Session Y18: Focus Session: Interfaces in Complex Oxides - Functional Interfaces

Tunable ferroelectricity and magnetoelectricity at the interfaces of superlattices of antiferromagnets

In heterostructures composed of transition metal oxides, the disruption introduced even by an ideal interface can drastically upset the delicate balance of the competing interactions among electronic spins, charges and orbitals, leading to a range of exotic phenomena. These include interfacial conduction, magnetism, superconductivity but also improved ferreoelectric properties. Possible incorporation of magnetoelectrics in these structures would further enable electrical control of magnetization as well as magnetic sensing of the polarization state. Naturally occurring magnetoelectrics are however, rare and generally with weak coupling. In this work we report a strategy and evidence for the emergence and control of ferroelectricity and strong magnetoelectric coupling in superlattices \textit{consisting of solely non-ferroelectric antiferromagnetic layers}. By selecting appropriate yet abundant starting materials one may now design a plethora of low dimensional heterostructures with the desired relative arrangement of the oxide layers, of potential utility to miniature valves with electric and magnetic field tunable functions, promising to set new standards in future electronics.   

Interfacial Properties of Ultra-thin YBCO/LCMO Superlattices

The rational design of complex oxide heterostructures enables the investigation of novel materials with antagonistic order parameters. Our previous work using resonant x-ray spectroscopies has provided insight into the role of orbital reconstruction and covalent bonding at the interface of such heterostructures. In this talk, we will further address the intriguing interfacial electronic and magnetic properties and possible coupling between layers in superlattices composed of alternating superconductive YBa$_{2}$Cu$_{3}$O$_{7}$ and ferromagnetic La$_{2/3}$Ca$_{1/3}$MnO$_{3}$ layers upon approaching the ultra-thin limit, where a superconductor to insulator transition with increasing thickness N is observed in [YBCO (1 u.c)/LCMO (N u.c.)] superlattices.   

Scanning tunneling microscope investigation of the interfacial electronic properties of YBCO/LCMO

Direct measurements of the interfacial electronic structures in YBCO/LCMO hetero-structures have been performed using cross-sectional scanning tunneling microscopy (STM) and spectroscopy (STS). Both scanning tunneling spectroscopy and analysis of the local electronic states across the interface of YBCO/LCMO reveal the evolution of the energy-band structures. Closely examining the recognition of the electronic structure by the unique combination of STM and STS reveals the direct information on the local interaction between superconductivity and magnetism in this work.   

Quasiparticle dynamics in YBCO and YBCO/LSMO Using Femtosecond Optical Pulses

The properties of various complex oxide systems, such as multiferroics, high-T$_{c}$ superconductors and colossal magnetoresistance manganites, have been extensively studied for the past $\sim $25 years. In particular, the interplay between superconductivity (SC) and ferromagnetism (FM) is interesting from both academic and applied viewpoints. we have temporally resolved quasiparticle dynamics in multilayered films composed of the high-temperature superconductor YBCO and the ferromagnetic manganite LaSrMnO$_{3}$ (LSMO) by performing temperature-dependent UOS experiments. In YBCO alone, we observed two distinct decay relaxation channels that have previously been related to the pseudogap and superconducting gaps and can be explained with the phenomenological Rothwarf-Taylor (RT) model. However, the fast sub-picosecond relaxation related to the pseudogap was not observed in our YBCO/LSMO heterostructures, possibly due to the influence of FM order These first UOS experiments on SC/FM heterostructures demonstrate the ability of UOS to quantify the influence of ferromagnetism on superconductivity through time domain measurements.   

Ba and Sr templates on Ge(001) for epitaxial oxide growth

In the epitaxial growth of oxides on semiconductors, Ba and Sr templates are used as transition layers for perovskite deposition. Furthermore, the interface between complex oxides and semiconductors are relatively unexplored. The ability to control the interface between them can lead to new phenomena and new combinations of oxide-semiconductor heterostructures. In this work, we compare the differences between the Sr template with Ba templates on Ge(001) at various temperatures. The sub-monolayer coverage of Ba and Sr shows different morphology and nucleation behavior depending on the deposition temperature. For example, at higher deposition temperature, the electron diffraction pattern of Sr on Ge shows a 9x1 phase which was not observed at lower deposition temperature. We perform an atomic-scale study of Ba and Sr templates grown on Ge(001) by molecular beam epitaxy as a function of deposition temperature and coverage, using a combination of reflection high energy electron diffraction and \textit{in situ} scanning tunneling microscopy (STM). An understanding of the behavior of Ba and Sr template formation on germanium can potentially be used as a means of better controlling the subsequent oxide growth.   

