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...

We investigated magnetoelastic coupling through the field-driven transition to the fully polarized magnetic state in quasi-two-dimensional [Cu(HF2)(pyz)2]BF4 by magnetoinfrared spectroscopy. This transition modifies out-of-plane ring distortion and bending vibrational modes of the pyrazine ligand. The extent of these distortions increases with the field, systematically tracking the low-temperature magnetization. These distortions weaken the antiferromagnetic spin exchange, a finding that provides important insight into magnetic transitions in other copper halides.

We present a detailed study of the zone-center phonons and magnetoelectric interactions in Ni 3 V 2 O 8 . Using combined neutron scattering, polarized infrared (IR) measurements and first-principles LDA + U calculations, we successfully assigned all IR-active modes, including eleven B 2u phonons which can induce the observed spontaneous electric polarization. We also calculated the Born-effective charges and the IR intensities which are in surprisingly good agreement with the experimental data. Among the eleven B 2u phonons, we find that only a few of them can actually induce a significant dipole moment. The exchange interactions up to a cutoff of 6.5 Å are also calculated within the LDA + U approach with different values of U for Ni, V and O atoms. We find that LSDA (i.e. U = 0) gives excellent results concerning the optimized atomic positions, bandgap and phonon energies. However, the magnitudes of the exchange constants are too large compared to the experimental Curie-Weiss constant, . Including U for Ni corrects the magnitude of the superexchange constants but opens a too large electronic bandgap. We observe that including correlation at the O site is very important to get simultaneously the correct phonon energies, bandgap and exchange constants. In particular, the nearest and next-nearest exchange constants along the Ni-spine sites result in incommensurate spin ordering with a wavevector that is consistent with the experimental data. Our results also explain how the antiferromagnetic coupling between sublattices in the b and c directions is consistent with the relatively small observed value of . We also find that, regardless of the values of U used, we always get the same five exchange constants that are significantly larger than the rest. Finally, we discuss how the B 2u phonons and the spin structure combine to yield the observed spontaneous polarization. We present a simple phenomenological model which shows how trilinear (and quartic) couplings of one (or two) phonons to two spin operators perturbatively affects the magnon (i.e. electromagnon) and phonon energies.

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