Session A37: Focus Session: Complex Oxide Thin Films -- Conductivity at Oxide Interfaces

Maximally anisotropic point Fermi surface system: VO$_2$ films embedded in TiO$_2$

Oxide heterostructures provide an unusually rich canvas for the design of unprecedented electronic states. Here we will discuss multilayer (TiO$_2$)$_m$/(VO$_2$)$_n$ nanostructures, namely V$^{4+}$:d$^1$ - Ti$^{4+}$:d$^0$ interfaces, with no polar discontinuity, studied by density functional theory techniques[1]. This system shows a metal-insulator transition with respect to the VO$_2$ layer thickness in our first principles calculations[2]. For n = 1 and 2 VO$_2$ layers, the system is insulating. For 5 and more layers, it is ferromagnetic and half-metallic. For the quantum confined cases of n = 3 and 4 the system is neither insulating nor conducting, instead an unexpected state arises: the Fermi surface is point-like as in graphene, except that extreme anisotropy is present[3]. The electrons (or holes, depending on doping) behave as massless fermions along the zone diagonal in k-space, and as conventional (massive) fermions along the perpendicular direction. Certain characteristics identify this ``semi-Dirac'' phase as resulting from quantum confinement, rather than being an interface phenomenon. This point Fermi surface system differs from graphene not only in its extreme anisotropy, but that it arises in a half-metallic system, so spin degrees of freedom are removed. In this presentation an analysis of the evolution of the electronic structure through this unprecedented insulator-to-metal transition will be provided, and the role of a non-intuitive orbital ordering of the V d$^1$ ions will be discussed. Also the robustness of the semi-Dirac electronic structure to interfacial disorder and the introduction of spin-orbit coupling in the calculations will be analyzed. \\[4pt] [1] V. Pardo and W.E. Pickett, Phys. Rev. Lett. 102, 107003 (2009).\\[0pt] [2] V. Pardo and W.E. Pickett, arXiv:0910.4411.\\[0pt] [3] S. Banerjee, R.R.P. Singh, V. Pardo and W.E. Pickett, Phys. Rev. Lett. 103, 016402 (2009).   

Electric Field Tuned Crossover From Semi-Classical to Weakly Localized Quantum Transport in a Quasi 2-Dimensional Electron Gas in SrTiO$_3$

Quasi 2-dimensional electron gases (Q2DEG) created by doping SrTiO$_3$ (STO) near interfaces and surfaces, have attracted considerable attention. These oxide Q2DEG's potentially offer a wide range of tunable behavior due to the extraordinary properties of STO. Here, we present electric field tuned magnetotransport measurements of a Q2DEG created by Ar- irradiation. The electric field, applied through a back gate bias, modulates both the mobility and sheet density, with a greater effect on the former. For high mobilities, the magnetotransport obeys Kohler's rule, consistent with semi- classical transport. As the mobility is decreased, violation of Kohler's rule is observed and weakly localized quantum transport emerges. This electric field tuned crossover arises from a modulation in the carrier confinement, which is enhanced by the electric field dependent dielectric constant of STO, as shown through self-consistent solutions to the Poisson and Schr\"odinger equations. The implications of our results on the development of oxide electronic devices will be discussed.   

Optical spectroscopic investigation in Nb:SrTiO$_{3}$/SrTiO$_{3}$ oxide heterostructures

