Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session G12: Focus Session: Complex Oxide Interfaces - Polar interfaces III |
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Sponsoring Units: DMP Chair: Guneeta Singh Balla, University of California, Berkeley Room: 314 |
Tuesday, March 19, 2013 11:15AM - 11:27AM |
G12.00001: Gated LaAlO$_3$/SrTiO$_3$ based superconducting nanowires Michelle Tomczyk, Guanglei Cheng, Joshua Veasey, Shicheng Lu, Chang-Beom Eom, Patrick Irvin, Jeremy Levy Oxide heterostructures have been shown to support a metal-insulator transition; additionally, below T$_c$, interface transport becomes superconducting. Control of this transition has been demonstrated at the nanoscale level in LaAlO$_3$/SrTiO$_3$ by AFM lithography\footnote{Cen, C. \textit{et al}. \textit{Nature Mater}. \textbf{7}, 298--302 (2008).}. Electrical transport at the 2D interface can be controlled through backgating. Tunability of one dimensional nanostructures created by AFM lithography is demonstrated through backgating of the heterostructure and through local capacitive effects from side gates. Side gates running parallel to the main channel can tune the Fermi level within the channel, facilitating modulation of a normal-superconducting transition in the wire. Local tuning of the carrier density may enable novel superconducting-normal junctions that could be useful for topological quantum computation. [Preview Abstract] |
Tuesday, March 19, 2013 11:27AM - 11:39AM |
G12.00002: Tuning the carrier density at SrTiO3/LaAlO3 interface by La1-xSrxMnO3 capping layer Yujun Shi, Di Wu The observation of a high-mobility quasi-two-dimensional electron gas (q2-DEG) at the interface between the insulators of SrTiO3 (STO) and LaAlO3 (LAO) has gained significant attention in recent years. The carrier density at these interfaces is usually tuned by controlling the growth conditions or applying an electric field in a three-terminal device. According to the polar catastrophe model, which is used to interpret the origin of the q2-DEG at the LAO-STO interfaces, the carrier density and the critical thickness of LAO for the metallic interfaces are related with the net charge of LaO and AlO2 layer in LAO. Here, we systematically study the growth of La1-xSrxMnO3 (LSMO-x), whose net charge is 1-x in each layer, on LAO (\textless\ 4 u.c.)/STO to tune the interfacial carrier density and critical thickness. For LAO (3 u.c.)/STO, we found that the threshold thickness of LSMO (x$=$0.33) for the observation of q2-DEG is 2 u.c. The LAO (3 u.c.)/STO interfaces show a metal-insulator transition for x between 2/3 and 7/8. Importantly, the carrier density monotonically decreases as increasing Sr doping. Our results strongly support the polar catastrophe model and provide a new approach to tune the interfacial carrier density.. [Preview Abstract] |
Tuesday, March 19, 2013 11:39AM - 11:51AM |
G12.00003: Simultaneous electromechanical and capacitance characterization of top-gated LaAlO$_3$/SrTiO$_3$ heterostructures Feng Bi, Mengchen Hung, Chung Wung Bark, Sangwoo Ryu, Chang-Beom Eom, Jeremy Levy LaAlO$_3$/SrTiO$_3$ (LAO/STO) heterostructures exhibit a sharp, hysteretic metal-insulator transition (MIT) with enhanced capacitance beyond the geometric limit when the interface is tuned by a biased top gate. To understand the physical origin of these behaviors, we investigate the electromechanical response and capacitance spectroscopy of top-gated LAO/STO heterostructures. Piezoelectric Force Microscopy (PFM) measurements demonstrate local variations in the hysteretic response, and capacitance measurements show carrier density changes at the LAO/STO interface as the top gate bias is varied. A strong correlation between PFM signals and capacitance signals is established by doing simultaneous measurements. The enhanced capacitance at the MIT is correlated with charging/discharging dynamics of nanoscale conducting islands at the interface, which can be imaged by spatially-resolved PFM. [Preview Abstract] |
Tuesday, March 19, 2013 11:51AM - 12:03PM |
G12.00004: A Gate-tunable Polarized Phase of Two-Dimensional Electrons at the LaAlO$_{3}$/SrTiO$_{3}$ Interface Arjun Joshua, Jonathan Ruhman, Sharon Pecker, Ehud Altman, Shahal Ilani We show using anisotropic magnetoresistance and anomalous Hall effect measurements that the LaAlO$_{3}$/SrTiO$_{3}$ interface has an unconventional phase diagram in the space of electron density and magnetic field. At high densities and fields we observe a polarized phase with crystalline anisotropy. Surprisingly, below a density-dependent critical field the polarization and anisotropy vanish and the resistivity sharply rises. This behavior, unobserved in other magnetic systems, indicates strong coupling between itinerant electrons and localized magnetic moments, enabling gate-tunable magnetism at this interface. [Preview Abstract] |
