Jason Kawasaki, University of Wisconsin
Elena Hassinger, Technische Universität München
Anderson Janotti, University of Delaware
National Science Foundation
NM03.01: Topology and Magnetism
Thursday PM, April 22, 2021
8:00 AM - *NM03.01.01
Manipulating of Magnetism and Band Structure in the Topological Semimetal EuCd2As2
Na Hyun Jo1,2,3,Brinda Kuthanazhi1,2,Yun Wu1,2,Thais Trevisan1,2,Erik Timmons1,2,Tae-Hoon Kim1,Lin Zhou1,Lin-Lin Wang1,Benjamin Ueland1,2,Andriy Palasyuk1,2,Dominic Ryan4,Robert McQueeney1,2,Kyungchan Lee1,2,Benjamin Schrunk1,2,Anton Burkov5,Ruslan Prozorov1,2,Peter Orth1,2,Sergey Budko1,2,Adam Kaminski1,2,Paul Canfield1,2
Ames Laboratory1,Iowa State University2,Lawrence Berkeley National Laboratory (current affiliation)3,McGill University4,University of Waterloo5Show Abstract
With the steady rise of topological materials, attention of the community is now shifting to magnetic incarnations that host a largely unexplored territory from both a theoretical and experimental perspective. Magnetic topological materials allow for the formation of new
structures (as the spin can introduce new periodicities) and are yet to be mapped out in generality and resultantly harbor a lot of potential for novel effects. Hence, having a stable material platform is of tremendous interest to underpin both theory and experimental progress in this direction. So far, several materials are reported as magnetic Weyl semimetal though a material with a single pair of Weyl points that readily offers tuning of its topological states is yet to be found. EuCd2As2 is a magnetic semimetal that has the potential of manifesting nontrivial electronic states, depending on its low temperature magnetic ordering. By discovering and taking advantage of the chemical tunability of EuCd2As2, we report the successful growths of single crystals of EuCd2As2 with the ferromagnetic and antiferromagnetic ground state. In addition, we will discuss a detailed temperature-dependent Angle-resolved photoemission spectroscopy (ARPES) study on the ferromagnetic EuCd2As2.
This work was supported by the Center for Advancement of Topological semimetals, an Energy Frontier Research Center funded by the U.S.DOE, Office of Basic Energy Sciences. Work at the Ames Laboratory was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division. The Ames Laboratory is operated for the U.S. DOE by Iowa State University under Contract No. DEAC0207CH11358.
8:25 AM - *NM03.01.03
Exotic Quantum Magnets and Competing States—New Yb Kondo Systems
Rice University1Show Abstract
Novel physics discoveries heavily rely on the design and synthesis of new materials, which, in turn, depends a lot on the growth method. I will use the backdrop of my group’s discoveries of several Kondo systems, to emphasize the importance of the interplay between chemistry, materials science and physics in unveiling new materials’ properties. In particular, I will focus on YbT3M7 where the transition metal T is either Rh or Ir, and the metal M is Si or Ge. These are rhombohedral compounds, chemically and structurally very similar, with some similar physical properties and substantive differences. They all show evidence for strong electronic correlations and Kondo physics; one orders ferromagnetically (YbIr3Ge7) while the other two have antiferromagnetic ground states. Most remarkable, YbIr3Si7 shows long range antiferromagnetic order and non-Fermi liquid behavior even within the ordered state, while the electrical resistivity indicates bulk insulating behavior with ARPES and DFT calculations pointing to a conductive surface. We reconcile the observed Kondo physics, long range magnetic order and the insulating-to-metal crossover from bulk to surface with a proposal of Kondo exhaustion and evience for a Yb valence transition from magnetic (3+ in the bulk) to non-magnetic (2+ on the surface).
8:50 AM - NM03.01.04
Late News: Tunable Chiral Symmetry Breaking in Symmetric Weyl Materials
Sahal Kaushik1,Evan Philip2,Jennifer Cano1
Stony Brook University1,Brookhaven National Laboratory2Show Abstract
Asymmetric Weyl semimetals, which possess an inherently chiral structure, have different energies and dispersion relations for left- and right-handed fermions. They exhibit certain effects not found in symmetric Weyl semimetals, such as the quantized circular photogalvanic effect and the helical magnetic effect. In this work, we derive the conditions required for breaking chiral symmetry by applying an external field in symmetric Weyl semimetals. We explicitly demonstrate that in certain materials with the Td point group, magnetic fields along low symmetry directions break the symmetry between left- and right-handed fermions; the symmetry breaking can be tuned by changing the direction and magnitude of the magnetic field. In some cases, we find an imbalance between the number of type I left- and right-handed Weyl cones (which is compensated by the number of type II cones of each chirality.)
9:05 AM - NM03.01.05
Late News: A New Cubic Hall Viscosity in Three-Dimensional Topological Semimetals
Iñigo Robredo1,2,Pranav Rao3,Fernando de Juan1,4,Aitor Bergara1,2,Juan Luis Mañes2,Alberto Cortijo5,Maia Vergniory1,4,Barry Bradlyn3
Donostia International Physics Center1,University of the Basque Country2,University of Illinois at Urbana-Champaign3,Basque Foundation for Science4,Universidad Autónoma de Madrid5Show Abstract
In this work, we study the non-dissipative viscoelastic response of three dimensional crystals. We show that for systems with tetrahedral symmetries, there exist new, intrinsically three-dimensional Hall viscosity coefficients that cannot be obtained via a reduction to a quasi-two-dimensional system. To study these coefficients, we specialize to a tight binding model for a chiral magentic metal inspace group P213 (198), which features a threefold degenerate “spin-1” fermion at the Fermi level. Using the Kubo formula for viscosity, we compute the non-dissipative Hall viscosity for the spin-1 fermion in two ways. First we use an electron-phonon coupling ansatz to derive the “phonon” strain coupling and associated phonon Hall viscosity. Second we use a momentum continuity equation to derive the viscosity corresponding to the conserved momentum density. We conclude by discussing the implication of our results for hydrodynamic transport in three-dimensional magnetic metals,and discuss some candidate materials in which these effects may be observed.
