Using local density approximation (LDA) calculations we predict GdFeO$_3$-like rotation of TiO$_6$ octahedra at the $n$-type interface between LaAlO$_3$ and SrTiO$_3$. The narrowing of the Ti $d$ bandwidth which results means that for very modest values of $U$, LDA$+U$ calculations predict charge and spin ordering at the interface. Recent experimental evidence for magnetic interface ordering may be understood in terms of the close proximity of an antiferromagnetic insulating ground state to a ferromagnetic metallic excited state.
A two-dimensional electron liquid is formed at the n-type interface between SrTiO3 and LaAlO3. Here we report on Kelvin probe microscopy measurements of the electronic compressibility of this electron system. The electronic compressibility is found to be negative for carrier densities of \approx10^13/cm^2. At even smaller densities, a metal-to-insulator transition occurs. These local measurements corroborate earlier measurements of the electronic compressibility of LaAlO3-SrTiO3 interfaces obtained by measuring the capacitance of macroscopic metal-LaAlO3-SrTiO3 capacitors.
Perovskite oxide interfaces have attracted tremendous research interest for their fundamental physics and promising all-oxide electronics applications. Here, based on first-principles calculations, we propose a novel surface La interstitial promoted interface insulator-metal transition in LaAlO3/SrTiO3 (110). Compared with surface oxygen vacancies, which play a determining role on the insulator-metal transition of LaAlO3/SrTiO3 (001) interfaces, we find that surface La interstitials can be more experimentally realistic and accessible for manipulation and more stable in ambient atmospheric envi…
Strontium titanate, SrTiO3, a widely used substrate material for electronic oxide thin film devices, has provided many interesting features. In a combination with a similar oxide material, LaAlO3, it has recently received great interest. It was suggested that two-dimensional electron gas is formed at the interface between SrTiO3 and LaAlO3, resulting in high electrical conductivity and mobility. In this report we demonstrate that the transport properties in those heterostructures are very sensitive to the deposition parameters during thin film growth. Using cathode- and photoluminescence studi…
Possible ferromagnetism induced in otherwise non-magnetic materials has been motivating intense research in complex oxide heterostructures. Here we show that a confined magnetism is realized at the interface between SrTiO3 and two insulating polar oxides, BiMnO3 and LaAlO3. By using polarization dependent x-ray absorption spectroscopy, we find that in both cases the magnetic order is stabilized by a negative exchange interaction between the electrons transferred to the interface and local magnetic moments. These local magnetic moments are associated to Ti3+ ions at the interface itself for LaA…
Using LaAlO$_3$-SrTiO$_3$ bilayers, we have fabricated field-effect devices that utilize the two-dimensional electron liquid generated at the bilayers' {\textit n}-type interfaces as drain-source channels and the LaAlO$_3$ layers as gate dielectrics. With gate voltages well below 1\,V, the devices are characterized by voltage gain and current gain. The devices were operated at temperatures up to 100\,°C.