Tuesday, March 19, 2013 12:03PM - 12:39PM |
G12.00005: Two-dimensional electron gas at the atomically smooth LaAlO$_{3}$/SrTiO$_{3}$ (111) interface Invited Speaker: Chang-Beom Eom The two-dimensional electron gas (2DEG) at the LaAlO$_{3}$/SrTiO$_{3}$ (001) heterointerface has been widely investigated due to its diverse functionalities such as conductivity, ferromagnetism, and superconductivity. In this orientation, the SrTiO$_{3}$ is nonpolar, with charge-neutral AO and BO$_{2}$ planes, while $+$e of charge is transferred between AO and BO$_{2}$ planes in the LaAlO$_{3}$ layer. The (111) orientation is, however, qualitatively different in that the AO$_{3}$ and B lattice planes in both materials exhibit charge transfer between layers, and both have in principle a polar character. We have found that LaAlO$_{3}$ deposited on the (111) SrTiO$_{3}$ polar surface also supports an interfacial 2DEG. An atomically smooth step and terrace structure of (111) SrTiO$_{3}$ surface was prepared by buffered-HF and heat treatment. The step height of the treated (111) SrTiO$_{3}$ is $\sim$2.25{\AA}, which is 1/3 of the diagonal of the cubic SrTiO$_{3}$ lattice along the [111] direction, consistent with the thickness of one AO$_{3}$/B (111) bilayer. LaAlO$_{3}$ was grown epitaxially in a layer-by-layer growth mode, with one oscillation of the reflection-high energy electron diffraction (RHEED) specular spot corresponding to this single step height. The (111) interfacial 2DEG shows a higher carrier concentration than LAO/STO (001) at room temperature. We find a LaAlO$_{3}$ critical thickness between 11.3 and 16 {\AA}, with the transition between insulating and conducting regimes broader than that of LAO/STO (001). Surface X-ray diffraction with COherent Bragg Rod Analysis (COBRA) has been carried out to explore the possible structural reconstruction of (111) SrTiO$_{3}$. We will discuss the origin of 2DEG at this polar-polar interface. This work has been done in collaboration with S. Ryu, C. W. Bark, T. Hernandez, M. S. Rzchowski, H. Zhou and D. D. Fong, T. R. Paudel and E.Y. Tsymbal. [Preview Abstract] |
Tuesday, March 19, 2013 12:39PM - 12:51PM |
G12.00006: Two-dimensional electron gasat the interface between two polar oxide materials Tula Paudel, Evgeny Tsymbal Following the discovery of a two-dimensional electron gas (2DEG) at the interface between polar LaAlO$_{3}$ (LAO) and non-polar SrTiO$_{3}$ (STO) grown in the [001] direction many related heterostructures with interesting physical phenomena have been proposed and explored. Here using the first-principles theory, we investigate the electronic band structure of the interface between two polar oxide materials -- a wide materials group that can broaden the field for designing conducing interfaces with novel properties. As a model system, we consider a LAO/STO heterostructure stacking in the [111] direction. In this direction both free standing LAO and STO are polar with alternatively charged planes -- (LaO$_{3})^{3-}$ and Al$^{3+}$ in LAO and (SrO$_{3})^{4-}$ and Ti$^{4+}$ in STO leading to inevitable interface reconstruction. Simple electrostatic arguments suggest that at the Ti/LaO$_{3}$ terminated interface of the LAO/STO(111) heterostructure this reconstruction may be achieved through depositing electron surface charge of 0.5e/$\surd $3a$^{2}$ at the interface. This is by a factor of $\surd $3 smaller than that for the LaO/STO(001) interface which is expected to lead to a larger critical thickness of LAO(111) compared to LAO(001). These arguments are consistent with our first-principles calculations which predict a critical thickness of LAO(111) to be eight (LaO$_{3}$-Al) bilayers. Our findings are consistent with the experimental studies performed by S. Ryu, C. W. Bark, T. Hernandez, M. S. Rzchowski, H. Zhou, D. D. Fong, and C.-B. Eom. [Preview Abstract] |
Tuesday, March 19, 2013 12:51PM - 1:03PM |
