Session L5: Focus Session: Interfaces in Complex Oxides - Nickelates

Exchange Bias in LaNiO$_{3}$-based heterostructures

The wide spectrum of exotic properties exhibited by transition-metal based oxides is triggered by the interplay between the spin, charge, orbital and lattice degrees of freedom. In this context, interface engineering in complex oxide heterostructures enables not only further tuning of the exceptional properties of these materials, but also gives access to hidden phases and emergent physical phenomena. Here, we show how interface engineering can induce a complex magnetic structure in a non-magnetic material. We specifically show that exchange bias phenomenon can unexpectedly emerge in (111)-oriented heterostructures involving materials a priori non-candidates for the development of such behavior, namely paramagnetic LaNiO$_{3}$ (LNO) and ferromagnetic LaMnO$_{3}$ (LMO). The observation of the exchange bias in LNO/LMO superlattices not only implies the development of interface-induced magnetism in the paramagnetic LNO layers, but also provides us with a very subtle tool for probing the interfacial coupling between the LNO and the LMO layers. First-principles calculations indicate that this interfacial interaction may give rise to an unusual spin order resembling a spin density wave within the LNO layers. Other possible magnetic orders are also discussed.   

Resonant x-ray reflectivity study of interface reconstructions in LaCoO(3) and other transition-metal-oxide heterostructures

Transition-metal-oxide (TMO) heterostructures offer the opportunity to combine the study of unconventional physical properties with the design of novel functionalities, which are not observed in simple semiconductor or metal heterostructures. In my talk I will concentrate on electronic and chemical reconstructions observed at the interfaces of heterostructures based on LaCoO(3) and other correlated oxides. After briefly summarizing the properties of the novel resonant x-ray technique of orbital reflectometry which we recently used to study the orbital reconstruction in LaNiO(3) heterostructures (Benckiser et al., Nature Materials 10, 189, (2010)), I will discuss the application of this technique, and of usual resonant x-ray reflectometry, to study the electronic and orbital properties at the interfaces and at the surface of LaCoO(3) / LaAlO(3) heterostructures. Finally, I will offer an outlook to the application of orbital reflectometry to other systems.   

Quantum confinement of correlated e$_{g}^{1}$ electrons in rare earth nickelate heterostructures

Complex oxide heterostrutures have emerged as a new playground for controlling the mutually coupled charge, spin, orbital and lattice degrees of freedom, and a promising route to stabilize unusual phases not existing in the bulk. In particular, quantum well structures have recently attracted attention due to the potential in creating novel two-dimensional systems with confined correlated electrons. To this end, we have studied the e$_{g}^{1}$ system based on the 3d$^{7}$ low-spin state in perovskite rare earth nickelates which are artificially confined by wide-gap dielectrics LaAlO$_{3}$. The combination of transport measurements and dynamical-mean-field calculations indicate that, a Mott-type metal-insulator transition can be induced by confinement via dimensionality-control. X-ray absorption spectroscopy reveals that the electronic modification in proximity to the confining interfaces is caused by modulated covalency, which is in good agreement with cluster calculations. J.C. was supported by DOD-ARO under the Contract No. 0402-17291 and NSF Contract No. DMR-0747808.   

Epitaxial Stabilization of Ultrathin Rare-Earth Nickelates

The nickelate family of perovskite materials has attracted great attention in recent years due to the interesting range of properties they exhibit. In this talk we report on the successful synthesis of EuNiO$_{3}$, YNiO$_{3}$, and PrNiO$_{3}$ films grown by interrupted pulse laser epitaxy on various substrates. Investigation of the phase space of nickelate thin film epitaxy revealed a linear trend between the optimized growth temperature and the Goldschmidt tolerance factor. This correlation is explained through epitaxial stabilization and the increase of the lattice energy via distortion of the ideal perovskite cell. This explanation offers the additional benefit of not being restricted only to the nickelate perovskite family, giving it possible applicability for a wide range of perovskite-structured materials.   

Unipolar Field-Effect Diode Based on a Complex Oxide

We demonstrate rectifying behavior in a field-effect device structure fabricated from thin NdNiO$_{3}$ films grown on SrTiO$_{3}$ substrates by the pulsed laser deposition technique. In contrast to the conventional three-terminal field effect devices, the device has only two terminals with the field gate electrode connected to one of the terminal electrodes. The active device area is a $10\mu$m$\times$$10\mu$m square with a Au/Al$_{2}$O$_{3}$/NdNiO$_{3}$/SrTiO$_{3}$ structure, where Au and Al$_{2}$O$_{3}$ are the gate and the gate insulator, respectively. At small bias voltages, the device exhibits a metal-insulator transition near T=150K, similar to extended NdNiO$_{3}$ films. I-V measurements reveal a strong dependence of device characteristics on temperature, applied bias, and both thermal and applied bias histories. We analyze the IV characteristics by using a modified charge-control model based on accumulation of charges in the channel near the gate oxide interface. We deduce the temperature dependence of the effective zero-field charge carrier mobility for the channel by including a field-dependence mobility. The observed hysteretic effects can be utilized in complex oxide devices that combine together both the diode and the memory functionalities.   

