Bulletin of the American Physical Society
APS March Meeting 2021
Volume 66, Number 1
Monday–Friday, March 15–19, 2021; Virtual; Time Zone: Central Daylight Time, USA
Session P55: Oxide Two Dimensional GasesLive
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Sponsoring Units: DMP Chair: Mark Rzchowski, University of Wisconsin - Madison |
Wednesday, March 17, 2021 3:00PM - 3:12PM Live |
P55.00001: Quantum transport in periodically modulated electron waveguides Elliott Mansfield, François Damanet, Andrew Daley, Megan K Briggeman, Patrick R Irvin, Jeremy Levy, Hyungwoo Lee, Jungwoo Lee, Kitae Eom, Chang-Beom Eom, Jianan Li, Mengchen Huang Recent experiments on nanowires written on LAO/STO interfaces have shown their conductive properties are highly controllable and display a range of interesting physics. For example, it has been observed that these display quantized conductance. New experiments apply a periodic potential in the nanowire, displaying a fracturing of the lowest transconductance peak and a 2e^2/h first peak that is not split by magnetic fields up to 16T. We present theoretical descriptions of how these features occur. We first study how the single-particle features change under a periodic potential modulation, and show that this can fracture the transconductance peak. Additionally, we consider that the modulations engineer a spin-orbit coupling, and we study its effect on the single particle regime. Finally, to describe the conductance plateau of 2e^2/h, we introduce electron-electron interactions, and show that the engineered spin-orbit coupling enhances the interactions and leads to a pairing of the lowest energy spin-subbands. |
Wednesday, March 17, 2021 3:12PM - 3:24PM Live |
P55.00002: Conductance of LaAlO3/SrTiO3 Nanostructures at Temperatures Between the Freezing Point and Boiling Point of Water Aaron Greenberg, Dengyu Yang, Philip H Shenk, Joseph A Albro, Kitae Eom, Chang-Beom Eom, Patrick R Irvin, Jeremy Levy The discovery of a tunable 2DEG at the interface of LaAlO3/SrTiO3 (LAO/STO) has spurred interest in LAO/STO nanoelectronics. The technique of conductive atomic force microscopy (c-AFM) lithography is capable of creating conductive nanostructures by controlling the charge state of the LAO surface [1]. It is also known that a “water cycle” mediates conductivity at the oxide interface [2] and causes premature self-erasure of nanostructures. Here we describe experiments in which we measure the properties of conductive nanostructures in the temperature range 273 K < T < 400 K and as a function of ambient pressure. Understanding the properties of LAO/STO nanostructures in these conditions is important for applications as well as for subsequent measurements at low temperatures. |
Wednesday, March 17, 2021 3:24PM - 3:36PM Live |
P55.00003: Persistent Photocarrier Accumulation and Depletion in LaAlO3/SrTiO3 Quantum Wells Yu Chen, Yoann Lechaux, Blai Casals, Bruno Guillet, Albert Minj, Jaume Gazquez, Laurence Mechin, Gervasi Herranz Many semiconductors exhibit persistent photoconductance, whereby light induces long-term changes of conductance. Generally, this phenomenon entails carrier accumulation, while occasionally it can lead to depletion. Yet, a simultaneous control of accumulation and depletion by optical means has remained elusive. Here, we present the quantum well at the LaAlO3/SrTiO3 interface, where in addition to accumulation, a secondary photoexcitation enables carrier depletion. The balance between both processes is wavelength dependent, and allows tunable accumulation or depletion in an asymmetric manner, depending on the relative arrival time of photons of different frequencies. The role played by time in this unconventional photoresponse paves the way to an optical implementation of spike-timing dependent plasticity. Interesting, it may also open the way to an optical control of amplitude and sign of the spin-charge conversion efficiency in LaAlO3/SrTiO3 quantum wells. |
Wednesday, March 17, 2021 3:36PM - 3:48PM Live |
P55.00004: Concomitant appearance of conductivity and superconductivity in (111) LaAlO3/SrTiO3 interface with metal capping Ravindra Bisht, Michael Mograbi, Prasanna Rout, Gal Tuvia, Hyeok Yoon, Adrian G Swartz, Harold Hwang, Yoram Dagan In (111) SrTiO3/LaAlO3 interface, a critical thickness of nine monolayers (ML) of epitaxial LaAlO3 is required for conductivity to appear. We show that this critical thickness always decreases when depositing a metallic over-layer. The critical thickness is only 3ML for cobalt overlayer and 6ML for platinum. The latter result contrasts with the (100) interface where a platinum overlayer increases the critical thickness beyond that of the bare interface [1]. These results suggest that the work function of the metallic overlayer plays an important role in both interfaces. Interestingly, for (111) SrTiO3/LaAlO3/Metal interfaces conductivity appears concomitantly with superconductivity in contrast with the (100) SrTiO3/LaAlO3/Metal interface with LaAlO3 layer smaller than four unit-cells, which are conducting but not superconducting. We suggest that this difference is related to the different sub-bands involved in conductivity for the (111) interfaces comparing to the (100) interfaces [2]. Our findings can be useful for superconducting tunnelling devices. |
