Electron–Lattice Coupling in Correlated Materials of Low Electron Occupancy (original) (raw)
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Bandgap Controlling of the Oxygen-Vacancy-Induced Two-Dimensional Electron Gas in SrTiO3
Strongly correlated oxides are full of fascinating phenomena owing to their interacting lattice, charge, spin and orbital degrees of freedom.[1–5] Bandgap, a critical parameter for an oxide insulator, is well determined by those degrees of freedom and in turn directly affects electronic, magnetic and optical properties of the material. Typically, tunability of the bandgap in an oxide insulator can be achieved through chemical doping,[6] which is important for electronic and photonic device applications. Here we report large bandgap enhancement in SrTiO3 (STO) thin films, which can be up to 20% greater than the bulk value, depending on the deposition temperature. There is no significant change in density and cationic ratio of the oxide so the effect is attributed to Sr/Ti antisite defects, an attribution supported by density functional theory calculations. It was found that the bandgap enhancement significantly changes the electronic and magnetic phases in the oxygen-vacancy-induced two-dimensional electron gas at the interface between amorphous LaAlO3 (LAO) and STO. This opens an attractive path to tailor electronic, magnetic and optical properties of STO-based oxide interface systems under intensive focus in the oxide electronics community. Meanwhile, our study provides key insight into the origin of the fundamental issue that STO thin films are difficult to convert into metals by oxygen vacancy doping
Symmetry breaking at the (111) interfaces of SrTiO3 hosting a two-dimensional electron system
Physical Review B
We used x-ray absorption spectroscopy to study the orbital symmetry and the energy band splitting of (111) LaAlO 3 /SrTiO 3 and LaAlO 3 /EuTiO 3 /SrTiO 3 heterostructures, hosting a quasi two-dimensional electron system (q2DES), and of a Ti-terminated (111) SrTiO 3 single crystal, also known to form a q2DES at its surface. We demonstrate that the bulk tetragonal Ti-3d D 4h crystal field is turned into trigonal D 3d crystal field in all cases. The symmetry adapted a 1g and e π g orbitals are non-degenerate in energy and their splitting, Δ, is positive at the bare STO surface but negative in the heterostructures, where the a 1g orbital is lowest in energy. These results demonstrate that the interfacial symmetry breaking induced by epitaxial engineering of oxide interfaces has a dramatic effect on their electronic properties, and it can be used to manipulate the ground state of the q2DES.
Origin of the asymmetric orbital--lattice interactions in correlated oxide heterostructures
Arxiv preprint arXiv: …, 2010
Using resonant X-ray spectroscopies combined with density functional calculations, we find an asymmetric bi-axial strain-induced d-orbital response in ultra-thin films of the correlated metal LaNiO3 which are not accessible in the bulk. The sign of the misfit strain governs the stability of an octahedral "breathing" distortion, which, in turn, produces an emergent charge-ordered ground state with an altered ligand-hole density and bond covalency. Control of this new mechanism opens a pathway to rational orbital engineering, providing a platform for artificially designed Mott materials. 71.20.Be Heteroepitaxial synthesis is a powerful avenue to modify orbital-lattice interactions in correlated materials with strong electron-electron interactions derived from transition metals with open d-shell configurations [1-3]. Epitaxial strain allows access to latent electronic functionalities and phases that do not exist in bulk equilibrium phase diagrams . However, efforts to rationally control properties that are exceedingly sensitive to small perturbations [9] through the orbital-lattice interaction are impeded by the poor understanding of how heteroepitaxy imposes constraints on the orbital response . Despite the recent progress in strain-induced orbital engineering, a crucial fundamental question remains: when a single electron occupies a doubly degenerate d-orbital in a cubic crystal field as in perovskites with Cu 2+ , Mn 3+ or low spin Ni 3+ cations, how does the substrate imposed epitaxial constraints dictate the correlated orbital responses of ultrathin films?
Enhanced electron-electron correlations in nanometric SrRuO3 epitaxial films
Physical Review B, 2003
Epitaxial and fully strained SrRuO 3 thin films have been grown on SrTiO 3 (100). At initial stages the growth mode is three-dimensional-͑3D-͒like, leading to a finger-shaped structure aligned with the substrate steps and that eventually evolves into a 2D step-flow growth. We study the impact that the defect structure associated with this unique growth mode transition has on the electronic properties of the films. Detailed analysis of the transport properties of nanometric films reveals that microstructural disorder promotes a shortening of the carrier mean free path. Remarkably enough, at low temperatures, this results in a reinforcement of quantum corrections to the conductivity as predicted by recent models of disordered, strongly correlated electronic systems. This finding may provide a simple explanation for the commonly observed-in conducting oxidesresistivity minima at low temperature. Simultaneously, the ferromagnetic transition occurring at about 140 K, becomes broader as film thickness decreases down to nanometric range. The relevance of these results for the understanding of the electronic properties of disordered electronic systems and for the technological applications of SrRuO 3-and other ferromagnetic and metallic oxides-is stressed.
