Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session W49: Focus Session: Titanate Interfaces, Layered Materials |
Hide Abstracts |
Sponsoring Units: DMP Chair: Emilio Artacho, CIC nanoGUNE, Spain Room: Mile High Ballroom 1C |
Thursday, March 6, 2014 2:30PM - 2:42PM |
W49.00001: Induced ferromagnetism and antiferromagnetism in perovskite quantum wells Clayton Jackson, Jack Zhang, Susanne Stemmer We report on induced magnetism in thin SrTiO$_{3}$ quantum wells embedded in ferrimagnetic GdTiO$_{3}$ and antiferromagnetic SmTiO$_{3}$, respectively. The SrTiO$_{3}$ quantum wells contain a high density of mobile electrons (7x10$^{14}$ cm$^{-2})$. We show that the longitudinal and transverse magnetoresistance in the structures with GdTiO$_{3}$ are consistent with anisotropic magnetoresistance, and thus indicative of induced ferromagnetism in the SrTiO$_{3}$. Measurements of the sheet and Hall resistances as a function of temperature in the structures with SmTiO$_{3}$ are consistent with two-dimensional itinerant antiferromagnetism induced in the SrTiO$_{3}$ layer as a result of the confinement of an extreme charge density coupled with proximity affects from the neighboring SmTiO$_{3}$. The studies show that the properties of thin SrTiO$_{3}$ quantum wells can be tuned to obtain magnetic states that do not exist in the bulk material. [Preview Abstract] |
Thursday, March 6, 2014 2:42PM - 2:54PM |
W49.00002: Itinerant electron ferromagnetism at GdTiO$_{3}$/SrTiO$_{3}$ heterostructure Ming Xie, Allan MacDonald Interfaces between perovskite oxides have known to support conducting two dimensional electron gases (2DEGs) even when the parent materials are insulators. When one of the insulating parent materials is magnetic, the magnetism is inherited by the 2DEG. We will discuss the itinerant-electron magnetic 2DEGs which occur at GdTiO$_{3}$/SrTiO$_{3}$ interfaces in which the 2DEG resides on the otherwise non-magnetic SrTiO$_{3}$ side of the interface. Experimental studies [1,2] have shown magnetoresistance effects that are characteristic of itinerant electron ferromagnets, but the origin of the magnetic coupling is still not resolved. In this study, we propose that the ferromagnetic state of the 2DEG originates from exchange coupling between 2DEG electrons and electrons localized on GdTiO$_{3}$ Ti sites near the interface. We develop a tight binding model that accounts for this coupling and its relation to structural distortion of GdTiO$_{3}$ near the interface, and address its influence on 2DEG properties. \\[4pt] [1] P. Moetakef, J. R. Williams, D. G. Ouellette, A. P. Kajdos, D. Goldhaber-Gordon, S. J. Allen, and S. Stemmer, Phys. Rev. X 2, 021014 (2012). \\[0pt] [2] C. A. Jackson and S. Stemmer, arXiv:1311.0337 [Preview Abstract] |
Thursday, March 6, 2014 2:54PM - 3:06PM |
W49.00003: Evolution of the electronic structure of SrTiO$_3$/GdTiO$_3$ heterostructures with layer thickness Lars Bjaalie, Anderson Janotti, Chris G. Van de Walle A two-dimensional electron gas (2DEG), with density of 3e14cm$^{-2}$ (0.5 electrons per interface unit cell), has been observed at the SrTiO$_3$/GdTiO$_3$ interface, with potential applications in electronic devices [P. Moetakef, T.A. Cain, D.G. Ouellette, J.Y. Zhang, D.O. Klenov, A. Janotti, C.G. Van de Walle, S. Rajan, S.J. Allen, and S. Stemmer, Appl. Phys. Lett. 99, 232116 (2011)]. Yet, basic properties of the 2DEG is still poorly understood, in particular the variation of the electrical conductivity with the SrTiO$_3$ layer thickness. We performed density functional calculations with a hybrid functional to study the electronic structure of SrTiO$_3$/GdTiO$_3$ superlattices. We address the insulator to metal transition as a function of layer thickness, analyzing the effects of quantum confinement, charge ordering, and lattice distortions. Work supported by NSF and ARO. [Preview Abstract] |
Thursday, March 6, 2014 3:06PM - 3:18PM |
W49.00004: Metal-insulator transitions in GdTiO$_3$/SrTiO$_3$ superlattices Se Young Park, Andrew Millis The density functional plus U (DFT+U) method is used to obtain electronic structures and metal-insulator phase diagrams of metal-insulator transition of (001) (GdTiO$_3$)$_m$/(SrTiO$_3$)$_n$ superlattices. In metallic phases, the mobile electrons are found in the SrTiO$_3$ layers, with near-interface electrons occupying $xy$-derived bands, while away from the interface the majority of electrons reside in $xz/yz$ bands. As the thickness $n$ of the SrTiO$_3$ layers decreases a metal-insulator transition occurs. Two insulating phases are found. At $n=1$ the hybridization of two TiO$_2$ layers across the SrO layer leads to a dimerized insulating state as previously proposed\footnote{Ru Chen, SungBin Lee, and Leon Balents, Phys. Rev. B {\bf 87}, 161119(R) (2013).} with relatively small U$_c \sim 2.5$ eV. For $n>1$ we find that insulating phases occur together with checkerboard charge and orbital ordering and variation of Ti-O bonds at a larger U$_c\sim3.5$eV. Inconsistencies with experiment suggest that many-body correlations are important. [Preview Abstract] |
