Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session X09: Topological Materials -- Thin FilmsFocus
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Sponsoring Units: DCMP Chair: Hang Chi, Massachusetts Institute of Technology Room: BCEC 151A |
Friday, March 8, 2019 8:00AM - 8:12AM |
X09.00001: Quantum transport in ultra-thin topological crystalline insulator films Stephen D Albright, Frederick J Walker, Charles H Ahn Recent developments have demonstrated the unique properties of conventional topological insulators, such as from dissipation-free transport and the creation of Majorana fermions by interfacing with superconductors, and offered a method for controlling them spatially with magnetic fields. Topological crystalline insulators (TCIs), such as SnTe, offer the same unique properties but an alternate mode of control: crystal symmetry breaking through strain or electric fields. Growth of TCIs thin and uniform enough to access these unique properties is challenging to achieve. This work presents structural and electronic characterization of ultra-thin SnTe films, down to 5nm, grown by molecular beam epitaxy on SrTiO3 substrates. X-ray diffraction and atomic force microscopy reveal that SnTe films are single-domain and uniform, which is critical to realizing controllable topological properties. Quantum transport effects observed in magnetotransport measurements are consistent with the existence of topological states. To better understand their behavior and inform next steps towards their control, we extract details of the topological states by fitting the magnetotransport over a range of film thicknesses. |
Friday, March 8, 2019 8:12AM - 8:24AM |
X09.00002: Suppression of the Spectral Weight of Topological Surface States on the Nanoscale via Local Symmetry Breaking Omur Dagdeviren, Subhasish Mandal, Ke Zou, Chao Zhou, Georg Simon, Stephen Albright, Sohrab Ismail-Beigi, Frederick J Walker, Charles H Ahn, Udo Dietmar Schwarz, Eric Altman In topological crystalline insulators the topological conducting surface states are protected by crystal symmetry. Here, We show using scanning tunneling microscopy/spectroscopy that defects that break local mirror symmetry of SnTe suppress electron tunneling over an energy range as large as the bulk band gap, an order of magnitude larger than that produced globally via magnetic fields or uniform structural perturbations [1]. The results reveal the influence of various defects on the electronic properties, including screw dislocations, point defects, and tilt boundaries that lead to dislocation arrays that serve as periodic nucleation sites for pits grown on SrTiO3 [2,3]. Complementary ab initio calculations show how local symmetry breaking obstructs topological surface states as shown by a threefold reduction of the spectral weight of the topological surface states. The findings highlight the potential benefits of manipulating the surface morphology to create devices that take advantage of the unique properties of surface states and can operate at practical temperatures. |
Friday, March 8, 2019 8:24AM - 8:36AM |
X09.00003: Suppressing metallicity of topological insulator GdBi thin films by quantum confinement Hisashi Inoue, Minyong Han, Mengli Hu, Takehito Suzuki, Liang Fu, Junwei Liu, Joseph Checkelsky GdBi is an antiferromagnetic semimetal which has been proposed to have non-trivial band topology.[1,2] Despite attempts to experimentally observe its topological nature, it is challenging due to the large band overlap between the conduction and valence bands. Here we show via electrical transport measurements that quantum confinement can suppress the band overlap in GdBi thin films. The semimetallicity is lifted below a film thickness of approximately 8 crystallographic unit cells while the antiferromagnetic order is preserved down to our minimum thickness of 5 crystallographic unit cells. This is a step toward realizing a bulk insulating antiferromagnetic topological insulator, which can potentially realize a Chern insulating state in the monolayer limit. |
Friday, March 8, 2019 8:36AM - 8:48AM |
