We present joint theoretical-experimental study of the correlation effects in the electronic structure of (pyH)3[Mn4O3Cl7(OAc)3]·2MeCN molecular magnet (Mn4). Describing the many-body effects by cluster dynamical mean-field theory, we find that Mn4 is predominantly Hubbard insulator with strong electron correlations. The calculated electron gap (1.8 eV) agrees well with the results of optical conductivity measurements, while other methods, which neglect many-body effects or treat them in a simplified manner, do not provide such an agreement. Strong electron correlations in Mn4 may have important implications for possible future applications.PACS numbers: 75.50. Xx,71.15.Mb, Single molecule magnets (SMMs), made of exchangecoupled magnetic ions surrounded by large organic ligands, represent a novel interesting class of magnetic materials. They are of fundamental interest as test systems for studying magnetism at nanoscale, and interplay between the structural, electronic, and magnetic properties. SMMs demonstrate fascinating mixture of clasiscal and quantum properties: as classical superparamagnets, they possess large anisotropy and magnetic moment, but also exhibit interesting mesoscopic quantum spin effects [1,2,3]. Moreover, recent experiments on the electron transport through SMMs [4], and predicted connection between the transport and spin tunneling [5], make SMMs good candidates for interesting spintronics studies. Progress in this area -synthesis of novel SMMs with optimized properties, design and analysis of the transport experiments, possible uses in information processing -demands detailed theoretical investigations of the magnetic and electronic structure of SMMs [6,7,8,9,10]. Among other factors, the many-body correlations caused by the Coulomb repulsion between electrons, may be important. E.g., in transition metal-oxide systems [11], which share many similarities with SMMs, strong correlations may form the Mott-Hubbard insulating state [12], where the nature of the charge and spin excitations is drastically different from the predictions of standard band-insulator theory. This affects the basic properties of the system (e.g., exchange interactions), and drastically changes charge and spin transport.In this joint experimental-theoretical work, we present a detailed study of the many-body effects in electronic structure of SMMs (pyH) 3 [Mn 4 O 3 Cl 7 (OAc) 3 ]·2MeCN (denoted below as Mn 4 for brevity) [19]. We use the cluster LDA+DMFT method [13] which combines the realistic ab initio calculations based on the local density approximation (LDA), and the accurate description of the correlation effects within the cluster dynamical mean field theory (CDMFT). Using the electron gap as a most convenient benchmark, we show that the gap value (1.8 eV) calculated within LDA+CDMFT is in good agreement with the optical conductivity measurements (showing the peak corresponding to vertical transitions at ∼1.8 eV). The approaches which neglect the electron correlations (LDA), or treat these correlation in a simplifi...

The optical properties of epitaxial BiFeO 3 thin films have been characterized in the visible range. Variable temperature spectra show an absorption onset near 2.17 eV, a direct gap ͑2.667Ϯ 0.005 eV at 300 K͒, and charge transfer excitations at higher energy. Additionally, we report photoconductivity in BiFeO 3 films under illumination from a 100 mW/ cm 2 white light source. A direct correlation is observed between the magnitude of the photoconductivity and postgrowth cooling pressure. Dark conductivities increased by an order of magnitude when comparing films cooled in 760 and 0.1 Torr. Large increases in photoconductivity are observed in light.

We report a photovoltaic effect in ferroelectric BiFeO3 thin films. The all-oxide heterostructures with SrRuO3 bottom and tin doped indium oxide top electrodes are characterized by open-circuit voltages ∼0.8–0.9 V and external quantum efficiencies up to ∼10% when illuminated with the appropriate light. Efficiencies are at least an order of magnitude larger than the maximum efficiency under sunlight (AM 1.5) thus far reported for ferroelectric-based devices. The dependence of the measured open-circuit voltage on film thickness suggests contributions to the large open-circuit voltage from both the ferroelectric polarization and band offsets at the BiFeO3/tin doped indium oxide interface.

We present the Raman scattering results on layered 2D semiconducting ferromagnetic compound CrSiTe3. Four Raman active modes, predicted by symmetry, have been observed and assigned. The experimental results are supported by DFT calculations. The self-energies of the A 3 g and the E 3 g symmetry modes exhibit unconventional temperature evolution around 180 K. In addition, the doubly degenerate E 3 g mode shows clear change of asymmetry in the same temperature region. The observed behaviour is consistent with the presence of the previously reported short-range magnetic order and the strong spin-phonon coupling.

BiFeO 3 thin films have been deposited on ͑001͒ SrTiO 3 substrates by adsorption-controlled reactive molecular-beam epitaxy. For a given bismuth overpressure and oxygen activity, single-phase BiFeO 3 films can be grown over a range of deposition temperatures in accordance with thermodynamic calculations. Four-circle x-ray diffraction reveals phase-pure, epitaxial films with rocking curve full width at half maximum values as narrow as 29 arc sec ͑0.008°͒. Multiple-angle spectroscopic ellipsometry reveals a direct optical band gap at 2.74 eV for stoichiometric as well as 5% bismuth-deficient single-phase BiFeO 3 films.