○ Date : 2015년 11월 13일(금) 오후 4:00
○ Place :  기초학문관 3층 창조존

○ Speaker : 이  현  정  박사님 (KAIST)
○ Topic : Kondo screening in mixed valence and heavy fermion compounds and the signatures of Nozieres’ exhaustion

○ Abstract: 

Whereas most rare-earth compounds with the stable valence show concomitant local magnetic moments which order magnetically at low temperature, the mixed-valence compounds have avoided local magnetism of the rare earths through the hybridization process between the degenerate valence states. To understand the relation of this hybridization with the Kondo effect, it is necessary to have a low energy Hamiltonian for the lattice problem with which one can answer questions like what determines the coherence scale in the Kondo problem and answer questions such as the exhaustion paradox raised by P. Nozieres [Ann. Phys. (Paris) 10, 19, (1985)].

To investigate the exhaustion problem in Kondo lattices, we examine the dynamical properties of the periodic Anderson model such as the local spectral density and the spin excitation spectrum within the dynamical mean-field theory using the numerical renormalization-group technique. Pursuing the high impurity-concentration and the strong coupling limit simultaneously, we find a transition between coherent and incoherent Kondo regime, characterized by closing of the hybridization gap and appearance of Fano resonance. The appearance of the Fano resonance is attributed to coherent valence fluctuation which leads to a polaron like depression of the density of states near the van Hove singularity in the conduction band. We discuss the competing mechanism between the two screening channels, i.e. spin and valence fluctuations, and the relevance to the quantum criticality exhibited in Yb based heavy fermion superconductors.