Wednesday, March 17, 2021 3:48PM - 4:00PM Live |
P55.00005: Charge-transfer engineering of oxygen vacancies for tailored ionic conductivity at oxide interfaces Felix Gunkel, Dennis Christensen, Yunzhong Chen, Nini Pryds Exploiting the electronic charge-transfer across oxide interfaces has emerged as a versatile tool to tailor the electronic and magnetic properties of oxides. While the richness in the electronic and magnetic properties of these systems is a main focus of research, the implications for the ionic transport at oxide interfaces have not received much attention so far. In this contribution, we propose that charge-transfer strategies can also be applied to boost ionic charge carrier concentrations at interfaces by orders of magnitude. Based on numerical space-charge modeling, we will illustrate how the ‘p-type’ charge-transfer predicted between SrO-terminated SrTiO3 and LaAlO3 may foster 2-dimensional oxygen ion conduction at the interface. The ion conduction is effectively separated from impurity dopants, which may allow large concentrations of oxygen vacancies to be achieved in the absence of trapping phenomena. The interface promises high ionic conductivity with nanoscale confinement, potentially allowing the |
Wednesday, March 17, 2021 4:00PM - 4:12PM Live |
P55.00006: Metal/SrTiO3 two-dimensional electron gases for spin-to-charge conversion Srijani Mallik, Luis Moreno, Maxen Cosset-Cheneau, Paul Noel, Diogo Vaz, Felix Trier, Anke Sander, Agnès Barthélémy, Jean-Phillippe Attane, Laurent Vila, Manuel Bibes An emerging direction in oxide research aims at probing interfaces with two-dimensional electron gases (2DEGs) possessing a finite Rashba spin-orbit coupling. This allows the interconversion of spin and charge through the direct and inverse Edelstein and spin Hall effects which play a key role for future spintronics devices. A very simple path to achieve SrTiO3-based 2DEGs is by deposition of reactive metals on SrTiO3. Here we compare the formation and properties of 2DEGs generated in SrTiO3 by the growth of Al, Ta, and Y ultrathin films by magnetron sputtering. X-ray photoelectron spectroscopy study revealed the quantification of reduced Ti states associated with 2DEG formation. The carrier densities are extracted from magnetotransport measurements. Finally, spin-charge conversion has been measured and compared between different metal/SrTiO3 systems by spin-pumping experiments where the samples were capped with an extra NiFe layer. The trends observed for different metal/SrTiO3 systems provide a systematic study as a function of the carrier density and the transparency of the metal oxide tunnel barrier. |
Wednesday, March 17, 2021 4:12PM - 4:24PM Live |
P55.00007: Electrical gate control of carrier type of the two-dimensional carrier gas at the FeOy/SrTiO3 interface and its field-effect-transistor operation Theodorus Jonathan Wijaya, Le Duc Anh, Shingo Kaneta-Takada, Masaaki Tanaka, Shinobu Ohya The recently found two-dimensional (2D) hole gas (2DHG), which has ultrahigh mobility up to 24000 cm2/Vs at 4 K, and 2D electron gas (2DEG) formed at the FeOy/SrTiO3 (STO) interface are expected to provide a new platform for oxide-based electronics [1]. Here, using a back-gate configuration, we demonstrate for the first time a gate control of the carrier type and mobility of the 2D carrier gas at this interface. The samples were formed by depositing Al (1 nm)/Fe (0.075–0.4 nm) on STO (001) substrates using molecular beam epitaxy. For all the samples, the carrier type was transformed from n-type to p-type with increasing VG above a threshold value, which varies among samples. This suggests a complicated band structure of this 2D system. Furthermore, at low temperatures (~3.5 K), these devices exhibit excellent transistor features with subthreshold swing values of ~30 mV/dec and on-off ratios of ~107. These results provide insights into the formation mechanism of the 2DHG at the FeOy/SrTiO3 interface, as well as highlight the possibility of high-performance field-effect transistors based on the 2D carrier gas on STO substrates. |
Wednesday, March 17, 2021 4:24PM - 4:36PM Live |
