Session D11: Focus Session: Correlated Electron Superlattices

A tight-binding LDA+DMFT study of manganite superlattices

The combination of {\it ab initio} density functional theory with model ``many-body'' calculations provides a very promising way for a realistic theoretical treatment of surface and interface effects of strongly correlated electron materials. Here we show in detail how the electronic structure of LaMnO$_3$ calculated within the local density approximation (LDA) can be efficiently parametrized using a physically transparent tight binding model and considering both nearest and next nearest neighbor hoppings. In particular, we address effects due to rotations of the oxygen octahedra surrounding the Mn cations. The resulting two-band model is then applied to study LaMnO$_3$-SrMnO$_3$ superlattices using dynamical mean-field theory (DMFT) and the predicted ground state phases for superlattices with a small number of individual LaMnO$_3$ and SrMnO$_3$ layers are compared with the results obtained by density functional theory.   

A Realistic Model Calculation on Manganite Superlattice

We present a realistic model calculation for (0,0,1) (LaMnO$_3$)$_m$ (SrMnO$_3$)$_n$ superlattices. In this model, the superlattice is defined by the long-range Coulomb interaction generated by different ion charges of La (3+) and Sr (2+). The electronic degree of freedom contains two Manganese-Oxygen hybridized e$_g$ bands coupled to localized Mn $t_{2g}$ spins and to Jahn-Teller phonon modes, and the most general on-site electron-electron interactions within those $e_g$ bands. We will show how charge reconstruction, structural constraints and the symmetry breaking induced by the superlattice affect the orbital and magnetic orders. This work is supported by DMR-0213574.   

Two-Dimensional Spin-Polarized Electron Gas at the Perovskite Manganite Interface : SrMnO$_3$/LaMnO$_3$

Electronic structure calculations for the perovskite manganite heterostructure (SrMnO$_3$)$_{n}$/(LaMnO$_3$)$_{1}$/(SrMnO$_3$)$_{n}$ reveal the presence of a novel spin-polarized electron gas at the interface, generated from the stripped-off La (5d$^1$) electrons, which become confined in the electrostatic V-shaped potential well of the positively charged (LaO) sheet, occupying the Mn(e$_g$) states near the interface. The presence of these electrons turns the interaction between the interfacial Mn atoms to be ferromagnetic via the Anderson-Hasegawa double exchange, overcoming the original antiferromagnetic superexchange present in the SrMnO$_3$ bulk. The FM Mn atoms at the interface in turn make the electron gas spin-polarized, as confirmed by the total energy calculations, and the type G AFM of the bulk is resumed a few layers into the bulk.   

First-principles study of LaAlO$_3$/SrTiO$_3$ thin films

Although the perovskite oxides LaAlO$_3$ (LAO) and SrTiO$_3$ (STO) are conventional band insulators, an electron gas can form at their interface. Several mechanisms have been proposed to produce the electron gas, including the electrostatic divergence within LAO that results from the growth of alternating charged (LaO)$^+$ and (AlO$_2$)$^-$ layers and an electronic reconstruction in which Ti$^{3+}$ and Ti$^{4+}$ is formed at the interface. Of practical interest, thin films of LAO on STO have been observed to have highly mobile carriers and a carrier density that is tunable via LAO thickness---in recent work, Thiel et al. observed an insulator-metal transition between 3--4 MLs. In this talk, first-principles electronic structure calculations are performed on LAO thin films grown on STO to investigate surface reconstructions and the penetration depth of the electron gas into the substrate. An insulator-metal transition is found in good agreement with experiment.   

Theory of band alignment at the LaAlO$_{3}$/SrTiO$_{3}$ interface

A polar discontinuity at the abrupt oxide/oxide interface is one of several problems that need to be addressed before we can realize the promise of multiferroic oxide structures. To avoid the so-called polar catastrophe the interface undergoes roughening which renders the structure useless, unless the system finds a mechanism for compensating the interface charges. Recent experiments of Hwang and co-workers (\textit{Nature }\textbf{427}, 423-426 (2004) and \textit{Nature }\textbf{430}, 657-661 (2004)) suggest that in the case of perovskite oxides two quite different compensatory mechanisms are at play at the heterojunction. For the n-type LaAlO$_{3}$/SrTiO$_{3}$ interface it is purely electronic involving mixed valence Ti ions, while for the p-type it is an actual ionic reconstruction involving oxygen vacancies. We report a first-principles study of both interfaces within density functional theory. We consider the energetics and electronic structure of the interface, including the role of oxygen vacancies and band offsets. In addition we consider the interface stability with respect to inter diffusion of La and Sr across the interface.   

