Covalent bonding and hybridization effects in the corundum-type transition-metal oxides V2O3 and Ti2O3

The electronic structure of the corundum-type transition-metal oxides V2O3 and Ti2O3 is studied by means of the augmented spherical wave method, based on density-functional theory and the local density approximation. Comparing the results for the vanadate and the titanate allows us to understand the peculiar shape of the metal 3d a_{1g} density of states, which is present in both compounds. The a_{1g} states are subject to pronounced bonding-antibonding splitting due to metal-metal overlap along the c-axis of the corundum structure. However, the corresponding partial density of states is strongly asymmetric with considerably more weight on the high energy branch. We argue that this asymmetry is due to an unexpected broadening of the bonding a_{1g} states, which is caused by hybridization with the e_g^{pi} bands. In contrast, the antibonding a_{1g} states display no such hybridization and form a sharp peak. Our results shed new light on the role of the a_{1g} orbitals for the metal-insulator transitions of V2O3. In particular, due to a_{1g} - e_g^{pi} hybridization, an interpretation in terms of molecular orbital singlet states on the metal-metal pairs along the c-axis is not an adequate description.Comment: 7 pages, 3 figures, more information at http://www.physik.uni-augsburg.de/~eyert

Electronic structure of spinel-type LiV_2O_4

The band structure of the cubic spinel compound LiV_2O_4, which has been reported recently to show heavy Fermion behavior, has been calculated within the local-density approximation using a full-potential version of the linear augmented-plane-wave method. The results show that partially-filled V 3d bands are located about 1.9 eV above the O 2p bands and the V 3d bands are split into a lower partially-filled t_{2g} complex and an upper unoccupied e_{g} manifold. The fact that the conduction electrons originate solely from the t_{2g} bands suggests that the mechanism for the mass enhancement in this system is different from that in the 4f heavy Fermion systems, where these effects are attributed to the hybridization between the localized 4f levels and itinerant spd bands.Comment: 5 pages, revte

On the nature of the magnetic ground-state wave function of V_2O_3

After a brief historical introduction, we dwell on two recent experiments in the low-temperature, monoclinic phase of V_2O_3: K-edge resonant x-ray scattering and non-reciprocal linear dichroism, whose interpretations are in conflict, as they require incompatible magnetic space groups. Such a conflict is critically reviewed, in the light of the present literature, and new experimental tests are suggested, in order to determine unambiguously the magnetic group. We then focus on the correlated, non-local nature of the ground-state wave function, that is at the basis of some drawbacks of the LDA+U approach: we singled out the physical mechanism that makes LDA+U unreliable, and indicate the way out for a possible remedy. Finally we explain, by means of a symmetry argument related to the molecular wave function, why the magnetic moment lies in the glide plane, even in the absence of any local symmetry at vanadium sites.Comment: 7 pages, 1 figur

The Metal-Insulator Transition of the Magneli phase V_4O_7: Implications for V_2O_3

The metal-insulator transition (MIT) of the Magneli phase V_4O_7 is studied by means of electronic structure calculations using the augmented spherical wave method. The calculations are based on density functional theory and the local density approximation. Changes of the electronic structure at the MIT are discussed in relation to the structural transformations occuring simultaneously. The analysis is based on a unified point of view of the crystal structures of all Magneli phase compounds V_nO_2n-1 (3 =< n =< 9) as well as of VO_2 and V_2O_3. This allows to group the electronic bands into states behaving similar to the dioxide or the sesquioxide. In addition, the relationship between the structural and electronic properties near the MIT of these oxides can be studied on an equal footing. For V_4O_7, a strong influence of metal-metal bonding across octahedral faces is found for states both parallel and perpendicular to the hexagonal c_hex axis of V_2O_3. Furthermore, the structural changes at the MIT cause localization of those states, which mediate in-plane metal-metal bonding via octahedral edges. This band narrowing opens the way to an increased influence of electronic correlations, which are regarded as playing a key role for the MIT of V_2O_3.Comment: 7 pages, 3 figures, more information at http://www.physik.uni-augsburg.de/~eyert

Doping Dependence of the Electronic Structure of Ba_{1-x}K_{x}BiO_{3} Studied by X-Ray Absorption Spectroscopy

We have performed x-ray absorption spectroscopy (XAS) and x-ray photoemission spectroscopy (XPS) studies of single crystal Ba_{1-x}K_{x}BiO_{3} (BKBO) covering the whole composition range $0 \leq x \leq 0.60$. Several features in the oxygen 1\textit{s} core XAS spectra show systematic changes with $x$. Spectral weight around the absorption threshold increases with hole doping and shows a finite jump between $x=0.30$ and 0.40, which signals the metal-insulator transition. We have compared the obtained results with band-structure calculations. Comparison with the XAS results of BaPb_{1-x}Bi_{x}O_{3} has revealed quite different doping dependences between BKBO and BPBO. We have also observed systematic core-level shifts in the XPS spectra as well as in the XAS threshold as functions of $x$, which can be attributed to a chemical potential shift accompanying the hole doping. The observed chemical potential shift is found to be slower than that predicted by the rigid band model based on the band-structure calculations.Comment: 8 pages, 8 figures include

