Imaging The Realm Of The Strongly Correlated: Visualizing Heavy Fermion Formation And The Impact Of Kondo Holes In Uru2Si2

Abstract

The methods of spectroscopic imaging scanning tunneling microscopy (SI-STM) are used to study the physics of heavy fermion compounds. The representative sample, URu2 Si2 , notable for its elusive 'hidden' order state below T 0 = 17.5K , exhibits the spectroscopic spatial image of a Kondo lattice via the Fano lattice observed through conductance measurements on the Si-terminated surface. The periodic array of Fano spectra observed above T 0 describe a system of manybody states commensurate with the U-atom matrix. Descending into the 'hidden' order phase these spectra develop a soft gap and the local density of states on the U-terminated surface show a simultaneous redistribution of spectral weight near the chemical potential. At the same time that spatial signatures accompany the onset of the 'hidden' order, the momentum space structure shows the splitting of a light band, crossing the chemical potential at Q* [ALMOST EQUAL TO] 0.3(2[pi]/a0 ) into two new heavy bands in splendid accord with Kondo lattice theory. The ensuing effective mass of m* [ALMOST EQUAL TO] 30me , is in accord with bulk thermodynamic measurements demonstrating that most if not all of the density of states about the chemical potential is contained in the newly formed heavy bands. The exact relationship between the 'hidden' order and the formation of a heavy fermion electronic structure is most likely a bootstrap effect and further investigations are needed to understand the link. Having established the robust heavy fermion character of URu2 Si2 at low temperature, the first known spectroscopic visualization studies of the effect of Kondo holes is made. The consequence of removing local moments, which contribute to the formation of heavy quasiparticles, achieved by replacing a small number of U-atoms with Th-atoms, produces spatial ripples in the hybridization strength between the Fermi sea and the f -states. Astonishingly, the period of oscillations is not associated with any wavevector on the Fermi surface of the heavy state, but rather with the Fermi wavevector Q* of the unhybridized electronic structure. Calculations of the Kondo hole response of an Anderson lattice model directly confirm these observations and further support that URu2 Si2 exhibits an almost conventional behavior of a heavy fermion material.