Structure of SrTiO$_3$ films on Si

The epitaxial deposition of oxides on silicon opens the possibility of incorporating their diverse properties into silicon-device technology. Deposition of SrTiO$_3$ on silicon was first reported over a decade ago, but growing the coherent, lattice-matched films that are critical for many applications has been difficult for thicknesses beyond 5 unit cells. Using a combination of density functional calculations and x-ray diffraction measurements, we determine the atomic structure of coherent SrTiO$_3$ films on silicon, finding that the Sr concentration at the interface varies with the film thickness. The structures with the lowest computed energies best match the x-ray diffraction. During growth, Sr diffuses from the interface to the surface of the film; the increasing difficulty of Sr diffusion with film thickness may cause the disorder seen in thicker films. The identification of this unique thickness-dependent interfacial structure opens the possibility of modifying the interface to improve the thickness and quality of metal oxide films on silicon.   

Low kV Atomic Resolution and First Principles Study of the Structure and Bonding at SrTiO$_{3}$/GaAs Hetero-interfaces

Ultrathin transition-meal oxide films on polar substrates have attracted increasing attention in recent years, due to the emergence of novel interfacial phases, not seen in the bulk of either material. In this study, we have combined aberration-corrected atomic-resolution Z-contrast imaging, electron energy loss spectroscopy (EELS) with first-principles density functional theory calculations to examined the atomic and electronic structures of epitaxially grown, ultrathin SrTiO$_{3}$ (100) films on GaAs (001). We find that the interface is atomically abrupt and no surface reconstruction of the GaAs (001) surface is observed. Using atomic-column resolved EELS, we show that Ti diffuses into the first few monolayers of GaAs and we will present evidence for the formation of As-oxides at the interface depending on the thin film growth conditions. First-principles DFT calculations will be used to analyze the formation energies of Ti-related impurity defects in the bulk and surface regions of GaAs, as well as the stability of any surface reconstruction at the SrTiO$_{3}$/GaAs interface. These findings are used to explain transport behavior of the SrTiO$_{3}$ films as a function of deposition conditions.   

Ultimate photovoltage in perovskite oxide heterostructures with critical film thickness

One order larger photovoltage is obtained with critical thicknesses of La$_{0.9}$Sr$_{0.1}$MnO$_{3}$ films in both kinds of heterostructures of La$_{0.9}$Sr$_{0.1}$MnO$_{3}$/SrTiO$_{3}$ (0.8 wt {\%} Nb-doped) and La$_{0.9}$Sr$_{0.1}$MnO$_{3}$/Si fabricated at various oxygen pressures. Our self-consistent calculation reveals that the critical thickness of the La$_{0.9}$Sr$_{0.1}$MnO$_{3}$ film with the ultimate value of photovoltage is just the thickness of the depletion layer of La$_{0.9}$Sr$_{0.1}$MnO$_{3}$ in the $p-n$ heterostructures of La$_{0.9}$Sr$_{0.1}$MnO$_{3}$/SrTiO$_{3}$ (0.8 wt {\%} Nb-doped) and La$_{0.9}$Sr$_{0.1}$MnO$_{3}$/Si, respectively.   

Topological insulators from complex orbital order in transition-metal oxides heterostructures

Topological band insulators which are dynamically generated by electron-electron interactions have been the- oretically proposed in two and three dimensional lattice models. We present evidence that the two-dimensional version can be stabilized in digital (111) heterostructures of transition-metal oxides as a result of purely local interactions. The topological phases are accompanied by spontaneous ordering of complex orbitals and we discuss their stability with respect to the Hund's rule coupling, Jahn-Teller interaction and inversion symmetry breaking terms. As main competitors we identify spin-nematic and magnetic phases.   

The electronic structure of epitaxially strained iridate thin films and superlattices from first principles

Within the Ruddlesden-Popper iridates Sr$_{n+1}$Ir$_n$O$_{3n+1}$, strong spin-orbit interactions lead to the formation of a half-filled, narrow $J_\textrm{eff}=1/2$ band and filled $J_\textrm{eff}=3/2$ bands. This places the iridates in the vicinity of a Mott transition, which is sensitive to perturbations in crystal structure, despite relatively weak on-site Coulomb interactions [1]. For example, Sr$_2$IrO$_4$ ($n=1$) is an antiferromagnetic Mott insulator that displays an almost rigid coupling between spin canting and IrO$_6$ octahedron rotations [2], while epitaxially stabilized SrIrO$_3$ ($n=\infty$) is a correlated metal. In this talk, we will discuss from first-principles within the LDA+SO+$U$ approach the possibility to engineer the electronic structure of SrIrO$_3$ and CaIrO$_3$ thin films using epitaxial strain and by creating superlattices of the form ($A$IrO$_3$)$_m$($A'B$O$_3$)$_{m'}$ with $A$, $A'$ = Ca, Sr. [1] S.J. Moon, H. Jin, K.W. Kim, W.S. Choi, Y.S. Lee, J. Yu, G. Cao, A. Sumi, H. Funakubo, C. Bernhard, and T.W. Noh, PRL 101, 226402 (2008). [2] B.J. Kim, H. Jin, S.J. Moon, J.-Y. Kim, B.-G. Park, C.S. Leem, J. Yu, T.W. Noh, C. Kim, S.-J. Oh, J.-H. Park, V. Durairaj, G. Cao, and E. Rotenberg, PRL 101, 076402 (2008).   