Asymmetric Orbital-Lattice Interactions in Ultrathin Correlated Oxide Films

Epitaxial control of strongly correlated electrons offers opportunities to push beyond the bulk phase diagram and access new ground states. Using resonant x-ray spectroscopies combined with density functional calculations, we report the discovery of an asymmetric biaxial strain-induced 3d-orbital response in ultrathin films of the correlated metal LaNiO3 that are not accessible in the bulk [J. Chakhalian et al., PRL, 107, 116805 (2011)]. Compressive strain results in an orbital polarization due to structural induced changes in the crystal field, but tensile strain shows no orbital response. This is accompanied by a strong change in the oxygen hole states due a systematic change of the charge transfer energy as a function of strain. We suggest that knowledge of this asymmetric orbital-lattice interaction is fundamental to the rational design of quantum materials with exotic correlated phases and enhanced critical temperatures.   

\textit{in situ} studied correlated oxide LaNiO$_{3}$ ultra thin film by angle resolved photoemission spectroscopy

Recently, $R$NiO$_{3}$ ($R$: rare earth) attracted increasing attention due to the possible realization of the electronic band structure similar to the high-temperature superconductor cuprates. Among them, LaNiO$_{3}$ based heterostructures have shown various fascinating physical properties such as dimensionality controlled electronic phase transitions. Theoretical works on confined LaNiO$_{3}$ through heterostructuring predicted cuprate-like band structure and magnetic properties. Here, we reports \textit{in-situ} angle resolved photoemission spectroscopy results on the LaNiO$_{3}$ films grown by pulsed laser deposition method. We carefully controlled the thickness of LaNiO$_{3}$ films from one to 30 unit cells and measured the thickness dependent band dispersions. First, we will discuss the strong electronic correlation effect in bulk-like band structure of thick LaNiO$_{3}$ films comparing to the previously reported LDA+DMFT calculation. Moreover, we will discuss the thickness dependent band structure. As decreasing the film thickness, we observed the charge redistribution of two Ni $e_{g}$ orbitals at the Fermi surface. The origins of thickness dependent electronic structure will be discussed.   

Influence of Symmetry on the Octahedral Rotations of Epitaxial RNiO$_3$ Thin Films

Understanding the structural and electronic behavior of ABO$_3$ thin films subjected to confinement, lattice misfit and broken symmetry at the interface in the ultra-thin limit is fundamentally important for the rational design of new materials [1]. However, the epitaxial strain will not only due to a change the in-plane lattice constants but also the octahedral rotations connected to bond angles and crystallographic symmetry. Here we present a study of the effect of the bulk lattice symmetry on octahedral rotations under epitaxial strain in thin films of RNiO$_3$ (R=La, Pr, Nd) grown on various substrates by pulsed laser deposition. A combination of high-resolution x-ray diffraction, polarization-dependent soft x-ray absorption spectroscopy, and first-principles density functional calculations has been applied to elucidate structural and electronic properties of the samples. Work at the Advanced Photon Source, Argonne is supported by the U.S. Department of Energy, Office of Science under Contract No. DE-AC02-06CH11357.\\[4pt] [1] J. Chakhalian et al., Phys. Rev. Lett., 107, 116805 (2011).   

Probing the Nickelate Ground State in NdNiO3 Superlattices

The rare-earth nickelates of the type RNiO$_3$ exhibit tunable, sharp metal-insulator transitions as a function of R size, film thickness, and external fields. The nature of these transitions has been the subject of much study, including examination of the insulating ground state. NdNiO$_3$ has garnered particular interest due to a concomitant magnetic crossover to an antiferromagnetic state occurring at the metal-insulator transition temperature. Several previous studies have focused on thin films; in this work, we examine NdNiO$_3$ layers confined in heterostructures. The metal-insulator transition temperature can be manipulated based on the thickness of the NdNiO$_3$ layers within the heterostructure, and a suppression of the high-temperature metallic phase leads to a crossover to localization related to a change in dimensionality. The electronic structure of these phases probed using x-ray absorption spectroscopy enables us to elucidate the link between potential charge ordering and dimensionality in the ground state of this system.   