Thursday, March 6, 2014 3:18PM - 3:30PM |
W49.00005: Polarized Infrared Response of Subband Transitions in High Density 2DEG in GdTiO$_{3}$/SrTiO$_{3}$ Interfaces Bill Flaherty, Daniel Ouellette, Pouya Moetakef, Clayton Jackson, Susanne Stemmer, S. James Allen The 2-D electron gas at the interface between GdTiO$_{3}$ and SrTiO$_{3}$ layers has an electron density comparable to 3.4 x 10$^{14}$ cm$^{-2}$ per interface with potential applications for tunable plasmonic devices. Experiments are currently underway to measure the infrared response of this electron gas, with infrared electric fields perpendicular to the interface as well as parallel. The former may provide insight into the electric subband states. Using angle-resolved Fourier transform infrared spectroscopy with s- and p-polarized beams, we can compare the in- and out-of-plane response of the 2DEG. Normalizing it against the response of the bare substrate will allow us to extract the 2DEG contribution. These results will be compared to those predicted by Park and Millis, Phys. Rev. B87, 205145 (2013). Results to date display in-plane but little out-of-plane response. We will look at various GTO/STO interfaces, such as single interfaces and superlattices of alternating layers. [Preview Abstract] |
Thursday, March 6, 2014 3:30PM - 3:42PM |
W49.00006: Electronic and structural reconstruction in titanate heterostructures from first principles Andrew T. Mulder, Craig J. Fennie Recent advances in transition metal oxide heterostructures have opened new routes to create materials with novel functionalities and properties. One direction has been to combine a Mott insulating perovskite with an electronic d$^1$ configuration, such as LaTiO$_3$, with a band insulating d$^0$ perovskite, such as SrTiO$_3$. An exciting recent development is the demonstration of interfacial conductivity in GdTiO$_3$/SrTiO$_3$ heterostructures that display a complex structural motif of octahedral rotations and ferromagnetic properties similar to bulk GdTiO$_3$. In this talk we present our first principles investigation of the interplay of structural, electronic, magnetic, and orbital degrees of freedom for a wide range of d$^1$/d$^0$ titanate heterostructures. We find evidence for both rotation driven ferroelectricity and a symmetry breaking electronic reconstruction with a concomitant structural distortion at the interface. We argue that these materials represent an ideal platform to realize novel functionalities such as the electric field control of electronic and magnetic properties. [Preview Abstract] |
Thursday, March 6, 2014 3:42PM - 3:54PM |
W49.00007: Stoichiometry-Control of Electronic Transport at Complex Oxide Interface Peng Xu, Bharat Jalan Employing the hybrid molecular beam epitaxy approach to grow NdTiO$_{\mathrm{3}}$/SrTiO$_{\mathrm{3}}$ heterostructures - a polar/nonpolar system sharing many similarities with LaAlO$_{\mathrm{3}}$/SrTiO$_{\mathrm{3}}$ with an added functionality of NdTiO$_{\mathrm{3}}$ being an antiferromagnetic Mott insulator- we will present a detailed film growth and transport study as a function of cation stoichiometry in NdTiO$_{\mathrm{3}}$. Irrespective of the cation stoichiometry (measured by high resolution x-ray diffraction and x-ray photoelectron spectroscopy), films grew in an atomic layer-by-layer fashion as evidenced by the reflection high-energy electron diffraction intensity oscillations, and films showed a temperature dependent metal-to-insulator (M-I) type behavior. Remarkably, T$_{\mathrm{MI}}$ was found to increase irrespective of whether films were Nd- or Ti-rich. Furthermore, hall measurement of a 3.5 nm NdTiO$_{\mathrm{3}}$ film grown on 3 nm SrTiO$_{\mathrm{3}}$ layer on LSAT substrate revealed n type carrier density, 3 x 10$^{\mathrm{14}}$ cm$^{\mathrm{-2}}$ for stoichiometric samples, which would be consistent with the interface conduction due to an interfacial polar discontinuity effect. Using detailed temperature dependent magneto-transport measurements, we will present a comprehensive study of correlation between film stoichiometry, interface conduction, and transport mechanisms. [Preview Abstract] |
