Spin-1 chain doped with mobile S=12 fermions.pdfVIP

a r X i v : c o n d - m a t / 9 9 1 2 2 7 3 v 1 1 5 D e c 1 9 9 9 Spin-1 chain doped with mobile S = 1/2 fermions Beat Ammon? and Masatoshi Imada? ISSP, University of Tokyo, 7-22-1 Roppongi, Minato-ku, Tokyo 106, Japan We investigate doping of a two-orbital chain with mobile S = 1/2 fermions as a valid model for Y2?xCaxBaNiO5. 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. Since Haldane’s famous conjecture about the existence of a spin-gap in one-dimensinal integer Heisenberg (HB) spin-chains [1], the intriguing physics of S = 1 chains and S = 1/2 ladders has been investigated in numer- ous theoretical and experimental studies and especially hole doping of a spin-liquid is of great importance for the Mott-transition in the context of high-TC supercon- ductivity. However, doping of a Haldane S = 1 chain with mobile holes has only recently been realized in the material Y2BaNiO5 [2]. In the undoped case, the two active Ni2+ orbitals are coupled by a strong fm Hund’s rule coupling JH , giving an almost ideal S = 1 HB chain with a large spin gap of ?S=1 ≈ 100K and very weak inter-chain interactions. By substituting Ca2+ by Y3+, mobile holes can be introduced in the chain. The most remarkable experimental features are a reduction of the resistivity ρdc by several orders of magnitude, and the creation of states with S between 1 and 3/2 per impu- rity inside the Haldane gap [3]. Further the temperature dependence of the resistivity no longer follows thermal activation over a barrier. While doping of S = 1 HB chains by stati