NM03.02/NM04.07: Joint Session: Skyrmions
Thursday PM, April 22, 2021
10:30 AM - *NM03.02/NM04.07.01
Atomic-Scale Studies and Design of Topological Spin Textures and Majorana States in Magnet-Superconductor Hybrid Systems
University of Hamburg1Show Abstract
Since the discovery of the existence and manipulation possibilities of individual nanoscale skyrmions in ultrathin transition metal films on heavy-element substrates  by spin-polarized scanning tunneling microscopy (SP-STM) , the field of magnetic skyrmions [3,4] has gained significant interest due to their great potential for future magnetic memory and logic devices [5,6]. In particular, the small size, enhanced stability and unidirectional current-driven movement of chiral magnetic skyrmions make them interesting for novel types of magnetic device concepts. These unique properties of skyrmions are intimately linked to interfacial Dzyaloshinskii-Moriya (DM) interactions [7,8] which recently have been studied directly at the level of individual magnetic atoms on surfaces of heavy-element substrates . Based on these fundamental investigations of the distance- and material dependencies of interfacial DM interactions, it has become possible to design non-collinear spin textures in low-dimensional systems based on the powerful combination of SP-STM and single-atom manipulation techniques [10,11]. Proximity-coupling of such well-defined low-dimensional non-collinear spin-states with elemental superconductors allows the design of yet another type of interesting topological states, i.e. Majorana zero modes, which offer great potential for topological quantum computation [11,12]. Besides the emergence of Majorana states in quasi-1D hybrid systems consisting of spin-spirals interacting with elemental superconductors, we will also discuss most recent progress towards the observation of Majorana states in quasi-2D skyrmion-superconductor hybrid systems .
 N. Romming et al., Science 341, 6146 (2013).
 R. Wiesendanger, Rev. Mod. Phys. 81, 1495 (2009).
 A.N. Bogdanov and Ch. Panagopoulos, Nature Reviews Physics 2, 492 (2020).
 A.N. Bogdanov and Ch. Panagopoulos, Physics Today 73(3), 44 (2020).
 R. Wiesendanger, Nature Reviews Materials 1, 16044 (2016).
 A. Fert, N. Reyren, V. Cros, Nature Reviews Materials 2, 17031 (2017).
 M. Bode et al., Nature 447, 190 (2007).
 S. Heinze et al., Nature Physics 7, 713 (2011).
 A. A. Khajetoorians et al., Nature Commun. 7, 10620 (2016).
 M. Steinbrecher et al., Nature Commun. 9, 2853 (2018).
 H. Kim et al., Science Advances 4, eaar5251 (2018).
 A. Palacio-Morales et al., Science Advances 5, eaav6600 (2019).
 A. Kubetzka et al., Phys. Rev. Mater. 4, 081401(R) (2020).
10:55 AM - *NM03.02/NM04.07.02
Skyrmions and Chiral Spin Textures in Non-Centrosymmetric Magnetic Materials
The Ohio State University1Show Abstract
Magnetic domain structures are fundamentally altered by the presence of the Dzyaloshinskii-Moriya interaction (DMI), which introduces a preference for the twisting of neighboring spins. This produces various chiral spin textures including topological skyrmion bubbles, helical phases, and chiral domain walls. Two important directions for the field are to (1) explore and visualize the complex magnetic phases down to the atomic scale and (2) develop small skyrmions at room temperature for potential high-density memory applications. For this, we are investigating skyrmions and chiral spin textures in thin films of FeGe, MnGe, and Fe-rich FeGe grown by molecular beam epitaxy. These materials have a B20 structure with broken inversion symmetry to generate a bulk-like DMI, which yields skyrmions that are often studied by the topological Hall effect or other macroscopic probes. In our work, we visualize the spin textures from the submicron-scale down to the atomic-scale using spin-polarized scanning tunneling microscopy, magnetic force microscopy, and Lorentz transmission electron microscopy. I will discuss our team’s advances in atomic-layer engineering for FeGe to boost the Curie temperature above room temperature while shrinking the skyrmion size down to < 20 nm, the atomic-scale visualization of helical spin textures with topological defects in MnGe, and initial work toward skyrmions in epitaxial 2D magnets.
The work was done in collaboration with Tao Liu, Camelia Selcu, Jake Repicky, Jay Gupta, David McComb, Mohit Randeria, Prasanna Balachandran, Nuria Bagues Salguero, Binbin Wang, Po-Kuan Wu, Shuyu Cheng, Tim Hartnett, Denis Pelekhov, Perry Corbett, Brendan McCullian, Adam Ahmed, Chris Hammel.
