A spin-orbit playground: surfaces and interfaces of transition metal oxides.

Within the last twenty years, the status of the spin-orbit interaction has evolved from that of a simple atomic contribution to a key effect that modifies the electronic band structure of materials. It is regarded as one of the basic ingredients for spintronics, locking together charge and spin degrees of freedom and recently it is instrumental in promoting a new class of compounds, the topological insulators. In this review, we present the current status of the research on the spin-orbit coupling in transition metal oxides, discussing the case of two semiconducting compounds, [Formula: see text] and [Formula: see text], and the properties of surface and interfaces based on these. We conclude with the investigation of topological effects predicted to occur in different complex oxides.

Rare-earth nickelates RNiO3: thin films and heterostructures.

This review stands in the larger framework of functional materials by focussing on heterostructures of rare-earth nickelates, described by the chemical formula RNiO3 where R is a trivalent rare-earth R = La, Pr, Nd, Sm, …, Lu. Nickelates are characterized by a rich phase diagram of structural and physical properties and serve as a benchmark for the physics of phase transitions in correlated oxides where electron-lattice coupling plays a key role. Much of the recent interest in nickelates concerns heterostructures, that is single layers of thin film, multilayers or superlattices, with the general objective of modulating their physical properties through strain control, confinement or interface effects. We will discuss the extensive studies on nickelate heterostructures as well as outline different approaches to tuning and controlling their physical properties and, finally, review application concepts for future devices.

Multiple Supersonic Phase Fronts Launched at a Complex-Oxide Heterointerface.

Selective optical excitation of a substrate lattice can drive phase changes across heterointerfaces. This phenomenon is a nonequilibrium analogue of static strain control in heterostructures and may lead to new applications in optically controlled phase change devices. Here, we make use of time-resolved nonresonant and resonant x-ray diffraction to clarify the underlying physics and to separate different microscopic degrees of freedom in space and time. We measure the dynamics of the lattice and that of the charge disproportionation in NdNiO_{3}, when an insulator-metal transition is driven by coherent lattice distortions in the LaAlO_{3} substrate. We find that charge redistribution propagates at supersonic speeds from the interface into the NdNiO_{3} film, followed by a sonic lattice wave. When combined with measurements of magnetic disordering and of the metal-insulator transition, these results establish a hierarchy of events for ultrafast control at complex-oxide heterointerfaces.

Spatially resolved ultrafast magnetic dynamics initiated at a complex oxide heterointerface.

Static strain in complex oxide heterostructures has been extensively used to engineer electronic and magnetic properties at equilibrium. In the same spirit, deformations of the crystal lattice with light may be used to achieve functional control across heterointerfaces dynamically. Here, by exciting large-amplitude infrared-active vibrations in a LaAlO3 substrate we induce magnetic order melting in a NdNiO3 film across a heterointerface. Femtosecond resonant soft X-ray diffraction is used to determine the spatiotemporal evolution of the magnetic disordering. We observe a magnetic melt front that propagates from the substrate interface into the film, at a speed that suggests electronically driven motion. Light control and ultrafast phase front propagation at heterointerfaces may lead to new opportunities in optomagnetism, for example by driving domain wall motion to transport information across suitably designed devices.

Interfacial Control of Magnetic Properties at LaMnO3/LaNiO3 Interfaces.

The functional properties of oxide heterostructures ultimately rely on how the electronic and structural mismatches occurring at interfaces are accommodated by the chosen materials combination. We discuss here LaMnO3/LaNiO3 heterostructures, which display an intrinsic interface structural asymmetry depending on the growth sequence. Using a variety of synchrotron-based techniques, we show that the degree of intermixing at the monolayer scale allows interface-driven properties such as charge transfer and the induced magnetic moment in the nickelate layer to be controlled. Further, our results demonstrate that the magnetic state of strained LaMnO3 thin films dramatically depends on interface reconstructions.

Electric field control of the LaAlO3/SrTiO3 interface ground state.

Interfaces between complex oxides are emerging as one of the most interesting systems in condensed matter physics. In this special setting, in which translational symmetry is artificially broken, a variety of new and unusual electronic phases can be promoted. Theoretical studies predict complex phase diagrams and suggest the key role of the charge carrier density in determining the systems' ground states. A particularly fascinating system is the conducting interface between the band insulators LaAlO(3) and SrTiO(3) (ref. 3). Recently two possible ground states have been experimentally identified: a magnetic state and a two-dimensional superconducting condensate. Here we use the electric field effect to explore the phase diagram of the system. The electrostatic tuning of the carrier density allows an on/off switching of superconductivity and drives a quantum phase transition between a two-dimensional superconducting state and an insulating state. Analyses of the magnetotransport properties in the insulating state are consistent with weak localization and do not provide evidence for magnetism. The electric field control of superconductivity demonstrated here opens the way to the development of new mesoscopic superconducting circuits.

