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We discuss the consequences of including both electron-phonon and electron-electron couplings in multiband models, focusing on numerical studies of a one-dimensional two-band model in the intermediate regime for both coupling strengths. Spin-Peierls as well as long-period, frustrated ground states are identified, reminiscent of those found in axial next-nearest-neighbor Ising models. We speculate on experimentally observable signatures of this rich phase diagram.PACS numbers: 75.30.Fv, 64.70.Rh, 71.38.+i, 71.45.Lr Multiband and multiorbital models have recently been much studied, primarily due to their relevance for hightemperature superconductors (HTS) [1]. At stoichiometry, such models can exhibit interesting and unusual ground states, such as elect ron-phonon (e-ph) driven incommensurate long-period or superlattice phases [2]. Here we show that a one-dimensional (ID), two-band model with competing electron-electron (e-e) and e-ph interactions also exhibits at stoichiometry qualitatively new magnetic behavior, namely, frustrated or spin-Peierls phases in the intermediate regime between the strongly electron-electron (e-e) correlated antiferromagnetic limit and the strongly e-ph correlated large lattice distortion limit. Similar complex phases are found in, e.g., axial next-nearest neighbor-Ising (ANNNI) models [3].Considerable effort has gone into solving effective oneband models. For dominant e-e interactions Zhang and Rice [4] derived for HTS, due to the separation of energy scales, an effective one-band t-J Hamiltonian. Imada [5] suggested that inclusion of an e-ph dependent effective spin interaction J is crucial, as it opens a spin gap, important for singlet superconductivity. Here we stress phenomena which extend to several interaction forms and parameter ranges, and for which keeping the full two-band model with both e-e and e-ph interactions is essential.We study the ID, two-band, 3/4-filled, tight-binding Peierls-Hubbard Hamiltonian developed [6, 7] to model an interesting class of ID compounds-halogen-bridged transition-metal linear chain complexes (MX chains) -which exhibit tunable behavior ranging from antiferromagnetic or spin-density wave to charge-density wave (CDW) to semimetallic. This same model can be considered as follows: a model of CuO chains; a ID analog of the models used for CuC>2 planes in HTS [8]; a 3/4-filled analog of the the organic conductor polyacetylene; a model for charge-transfer salts such as TTF-TCNQ; or a model of neutral-ionic transitions [9], If one considers the two orbitals to be on the same site, it is also related to the Kondo Hamiltonian used to describe heavy fermions. This model yields quantitative fits for the MX chain compounds to a variety of experimental data (optical absorption, Raman spectra, ESR, etc.) [6,7].Our ID, two-band, Peierls-Hubbard Hamiltonian is [6,where c] a creates an electron at site / with spin a. M (d z 2) and X (p z ) Wannier orbitals are situated on even and odd sites, respectively. Each M2X2 unit cell has 6 electrons, or 3/4 band fillin...

The hyperfine structure constants of 9SMo and 97Mo and the gj-factors have been measured in the metastable multiplets # 2 s 4d 5s a D, 4dS5s aSs, asp, and 4d 6 cSD by atomic beam magnetic resonance combined with state-selective laser-induced detection of the rf-resonant atoms. Line isotope shifts and the A-and B-factors of further metastable levels and of high-lying odd-parity states were obtained from high-resolution Doppler-free laser spectroscopy.For a detailed study of configuration interaction effects in atomic hyperfine structure (hfs) the 4d-shell transition element molybdenum with its numerous metastable fine structure levels is very favourable. Because of the rather high excitation energies of 10000-26000 cm -i for these states, however, only the hfs constants A and B of the states 4d45s 2 aSD. ^ could be measured by precision rf spectrosco-9 9 i~z 97 py in ~o (16~ abundance, I=5/2) and Mo (9%, I=5/2) until now /i/. Tqe metastable states arise from the three configurations (4d+5s) 6 , and intermediate coupling eigenvectors were obtained by a least-squares fit of one-and twobody effective operators to 78 known energy levels. The mean deviation (89 cm -I) of the fit with 18 independent parameters is already satisfactory, but we hope to improve it further by including three-body interactions.In order to test the quality of the eigenvectors, we deter-5 5 mined the gj-factors of the states 4d 5s a S 2 and 4d45s 2 aSDl_4 applying the atomic beam magnetic resonance (ABMR) method for measurements of the double-quantum Zeeman transition Mj=l <---> ~ =-l at 700 Oe and 1800 Oe in 98Mo (24%, I=0). The molybdenum atomic beam was produced crucible-free using the rotating-target method /2,3/, and the population of these metastable levels in 98Mo was high enough to allow rf resonance detection by electron-impact ionization and subsequent mass separation. The ABMR apparatus is conventionally equipped with inhomogeneous deflection magnets A, B and a homogeneous C-field region where the rf transitions are induced. For rf spectroscopy hfs investigations of 9SMo or 97140 in the states a5S2, aSD3 ,. , and in the 5 5 ~-1 6 5 high-lying levels 4d 5s a P1-3 at 18000 cm and 4d c D 4 at 25456 cm -1 the ABMR technique was combined with a sensitive state-and isotope-selective laser-induced detection scheme for the rf-resonant atoms /4,5/. The optical system consists of an actively stabilized cw ring dye laser, a direct-counting Michelson-type interferometer for wavelength measurements and a non-confocal spherical mirror imterferometer ("marker interferometer") with 60 MHz free spectral range /6/. In a molybdenum absorption line the laser was tuned to a certain hfs component of 9SMo or 97M0 which excites one F-term of the considered AF=I rf transition and the fluorescence light was registered during the rf scan. The magnetic dipole and electric quadrupole hfs constants A and B of both isotopes were determined from AF=l transitions at low magnetic fields in at least three hfs intervals of the respective metastable state, with ...