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$_2…
Using a combination of vertical transport measurements across and lateral transport measurements along the LaAlO$_{3}$/SrTiO$_{3}$ heterointerface, we demonstrate that significant potential barrier lowering and band bending are the cause of interfacial metallicity. Barrier lowering and enhanced band bending extends over 2.5 nm into LaAlO$_{3}$ as well as SrTiO$_{3}$. We explain origins of high-temperature carrier saturation, lower carrier concentration, and higher mobility in the sample with the thinnest LaAlO$_{3}$ film on a SrTiO$_{3}$ substrate. Lateral transport results suggest that parasi…
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 $3d_{xy}$ orbit…
The relative significance of quantum conductivity correction and magnetic nature of electrons in understanding the intriguing low-temperature resistivity minimum and negative magnetoresistance of the two-dimensional electron gas at LaAlO3/SrTiO3 interfaces has been a long outstanding issue since its discovery. Here we report a comparative magnetotransport study on amorphous and oxygen-annealed crystalline LaAlO3/SrTiO3 heterostructures at a relatively high-temperature range, where the orbital scattering is largely suppressed by thermal fluctuations. Despite of a predominantly negative out-of-p…
The time-resolved photoconductance of amorphous and crystalline LaAlO$_3$/SrTiO$_3$ interfaces, both hosting an interfacial 2-dimensional electron gas, is investigated under irradiation by variable-wavelengths, visible or ultraviolet photons. Unlike bare SrTiO$_3$ single crystals, showing relatively small photoconductance effects, both kinds of interfaces exhibit an intense and highly persistent photoconductance with extraordinarily long characteristic times. The temporal behaviour of the extra photoinduced conductance persisting after light irradiation shows a complex dependence on interface …
When insulator LaAlO3 is grown by epitaxy onto a TiO2-terminated {100} surface of insulator SrTiO3, the resulting system has a metallic character. This phenomenon has been associated with an electrostatic frustration at the interface, as {100} surfaces of SrTiO3 are neutral while those of LaAlO3 are polar, but its microscopic mechanism is not quite understood. Here, we present a structural characterisation of this interface by aberration-corrected transmission electron microscopy. The unit cells at the interface appear elongated: we discuss this distortion in terms of electrostatic charge and …
LaAlO$_{3}$ and NdGaO$_{3}$ thin films of different thickness have been grown by pulsed laser deposition on TiO$_2$-terminated SrTiO$_{3}$ single crystals and investigated by soft X-ray photoemission spectroscopy. The surface sensitivity of the measurements has been tuned by varying photon energy $hν$ and emission angle $Θ$. In contrast to the core levels of the other elements, the Sr $3d$ line shows an unexpected splitting for higher surface sensitivity, signaling the presence of a second strontium component. From our quantitative analysis we conclude that during the growth process Sr atoms d…
Recently, it was established that a two-dimensional electron system can arise at the interface between two oxide insulators LaAlO3 and SrTiO3. This paradigmatic example exhibits metallic behaviour and magnetic properties between non-magnetic and insulating oxides. Despite a huge amount of theoretical and experimental work a thorough understanding has yet to be achieved. We analyzed the structural deformations of a LaAlO3 (001) slab induced by hydrogen ad-atoms and oxygen vacancies at its surface by means of density functional theory. Moreover, we investigated the influence of surface reconstru…
We find that the sheet resistance ($R_s$) of patterned samples of ${\rm LaAlO_3}/{\rm SrTiO_3}$ with a length scale of several microns may increase significantly at low temperatures in connection with driving electrical currents and applying in-plane magnetic fields. As the samples are warmed up, $R_s$ recovers to its original value with accelerated recovery near ${\rm 70 \ K}$ and ${\rm 160 \ K}$. Concomitantly with the increase in $R_s$, the carrier density and the mobility decrease and magnetotransport properties which may be linked to magnetism are suppressed.
Localization of electrons in the two-dimensional electron gas at the LaAlO$_3$/SrTiO$_3$ interface is investigated by varying the channel thickness in order to establish the nature of the conducting channel. Layers of SrTiO$_3$ were grown on NdGaO$_3$ (110) substrates and capped with LaAlO$_3$. When the SrTiO$_3$ thickness is $\leq 6$ unit cells, most electrons at the interface are localized, but when the number of SrTiO$_3$ layers is 8-16, the free carrier density approaches $3.3 \times 10^{14}$ cm$^{-2}$, the value corresponding to charge transfer of 0.5 electron per unit cell at the interfa…