New magnetic phase in metallic V_{2-y}O_3 close to the metal insulator transition

We have observed two spin density wave (SDW) phases in hole doped metallic V_{2-y}O_3, one evolves from the other as a function of doping, pressure or temperature. They differ in their response to an external magnetic field, which can also induce a transition between them. The phase boundary between these two states in the temperature-, doping-, and pressure-dependent phase diagram has been determined by magnetization and magnetotransport measurements. One phase exists at high doping level and has already been described in the literature. The second phase is found in a small parameter range close to the boundary to the antiferromagnetic insulating phase (AFI). The quantum phase transitions between these states as a function of pressure and doping and the respective metamagnetic behavior observed in these phases are discussed in the light of structurally induced changes of the band structure.Comment: REVTeX, 8 pages, 12 EPS figures, submitted to PR

Semimetalic antiferromagnetism in the half-Heusler compound CuMnSb

The half-Heusler compound CuMnSb, the first antiferromagnet (AFM) in the Mn-based class of Heuslers and half-Heuslers that contains several conventional and half metallic ferromagnets, shows a peculiar stability of its magnetic order in high magnetic fields. Density functional based studies reveal an unusual nature of its unstable (and therefore unseen) paramagnetic state, which for one electron less (CuMnSn, for example) would be a zero gap semiconductor (accidentally so) between two sets of very narrow, topologically separate bands of Mn 3d character. The extremely flat Mn 3d bands result from the environment: Mn has four tetrahedrally coordinated Cu atoms whose 3d states lie well below the Fermi level, and the other four tetrahedrally coordinated sites are empty, leaving chemically isolated Mn 3d states. The AFM phase can be pictured heuristically as a self-doped Cu$^{1+}$Mn$^{2+}$Sb$^{3-}$ compensated semimetal with heavy mass electrons and light mass holes, with magnetic coupling proceeding through Kondo and/or antiKondo coupling separately through the two carrier types. The ratio of the linear specific heat coefficient and the calculated Fermi level density of states indicates a large mass enhancement $m^*/m \sim 5$, or larger if a correlated band structure is taken as the reference

Vertex-corrected tunneling inversion in superconductors: Pb

The McMillan-Rowell tunneling inversion program, which extracts the electron-phonon spectral function $\alpha^2F(\Omega)$ and the Coulomb pseudopotential $\mu^*$ from experimental tunneling data, is generalized to include the lowest-order vertex correction. We neglect the momentum dependence of the electron-phonon matrix elements, which is equivalent to using a local approximation. The perturbation theory is performed on the imaginary axis and then an exact analytic continuation is employed to produce the density of states on the real axis. Comparison is made with the experimental data for Pb.Comment: 14 pages, typeset in ReVTeX, including three encapsulated postscript figure

Implications of the B20 Crystal Structure for the Magneto-electronic Structure of MnSi

Due to increased interest in the unusual magnetic and transport behavior of MnSi and its possible relation to its crystal structure (B20) which has unusual coordination and lacks inversion symmetry, we provide a detailed analysis of the electronic and magnetic structure of MnSi. The non-symmorphic P2_13 spacegroup leads to unusual fourfold degenerate states at the zone corner R point, as well as ``sticking'' of pairs of bands throughout the entire Brillouin zone surface. The resulting Fermi surface acquires unusual features as a result of the band sticking. For the ferromagnetic system (neglecting the long wavelength spin spiral) with the observed moment of 0.4 \mu_B/Mn, one of the fourfold levels at R in the minority bands falls at the Fermi energy (E_F), and a threefold majority level at k=0 also falls at E_F. The band sticking and presence of bands with vanishing velocity at E_F imply an unusually large phase space for long wavelength, low energy interband transitions that will be important for understanding the unusual resistivity and far infrared optical behavior.Comment: Nine two-column pages with eight figures include

Photogenerated Carriers in SrTiO3 Probed by Mid-Infrared Absorption

Infrared absorption spectra of SrTiO$_3$ have been measured under above-band-gap photoexcitations to study the properties of photogenerated carriers, which should play important roles in previously reported photoinduced phenomena in SrTiO$_3$. A broad absorption band appears over the entire mid-infrared region under photoexcitation. Detailed energy, temperature, and excitation power dependences of the photoinduced absorption are reported. This photo-induced absorption is attributed to the intragap excitations of the photogenerated carriers. The data show the existence of a high density of in-gap states for the photocarriers, which extends over a wide energy range starting from the conduction and valence band edges.Comment: 5 pages, 5 figures, submitted to J. Phys. Soc. Jp