Bulletin of the American Physical Society
APS March Meeting 2022
Volume 67, Number 3
Monday–Friday, March 14–18, 2022; Chicago
Session T70: Tuning and Manipulation of Topological Materials IIFocus Recordings Available
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Chair: Fazel Tafti, Boston College Room: Hyatt Regency Hotel -Jackson Park B |
Thursday, March 17, 2022 11:30AM - 11:42AM |
T70.00001: X-ray and Raman Characterization of Bi-Sb Thin Films Grown by Molecular Beam Epitaxy Yu-Sheng Huang, Yongxi Ou, Supriya Ghosh, Andre Mkhoyan, Nitin Samarth The material Bi x Sb 1−x is a promising candidate for spin-orbit torque (SOT) applications due to its potentially high figure of merit for spin-charge interconversion [Nature Mater. 17, 808 (2018)]. This is due to the strong spin-orbit coupling and topological features in the electronic band structure in the appropriate composition range. We describe the synthesis of Bi x Sb 1−x thin films using molecular beam epitaxy and their detailed structural characterization via x-ray diffraction, transmission electron microscopy, and reflection high energy electron diffraction. We discuss the crystalline defects in these films over a wide range of alloy compositions. Finally, we use Raman spectroscopy to investigate the phonon modes in these thin films and compare the results with those from bulk-grown Bi x Sb 1−x crystals previously reported in the literature. |
Thursday, March 17, 2022 11:42AM - 11:54AM |
T70.00002: Charge carriers and morphology of thin film Bi(111) grown on mica and on Si(111) Victoria Soghomonian, Zijian Jiang, Jean J Heremans Carriers in Bi and Bi(111) surfaces possess electronic and spin properties that render them compelling for the study of new quantum states of matter and topological systems. Strong spin-orbit interaction exists in the Bi(111) surface electron states and hyperfine interaction usefully couples electron and nuclear spin systems [1]. High-quality thin films are indicated to use the quantum properties of Bi(111) but the growth is challenging, given the aim to approach the carrier qualities of pure single-crystals. Bi films can also undergo a semimetal to semiconductor transition due to the formation of a quantum-confinement energy gap. We deposited Bi(111) films of thickness 10, 20, 40 or 60 nm on mica (van der Waals epitaxy) [1] and on Si(111)7x7 (Stranski-Krastanov epitaxy) and performed detailed magnetotransport measurements over 4-296 K. A 3-carrier model including metallic electrons in surface states and electrons and holes in the films' interiors provided densities, mobilities and mean-free paths of the 3 carrier types, and provided estimates of the quantum-confinement energy gap. A higher overall crystalline quality of van der Waals epitaxy on mica is evidenced by superior electronic transport properties compared to growth on Si(111), as will be discussed. |
Thursday, March 17, 2022 11:54AM - 12:06PM |
T70.00003: Ultrathin topological bismuth crystals grown inside van der Waals materials (Part I) Amy X Wu, Laisi Chen, Chaitanya A Gadre, Adrian Juanson, Naol Tulu, Joshua Wang, Kenji Watanabe, Takashi Taniguchi, Xiaoqing Pan, Ruqian Wu, Javier D Sanchez-Yamagishi Two-dimensional bismuth <111> is theoretically predicted to be a room temperature topological insulator (TI) with a large bandgap (~0.5eV). However, growth of Bi thin films is limited to sub-hundred nanometer grain size, providing a major obstacle for electronic studies. We have developed a new approach to 2D crystal growth in which bismuth is molded between van der Waals materials. By applying heat and pressure to Bi encapsulated between crystals of hexagonal boron nitride (hBN), we consistently produce 5-20 nm thick Bi crystals with atomically flat surfaces. Transmission electron microscopy (TEM) analysis reveals 10 um size single crystal domains with the <111> orientation corresponding to stacked hexagonal layers of Bi. We will present preliminary electrical transport measurements in this exciting unexplored regime of ultra-thin and atomically flat Bi. We anticipate that the squeeze-melt growth method with hBN can provide a general way to produce 2D crystals of soft materials. |
Thursday, March 17, 2022 12:06PM - 12:18PM |
