Nearest-Neighbor Correlations in Hubbard Model

The Hubbard Hamiltonian is investigated by means of a variational trial wave function of Gutzwiller's type. The wave function includes nearest - neighbor correlations in an explicit form. To calculate density matrices the method of Kikuchi's pseudoensemble is used. The case of half-filled fermionic band carefully investigated in the limit of a large number of lattice sites. The ground state energy and correlation functions are determined for lattices with z=2,4 and 6 nearest neighbors.Comment: minor revisions (additional figure, fig2 is redrawn, et al.) 8 pages, 3 Post Script figures, RevTex; accepted to Phys.Lett.

Enhancement of bichromatic high-harmonic generation with a high-frequency field

Using a high-frequency field superposed to a linearly polarized bichromatic laser field composed by a wave with frequency $\omega$ and a wave with frequency $2\omega$, we show it is possible to enhance the intensity of a group of high harmonics in orders of magnitude. These harmonics have frequencies about 30% higher than the monochromatic-cutoff frequency, and, within the three-step-model framework, correspond to a set of electron trajectories for which tunneling ionization is strongly suppressed. Particular features in the observed enhancement suggest that the high-frequency field provides an additional mechanism for the electron to reach the continuum. This interpretation is supported by a time-frequency analysis of the harmonic yield. The additional high frequency field permits the control of this group of harmonics leaving all other sets of harmonics practically unchanged, which is an advantage over schemes involving only bichromatic fields.Comment: 6 pages RevTex, 5 figures (ps files), Changes in text, figures, references and equations include

Induced Rashba splitting of electronic states in monolayers of Au, Cu on a W 110 substrate

The paper sums up a theoretical and experimental investigation of the influence of the spin orbit coupling in W 110 on the spin structure of electronic states in deposited Au and Cu monolayers. Angle resolved photoemission spectroscopy reveals that in the case of monolayers of Au and Cu spin orbit split bands are formed in a surface projected gap of W 110 . Spin resolution shows that these states are spin polarized and that, therefore, the spin orbit splitting is of Rashba type. The states evolve from hybridization of W 5d, 6p derived states with the s, p states of the deposited metal. Interaction with Au and Cu shifts the original W 5d derived states from the edges toward the center of the surface projected gap. The size of the spin orbit splitting of the formed states does not correlate with the atomic number of the deposited metal and is even higher for Cu than for Au. These states can be described as W derived surface resonances modified by hybridization with the p, d states of the adsorbed metal. Our electronic structure calculations performed in the framework of the density functional theory correlate well with the experiment and demonstrate the crucial role of the W top layer for the spin orbit splitting. It is shown that the contributions of the spin orbit interaction from W and Au act in opposite directions which leads to a decrease of the resulting spin orbit splitting in the Au monolayer on W 110 . For the Cu monolayer with lower spin orbit interaction the resulting spin splitting is higher and mainly determined by the