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Thickness Dependent Carrier Density at the Surface of SrTiO3 (111) Slabs  [PDF]
N. Sivadas,H. Dixit,Valentino R. Cooper,Di Xiao
Physics , 2013, DOI: 10.1103/PhysRevB.89.075303
Abstract: We investigate the surface electronic structure and thermodynamic stability of the SrTiO3 (111) slabs using density functional theory. We observe that, for Ti-terminated slabs it is indeed possible to create a two-dimensional electron gas (2DEG). However, the carrier density of the 2DEG displays a strong thickness dependence due to the competition between electronic reconstruction and polar distortions. As expected, having a surface oxygen atom at the Ti termination can stabilize the system, eliminating any electronic reconstruction, thereby making the system insulating. An analysis of the surface thermodynamic stability suggests that the Ti terminated (111) surface should be experimentally realizable. This surface may be useful for exploring the behavior of electrons in oxide (111) interfaces and may have implications for modern device applications.
Atomic oxygen adsorption and incipient oxidation of the Pb(111) surface: A density-functional theory study  [PDF]
Bo Sun,Ping Zhang,Zhigang Wang,Suqing Duan,Xian-Geng Zhao,Xuchun Ma,Qi-Kun Xue
Physics , 2007, DOI: 10.1103/PhysRevB.78.035421
Abstract: We study the atomic oxygen adsorption on Pb(111) surface by using density-functional theory within the generalized gradient approximation and a supercell approach. The atomic and energetic properties of purely on-surface and subsurface oxygen structures at the Pb(111) surface are systematically investigated for a wide range of coverages and adsorption sites. The fcc and tetra-II sites (see the text for definition) are found to be energetically preferred for the on-surface and subsurface adsorption, respectively, in the whole range of coverage considered. The on-surface and subsurface oxygen binding energies monotonically increase with the coverage, and the latter is always higher than the former, thus indicating the tendency to the formation of oxygen islands (clusters) and the higher stability of subsurface adsorption. The on-surface and subsurface diffusion-path energetics of atomic oxygen, and the activation barriers for the O penetration from the on-surface to the subsurface sites are presented at low and high coverages. In particular, it is shown that the penetration barrier from the on-surface hcp to the subsurface tetra-I site is as small as 65 meV at low coverage ($\Theta $=0.25). The other properties of the O/Pb(111) system, including the charge distribution, the lattice relaxation, the work function, and the electronic density of states, are also studied and discussed in detail, which consistently show the gradually stabilizing ionic O-Pb bond with increase of the oxygen coverage.
Origin of the two-dimensional electron gas at LaAlO3/SrTiO3 interfaces: The role of oxygen vacancies and electronic reconstruction  [PDF]
Z. Q. Liu,C. J. Li,W. M. Lü,X. H. Huang,Z. Huang,S. W. Zeng,X. P. Qiu,L. S. Huang,A. Annadi,J. S. Chen,J. M. D. Coey,T. Venkatesan,Ariando
Physics , 2013, DOI: 10.1103/PhysRevX.3.021010
Abstract: The relative importance of atomic defects and electron transfer in explaining conductivity at the crystalline LaAlO3/SrTiO3 interface has been a topic of debate. Metallic interfaces with similar electronic properties produced by amorphous oxide overlayers on SrTiO3 have called in question the original polarization catastrophe model. We resolve the issue by a comprehensive comparison of (100)-oriented SrTiO3 substrates with crystalline and amorphous overlayers of LaAlO3 of different thicknesses prepared under different oxygen pressures. For both types of overlayers, there is a critical thickness for the appearance of conductivity, but its value is always 4 unit cells (around 1.6 nm) for the oxygen-annealed crystalline case, whereas in the amorphous case, the critical thickness could be varied in the range 0.5 to 6 nm according to the deposition conditions. Subsequent ion milling of the overlayer restores the insulating state for the oxygen-annealed crystalline heterostructures but not for the amorphous ones. Oxygen post-annealing removes the oxygen vacancies, and the interfaces become insulating in the amorphous case. However, the interfaces with a crystalline overlayer remain conducting with reduced carrier density. These results demonstrate that oxygen vacancies are the dominant source of mobile carriers when the LaAlO3 overlayer is amorphous, while both oxygen vacancies and polarization catastrophe contribute to the interface conductivity in unannealed crystalline LaAlO3/SrTiO3 heterostructures, and the polarization catastrophe alone accounts for the conductivity in oxygen-annealed crystalline LaAlO3/SrTiO3 heterostructures. Furthermore, we find that the crystallinity of the LaAlO3 layer is crucial for the polarization catastrophe mechanism in the case of crystalline LaAlO3 overlayers.
