1. Condensed-Matter Physics & Materials Science Seminar

    "The Kondo effect from an STM perspective:Comparison between metals and insulators"

    Presented by Steffen Wirth, Max-Planck-Institute for Chemical Physics of Solids Dresden, Germany

    Wednesday, January 29, 2014, 1:30 pm
    ISB Bldg. 734, Conf. Room 201 (upstairs)

    Hosted by: Meigan Aronson

    Hybridization is a fundamental concept in strongly correlated electron physics. In heavy fermion metals, it may result in the generation of low-energy scales that can give rise to quantum criticality and unconventional superconductivity. We discuss the effects of hybridization and of the Kondo interaction as probed by tunneling and magnetotransport studies, and focus on the heavy fermion metal YbRh2Si2 and the intermediate valence insulator SmB6. We investigated YbRh2Si2 and its doped counterparts Yb(Rh1��'xMx)2Si2. Hints toward the existence of a quantum critical point are discussed in terms of an antiferromagnetic instability and the Kondo break-down of the heavy quasiparticles [1,2]. Recent Scanning Tunneling Microscopy (STM) and Spectroscopy (STS) studies identified a hybridization-induced gap-like feature of the tunneling conductance as well as crystal field excitations [3]. Here we focus on the evolution of the signatures of the Kondo lattice. While the Kondo lattice starts forming already at the single-ion Kondo temperature it dominates the material properties only at much lower temperatures. These findings by STS are augmented by band structure calculations and additional measurements [4,5]. Hybridization also plays a decisive role in the low temperature properties of the intermediate valence system SmB6. The hybridization gap, inter-multiplet transitions and possible crystal field excitations are observed by STS. The temperature evolution of these spectra again points toward the Kondo effect being at play. STS conducted on non-reconstructed surfaces gave results in excellent agreement with expectations for a Fano resonance. The impact of surface properties and reconstructions is discussed, also in relation to the recent proposal of a topologicall