Bulletin of the American Physical Society
APS March Meeting 2023
Las Vegas, Nevada (March 510)
Virtual (March 2022); Time Zone: Pacific Time
Session Y42: Topological Crystalline InsulatorsFocus

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Sponsoring Units: DMP Chair: Sayed Ghorashi, Stony Brook University Room: Room 318 
Friday, March 10, 2023 8:00AM  8:36AM 
Y42.00001: Higherorder topological insulators Invited Speaker: Frank Schindler The discovery of topological insulators has rejuvenated band theory and led to fruitful collaboration between theory and experiment. Capitalizing on their inherent insensitivity to local noise, the (d1)dimensional surface states of ddimensional firstorder topological insulators remain gapless as long as their protecting symmetries are preserved. In my talk, I will review higherorder topological insulators (HOTIs), which comprise the most recent chapter of topological band theory. They are ddimensional materials with (dn)dimensional gapless boundary states where n > 1. Using minimal examples, I will explain the fundamental types of HOTIs and their symmetry protection. I will also recount some of the exciting experimental results on HOTI (meta)materials. My talk will be a pedagocical invitation to the field, directed at a broad audience, rather than a comprehensive technical review. 
Friday, March 10, 2023 8:36AM  8:48AM 
Y42.00002: Twoatomthin topological crystalline insulators lacking out of plane inversion symmetry Salvador BarrazaLopez A twodimensional topological crystalline insulator (TCI) with a single unit cell (u.c.) thickness is demonstrated here. The chiral nature of the bulk TCI Hamiltonian from Fu [1] permits creating a 2x2 square Hamiltonian, whose topological properties hold invariant at both the bulk and at the single u.c.~thickness limits. The identical topological characterization for bulk and u.c.thick phases is further guaranteed by a calculation involving Pfaffians. 
Friday, March 10, 2023 8:48AM  9:00AM 
Y42.00003: MetalOrganic Molecular Beam Epitaxy of WTe_{2} Kevin Hauser, Gregory Lapit, Christian E Matt, Jason D Hoffman, Jennifer E Hoffman Monolayer transition metal dichalcogenides (TMDs) are strong candidates for future technological applications. Such applications require controlled means of ultrahigh purity synthesis, as provided by molecular beam epitaxy (MBE). So far, MBEsynthesized TMD monolayers have been limited to islands with lateral sizes on the order of tens of nanometers, due to the large difference of surface diffusivity of the transition metal and chalcogen species. Here, we introduce a molecular beam epitaxy method, based on the metalorganic precursor W(CO)_{6}, for growing monolayer WTe_{2} [1]. We characterize our growth using xray photoelectron spectroscopy, scanning electron microscopy, and atomic force microscopy. We observe onedimensional nanostructures of nonstoichiometric WTe_{x} localized at the substrate step edges, and twodimensional islands on the substrate terraces. 
Friday, March 10, 2023 9:00AM  9:12AM 
Y42.00004: Tuning topological properties in Bi_{2}Se_{3}/VSe_{2} heterostructures Xuance Jiang, Elio Vescovo, Turgut Yilmaz, Deyu Lu Topological materials have drawn great attention due to their exotic physical properties and various applications in quantum information science. Central to the fundament research and practical applications are the control and tunability of the topological states through material engineering, such as doping, strain and stacking of different 2D van der Waals materials. Here we present a combined angleresolved photoemission spectroscopy (ARPES) and firstprinciples study of the topological properties of fewlayer 1T VSe_{2 }on fewquintuple layer Bi_{2}Se_{3} heterojunctions. ARPES measurements show that details of the topological band structure of the heterojunction strongly depend on the thickness of the quintuple layers of Bi_{2}Se_{3}. Density functional theory is used to interpret the ARPES measurement and understand the evolution of the topological states. This study gains insights into the topological properties of VSe_{2}/ Bi_{2}Se_{3} heterojunctions, which have important implications on tunability of the topological materials. 
