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Insulator-to-metal transition of SrTiO3:Nb single crystal surfaces induced by Ar+ bombardment  [PDF]
C. Rodenbücher,S. Wicklein,R. Waser,K. Szot
Physics , 2013, DOI: 10.1063/1.4795611
Abstract: In this paper, the effect of Ar+ bombardment of SrTiO3:Nb surface layers is investigated on the macro- and nanoscale using surface-sensitive methods. After bombardment, the stoichiometry and electronic structure are changed distinctly leading to an insulator-to-metal transition related to the change of the Ti "d" electron from d0 to d1 and d2. During bombardment, conducting islands are formed on the surface. The induced metallic state is not stable and can be reversed due to a redox process by external oxidation and even by self-reoxidation upon heating the sample to temperatures of 300{\deg}C.
Efficient resistive memory effect on SrTiO3 by ionic-bombardment  [PDF]
Heiko Gross,Seongshik Oh
Physics , 2011, DOI: 10.1063/1.3633114
Abstract: SrTiO3 is known to exhibit resistive memory effect either with cation-doping or with high-temperature thermal reduction. Here, we add another scheme, ionic-bombardment, to the list of tools to create resistive memory effect on SrTiO3 (STO). In an Ar-bombarded STO crystal, two orders of resistance difference was observed between the high and low resistive states, which is an order of magnitude larger than those achieved by the conventional thermal reduction process. One of the advantages of this new scheme is that it can be easily combined with lithographic processes to create spatially-selective memory effect.
Metal-insulator transitions in GdTiO3/SrTiO3 superlattices  [PDF]
Se Young Park,Andrew J. Millis
Physics , 2015,
Abstract: The density functional plus U method is used to obtain the electronic structure, lattice relaxation and metal-insulator phase diagram of superlattices consisting of $m$ layers of Gadolinium Titanate (GdTiO$_{3}$) alternating with $n$ layers of Strontium Titanate (SrTiO$_{3}$). Metallic phases occur when the number of SrTiO$_3$ layers is large or the interaction $U$ is small. In metallic phases, the mobile electrons are found in the SrTiO$_3$ layers, with near-interface electrons occupying $xy$-derived bands, while away from the interface the majority of electrons reside in $xz/yz$ bands. As the thickness of the SrTiO$_3$ layers decreases or the on-site interaction U increases a metal-insulator transition occurs. Two different insulating states are found. When the number of SrTiO$_{3}$ layers is larger than one, we find an insulating state with two sublattice charge and orbital ordering and associated Ti-O bond length disproportionations. When the number of SrTiO$_{3}$ units per layer is one, a different insulating phase occurs driven by orbital ordering within the quasi one-dimensional $xz/yz$ bonding bands connecting Ti atoms across the SrO layer. In this phase there is no sublattice charge ordering or bond disproportionation. The critical U for the single-layer insulator is $\sim$ 2.5 eV, much less than critical U $\sim$ 3.5 eV required to drive the metal-insulator transition when the number of SrTiO$_3$ is larger than one. Inconsistencies between the calculation and the experiment suggest that many-body correlations may be important. A local inversion symmetry breaking around Ti atoms suggests the possibility of in-plane ferroelectric polarization in the insulating phase.
Diodes with Breakdown Voltages Enhanced by the Metal-Insulator Transition of LaAlO$_3$-SrTiO$_3$ Interfaces  [PDF]
R. Jany,M. Breitschaft,G. Hammerl,A. Horsche,C. Richter,S. Paetel,J. Mannhart,N. Stucki,N. Reyren,S. Gariglio,P. Zubko,A. D. Caviglia,J. -M. Triscone
Physics , 2010, DOI: 10.1063/1.3428433
Abstract: Using the metal-insulator transition that takes place as a function of carrier density at the LaAlO$_3$-SrTiO$_3$ interface, oxide diodes have been fabricated with room-temperature breakdown voltages of up to 200 V. With applied voltage, the capacitance of the diodes changes by a factor of 150. The diodes are robust and operate at temperatures up to 270 C.
Tuning of metal-insulator transition of two-dimensional electrons at parylene/SrTiO$_3$ interface by electric field  [PDF]
H. Nakamura,H. Tomita,H. Akimoto,R. Matsumura,I. H. Inoue,T. Hasegawa,K. Kono,Y. Tokura,H. Takagi
Physics , 2008, DOI: 10.1143/JPSJ.78.083713
Abstract: Electrostatic carrier doping using a field-effect-transistor structure is an intriguing approach to explore electronic phases by critical control of carrier concentration. We demonstrate the reversible control of the insulator-metal transition (IMT) in a two dimensional (2D) electron gas at the interface of insulating SrTiO$_3$ single crystals. Superconductivity was observed in a limited number of devices doped far beyond the IMT, which may imply the presence of 2D metal-superconductor transition. This realization of a two-dimensional metallic state on the most widely-used perovskite oxide is the best manifestation of the potential of oxide electronics.