T70.00004: Ultrathin topological bismuth crystals grown inside van der Waals materials (Part II) Laisi Chen, Amy X Wu, Chaitanya A Gadre, Adrian Juanson, Naol Tulu, Joshua Wang, Kenji Watanabe, Takashi Taniguchi, Xiaoqing Pan, Ruqian Wu, Javier Sanchez-Yamagishi Two-dimensional bismuth <111> is theoretically predicted to be a room temperature topological insulator (TI) with a large bandgap (~0.5eV). However, growth of Bi thin films is limited to sub-hundred nanometer grain size, providing a major obstacle for electronic studies. We have developed a new approach to 2D crystal growth in which bismuth is molded between van der Waals materials. By applying heat and pressure to Bi encapsulated between crystals of hexagonal boron nitride (hBN), we consistently produce 5-20 nm thick Bi crystals with atomically flat surfaces. Transmission electron microscopy (TEM) analysis reveals 10 um size single crystal domains with the <111> orientation corresponding to stacked hexagonal layers of Bi. We will present preliminary electrical transport measurements in this exciting unexplored regime of ultra-thin and atomically flat Bi. We anticipate that the squeeze-melt growth method with hBN can provide a general way to produce 2D crystals of soft materials. |
Thursday, March 17, 2022 12:18PM - 12:30PM |
T70.00005: Anomalous Zeeman shift of the defect bound states in epitaxial FeSn films on SrTiO3(111) Lian Li, Huimin Zhang The crystal structure of Kagome magnet FeSn consists of alternating planes of Fe3Sn Kagome and Sn honeycomb lattices, which provides a versatile platform for correlated topological phases hosting symmetry-protected electronic excitations and magnetic states. To date most work on FeSn has been carried out on bulk materials. In this work, we synthesize high quality FeSn thin films on SrTiO3(111) substrates by molecular beam epitaxy. The growth mode is three-dimensional, and x-ray diffraction confirms single crystalline FeSn structure. Using in situ low temperature scanning tunneling microscopy/spectroscopy, we observe honeycomb lattice on the Sn-terminated films, and close-packed lattice for the Fe3Sn termination. In addition, we also directly visualize single Sn vacancy in Fe3Sn layer and di-vacancy in Sn layer, as well as their response to applied magnetic fields. The bound states induced by these defects exhibit anomalous Zeeman shift under out-of-plane magnetic fields, where the energy of the bound states shifts linearly towards higher energy independent of the direction of the magnetic field. These findings provide critical insights to the understanding of magnetism in Kagome magnetic materials. |
Thursday, March 17, 2022 12:30PM - 12:42PM |
T70.00006: Vacancy Tuned Antiferromagnetism in the Candidate Topological Semimetal LaMnxSb2 Tyler J Slade, Aashish Sapkota, Juan Schmidt, John M Wilde, Qiang Zhang, Lin-Lin Wang, Sergey L Budko, Paul C Canfield We report the magnetic and transport properties of LaMnxSb2, an underexplored member of the 1-1-2 family of topological semimetals. LaMnxSb2 crystalizes in the tetragonal P4/nmm space group with intrinsic Mn vacancies (x < 1). We use a Sb-flux to prepare single crystals of LaMnxSb2 with x = 0.76–0.92 and find LaMnxSb2 has a rich temperature–composition magnetic phase diagram, ordering antiferromagnetically below TN1 = 130–170 K with additional transitions at lower temperatures. Depending on x, our magnetic and transport measurements provide evidence for at least three unique antiferromagnetic states in LaMnxSb2. This contradicts previous reports of ferromagnetism found in polycrystalline LaMnxSb2,1 which we ascribe to a MnSb impurity. Results of powder neutron diffraction to determine the ordering wave vectors of LaMnxSb2 (x = 0.92) will be discussed. |
Thursday, March 17, 2022 12:42PM - 12:54PM Withdrawn |
T70.00007: Manipulating topology in tailored artificial graphene nanoribbons Daniel J Trainer, Srilok Srinivasan, Brandon Fisher, Yuan Zhang, Constance R Pfeiffer, Saw-Wai Hla, Pierre Darancet, Nathan Guisinger Topological phases of matter give rise to exotic physics that can be leveraged for next generation quantum computation1,2 and spintronic devices3,4. Thus, the search for topological phases and the quantum states that they exhibit have become the subject of a massive research effort in condensed matter physics. Topologically protected states have been produced in a variety of systems, including artificial lattices6,7, graphene nanoribbons (GNRs)8,9 and bismuth bilayers10. Despite these advances, the real-time manipulation of individual topological states and their relative coupling, a necessary feature for the realization of topological qubits, remains elusive. Guided by first-principles calculations, we spatially manipulate robust, zero-dimensional topological states by altering the topological invariants of quasi-one-dimensional artificial graphene nanostructures. This is achieved by positioning carbon monoxide molecules on a copper surface to confine its surface state electrons into artificial atoms positioned to emulate the low-energy electronic structure of graphene derivatives. Ultimately, we demonstrate control over the coupling between adjacent topological states that are finely engineered and simulate complex Hamiltonians. Our atomic synthesis gives access to nanoribbon geometries beyond the current reach of synthetic chemistry, and thus provides an ideal platform for the design and study of novel topological and quantum states of matter. |
