We study the ground-state properties of spin-1/2 fermionic atoms confined in a one-dimensional optical superlattice with harmonic confinement by using the density-matrix renormalization group method. For this purpose, we consider an ionic Hubbard model that has superlattice potentials with two-site periodicity. We find that several different types of insulating regimes coexist even if the number of atoms at each site is not an integer, but its average within the unit cell is an integer or half integer. This is contrasted to the coexisting phase of the metallic and Mott-insulating regimes known for the ordinary Hubbard model in an optical lattice. The phase characteristics are elucidated by investigating the profiles of the atom density, the local density ͑spin͒ fluctuations, the double occupation probability, and the spin correlations in detail.

PHYSICAL REVIEW LETTERS 24APRIL1972elastic-constant data, 5 is very similar. We can make an estimate for the distance in energy that the second band lies below E ¥ at absolute zero from Schirber and Switendick's volume-dependent band calculation. 7 This gave a difference between the increase in energy of the second band at T and the energy of the other bands at L of 0.067 V with ~30 kbar. Assuming this difference is a measure of the rate at which the second band moves relative to E F , this equates -0.002 V to 1 kbar and puts the band ~0.012 V below the Fermi energy at absolute zero. This value is roughly consistent with the thermal behavior of the Knight shift and elastic-constant data which change in a temperature region centered around 80 K or ~ 0.007 V. This reasoning also indicates that the peak in the density of states is quite sharp (width at half-height of perhaps 0.005 V) since T c drops appreciably between 6 and 8 kbar.In conclusion, we feel that the combination of the unusual pressure dependence of the thirdzone Fermi-surface cross sections and the almost discontinuous pressure dependence of the superconducting transition temperature argue very strongly for a pressure-induced electron transition in AuGag. Such a transition supports both the experimental inference from the studies of AuGag, AuALj, and Aula^ series a*id the theoretical predictions of Schirber and Switendick's volume-dependent band model. We further propose that the anomalous temperature dependence of magnetic properties of AuGa^j stems from a purely thermal depopulation of the second band, thus requiring no explicit phonon effects such as suggested earlier. 7 The technical assistance of R. L. White is gratefully acknowledged, as are helpful discussions with A. C. Switendick, J. P. Van Dyke, and A. Narath.Magnetophonon resonances in the hot-electron mobility have been observed in pure n-InSb at 77°K in transverse magnetic fields with the application of sufficiently small voltage to produce a slight change in the mobility. The minima in the quantity /3 in the formula A* =Mo(l -fiE 2 ) are attributed to resonant cooling of hot electrons due to optical-phonon-induced transitions between Landau levels.We report here a new type of resonance experiment consisting of the observation of magnetophonon resonances in the hot-electron mobility in w-InSb at 77°K. Magnetophonon resonances were first predicted by Gurevich and Firsov, 1 and subsequently confirmed experimentally by many workers. 2 Magnetophonon phenomena in the electrical resistance reflect the resonant inelastic scattering of electrons between the high densities of states in Landau levels close to k 9 1129

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