X09.00004: Growth and Characterization of Antiferromagnetic Rare-earth Bismuth thin films Minyong Han, Hisashi Inoue, Mengli Hu, Takehito Suzuki, Liang Fu, Junwei Liu, Joseph Checkelsky Recent studies have shown that Rare-earth Bismuth (RBi) hosts topologically protected Dirac surface states [1,2]. However, spectroscopic measurements on bulk single crystals thus far have been limited to (001) natural cleavage surface of RBi. Here we report the first successful realization and characterization of GdBi (111) films grown on BaF2 substrates with Molecular Beam Epitaxy. GdBi, in a bulk form, is a semimetal with type-II antiferromagnetic (AFM) order below TN,bulk = 28 K. With torque magnetometry, we confirm that type-II AFM order is preserved in our films with a Neel temperature TN,film = 30 K. From electrical transport measurements, we observe a non-linear Hall effect and non-saturating magnetoresistance, the characteristics of a compensated semimetallic electronic structure. Stabilization of GdBi (111) films enables previously unexplored studies of the interplay of symmetry-breaking and Dirac crossings on the (111) surface of RBi. We further anticipate a possible Chern insulator phase in the ultrathin limit, when quantum confinement lifts semimetallicity while retaining non-trivial band topology. Reference: [1] M. Zeng and C. Fang et al., arXiv:1504.03492 (2015), [2] J. Nayak et al., Nat. Comm. 8, 13942 (2017). |
Friday, March 8, 2019 8:48AM - 9:24AM |
X09.00005: Nanoscale Synthesis and Characterization of Topological Materials Invited Speaker: Ilija Zeljkovic Interest in the superconducting proximity effect (SPE) has recently been reignited by theoretical predictions that it could be used to achieve topological superconductivity. However, small proximity-induced gaps (Δind) of ~1 meV have predominantly been obtained so far using select few low-Tc superconductors. In this talk, I will discuss how we use a combination of molecular beam epitaxy and scanning tunneling microscopy/spectroscopy to study topological insulators grown on high-Tc superconductors Fe(Te,Se) and Bi2Sr2CaCu2O8+x. On the surface of Bi2Te3 grown on Fe(Te,Se), we detect Δind as high as ~3.5 meV, which is the largest reported gap induced by proximity to an s-wave superconductor to-date. We find that Δind exponentially decays with Bi2Te3 thickness, but remains finite even after the topological surface states had been formed. By imaging the scattering and interference of surface state electrons, we provide a microscopic visualization of the fully gapped Bi2Te3 surface state due to Cooper pairing. We contrast this observation with the lack of observed superconducting gap in Bi2Te3/Bi2Sr2CaCu2O8+x heterostructures. We conclude by discussing the roles of various parameters in driving the SPE effect across complex interfaces. |
Friday, March 8, 2019 9:24AM - 9:36AM |
X09.00006: In-situ analysis of MBE-grown ZrTe5 thin films Timothy Pillsbury, Anthony Richardella, Yanan Li, Nitin Samarth Originally of interest for its resistivity anomaly, ZrTe5 has been predicted by recent theoretical work to be a quantum spin Hall insulator (QSHI) with band-gap of ~100meV [PRX 4, 011002 (2014)]. Additionally, bulk measurements of ZrTe5 have shown a variety of states from a strong topological insulator (TI) to a weak TI and even a Dirac Semimetal depending on the experimental methods [Scientific Reports 7, 45667 (2017)]. This makes ZrTe5 also interesting for studying the transition from a weak to strong TI. Since experiments so far have used films exfoliated from bulk crystals, they are complicated by the difficulty of exfoliating large area crystals. Moreover, rapid oxidization of ZrTe5 films prevents thorough analysis of exfoliated thin films. To help elucidate the nature of the topological properties of ZrTe5 and achieve a large band-gap QSHI, we have grown epitaxial thin films under ultra-high vacuum using molecular beam epitaxy (MBE) and performed in-vacuo analysis using ARPES and STM to confirm the growth quality and analyze the band structure, laying the groundwork for future MBE studies that utilize strain engineering to study the strong to weak TI transition and the observation of a large gap QSHI. |