Interface Fermi states of LaAlO3/SrTiO3 and related heterostructures.

The interfaces of LaAlO3/SrTiO3 and (LaAlO3)(x)(SrTiO3)(1-x)/SrTiO3 heterostructures have been investigated by soft x-ray photoelectron spectroscopy for different layer thicknesses across the insulator-to-metal interface transition. The valence band and Fermi edge were probed using resonant photoemission across the Ti L(2,3) absorption edge. The presence of a Fermi-edge signal originating from the partially filled Ti 3d orbitals is only found in the conducting samples. No Fermi-edge signal could be detected for insulating samples below the critical thickness. Furthermore, the angular dependence of the Fermi intensity allows the determination of the spatial extent of the conducting electron density perpendicular to the interface.

Electrostatic coupling and local structural distortions at interfaces in ferroelectric/paraelectric superlattices.

The performance of ferroelectric devices is intimately entwined with the structure and dynamics of ferroelectric domains. In ultrathin ferroelectrics, ordered nanodomains arise naturally in response to the presence of a depolarizing field and give rise to highly inhomogeneous polarization and structural profiles. Ferroelectric superlattices offer a unique way of engineering the desired nanodomain structure by modifying the strength of the electrostatic interactions between different ferroelectric layers. Through a combination of X-ray diffraction, transmission electron microscopy, and first-principles calculations, the electrostatic coupling between ferroelectric layers is studied, revealing the existence of interfacial layers of reduced tetragonality attributed to inhomogeneous strain and polarization profiles associated with the domain structure.

Ultrafast strain engineering in complex oxide heterostructures.

We report on ultrafast optical experiments in which femtosecond midinfrared radiation is used to excite the lattice of complex oxide heterostructures. By tuning the excitation energy to a vibrational mode of the substrate, a long-lived five-order-of-magnitude increase of the electrical conductivity of NdNiO(3) epitaxial thin films is observed as a structural distortion propagates across the interface. Vibrational excitation, extended here to a wide class of heterostructures and interfaces, may be conducive to new strategies for electronic phase control at THz repetition rates.

Local switching of two-dimensional superconductivity using the ferroelectric field effect.

Correlated oxides display a variety of extraordinary physical properties including high-temperature superconductivity and colossal magnetoresistance. In these materials, strong electronic correlations often lead to competing ground states that are sensitive to many parameters--in particular the doping level--so that complex phase diagrams are observed. A flexible way to explore the role of doping is to tune the electron or hole concentration with electric fields, as is done in standard semiconductor field effect transistors. Here we demonstrate a model oxide system based on high-quality heterostructures in which the ferroelectric field effect approach can be studied. We use a single-crystal film of the perovskite superconductor Nb-doped SrTiO3 as the superconducting channel and ferroelectric Pb(Zr,Ti)O3 as the gate oxide. Atomic force microscopy is used to locally reverse the ferroelectric polarization, thus inducing large resistivity and carrier modulations, resulting in a clear shift in the superconducting critical temperature. Field-induced switching from the normal state to the (zero resistance) superconducting state was achieved at a well-defined temperature. This unique system could lead to a field of research in which devices are realized by locally defining in the same material superconducting and normal regions with 'perfect' interfaces, the interface being purely electronic. Using this approach, one could potentially design one-dimensional superconducting wires, superconducting rings and junctions, superconducting quantum interference devices (SQUIDs) or arrays of pinning centres.

Metal-insulator transition in ultrathin LaNiO3 films.

Transport in ultrathin films of LaNiO(3) evolves from a metallic to a strongly localized character as the film's thickness is reduced and the sheet resistance reaches a value close to h/e(2), the quantum of resistance in two dimensions. In the intermediate regime, quantum corrections to the Drude low-temperature conductivity are observed; they are accurately described by weak localization theory. Remarkably, the negative magnetoresistance in this regime is isotropic, which points to magnetic scattering associated with the proximity of the system to either a spin-glass state or the charge ordered antiferromagnetic state observed in other rare earth nickelates.

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.

X-ray diffraction studies of 180 degrees ferroelectric domains in PbTiO3/SrTiO3 superlattices under an applied electric field.