11:20 AM - *NM03.02/NM04.07.03
Skyrmions and Topology in Magnetic Materials
Max Planck Institute Chemical Phyics of Solids1Show Abstract
Topology a mathematical concept became recently a hot topic in condensed matter physics and materials science, a complete classification of all non-magnetic inorganic materials are availbale [1,2]. Recently the focus have shifted to magnetic materials, first antiferromagnetic materials are classified, too . In magnetic materials the Berry curvature in real and reciprocal space lead to new topological properties such as Skyrmions  and Antiskyrmions  and in magnetic Weyl semimetals to giant responses in antiferromagnetic  and ferromagnetic compounds . Heusler compounds as tunable materials are interesting for both categories, they might even allow for the investigation the relation between Berry curvatures in real and reciprocal space. In materials hosting Antikyrmions the Dzyaloshinskii Moriya-Exchange and the dipol-dipol interaction is important. The dipol-dipol interactions allows for interesting mesoscopic effects. New materials can be designed by changing the symmetry, magnetic moments (ferro, ferri, antiferro, compensated ferri) and magnetic crystalline anisotropy .
 Bradlyn et al., Nature 547, 298, (2017)
 Vergniory, etal., Nature 566, 480 (2019)
 Xu, et al., Nature (2020) accepted, preprint arXiv:2003.00012
 Mühlbauer, et al. Science 323, 915 (2009)
 Nayak, et al., Nature 548, 561 (2017)
 Nayak, et al., Science Advances 2 e1501870 (2016)
 Liu, et al. Nature Physics 14, 1125 (2018)
 Manna, et al., Nature Reviews Materials 3, 244 (2018)
11:45 AM - *NM03.02/NM04.07.04
Anti-Skyrmions and Skyrmions in Heusler Compounds
Max Planck Institute of Microstructure Physics1Show Abstract
Heusler compounds are a large family of compounds which exhibit a wide range of properties. The inverse tetragonal Heusler compounds exhibit unidirectional magnetic anisotropy. Some of these compounds, in particular those containing heavy elements, exhibit non-collinear spin structures, that are derived from a Dzyaloshinskii-Moriya (DMI) vector exchange interaction. We show that such materials can exhibit a range of spin textures including both anti-skyrmions and elliptical Bloch skyrmions in the same compound, as well as helical spin textures that propagate along particular crystal directions. Another fascinating aspect of these structures is that the size of the anti-skyrmion and the wavelength of the helices can be varied from sub 100 nm to more than a micron by varying the thickness of the lamella in which the structures are observed. This is due to the important role of long-range magneto-dipole interactions that are more much important in these Heusler compounds than, for example, in the widely studied cubic B20 compounds in which skyrmions have been extensively explored. Chiral spin textures in ferro-, ferri- and anti-ferrimagnetic materials and thin film heterostructures are of fundamental interest with great potential for spintronic applications especially Racetrack Memory.
NM03.03: Layered Compounds + 2D Magnets
Thursday PM, April 22, 2021
1:00 PM - *NM03.03.01
Modulation Doping via the 2D Crystalline Acceptor RuCl3
Boston College1Show Abstract
Two-dimensional (2d) nano-electronics, plasmonics, and emergent phases require clean and local charge control, calling for layered, crystalline acceptors or donors. Here I will describe how the Relativistic Mott Insulating state of RuCl3 provides a new opportunity to introduce modulation doping into 2D materials. Specifically, we demonstrate and optimize this charge transfer with extensive Raman, photovoltage, and electrical conductance measurements combined with ab initio calculations. Also, we find the doping is exceptionally local, can occur through hBN, works with various exfoliated, CVD, and MBE materials. Time permitting, I will discuss new opportunities this opens for nanoplasmonic, optoelectronics, and correlated phases.
1:25 PM - *NM03.03.02
Exploring and Expanding the Compositional and Microstructural Space of Transition Metal-Based 3D and 2D Carbides and Nitrides
Christina Birkel1,2,Jan Paul Siebert1,Niels Kubitza2,Minh Hai Tran2,Andreas Reitz1,Robert Brilmayer2,Annette Andrieu-Brunsen2
Arizona State University1,Technische Universität Darmstadt2Show Abstract
Transition metal-based carbides that belong to the family of MAX phases are an intriguing class of materials – particularly in terms of their mechanical properties – as they combine characteristics of metals and ceramics. More recently, some of their members have also been discussed in terms of their magnetic behavior, whereas most studies are conducted on thin film samples and the synthesis of bulk materials with later transition metals (e.g. Mn) is still a real challenge. Besides, MAX phases are used as precursors for a relatively new class of 2D materials, the so-called MXenes that are investigated in the context of a plethora of potential applications, for example in catalysis, battery and biomedical research.
Our group utilizes a diverse set of synthesis techniques to prepare new MAX phases as well as known ones with unique morphologies. In this talk, I will focus on the following two examples: (i) The synthesis of MAX phase Cr2GaC by an initially wet chemical approach that allows a new type of processability of the soluble precursors. (ii) Design of a “smart” hybrid MXene with switchable conductivity and temperature as an external stimulus. We employ X-ray powder diffraction and electron microscopy techniques to study the structure and microstructure of the products, respectively. In the case of the wet chemical-based synthesis of Cr2GaC, we conducted detailed thermal analysis and ex-situ Neutron diffraction to propose a formation mechanism of the MAX phase particles.
1:50 PM - *NM03.03.03
Near-Fermi-Level Electronic States in Hexagonal ABC Intermetallic Compounds from First Principles
Rutgers, The State University of New Jersey1Show Abstract
Ternary ABC intermetallic compounds exhibit a rich variety of crystal structures and electronic properties. In this work, we study the structural energetics and band structures of real and hypothetical ABC intermetallic phases with structures obtained by stacking binary honeycomb layers with single layers of interstitial atoms in various ways, using first principles calculations to determine the structural parameters and the bands in each phase. We use this dataset to analyze and model the bands near the Fermi level to classify the systems considered and to extract a set of rules that allows us to to predict and design hexagonal ABC intermetallic materials with targeted transport and optical properties, connecting to experimental measurements on known hexagonal ABC phases. in particular, we investigate the rich physics of polar metals in this family of materials, which offer the promise of functional properties switchable by appropriate applied fields and stresses.
