Session F30: Orbital and Electronic Transitions in Oxide Heterostructures

Orbital reconstruction through interlayer cation charge imbalance: insights from wave-function-based quantum chemistry calculations

A promising route to engineering the electronic properties of quantum materials and devices rests on the idea of orbital reconstruction in multilayered oxide heterostructures. In this context, we identify and discuss in detail one simple, appealing mechanism for tailoring the sequence of d-electron energy levels: interplanar cation charge imbalance (ICCI) along successive metal-oxygen layers [1,2]. Through interplay with distortions of the ligand cages, it provides a knob for tunning the order of electronic levels in even intrinsically stacked oxides. We analyze in this regard electron spin resonance data for the 214 Sr-iridate oxide compound [1]. While canonical ligand-field theory predicts z-axis g factors smaller than 2 for positive tetragonal distortions as present in Sr214, the experiment indicates values larger than 2. This implies that the iridium d levels are inverted with respect to their normal ordering. State-of-the-art quantum chemistry calculations confirm the level switching in Sr214, whereas we find them to be instead normally ordered in the sister compound Ba214. Given the nonpolar character of the metal-oxygen layers, our findings highlight the tetravalent transition-metal 214 oxides as ideal platforms to explore d-orbital engineering in the context of oxide electronics. The crucial role of internal anisotropic fields related to the environment beyond nearest-neighbor ligands is further highlighted by ab initio quantum chemistry calculations on 3D pyrochlore osmates and iridates [3,4]. \ \ \ [References: [1] N. Bogdanov et al., Nature Commun. 6, 7306 (2015); [2] V. Katukuri et al., Inorg. Chem. 53, 4833 (2014); [3] L. Hozoi et al., Phys. Rev. B 89, 115111 (2014); [4] N. Bogdanov et al., Phys. Rev. Lett. 110, 127206 (2013).]   

Enhancement of conductivity in ultrathin films of LaNiO$_{3}$.

In order to achieve tailor-made properties in oxide heterostructures it is essential to first fully understand the physics of these materials and their behavior when pushing the 2-D limit. That is the principle goal of this work. LaNiO$_{3}$ (LNO), a metal in bulk, has previously been found to be insulating in ultrathin form and this behavior was attributed to the onset of weak, and then strong, localization. A key feature of ultrathin films is that a significant portion of the sample is under the influence of the two "boundaries", these are the interfaces with the substrate and with the vacuum respectively. The effects of both of these boundaries have been studied previously by changing substrate material and with the addition of an encapsulating layer. In this work we show that, for LNO grown on LaAlO$_{3}$ (001) substrates, there is an enhancement of conductivity that occurs on the verge of the ultrathin regime (less than 10 unit cells) and we hypothesize that this stems from a structural distortion imposed at the substrate-film interface.   

Metal-insulator transition and emergence of spontaneous polarization in (La$_{1-x}$Sr$_x$MnO$_3$)$_m$/(LaNiO$_3$)$_n$ superlattices constructed from metallic building blocks

La$_{0.7}$Sr$_{0.3}$MnO$_3$ and LaNiO$_3$ are metallic oxides. However, short-period superlattices of the form (La$_{0.7}$Sr$_{0.3}$MnO$_3$)$_m$ /(LaNiO$_3$)$_n$ show insulating behavior depending on $m$ and $n$. In particular, the insulating property is robust when $m=n=2$ (SL 2/2). Here, using first-principles density functional theory (DFT) and DFT+U (static $d-d$ Coulomb interaction), we explain the experimental observation and discuss a general mechanism that underlies such metal-insulator transition for different $m$ and $n$. The general mechanism is based on the finding that disproportionation on Ni sites is the key. Further, we predict that insulating SL 2/2 is ferroelectric with large spontaneous polarization. The ferroelectric distortion persists in the cases where the superlattices are metallic, which leads to the possibility of designing a new family of 'polar metals'. Finally, we discuss the origin of such polar distortion and its coupling with the magnetic properties (by considering spin-phonon coupling) of the material.   

Polarity and the Metal-Insulator Transition in ultrathin LaNiO$_3$ on SrTiO$_3$

Dimensionality and strain control of nickelates has been shown as a route for control of interesting electronic and magnetic phases. However, little is know about the evolution of atomic structure in these layered architectures and the interplay with these states. Here we present, a detailed study of lattice structures measured real time during the layer-by-layer growth of LaNiO3 on SrTiO3. Using hard X-rays coupled with oxide MBE, we have tracked the lattice structure evolution as a function of depth across the regime where transport shows a clear metal to insulator transition. At the same time X-ray absorption shows the films are closer to LaNiO$_{2.5}$ when thin and evolve to LaNiO$_3$ by 10 unit cells thickness. Analysis of the structure during growth displays a very complex evolution throughout the film of the lattice parameter and displacement of the B-site from the unit cell center, which theory connects with pathways of compensating the polar mismatch at the surface and interface. Work at the APS, Argonne is supported by the U.S. Department of Energy, Office of Science, and Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357.   

