We investigate the ground-state phase diagram of the one-dimensional "ionic" Hubbard model with an alternating periodic potential at half-filling by numerical diagonalization of finite systems with the Lanczos and density matrix renormalization group (DMRG) methods. We identify an insulatorinsulator phase transition from a band to a correlated insulator with simultaneous charge and bondcharge order. The transition point is characterized by the vanishing of the optical excitation gap while simultaneously the charge and spin gaps remain finite and equal. Indications for a possible second transition into a Mott-insulator phase are discussed.

We investigate the influence of a cyclic spin exchange Jcyc on the one-and two-triplet excitations of an undoped two-leg S=1/2 ladder, using the density matrix renormalization group (DMRG). The dispersion of the S=0 two-triplet bound state is dramatically reduced by Jcyc due to a repulsion between triplets on neighboring rungs. In (La,Ca)14Cu24O41 a consistent description of both the spin gap and the S=0 bound state requires Jcyc/J ⊥ ≈ 0.20-0.27 and J/J ⊥ ≈ 1.25-1.35. With these coupling ratios the recently developed dynamical DMRG yields an excellent description of the entire S=0 excitation spectrum observed in the optical conductivity, including the continuum contribution.PACS numbers: 75.10. Jm, 75.40.Gb, 75.40.Mg, 74.72.Jt, 75.30.Et The antiferromagnetic parent compounds of the high-T c cuprates are thought to be the best representatives of the two-dimensional S=1/2 square-lattice Heisenberg model. Understanding their magnetic properties is of utmost importance due to the intimate relationship of magnetic correlations and high-T c superconductivity. Recently, the question how to set up a minimal model which accounts for these magnetic properties has been readdressed. High-resolution inelastic neutron scattering experiments performed on the two-dimensional S=1/2 antiferromagnet La 2 CuO 4 [1] exhibit a magnon dispersion at the zone boundary which cannot be obtained within a nearest-neighbor Heisenberg model. It has been argued that the inclusion of a cyclic spin exchange term of about 20% would reproduce this dispersion [2]. This cyclic spin exchange emerges as a correction to the nearestneighbor Heisenberg Hamiltonian in order t 4 /U 3 from a t/U -expansion of the one-band Hubbard model [3]. It is expected to be the dominant correction within a more realistic three-band description of the CuO 2 -planes because there the cyclic permutation of 4 spins on a plaquette can take place without double occupancy [4,5]. Similar cyclic spin exchange processes have proven to be significant in other systems, e.g. in cuprate spin chains a ferromagnetic 2-spin cyclic exchange is responsible for the unusually strong exchange anisotropy [6].Cuprate spin ladders offer an alternative approach to decide about the existence and potential implications of a cyclic spin exchange term. They are composed of the same corner-sharing Cu-O plaquettes as the 2D cuprates, thus similar exchange couplings are expected for all spin products. In fact the inclusion of a cyclic spin exchange has been suggested in order to explain the smallness of the ladder spin gap observed in (La,Ca) 14 Cu 24 O 41 [7][8][9]. However, it is impossible to extract a unique set of coupling parameters or even to settle the existence of a cyclic spin exchange term from an analysis of the spin gap only. Here, the optical conductivity σ(ω) [10,11] can provide the missing information. Magnetic excitations can be observed in σ(ω) via the simultaneous excitation of a phonon [12,13]. In (La,Ca) 14 Cu 24 O 41 , two peaks in σ(ω) were identified as S=0 bound states of tw...

The ground state phase diagram of the 1D Hubbard chain with pair-hopping interaction is studied. The analysis of the model is performed using the continuum-limit field theory approach and exact diagonalization studies. At half-filling the phase diagram is shown to consist of two superconducting states with Cooper pair center-of-mass momentum Q = 0 (BCS-η0 phase) and Q = π (ηπ-phase) and four insulating phases corresponding to the Mott antiferromagnet, the Peierls dimerized phase, the charge-density-wave (CDW) insulator as well as an unconventional insulating phase characterized by the coexistence of a CDW and a bond-located staggered magnetization. Away from half-filling the phase diagram consists of the superconducting BCS-η0 and ηπ phases and the metallic Luttingerliquid phase. The BCS-η0 phase exhibits smooth crossover from a weak-coupling BCS type to a strong coupling local-pair regime. The ηπ phase shows properties of the doublon (zero size Cooper pair) superconductor with Cooper pair center-of-mass momentum Q = π. The transition into the ηπ-paired state corresponds to an abrupt change in the groundstate structure. After the transition the conduction band is completely destroyed and a new ηπ-pair band corresponding to the strongly correlated doublon motion is created.

We study transport through one or two ultrasmall quantum dots with discrete energy levels to which a time-dependent field is applied (e.g., microwaves). The AC field causes photon-assisted tunneling and also transitions between discrete energy levels of the dot. We treat the problem by introducing a generalization of the rotating-wave approximation to arbitrarily many levels. We calculate the dc-current through one dot and find satisfactory agreement with recent experiments by Oosterkamp et al. [Phys. Rev. Lett. 78, 1536(1997]. In addition, we propose a novel electron pump consisting of two serially coupled single-level quantum dots with a time-dependent interdot barrier.

PACS. 73.23.Ra -Persistent currents. PACS. 71.10.Fd -Lattice fermion models (Hubbard model, etc.).Abstract. -We show that disorder increases the persistent current of a half-filled onedimensional Hubbard-Anderson ring at strong interaction. This unexpected effect results from a perturbative expansion starting from the strongly interacting Mott insulator ground state. The analytical result is confirmed and extended by numerical calculations.