We report a detailed analysis of magneto-transport properties of top- and back-gated LaAlO$_3$/SrTiO$_3$ heterostructures. Efficient modulation in magneto-resistance, carrier density, and mobility of the two-dimensional electron liquid present at the interface is achieved by sweeping top and back gate voltages. Analyzing those changes with respect to the carrier density tuning, we observe that the back gate strongly modifies the electron mobility while the top gate mainly varies the carrier density. The evolution of the spin-orbit interaction is also followed as a function of top and back gati…
We investigate the effect of oxygen vacancies and hydrogen dopants at the surface and inside slabs of LaAlO3, SrTiO3, and LaAlO3/SrTiO3 heterostructures on the electronic properties by means of electronic structure calculations as based on density functional theory. Depending on the concentration, the presence of these defects in LaAlO3 slab can suppress the surface conductivity. In contrast, in insulating SrTiO3 slabs already very small concentrations of oxygen vacancies or hydrogen dopant atoms induce a finite occupation of the conduction band. Surface defects in insulating LaAlO3/SrTiO3 het…
We shed light on the interplay between structure and many-body effects relevant for itinerant ferromagnetism in LaAlO$_3$/SrTiO$_3$ heterostructures. The realistic correlated electronic structure is studied by means of the (spin-polarized) charge self-consistent combination of density functional theory (DFT) with dynamical mean-field theory (DMFT) beyond the realm of static correlation effects. Though many-body behavior is also active in the defect-free interface, a ferromagnetic instability occurs only with oxygen vacancies. A minimal Ti two-orbital $e_g$-$t_{2g}$ description for the correlat…
We investigate the electronic properties of the LaAlO3/SrTiO3 interface using density functional theory. In contrast to previous studies, which relied on (semi-)local functionals and the GGA+U method, we here use a recently developed hybrid functional to determine the electronic structure. This approach offers the distinct advantage of accessing both the metallic and insulating multilayers on a parameter-free equal footing. As compared to calculations based on semilocal GGA functionals, our hybrid functional calculations lead to a considerably increased band gap for the insulating systems. The…
Conventional two-dimensional electron gases are realized by engineering the interfaces between semiconducting compounds. In 2004, Ohtomo and Hwang discovered that an electron gas can be also realized at the interface between large gap insulators made of transition metal oxides [1]. This finding has generated considerable efforts to clarify the underlying microscopic mechanism. Of particular interest is the LaAlO3/SrTiO3 system, because it features especially striking properties. High carrier mobility [1], electric field tuneable superconductivity [2] and magnetic effects [3], have been found. …
Realization of a fully metallic two-dimensional electron gas at the interface between artificially-grown LaAlO$_3$ and SrTiO$_3$ thin films has been an exciting challenge. Here we present for the first time the successful realization of a superconducting 2DEG at interfaces between artificially-grown LaAlO$_3$ and SrTiO$_3$ thin films. Our results highlight the importance of two factors-the growth temperature and the SrTiO$_3$ termination. We use local friction force microscopy and transport measurements to determine that in normal growth conditions the absence of a robust metallic state at low…
We investigated the electronic structure of the SrTiO$_3$/LaAlO$_3$ superlattice (SL) by resonant soft x-ray scattering. The (003) peak, which is forbidden for our "ideal" SL structure, was observed at all photon energies, indicating reconstruction at the interface. From the peak position analyses taking into account the effects of refraction, we obtained evidence for electronic reconstruction of Ti 3d and O $2p$ states at the interface. From reflectivity analyses, we concluded that the AlO$_2$/LaO/TiO$_2$/SrO and the TiO$_2$/SrO/AlO$_2$/LaO interfaces are quite different, leading to highly as…
Understanding the role of defects in oxide heterostructures is crucial for future materials control and functionalization. We hence study the impact of oxygen vacancies (OVs) at variable concentrations on orbital- and spin exchange in the LaAlO$_3$/SrTiO$_3$ interface by first principles many-body theory and real-space model-Hamiltonian techniques. Intricate interplay between Hubbard $U$ and Hund's coupling $J_{\rm H}$ for OV-induced correlated states is demonstrated. Orbital polarization towards an effective $e_g$ state with predominant local antiferromagnetic alignment on Ti sites near OVs i…
We have studied the electronic structure at the heterointerface between the band insulators LaAlO$_3$ and SrTiO$_3$ using $in situ$ photoemission spectroscopy. Our experimental results clearly reveal the formation of a notched structure on the SrTiO$_3$ side due to band bending at the metallic LaAlO$_3$/TiO$_2$-SrTiO$_3$ interface. The structure, however, is absent at the insulating LaAlO$_3$/SrO-SrTiO$_3$ interface. The present results indicate that the metallic states originate not from the charge transfer through the interface on a short-range scale but from the accumulation of carriers on …