SrTiO$_{3}$ based heterostructures show various fascinating properties ranging from anomalous magnetism to field effect controlled superconductivity. In this talk, we used optical spectroscopy to study electron-phonon coupling in Nb:SrTiO$_{3}$/SrTiO$_{3}$ heterostructures, which governs the electrodynamics of the system. First, we will discuss on the heavily doped SrTiO$_{3}$ superlattices (SrTi$_{0.8}$Nb$_{0.2}$O$_{3}$/SrTiO$_{3})$, which show large enhancement of thermoelectric Seebeck coefficient.[1] As the conducting Nb:SrTiO$_{3}$ layer thickness decreases, the polaron effective mass and relaxation time increase due to the dimensional crossover of polaron. It is shown that such changes due to the electron-phonon coupling can benefit the thermoelectric properties. Second, we will discuss on electrodynamics of the \textit{$\delta $}-doped SrTiO$_{3}$ (SrTiO$_{3}$/SrTi$_{0.99}$Nb$_{0.01}$O$_{3}$/SrTiO$_{3})$ heterostructures.[2] As the temperature decreases, these heterostructures show a large enhancement of mobility, while charge carrier density remains almost constant. The role of strong electron-phonon coupling in the quasi 2-dimensional SrTiO$_{3}$ heterostructure will be also discussed. [1] H. Ohta, \textit{et al}., Nature materials \textbf{6}, 129 (2007) [2] Y.Kozuka, \textit{et al}., unpublished   

Metal-insulator transition by rare-earth substitution in oxide two-dimensional electron gas

The effect of rare-earth (R) ion in the formation of a two-dimensional electron gas (2DEG) in epitaxial SrTiO$_{3}$/RO/SrTiO$_{3}$ (R = La, Pr, Nd, Sm, Y) heterostructures was studied. The interfacial RO monolayer with a +1 valence is expected to donate one electron per unit cell to neighboring TiO$_{2}$ layers in SrTiO$_{3}$, resulting in a conducting 2DEG confined near the $R$O monolayer. We observe such conducting interfaces with LaO, PrO, and NdO monolayers, but find that the heterostructures doped with SmO and YO monolayers are insulating. Also, there is a critical thickness of SrTiO$_{3}$ needed for the interfaces to become conducting. The temperature dependence of the carrier concentration is similar for samples doped with La and Pr, but for samples doped with Nd, the carrier concentration decreases dramatically with decreasing temperature. We probe the interface structure in the LaO and SmO doped samples with EELS and X-ray diffraction in order to find a correlation between the interface structure and conductivity.   

Spin-polarized 2DEG through electrostatic field doping in LaAlO$_{3}$-EuO heterostructures

We recently presented theoretical calculations in support of the \textbf{electrostatic doping model} for the $n$-type LaAlO$_{3}$/SrTiO$_{3}$ interface and offered a consistent picture explaining the origin of charge and its localization at the interface\textbf{:} the strong internal electric field of the polar oxide (LAO) causes charge transfer to the conduction band of the adjacent oxide STO. In this talk we extend this concept to explore theoretically the possibility of creating \textbf{a spin polarized 2DEG} at the heterointerface between LAO and ferromagnetic semiconducting EuO. We use the density functional theory within the generalized gradient approximation (GGA) as implemented in the VASP code. We reports that the amount of spin polarized charge transferred from LaAlO$_{3}$ to EuO is of the order of 4.0$\times $10$^{13}$ cm$^{-2}$ and then exponentially decays to 5-6 EuO ($\sim $ 10 {\AA}) with 2 DEG characteristic.   

Prediction of a Spin-Polarized Two-Dimensional Electron Gas at the LaAlO3/EuO(001) Interface

The discovery of a two-dimensional electron gas (2DEG) at the interface between insulating oxides like LaAlO3/SrTiO3 is promising in designing novel electronic devices. Making a spin-polarized 2DEG is a very exciting prospect for spintronics applications, where the involvement of the spin degree of freedom broadens the spectrum of potential applications. Here we propose a method to achieve a spin- polarized 2DEG by employing a ferromagnetic insulator as one of the constituents in the oxide heterostructure and consider EuO as a representative ferromagnetic insulator in conjunction with LaAlO3 to form a spin-polarized 2DEG at the LaAlO3/EuO (001) interface.[1] This polar interface favors electron doping into the Eu-5d conduction bands. Due to the exchange splitting of the Eu-5d states in the ferromagnetic insulator the 2DEG is spin-polarized below the Curie temperature of EuO. Spin-polarized properties of this 2DEG at the interface between polar and ferromagnetic insulators inherited from the ferromagnetism of the oxide may provide a robust magnetism of the 2DEG which is beneficial and interesting for spintronics applications. [1] Y. Wang et al, Phys. Rev. B 79, 212408 (2009).   

