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Register a new userStructure and ferromagnetic instability of the oxygen-deficient SrTiO$_3$ surface
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Added by
Efstratios Manousakis
Publication date
2015
fields
Physics
and research's language is
English
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SrTiO$_3$ (STO) is the substrate of choice to grow oxide thin-films and oxide heterojunctions, which can form quasi-two-dimensional electronic phases that exhibit a wealth of phenomena, and, thus, a workhorse in the emerging field of metal-oxide electronics. Hence, it is of great importance to know the exact character of the STO surface itself under various oxygen environments. Using density functional theory within the spin generalized gradient approximation we have investigated the structural, electronic and magnetic properties of the oxygen-deficient STO surface. We find that the surface oxygen vacancies order in periodic arrays giving rise to surface magnetic moments and a quasi two-dimensional electron gas in the occupied Ti 3-d orbitals. The surface confinement, the oxygen-vacancy ordering, and the octahedra distortions give rise to spin-polarized $t_{2g}$ dispersive sub-bands; their energy split near the Brillouin zone center acts as an effective Zeeman term, which, when we turn on a Rashba interaction, produces bands with momentum-spin correlations similar to those recently discovered on oxygen deficient STO surface.
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Motivated by recent spin- and angular-resolved photoemission (SARPES) measurements performed on the two-dimensional electronic states confined near the (001) surface of SrTiO$_3$ in the presence of oxygen vacancies, we explore their spin structure by means of ab initio density functional theory (DFT) calculations of slabs. Relativistic nonmagnetic DFT calculations display Rashba-like spin winding with a splitting of a few meV and when surface magnetism on the Ti ions is in- cluded, bands become spin-split with an energy difference ~100 meV at the $Gamma$ point, consistent with SARPES findings. While magnetism tends to suppress the effects of the relativistic Rashba interaction, signatures of it are still clearly visible in terms of complex spin textures. Furthermore, we observe an atomic specialization phenomenon, namely, two types of electronic contributions: one is from Ti atoms neighboring the oxygen vacancies that acquire rather large magnetic moments and mostly create in-gap states; another comes from the partly polarized t$_{2g}$ itinerant electrons of Ti atoms lying further away from the oxygen vacancy, which form the two-dimensional electron system and are responsible for the Rashba spin winding and the spin splitting at the Fermi surface.
We report on the observation of metallic behavior in thin films of oxygen-deficient SrTiO$_3$ - down to 9 unit cells - when coherently strained on (001) SrTiO$_3$ or DyScO$_3$-buffered (001) SrTiO$_3$ substrates. These films have carrier concentrations of up to 2$times10^{22}$ cm$^{-3}$ and mobilities of up to 19,000 cm$^2$/V-s at 2 K. There exists a non-conducting layer in our SrTiO$_{3-delta}$ films that is larger in films with lower carrier concentrations. This non-conducting layer can be attributed to a surface depletion layer due to a Fermi level pinning potential. The depletion width, transport, and structural properties are not greatly affected by the insertion of a DyScO$_3$ buffer between the SrTiO$_3$ film and SrTiO$_3$ substrate.
The 2-dimensional electron system at the interface between LaAlO$_{3}$ and SrTiO$_{3}$ has several unique properties that can be tuned by an externally applied gate voltage. In this work, we show that this gate-tunability extends to the effective band structure of the system. We combine a magnetotransport study on top-gated Hall bars with self-consistent Schrodinger-Poisson calculations and observe a Lifshitz transition at a density of $2.9times10^{13}$ cm$^{-2}$. Above the transition, the carrier density of one of the conducting bands decreases with increasing gate voltage. This surprising decrease is accurately reproduced in the calculations if electronic correlations are included. These results provide a clear, intuitive picture of the physics governing the electronic structure at complex oxide interfaces.
We report the effect of oxygen pressure during growth ($P_{O_{2}}$) on the electronic and magnetic properties of PrAlO$_3$ films grown on $rm TiO_{2}$-terminated SrTiO$_3$ substrates. Resistivity measurements show an increase in the sheet resistance as $P_{O_{2}}$ is increased. The temperature dependence of the sheet resistance at low temperatures is consistent with Kondo theory for $P_{O_{2}} ge 10^{-5}$ torr. Hall effect data exhibit a complex temperature dependence that suggests a compensated carrier density. We observe behavior consistent with two different types of carriers at interfaces grown at $P_{O_{2}} ge 10^{-4}$ torr. For these interfaces, we measured a moderate positive magnetoresistance (MR) due to a strong spin-orbit (SO) interaction at low magnetic fields that evolves into a larger negative MR at high fields. Positive high MR values are associated with samples where a fraction of carriers are derived from oxygen vacancies. Analysis of the MR data permitted the extraction of the SO interaction critical field ( e.g. $ H_{SO}=$1.25 T for $P_{O_{2}}=10^{-5}$ torr). The weak anti-localization effect due to a strong SO interaction becomes smaller for higher $P_{O_{2}}$ grown samples, where MR values are dominated by the Kondo effect, particularly at high magnetic fields.
Inspired by the experimental findings of an exotic ferromagnetic insulating state in LaMnO$_3$/SrTiO$_3$ heterostructures, we calculate the electronic and magnetic state of LaMnO$_3$/SrTiO$_3$ superlattices with comparable thicknesses employing ab-initio dynamical mean-field theory. Projecting on the low-energy subspace of Mn $3d$ and Ti $3d$ states, and solving a multi-impurity problem, our approach emphasizes on local correlations at Mn and Ti sites. We find that a ferromagnetic insulating state emerges due to intrinsic effects of strong correlations in the system, in agreement with experimental studies. We also predict that, due to electronic correlations, the emerging 2D electron gas is located at the LMO side of the interface. This is in contrast to DFT results that locate the electron gas on the STO side. We estimate the transition temperature for the paramagnetic to ferromagnetic phase transition, which may be verified experimentally. Importantly, we also clarify that the epitaxial strain is a key ingredient for the emergence of the exotic ferromagnetic insulating state. This becomes clear from calculations on a strained LaMnO$_3$ system, also showing ferromagnetism which is not seen in the unstrained bulk material.
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