Friday, March 8, 2019 9:36AM - 9:48AM |
X09.00007: Remote Doping of Sb Quantum Wells with Te Ryan P O'Toole, Kaushini Wickramasinghe, Tetsuya D Mishima, Michael Santos Antimony (Sb) has topological surface states, but its bulk band structure is semimetallic. Our goal is to study the transport properties of the topological states by suppressing the bulk conductivity through quantum confinement and enhancing the surface conductivity through remote n-type doping. A series of Sb quantum-well structures was grown by molecular beam epitaxy with AlxGa1-xSb barrier layers and GaAs (111) substrates. The Sb layer thickness of 4 nm was chosen to be thin enough to open a bandgap in the bulk band structure. To populate the topological electron states in the Sb quantum well, a section of one AlxGa1-xSb barrier was doped with Te atoms, which act as donor impurities. We report on Hall effect measurements that characterize the effectiveness of our approach in increasing the contribution of topological surface states to the conductivity of the structure. |
Friday, March 8, 2019 9:48AM - 10:00AM |
X09.00008: Nanoscale strain induced effects on the optoelectronic properties of monolayer transition metal dichalcogenides. Marcus Teague, Jiaqing Wang, Wei-Hsiang Lin, Duxing Hao, Chen-Chih Hsu, Nai-Chang Yeh We perform simulations and scanning tunneling spectroscopic (STS) studies of the strain effects on the optoelectronic properties of monolayer TMDC materials MS2, where M = Mo and W. We use molecular dynamics to simulate the strain effect on MS2 by relaxing a (60x60) nm2 monolayer MS2 sheet on top of a 2.5 nm radius semi-sphere gold nanoparticle. We find that the strain tensor component εxy changes between the top S-layer (positive) and bottom S-layer (negative), indicating the top S-layer experiences an effective stretching force whereas the bottom S-layer is compressed. For STS studies, high quality monolayer MoS2 single crystals with areas to 200x200nm2 are synthesized by CVD and transferred to Si substrates with triangular arrays of nanostructures. Nanostructures are ~20nm in size with a lattice constant of ~400nm. Spatially resolved STS studies on the LDOS in MS2 are carried out to investigate spatial variations resulting from strain and temperature evolution of the local electronic bandgap. Using a variable-wavelength light source, circularly polarized light is also applied to the sample under STS studies, and the spatially resolved LDOS in strained MS2 are investigated as a function of temperature, optical wavelength and polarization.This work is supported by NSF and ARO |
Friday, March 8, 2019 10:00AM - 10:12AM |
X09.00009: Electrical Transport Study in Wafer Scale Epitaxial WTe2 Film Grown by Molecular Beam Epitaxy Jason Tran, Junxue Li, Justin Horowitz, Jing Shi, Peng Wei WTe2, a member of the layered transition metal dichalcogenides (TMDs), has recently garnered attention due to its topological properties. Current studies of WTe2 have mainly focused on samples produced by mechanical exfoliation. Recent success has shown that WTe2 is able to grow on graphene using molecular beam epitaxy (MBE). As a conducting layer, graphene may weaken or smear any signatures of topological transport in WTe2. In this talk, we present our electrical transport studies in wafer scale WTe2 grown on insulating sapphire substrate. The 1T’ structural phase of the MBE grown samples is confirmed by in situ reflection high energy electron diffraction (RHEED). Electrical transport studies show a sharply enhanced magnetoresistance (MR) at low temperature. The MR has a clear linear magnetic field dependence over a range of magnetic fields up to 12 Tesla. In the low field regime, we observe signatures of weak antilocalization which suggest the presence of strong spin-orbit coupling. We will also present the effect of varying thickness of WTe2 on magnetotransport, and discuss potential signatures related to topological order. |
Friday, March 8, 2019 10:12AM - 10:24AM |