The interplay of electronic reconstructions, lattice distortions, and surface oxygen vacancies in insulator-metal transition of LaAlO$_{3}$/SrTiO$_{3}$  [PDF]
Jun Zhou,Teguh Citra Asmara,Ming Yang,George A. Sawatzky,Yuan Ping Feng,Andrivo Rusydi
Physics , 2015,
Abstract: The mechanism responsible for the extraordinary interface conductivity of LaAlO$_{3}$ on SrTiO$_{3}$ and its insulator-metal transition remains controversial. Here, using density functional theory calculations, we establish a comprehensive and coherent picture that the interplay of electronic reconstructions, lattice distortions, and surface oxygen vacancies fully compensates the polarization potential divergence in LaAlO$_{3}$/SrTiO$_{3}$, explaining naturally the experimental observations under different conditions. While lattice distortions and a charge redistribution between LaO and AlO$_2$ sub-layers play a dominant role in insulating state, a spontaneous appearance of 1/4 oxygen vacancies per AlO$_{2}$ sub-layer at the LaAlO$_{3}$ surface accompanied by 0.5$e^{-}$ charge-transfer into the interface is responsible for interface conductivity and the discontinuous transition in LaAlO$_{3}$/SrTiO$_{3}$. Our model also explain properties of LaAlO$_{3}$/SrTiO$_{3}$ prepared with different growth conditions.
Metal-Insulator Transition of the LaAlO3-SrTiO3 Interface Electron System  [PDF]
Y. C. Liao,T. Kopp,C. Richter,A. Rosch,J. Mannhart
Physics , 2010, DOI: 10.1103/PhysRevB.83.075402
Abstract: We report on a metal-insulator transition in the LaAlO3-SrTiO3 interface electron system, of which the carrier density is tuned by an electric gate field. Below a critical carrier density n_c ranging from 0.5-1.5 * 10^13/cm^2, LaAlO3-SrTiO3 interfaces, forming drain-source channels in field-effect devices are non-ohmic. The differential resistance at zero channel bias diverges within a 2% variation of the carrier density. Above n_c, the conductivity of the ohmic channels has a metal-like temperature dependence, while below n_c conductivity sets in only above a threshold electric field. For a given thickness of the LaAlO3 layer, the conductivity follows a sigma_0 ~(n - n_c)/n_c characteristic. The metal-insulator transition is found to be distinct from that of the semiconductor 2D systems.
Structural relaxation and metal-insulator transition at the interface between SrTiO3 and LaAlO3  [PDF]
Natalia Pavlenko,Thilo Kopp
Physics , 2009, DOI: 10.1016/j.susc.2011.03.016
Abstract: The electronic structure of interfaces between LaAlO$_3$ and SrTiO$_3$ is studied using local spin density approximation (LSDA) with intra-atomic Coulomb repulsion (LSDA+U). We find that the nature of the interface metallic states is strongly affected by the type of the structure (sandwich or bilayer) and by the termination surface of LaAlO$_3$. In all structures the atomic relaxation plays a crucial role in the electronic properties of the system. While in sandwiches the structural relaxation produces a significant polarization in SrTiO$_3$ and Jahn-Teller like splitting of Ti $3d_{xy}$ orbitals, in AlO$_2$-terminated bilayers the relaxation occurs primarily in LaAlO$_3$ and results in an insulator-metal transition which has been observed experimentally with increasing thickness of the LaAlO$_3$ layer.
Carrier freeze-out induced metal-insulator transition in oxygen deficient SrTiO3 films  [PDF]
Z. Q. Liu,D. P. Leusink,X. Wang,W. M. Lü,K. Gopinadhan,A. Annadi,Y. L. Zhao,X. H. Huang,S. W. Zeng,Z. Huang,A. Srivastava,S. Dhar,T. Venkatesan,Ariando
Physics , 2011, DOI: 10.1103/PhysRevLett.107.146802
Abstract: We report the optical, electrical transport, and magnetotransport properties of high quality oxygen deficient SrTiO3 (STO) single crystal film fabricated by pulsed laser deposition and reduced in the vacuum chamber. The oxygen vacancy distribution in the thin film is expected to be uniform. By comparing the electrical properties with oxygen deficient bulk STO, it was found that the oxygen vacancies in bulk STO is far from uniform over the whole material. The metal-insulator transition (MIT) observed in the oxygen deficient STO film was found to be induced by the carrier freeze-out effect. The low temperature frozen state can be re-excited by an electric field, Joule heating, and surprisingly also a large magnetic field.
Magnetism and metal-insulator transition in oxygen deficient SrTiO$_3$  [PDF]
Alejandro Lopez-Bezanilla,P. Ganesh,Peter B. Littlewood
Physics , 2014, DOI: 10.1103/PhysRevB.92.115112
Abstract: First-principles calculations to study the electronic and magnetic properties of bulk, oxygen-deficient SrTiO$_3$ (STO) under different doping conditions and densities have been conducted. The appearance of magnetism in oxygen-deficient STO is not determined solely by the presence of a single oxygen vacancy but by the density of free carriers and the relative proximity of the vacant sites. We find that while an isolated vacancy behaves as a non-magnetic double donor, manipulation of the doping conditions allows the stability of a single donor state, with emergent local moments coupled ferromagnetically by carriers in the conduction band. Strong local lattice distortions enhance the binding of this state. The energy of the in-gap local moment can be further tuned by orthorhombic strain. Consequently we find that the free-carrier density and strain are fundamental components to obtaining trapped spin-polarized electrons in oxygen-deficient STO, which may have important implications in the design of optical devices.
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