Bandgap Controlling of the Oxygen-Vacancy-Induced Two-Dimensional Electron Gas in SrTiO3  [PDF]
Z. Q. Liu,W. Lu,S. W. Zeng,J. W. Deng,Z. Huang,C. J. Li,M. Motapothula,W. M. Lü,L. Sun,K. Han,J. Q. Zhong,P. Yang,N. N. Bao,W. Chen,J. S. Chen,Y. P. Feng,J. M. D. Coey,T. Venkatesan,Ariando
Physics , 2014, DOI: 10.1002/admi.201400155
Abstract: We report very large bandgap enhancement in SrTiO3 (STO) films (fabricated by pulsed laser deposition below 800 {\deg}C), which can be up to 20% greater than the bulk value, depending on the deposition temperature. The origin is comprehensively investigated and finally attributed to Sr/Ti antisite point defects, supported by density functional theory calculations. More importantly, the bandgap enhancement can be utilized to tailor the electronic and magnetic phases of the two-dimensional electron gas (2DEG) in STO-based interface systems. For example, the oxygen-vacancy-induced 2DEG (2DEG-V) at the interface between amorphous LaAlO3 and STO films is more localized and the ferromagnetic order in the STO-film-based 2DEG-V can be clearly seen from low-temperature magnetotransport measurements. 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 films are difficult to be doped into the fully metallic state by oxygen vacancies.
Anisotropic magnetotransport of electron gases at SrTiO3 (111) and (110) surfaces with high mobility  [PDF]
Ludi Miao,Renzhong Du,Yuewei Yin,Qi Li
Physics , 2015,
Abstract: Electron gases at the surfaces of (001), (110), and (111) oriented SrTiO3 (STO) have been created using Ar+-irradiation with fully metallic behavior and low-temperature-mobility as large as 5500 cm2V-1s-1, 1300 cm2V-1s-1 and 8600 cm2V-1s-1 for (001)-, (110)-, and (111)-surfaces, respectively. The in-plane anisotropic magnetoresistance (AMR) have been studied for the samples with the current along different crystal axis directions to subtract the Lorentz Force effect. The AMR shows features which coincide with the fixed orientations to the crystalline axes, with 4-fold, 2-fold and nearly-6-fold symmetries for (001)-, (110) and (111)-surfaces, respectively, independent of the current directions. These features are possibly caused by the polarization of spin orbit texture of the 2D Fermi surfaces. In addition, a 6-fold to 2-fold symmetry breaking for (111)-surfaces is observed. Our results demonstrate the effect of symmetry of two-dimensional electronic structure on the transport behaviors for the electron gases at STO surfaces.
Resonances in Electron Impact on Atomic Oxygen

WANG Yang,ZHOU Ya-Jun,JIAO Li-Guang,Kuru Ratnavelu,

中国物理快报 , 2008,
Abstract: The momentum-space coupled-channels-optical (CCO) method is used to study the resonances in electron--oxygen collision in the energy region of 9--12eV. Present results have shown agreement with the available experimentaland theoretical results, and new positions of resonances are found by the comparison of total cross sections.
Unexpected Anisotropic Two Dimensional Electron Gas at the LaAlO3/SrTiO3 (110) Interface  [PDF]
A. Annadi,X. Wang,K. Gopinadhan,W. M. Lu,A. Roy Barman,Z. Q. Liu,A. Srivastava,S. Saha,Y. L. Zhao,S. W. Zeng,S. Dhar,N. Tuzla,E. Olsson,Q. Zhang,B. Gu,S. Yunoki,S. Maekawa,H. Hilgenkamp,T. Venkatesan,Ariando
Physics , 2012, DOI: 10.1038/ncomms2804
Abstract: The observation of a two dimensional electron gas (2DEG) (1, 2), superconductivity (3, 4), magnetic effects (5) and electronic phase separation (6-8) at the interfaces of insulating oxides, especially LaAlO3/SrTiO3, has further enhanced the potential of complex oxides for novel electronics. The occurrence of the 2DEG is strongly believed to be driven by the polarization discontinuity (9) at the interface between the two oxides. In this scenario, the crystal orientation plays an important role and no conductivity would be expected for e.g., the interface between LaAlO3 and (110)-oriented SrTiO3, which should not have a polarization discontinuity (10, 11). Here, we report the observation of unexpected conductivity at the LaAlO3/SrTiO3 interface prepared on (110)-oriented SrTiO3. The conductivity was further found to be strongly anisotropic, with the ratio of the conductance along the different directions parallel to the substrate surface showing a remarkable dependence on the oxygen pressure during deposition. The conductance and its anisotropy are discussed based on the atomic structure at the interface, as revealed by Scanning Transmission Electron Microscopy (STEM) and further supported by density functional theory (DFT) calculations.
