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Register a new userInfrared Optical Absorption in Low-spin Fe$^{2+}$-doped SrTiO${}_{3}$
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Band gap engineering in SrTiO${}_{3}$ and related titanate perovskites has long been explored due to the intriguing properties of the materials for photocatalysis and photovoltaic applications. A popular approach in the materials chemistry community is to substitutionally dope aliovalent transition metal ions onto the B site in the lattice to alter the valence band. However, in such a scheme there is limited control over the dopant valence, and compensating defects often form. Here we demonstrate a novel technique to controllably synthesize Fe$^{2+}$- and Fe$^{3+}$-doped SrTiO${}_{3}$ thin films without formation of compensating defects by co-doping with La$^{3+}$ ions on the A site. We stabilize Fe$^{2+}$-doped films by doping with two La ions for every Fe dopant, and find that the Fe ions exhibit a low-spin electronic configuration, producing optical transitions in the near infrared regime and degenerate doping. The novel electronic states observed here offer a new avenue for band gap engineering in perovskites for photocatalytic and photovoltaic applications.
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A magnetic field parallel to an electrical current does not produce a Lorentz force on the charge carriers. Therefore, orbital longitudinal magnetoresistance is unexpected. Here we report on the observation of a large and non saturating magnetoresistance in lightly doped SrTiO$_{3-x}$ independent of the relative orientation of current and magnetic field. We show that this quasi-isotropic magnetoresistance can be explained if the carrier mobility along all orientations smoothly decreases with magnetic field. This anomalous regime is restricted to low concentrations when the dipolar correlation length is longer than the distance between carriers. We identify cyclotron motion of electrons in a potential landscape tailored by polar domains as the cradle of quasi-isotropic orbital magnetoresistance. The result emerges as a challenge to theory and may be a generic feature of lightly-doped quantum paralectric materials.
We present an approach to tune the effective mass in an oxide semiconductor by a double doping mechanism. We demonstrate this in a model oxide system Sr$_{1-x}$La$_x$TiO$_{3-delta}$, where we can tune the effective mass ranging from 6--20$mathrm{m_e}$ as a function of filling or carrier concentration and the scattering mechanism, which are dependent on the chosen lanthanum and oxygen vacancy concentrations. The effective mass values were calculated from the Boltzmann transport equation using the measured transport properties of thin films of Sr$_{1-x}$La$_x$TiO$_{3-delta}$. Our method, which shows that the effective mass decreases with carrier concentration, provides a means for understanding the nature of transport processes in oxides, which typically have large effective mass and low electron mobility, contrary to the tradional high mobility semiconductors.
We combined photoelemission spectroscopy with first-principle calculations to investigate structural and electronic properties of SrTiO$_{3}$ doped with Ni impurities. In SrTiO$_{3}$ polycrystalline thin films, grown by magnetron sputtering, the mean size of the crystallites increases with the concentration of Ni. To determine the electronic band structure of SrTiO$_{3}$ films doped with Ni, high quality ordered pristine and SrTiO3:Ni$_{x}$ films with x=0.06 and 0.12 were prepared by pulsed laser deposition. Electronic band structure calculations for the ground state, as well as one-step model photoemission calculations, which were obtained by means of the Korringa-Khon-Rostoker Greenss function method, predicted the formation of localised $3d$-impurity bands in the band gap of SrTiO$_{3}$ close to the valence band maxima. The measured valence bands at the resonance Ni2p excitation and band dispersion confirm these findings.
We use density functional theory (DFT) calculations to show that oxygen vacancies ($v_mathrm{O}$) induce noncentrosymmetric polar structures in SrTi$_{0.75}$Fe$_{0.125}$Co$_{0.125}$O$_{3-delta}$ (STFC) with $delta = {0.125, 0.25}$, enhance the magnetic moment and give rise to large changes in the electric polarization $vertDelta Pvert$. Variations of $delta$ or oxygen vacancy migration for a given deficiency are shown to be effective mechanisms to tune the ferroic order parameters, with the former yielding $vertDelta Pvert$ values up to $sim{8mu}$C/c$m^{2}$ while the latter yields $vertDelta Pvert$ up to $sim{23mu}$C/c$m^{2}$. The underlying mechanism is the differentiated self-regulatory-like ferroic response of Fe and Co through the (Fe/Co)-$v_mathrm{O}$ and Fe-$v_mathrm{O}$-Co interactions, which drive B-site off-centering, bending of O$_{4,5}$ incomplete octahedra and B-$v_mathrm{O}$ aligned distortions, all with characteristic charge redistributions. Our results capture characteristics observed in the end-members of the series SrTi(Co,Fe)O$_{3}$, and predict multiferroic behavior that could also be present in other ABO$_{3-delta}$ magnetic oxides.
We report the effect of $delta$-doping at LaAlO$_{3}$/SrTiO$_{3}$ interface with LaMnO$_{3}$ monolayers on the photoconducting (PC) state. The PC is realized by exposing the samples to broad band optical radiation of a quartz lamp and 325 and 441 nm lines of a He-Cd laser. Along with the significant modification in electrical transport which drives the pure LaAlO$_{3}$/SrTiO$_{3}$ interface from metal-to-insulator with increasing LaMnO$_{3}$ sub-monolayer thickness, we also observe an enhancement in the photo-response and relaxation time constant. Possible scenario for the PC based on defect-clusters, random potential fluctuations and large lattice relaxation models have been discussed. For pure LaAlO$_{3}$/SrTiO$_{3}$, the photoconductivity appears to originate from inter-band transitions between Ti-derived $3d$ bands which are $e_{g}$ in character and O 2p - Ti $t_{2g}$ hybridized bands. The band structure changes significantly when fractional layers of LaMnO$_{3}$ are introduced. Here the Mn $e_{g}$ bands ($approx1.5$ eV above the Fermi energy) within the photo-conducting gap lead to a reduction in the photo-excitation energy and a gain in overall photoconductivity.
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