Electrostriction at the LaAlO3/SrTiO3 interface

We present a direct comparison between experimental data and ab initio calculations for the electrostrictive effect in the polar LaAlO(3) layer grown on SrTiO(3) substrates. From the structural data, a complete screening of the LaAlO(3) dipole field is observed for film thicknesses between 6 and 20 uc. For thinner films, an expansion of the c axis of 2% matching the theoretical predictions for an electrostrictive effect is observed experimentally.     

Structural relaxation and metal-insulator transition at the interface between SrTiO3 and LaAlO3

The electronic structure of interfaces between LaAlO₃ and SrTiO₃ 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 LaAlO₃. 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 SrTiO₃ and a splitting of Ti 3d_xy orbitals, in AlO₂-terminated bilayers the relaxation occurs primarily in LaAlO₃ and results in an insulator-metal transition which has been observed experimentally with increasing thickness of the LaAlO₃ layer.     

Origin of charge density at LaAlO3 on SrTiO3 heterointerfaces: possibility of intrinsic doping

As discovered by Ohtomo and Hwang, a large sheet charge density with high mobility exists at the interface between SrTiO3 and LaAlO3. Based on transport, spectroscopic, and oxygen-annealing experiments, we conclude that extrinsic defects in the form of oxygen vacancies introduced by the pulsed laser deposition process used by all researchers to date to make these samples is the source of the large carrier densities. Annealing experiments show a limiting carrier density. We also present a model that explains the high mobility based on carrier redistribution due to an increased dielectric constant.     

Highly spin-polarized conducting state at the interface between nonmagnetic band insulators: LaAlO3/FeS2 (001)

First-principles density functional calculations demonstrate that a spin-polarized two-dimensional conducting state can be realized at the interface between two nonmagnetic band insulators. The (001) surface of the diamagnetic insulator FeS(2) (pyrite) supports a localized surface state deriving from Fe d orbitals near the conduction band minimum. The deposition of a few unit cells of the polar perovskite oxide LaAlO(3) leads to electron transfer into these surface bands, thereby creating a conducting interface. The occupation of these narrow bands leads to an exchange splitting between the spin subbands, yielding a highly spin-polarized conducting state distinct from the rest of the nonmagnetic, insulating bulk. Such an interface presents intriguing possibilities for spintronics applications.     

Gate-controlled spin injection at LaAlO3/SrTiO3 interfaces

We report results of electrical spin injection at the high-mobility quasi-two-dimensional electron system (2-DES) that forms at the LaAlO3/SrTiO3 interface. In a nonlocal, three-terminal measurement geometry, we analyze the voltage variation associated with the precession of the injected spin accumulation driven by perpendicular or transverse magnetic fields (Hanle and inverted Hanle effect). The influence of bias and back-gate voltages reveals that the spin accumulation signal is amplified by resonant tunneling through localized states in the LaAlO3 strongly coupled to the 2-DES by tunneling transfer.     

Structural basis for the conducting interface between LaAlO3 and SrTiO3

The complete atomic structure of a five-monolayer film of LaAlO3 on SrTiO3 has been determined for the first time by surface x-ray diffraction in conjunction with the coherent Bragg rod analysis phase-retrieval method and further structural refinement. Cationic mixing at the interface results in dilatory distortions and the formation of metallic La(1-x)SrxTiO3. By invoking electrostatic potential minimization, the ratio of Ti{4+}/Ti{3+} across the interface was determined, from which the lattice dilation could be quantitatively explained using ionic radii considerations. The correctness of this model is supported by density functional theory calculations. Thus, the formation of a quasi-two-dimensional electron gas in this system is explained, based on structural considerations.     

Gate tunable Rashba spin-orbit coupling at CaZrO3/SrTiO3 heterointerface

High mobility quasi two-dimensional electron gas (2DEG) found at the CaZrO₃/SrTiO₃ nonpolar heterointerface is attractive and provides a platform for the development of functional devices and nanoelectronics. Here we report that the carrier density and mobility at low temperature can be tuned by gate voltage at the CaZrO₃/SrTiO₃ interface. Furthermore, the magnitude of Rashba spin-orbit interaction can be modulated and increases with the gate voltage. Remarkably, the diffusion constant and the spin-orbit relaxation time can be strongly tuned by gate voltage. The diffusion constant increases by a factor of ～ 19.98 and the relaxation time is reduced by a factor of over three orders of magnitude while the gate voltage is swept from -50 V to 100 V. These findings not only lay a foundation for further understanding the underlying mechanism of Rashba spin-orbit coupling, but also have great significance in developing various oxide functional devices.     