Thursday, March 6, 2014 3:54PM - 4:06PM |
W49.00008: Growth and Transport Studies of LaTiO$_{3}$ / KTaO$_{3}$ Heterostructures K. Zou, F.J. Walker, C.H. Ahn Perovskite oxide heterostructures provide a rich platform for exploring emergent electronic properties, such as 2D electron gases (2DEGs) at interfaces. In this talk, we present results on the growth of LaTiO$_{3}$ / KTaO$_{3}$ heterostructures by molecular beam epitaxy and subsequent measurements of transport properties. Although both oxide materials are insulating in the bulk, metallic conduction is observed from T = 2 - 300 K. We achieve a room temperature carrier mobility of $\sim$ 25 cm$^{2}$ /Vs at a carrier density of $\sim$ 10$^{14}$ /cm$^{2}$. By comparison, 2DEGs in LaTiO$_{3}$ / SrTiO$_{3}$ and LaAlO$_{3}$ / SrTiO$_{3}$ have lower carrier mobility, but the same carrier density. We attribute some of the increase in mobility to the smaller band effective mass of the Ta 4d electrons compared to the Ti 3d electrons. [Preview Abstract] |
Thursday, March 6, 2014 4:06PM - 4:18PM |
W49.00009: Two-dimensional electron gas in tricolor oxide interfaces Yanwei Cao, Michael Kareev, Xiaoran Liu, Srimanta Middey, Derek Meyers, Jak Tchakhalian Understanding and manipulating spin of electrons in nanometer scale is the main challenge of current spintronics, recent emergent two-dimensional electron gas in oxide interface provides a good platform to investigate the spin behavior by covering an insulating magnetic oxide layer. In this work, take titanates as an example, ultra-thin tricolor (tri-compound) titanate superlattices ([LaTiO3/SrTiO3/YTiO3]) were grown in a layer-by-layer way by pulsed laser deposition. High sample quality and their electronic structures were characterized by the combination of in-situ photoelectron and ex-situ structure and surface morphology probes. Temperature-dependent sheet resistance indicates the presence of metallic interfaces in both [LaTiO3 /SrTiO3 ] and all the tricolor structures, whereas a [YTiO3 /SrTiO3] bi-layer shows insulating behavior. The tricolor titanate superlattices provide an opportunity to induce tunable spin-polarization into the two-dimensional electron gas (2DEG) with Mott carriers. [Preview Abstract] |
Thursday, March 6, 2014 4:18PM - 4:30PM |
W49.00010: Controllable strain fields in multimonolayer 2D-layered TiO2 (110) crystals studied by STM Zhisheng Li, Denis Potapenko, Richard Osgood Strain of crystal lattice can change the electronic property of materials, such as oxides and semiconductors, significantly. However, experimental studies of lattice effects in oxides are limited especially in atomic scale, due to the difficulty of generating strain field experimentally. In this work, we generate a strain field in multiple monolayer sample of at TiO2 (110) by very low energy bombardment of single crystal TiO2 samples with argon ions at 1000$^{\mathrm{o}}$C. The interstitial argon diffuses so as to form nanometer scale regions of local exfoliated TiO2 layers. These layers retain their unstressed surface reconstruction although the top-most surface layers have a convex morphology. We use STM studies along with a continuum model to show the strain field. Our studies also show that the strained surface layers are free of oxygen vacancies and that the adsorption energy of hydrogen is altered by the local strain field. [Preview Abstract] |
Thursday, March 6, 2014 4:30PM - 4:42PM |
W49.00011: The effect of hetero-structure material MoS$_{2}$-TiO$_{2}$(110) on CO and NO adsorption: insights from \textit{ab-initio} calculations Takat Rawal, Duy Le, Talat Rahman Using first-principles simulation based on the density functional theory, we show the effect of substrate on the adsorption of small gas molecules (CO and NO) on molybdenum disulfide (MoS$_{2})$ by investigating the adsorption on bare MoS$_{2}$ and on MoS$_{2}$-TiO$_{2}$(110) systems. First, our results show that MoS$_{2}$ binds to the rutile TiO$_{2}$ surface by forming bonds between unsaturated edge sulfur atoms of MoS$_{2}$ with both bridge oxygen atoms and five-fold titanium atoms of TiO$_{2}$. Second, results from structural optimizations show that CO prefers to adsorb on ($\bar{1}$010) edge (S-edge) of bare MoS$_{2}$ but on (10$\bar{1}$0) edge (Mo-edge) of MoS$_{2}$ when coupled to the TiO$_{2}$(110) surface. Third, results from Bader analysis indicate a very small difference in charge transfer (i.e. 0.01e) to CO molecule from these two systems. We also present detailed analysis of the electronic density of states and the charge density of adsorbate-substrate systems to explain the effect of substrates on the adsorption of CO. We compare and contrast the adsorption characteristics of NO with those of CO on these surfaces. [Preview Abstract] |
