice. Heavy-quasiparticle interference imaging within this gap reveals its cause as the rapid splitting below To of a light k-space band into two new heavy fermion bands. Thus, the URu2Si2 'hidden order' state emerges directly from the Fano lattice electronic structure and exhibits characteristics, not of a conventional density wave, but of sudden alterations in both the hybridization at each U atom and the associated heavy fermion states.