2:15 PM - NM03.03.04
Berry Curvature and Topological Nernst Effect in Biased Bilayer WSe2
Vassilios Vargiamidis1,Panagiotis Vasilopoulos2,Neophytos Neophytou1
University of Warwick1,Concordia University2Show Abstract
We investigate the anomalous thermoelectric transport in bilayer WSe2 with broken inversion symmetry, due to a gate electric field, regardless of time-reversal symmetry. We compare the cases in which the spin-orbit coupling (SOC) is absent or present. In the presence of SOC and of a valley-contrasting Berry curvature, anomalous spin and valley Nernst responses are generated. The Nernst signals exhibit peaks and dips, as the chemical potential is varied, that have the signs of the Berry curvatures of the bands and are proportional to their magnitudes. In the absence of SOC but with an electric field present, the Nernst responses are the same in the conduction and valence bands due to particle-hole symmetry. The anomalous valley Nernst coefficient is enhanced by increasing the electric field strength. When time-reversal symmetry is violated, e.g., upon using an insulating magnetic substrate, the total Nernst coefficient is finite and exhibits a dip-peak feature. We also analyze the orbital magnetization and the orbital magnetic moment. In the absence of a gate electric field the magnetization vanishes due to the spin degeneracy of the bands. In the presence of electric field, the magnetization and its two contributions, one due to the magnetic moment and one due to the Berry curvature, are calculated and interpreted in terms of opposite circulating currents of the bands in the two layers. The results are pertinent to other transition metal dichalcogenides and future caloritronic applications.
2:30 PM - NM03.03.05
Late News: Structural Defects and Low Temperature Twisting of Kagomé Layers in Intermetallics
Mekhola Sinha1,Hector Vivanco1,Cheng Wan1,Maxime Siegler1,Veronica Stewart1,Lucas Pressley1,Tanya Berry1,Ziqian Wang1,Isaac Johnson1,Mingwei Chen1,2,Thao Tran1,3,W. Adam Phelan1,4,Tyrel McQueen1
The Johns Hopkins University1,Tohoku University2,Clemson University3,Los Alamos National Laboratory4Show Abstract
Kagomé intermetallics with 2D lattices have served as an ideal platform to study the exotic quantum phenomenon associated with flat bands and Dirac-type dispersions. It is essential to explore the electronic properties of these structures for the experimental realization of 2D kagomé lattice in bulk materials. We explore a specific family of kagomé intermetallics with the formula MT6X6 compound where M = Mg, Lu, Y; T = Co, Fe, Cr; and X = Ge. Single crystals of MgCo6Ge6 were grown by laser Bridgman technique. X-ray precession images and electron diffraction measurements collected at T = 293(2) K show that the compound crystallizes in space group P6/mmm, with a = 5.06094(15) Å, c = 7.7271(2) Å. Residual electron maps provide evidence of columnar disorder along c axis while no defects were found in the electronic structures of flux-grown LuFe6Ge6 and YCr6Ge6 crystals. Further investigation of the crystal structure reveals spontaneous twisting of the Co kagomé layers in MgCo6Ge6. We look into crystal orbital hamiltonian population analysis to understand the cooperative twisting between layers within the Co-kagomé network and the interlayer tetragonal bonding. Despite the appearance of static tilting, no evidence of a phase transition was found in magnetization, resistivity, or specific heat measurements, implying that the twisting exists at all temperatures, but is thermally fluctuating at room temperature. This behavior is further supported by looking into related materials to understand the pairwise twisting and bonding in layered structures.
This work was supported by the David and Lucile Packard Foundation. HKV, VJS and TTT acknowledge support of the Institute for Quantum Matter, an Energy Frontier Research Center funded by the United States Department of Energy, Office of Science, Office of Basic Energy Sciences, under Award DE-SC0019331. MKS, LAP, TB, and WAP acknowledge support of the Platform for the Accelerated Realization, Analysis, and Discovery of Interface Materials (DMR-1539918), a National Science Foundation Materials Innovation Platform. ZW, IJ and MC acknowledge support of Whiting School of Engineering, the Johns Hopkins University, and the NSF (NSF NSF-DMR-1804320). CW acknowledges the support of Hopkins Extreme Materials Institute (HEMI). Access to the Bruker 1172 instrument was also possible via the Hopkins Extreme Materials Institute (HEMI).
2:45 PM - NM03.03.06
Late News: Temperature and Composition Evolution of Charge Density Wave and Superconducting Orders in Ta-Based Dichalcogenides by Total X-Ray Scattering
Central Michigan University1Show Abstract
A characteristic feature of quantum materials is the presence of various lattice degrees of freedom manifesting themselves as local structural distortions leading to competing ground state phases of the electronic system and exotic behavior. More often than not, the distortions are not well expressed and/or perfectly periodic, making it difficult to identify and quantify them using traditional crystallographic techniques. We will demonstrate the advantages of total x-ray scattering and large-scale structure modeling in studying lattice instabilities in archetypal quantum materials such as Ta-based dichalcogenides. In particular, we will show that the low-temperature charge density wave state in trigonal prismatic 2H-TaSe2 emerges via a gradual buildup of locally correlated clusters of Ta atoms, and not via a spontaneous emerging of Ta superstructure at the transition temperature . We will also show the presence of a hierarchical relationship among the crystal lattice, charge density wave and superconducting orders in ternary Ta-Te-Se solid solutions, where different degrees of crystal lattice order-disorder appear to promote and maintain the different orders to a different extent. The relationship may well explain the observed irregular evolution of the superconducting transition temperature with the relative Te to Se ratio .