Tailoring the electronic transitions of NdNiO$_{\mathrm{3}}$ films through (111)$_{\mathrm{pc}}$-oriented interfaces

Bulk NdNiO$_{\mathrm{3\thinspace }}$displays a 1$^{\mathrm{st}}$ order metal to insulator transition (MIT) that occurs simultaneously with a paramagnetic to antiferromagnetic N\'{e}el transition. For NdNiO$_{\mathrm{3}}$ epitaxial thin films grown along the (001)$_{\mathrm{pc}}$ axis, the MIT can be tuned between 0 and 200K through a variety of parameters, such as epitaxial strain or electrostatic carrier doping. Here, we extend the control of the electronic transitions of NdNiO$_{\mathrm{3}}$ thin films over an unprecedented temperature range by selecting (111)$_{\mathrm{pc}}$-oriented substrates as a template for the growth. We show that (111)$_{\mathrm{pc}}$ NdNiO$_{\mathrm{3}}$/NdGaO$_{\mathrm{3}}$ heterostructures exhibit a MIT above room temperature, at T$=$335K, and a N\'{e}el transition at T$=$230K. By comparing the behavior of NdNiO$_{\mathrm{3}}$ layers grown on substrates with different symmetries and lattice parameters, we conclude that the particularly large tuning of the critical temperatures of the system is produced by the specific lattice matching conditions imposed along the (111)$_{\mathrm{pc}}$ axis of orthorhombic substrates.   

Electronic and magnetic structure of ultra-thin film of EuNiO3

In order to uncover the effect of reduced dimensionality on electronic and magnetic structure of rare earth nickelates series, we have investigate ultra thin film of EuNiO$_3$ (ENO) using hard and soft resonant x-ray scattering. Despite of such small thickness, it exhibits checker board type charge ordering, and E’ antiferromagnetic transition highlighting bulk-like electronic and magnetic structure can be retained for highly distorted member of $RE$NiO$_3$ family. The presence of charge ordering at room temperature also provides opportunity for their use in novel electric field controlled devices.   

Modeling Correlation Effects in Nickelates with Slave Particles

Nickelate interfaces display interesting electronic properties including orbital ordering similar to that of cuprate superconductors and thickness dependent metal-insulator transitions. One-particle band theory calculations do not include dynamic localized correlation effects on the nickel sites and thus often incorrectly predict metallic systems or incorrect ARPES spectra. Building on two previous$^1$,$^2$ successful slave-particle treatments of local correlations, we present a generalized slave-particle method that includes prior models and allows us to produce new intermediate models$^3$. The computational efficiency of these slave-boson methods means that one can readily study correlation effects in complex heterostructures. We show some predictions of these methods for the electronic structure of bulk and thin film nickelates.\\ 1. Florens and Georges, PRB (2002); Lau and Millis, PRL (2013).\\ 2. de’Medici, Georges and Biermann, PRB (2005); de’Medici, Giovannetti and Capone, PRL (2014).\\ 3. Georgescu and Ismail-Beigi, arXiv:1506.03515, in press at PRB (2015)   

Testing for Quantum Criticality in the (Nd,La)NiO$_{\mathrm{3}}$ phase space by Inelastic Tunneling Electron Spectroscopy (IETS)

Continuous (2$^{\mathrm{nd}}$ order) T$=$0K Mott transitions are highly sought after because they are predicted to produce interesting quantum critical phenomena at higher temperatures such as quantum spin liquid states and marginal Fermi behavior. The rare earth nickelate system has generated significant interest for understanding charge and spin ordering phenomena in correlated materials and for potential application in novel switching devices. We will present Inelastic Electron Tunneling Spectroscopy (IETS) measurements from a series of films across the (Nd,La)NiO$_{\mathrm{3}}$ compositional system, yielding the single particle density of states at and across the quantum phase transition. Previous work has shown that pure NdNiO$_{\mathrm{3}}$ thin films have a characteristic bandgap, and pure LaNiO$_{\mathrm{3}}$ films have a characteristic pseudo-gap, satisfying a major requirement for a quantum critical transition point. Accordingly, our films undergo a metal-to-insulator transition upon cooling for high Nd content, while remaining metallic at all temperatures for high La content.   

