Electron-Phonon Interaction and Charge Carrier Mass Enhancement inSrTiO3

Abstract

We report a comprehensive THz, infrared and optical study of Nb doped SrTiO3 as well as DC conductivity and Hall effect measurements. Our THz spectra at 7 K show the presence of a very narrow (< 2 meV) Drude peak, the spectral weight of which shows approximately a factor of three enhancement of the band mass for all carrier concentrations. The missing spectral weight is regained in a broad 'mid-infrared' band which originates from electron-phonon coupling. We find no evidence of a particularly large electron-phonon coupling that would result in small polaron formation. Analysis of the results yields an electron-phonon coupling parameter of an intermediate strength, α ≈ 4.PACS numbers: 71.38.-k, 72.20.-i, 78.20.-e Electron-phonon coupling in the perovskites is a subject of much recent interest due to the controversy over its relevance in the phenomena of multiferroicity, ferroelectricity, superconductivity and colossal magnetoresistance [1,2,3,4,5]. Despite much progress, full understanding of the physics of electron-phonon coupling in perovskites is still lacking because of additional crystallographic complexities of many materials involved (breathing, tilting and rotational distortions, ferroelectric symmetry breaking), magnetism, complex electronic effects (strong correlations), and also because of the lack of high-accuracy spectroscopic measurements specifically designed to probe electron-phonon coupling.With this in mind, we have studied a prototypical perovskite oxide, SrTi 1−x Nb x O 3 with 0 ≤ x ≤ 0.02. SrTiO 3 is an insulator (∆ = 3.25 eV) with the conduction band formed by the Ti 3d states. These are split by the crystal field so that the three t 2g states become occupied when the material is electron-doped by substituting pentavalent Nb for tetravalent Ti. For 0.0005 ≤ x ≤ 0.02 SrTi 1−x Nb x O 3 becomes superconducting at a T c of typically ∼ 0.3 K [6,7], and at most 1.2 K [5]. Characterized by the threefold degeneracy of the conduction bands and the high lattice polarizability, electron doped SrTiO 3 provides a perfect opportunity for the study of electronphonon coupling and polaron formation in an archetypal perovskite [8,9].

First-Principles Modeling of SrTiO3 Based Oxides for Thermoelectric Applications

“…Therefore, the underestimation of experimental power factors P F exp of ∼2-3 mW/mK 2 [2,81,84] We assign the underestimation of P F exp to the enhancement of carrier effective mass due to the electronphonon coupling interaction, which is compatible with the fact that the electronic transport in n-type SrTiO 3 has a polaronic nature. [85] A factor of 3 larger inertial effective mass m * i was obtained from experimental optical conductivity relative to the theoretical m * i value of ∼0.63m e estimated within LDA. [85] At a given carrier concentration, larger experimental effective masses generate larger S exp by lowering the chemical potential relative to CB bottom.…”

“…[85] A factor of 3 larger inertial effective mass m * i was obtained from experimental optical conductivity relative to the theoretical m * i value of ∼0.63m e estimated within LDA. [85] At a given carrier concentration, larger experimental effective masses generate larger S exp by lowering the chemical potential relative to CB bottom. We have estimated m * i according to the relation [86]:…”

Clocking the Ultrafast Electron Cooling in Anatase Titanium Dioxide Nanoparticles

“…Within the single-particle limit probed by ARPES, the crossover takes place around n = 10 19 cm −3 [11]. In the two-particle limit of optical absorption, this threshold may be even lower, leading to the emergence of a well-defined Drude response for the photocarriers [34] instead of the characteristic absorption features of large polarons [60]. Even in this diffusive regime, the electron-phonon coupling can lead to an efficient and fast transfer of electron excess energies to the phonon bath within a few tens of fs.…”

Effect of oxygen vacancy distribution on the thermoelectric properties of La-doped SrTiO3 epitaxial thin films

“…This assumption, however, is valid only for high concentration of oxygen vacancies. By analyzing the transport properties, assuming a six-fold degenerate conduction band and r ¼ 2 (scattering by impurity ions), Okuda et al 24 Conventionally, m* of semiconductors have been determined by the Shubnikov-de Hass effect, 25 optical methods, 26 photoemission spectroscopy, 27 and cyclotron resonance techniques, 28 or by solving the Boltzmann transport equation using a four coefficients method. 29 Alternatively, m* may also be determined using spectroscopic ellipsometry (SE).…”