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Register a new userDensity functional characterization of the antiferromagnetism in oxygen-deficient anatase and rutile TiO2
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We present theoretical evidence for local magnetic moments on Ti3+ ions in oxygen-deficient anatase and rutile TiO2 observed in a recent experiment [S. Zhou, et al., Phys. Rev. B 79, 113201 (2009)]. Results of our first-principles GGA+U calculations reveal that an oxygen vacancy converts two Ti4+ ions to two Ti3+ ions in anatase phase, which results in a local magnetic moment of 1.0 $mu_B$ per Ti3+. The two Ti3+ ions, however, form a stable antiferromagnetic state, and similar antiferromagnetism is also observed in oxygen-deficient rutile phase TiO2. The calculated results are in good agreement with the experimentally observed antiferromagnetic-like behavior in oxygen-deficient Ti-O systems.
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Oxygen vacancies created in anatase TiO2 by UV photons (80 - 130 eV) provide an effective electron-doping mechanism and induce a hitherto unobserved dispersive metallic state. Angle resolved photoemission (ARPES) reveals that the quasiparticles are large polarons. These results indicate that anatase can be tuned from an insulator to a polaron gas to a weakly correlated metal as a function of doping and clarify the nature of conductivity in this material.
Elucidating the carrier density at which strongly bound excitons dissociate into a plasma of uncorrelated electron-hole pairs is a central topic in the many-body physics of semiconductors. However, there is a lack of information on the high-density response of excitons absorbing in the near-to-mid ultraviolet, due to the absence of suitable experimental probes in this elusive spectral range. Here, we present a unique combination of many-body perturbation theory and state-of-the-art ultrafast broadband ultraviolet spectroscopy to unveil the interplay between the ultraviolet-absorbing two-dimensional excitons of anatase TiO$_2$ and a sea of electron-hole pairs. We discover that the critical density for the exciton Mott transition in this material is the highest ever reported in semiconductors. These results deepen our knowledge of the exciton Mott transition and pave the route toward the investigation of the exciton phase diagram in a variety of wide-gap insulators.
The nuclear quadrupole interaction of the I=5/2 state of the nuclear probes 111Cd and 181Ta in the anatase and rutile polymorphs of bulk TiO2 was studied using the time differential perturbed angular correlation (TDPAC). The fast-slow coincidence setup is based on the CAMAC electronics. For anatase, the asymmetry of the electric field gradient was eta=0.22(1) and a quadrupole interaction frequency: 44.01(3) Mrad/s was obtained for 181Ta. For rutile, the respective values are eta=0.56(1) and quadrupole frequency=130.07(9) Mrad/s. The values for rutile match closely with the literature values. In case of the 111Cd probe produced from the beta decay of 111Ag, the quadrupole interaction frequency and the asymmetry parameter for anatase was negligible. This indicates an unperturbed angular correlation in anatase. On the other hand for rutile, the quadrupole frequency is 61.74(2) Mrad/s and the asymmetry is 0.23(1) for 111Cd probe. The results have been interpreted in terms of the surrounding atom positions in the lattice and the charge state of the probe nucleus.
Thermopower (S) for anatase TiO2 epitaxial films (n3D: 1E17-1E21 /cm3) and the gate voltage (Vg) dependence of S for thin film transistors (TFTs) based on TiO2 films were investigated to clarify the electronic density of states (DOS) around the conduction band bottom. The slope of the |S|-log n3D plots was -20 {mu}V/K, which is an order magnitude smaller than that of semiconductors (-198 {mu}V/K), and the |S| values for the TFTs increased with Vg in the low Vg region, suggesting that the extra tail states are hybridized with the original conduction band bottom.
The ferroelectricity of the spiral magnets LiCu2O2 and LiCuVO4 was examined by calculating the electric polarizations of their spin spiral states on the basis of density functional theory with spin-orbit coupling. Our work unambiguously reveals that spin-orbit coupling is responsible for the ferroelectricity with the primary contribution from the spin-orbit coupling on the Cu sites, but the asymmetric density distribution responsible for the electric polarization occurs mainly around the O atoms. The electric polarization is calculated to be much greater for the ab- than for the bc-plane spin spiral. The observed spin-spiral plane is found to be consistent with the observed direction of the electric polarization for LiCuVO4, but inconsistent for LiCu2O2.
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