We study the decay of interface induced structural and electronic relaxation effects in epitaxial LaAlO3/SrTiO3 heterostructures. The results are based on first-principles band structure calculations for a multilayer configuration with an ultrathin LaAlO3 layer sandwiched between bulk-like SrTiO3 layers. We carry out the structure optimization for the heterointerface and investigate the electronic states of the conducting interface layer, which is found to extend over two SrTiO3 unit cells. The decay of atomic displacements is analyzed as a function of the distance to the interface, and the re…
Quantum magnetic oscillations in SrTiO3/\LaAlO3 interface are observed. The evolution of their frequency and amplitude at various gate voltages and temperatures is studied. The data are consistent with the Shubnikov de-Haas theory. The Hall resistivity rho exhibits nonlinearity at low magnetic field. It is fitted assuming multiple carrier contributions. The comparison between the mobile carrier density inferred from the Hall data and the oscillation frequency suggests multiple valley and spin degeneracy. The small amplitude of the oscillations is discussed in the framework of the multiple band…
We studied LaAlO3/SrTiO3 interfaces for varying LaAlO3 thickness by core-level photoemission spectroscopy. In Ti 2p spectra for conducting "n-type" interfaces, Ti3+ signals appeared, which were absent for insulating "p-type" interfaces. The Ti3+ signals increased with LaAlO3 thickness, but started well below the critical thickness of 4 unit cells for metallic transport. Core-level shifts with LaAlO3 thickness were much smaller than predicted by the polar catastrophe model. We attribute these observations to surface defects/adsorbates providing charges to the interface even below the critical t…
We present low-temperature and high-field magnetotransport data on SrTiO3-LaAlO3 interfaces. The resistance shows hysteresis in magnetic field and a logarithmic relaxation as a function of time. Oscillations in the magnetoresistance are observed, showing a square root periodicity in the applied magnetic field, both in large-area unstructured samples as well as in a structured sample. An explanation in terms of a commensurability condition of edge states in a highly mobile two-dimensional electron gas between substrate step edges is suggested.
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 thicknes…
We report the operation of LaAlO_3 / SrTiO_3 depletion mode top-gated junction field-effect transistors using a range of LaAlO_3 thicknesses as the top gate insulator. Gated Hall bars show near ideal transistor characteristics at room temperature with on-off ratios greater than 1000. Lower temperature measurements demonstrate a systematic increase in the Hall mobility as the sheet carrier density in the channel is depleted via the top gate, providing a route to higher mobility, lower density electron gases in this system.
Oxide growth with semiconductor-like accuracy has led to atomically precise thin films and interfaces that exhibit a plethora of phases and functionalities not found in the oxide bulk material. This yielded spectacular discoveries such as the conducting, magnetic or even superconducting LaAlO$_3$/SrTiO$_3$ interfaces separating two prototypical insulating perovskite materials. All these investigations, however, consider the static state at the interface, although studies on fast oxide interface dynamics would introduce a powerful degree of freedom to understanding the nature of the LaAlO$_3$/S…
The tetragonal domain structure in SrTiO$_{3}$ (STO) is known to modulate the normal-state carrier density in LaAlO$_{3}$/SrTiO$_{3}$ (LAO/STO) heterostructures, among other electronic properties, but the effect of STO domains on the superconductivity in LAO/STO has not been fully explored. Using a scanning SQUID susceptometer microscope to map the superconducting response as a function of temperature in LAO/STO, we find that the superconducting transition temperature is spatially inhomogeneous and modulated in a pattern that is characteristic of structural domains in the STO.
This book chapter reviews the experimental evidence for magnetic phenomena at the LaAlO3/SrTiO3 interface. We argue that essentially all of the signatures of magnetism can be sorted into two distinct categories: (1) magnetic phases (e.g., ferromagnetic or Kondo) involving local magnetic moments and their coupling to itinerant electrons; (2) metamagnetic effects that are mediated by attractive electron-electron interactions that do not involve local moments. We review possible candidates for the local moments that give rise to the ferromagnetic phases and focus on arguments for one potential so…
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.