X09.00010: Epitaxial growth and electronic properties of 2D topological insulator 1 WSe2 Chenhui Yan, Huiming Zhang, Lian Li Monolayer 1T' WSe2 has recently been predicted to be a two-dimensional topological insulator (2D TI) with a larger bulk bandgap than 1T' WTe2, more preferable for practical applications. However, the energetically stable structure of WSe2 is the 2H phase, and hence the 1T' WSe2 doesn’t exist in the bulk. Here, we report the molecular beam epitaxial growth of monolayer 1T' WSe2 on epitaxial graphene/SiC(0001). We find the growth of homogeneous monolayer WSe2 is strongly dependent on the substrate temperature: while the 1T' phase can be grown below 300, a mixed 1T’ and 1H is obtained with increasing substrate temperature, with a complete 1H phase formation above 400oC. Using scanning tunneling microscopy/spectroscopy and angle resolved photoemission spectroscopy, we determine a bulk bandgap of 130 meV in monolayer 1T' WSe2, almost twice that of 1T' WTe2 (70 meV). We further observe one-dimensional edge states within the bulk bandgap in tunneling spectroscopy, confirming that monolayer 1T' WSe2 is indeed a 2D TI. |
Friday, March 8, 2019 10:24AM - 10:36AM |
X09.00011: Effect of epitaxial strain on topological properties of LaSb Shoaib Khalid, Fernando Sabino, Anderson Janotti Rare-earth mono-pnictides display interesting properties that include extreme magneto resistance and non-trivial topological band structures, yet have simple crystal structure. For instance, LaSb is on the verge of a transition from trivial to non-trivial topological semimetal, LaBi being a non-trivial semi-metal, and LaAs a trivial semimetal. Since these materials can be epitaxially grown on III-V substrates and, therefore, easily integrated to well-known electronic materials, it is imperative to understand the effects of epitaxial strain on their band structures and topological properties. Using DFT with the HSE06 hybrid functional, we study the effects of epitaxial strain on the electronic properties of these rare-earth mono-pnictides. For example, we find that at around 1.5% compressive epitaxial strain there is a band crossing between La d band and Sb p band near the Z point of the BCT Brillouin zone of strained LaSb. This band crossing indicates a topologically non-trivial behavior of LaSb under applied epitaxial strain. These results help understanding the novel transport and topological properties of epitaxially grown strained thin films of these rare earth mono-pnictides. |
Friday, March 8, 2019 10:36AM - 10:48AM |
X09.00012: Electric Field Tuned Quantum Phase Transition from Topological to Conventional Insulator in Few-Layer Na3Bi James Collins, Anton Tadich, Lidia Gomes, João Rodrigues, John Hellerstedt, Chang Liu, Hyejin Ryu, Shujie Tang, Weikang Wu, Shengyuan Yang, Shaffique Adam, Sung-Kwan Mo, Michael Fuhrer, Mark T Edmonds Na3Bi in bulk is a zero-bandgap topological Dirac semimetal (TDS), but when confined to a few layers it is predicted to be a large-gap (~300 meV) topological insulator. Application of an electric field to few-layer Na3Bi has been predicted to drive a band inversion due to Stark effect and induce a topological phase transition, which could be the basis of a topological transistor. Here we demonstrate the growth of epitaxial few-layer Na3Bi via MBE, and probe its electronic structure and response to an electric field using scanning probe microscopy/spectroscopy (STM/STS) and angle-resolved photoelectron spectroscopy, and compare with results from DFT.1 Both monolayer and bilayer Na3Bi show bandgaps >300 meV in STS, and the observation of an edge state with exponential decay into the bulk confirms their topological nature, consistent with DFT. With application of an electric field via potassium doping or approach of the STM tip the bandgap can be tuned to semi-metallic and then re-opened to greater than 100 meV. The electric fields required to induce this transition are below the breakdown field of many conventional dielectrics, making the creation of a topological transistor based on a few-layer TDS within reach. |
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