Two-dimensional quantum oscillations of the conductance at LaAlO3/SrTiO3 interfaces

We report on a study of magnetotransport in LaAlO3 /SrTiO3 interfaces characterized by mobilities of the order of several thousands cm2/V s. We observe Shubnikov-de Haas oscillations whose period depends only on the perpendicular component of the magnetic field. This observation directly indicates the formation of a two-dimensional electron gas originating from quantum confinement at the interface. From the temperature dependence of the oscillation amplitude we extract an effective carrier mass m* ? 1.45 m(e). An electric field applied in the back-gate geometry increases the mobility, the carrier density, and the oscillation frequency.     

Strain-Gradient-Induced Modulation of Carrier Density and Mobility at the LaAlO3/SrTiO3 Heterointerface

A strain gradient induced by mechanical bending on a SrTiO₃ substrate is demonstrated, and has a pronounced influence on the carrier density and mobility of the interfacial 2D electron gas at the LaAlO₃/SrTiO₃ heterointerface. Tensile and compressive strain gradients represent two states of upward and downward bending. Under the tensile strain gradient, the carrier density decreases and the mobility has about 200% increase. Conversely, under the compressive strain gradient, the mobility decreases and the carrier density increases by up to 107%. These results demonstrate a range of opportunities to modulate the carrier density and mobility at oxide heterointerface and open up a promising way for further research on application of oxide devices.     

Spontaneous 2-dimensional carrier confinement at the n-type SrTiO3/LaAlO3 interface

We describe the intrinsic mechanism of 2-dimensional electron confinement at the n-type SrTiO3/LaAlO3 interface as a function of the sheet carrier density n(s) via advanced first-principles calculations. Electrons localize spontaneously in Ti 3d(xy) levels within a thin (?2 nm) interface-adjacent SrTiO3 region for n(s) lower than a threshold value n(c)～10(14) cm(-2). For n(s)>n(c) a portion of charge flows into Ti 3d(xz)-d(yz) levels extending farther from the interface. This intrinsic confinement can be attributed to the interface-induced symmetry breaking and localized nature of Ti 3d t(2g) states. The sheet carrier density directly controls the binding energy and the spatial extension of the conductive region. A direct, quantitative relation of these quantities with n(s) is provided.     

Understanding the mechanism of conductivity at the LaAlO3/SrTiO3(001) interface

The observation of conductivity at (001)-oriented interfaces of the 2 band insulators LaAlO₃ and SrTiO₃ is both fascinating and potentially useful for next-generation electronics. The paradigm commonly used to explain this phenomenon is an electronic reconstruction resulting from the instability created by forming an interface of polar and nonpolar perovskites, leading to the formation of a two-dimensional electron gas. This explanation has typically been conceptualized within the framework of an atomically abrupt interface. However, a significant and growing body of data now exists which reveals that the interface is not abrupt, and that all four cations diffuse across the interface. Yet, the potential roles of the resulting defects and dopants in alleviating the polar catastrophe and promoting conductivity are rarely considered. The purpose of this prospective is to take an overview of the field from outside the reigning paradigm and consider ways in which dopants and defects might affect the electronic structure.     

Evolution of the interfacial structure of LaAlO3 on SrTiO3

The evolution of the atomic structure of LaAlO_{3} grown on SrTiO_{3} was investigated using surface x-ray diffraction in conjunction with model-independent, phase-retrieval algorithms between two and five monolayers film thickness. A depolarizing buckling is observed between cation and oxygen positions in response to the electric field of polar LaAlO_{3}, which decreases with increasing film thickness. We explain this in terms of competition between elastic strain energy, electrostatic energy, and electronic reconstructions. Based on these structures, the threshold for formation of a two-dimensional electron system at a film thickness of 4 monolayers is quantitatively explained. The findings are also qualitatively reproduced by density-functional-theory calculations.     

Estimation of the concentration of charge carriers in a conductive layer between LaAlO3 and SrTiO3 dielectrics

Using the capacitor model by Goetzberger, the concentration of charge carriers on the contact layer LAO’STO is evaluated. The obtained results are shown to be comparable with those obtained within the framework of “polar catastrophe” model. Some possible interpretations of charge carriers, concentration variations, depending on the thickness of deposited layers, are presented as well, both for the “pure” (LAO) and composite (LASTO) ones.     

Structural comparison of n-type and p-type LaAlO3/SrTiO3 interfaces

Using a surface x-ray diffraction technique, we investigated the atomic structure of two types of interfaces between LaAlO3 and SrTiO3, that is, p-type (SrO/AlO2) and n-type (TiO2/LaO) interfaces. Our results demonstrate that the SrTiO3 in the sample with the n-type interface has a large polarized region, while that with the p-type interface has a limited polarized region. In addition, atomic intermixing was observed to extend deeper into the SrTiO3 substrate at the n-type interface compared to the p type. These differences result in distinct degrees of band bending, which likely contributes to the striking contrast in electrical conductivity between the two types of interfaces.     