The dielectric response of PbTiO(3)/SrTiO(3) superlattices is studied using electrical and structural measurements. While the dielectric response of paraelectric superlattices is well accounted for by the lattice contribution, superlattices with ferroelectric compositions exhibit an enhanced permittivity. X-ray diffraction allowed the presence of ordered nanodomains in ferroelectric superlattices to be established and their displacement under an applied bias to be directly probed, demonstrating that the enhanced permittivity in these artificial materials is due to domain wall motion.

Tunable Rashba spin-orbit interaction at oxide interfaces.

The quasi-two-dimensional electron gas found at the LaAlO{3}/SrTiO{3} interface offers exciting new functionalities, such as tunable superconductivity, and has been proposed as a new nanoelectronics fabrication platform. Here we lay out a new example of an electronic property arising from the interfacial breaking of inversion symmetry, namely, a large Rashba spin-orbit interaction, whose magnitude can be modulated by the application of an external electric field. By means of magnetotransport experiments we explore the evolution of the spin-orbit coupling across the phase diagram of the system. We uncover a steep rise in Rashba interaction occurring around the doping level where a quantum critical point separates the insulating and superconducting ground states of the system.

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.

Electric field effect in correlated oxide systems.

Semiconducting field-effect transistors are the workhorses of the modern electronics era. Recently, application of the field-effect approach to compounds other than semiconductors has created opportunities to electrostatically modulate types of correlated electron behaviour--including high-temperature superconductivity and colossal magnetoresistance--and potentially tune the phase transitions in such systems. Here we provide an overview of the achievements in this field and discuss the opportunities brought by the field-effect approach.

A-Axis--Oriented YBa2Cu3O7/PrBa2Cu3O7 Superlattices.

A modulated structure has been fabricated from high transition temperature superconductors where the individual CuO(2) planes are composed of alternating superconducting and insulating strips. This structure is made by growing a-axis-oriented YBa(2)Cu(3)O(7)/PrBa(2)Cu(3)O(7) superlattices by 90 degrees off-axis sputtering on (100)SrTiO(3) and (100)LaAlO(3) substrates. Superlattice modulation is observed to a modulation wavelength of 24 angstroms (12 angstroms-YBa(2)Cu(3)O(7)/12 angstroms-PrBa(2)Cu(3)O(7)), both by x-ray diffraction and by cross-sectional transmission electron microscopy. Rutherford backscattering spectroscopy indicates a high degree of crystalline perfection with a channeling minimum yield of 3 percent. Quasi-one-dimensional conductivity should be obtainable in these structures.

Ferroelectric Field Effect in Epitaxial Thin Film Oxide SrCuO2/Pb(Zr0.52Ti0.48)O3 Heterostructures.

A ferroelectric field effect in epitaxial thin film SrCuO(2)/Pb(Zr(0.52)Ti(0.48))O(3) heterostructures was observed. A 3.5 percent change in the resistance of a 40 angstrom SrCuO(2) layer (a parent high-temperature superconducting compound) was measured when the polarization field of the Pb(Zr(0.52)Ti(0.48))O(3) layer was reversed by the application of a pulse of small voltage (<5 volts). This effect, both reversible and nonvolatile, is attributed to the electric field-induced charge at the interface of SrCuO(2) and Pb(Zr(0.52)Ti(0.48))O(3). This completely epitaxial thin film approach shows the possibility of making nonvolatile, low-voltage ferroelectric field effect devices for both applications and fundamental studies of field-induced doping in novel compounds like SrCuO(2).

Superconductivity at the LaAlO(3)/SrTiO(3) interface.

We report on the structural characterization of LaAlO(3)/SrTiO(3) interfaces and on their transport properties. LaAlO(3) films were prepared using pulsed laser deposition onto TiO(2) terminated (001) SrTiO(3) substrates inducing a metallic conduction at the interface. Resistance and Hall effect measurements reveal a sheet carrier density between 0.4 and 1.2 × 10(14) electrons cm(-2) at room temperature and a mobility of ∼300 cm(2) V(-1) s(-1) at low temperatures. A transition to a superconducting state is observed at a temperature of ∼200 mK. The superconducting characteristics display signatures of 2D superconductivity.

New phenomena at the interfaces of very thin ferroelectric oxides.

We present a brief review of the role of interfacial physics in ferroelectric oxides, with an emphasis on the importance of boundary conditions that determine the properties of very thin ferroelectric films and superlattices. As well as discussing the screening problem, and the role of strain and electrostatics in ferroelectrics, we highlight some of the possibilities in fine period superlattices where the high density of interfaces can lead to new and potentially useful phenomena.