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Register a new userOptical properties and electronic structure of Ca-doped alpha-NaV2O5
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The dielectric function of alpha-Na(1-x)Ca(x)V2O5 (0 < x < 20%) was measured for the a and b axes in the photon energy range 0.8-4.5 eV at room temperature. By varying the Ca-concentration we control the relative abundancy of V4+ and V5+. We observe that the intensity of the main optical absorption peak at 1 eV is proportional to the number of V5+ ions. This rules out the interpretation as a V4+ d-d excitation, and it establishes that this is the on-rung bonding-antibonding transition.
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The complicated electronic, magnetic, and colossal magnetoresistant (CMR) properties of Sr and Ca doped lanthanum manganites can be understood by spin-polarized first-principles calculations. The electronic properties can be attributed to a detailed balancing between Sr and Ca induced metal-like O 2p and majority-spin (majority-spin) Mn eg delocalized states and the insulator-like minority-spin (minority-spin) Mn t2g band near the Fermi level (EF). The magnetic properties can be attributed to a detailed balancing between O mediated antiferromagnetic superexchange and delocalized majority-spin Mn eg-state mediated ferromagnetic spin-spin couplings. While CMR can be attributed to the lining up of magnetic domains trigged by the applied magnetic field, which suppresses the trapping ability of the empty Mn t2g states that resists the motion of conducting Mn majority-spin eg electrons.
The complex optical properties of a single crystal of hexagonal FeCrAs ($T_N simeq 125$ K) have been determined above and below $T_N$ over a wide frequency range in the planes (along the $b$ axis), and along the perpendicular ($c$ axis) direction. At room temperature, the optical conductivity $sigma_1(omega)$ has an anisotropic metallic character. The electronic band structure reveals two bands crossing the Fermi level, allowing the optical properties to be described by two free-carrier (Drude) contributions consisting of a strong, broad component and a weak, narrow term that describes the increase in $sigma_1(omega)$ below $simeq 15$ meV. The dc-resistivity of FeCrAs is ``non-metallic, meaning that it rises in power-law fashion with decreasing temperature, without any signature of a transport gap. In the analysis of the optical conductivity, the scattering rates for both Drude contributions track the dc-resistivity quite well, leading us to conclude that the non-metallic resistivity of FeCrAs is primarily due to a scattering rate that increases with decreasing temperature, rather than the loss of free carriers. The power law $sigma_1(omega) propto omega^{-0.6}$ is observed in the near-infrared region and as $Trightarrow T_N$ spectral weight is transferred from low to high energy ($gtrsim 0.6$ eV); these effects may be explained by either the two-Drude model or Hunds coupling. We also find that a low-frequency in-plane phonon mode decreases in frequency for $T < T_N$, suggesting the possibility of spin-phonon coupling.
Infrared reflectance of alpha-NaV2O5 single crystals in the frequency range from 50 cm-1 to 10000 cm-1 was studied for a, b and c-polarisations. In addition to phonon modes identification, for the a-polarised spectrum a broad continuum absorption in the range of 1D magnetic excitation energies was found. The strong near-IR absorption band at 0.8 eV shows a strong anisotropy with vanishing intensity in c-polarisation. Activation of new phonons due to the lattice dimerisation were detected below 35K as well as pretransitional structural fluctuations up to 65K.
We present results for the electronic structure of alpha uranium using a recently developed quasiparticle self-consistent GW method (QSGW). This is the first time that the f-orbital electron-electron interactions in an actinide has been treated by a first-principles method beyond the level of the generalized gradient approximation (GGA) to the local density approximation (LDA). We show that the QSGW approximation predicts an f-level shift upwards of about 0.5 eV with respect to the other metallic s-d states and that there is a significant f-band narrowing when compared to LDA band-structure results. Nonetheless, because of the overall low f-electron occupation number in uranium, ground-state properties and the occupied band structure around the Fermi energy is not significantly affected. The correlations predominate in the unoccupied part of the f states. This provides the first formal justification for the success of LDA and GGA calculations in describing the ground-state properties of this material.
We present a theoretical investigation of the effects of correlations on the electronic structure of the Mott insulator Sr$_2$IrO$_4$ upon electron doping. A rapid collapse of the Mott gap upon doping is found, and the electronic structure displays a strong momentum-space differentiation at low doping level: The Fermi surface consists of pockets centered around $(pi/2,pi/2)$, while a pseudogap opens near $(pi,0)$. Its physical origin is shown to be related to short-range spin correlations. The pseudogap closes upon increasing doping, but a differentiated regime characterized by a modulation of the spectral intensity along the Fermi surface persists to higher doping levels. These results, obtained within the cellular dynamical mean-field theory framework, are discussed in comparison to recent photoemission experiments and an overall good agreement is found.
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