Many-body models for molecular nanomagnets

We present a flexible and effective ab-initio scheme to build many-body models for molecular nanomagnets, and to calculate magnetic exchange couplings and zero-field splittings. It is based on using localized Foster-Boys orbitals as one-electron basis. We apply this scheme to three paradigmatic systems, the antiferromagnetic rings Cr8 and Cr7Ni and the single molecule magnet Fe4. In all cases we identify the essential magnetic interactions and find excellent agreement with experiments.Comment: 5 pages, 3 figure

Mott transition and suppression of orbital fluctuations in orthorhombic 3d1d^{1} perovskites

Using $t_{2g}$ Wannier-functions, a low-energy Hamiltonian is derived for orthorhombic $3d^{1}$ transition-metal oxides. Electronic correlations are treated with a new implementation of dynamical mean-field theory for non-cubic systems. Good agreement with photoemission data is obtained. The interplay of correlation effects and cation covalency (GdFeO$_{3}$-type distortions) is found to suppress orbital fluctuations in LaTiO$_{3},$ and even more in YTiO$_{3}$, and to favor the transition to the insulating state.Comment: 4 pages, 3 figures; revised manuscrip

Frustration driven structural distortion in VOMoO4

Nuclear magnetic resonance (NMR), electron paramagnetic resonance (EPR), magnetization measurements and electronic structure calculations in VOMoO4 are presented. It is found that VOMoO4 is a frustrated two-dimensional antiferromagnet on a square lattice with competing exchange interactions along the side J1 and the diagonal J2 of the square. From magnetization measurements J1+J2 is estimated around 155 K, in satisfactory agreement with the values derived from electronic structure calculations. Around 100 K a structural distortion, possibly driven by the frustration, is evidenced. This distortion induces significant modifications in the NMR and EPR spectra which can be accounted for by valence fluctuations. The analysis of the spectra suggests that the size of the domains where the lattice is distorted progressively grows as the temperature approaches the transition to the magnetic ground state at Tc=42 K

Quantum oscillations in YBa2Cu3O6+δ\mathrm{YBa_{2}Cu_{3}O_{6+\delta}} from an incommensurate dd-density wave order

We consider quantum oscillation experiments in $\mathrm{YBa_{2}Cu_{3}O_{6+\delta}}$ from the perspective of an incommensurate Fermi surface reconstruction using an exact transfer matrix method and the Pichard-Landauer formula for the conductivity. The specific density wave order considered is a period-8 $d$-density wave in which the current density is unidirectionally modulated. The current modulation is also naturally accompanied by a period-4 site charge modulation in the same direction, which is consistent with recent magnetic resonance measurements. In principle Landau theory also allows for a period-4 bond charge modulation, which is not discussed, but should be simple to incorporate in the future. This scenario leads to a natural, but not a unique, explanation of why only oscillations from a single electron pocket is observed, and a hole pocket of roughly twice the frequency as dictated by two-fold commensurate order, and the corresponding Luttinger sum rule, is not observed. However, it is possible that even higher magnetic fields will reveal a hole pocket of half the frequency of the electron pocket or smaller. This may be at the borderline of achievable high field measurements because at least a few complete oscillations have to be clearly resolved.Comment: 8 pages, 7 figure

Role of covalency in the ground state properties of perovskite ruthenates: A first principle study using local spin density approximations

We investigate the electronic structure of SrRuO3 and CaRuO3 using full potential linearized augmented plane wave method within the local spin density approximations. The ferromagnetic ground state in SrRuO3 could exactly be described in these calculations and the calculated spin magnetic moment is found to be close to the experimentally observed values. Interestingly, the spin polarized calculations for CaRuO3 exhibit large spin moment as observed in the experiments but the magnetic ground state has higher energy than that in the non-magnetic solution. Various calculations for different structural configurations indicate that Ca-O covalency plays the key role in determining the electronic structure and thereby the magnetic ground state in this system.Comment: 8 figure

Electronic structure induced reconstruction and magnetic ordering at the LaAlO3∣_3|SrTiO3_3 interface

Using local density approximation (LDA) calculations we predict GdFeO$_3$-like rotation of TiO$_6$ octahedra at the $n$-type interface between LaAlO$_3$ and SrTiO$_3$. The narrowing of the Ti $d$ bandwidth which results means that for very modest values of $U$, LDA$+U$ calculations predict charge and spin ordering at the interface. Recent experimental evidence for magnetic interface ordering may be understood in terms of the close proximity of an antiferromagnetic insulating ground state to a ferromagnetic metallic excited state

Dynamical mean-field theory of indirect magnetic exchange

To analyze the physical properties arising from indirect magnetic exchange between several magnetic adatoms and between complex magnetic nanostructures on metallic surfaces, the real-space extension of dynamical mean-field theory (R-DMFT) appears attractive as it can be applied to systems of almost arbitrary geometry and complexity. While R-DMFT describes the Kondo effect of a single adatom exactly, indirect magnetic (RKKY) exchange is taken into account on an approximate level only. Here, we consider a simplified model system consisting of two magnetic Hubbard sites ("adatoms") hybridizing with a non-interacting tight-binding chain ("substrate surface"). This two-impurity Anderson model incorporates the competition between the Kondo effect and indirect exchange but is amenable to an exact numerical solution via the density-matrix renormalization group (DMRG). The particle-hole symmetric model at half-filling and zero temperature is used to benchmark R-DMFT results for the magnetic coupling between the two adatoms and for the magnetic properties induced in the substrate. In particular, the dependence of the local adatom and the nonlocal adatom-adatom static susceptibilities as well as the magnetic response of the substrate on the distance between the adatoms and on the strength of their coupling with the substrate is studied. We find both, excellent agreement with the DMRG data even on subtle details of the competition between RKKY exchange and the Kondo effect but also complete failure of the R-DMFT, depending on the parameter regime considered. R-DMFT calculations are performed using the Lanczos method as impurity solver. With the real-space extension of the two-site DMFT, we also benchmark a simplified R-DMFT variant.Comment: 14 pages, 8 figure

Materials Design using Correlated Oxides: Optical Properties of Vanadium Dioxide

Materials with strong electronic Coulomb interactions play an increasing role in modern materials applications. "Thermochromic" systems, which exhibit thermally induced changes in their optical response, provide a particularly interesting case. The optical switching associated with the metal-insulator transition of vanadium dioxide (VO2), for example, has been proposed for use in "intelligent" windows, which selectively filter radiative heat in hot weather conditions. In this work, we develop the theoretical tools for describing such a behavior. Using a novel scheme for the calculation of the optical conductivity of correlated materials, we obtain quantitative agreement with experiments for both phases of VO2. On the example of an optimized energy-saving window setup, we further demonstrate that theoretical materials design has now come into reach, even for the particularly challenging class of correlated electron systems.Comment: 4+x pages, 2 figure

First-Principles Computation of YVO3; Combining Path-Integral Renormalization Group with Density-Functional Approach

We investigate the electronic structure of the transition-metal oxide YVO3 by a hybrid first-principles scheme. The density-functional theory with the local-density-approximation by using the local muffin-tin orbital basis is applied to derive the whole band structure. The electron degrees of freedom far from the Fermi level are eliminated by a downfolding procedure leaving only the V 3d t2g Wannier band as the low-energy degrees of freedom, for which a low-energy effective model is constructed. This low-energy effective Hamiltonian is solved exactly by the path-integral renormalization group method. It is shown that the ground state has the G-type spin and the C-type orbital ordering in agreement with experimental indications. The indirect charge gap is estimated to be around 0.7 eV, which prominently improves the previous estimates by other conventional methods