Friday, March 10, 2023 9:12AM  9:24AM 
Y42.00005: Weak antilocalization in selectivearea grown Pb_{1x}Sn_{x}Te nanowire networks Maarten Kamphuis, Femke Witmans, Joost Ridderbos, Jonas Kareem, Sander G Schellingerhout, Chuan Li, Erik P. A. M. Bakkers, Floris Zwanenburg, Alexander Brinkman Lead Tin Telluride (Pb_{1x}Sn_{x}Te) is predicted to be a topological crystalline insulator (TCI) [1], meaning that the surface states are protected by crystal symmetries. The topological phase transition from a trivial narrow bandgap semiconductor to a TCI can be controlled through the Pb/Sn ratio, and has been observed experimentally at Sn concentrations of 3040% [2][3]. 
Friday, March 10, 2023 9:24AM  9:36AM 
Y42.00006: ClosetoDirac point shift of largearea MOCVDgrown Bi_{2}Te_{3}’s Fermi level following growth on Sb_{2}Te_{3} Roberto Mantovan, Lorenzo Locatelli, Polychronis Tsipas, Athanasios Dimoulas, Akylas Lintzeris, Emanuele M Longo, Arun Kumar, Massimo Longo We developed a Metal Organic Chemical Vapour Deposition (MOCVD) process to grow the bilayered Bi_{2}Te_{3}(top)/Sb_{2}Te_{3} topological insulator on top of 4’’ Si(111). In singlephase Bi_{2}Te_{3}/Si(111), we have previously demonstrated clear topologicallyprotected surface states (TSS) by angleresolved photoemission spectroscopy (ARPES) and magnetotransport, but a strong bulk contribution was still present with the Fermi level (E_{F}) crossing the conduction band at ~0.5 eV above the Dirac point [1]. Following the growth of 90 nm Bi_{2}Te_{3} in direct contact with 30 nm Sb_{2}Te_{3}, we observed by ARPES a remarkable shift of E_{F} towards the Dirac point, totally suppressing the bulk states’ contribution. This was confirmed by magnetotransport, where, within the HikamiLarkinNagaoka model, we measured an ideal α=0.5 due to the optimized topologicallyprotected surface states. We will also present the first attempts in making use of such closetoideal positioning of the Bi_{2}Te_{3}’s Fermi level to manipulate magnetic figures in 2Dferromagnet CrTe_{2} grown on top of the developed Bi_{2}Te_{3}/Sb_{2}Te_{3}/Si(111) heterostructure. 
Friday, March 10, 2023 9:36AM  9:48AM 
Y42.00007: Exploration of topological electronic structures in bismuth halide series Ji Seop Oh, Tianyi Xu, Nikhil Uday Dhale, Jianwei Huang, Sheng Li, Chiho Yoon, Chao Lei, Hanlin Wu, Makoto Hashimoto, Donghui Lu, Jonathan D Denlinger, Christopher Jozwiak, Aaron Bostwick, Eli Rotenberg, Chun Ning Lau, Bing Lv, Fan Zhang, Robert J Birgeneau, Ming Yi Quasionedimensional bismuth halides, Bi_{4}X_{4} (X = I, Br), constitute a promising material platform to realize various nontrivial topological phases. Bi_{4}X_{4} can be categorized by its stacking along c axis. Weak topological insulators (TIs) in singlelayer stacking, highorder TI in double stacking, and topological phases transitions opportunities in between are on the table. In addition, discovery of gatetunable Josephson supercurrent as well as existence of electronic instabilities driven by topological surface states (TSS) bring Bi_{4}X_{4} a fertile candidate to study topological manybody physics and emergent quantum physics. In this talk, I will present experimental characterizations for electronic structures among the Bi_{4}X_{4} series in terms of angleresolved photoemission spectroscopy (ARPES), which is smokinggun evidence to identify TSS and instabilities in them. Stateoftheart ARPES at synchrotrons enable us to explore (001) and (100) surfaces distinguishably, thus can provide reliable topological characterizations among the series. Taking advantages of it, we discovered a roomtemperature topological phase transition between weak TI and highorder TI, an ideal weak TI with spinpolarized electronic instabilities derived from TSS within bulk gap, and gapped TSS due to hinge states in highorder TI. Future efforts to induce topological phase transitions will be also discussed. 
