We investigate the superfluid-Mott-insulator quantum phase transition of spin-1 bosons in an optical lattice created by pairs of counterpropagating linearly polarized laser beams, driving an Fg = 1 to Fe = 1 internal atomic transition. The whole parameter space of the resulting two-component Bose-Hubbard model is studied. We find that the phase transition is not always second order as in the case of spinless bosons, but can be first order in certain regions of the parameter space. The calculations are done in the mean-field approximation by means of exact numerical diagonalization as well as within the framework of perturbaton theory. PACS numbers: 03.75.Lm,03.75.Mn,71.35.Lk The superfluid-Mott-insulator quantum phase transition (SMQPT) of spinless bosons in periodic lattices is a secondorder transion, which is characterized by a continuous variation of the order parameter ψ from ψ = 0 (superfluid phase) to ψ = 0 (Mott-insulator phase) if the amplitude of the lattice potential increases [1, 2]. In our recent paper [3], we have shown that the SMQPT in a system of spin-1 bosons can be first order as well [4]. By means of numerical calculations within the framework of the mean-field theory, it was found that in the case of 23 Na the SMQPT is second order if the number of atoms per lattice site n = 1, 3, and it is first order for n = 2. In the case of 87 Rb, the SMQPT was found to be second order for n = 1, 2, 3. In the present work, we continue the study of Ref. [3]. The main purpose is to investigate the whole parameter space of spin-1 bosons and to find the regions where the SMQPT is first and second order for arbitrary n.We consider spin-1 neutral polarizable bosons, possessing three Zeeman-degenerate internal ground and excited electronic states characterized by the magnetic quantum number m = 0, ±1 (F g = F e = 1) in a d-dimensional (d = 1, 2, 3) optical lattice. The lattice is assumed to be created by d pairs of counterpropagating linearly polarized laser waves running in d orthogonal directions and having different frequencies in different directions. The beams propagating along the 3-axis, which is chosen to be a quantization axis, are polarized along the 1or 2-axis, and the beams propagating along the 1and 2-axis are polarized along the 2and 1-axis, respectively.The running laser waves form left-and rightpolarized standing waves with Rabi frequencies Ω ν (r ν ) = Ω 0ν cos(k L r ν ), which couple internal ground and excited states by V -and Λtransitions. In order to avoid decoherence due to spontaneous emission, the detunings ∆ i must satisfy the conditions |∆ ν | ≫ γ, where γ is the spontaneous emission rate. If the laser intensities and the detunings are chosen in such a manner that Ω 2 0ν /∆ ν = Ω 2 0 /∆, ν = 1, . . . , d, the laser potential acting on the atomic ground states is given by the matrixwhich determines the isotropic lattice potential with the pe-riod π/k L and at the same time couples the atomic ground states with m = ±1. In the case of red detuning ∆ < 0, the system is described by the tw...

Nonadiabaticity in adsorption on metal surfaces gives rise to a number of measurable effects such as chemicurrents and exoelectron emission. Here we present a quantitative theory of chemicurrents on the basis of ground-state density functional theory ͑DFT͒ calculations of the effective electronic potential and the Kohn-Sham band structure. Excitation probabilities are calculated both for electron-hole pairs and for electrons and holes separately from first-order time-dependent perturbation theory. This is accomplished by evaluating the matrix elements ͑between Kohn-Sham states͒ of the rate of change of the effective electronic potential between subsequent ͑static͒ DFT calculations. Our approach is related to the theory of electronic friction, but allows for direct access to the excitation spectra. The method is applied to adsorption of atomic hydrogen isotopes on the Al͑111͒ surface. The results are compatible with the available experimental data ͑for noble metal surfaces͒; in particular, the observed isotope effect in H versus D adsorption is described by the present theory. Moreover, the results are in qualitative agreement with computationally elaborate calculations of the full dynamics within time-dependent density functional theory, with the notable exception of effects due to the spin dynamics. Being a perturbational approach, the method proposed here is simple enough to be applied to a wide class of adsorbates and surfaces, while at the same time allowing us to extract system-specific information.

We determine the leading shift of the Bose-Einstein condensation temperature for an ultracold dilute atomic gas in a harmonic trap due to weak disorder by treating both a Gaussian and a Lorentzian spatial correlation for the quenched disorder potential. Increasing the correlation length from values much smaller than the geometric mean of the trap scale and the mean particle distance to much larger values leads first to an increase of the positive shift to a maximum at this critical length scale and then to a decrease.

The nonadiabatic response of the electronic system during growth of Mg films is investigated both experimentally by measuring chemicurrents in Mg/ p-Si͑001͒ Schottky diodes, and theoretically by time-dependent perturbation theory applied to first-principles electronic-structure calculations. Reverse currents are detected in the diodes when they are exposed to thermally evaporated Mg atoms. Dissipation of condensation energy to the electronic system as well as absorption of infrared photons due to heat radiation are the current-generating mechanisms. They can be distinguished by studying the dependence of the currents on the evaporator temperature and on the Mg film thickness. In contrast to the photocurrents, the chemicurrent is proportional to the Mg atom flux as it reproduces the enthalpy of Mg sublimation in an Arrhenius diagram. Independent measurements of photocurrents by use of an empty evaporator as a source of heat radiation provide further evidence for a chemicurrent contribution to the overall signal. The presence of chemicurrents in Mg epitaxy is further supported by simulations of monolayer growth and calculations of the pertinent rates for nonadiabatic electronic transitions in Mg adsorption. The simulations show that the grown surface is atomically rough with many step and kink sites. Adsorption at these sites is sufficiently exothermic to induce energetic electron-hole pairs that give rise to a detectable current across the Schottky barrier of the diode. The calculated spectra of the excited electrons and holes are found to display high-energy tails above 0.4 eV. While the contribution of the electronic channel to the dissipation of condensation energy is very small ͑less than 1%͒, the calculated probability for high-energy electronic excitations in Mg epitaxy is compatible with the chemicurrent contribution extracted from the experimental data.

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