1. V. Petkov et al. Phys. Rev. B 101, 121114(R) (2020).
2. V. Petkov et al. Phys. Rev. B 102, 134119 (2020).
NM03.04: Topological Materials I
Thursday PM, April 22, 2021
4:00 PM - *NM03.04.02
Frontiers of Crystal Growth and Characterization of Topological and Quantum Intermetallics
Johns Hopkins University1Show Abstract
Materials by design is the rational prediction and creation of functional materials with defined properties. Its goal is to meet current and future societal needs for better or more complex materials, from biocompatible materials in medicine to lightweight alloys for space applications and energy generation, storage, and transport. Unfortunately the chemistry underlying modern materials science and engineering has lagged other sub-fields in an extremely critical area: the ability to selectively make and break bonds in the solid state. This is due to limited synthetic methodology and method development. True materials by design cannot be achieved until reliable synthetic capabilities are developed that can actually produce the specified materials. In this talk, I will highlight the progress being made in such synthesis by design, with a particular focus on intermetallic quantum materials necessary to realize new electronic and magnetic states of matter including topological superconductivity and axion insulators. Efforts to build and maintain US leadership in the area of new materials discovery and synthesis – particularly via the JHU Institute for Quantum Matter and PARADIM, the Platform for Accelerated Realization, Analysis, and Discovery of Interface Materials (PARADIM), an NSF-MIP National User Facility, and opportunities for use of these capabilities by the community, will also be presented.
4:25 PM - *NM03.04.03
Weyl Nodes and Magnetostructural Instability
Ram Seshadri1,Samuel Teicher1
University of California, Santa Barbara1Show Abstract
The room temperature ferromagnetic phase of the cubic antiperovskite Mn3ZnC is known from from first-principles calculation to be a nodal line Weyl semimetal. Features in the electronic structure that are the hallmark of a nodal line Weyl state—a large density of linear band crossings near the Fermi level—can also be interpreted as signatures of a structural and/or magnetic instability. We examine this system carefully, and use it as a jumping off point to understand the possible implications of Weyl-like features in the electronic structures, and Peierls-like structural distortions and related structural and magnetic instabilities.
4:50 PM - NM03.04.06
Exciting Features of Electronic Band Dispersion of IrGa & RhGa Compounds from First-Principles
Joshua Steier1,2,David Gordon3,JeanPierre Alvarez3,Kalani Hettiarachchilage1,Neel Haldolaarachchige3
Seton Hall University1,Stony Brook University, The State University of New York2,Bergen Community College3Show Abstract
Dirac materials have recently been one of the most significant attractions of the scientific community. We present an ab initio study of electronics properties of IrGa and RhGa compounds, with the absence and presence of spin-orbit interaction using first-principles calculations. Linearly dispersed band crossings, which are characteristic of topological semimetals, were identified near Fermi energy. These include type I and II Dirac points and nodal lines. Additionally, the sensitivity of Dirac points to physical pressure was studied by applying compressive and tensile stress to the lattice axis. The unusual electronic structure of IrGa will be useful to search for novel Dirac Fermions and it is a good candidate for further research and experimental studies.
NM03.05: Novel Superconductivity and Magnetism I
Friday AM, April 23, 2021
8:15 PM - *NM03.05.01
Insulator-Metal Transition, Topological Superconductivity and Parity Violation in UTe2
Youichi Yanase1,Jun Ishizuka1,Shuntaro Sumita2,Akito Daido1
Kyoto University1,RIKEN2Show Abstract
We theoretically study magnetism and superconductivity in UTe2, which is a recently discovered strong candidate for an odd-parity spin-triplet superconductor. Theoretical studies for this compound faced difficulty because first-principles calculations predict an insulating electronic state, incompatible with superconducting instability. To overcome this problem, we take into account electron correlation effects by a GGA+U method and show the insulator-metal transition by Coulomb interaction. Using Fermi surfaces obtained as a function of U, we clarify the topological properties of possible superconducting states. Fermi surface formulas for the three-dimensional winding number and three two-dimensional Z2 numbers indicate topological superconductivity at an intermediate U for all the odd-parity pairing symmetry in the Immm space group. Symmetry and topology of superconducting gap nodes are analyzed, and the gap structure of UTe2 is predicted. Topologically protected low-energy excitations are highlighted, and experiments by bulk and surface probes are proposed to link Fermi surfaces and pairing symmetry. We show that a recent ARPES experiment is consistent with the topological superconductivity.
Next, we provide and analyze a periodic Anderson model for UTe2. The 24-band tight-binding model reproduces the band structure obtained from a GGA+U calculation consistent with an ARPES experiment. The Coulomb interaction of f-electrons enhances Ising ferromagnetic fluctuation along the a-axis and stabilizes the spin-triplet superconductivity of either B3u or Au symmetry. When effects of pressure are taken into account in hopping integrals, the magnetic fluctuation changes to an antiferromagnetic one, and accordingly, the spin-singlet superconductivity of Ag symmetry is stabilized. Based on the results, we propose pressure-temperature and magnetic field-temperature phase diagrams revealing multiple superconducting phases. Interestingly, a mixed-parity superconducting state with spontaneous parity violation is predicted.