G12.00007: Effect of Surface Engineering on LaAlO$_3$/SrTiO$_3$ Interfaces Sanjay Adhikari, Chang-Beom Eom, Michael klopf, Cheng Cen Carrier properties at the LaAlO$_{3}$/SrTiO$_{3}$ interfaces are highly sensitive to potential profile generated by LaAlO$_{3}$ top surface termination. In ambient environment, the uncontrolled surface exposure may introduce randomly distributed charge or polarization and therefore significantly impact interfacial transport by disorder related effect. As evidence, local fluctuation in carrier density and mobility has been observed in nanostructures defined by atomic force microscope (AFM) lithography. Here we report controlled modification of LaAlO$_{3}$ surface by solvent deposition. Surface desorption is first carried out by sample annealing in O$_{2}$ environment. The annealed LaAlO$_{3}$ surfaces are later coated with various solvents of controlled thicknesses by pulsed laser deposition using frozen targets. Coated surfaces are analyzed by pulsed force and frictional force microscopy. AFM lithography is also carried out to locally alter the surface charge state and modulate the potential disorder level. Effect of different controlled surface coatings on interface are studied by magneto-transport measurement [Preview Abstract] |
Tuesday, March 19, 2013 1:03PM - 1:15PM |
G12.00008: A spectroscopic study of the superconductor at the LaAlO$_3$/SrTiO$_3$ interface Hans Boschker, Christoph Richter, Jochen Mannhart The electron liquid at the LaAlO$_3$/SrTiO$_3$ interface is a model system for the study of superconductivity as it provides a two-dimensional superconductor whose properties can be tuned with an electrical gate field. We developed planar tunnel junctions to study the superconductivity spectroscopically. Our tunnel junctions give access to two important physical parameters: the size of the superconducting gap and the electron-phonon spectral function. We will present measurements of both as a function of the electric gate field. The likelihood of the convential electron-phonon coupling mechanism for superconducting pairing will be discussed. [Preview Abstract] |
Tuesday, March 19, 2013 1:15PM - 1:27PM |
G12.00009: Nature of Charge Compensation Mechanism in Devices with Polar Catastrophe Raphael Tsu, Wattaka Sitaputra Polar catastrophe (PC) is well-known in surface science driving a charge compensation mechanism (CCM) at surface/interface, with properties unfound in natural solids [1]. Combining PC with superlattice and resonant tunneling, new device opportunity is wide opened. The strange results of oscillations in conductance, between two limits of $G=gG_{o} $ with $g=2,...$in units of $G_{o} =e^{2}/h=39\mu S$, and hysteresis, were observed in nano-sized (tens of nm) crystalized silicon in amorphous silicon matrix, having native oxides, can be explained [2]. Recent observation of substantial enhanced mobility for very large transfer of carrier from Gd$_{2}$O$_{3}$ (100) / Si(100), in the order of $n\sim 10^{20}$ cm$^{-3}$ near the interface may apply to high current MOSFET [3]. The field of PC is dominated by heterostructures. 3D structures are lacking defined interfacial orientation; it is compatible with the CCM incorporating resonant tunneling.\\[4pt] [1] Hwang et al., Nat Mater 11 (2), 103(2012).\\[0pt] [2] Thesis, X. Li, 1993, UNC Charlotte; Thesis, A. Bowhill, 1994, UNC Charlotte; Tsu, Superlattice to nanoelectronics, 2$^{\mathrm{nd}}$ (2011) Elsevier\\[0pt] [3] W. Sitaputra, R. Tsu, (2012). Submitted. [Preview Abstract] |
Tuesday, March 19, 2013 1:27PM - 1:39PM |
G12.00010: Band alignments at the interface of complex oxides Lars Bjaalie, Anderson Janotti, Chris G. Van de Walle The realization of a two-dimensional electron gas at the SrTiO$_3$/LaAlO$_3$ interface has spurred interest in the development of electronic devices based on complex oxides. In the design of such devices it is crucial to know the band alignment at the interface of the different oxides, a key quantity that governs carrier barrier heights and carrier confinement. Reported values for the valence-band alignment at the SrTiO$_3$/LaAlO$_3$ interface vary by more than 1 eV. Using first-principles calculations based on a hybrid functional we calculate the band alignment at the interface between various complex oxides, including the band insulators SrTiO$_3$, SrZrO$_3$, LaAlO$_3$, CaTiO$_3$, and GaAlO$_3$ and the Mott insulators GdTiO$_3$ and YTiO$_3$. This choice of materials allows us to analyze the effects of cation size, lattice parameters, band gaps, and lattice orientation on the band alignment. [Preview Abstract] |
Tuesday, March 19, 2013 1:39PM - 1:51PM |