The Structure of Y$_2$O$_3$:ZrO$_2$ / SrTiO$_3$ Layered Heterostructures

Since the discovery of greatly increased conductivity in layered heterostructures of yttria stabilized zirconia (YSZ) and strontium titanate (STO) there has been considerable study and discussion regarding the origin of the high level of conductivity. The detailed atomic structure of layered YSZ-STO heterostructures is likely to be a crucial source of understanding, and as yet remains undetermined, particularly in the region of the buried interfaces between YSZ and STO. We present the results of density functional theory calculations on YSZ-STO heterostructures layered in the [001] direction, with different structures and compositions. We find that structures built using the terminations of the fluorite crystal structure of YSZ do not produce heterostructures which follow conventional solid state chemistry. These heterostructures contain Ti and Zr ions with unlikely O coordination geometries, reduced Ti$^{3+}$ and Zr$^{3+}$ ions and O-O bonding. In contrast, heterostructures built using a reconstructed YSZ termination, based on a rock-salt ordered ZrO layer, give more stable and more conventional atomic structures and are calculated to be electrically insulating. We describe these structures in detail, and the reasons behind the necessary reconstruction of YSZ at the interface.   

Structural Rearrangement of Niobium Oxides from Lamellar Phases to Discrete Nanosheets and Nanoscrolls Probed by DFT Calculations

Inorganic niobates ACa$_{2}$Nb$_{3}$O$_{10}$ (A= H and K) with layered structures are good photocatalytic materials due to their high surface areas accommodating a larger number of active sites and ease of processing through soft chemical techniques like exfoliation and restacking. Alkali metal phases can be ion-exchanged to the acid phase, which in turn can be easily exfoliated to individual nanosheets. The nanosheets can change their form to nanoscrolls with a curled geometry instead of a flat surface. During these morphological transformations, the local structure at the Nb-atom, H-atom and the interface may undergo rearrangement which is responsible for the alteration of properties of the materials. This presentation highlights the preliminary results on these structural modifications (interface variation, stacking of layers, lattice contraction and space group settings) and the possible positions of the proton. Our calculations show that the protons in the acid form are non-bridging and bonded to the same layer oxygen atoms unlike the K-atoms in its parent compound. The Electric field gradient (EFG) is a parameter very sensitive to the electron density around a quadrupolar nucleus $^{2}$H and $^{93}$Nb that can be detected using NMR. Changes in its magnitude/sign can be correlated to the change in the local environment (bond lengths and angles) around the sites of interest. EFG values from DFT calculations based on the proposed structural models will be used for the characterization of surface O-H bond lengths, H-bonding and Nb-O bond lengths and can be used to interpret NMR studies.   

Study of interface interactions in ZnO/Mesoporous silica nanocomposite

The Photoluminescence (PL) properties of ZnO/Mesoporous silica (MPS) nanocomposite annealed under different temperatures were studied. A broad PL band at 395 nm has been observed in all samples and analysis was made by using Gaussian fitting. As the temperature increased, emission bands were blue shifted and the relative intensity ratio of the oxygen vacancies at ZnO-SiO$_{2}$ interface to the oxygen vacancies in inner ZnO crystallites was increased. The emission peaks at 363 nm and 384 nm are attributed to the near band edge emission (NBE) and to the phonon replica emission. At 550C the exciton confinement effect disappears due to the large amount of surface effects. The influence of porosity of host media on Si-O-Zn cross linking bonds was also investigated. ZnO nanoparticles were loaded into nanocrystalline silica (NCS) and silica gel (SG). The surface area increases monotonously from NCS to MPS through SG. Si-O-Zn cross linking bonds were almost absent in the sample prepared with NCS. It exhibits NBE emission at 360 nm which was found to be absent in other samples prepared with SG and MPS. It was also found that the emission intensity of the samples decreases with aging. This is due to diffusion of oxygen atoms from the atmosphere to interior of the sample.