Friday, March 10, 2023 9:48AM  10:00AM 
Y42.00008: Shubnikovde Haas oscillations in Pb_{1x}Sn_{x}Te epitaxial films Samuel J Poage, Antonio Gonzalez, Athby H AlTawhid, Ian Mercer, Andrew Liao, Shalinee Chikara, Kaveh Ahadi Topological crystalline insulators (TCIs) are a class of materials prized in recent years for their ability to form symmetryprotected massless surface states. SnTe is a TCI while PbTe is a trivial insulator, but Pb1xSnxTe shows a topological phase transition at X~0.4. Higher Pb concentrations yield a trivial insulator while lower concentrations yield a TCI. Characterizing the surface states at Sn rich side of the transition remains elusive due to parasitic transport from. We grew high quality Pb1xSnxTe heterostructures in a chalcogenide molecular beam epitaxy system. XRD measurements confirm single crystal thin films grown epitaxially on BaF2 (111) substrates. High field magnetoresistance measurements were carried out at various temperatures (10mK700mK) and angles. We analyzed Shubnikov deHaas (SdH) oscillations in samples with various Sn concentrations at various temperatures and field angles. The summed frequencies contained in the SdH oscillations elucidate the nature of topological surface states. 
Friday, March 10, 2023 10:00AM  10:12AM 
Y42.00009: gfactor of topological states from magnetooptical spectroscopy and quantum oscillations in Pb_{1x}Sn_{x}Se quantum well Jiashu Wang, Tianyi Wang, Mykhaylo Ozerov, Maksym Zhukovskyi, Tatyana Orlova, Dmitry Smirnov, Xinyu Liu, Badih A Assaf Large gfactors in system with strong spinorbit coupling are ideal for spintronic and quantum computing devices. Here, we report the high gfactor of the surface state in Pb_{1x}Sn_{x}Se quantum wells. We successfully grow high quality Pb_{0.85}Eu_{0.15}Se/Pb_{0.7}Sn_{0.3}Se/ Pb_{0.85}Eu_{0.15}Se single quantum wells with mobility larger than 4000cm^{2}V^{1}s^{1}. The high carrier mobility allows us to reach the quantum limit at reasonably achievable magnetic fields. We exploit this advantage to combine magnetooptical Landau level spectroscopy, ShubnikovdeHaas transport measurement and consistent modelling of the two experiments to precisely extract the band parameters of the topological states of this system along with their spin and orbital gfactor. The obtained effective gfactor (g~80 at 1T for index n=1) is comparable with what is found for the InSb system, and is much larger than the InAs system (~15). This information is vital to the realization and understanding of novel quantized Hall effect stemming from this material family, and also shed light onto their future applications to quantum devices utilizing topological surface states. 
Friday, March 10, 2023 10:12AM  10:24AM 
Y42.00010: Highly Tunable Band Inversion in AB_{2}X_{4} (A=Ge, Sn, Pb; B=As, Sb, Bi; X=Se, Te) Compounds LinLin Wang Topological materials have been discovered so far largely by searching for existing compounds in crystallographic databases, but there are potentially new topological materials with desirable features that have not been synthesized. One of the desirable features is high tunability resulted from the band inversion with a very small direct band gap, which can be tuned by changes in pressure or strain to induce a topological phase transition. Using density functional theory (DFT) calculations, we have studied the septuple layered AB_{2}X_{4} series compounds, where A=(Ge, Sn and Pb), B=(As, Sb and Bi) and X=(Se and Te). With the DFT thermodynamic stability validated by the already reported compounds in these series, we predict new stable Se compounds, which are not found in crystallographic database. Among them, we find that GeBi_{2}Se_{4} and GeSb_{2}Se_{4} having a small direct band gap at the Z point are very close to a strong topological insulator, which can be tuned by a moderate pressure to induce the band inversion. Importantly, the topological features with the small direct band gap are well isolated in both momentum and energy windows, which offers high tunability for studying the topological phase transition. 