Polar catastrophe, electron leakage, and magnetic ordering at the LaMnO$_3$/SrMnO$_3$ interface

Issues such as polar catastrophe, two-dimensional electron gas, etc., which are well studied for the prototypical LaAlO$_3$/SrTiO$_3$ interface also arise at the (100) interface between LaMnO$_3$ (LMO) and SrMnO$_3$ (SMO). We have studied the nature of electronic reconstruction and its effect on the polar discontinuity and magnetic ordering for the latter interface from density-functional calculations and model studies. We show that the polar catastrophe, originating from the charged LMO layers, is avoided by the accumulation of extra 0.5 electrons at the interface just like the LaAlO$_3$/SrTiO$_3$ interface. In addition, the Mn e$_g$ electrons leak out from the LMO side to the SMO side, the extent of the leakage being controlled by the interfacial potential barrier and the substrate induced epitaxial strain. The leaked electrons mediate a ferromagnetic Zener double exchange, making the interfacial layers ferromagnetic, while the two bulks retain their original type A or G antiferromagnetic structures. The interfacial electrons form a half-metallic conduction band, while both bulks remain insulating, resulting in an interesting interface system.   

In-situ Angle-Resolved Photoelectron Spectroscopy through the Metal-Insulator Transition in (LaMnO$_{3}$)$_{2n}$(SrMnO$_{3}$)$_{n}$ Superlattices

We report in-situ Angle-Resolved Photoelectron Spectroscopy (ARPES) studies of (LaMnO$_{3}$)$_{2n}$(SrMnO$_{3}$)$_{n}$ superlattices. Our combined Molecular Beam Epitaxy and ARPES system permits the growth and measurement of (LaMnO$_{3}$)$_{2n}$(SrMnO$_{3}$)$_{n}$ under ultra-high vacuum conditions, permitting high-resolution ARPES to be performed on these materials for the first time. Superlattices of this form exhibit a variety of electronic states as a function of ``n'' and temperature, including a transition from metallic to insulating behaviour for n $<$ 3 to n $\ge$ 3. We present ARPES measurements of the Fermi surface and remnant Fermi surface for metallic and insulating superlattices, and discuss the suppression of spectral weight at the Fermi level across the metal-insulator transition. We have directly observed band-mass renormalization in high-resolution ARPES data on metallic samples, and will discuss the implications to interactions with collective modes.   

Stray field and superconducting spin valve effect in La$_{0.7}$Ca$_{0.3}$Mn$O_3$/YBa$_2$Cu$_3$O$_{7-\delta}$ bilayers

Electronic transport and magnetization measurements were performed on La$_{0.7}$Ca$_{0.3}$Mn$O_3$/YBa$_2$Cu$_3$O$_{7- \delta}$ (LCMO/YBCO) bilayers below the superconducting transition temperature. Two types of angular dependent magnetoresistance (MR) behavior were observed. One MR comes from the motion of weakly pinned vortices, which are induced by the out-of-plane stray field of the domain walls. The other MR depends on the relative angle between the magnetization at the LCMO/YBCO interface and the magnetization present in the interior of the LCMO layer and gives rise to the superconducting surface spin valve effect.   

Spectroscopic magnetrotransport across YBa$_{2}$Cu$_{3}$O$_{7-x}$/La$_{0.7}$Ca$_{0.3}$MnO$_{3}$ microjunctions

Superconducting/Ferromagnetic YBCO/LCMO heterostructures exhibit a number of novel behaviors, such as unexpected long-range proximity effects, and giant magnetoresistance. The microscopic mechanisms behind these effects are under debate and include, among others, different types of charge and spin transport processes across the YBCO/LCMO interface. Characterizing these and determining the electronic density of states nearby the interfaces is key to understand the nature of the S/F interactions. With this motivation, we have fabricated vertical YBCO/LCMO and YBCO/LCMO/YBCO junctions (of areas down to 8 $\mu $m$^{2})$ using optical lithography and ion etching. The current-perpendicular-to-plane differential conductance shows salient features such as i) a sizable interface resistivity $\sim $ 10$^{-3 }\Omega \cdot $cm$^{2}$, ii) a prominent zero-bias conductance peak, and iii) a symmetric series of resonances at quasiparticle sup-gap energies, reminiscent of Tomasch and McMillan-Rowell oscillations. We will discuss the origin of these behaviors and their implications in regard of proximity effects.   