Instability,intermixing and electronic structure at the epitaxial LaAlO3/SrTiO3(001) heterojunction

The question of stability against diffusional mixing at the prototypical LaAlO₃/SrTiO₃(001) interface is explored using a multi-faceted experimental and theoretical approach. We combine analytical methods with a range of sensitivities to elemental concentrations and spatial separations to investigate interfaces grown using on-axis pulsed laser deposition. We also employ computational modeling based on the density function theory as well as classical force fields to explore the energetic stability of a wide variety of intermixed atomic configurations relative to the idealized, atomically abrupt model. Statistical analysis of the calculated energies for the various configurations is used to elucidate the relative thermodynamic stability of intermixed and abrupt configurations. We find that on both experimental and theoretical fronts, the tendency toward intermixing is very strong. We have also measured and calculated key electronic properties such as potential energy gradients and valence band discontinuity at the interface. We find no measurable electric field in either the LaAlO₃ or SrTiO₃, and that the valence band offset is near zero, partitioning the band discontinuity almost entirely to the conduction band edge. Significantly, we find it is not possible to account for these electronic properties theoretically without including extensive intermixing in our physical model of the interface. The atomic configurations which give the greatest electrostatic stability are those that eliminate the interface dipole by intermixing, calling into question the conventional explanation for conductivity at this interface—electronic reconstruction. Rather, evidence is presented for La indiffusion and doping of the SrTiO₃ below the interface as being the cause of the observed conductivity.     

Electro- and Magnetotransport near a LaAlO3/SrTiO3 Interphase Boundary

Comparison of the channel spectra of medium energy ion scattering, visualized for LaAlO₃/(001)SrTiO₃ heterostructures with a thickness of the lanthanum aluminate layer of one to six unit cells, indicates that the lanthanum aluminate layer grows coherently on a TiO₂-terminated surface of a strontium titanate substrate. The resistance of the interphase boundary in the heterostructure with a thickness of the LaAlO₃ layer of six unit cells decreased with temperature. At T < 100 K, the positive magnetoresistance and Hall mobility of electrons increased sharply with decreasing temperature.

     

Stable pH sensitivity of LaAlO3/SrTiO3 interfacial electronic gas

Probing pH value is essential for many applications. The request of developing stable and highly sensitive pH sensor with small size are increased recently, this is a challenge to the traditional glass pH sensors. This work investigated the pH sensing performance of the novel two-dimensional electronic gas (2DEG) at the LaAlO₃/SrTiO₃ heterostructure interface discovered recently. The experimental results demonstrated that the devices host excellent sensing ability to the pH value of aqueous solutions. Quite stable output current is realized for a given pH value, and the output current is linearly dependent on pH value as required by sensor applications. This work would prompt the application research of LaAlO₃/SrTiO₃ heterostructure and design of new generation of advanced pH sensors.     

Synthesis of LaAlO3 by metal organic decomposition on SrTiO3 substrates

LaAlO₃ (LAO) films on SrTiO₃ (STO) substrates have been produced by metal organic decomposition using La(C₅H₇O₂)₃ and Al(C₅H₇O₂)₃ as precursors dissolved in propionic acid. The process consists of growing thin layers through dip coating and subsequent annealing. After testing different cationic ratios of La and Al, it was determined that an optimal ratio leads to a single LAO phase film that grows epitaxially (cube on cube) on top of the STO. This was shown by X-ray diffraction (XRD) and transmission electron microscopy (TEM) analyses. These analyses, as well as additional X-ray reflectivity analysis, also revealed that the LAO's thickness obtained in one dip ranges from 8 nm to 16 nm. Taking advantage of the epitaxial conditions, several layers can be stacked by successive dip coatings and annealing to form an epitaxial structure.     

Quantum criticality between topological and band insulators in 3+1 dimensions

Four-component massive and massless Dirac fermions in the presence of long range Coulomb interaction and chemical potential disorder exhibit striking fermionic quantum criticality. For an odd number of flavors of Dirac fermions, the sign of the Dirac mass distinguishes the topological and the trivial band insulator phases, and the gapless semimetallic phase corresponds to the quantum critical point that separates the two. Up to a critical strength of disorder, the semimetallic phase remains stable, and the universality class of the direct phase transition between two insulating phases is unchanged. Beyond the critical strength of disorder the semimetallic phase undergoes a phase transition into a disorder controlled diffusive metallic phase, and there is no longer a direct phase transition between the two types of insulating phases.