Surface composition of BaTiO3/SrTiO3(001) films grown by atomic oxygen plasma assisted molecular beam epitaxy  [PDF]
A. Barbier,C. Mocuta,D. Stanescu,P. Jegou,N. Jedrecy,H. Magnan
Physics , 2012, DOI: 10.1063/1.4768469
Abstract: We have investigated the growth of BaTiO3 thin films deposited on pure and 1% Nb-doped SrTiO3(001) single crystals using atomic oxygen assisted molecular beam epitaxy (AO-MBE) and dedicated Ba and Ti Knudsen cells. Thicknesses up to 30 nm were investigated for various layer compositions. We demonstrate 2D growth and epitaxial single crystalline BaTiO3 layers up to 10 nm before additional 3D features appear; lattice parameter relaxation occurs during the first few nanometers and is completed at {\guillemotright}10 nm. The presence of a Ba oxide rich top layer that probably favors 2D growth is evidenced for well crystallized layers. We show that the Ba oxide rich top layer can be removed by chemical etching. The present work stresses the importance of stoichiometry and surface composition of BaTiO3 layers, especially in view of their integration in devices.
Quantum oscillations in adsorption energetics of atomic oxygen on Pb(111) ultrathin films: A density-functional theory study  [PDF]
Ziyu Hu,Yu Yang,Bo Sun,Xiaohong Shao,Wenchuan Wang,Ping Zhang
Physics , 2009, DOI: 10.1063/1.3277674
Abstract: Using first-principles calculations, we have systematically studied the quantum size effects of ultrathin Pb(111) films on the adsorption energies and diffusion energy barriers of oxygen atoms. For the on-surface adsorption of oxygen atoms at different coverages, all the adsorption energies are found to show bilayer oscillation behaviors. It is also found that the work function of Pb(111) films still keeps the bilayer-oscillation behavior after the adsorption of oxygen atoms, with the values being enlarged by 2.10 to 2.62 eV. For the diffusion and penetration of the adsorbed oxygen atoms, it is found that the most energetically favored paths are the same on different Pb(111) films. And because of the modulation of quantum size effects, the corresponding energy barriers are all oscillating with a bilayer period on different Pb(111) films. Our studies indicate that the quantum size effect in ultrathin metal films can modulate a lot of processes during surface oxidation.
Spin-texture induced by oxygen vacancies in Strontium perovskites (001) surfaces: A theoretical comparison between SrTiO3 and SrHfO3  [PDF]
A. C. Garcia-Castro,M. G. Vergniory,E. Bousquet,A. H. Romero
Physics , 2015,
Abstract: The electronic structure of SrTiO3 and SrHfO3 (001) surfaces with oxygen vacancies is studied by means of first-principles calculations. We reveal how oxygen vacancies within the first atomic layer of the SrTiO3 surface (i) induce a large antiferrodistortive motion of the oxygen octahedra at the surface, (ii) drive localized magnetic moments on the Ti-3d orbitals close to the vacancies and (iii) form a two-dimensional electron gas localized within the first layers. The analysis of the spin-texture of this system exhibits a splitting of the energy bands according to the Zeeman interaction, lowering of the Ti-3dxy level in comparison with dxz and dyz and also an in-plane precession of the spins. No Rashba-like splitting for the ground state neither for ab-initio molecular dynamics trajectory at 400K is recognized as suggested recently by A. F. Santander-Syro et al. [1]. Instead, a sizeable Rashba-like splitting is observed when the Ti atom is replaced by a heavier Hf atom with a much larger spin-orbit interaction. However, we observe the disappearance of the magnetism and the surface two dimensional electron gas when full structural optimization of the SrHfO3 surface is performed. Our results uncover the sensitive interplay of spin-orbit coupling, atomic relaxations and magnetism when tuning these Sr-based perovskites.
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