Electrostatic control of the metal-insulator transition of ultrathin NdNiO$_{3}$ films

Rare earth nickelates (RNiO$_{3}$) exhibit a first order metal insulator transition upon cooling. Bulk studies on chemical doping indicated that both divalent and quatrovalent ions were effective in shifting T$_{MIT}$ to lower temperatures by $\sim$ 50 to 25 K for 1 \% hole and electron doping, respectively. However, separating the influence of structural distortions from band filling is particular important for the nickelates. Here we present a new approach to control the band-filling in nanoscale NdNiO$_{3}$ thin films by modulation doping. NdNiO$_{3}$ is remotely doped by interfacing it with a degenerately doped conventional band insulator, La-doped SrTiO$_{3}$. We show that the remote doping approach allows for purely electronic modulation of a carrier density in the absence of other structural changes. The proposed approach is experimentally tested using ultrathin (2.5 nm) NdNiO$_{3}$ films grown on La-doped SrTiO$_{3}$ films with different carrier concentrations. We show that remote doping systematically changes the charge carrier density in the NdNiO$_{3}$ film and causes a moderate shift ($\sim$ 20 K) in the metal-insulator transition temperature. These results will be discussed in the context of theoretical models of the materials exhibiting a metal-insulator transition.   

Tuning Electronic Conductivity in Nickelate Heterostructures

Bulk LaNiO$_{3}$ (LNO) is a metallic material, but for thin films below a critical thickness of about 5-6 unit cells of LNO, a transition occurs from metallic to insulating transport behavior. This transition is suppressed in superlattice structures with LaAlO$_{3}$ (LAO) spacers which are found to be metallic down to 3 unit cell thick LNO. To understand the differences between thin film and superlattices, we have identified structural differences in thin films and superlattice structures of LNO and LAO using a combination of first principles theory and synchrotron x-ray diffraction. Distortions are observed in the Ni-O-Ni bond angle in LNO epilayers grown on LAO resulting from relaxations at the vacuum-LNO interface. These distortions are suppressed in superlattice structures. Based on our observations, we propose that the electronic properties of nickelate superlattices can be designed by carefully selecting spacer layers that result in specific structural distortions in the LNO conducting layers. By tuning the observed distortions in this fashion, we identify a new pathway for controlling the electronic properties of rare earth nickelate compounds.   

Composition spread studies of (RE)NiO$_{3}$-LaNiO$_{3}$ thin films

Epitaxial thin films of La$_{x}$Nd$_{1-x}$NiO$_{3}$ and La$_{x}$Pr$_{1-x}$NiO$_{3}$ were grown via pulsed laser deposition on SrTiO$_{3}$ (001) with an automated~moving shutter system to create 100 nm thick composition spreads. These films have the advantage of providing a continuous spectrum of doping compositions at the same deposition conditions, such as temperature and oxygen pressure, without the need to prepare different stoichiometric targets or perform different depositions for desired compositions.~Composition evolution is verified via wavelength dispersive spectroscopy, and x-ray diffraction mapping shows that the lattice constant of the film varies continuously from LaNiO$_{3}$ to NdNiO$_{3}$. Simultaneous measurements of resistance versus temperature down to 4.2 K across the spreads show evolution in transport behavior. The details will be discussed.   

Conduction mechanisms in epitaxial and polycrystalline SmNiO$_{3}$ thin films

Correlated oxides that exhibit metal-insulator phase transitions are emerging as potential candidates for switching devices and their fundamental physical properties are of interest. One such material is SmNiO$_{3}$, which has a transition temperature above room temperature ($\sim$400 K in bulk crystals). We present temperature- and bias-dependent conduction mechanisms in epitaxial and polycrystalline SmNiO$_{3}$ thin films. In both cases, at low electric field we observe thermally assisted hopping conduction through defect states. At high electric field the conduction transitions to a space-charge limited regime controlled by an exponential trap distribution. The trap decay parameter in epitaxial films does not have the expected 1/T temperature dependence, which may be a signature of band gap narrowing at high temperature due to the insulator to metal transition. The role of defects in affecting charge transport parameters will be discussed.   

Many-body effects in the capacitance of multilayers composed of Mott insulators

Inhomogeneous dynamical mean-field theory is employed to investigate the non-linearity of the capacitance of multilayer nanostructures. The multilayer nanostructures are constructed with semi-infinite metallic leads coupled via a strongly correlated (Mott insulating) dielectric barrier. Results on the effects of varying barrier thickness temperature, potential difference, screening length, and chemical potential will be presented. In addition, we also intend to examine phase separation effects which have been experimentally measured to enhance the many-body capacitance over the geometric capacitance.