Thursday, March 6, 2014 4:42PM - 4:54PM |
W49.00012: First Principles Study of Monolayer MoS$_{2}$ with Defects and Vacancy Yingye Gan, Huijuan Zhao Unlike graphene and silicene, monolayer molybdenum disulfide (MoS$_{2}$) is a direct band gap transition metal with interesting electrical, mechanical, and optical properties. As a potential material in NEMS application, it is necessary to systematically study the defect effects to the material properties of MoS$_{2}$ under strain tuning. We will perform ab initio density functional theory based calculations to study the mechanical and electronic property variation of MoS$_{2}$ with different vacancy types and vacancy densities, such as Young's modulus, Poisson's ratio, fracture strength and band gap. The failure mechanism under various strain conditions will be investigated through the phonon dispersion curves. We expect to elucidate the relation between material properties of MoS$_{2}$ with strain tuning and defect tuning. [Preview Abstract] |
Thursday, March 6, 2014 4:54PM - 5:06PM |
W49.00013: Direct imaging of band profile in single layer MoS2 on graphite: metallic edge states and the lateral Schottky barrier Chendong Zhang, Chang-Lung Hsu, Yong-Huang Chang, Lain-Jong Li, Chih-Kang Shih Recently, single layer (SL) Transition metal dichalcogenides MX$_{2}$ has attracted intense interests as the band structures change from indirect to direct gap. In addition, the valley degeneracy is also lifted in SL MX$_{2}$. These properties have important implications in nanoelectronics and optoelectronics. The SL MX$_{2}$ islands often come with a triangular form with straight edges and it has been shown theoretically these are zig-zag edge with metallic states. Here we use scanning tunneling microscopy/spectroscopy (STM/S) to map out the electronic structure of single layer MoS$_{2}$ grown on HOPG (highly oriented pyrolytic graphite) using CVD. In the region away from the edge, the MoS$_{2}$ band profile shows a homogeneous band gap of about 1.95 $\pm$ 0.1 eV, consistent with the optical studies before. Moreover, the Fermi level locates at 0.15 $\pm$ 0.05 eV below the conduction band minimum (CBM), confirming its n-type nature. The band profile is bend upward by about 0.5 eV within 5 nm from the edge. At the edge, the metallic nature is observed from finite conductivity in the gap region. This study shows that the bulk SL MoS$_{2}$ and its metallic edge formed a lateral Schottky barrier with a narrow depletion region of 5 nm and the Fermi level is pinned at 0.65 eV below the CBM. [Preview Abstract] |
Thursday, March 6, 2014 5:06PM - 5:18PM |
W49.00014: Imaginary geometric phases of quantum trajectories in high-order terahertz sideband generation Fan Yang, Ren-Bao Liu Quantum evolution of particles under strong fields can be described by a small number of quantum trajectories that satisfy the stationary phase condition in the Dirac-Feynmann path integral. The quantum trajectories are the key concept to understand the high-order terahertz siedeband generation (HSG) in semiconductors [1]. Due to the nontrivial ``vacuum'' states of band materials, the quantum trajectories of optically excited electron-hole pairs in semiconductors can accumulate geometric phases under the driving of an elliptically polarized THz field [2]. We find that the geometric phase of the stationary trajectory is generally complex with both real and imaginary parts. In monolayer MoS2, the imaginary parts of the geometric phase leads to a changing of the polarization ellipticity of the sideband. We further show that the imaginary part originates from the quantum interference of many trajectories with different phases. Thus the observation of the polarization ellipticity of the sideband shall be a good indication of the quantum nature of the stationary trajectory.\\[4pt] [1] B. Zaks, R. B. Liu, and M. S. Sherwin, Nature 483, 580 (2012).\\[0pt] [2] F. Yang and R.-B. Liu, arXiv:1211.3021. [Preview Abstract] |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700