G12.00011: Understanding polarity compensation across polar LaAlO$_3$ films G. Singh-Bhalla, P. Rossen, S. Jaganath, G. Palsson, D. Yi, A. Dasgupta, J. Ravichandran, V. Ruiz, J. Heron, C. Fadley, A. Yadav, R. Pentcheva, R. Ramesh Dipole screening mechanisms for polar crystals can manifest in a variety of ways depending on bandgaps, surface energies and environmental conditions. Here we study the polarity compensation process in LaAlO$_{3}$ thin film grown on the two different surface terminations of [001] SrTiO$_{3}$ crystals (SrO and TiO$_{2})$. An electron gas that appears at the interface between LaAlO$_{3}$ and TiO$_{2}$-SrTiO$_{3}$ surface (n-type) potentially screens the LaAlO$_{3}$ polarity, while the interface between LaAlO$_{3}$ and SrO-SrTiO$_{3}$ (p-type) remains insulating. To understand this asymmetry, we probed the LaAlO$_{3}$ surface using a variety of element-specific probes and observe a change in the LaAlO$_{3}$ stacking structure in the p-type geometry. Tunneling measurements reveal remnants of a built-in field reflective of LaAlO$_{3}$'s intrinsic polarity across the n-type structure, but no such signatures are detected for the p-type structure. When combined with density functional theory simulations, the results suggest that while free charge screens the LaAlO$_{3}$ dipole in the n-type geometry, a change in LaAlO$_{3}$'s structure during growth nullifies the dipole in the p-type geometry. In essence, SrTiO$_{3}$ surface layers drastically affect LaAlO$_{3}$ polarity compensation and in turn the electronic properties. [Preview Abstract] |
Tuesday, March 19, 2013 1:51PM - 2:03PM |
G12.00012: Stoichiometry, defects, and the polar catastrophe in LaAlO$_3$ thin films on SrTiO$_3$ C. Stephen Hellberg Careful growth of LaAlO$_3$ thin films on SrTiO$_3$ by molecular beam epitaxy has shown that the La/Al ratio of the nominal LaAlO$_3$ layer is key to the formation of a two-dimensional electron liquid at the interface---metallic conductivity is only observed in Al-rich films. The interfacial electron liquid forms due to the polar catastrophe, the diverging potential caused by the atomic layer arrangement at the interface when polar LaAlO$_3$ is grown on TiO$_2$-terminated non-polar SrTiO$_3$. The system eventually reconstructs, moving negative charges to the interface to screen the diverging potential. I will present density functional calculations of the defects that form in LaAlO$_3$ on SrTiO$_3$ to accomodate variations in stoichiometry. In La-rich films, the lowest energy defects are extended and allow cation vacancies to move to the interface to screen the diverging potential. Thus the interface between La-rich LaAlO$_3$ and SrTiO$_3$ remains insulating. In Al-rich films, the defects are localized and block cation motion. In this case a conducting electron liquid forms to screen the diverging potential. [Preview Abstract] |
Tuesday, March 19, 2013 2:03PM - 2:15PM |
G12.00013: What can we learn from AC impedance study about the bipolar resistive switching effect in LaAlO$_3$/Nb:SrTiO$_3$ heterostructures Xingli Jiang, Yonggang Zhao, Xin Zhang, Meihong Zhu, Huiyun Zhang, Dashan Shang, Jirong Sun Recently, resistive switching (RS) effect has attracted much attention due to its importance in potential applications in resistance random access memory. It has been shown that traps play an important role in RS effect. However, a direct and in-depth study on the characteristics of traps is still lacking so far, including the spatial and energy distribution of traps, relaxation of trapped carriers and transport of carriers via traps, especially the effect of historical process on the transport of carriers, which are important for understanding the mechanism of RS effect and also essential for optimizing devices. We studied the RS effect in heterostructures composed of LaAlO$_3$ (LAO) and Nb:SrTiO$_3$ (NSTO) from 80 to 300 K by using AC impedance technique. It was demonstrated that the bipolar RS effect originates from the LAO/NSTO interface and the resistance states are controlled by the filling status of traps via the trapping/detrapping of electrons. Moreover, the spatial and energy distributions of traps and the effect of history on the transport of carriers were obtained. A model was proposed to explain the experimental results. This work demonstrates that AC impedance technique is powerful for uncovering the mechanism of RS effect. [Preview Abstract] |
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