8:40 PM - *NM03.05.02
Local Magnetic Measurements of Unconventional Superconductors
Cornell University1Show Abstract
A fundamental property of a superconductor is its response to an applied magnetic field. In this talk, I will discuss how we use scanning superconducting quantum interferences device (SQUID) microscopy to study the local magnetic response in two different types of superconductors. First, I will discuss measurements on focus ion beam defined microstructures fabricated from single crystals of the heavy-fermion superconductor CeIrIn5. By imaging the local diamagnetic response, we observe that the superconducting transition temperature, Tc, varies throughout the structure in a complex pattern. This pattern arises due to the interplay of a non-trivial strain field from the differential thermal contraction of the substrate and microstructure and the sensitivity of Tc in CeIrIn5 to the strength and direction of strain. Devices with different geometry show that the spatial modulation of Tc can be tailored in agreement with predictions based on finite element simulations. These results offer a new approach to manipulate strain-sensitive electronic order on micrometer length scales in strongly correlated matter. Second, I will show how we use scanning SQUID to perform local magnetic measurements on few-layer van der Waals superconductors. We can directly probe the diamagnetic response as a function of temperature and other tuning parameters despite the extremely small sample volume. I will discuss how we can extract the superfluid stiffness and other characteristics of the superconducting state from our measurements.
Imaging of CeIrIn5 microstructures was primarily supported by the Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering, under Award DE-SC0015947. Measurements of van der Waals superconductors were supported by the Cornell Center for Materials Research with funding from the NSF MRSEC program (DMR-1719875) and the NSF (DMR-2004864).
9:05 PM - *NM03.05.03
Topological and High-Field Reentrant Superconductivity in UTe2
NIST Center for Neutron Research1,University of Maryland2Show Abstract
Spin triplet superconductivity is found in UTe2 below 1.6 K. Remarkably, it coexists with strong spin fluctuations, has an extremely high upper critical field of 35 T, and breaks time reversal symmetry. I will describe how superconductivity in UTe2 differs from conventional superconductivity and I will present evidence that this phase is topologically nontrivial. I will also discuss how in UTe2 large magnetic fields give rise to a new, reentrant, superconducting phase, between 40 T and 65 T, the highest values reported for any material.
9:30 PM - NM03.05.04
Scanning SQUID Microscopy of the Quantum Anomalous Hall Effect
George Ferguson1,Run Xiao2,David Low1,Ling-Jie Zhou2,Anthony Richardella2,Cui-Zu Chang2,Nitin Samarth2,Katja Nowack1
Cornell University1,The Pennsylvania State University2Show Abstract
We report magnetic imaging of Cr-doped (Bi,Sb)2Te3 heterostructures in the quantum anomalous hall regime. We used a scanning superconducting quantum interference device (SQUID) microscope with micrometer scale spatial resolution to image the magnetic fields above current biased devices. Using these images we reconstructed the current density, allowing us to visualize where current flows in the devices. We also used top and back gates to study how the magnetization is affected by electrostatic gating. By performing these measurements as a function of current bias, gate voltage and magnetization direction we construct a comprehensive picture of electronic transport in our devices.
9:45 PM - NM03.05.06
Late News: Quantum Magnetism in EuPd3S4
Tanya Berry1,Vincent Morano1,Micheal Nickolas2,Xin (Jason) Zhang1,Qun Yang2,Topias Foerster3,Zhijun Zhu4,Rafal Wawrzynczak2,Tom Halloran1,Walter Schnelle2,Johannes Gooth2,Jeffrey Lynn4,Claudia Felser2,Collin Broholm1,Tyrel McQueen1
Johns Hopkins University1,Max Planck Institute for chemical Physics of Solids2,HZDR – Helmholtz-Zentrum Dresden-Rossendorf3,National Institute of Standards and Technology4Show Abstract
Double Dirac materials are a topological phase of matter that have an unprecedented eightfold electronic degeneracy. They display a host of exotic charge transport properties that are strongly impacted by magnetic fields. Quantum magnets are known to exhibit a variety of magnetic phases of matter with emergent quasiparticles. Here we report the discovery of quantum magnetism in the Dirac material EuPd3S4. The zero-field magnetic structure is found to be a 2-sublattice antiferromagnetic state with no magnetic coupling between sublattices at the mean-field level that results in an emergent degree of freedom corresponding to the relative orientation of the sublattice magnetizations. Application of a magnetic field drives domain ordering before collapsing to a fully ferromagnetic state. These magnetic states are found to dramatically impact the electronic structure of EuPd3S4, resulting in the destruction of the 8-fold degenerate state and formation of mostly likely Weyl states. Our findings open the door to the experimental and theoretical explanation of the interplay between novel magnetic states and topological phases of matter.
Jason Kawasaki, University of Wisconsin
Elena Hassinger, Technische Universität München
Anderson Janotti, University of Delaware
National Science Foundation
NM03.06: Novel Superconductivity and Magnetism II
Friday AM, April 23, 2021
8:00 AM - *NM03.06.01
Even and Odd Parity Superconductivity in CeRh2As2
Javier Landaeta1,Seunghyun Khim1,Jacintha Landaeta1,Nyantakyi Bannor1,Manuel Brando1,Philip Brydon2,Daniel Hafner1,Robert Küchler1,Raul Cardoso1,Ukrike Stockert1,Andrew Mackenzie1,3,Daniel Agterberg4,Christoph Geibel1,Elena Hassinger1,5
Max Planck Institute for Chemical Physics of Solids1,University of Otago2,University of St Andrews3,University of Wisconsin–Milwaukee4,Technical University Munich5Show Abstract
Multiple new forms of unconventional superconductivity have been discovered over the past four decades. In most cases, the strong electronic correlations and the interplay between superconductivity and other phases (magnetic ones, among others) leads to understanding that the simple frame of electron-phonon superconductivity breaks down. Because this implies that complex phase diagrams with different superconducting states are expected, it is striking that many materials only show a single-component superconducting phase diagram. Here, we report the discovery of two-phase unconventional superconductivity in CeRh2As2 with a transition temperature of 0.26 K. Using thermodynamic and magnetic probes, we establish that the superconducting critical field is as high as 14 T for magnetic fields along the c-axis. Furthermore, a c-axis field drives a transition between two different superconducting states. In spite of the fact that CeRh2As2 is globally centrosymmetric, we show that local inversion-symmetry breaking at the Ce sites enables Rashba spin-orbit coupling to play a key role in the underlying physics. More detailed analysis identifies the transition from the low- to high-field states to be associated with one between states of even and odd parity.