Correlation driven charge order at LaAlO$_{3}$/SrTiO$_{3}$ and LaTiO$_{3}$/SrTiO$_{3}$ Interfaces

Correlated behavior at complex oxide interfaces offers additional degrees of freedom to compensate charge imbalance not available $e.g$. in polar semiconductor heterostructures. This can result in electronic, charge and magnetic phases that do not exist in the bulk and offers new possibilities for device applications. For instance, the interfaces of LaTiO3 and SrTiO3 [1] as well as LaAlO3 and SrTiO3 [2] show metallic conductivity, although the respective bulk materials are Mott (LTO) and band insulating (STO, LAO). Here we present the results of material-specific correlated band theory (LDA+U) employing the FP-LAPW code in the WIEN2k implementation for a variety of (n,m) multilayers containing $n$ LTO (or LAO) and $m$ STO-layers. To explore the relaxation length towards bulk behavior $n$ and $m$ is varied between 1 and 9. We find that charge mismatch at the LTO/STO IF is accommodated by the formation of a charge and orbitally ordered (CO/OO) layer with a checkerboard arrangement of Ti$^{3+}$ and Ti$^{4+}$ and an antiferromagnetic coupling of the Ti$^{3+}$-spins [3]. Lattice relaxations lead to the observed conducting behavior. An analogous diluted layer of Ti$^{3+ }$spins is obtained for the $n$-type LAO/STO interface, although the corresponding bulk materials are nonmagnetic. For a structurally ideal $p$-type LAO/STO IF the measured insulating behavior can only be understood by a charge disproportionation on the oxygen sublattice and the formation of a CO/OO magnetic OP$\pi $ hole. Alternatively, charge compensation by oxygen vacancies and the formation of a charge conjugate F-center is considered. [1] A. Ohtomo, and H.Y. Hwang, Nature \textbf{423}, 378 (2002). [2] A. Ohtomo, D.A. Muller, J.L. Grazul, and H.Y. Hwang, Nature \textbf{419}, 378 (2002). [3] R. Pentcheva and W.E. Pickett, cond-mat/0608212. [4] R. Pentcheva and W.E. Pickett, Phys. Rev. B \textbf{74}, 035112 (2006).   

Origin of the charge carriers at LaAlO$_{3}$-on-SrTiO$_{3 }$ hetero-interfaces; possibility of intrinsic doping

We have made very thin films of LaAlO$_{3}$ on TiO$_{2}$ terminated SrTiO$_{3}$. \textit{In situ} UPS, XAS, vis-VUV-SE, transport measurements and annealing experiments results indicate that oxygen vacancies play an important role in the creation of the charge carriers and that these vacancies are introduced by the pulsed laser deposition process. Our results explain for the first time the origin of the large sheet carrier densities and high mobility observed previously. Simple model calculations confirm the plausibility of having defects at the origin of charge carriers while still maintaining a high mobility. By means of annealing experiments in atomic oxygen we examine the question whether an intrinsically doped interface does indeed exist at lower carrier concentrations. Work supported by the DoE BES and EPRI.   

Kondo effect and ferromagnetic ordering at the $n$-type SrTiO$_3$ - LaAlO$_3$ conducting interface

The intriguing phenomenon of electrical conductivity at the interface between two insulators is one of the possible consequences of electronic reconstruction of materials. In analogy with interface conduction, the question arises whether or not it is possible to induce magnetism at the contact between two nonmagnetic materials. We show how a polar discontinuity at the $n$-type conducting interface between the nonmagnetic perovskites SrTiO$_3$ and LaAlO$_3$ can induce a local magnetic moment on the Ti site. The resulting interface magnetism manifests itself in the form of ferromagnetic ordering at 0.3 K and the scattering of conduction electrons on the magnetic local moment, which provides a large negative magnetoresistance of 30\%. The scattering can be described in terms of the Kondo effect with a Kondo temperature of 50 K. Electronically reconstructed interfaces now provide another versatile class of solid state Kondo systems, next to dilute impurities in metals and artificial quantum dots.   