Suppression of charge and spin order in confined NdNiO$_3$ layers

Atomic-layer synthesis allows one to study and control the complex phase behavior correlated systems by controllably modifying dimensionality and interfacial constraints. To this end, the rare-earth nickelates ($R$NiO$_3$) embody a remarkable model system, as exhibited by the bulk metal-insulator and paramagnetic-antiferromagnetic ordering transitions, which are sensitive to structural and electronic conditions. We present evidence from transport, x-ray absorption, and resonant x-ray scattering measurements of NdNiO$_3$/NdAlO$_3$ superlattices of a suppression of charge and spin order induced by interfacial confinement. We find that the spectroscopic signatures of charge localization and antiferromagnetic ordering remain for NdNiO$_3$ layers thicker than a single unit cell. The disappearance of ground state order in single NdNiO$_3$ layers is attributed to enhanced $e_g$ orbital polarization from the interaction with the NdAlO$_3$ confining layers. We also observe a crossover region of thickness with no metal-insulator transition but distinct charge and spin ordering temperatures. These findings shed light on the interplay between competing ground states in nickelates and help guide efforts to controlling long-range order in such systems.   

Effect of strain on ferroelectric field effect in strongly correlated oxide Sm$_{\mathrm{0.5}}$Nd$_{\mathrm{0.5}}$NiO$_{\mathrm{3}}$

We report the effect of epitaxial strain on the magnitude and retention of the ferroelectric field effect in a prototype FerroFET based on a charge transfer-type Mott insulator, Sm$_{\mathrm{0.5}}$Nd$_{\mathrm{0.5}}$NiO$_{\mathrm{3}}$ (SNNO). It has been shown that epitaxial strain can change the transition temperature $T_{\mathrm{MI}}$ in SNNO by more than 100 K, and modify the metal-insulator transition (MIT) characteristic between first-order and second-order. We have fabricated epitaxial PbZr$_{\mathrm{0.3}}$Ti$_{\mathrm{0.7}}$O$_{\mathrm{3}}$ (PZT)/3.8--4.3 nm SNNO heterostructures on (001) LaAlO$_{\mathrm{3}}$ (LAO) and SrTiO$_{\mathrm{3}}$ (STO) substrates. The magnitude of the field effect modulation can differ by more than one order of magnitude in these two systems, which has been attributed to strain modified MIT characteristic in SNNO. In both systems, we also observe a pronounced relaxation of off state resistance $R_{\mathrm{off}}$, showing a thermally activated behavior with corresponding activation energy of 22 meV (28 meV) for devices on LAO (STO). The time dynamics and thermal response of the retention behavior suggest that strain-induced oxygen vacancies play a critical role in the ferroelectric field effect instability.   

Current-voltage profile of a strongly correlated materials heterostructure using non-equilibrium dynamical mean field theory

We investigate the nonlinear electronic transport across a multilayered heterostructure which consists of Mott insulator layers connected to ballistic metal leads on both sides. To create current flow, we turn on an electric field in the leads for a finite period of time and then turn it off and let the system reach the steady state by adding an electric field over the correlated region. We use nonequilibrium dynamical mean-field theory to obtain the current-voltage relation. To do so, we current bias the device, and adjust the voltage profile to ensure current conservation and charge conservation throughout. The calculation ultimately works directly in the steady-state limit.   

Photoinducing the hidden M2 phase in VO$_{\mathrm{2}}$

We used time-resolved x-ray diffraction to study photoinduced structural phase transitions in a 170-nm-thick VO$_{\mathrm{2}}$ film grown on sapphire (1,0,-1,0). Heating the unstrained film from room temperature induces the well-known phase transition from the monoclinic (M1) phase directly to the high-temperature tetragonal rutile (R) phase. In contrast, upon ultrafast optical excitation, the phase transition depends strongly on the laser intensity. At low fluences, the film is partially transformed into the monoclinic M2 phase, a phase which generally is observed only in doped or strained materials. Above a threshold at higher fluences, a small portion of the film is transformed into the M2 phase, decaying on a time scale of a few nanoseconds, while the majority of the film is transformed into the R phase which can persist for tens of nanoseconds. We further discuss the effects of laser wavelength on the efficiency of producing the M2 phase.   

Quantum capacitance in thin film vanadium dioxide metal insulator transition

We present capacitance measurements of the electronic density of states performed in high quality vanadium dioxide (VO$_2$) thin films on sapphire (Al$_2$O$_3$) substrate. These films show the expected metal insulator transition near 60 $^{\circ}$C with resistivity changing by 3 orders of magnitude with a hysteresis of 10 $^{\circ}$C. To make a capacitive probe, a gate is suspended above the film surface using a flip-chip method with microfabricated supports. The geometric capacitance per-area reached is 40 pF/mm$^2$. Such a large capacitance can be significantly modified by electron interaction and band charging/discharging which appear as an extra term known as the quantum capacitance ($C_q$). An AC signal applied to the gate allows measurement of the changing density of states (DOS) across the MIT. The DOS abruptly increases as the sample is heated through the transition point. Conversely the low temperature drop of d$\mu/$d$n$ is consistent with an energy gap opening in the insulating phase. These parameters shed light on the transition mechanism.