8:25 AM - NM03.06.02
Late News: Epitaxy, Exfoliation and Strain-Induced Magnetism in Rippled Heusler Membranes
Dongxue Du1,Sebastian Manzo1,Vivek Saraswat1,Konrad Genser2,Karin Rabe2,Paul Voyles1,Michael Arnold1,Jason Kawasaki1
University of Wisconsin1,Rutgers, The State University of New Jersey2Show Abstract
Single-crystalline membranes of functional materials enable the tuning of properties via extreme strain states; however, conventional routes for producing membranes require the use of sacrificial layers and chemical etchants, which can both damage and limit the ability to make membranes ultrathin. Here we demonstrate the epitaxial growth of the cubic Heusler compound GdPtSb on graphene-terminated Al2O3 substrates. The weak Van der Waals interactions of graphene enable the mechanical exfoliation to yield free-standing GdPtSb membranes. Despite the presence of the graphene interlayer, the Heusler films have epitaxial registry to the underlying sapphire, as revealed by x-ray diffraction, reflection high energy electron diffraction, and transmission electron microscopy. Whereas unstrained GdPtSb is antiferromagnetic, we show that the large strains or strain gradients in rippled membranes induce a spontaneous magnetic moment at room temperature, with a saturation magnetization of 5.2 bohr magneton per Gd atom, approaching the ~ 7 bohr magneton limit expected for ferromagnetic ordering. Our membranes provide a novel platform for tuning the magnetic properties of intermetallic compounds via strain (piezomagnetixm and magnetostriction) and strain gradients (flexomagnetism).
8:40 AM - NM03.06.04
Carrier Conduction and Magnetic Interactions in a Nanostructured Epitaxial Ferromagnet/Semiconductor Fe: FeVSb
Estiaque Haidar Shourov1,Chenyu Zhang1,Paul Voyles1,Jason Kawasaki1
University of Wisconsin–Madison1Show Abstract
Magnetic semiconductors are attractive for memory devices and spintronic applications . However, the practical utility of dilute magnetic semiconductors (DMS) such as Mn: GaAs, albeit having all key ingredients for these applications, are hindered due to low Curie temperature. On the other hand, narrow bandgap semiconductors often make excellent thermoelectric candidates. A conventional strategy for improving the thermoelectric figure of merit (ZT) is to decrease the phonon thermal conductivity via nano-structuring . Recently, it has been suggested that magnon drag (spin wave) , spin fluctuations , and enhanced effective mass  are alternative strategies to enhance thermoelectric performance by enhancing the power factor. Here we demonstrate the controllable synthesis of a ferromagnet/semiconductor nanostructured system: Fe nanoparticles embedded epitaxially within a semiconducting FeVSb matrix. Transmission electron microscopy confirms that the nanoparticle formation is mediated by bulk segregation for all composition greater than 10% excess Fe in stoichiometric FeVSb. Our Fe: FeVSb thin films, grown by molecular beam epitaxy, display magnetic moment and anomalous Hall effect that scale with the volume fraction of Fe nanoparticles. The parent stoichiometric FeVSb is predicted to be diamagnetic, but our stoichiometric films also exhibits magnetic signatures in transport and magnetometry at 300K. This suggests that within the solubility limit of Fe, Fe1+δVSb, where δ is the bound of solubility, is a potential dilute magnetic semiconductor with Curie temperature greater than room temperature. Additionally, the ferromagnetic nanoparticles can induce spin polarized charge carriers in the semiconducting matrix due to proximity effect, making this highly controllable and tunable system appealing for spintronics. Our angle-resolved photoemission spectroscopy (ARPES) measurements on the parent semiconductor FeVSb reveal an enhanced effective mass due to electronic correlations . The combination of nanostructure precipitates, magnetic interaction, and electronic correlation in this highly tunable heterostructure presented here offers a promising route for new thermoelectric application for practical waste heat recovery.
 Y. Ohno, D. K. Young, B. Beschoten, F. Matsukura, H. Ohno and D. D. Awschalom, Nature (London) 402, 790 (1999).
 H. Scherrer and S. Scherrer, in Handbook of Thermoelectrics, edited by D. M. Rowe (CRC Press, New York, 1994), pp. 211– 237.
