Spin-1 chain doped with mobile S=1/2 fermions

Abstract

We investigate doping of a two-orbital chain with mobile $S=1/2$ fermions as a valid model for $\rm Y_{2-x}Ca_xBaNiO_5$. The S=1 spins are stabilized by strong, ferromagnetic (fm) Hund's rule couplings. We calculate correlation functions and thermodynamic quantities by DMRG methods and find a new hierarchy of energy scales in the spin sector upon doping. Gapless spin-excitations are generated at a lower energy scale by interactions among itinerant polarons created by each hole and coexist with the larger scale of the gapful spin-liquid background of the S=1 chain accompanied by a finite string-order parameter.