Spin Dynamics of Hole DopedY2−xCaxBaNiO5

Abstract

We propose an electronic model for the recently discovered hole doped compound Y2−xCaxBaNiO5. From a multiband Hamiltonian with oxygen and nickel orbitals, a one band model is derived. Holes are described using Zhang-Rice-like S=1/2 states at the nickels propagating on a S=1 spin chain. Using numerical techniques to calculate the dynamical spin structure factor S(q, ω) in a realistic regime of couplings, spectral weight in the Haldane gap is observed in agreement with neutron scattering data. Low energy states with S=3/2 appear in the model. Several predictions are made to test these ideas. 75.25.+z, 75.50.Ee, 75.10.Jm Spin-liquid ground states in Heisenberg S=1 chains and S=1/2 ladders have been studied theoretically as paradigms of disordered nonclassical systems. [1,2] These spin models can be physically realized in several compounds. An important issue is the effect of hole doping on these systems. Theoretical studies of doped S=1/2 ladders have shown that the spin gap survives in the presence of holes, and that the spin-gapped phase is favorable for superconductivity.[2] Behavior indicating a spin gap has also been observed experimentally in some underdoped high-Tc cuprates. However, the case of the doped S=1 chains has been considered only very recently in the context of the S=1 metal oxide Y 2 BaNiO 5 .[3] Lightly doping this compound with Ca introduces hole carriers in the chains. Two remarkable experimental features were observed upon doping, namely (i) the reduction of the resistivity ρ dc by several orders of magnitude leading to a one dimensional conductor, and (ii) the creation of states inside the Haldane gap as revealed by inelastic neutron scattering (INS) data. [4] To understand these in-gap states, spin systems with site or bond impurities have been recently proposed. [5] Holes are assumed to be so strongly localized that their mobility is neglected. In another approach dynamics was provided to the holes [6] but the Ni-Ni exchange J was assumed larger than both the hole hopping amplitude and the short-bond NiO exchange. While studying these limits is instructive since gap states are produced, the drastic reduction in ρ dc observed experimentally upon doping [4] suggests that holes are likely mobile over several lattice spacings. [7] Thus, in this paper we propose a new model for Y 2−x Ca x BaNiO 5 with fully mobile S=1/2 holes interacting with S=1 spins, which is studied using realistic values for the couplings.In Ni ++ surrounded by oxygens, d 3z 2 −r 2 and d x 2 −y 2 are the two active orbitals if deviations from the perfect octahedral symmetry in NiO 6 are considered.[8] Then, as a Hamiltonian for the Ni − O chains in the hole notation we proposeat the Ni (O) sites, and ∆ is the charge-transfer energy. d iσα are hole operators corresponding to a Ni site, spin σ and orbital α, while p jσ are oxygen hole operators. The last term in Eq.(1) is a f erromagnetic coupling between the Ni holes on different orbitals (using S iα = d † iα σd iα /2), which enforces Hund's rule. This term is important...

Effect of the hole doping into the Haldane-gap state on the one-dimensional S = 1 t−J model

“…The doping causes the reduction of resistivity and creates states within the Haldane gap [17]. There are several theoretical proposals to describe such experimental results [18,19,20]. These proposals are based upon some model Hamiltonians in which each hole is regarded to have the inner freedom of the spin S = 1/2, and propagates along an S = 1 spin chain.…”

Electronic Hamiltonian for transition-metal oxide compounds

“…For charge-transfer compounds, the absence of an e g electron can be considered as caused by the Zhang-Rice singlet formation between oxygen and d electrons. [11] Thus, our results are not restricted to Mott-Hubbard compounds but are valid for transition metals in general in arbitrary dimensions, and contain as a special case the well-known t-J model widely used for cuprates.…”

“…Actually, the Hamiltonian is discussed for an arbitrary spin S. The ideas followed in this paper are a generalization of the calculation recently presented for NiO compounds, where "holes" doped into a S=1 background carry S=1/2 and they move following nontrivial hopping processes. [11] At large S, the model described here contains a complex effective coupling for electrons whose spin is aligned with the core spins. These electrons acquire a phase when they move in closed loops, an effect not taken into account in previous literature for Mn-oxides.…”

Doped Heisenberg chains: Spin-S generalizations of the supersymmetric t-J model

“…To obtain an effective lattice model on the 4S + 1 dimensional local Hilbertspace one has to eliminate the other allowed spin configurations in a perturbative analysis for J H ≫ t [4,5]. To leading order in J H the effective Hamiltonian of the resulting model is simply the projection of (1.1) onto the states listed above.…”

“…Hamiltonian operators of this type have been used as a starting point for studies the phase diagram of doped transition metal oxides by numerical diagonalization of small clusters [1,5,6].…”