Electron-Phonon Interaction and Charge Carrier Mass Enhancement in SrTiO3
Physical Review Letters, 2008
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
Asymmetric Orbital-Lattice Interactions in Ultrathin Correlated Oxide Films
Physical Review Letters, 2011
Using resonant x-ray spectroscopies combined with density functional calculations, we find an asymmetric biaxial strain-induced d-orbital response in ultrathin films of the correlated metal LaNiO 3 which are not accessible in the bulk. The sign of the misfit strain governs the stability of an octahedral ''breathing'' distortion, which, in turn, produces an emergent charge-ordered ground state with an altered ligand-hole density and bond covalency. Control of this new mechanism opens a pathway to rational orbital engineering, providing a platform for artificially designed Mott materials.
Physical Review B, 2013
A combined experimental and theoretical investigation of the electronic structure of the archetypal oxide heterointerface system LaAlO3 on SrTiO3 is presented. High-resolution, hard x-ray photoemission is used to uncover the occupation of Ti 3d states and the relative energetic alignment -and hence internal electric fields -within the LaAlO3 layer. Firstly, the Ti 2p core level spectra clearly show occupation of Ti 3d states already for two unit cells of LaAlO3. Secondly, the LaAlO3 core levels were seen to shift to lower binding energy as the LaAlO3 overlayer thickness, n, was increased -agreeing with the expectations from the canonical electron transfer model for the emergence of conductivity at the interface. However, not only is the energy offset of only ∼300meV between n = 2 (insulating interface) and n = 6 (metallic interface) an order of magnitude smaller than the simple expectation, but it is also clearly not the sum of a series of unit-cell by unit-cell shifts within the LaAlO3 block. Both of these facts argue against the simple charge-transfer picture involving a cumulative shift of the LaAlO3 valence bands above the SrTiO3 conduction bands, resulting in charge transfer only for n ≥ 4. We discuss effects which could frustrate this elegant and simple charge transfer model, concluding that although it cannot be ruled out, photodoping by the x-ray beam is unlikely to be the cause of the observed behavior. Turning to the theoretical data, our density functional simulations show that the presence of oxygen vacancies at the LaAlO3 surface at the 25% level reverses the direction of the internal field in the LaAlO3. Therefore, taking the experimental and theoretical results together, a consistent picture emerges for real-life samples in which nature does not wait until n = 4 and already for n = 2, mechanisms other than internal-electric-field-driven electron transfer from idealized LaAlO3 to near-interfacial states in the SrTiO3 substrate are active in heading off the incipient polarization catastrophe that drives the physics in these systems.
Electronic structure of electron dopedSrTiO3:SrTiO3−δandSr1−xLaxTiO3
Physical Review B
Electronic structures of electron-doped SrTiO 3Ϫ␦ and Sr 1Ϫx La x TiO 3 have been investigated within the ab initio band-structure approach using a supercell containing eight basic units. A small amount of electron doping is found to drive SrTiO 3 metallic, with the Fermi level moving into the conduction band in both the systems. Clustering of oxygen vacancies in the case of SrTiO 3Ϫ␦ gives rise to distinct non-rigid-band-like evolution of the electronic structure, trapping doped charge carriers in midgap states; similar effects are not observed for Sr 1Ϫx La x TiO 3 , explaining the difference in the Hall measurements of these two closely related compounds. ͓S0163-1829͑98͒01404-0͔
Role of trivalent Sr substituents and Sr vacancies in tetragonal and polar states of SrTiO3
Acta Materialia, 2011
Inelastic light scattering is used to study lattice dynamics of strontium titanate (STO) ceramics with several heterovalent dopants (La 3+ , Gd 3+ , Y 3+), which substitute Sr 2+ ions. An extraordinary shift of the antiferrodistortive transition temperature (T a) is ascertained when just a small percentage of any of the dopants is used. T a is dependent on the tolerance factor (t). In this work, it is clearly shown that, regardless of the dopant used, a common linear dependence of T a vs. t is obtained if strontium vacancies are taken into account. A vacancy size of 1.547A˚wasestimated,whichis1.547 Å was estimated, which is 1.547A˚wasestimated,whichis7% larger than the Sr 2+ radius. The vacancy size obtained can directly explain the increase in lattice parameter with increasing Bi 3+ content in Bi-doped STO, as opposed to the dopants referred to above. Furthermore, the introduction of La 3+ , Gd 3+ or Y 3+ ions at the Sr site causes a considerable stiffening of the transverse optic TO 1 mode at low temperatures, thereby decreasing the phonon contribution to the dielectric permittivity. Thus, no traces of a ferroelectric phase are found for any of the dopants used in this work.
Tuning the electronic effective mass in double-doped SrTiO_{3}
Physical Review B, 2011
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 Sr1−xLaxTiO 3−δ , where we can tune the effective mass ranging from 6-20me 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 Sr1−xLaxTiO 3−δ . 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.