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Register a new userHeavy non-degenerate electrons in doped strontium titanate
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Room-temperature metallicity of lightly doped SrTiO$_3$ is puzzling, because the combination of mobility and the effective mass would imply a mean-free-path (mfp) below the Mott Ioffe Regel (MIR) limit and a scattering time shorter than the Planckian time ($tau_P=hbar/k_BT$). We present a study of electric resistivity, Seebeck coefficient and inelastic neutron scattering extended to very high temperatures, which deepens the puzzle. Metallic resistivity persists up to 900 K and is accompanied by a large Seebeck coefficient whose magnitude (as well as its temperature and doping dependence) indicates that carriers are becoming heavier with rising temperature. Combining this with neutron scattering data, we find that between 500 K and 900 K, the Bohr radius and the electron wave-length become comparable to each other and twice the lattice parameter. According to our results, between 100 K and 500 K, metallicity is partially driven by temperature-induced amplification of the carrier mass. We contrast this mass amplification of non-degenerate electrons with the better-known case of heavy degenerate electrons. Above 500 K, the mean-free-path continues to shrink with warming in spite of becoming shorter than both the interatomic distance and the thermal wavelength of the electrons. The latter saturates to twice the lattice parameter. Available theories of polaronic quasi-particles do not provide satisfactory explanation for our observations.
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Monodispersed strontium titanate nanoparticles were prepared and studied in detail. It is found that ~10 nm as-prepared stoichiometric nanoparticles are in a polar structural state (with possibly ferroelectric properties) over a broad temperature range. A tetragonal structure, with possible reduction of the electronic hybridization is found as the particle size is reduced. In the 10 nm particles, no change in the local Ti-off centering is seen between 20 and 300 K. The results indicate that nanoscale motifs of SrTiO3 may be utilized in data storage as assembled nano-particle arrays in applications where chemical stability, temperature stability and low toxicity are critical issues.
The temperature dependent Hall mobility data from La-doped SrTiO3 thin films has been analyzed and modeled considering various electron scattering mechanisms. We find that a ~6 meV transverse optical phonon (TO) deformation potential scattering mechanism is necessary to explain the dependence of transport on temperature between 10-200 K. Also, we find that the low temperature electron mobility in intrinsic (nominally undoped) SrTiO3 is limited by acoustic phonon scattering. Adding the above two scattering mechanisms to longitudinal optical phonon (LO) and ionized impurity scattering mechanisms, excellent quantitative agreement between mobility measurement and model is achieved in the whole temperature range (2-300K) and carrier concentrations ranging over a few orders of magnitude (8x1017 cm-3 - 2x1020 cm-3).
The surface composition of polycrystalline niobium-doped strontium titanate (SrTiO3 : Nb) is studied using X-ray photoelectron emission microscopy (XPEEM) for many grain orientations in order to characterise the surface chemistry with high spatial resolution. The surface sensitivity is maximised by the use of soft X-ray synchrotron radiation (SR). The grain orientation is determined by electron backscattering diffraction (EBSD). Stereographic plots are used to show the correlation between surface composition and orientation for several grains. Predominant surface terminations are assigned to major orientations.
Room temperature ferromagnetism was characterized for thin films of SrTi$_{0.6}$Fe$_{0.4}$O$_{3-{delta}}$ grown by pulsed laser deposition on SrTiO$_{3}$ and Si substrates under different oxygen pressures and after annealing under oxygen and vacuum conditions. X-ray magnetic circular dichroism demonstrated that the magnetization originated from Fe$^{2+}$ cations, whereas Fe$^{3+}$ and Ti$^{4+}$ did not contribute. Films with the highest magnetic moment (0.8 {mu}B per Fe) had the highest measured Fe$^{2+}$:Fe${^3+}$ ratio of 0.1 corresponding to the largest concentration of oxygen vacancies ({delta} = 0.19). Post-growth annealing treatments under oxidizing and reducing conditions demonstrated quenching and partial recovery of magnetism respectively, and a change in Fe valence states. The study elucidates the microscopic origin of magnetism in highly Fe-substituted SrTi$_{1-x}$Fe$_x$O$_{3-{delta}}$ perovskite oxides and demonstrates that the magnetic moment, which correlates with the relative content of Fe$^{2+}$ and Fe$^{3+}$, can be controlled via the oxygen content, either during growth or by post-growth annealing.
The Landau theory of phase transitions of Ba0.8Sr0.2TiO3 thin film under external electric field applied in the planar geometry is developed. The interfacial van-der-Waals field Ez=1.1x10^8 V/m oriented normal to the film-substrate interface was introduced in to the model calculation to explain experimentally observed behavior of the polarization as a function of planar electric field. The Ez - misfit strain phase diagram of the film is constructed and discussed.
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