Thursday, March 17, 2022 12:54PM - 1:06PM |
T70.00008: Anomalous negative longitudinal magnetoresistance in the topological insulating regime in Bi1−xSbx Amit Vashist, R K Gopal, Yogesh Singh Bi1−xSbx is a topological insulator (TI) for x ≈ 0.03–0.20. Close to the Topological phase transition at x = 0.03, a magnetic field induced Weyl semi-metal (WSM) state is stabilized due to the splitting of the Dirac cone into two Weyl cones of opposite chirality. A signature of the Weyl state is the observation of a Chiral anomaly [negative longitudinal magnetoresistance (LMR)] and a violation of the Ohm’s law (non-linear I−V). We report the unexpected discovery of Chiral anomaly-like features in the whole range (x = 0.032, 0.072, 0.16) of the TI state. This points to a field-induced WSM state in an extended x range and not just near the topological transition at x = 0.03. Surprisingly, the strongest Weyl phase is found at x = 0.16 with a non-saturating negative LMR much larger than observed for x = 0.03. The negative LMR vanishes rapidly with increasing angle between B and I. Additionally, non-linear I–V is found for x = 0.16 indicating a violation of Ohm’s law. This unexpected observation of a strong Weyl state in the whole TI regime in Bi1−xSbx points to a gap in our understanding of the detailed crystal and electronic structure evolution in this alloy system. |
Thursday, March 17, 2022 1:06PM - 1:18PM |
T70.00009: Magnetotransport properties of thin pentatellurides Robert A Welser, Jinyu Liu, Luis A Jauregui, Javier D Sanchez-Yamagishi, Michael T Pettes Here we report the magnetotransport properties of thin flakes of HfTe5 and ZrTe5, which have been reported to be in multiple possible phases (Dirac semimetal, weak TI, and strong TI). We observed a nonlinear Hall effect across thin flakes (<200 nm) at temperatures down to 1.6K, and we see a sign-change as the majority charge carrier changes in temperature. We then tuned the magnetotransport through an ionic liquid gate placed directly onto the flakes. |
Thursday, March 17, 2022 1:18PM - 1:54PM |
T70.00010: Impurity states in crystalline topological materials Invited Speaker: Raquel Queiroz Nontrivial topology plays a crucial role in how a system behaves in the presence of impurities and crystalline defects. Topological phases are characterized by a lack of an exponentially localized Wannier representation that respects the local and crystalline symmetries of the system. In this talk, I will relate the lack of localizability to a universal structure of the real part of the local Green's function. I will show that this structure is manifested in defect-bound states with unique qualitative features, independent of the exact nature of the topological phase or which symmetry protects it. Universal defect states can provide critical cues to prove a material is in a topological phase, particularly in phases where crystalline symmetries protect the topology and the boundary states are hard to access. |
Thursday, March 17, 2022 1:54PM - 2:06PM |
T70.00011: Pressure-induced topological nodal line semimetal (TNLS) phase in SrAgP Vikrant Chaudhary, Hem C. Kandpal We have studied the effect of external pressure on the crystal structure and electronic properties of SrAgP. We found a structural phase transformation in SrAgP from space group P63/mmc (194) to P–62m (189) above 11 GPa. In our density functional theory (DFT) calculations, SrAgP shows a nodal line semimetal like band dispersion in the P–62m (189) space group similar to the well-studied compound CaAgP. We hope that experimentalists will find SrAgP to be an interesting topological nodal line semimetal (TNLS) candidate. |
Thursday, March 17, 2022 2:06PM - 2:18PM |
T70.00012: Strain engineering of topological semimetal WTe2 flakes Amanda L Coughlin, Kevin Gutierrez, Michael Hosek, Tongxie Zhang, Isaac Delgass, Shixiong Zhang Strain engineering is a powerful technique to manipulate the electronic properties of nanostructured materials such as thin layers of transition metal dichalcogenides (TMDs). In this talk, we report on the influence of externally applied strains on the physical properties of the topological Weyl semimetal WTe2, a prototypical TMD. We developed different methods of applying strain (i.e., a three-point bending rig and controlled stretching/buckling via PDMS) and have found that applying strain to WTe2 flakes has a negligible influence on the vibrational modes probed via Raman spectroscopy, whereas applying and releasing tensile strain can distinctly modulate the resistance of the flakes. We will further discuss how the charge carrier density, mobility, and magneto-transport properties vary as a function of the applied strains. |
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