P55.00008: Switchable transport properties of a two-dimensional electron gas based on ferroelectric Ca:SrTiO3 Julien Bréhin, Felix Trier, Luis Moreno, Pierre Hemme, Paul Noël, Maxen Cosset-Cheneau, Jean-Phillippe Attane, Laurent Vila, Anke Sander, Vincent Garcia, Stéphane Fusil, Agnès Barthélémy, Maximilien Cazayous, Manuel Bibes Two-dimensional electron gases (2DEGs) obtained at surfaces and interfaces of oxides such as SrTiO3 [1,2] form versatile platforms for nanoelectronics. Their gate-tunable carrier density, sheet resistance [3] and Rashba spin-orbit coupling [4] can be exploited for spin-orbitronics, as they promote highly efficient and electrically tunable spin-charge interconversion [5]. However, in most cases, the tunability of all these properties with an electric field remains volatile. In this talk we show the non-volatile modulation of the transport properties of a 2DEG, using the ferroelectricity of the Ca:SrTiO3 host material [6,7]. Finally, we suggest that this behavior may be extended to other oxide 2DEGs, leading to novel types of electronic and spintronic architectures. |
Wednesday, March 17, 2021 4:36PM - 4:48PM Live |
P55.00009: Study of 2DEGs at the interface of Hybrid MBE Grown SrNbO3 on BaSnO3 Suresh Thapa, Patrick Gamperline, Sydney Provence, Hanjong Paik, Darrell Schlom, Ryan B Comes Over a decade, two-dimensional electron gases (2DEGs) in complex oxide interfaces have emerged as an option for high speed electronics. Modulation doping is an popular technique to produce high carrier-concentration 2DEGs by separating the carriers from the donors at an interface to improve mobility. The search for good donors has led to the complex oxide perovskite, SrNbO3 (SNO), which has one free electron to donate. Moreover, BaSnO3 (BSO) is well known for its high mobility due to an availability of unoccupied 5s bands in Sn. Hybrid molecular beam epitaxy (hMBE) has been used to grow SNO film on as prepared BSO film on different substrates with Nb supplied using TDTBN precursor and Sr supplied as an elemental source. In situ RHEED is used to compare the crystal quality and stoichiometry of the sample right after growth and after cooling down. In vacuo XPS is used to quantify the oxidation state of Nb and band alignment at the interface. These studies help to understand the high-quality SNO film growth using hMBE approach and quantify the carrier concentration and mobility in SNO/BSO interface. In the future, this study could open the door for long awaited search of promising alternatives for traditional semiconductor in high mobility devices. |
Wednesday, March 17, 2021 4:48PM - 5:00PM Live |
P55.00010: Creating nano-scale “vias” in LaAlO3/SrTiO3 for integration of nanostructures Carl Wilson, Kitae Eom, Dengyu Yang, Muqing Yu, Aditi Nethwewala, Jung-Woo Yoo, Chang-Beom Eom, Patrick Ivrin, Jeremy Levy The 2D electron gas (2DEG) at the LaAlO3/SrTiO3 interface can be patterned with nanoscale dimensions using c-AFM lithography [1] and integrated with other 2D materials such as graphene [2]. While the surface of LaAlO3/SrTiO3 is only 1.2 nm from the interface, improved coupling of particles on the surface with the interface requires closer proximity. Here, we discuss methods to create nanoscale holes or “nano vias” in LaAlO3/SrTiO3 with the goal of integrating nanomaterials directly to the interface while also maintaining interfacial conductivity. These nano vias will allow us to study the effect of local coupling between the 2DEG at LaAlO3/SrTiO3 interface and a wide variety of interesting nanomaterials such as colloidal quantum dots or graphene nanoribbons. |
Wednesday, March 17, 2021 5:00PM - 5:12PM Live |
P55.00011: Origin of the 2D electron gas at the SrTiO3 surface Xi Yan, Friederike Wrobel, Hawoong Hong, Jirong Sun, Jessica L McChesney, Hua Zhou, Dillon D Fong Since the discovery of the 2D electron gas (2DEG) at the SrTiO3 (001) surface in 2011 via angle-resolved photoemission spectroscopy (ARPES), there have been several theories proposing its origin. In separate studies, scientists have demonstrated that many of reconstructions at the SrTiO3 surface are TiO2-rich. Others using X-ray photoelectron spectroscopy (XPS) have demonstrated the presence of oxygen vacancies at the surface, correlating them with a surface electronic state (in-gap state, IGS). Here we employed in situ synchrotron X-ray methods during growth by molecular beam epitaxy (MBE), ARPES, and XPS to study the origin of the 2DEG and its evolution. We discover that the 2DEG only appears when the surface has TiO2 double layer structure [1], for the first time linking the atomic surface structure to its 2DEG electronic states. Interestingly, by deposition of single layer of SrO or the subsequent growth of a TiO2 monolayer, we are able to completely control the disappearance/reappearance of both the 2DEG and the IGS, potentially providing a means of manipulating 2DEG-behavior in oxide electronics. |
Wednesday, March 17, 2021 5:12PM - 5:24PM Live |