Friday, March 10, 2023 10:24AM  10:36AM 
Y42.00011: Reducing the 3D2D crossover thickness in Bi2Se3 by heterostructure engineering Yuanxi Wang, Hemian Yi, Danielle Hickey, Nasim Alem, CuiZu Chang The 3D topological insulator Bi2Se3 crosses over to the 2D limit usually as layer thickness is reduced past 6 Bi2Se3 quintuple layers (QL), as can be measured by the opening of a hybridization gap from its two surface states. In common Bi2Se3 heterostructures with a bandbendinginduced potential gradient, the separation of the two surface states in real space and in energy both decreases as layer thickness is reduced, yielding the typical 6QL crossover thickness. Here, a combined firstprinciples theory and experimental study shows that an energy separation of the two surface states can be sustained to prevent surface state hybridization until 3 QLs. This is achieved by allowing the lower surface state to interact with a BiSe substrate (on top of NbSe2), which relieves the upper surface state from hybridizing with it. We further show that the 3QL Bi2Se3/BiSe system behaves similar to an asymmetric 4QL Bi2Se3 system, where Rashbatype quantum well bands are induced by a compositional gradient perpendicular to the layers. 
Friday, March 10, 2023 10:36AM  10:48AM 
Y42.00012: molecular beam epitaxy and characterization of Bi_{2}Se_{3} and Sb_{2}Te_{3 }on In_{2}Se_{3} layers via selenium passivation of nonvicinal InP(111)B substrates Kaushini S Wickramasinghe, Candice Forrester, Ido Levy, Maria Tamargo Topological Insulators (TI) have gained much attention due to their nontrivial topology facilitating a wide variety of applications. However, there is a large unintentional background doping that conceals their surface channels, and crystal defects such as twin domains are frequently observed. Although these are van der Waals materials, the influence of the substrate on the molecular beam epitaxy (MBE) material properties has been shown to be nonnegligible. Nonvicinal InP(111)B has been used before and reported by some to produce good material properties, yet it has been challenging to completely suppress the twin domains. In this study, we explore the quality of epitaxial Bi_{2}Se_{3 }and Sb_{2}Te_{3} on 5 nm ultrathin In_{2}Se_{3} layers grown by a selenium passivation technique during the oxide desorption of the InP(111)B substrates, without the use of an indium source. AFM shows the formation of a smooth In_{2}Se_{3 }layer and the HRXRD shows its high crystallinity, with the presence of satellite peaks from which the thickness could be extracted. Morphology of Bi_{2}Se_{3 }exhibits smooth surfaces with large terraces. AFM of Sb_{2}Te_{3} show planes with steps, with a larger roughness than Bi_{2}Se_{3. }Φ scans of the (015) plane of both the Bi_{2}Se_{3} and Sb_{2}Te_{3} layers show complete suppression of twin domains. Further investigation showed that the direction of the twin suppression is sometimes parallel to the (311) plane of the InP(111)B substrates and other times it is in the opposite direction. In either case we observe full twin suppression. 
Friday, March 10, 2023 10:48AM  11:00AM 
Y42.00013: Quantum Interference in SnTe nanowires Femke Witmans, Maarten Kamphuis, Joost Ridderbos, Job Noordkamp, Mathijs Mientjes, Max Hoskam, Xin Guan, Chuan Li, Erik P. A. M. Bakkers, Floris Zwanenburg, Alexander Brinkman SnTe is a topological crystalline insulator (TCI) [1] and is predicted to undergo a phase transition to a higherorder topological state [2] by breaking crystal symmetry. These higherorder topological insulators (HOTI) have not been experimentally realized; they host hinge states on the boundary of two gapped surfaces, leading to conducting hinges. When combined with a superconductor, this system could host parafermions on a pair of hinge states due to the onedimensional character of these states. Detecting the 2D surface and 1D hinge states in this material is therefore a key objective. 
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