Holes and vacancies across interfaces in superconducting Sr$_{2}$CuO$_{4}$-La$_{2}$CuO$_{4}$

Sr$_{2}$CuO$_{4}$ films with superconducting critical temperatures T$_{c}$ up to 90 K have a substantial number of apical oxygen vacancies. An increase in T$_{c}$ has been correlated in La$_{2}$CuO$_{4}$-La$_{2-x}$Sr$_{x}$CuO$_{4}$ bilayers with an increase in the apical oxygen -- Cu atom distance. This suggests that the apical oxygen in these structures is not an entirely passive participant in high-T$_{c}$ superconductivity. We studied Sr$_{2}$CuO$_{4}$-La$_{2}$CuO$_{4}$ superlattices with resonant soft x-ray scattering in order to quantify the apical oxygen scattering. At the O K edge we observed scattering resonances from mobile holes and apical oxygen vacancies. The apical vacancy resonant scattering factor was obtained. The distribution of apical vacancies measured at O K edge follows closely the structural imperfections measured with hard x-rays with a characteristic length 5.25 \underline {+} 0.75 A. The mobile holes diffuse with a characteristic length of $\lambda $=5.3 \underline {+} 1 A.   

Superconductivity at oxydes interface

Superconductivity has been discovered at the interface between two insulators, namely SrTiO$_{3}$ and LaAlO$_{3}$ [1], which can be tuned by applying a gate voltage [2]. We have recently measured the properties of such heterostructures at low temperature. The carrier density can be modulated by field effect, and so the superconducting and normal properties. We will present a study of the phase diagram and an analysis of the superconductor-insulator transition observed at low carrier density. We will also discuss the possibility to observe superconductivity in other oxides interfaces. \\[4pt] [1] N. Reyren et al, Science 317, 1196 (2007) \\[0pt] [2] A. Caviglia et al, Nature 456, 624 (2008)   

Ferrimagnetism in Ti and Cr doped $\alpha$-Fe$_2$O$_3$ due to heterostructuring

Based on DFT calculations including an on-site Coulomb repulsion term (GGA+$U$) we perform a comparative study of Ti and Cr substitution in $\alpha$-Fe$_2$O$_3$ by varying the concentration, distribution and magnetic alignment of the cations in the Fe$_2$O$_3$ host. In the FeTiO$_3$-Fe$_2$O$_3$ system the charge mismatch at the interface between the room temperature paramagnet ilmenite and the canted antiferrromagnet hematite is identified as the driving force towards ferrimagnetism as it leads to the formation of uncompensated spins in a mixed Fe$^{2+}$, Fe$^{3+}$ layer [1-3]. In contrast, in Cr$_2$O$_3$-Fe$_2$O$_3$ there is no valence discontinuity, but the formation of superlattices opens a path to stabilize a different magnetic order than in the Cr$_2$O$_3$ end member leading to a net magnetization in the system. The magnetic properties are discussed based on the magnetic interaction parameters for the interfaces extracted from the DFT calculations. Funding by the DFG (Pe883/4-1) and ESF are gratefully acknowledged. \\[4pt] [1] P. Robinson, et al., Nature {\bf 418}, 517 (2002).\\[0pt] [2] R. Pentcheva and H. Sadat Nabi, Phys. Rev. B {\bf 77}, 172405 (2008).\\[0pt] [3] H. Sadat Nabi and R. Pentcheva, J. Appl. Phys. {\bf 105}, 053905 (2009).