P55.00012: Spin-Orbit Coupled 2-Dimensional Electron Gases in SrTaO3 Heterostructures Patrick Gemperline, Suresh Thapa, Sydney Provence, Ryan B Comes 2-Dimensional electron gases (2DEGs) with high mobility at room temperature and low defect concentrations are highly sought after for future advancements in high speed electronics. According to theoretical modeling, in a SrTaO3/SrZrO3/SrTiO3 heterostructure, SrTaO3 (STaO) will substantially electron dope SrTiO3 (STO) through modulation doping, making it a prime candidate for a complex oxide 2DEG. STaO is also predicted to be metallic and possess strong spin orbit coupling, which should allow for a high mobility spin-orbit coupled 2DEG. Epitaxial films of STaO were grown on STO using hybrid molecular beam epitaxy, where Ta was introduced through a metal organic precursor and growth was monitored via reflected high energy electron diffraction. Stoichiometry and surface termination were determined by in situ x-ray photoelectron spectroscopy and Rutherford back scattering. Temperature dependent transport measurements and atomic force microscopy were used to determine electronic properties and surface topology, respectively. |
Wednesday, March 17, 2021 5:24PM - 5:36PM Live |
P55.00013: Spatially Tunable High-density electron gas at the KTaO3/TbScO3 (001) interface Bhubnesh Lama, Kitae Eom, Jungwoo Lee, Evgeny Y Tsymbal, Chang-Beom Eom, Tula Paudel The interface between 3d and 5d polar oxides has potentials for thickness independent high-density electron gas that is highly tunable in terms of spatial localization, orbital distribution, and density, much of which are not available widely studied interface between polar-nonpolar oxide interfaces. As a model system for observing such behavior, we studied the (001) interface between polar KTaO3(KTO) and polar TbScO3 (TSO) and found a large density (> 1e/uc) two-dimensional electron gas (2DEG) spread across interface in both materials. While the electron gas density at the TSO side of the interface is small (<3% of total) and localized within the unit cell (less than half a nm), the concentration of electron gas in the KTO side of the interface is much larger (remaining ~97%), and the spread is much longer (at least 3 nm). An applied external voltage can tune the spatial location of electron gas across the interface. The metallic nature of KTO across the interface and eventual charge compensation may need a capping layer for experimental detection of 2DEG when the KTO thickness is less than two unit cells, similar to what is required for detecting hole gas at the surface of LAO/STO (001) heterostructure. |
Wednesday, March 17, 2021 5:36PM - 5:48PM On Demand |
P55.00014: Exploring two-dimensional electron (or hole) gas at BaSnO3/LaInO3 interfaces: A first-principles study Wahib Aggoune, Claudia Draxl We explore the formation of a two-dimensional electron gas (2DEG) at interfaces formed by two wide-gap perovskites of emerging interest, i.e. BaSnO3 (BSO) and LaInO3 (LIO), the latter being a polar material. For periodic structures, we find that the polar discontinuity at the interface is mainly compensated by an electronic relaxation through a charge transfer from the LIO to the BSO side. This leads to the formation of a 2DEG of Sn-s character, confined within three BSO unit cells (u.c.), and a 2D hole gas, of O-p character confined within one LIO u.c. We also find that structural distortions through octahedra tilts induce a depolarization field to counteract the polar discontinuity. This causes a progressive charge transfer from the LIO to the BSO side beyond a critical thickness of 4 LIO u.c. The polar discontinuity increases with the LIO thickness, leading to a 2DEG density of 0.5 e/u.c-surface. In a non-periodic BSO/LIO interface, the distortions at the BSO side (substrate) are less pronounced. This enhances the polar discontinuity and allows for reaching higher 2DEG densities than in the heterostructure, for a similar LIO thickness and allows an extension of the 2DEG up to 5 u.c. in the BSO substrate compared to 3 u.c. in the heterostructure. |
Wednesday, March 17, 2021 5:48PM - 6:00PM On Demand |
P55.00015: Quasi-two-dimensional electron and hole gases at the head-to-head and tail-to-tail 180o domain walls in ferroelectric thin films JAMES SIFUNA, Pablo Garcia-Fernandez, George S Manyali, George Amolo, Javier Junquera Many schemes on screening of bound charges and stabilization of head-to-head (HH) and tail-to-tail (TT) 180o domain walls (DWs) in ferroelectrics have been proposed. Some studies have employed (i) substitutional atoms at one or two HH and TT domain walls in superlattices, (ii) altering defect concentration at the tails in thin films. However, such approaches say nothing regarding the intrinsic critical thickness for the stabilization of the 2D conducting layers for screening of polarization charges, the spatial extend in which the hole and electron gases cover nor the magnitude extend of polarization if the said domain walls would be induced in a slab. |
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