K. J. E. Vos

Theory of electron-hole asymmetry in doped CuO2 planes.

Consistent with experiment, for the hole-doped materials short-ranged incommensurate spin orderings are induced, whereas for the electron-doped system only commensurate spin correlations are found. Further, we propose that there is an important difference between the spatial distributions of mobile carriers for these two systems: for the hole-doped material the quasiparticles tend to stay far apart from one another, whereas for the electron-doped material we find tendencies consistent with the clustering of carriers, and possibly of low-energy fluctuations into an electronic phase-separated state. Phase separation in this material is consistent with the midgap states found by recent angle-resolved photoemission spectroscopy studies. Last, we propose the extrapolation of an approach based on the [ital t]-[ital t][prime]-[ital J] model to the hole-doped 123 system.

EXACT-DIAGONALIZATION DEMONSTRATION OF INCOMMENSURABILITY AND THE ASSOCIATED FERMI SURFACE FOR N HOLES IN THE T-J MODEL

We have calculated S(q) and the single particle distribution function for N holes in the t - J model on a non--square sqrt{8} X sqrt{32} 16--site lattice with periodic boundary conditions; we justify the use of this lattice in compariosn to those of having the full square symmetry of the bulk. This new cluster has a high density of vec k points along the diagonal of reciprocal space, viz. along k = (k,k). The results clearly demonstrate that when the single hole problem has a ground state with a system momentum of vec k = (pi/2,pi/2), the resulting ground state for N holes involves a shift of the peak of the systems structure factor away from the antiferromagnetic state. This shift effectively increases continuously with N. When the single hole problem has a ground state with a momentum that is not equal to k = (pi/2,pi/2), then the above--mentioned incommensurability for N holes is not found. The results for the incommensurate ground states can be understood in terms of rigid--band filling: the effective occupation of the single hole k = (pi/2,pi/2) states is demonstrated by the evaluation of the single particle momentum distribution function . Unlike many previous studies, we show that for the many hole ground state the occupied momentum states are indeed k = (+/- pi/2,+/- pi/2) states.

Magnetic Properties of Single-Layer Cuprates

The evolution of the magnetic properties of the insulating parent cuprate compounds as they are doped into the metallic and superconducting state continues to be an important area of research. Understanding this evolution and its relationship to the microscopic distribution of the doped holes may offer insights into interactions important in the superconducting materials. In addition, the nature of this evolution in strongly correlated electron systems is interesting in its own right. In this paper, we will review some of the progress made in attempting to address this issue in the La2CuO4-type and related systems.

Phase separation kinetics in La2CuO4+δ and inhomogeneous hole doping in the antiferromagnetic regime (0 <x < 0.02) of La2−xSr x CuO4

The diffusion of the excess oxygen during phase separation in La2CuO4+δ was studied using thermal history-dependent normal state magnetic susceptibilityϰ(T, t) measurements versus temperatureT and timet as a probe. A large thermal hysteresis ofϰ(T) was observed for La2CuO4.044 between data obtained after quenching to 5 K and then warming, and data obtained while or after slowly cooling from 300 K. A model for the excess oxygen diffusion is presented, from which theϰ(T, t) data yield aT-independent activation energy of 0.24(3) eV for the diffusion coefficient of the excess oxygen from 150 to 220 K. In related work, we have used139La NQR andμSR measurements to probe the antiferromagnetic (AF) region (x<0.02) of the La2−xSrxCuO4 system below the Néel temperatureTN(x), from which we extract the Cu+2 staggered magnetizationM†(x, T). M†(x, T=0), extrapolated from above 30 K, was successfully modeled with spin-wave theory, assuming that the doped holes are mobile and are situated in walls in the CuO2 plane which uncouple undoped AF domains; these domains are coupled to those in adjacent CuO2 planes. This agreement supports the previous hypothesis that microsegregation of the (mobile) doped holes into domain walls occurs above ∼30 K, consistent with the phenomenology of Emery and Kivelson. Below ∼30 K, an anomalous increase inM†(x, T) is observed, such thatM†(x, T=0) is nearly independent ofx. We interpret this effect as arising from localization of the doped holes below ∼30 K.

Spatial distribution of mobile holes in the t-J model.

We have calculated the hole-hole correlation function for two holes doped into a 5= 1/2 antiferromagnetic insulator described by the t Jmodel o-n a 16-site +8X ~32 lattice and a 24-site ~18X ~32 lattice using an exact diagonalization technique. We predict that the carriers described by this model, in the parameter range relevant to Cu02 planes, tend to stay as far apart as possible. This result is in direct contradiction to previously published exact diagonalization data [e.g., D. Poilblanc, Phys. Rev. B 49, 1477 (1994)].We suggest that if the carriers do tend to stay as far apart from one another as possible, then the electron momentum distribution function shows evidence for hole pockets at k=(~ m/2, ~ 7t/2), consistent with many recent studies of these models, including recent photoemission work.