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 g…
The theoretical understanding of the spin-orbit coupling (SOC) effects at LaAlO$_{3}$/SrTiO$_{3}$ interfaces and SrTiO$_{3}$ surfaces is still in its infancy. We perform first-principles density-functional-theory calculations and derive from these a simple tight-binding Hamiltonian, through a Wannier function projection and group theoretical analysis. We find striking differences to the standard Rashba theory for spin-orbit coupling in semiconductor heterostructures due to multi-orbital effects: by far the biggest SOC effect is at the crossing point of the $xy$ and $yz$ (or $zx$) orbitals; and…
The possible existence of short-channel effects in oxide field-effect transistors is investigated by exploring field-effect transistors with various gate lengths fabricated from LaAlO$_3$-SrTiO$_3$ heterostructures. The studies reveal the existence of channel-length modulation and drain-induced barrier lowering for gate lengths below 1 μm, with a characteristic behavior comparable to semiconducting devices. With the fabrication of field-effect transistors with gate lengths as small as 60 nm the results demonstrate the possibility to fabricate by electron-beam lithography functional devices bas…
The emerging field of complex oxide interfaces is generically built on one of the most celebrated substrates - strontium titanate (SrTiO3). This material hosts a range of phenomena, including ferroelasticity, incipient ferroelectricity, and most puzzlingly, contested giant piezoelectricity. Although these properties may markedly influence the oxide interfaces, especially on microscopic length scales, the lack of local probes capable of studying such buried systems has left their effects largely unexplored. Here we use a scanning charge detector - a nanotube single-electron transistor - to noni…
We investigated the effect of the tensile strain on the spin splitting at the n-type interface in LaAlO$_3$/SrTiO$_3$ in terms of the spin-orbit coupling coefficient $α$ and spin texture in the momentum space using first-principles calculations. We found that the $α$ could be controlled by the tensile strain and be enhanced up to 5 times for the tensile strain of 7%, and the effect of the tensile strain leads to a persistent spin helix, which has a long spin lifetime. These results support that the strain effect on LaAlO$_3$/SrTiO$_3$ is important for various applications such as spinFET and s…
We have measured the electronic structure of the two-dimensional electron system (2DES) found at the Al/SrTiO3 (Al/STO) and LaAlO3/SrTiO3 (LAO/STO) interfaces by means of laser angle resolved photoemission spectroscopy, taking advantage of the large photoelectron escape depth at low photon energy to probe these buried interfaces. We demonstrate the possibility of tuning the electronic density in Al/STO by varying the Al layer thickness and show that the electronic structure evolution is well described by self-consistent tight binding supercell calculations, but differs qualitatively from a rig…
With infrared ellipsometry and transport measurements we investigated the electrons at the interface between LaAlO3 and SrTiO3. We obtained a sheet carrier density of Ns~5-9x 10E13 cm^-2, an effective mass of m*~3m_e, and a strongly frequency dependent mobility. The latter are similar as in bulk SrTi1-xNbxO3 and therefore suggestive of polaronic correlations of the confined carriers. We also determined the vertical density profile which has a strongly asymmetric shape with a rapid initial decay over the first 2 nm and a pronounced tail that extends to about 11 nm.
