Emergence of the Hidden Order State from the Fano Lattice

Electronic Structure of the Heavy-fermion Material URu₂Si₂

Mohammad H. Hamidian and Andrew R. Schmidt

J.C. Séamus Davis Lab, Cornell University

The heavy fermion compound URu₂Si₂ boasts a 25 year old mystery. Its ‘hidden order’ phase transition at T₀=17.5K has eluded the onslaught of theory and experiment to describe the complex underlying mechanism. Whether the transition is due to conventional ordering of k-space heavy electrons or to a change in hybridization of the r-space states at each magnetic-moment-contributing U atom is unknown. Addressing the problem requires a probe which can simultaneously measure the real space and momentum space structure, making spectroscopic imaging STM (SI-STM) the natural choice.

SI-STM studies of URu₂Si₂ above T₀ reveal the first images of the ‘Fano lattice’ electronic structure, the real-space spectroscopic manifestation of a periodic array of localized Kondo resonances at the U sites. Below T₀, however, a hybridization gap opens in the density of states. Quasiparticle interference imaging reveals a concurrent rapid splitting of a light symmetric k-space band to form two new heavy bands exhibiting momentum space anisotropy. Thus, the ‘hidden order’ state emerges directly from the Fano lattice electronic structure and exhibits characteristics of alterations in the hybridization of states at each U atom.