Optical Conductivity of LaAlO$_{3}$/SrTiO$_{3}$ Superlattices

Precise contactless measurements of the optical conductivity of LaAlO$_{3}$/SrTiO$_{3}$ (LAO/STO) superlattices can be achieved using time-domain terahertz spectroscopy. We report the optical conductivity in the frequency range 3-30 cm$^{-1}$ as a function of temperature and concentration of oxygen vacancies in the STO layers. Superlattices were grown by laser-MBE, enabling control of the structure on the single-unit cell level. Optical measurements were made possible through the use of Si rather than STO substrates. To identify the interface contribution to the conductivity we compared the optical transmission of structures with different numbers of interfaces, while maintaining constant the total number of unit cells.   

Properties of epitaxial LaMnO$_{3}$/SrTiO$_{3}$ interfaces.

Electronic effects at interfaces between dissimilar oxides are known to have fundamental consequences on their transport and magnetic properties. Interfaces between the band insulator SrTiO$_{3}$ and the antiferromagnetic charge-transfer insulator LaMnO$_{3 }$ provide a particularly interesting platform to test such effects. Both perovskites are grown epitaxially by pulsed-laser deposition, and electron energy loss spectra (EELS) collected in a scanning transmission electron microscope (STEM) show interfacial valence changes on the Mn-sites, while Ti remains in a 4+ state even in direct contact with the LaO layer of the LaMnO$_{3}$. This observation is fully consistent with computational results obtained for such structures using the self-interaction corrected (SIC) local spin density (LSD) method. In this presentation, we discuss the physical origin and consequences of these valence changes in single interfaces as well as LaMnO$_{3}$/SrTiO$_{3}$ superlattices. This research was sponsored by the Office of Basic Energy Sciences, Division of Materials Sciences and Engineering, U.S. Department of Energy, under contract DE-AC05-00OR22725 with Oak Ridge National Laboratory (ORNL), managed and operated by UT-Battelle, LLC, and ORNL's Laboratory Directed Research and Development Program.   

Interface effect on the positive magnetoresistence in a heterostructure composed of two perovskite oxides

Different from the negative colossal magnetoresistance (CMR) of doped manganites, a positive CMR is discovered at low applied magnetic field and high temperature in the epitaxial $p-n$ heterostructure with Sr-doped LaMnO$_{3}$ and Nb-doped SrTiO$_{3}$ fabricated by laser molecular-beam epitaxy [1,2]. We have found that such unusual positive CMR is an interface effect which causes a charge redistribution in at the interface with different electron spin polarization at Fermi level from that in the corresponding bulk CMR materials. Self-consistent calculation was carried out the band structure around the interface of the heterostructure and confirms the unusual behavior. Other puzzling CMR features with bias voltage, temperature and even composition are well explained by the present scenario. 1. Kui-Juan Jin et al., Phys. Rev. B\textbf{ 71}, 184428 (2005). 2. Qing-li Zhou, et al., Europhys. Lett. \textbf{71}, 1-7 (2005)   

Band structure engineering of LaMnO3:SrTiO3 superlattices at the molecular level

We have made single crystal short period superlattices consisting of alternating slabs of LaMnO3 and SrTiO3, using atomic layer by layer molecular beam epitaxy. The supercells consist of N-layers of each component, with N=1, 2, 3, 4. Ellipsometric measurements of the new materials show that optical absorption in the visible light range is significantly different from bulk LaMnO3 or SrTiO3, and depends on N. The new band structure is dependent on the superlattice design. This is an example of engineered coherent ``meta-materials'', and this fabrication technique can be extended to other lattice matched transition metal oxides with a wide range of conducting and magnetic properties.   

Potential profiling of the nanometer-scale charge-depletion layer in $n$-ZnO/$p$-NiO junction using photoemission spectroscopy

We have performed a depth-profile analysis of an all-oxide $p-n$ junction $n$-ZnO/$p$-NiO [1] using photoemission spectroscopy combined with Ar-ion sputtering, and investigated the potential profile of the space-charge region embedded at the interface [2]. Systematic core-level shifts were observed during the gradual removal of the ZnO overlayer, and were interpreted using a model based on charge conservation. Spatial profile of the potential around the interface was deduced, including the charge-depletion width of 2.3 nm extending on the ZnO side and the built-in potential of 0.54 eV. [1] H. Ohta, \textit{et al}., APL \textbf{83}, 1029 (2003). [2] Y. Ishida, \textit{et al}., APL \textbf{89}, 153502 (2006).