LaAlO3/SrTiO3 structures showing high mobility conduction have recently aroused large expectations as they might represent a major step towards the conception of all-oxide electronics devices. For the development of these technological applications a full understanding of the dimensionality and origin of the conducting electronic system is crucial. To shed light on this issue, we have investigated the magnetotransport properties of a LaAlO3 layer epitaxially grown at low oxygen pressure on a TiO2-terminated (001)-SrTiO3 substrate. In agreement with recent reports, a low-temperature mobility of…
We present a first-principles study of the electronic structures and properties of ideal (atomically sharp) LaAlO3/SrTiO3 (001) heterointerfaces and their variants such as a new class of quantum well systems. We demonstrate the insulating-to-metallic transition as a function of the LaAlO3 film thickness in these systems. After the phase transition, we find that conduction electrons are bound to the n-type interface while holes diffuse away from the p-type interface, and we explain this asymmetry in terms of a large hopping matrix element that is unique to the n-type interface. We build a tight…
We investigate the effect of the laser parameters of pulsed laser deposition on the film stoichiometry and electronic properties of LaAlO_3/SrTiO_3 (001) heterostructures. The La/Al ratio in the LaAlO_3 films was varied over a wide range from 0.88 to 1.15, and was found to have a strong effect on the interface conductivity. In particular, the carrier density is modulated over more than two orders of magnitude. The film lattice expansion, caused by cation vacancies, is found to be the important functional parameter. These results can be understood to arise from the variations in the electrostat…
Transport measurements of the two-dimensional electron gas (2DEG) at the LaAlO$_3$/SrTiO$_3$ interface have found a density of carriers much lower than expected from the "polar catastrophe" arguments. From a detail density-functional study, we suggest how this discrepancy may be reconciled. We find that electrons occupy multiple subbands at the interface leading to a rich array of transport properties. Some electrons are confined to a single interfacial layer and susceptible to localization, while others with small masses and extended over several layers are expected to contribute to transport…
We examined the magneto-transport behavior of electrons confined at the conducting LaAlO3/SrTiO3 interface in the low sheet carrier density regime. We observed well resolved Shubnikov-de Haas quantum oscillations in the longitudinal resistance, and a plateau-like structure in the Hall conductivity. The Landau indices of the plateaus in the Hall conductivity data show spacing close to 4, in units of the quantum of conductance. These experimental features can be explained by a magnetic breakdown transition, which quantitatively explains the area, structure, and degeneracy of the measured Fermi s…
We investigate the role of lattice polarization in determination of induced carrier density at the $n$-type interface of LaAlO$_3$ overlayer on SrTiO$_3$ (001) by carrying out density-functional-theory calculations. When no oxygen vacancy or defect is present, the magnitude of polarization screening in the LaAlO$_3$ layers is found to be correlated with the carrier charge induced at the interface. For the interfaces with a few LaAlO$_3$ layers, the induced charge carrier is compensated by the electrostatic screening and consequently its density remains far less than 0.5 electrons per unit cell…
The polar discontinuity at the SrTiO3/LaAlO3 interface (STO/LAO) can in principle sustain an electron density of 3.3E14 cm-2 (0.5 electrons per unit cell). However, experimentally observed densities are more than an order of magnitude lower. Using a combination of first-principles and Schrodinger-Poisson simulations we show that the problem lies in the asymmetric nature of the structure, i.e., the inability to form a second LAO/STO interface that is a mirror image of the first, or to fully passivate the LAO surface. Our insights apply to oxide interfaces in general, explaining for instance why…
A wealth of intriguing properties emerge in the seemingly simple system composed of the band insulators LaAlO3 and SrTiO3 such as a two-dimensional electron gas, superconductivity and magnetism. In this paper we review the current insight obtained from first principles calculations on the mechanisms governing the behavior of thin LaAlO3 films on SrTiO3(001). In particular, we explore the strong dependence of the electronic properties on the surface and interface termination, the finite film thickness, lattice polarization and defects. A further aspect that is addressed is how the electronic be…
