The molecular Kondo effect occurs only when the magnetic impurity strongly hybridizes with conduction electrons at edge center sites of the Lieb lattice, and at the temperature range between the artificial strong coupling and the local moment regimes. | Communications in Physics, Vol. 28, No. 4 (2018), pp. 361-367 DOI: COMPETITION BETWEEN THE SOFT GAP AND THE MOLECULAR KONDO SINGLETS IN FLAT-BAND LATTICES DUONG-BO NGUYEN1,† , HONG-SON NGUYEN2 AND MINH-TIEN TRAN1,3 1 Graduate University of Science and Technology, Vietnam Academy of Science and Technology, Hanoi, Vietnam 2 Department of Occupational Safety and Health, Trade Union University, 169 Tay Son, Hanoi, Vietnam 3 Institute of Physics, Vietnam Academy of Science and Technology, Hanoi, Vietnam † E-mail: nguyenduongbo80@ Received 07 October 2018 Accepted for publication 04 November 2018 Published 15 December 2018 Abstract. The Kondo problem of a magnetic impurity embedded in the Lieb lattice is studied by the numerical renormalization group. The magnetic impurity hybridizes with conduction electrons from both the flat- and the soft-gap bands. We find a competition between the soft gap and the molecular Kondo singlet formations. The molecular Kondo effect occurs only when the magnetic impurity strongly hybridizes with conduction electrons at edge center sites of the Lieb lattice, and at the temperature range between the artificial strong coupling and the local moment regimes. Keywords: Kondo effect, numerical renormalization group, strongly correlated electron systems, flat band, soft gap. Classification numbers: , , , . I. INTRODUCTION Electron correlations in flat-band lattices have attracted a lot of research attention due to their special features. In flat-band lattices, the Coulomb interaction of electrons becomes dominant over the kinetic energy, and as a consequence intriguing phenomena such as the flat-band ferromagnetism or the molecular Kondo effect emerge [1,2]. When a magnetic impurity is coupled to conduction electrons from the flat band, it forms the molecular Kondo singlet with a single conduction electron and quenches all other conduction electrons in the Kondo singlet .