Using density-functional-theory (DFT) calculations with the HSE06 hybrid functional, we accurately evaluate the critical thickness of LaAlO3 film for the intrinsic doping in LaAlO3/SrTiO3 (LAO/STO) heterstructures. The calculated critical thickness of 6 unit-cell (uc) layers suggests to rule out the intrinsic doping mechanism. We also calculate the density of oxygen vacancies on the LAO surface at varying LAO thicknesses, preparation oxygen pressures and temperatures by using the condition of chemical equilibrium and DFT calculations. We find that once LAO thickness >=3 uc high-density (~ 10^1…
Surface photovoltage (SPV) spectroscopy, which is a versatile method to analyze the energetic distribution of electronic defect states at surfaces and interfaces of wide-bandgap semiconductor (hetero-)structures, is applied to comparatively investigate heterostructures made of 5-unit-cell-thick LaAlO$_3$ films grown either on TiO$_2$- or on SrO-terminated SrTiO$_3$. As shown in a number of experimental and theoretical investigations in the past, these two interfaces exhibit dramatically different properties with the first being conducting and the second insulating. Our present SPV investigatio…
Using first-principles density functional theory calculations, we find a strong position and thickness dependence of the formation energy of oxygen vacancies in LaAlO3|SrTiO3 (LAO|STO) multilayers and interpret this with an analytical capacitor model. Oxygen vacancies are preferentially formed at p-type SrO|AlO2 rather than at n-type LaO|TiO2 interfaces; the excess electrons introduced by the oxygen vacancies reduce their energy by moving to the n-type interface. This asymmetric behavior makes an important contribution to the conducting (insulating) nature of n-type (p-type) interfaces while p…
Using tunneling spectroscopy we have measured the spectral density of states of the mobile, two-dimensional electron system generated at the LaAlO3-SrTiO3 interface. As shown by the density of states the interface electron system differs qualitatively, first, from the electron systems of the materials defining the interface and, second, from the two-dimensional electron gases formed at interfaces between conventional semiconductors.
We study plasmon modes of the two-dimensional electron gas residing at the interface of band insulators $\rm{LaAlO_3}$ and $\rm{SrTiO_3}$ (LAO/STO) and the plasmon excitations of graphene-LAO/STO double layer as well. Considering the electron-electron interaction within random phase approximation, we calculate the plasmon dispersions of both systems numerically and in the long wavelength limit analytical expressions for collective modes are found. One optical mode and two (three) acoustic modes are predicted for the LAO/STO (graphene-LAO/STO) system where only the uppermost acoustic mode of bo…
We present a comprehensive first-principles study of the electronic charge redistribution in atomically sharp LaAlO$_3$/SrTiO$_3$(001) heterointerfaces of both n- and p-types allowing for non-stoichiometric composition. Using two different computational methods within the framework of the density functional theory (linear combination of atomic orbitals and plane waves) we demonstrate that conducting properties of LaAlO$_3$/SrTiO$_3$(001) heterointerfaces strongly depend on termination of LaAlO$_3$ (001) surface. We argue that both the polar "catastrophe" and the polar distortion scenarios may …
The two-dimensional electron system found between LaAlO3 and SrTiO3 hosts a variety of physical phenomena that can be tuned through external stimuli. This allows for electronic devices controlling magnetism, spin-orbit coupling, and superconductivity. Controlling the electron density by varying donor concentrations and using electrostatic gating are convenient handles to modify the electronic properties, but the impact on the microscopic scale, particularly of the former, remains underexplored. Here, we image the current distribution at 4.2 K in amorphous-LaAlO3/SrTiO3 using scanning supercond…
A superconducting phase with an extremely low carrier density of the order of 10^13 cm^-2 is present at LaAlO3-SrTiO3 interfaces. If depleted from charge carriers by means of a gate field, this superconducting phase undergoes a transition into a metallic/insulating state that is still characterized by a gap in the spectral density of states. Measuring and analyzing the critical field of this gap, we provide evidence that macroscopically phase-coherent Cooper pairs are present in the metallic/insulating state. This is characterized by fluctuating vortex-antivortex pairs, and not by individual, …
We propose a simple analytical model to explain possible appearance of the metallic conductivity in the two-dimensional (2D) LaAlO$_3$/SrTiO$_3$ interface. Our model considers the interface within a macroscopic approach which is usual to semiconductor heterojunctions and is based on drift-diffusion equations. The solution of these equations allows to obtain the positions of band edges as a function of distances from the interface. We show that for the 2D metallic conductivity to appear at the interface, the constituting substances should have the same type (either electronic or hole) of conduc…
As discovered by Ohtomo et al., 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.