 M. V. Costache, G. Bridoux, I. Neumann and S. O. Valenzuela, Magnon-drag thermopile, Nat. Mater. 11 (2012).
 N. Tsujii, A. Nishide, J. Hayakawa, and T. Mori, Science advances 5, eaat5935 (2019).
 Y. Pei, X. Shi, A. LaLonde, H. Wang, L. Chen, and G. J. Snyder, Nature 473, 66 (2011).
 E. H. Shourov et al, arXiv preprint, arXiv:2009.11489 (2020).
8:55 AM - NM03.06.05
Complex Magnetic Phases in Polar Tetragonal Intermetallic NdCoGe3
Binod Rai1,2,Ganesh Pokharel3,1,Hasitha Suriya Arachchige3,1,Seung-Hwan Do1,Qiang Zhang1,Masaaki Matsuda1,Matthias Frontzek1,Vasile Garlea1,Andrew Christianson1,Andrew May1
Oak Ridge National Laboratory1,Savannah River National Laboratory2,The University of Tennessee, Knoxville3Show Abstract
Polar materials can host a variety of topologically significant magnetic phases, which often emerge from a modulated magnetic ground state. Relatively few noncentrosymmetric tetragonal materials have been shown to host topological spin textures and new candidate materials are necessary to expand the current theoretical models. This presentation will discuss the anisotropic magnetism in the polar, tetragonal material NdCoGe3 via thermodynamic and neutron diffraction measurements. The H-T phase diagram shows several magnetic field-induced phases with an applied field in the plane. Neutron diffraction data reveal that NdCoGe3 hosts complicated magnetic order derived from modulated magnetic moments, with the ground state characterized by the propagation vector k = (0.494, 0.0044, 0.385) at 1.8 K.
9:10 AM - *NM03.06.06
Topology Enabled Unconventional Superconductivity in a Time-Reversal Symmetry-Breaking Bulk Superconductor
Valentin Taufour1,Jackson Badger1,Yundi Quan1,Matthew Staab1,Antonio Rossi1,Kasey Devlin1,Peter Klavins1,Susan Kauzlarich1,Inna Vishik1,Warren Pickett1
University of California, Davis1Show Abstract
In recent years, much efforts have been made towards the discovery of topological superconductors. Topological superconductivity can be artificially engineered in hybrid structures combining topological materials with conventional superconductors, or it can exist intrinsically in certain unconventional superconductors. Odd-parity superconductors are candidates for intrinsic topological superconductivity because topologically non-trivial gap functions naturally show up in these materials. These superconductors can be discovered in the proximity of magnetic instabilities such as in the superconducting ferromagnets, or in structural families that lack inversion symmetry. In this presentation, I will present our experimental and computational results on a new superconductor that breaks-time reversal symmetry because of its intrinsic topological properties. The normal state band structure includes Dirac lines and a Dirac loop at the Fermi level resulting from non-symmorphic symmetry operations, as well as Dirac points robust against splitting from SOC. The topology of the Fermi surface provides a platform for inter-band pairing and time-reversal symmetry breaking superconductivity. Our results illustrate a new route to realize spin-triplet superconductivity in a centro-symmetric structure without the need for a nearby magnetic instability.
9:35 AM - NM03.06.07
Pressure-induced suppression of axionic charge density wave and onset of superconductivity in the chiral Weyl semimetal Ta2Se8I
Qingge Mu1,Dennis Nenno2,Yanpeng Qi3,Fengren Fan1,Cuiying Pei3,Moaz ElGhazali1,Johannes Gooth1,Claudia Felser1,Prineha Narang2,Sergey Medvediev1
Max Planck Institute for Chemical Physics of Solids1,Harvard University2,ShanghaiTech University3Show Abstract
The Weyl points with opposite chiralities can be coupled by charge density wave (CDW) resulting in axion quasiparticle in condensed matter physics 1, 2. Recently, the Weyl semimetal Ta2Se8I was reported to exhibit axions in the collective mode of CDW, the sliding CDW which is detected with nonlinear V-I curves 3. Here we investigate the electrical transport property and crystal structure under pressure. Upon applying pressure, the axionic CDW is suppressed, which is manifested by the decrease of transition temperature and corresponding single particle energy gap. A superconducting transition appears at the vicinity of the complete suppression of the CDW. The superconducting Tc is enhanced monotonically to 4.5 K with pressure up to 47 GPa. No main structural transition is detected from Raman spectra and synchrotron XRD patterns. However, partial amorphization is observed in which the stretching mode of Se-Se survives, and Ta2Se8I evolves into superconducting state at low temperature indicating that the TaSe4 chains may play an important role in the pressure-induced superconductivity. Our investigations construct a complete phase diagram depicting the evolution of anionic CDW and superconductivity with respect to pressure, giving further understanding of correlated topological states.
1. Li R, Wang J, Qi X L, Zhang S C. Dynamical axion field in topological magnetic insulators. Nature Physics 6, 284-288 (2010).
2. Wang Z, Zhang S C. Chiral anomaly, charge density waves, and axion strings from Weyl semimetals. Physical Review B 87, (2013).
3. Gooth J, et al. Axionic charge-density wave in the Weyl semimetal (TaSe4)2I. Nature, (2019).
NM03.07: Topological Materials II
Friday PM, April 23, 2021
11:45 AM - *NM03.07.01
Tunable Quantum Anomalous Hall Effect by the Exchange Coupling Between vdW Magnetic Layers
Binghai Yan1,Huixia Fu1,Jiewen Xiao1,Chaoxing Liu2
Weizmann Institute of Science1,The Pennsylvania State University2Show Abstract
The layered antiferromagnetic MnBi2Te4 films have been proposed to be an intrinsic quantum anomalous Hall (QAH) insulator with a large gap. It is crucial to open a magnetic gap of surface states. However, recent experiments have observed gapless surface states, indicating the absence of out-of-plane surface magnetism, and thus, the quantized Hall resistance can only be achieved at the magnetic field above 6 T. We propose to induce out-of-plane surface magnetism of MnBi2Te4 films via the magnetic proximity with magnetic insulator CrI3. A strong exchange bias of ∼40 meV originates from the long Cr-eg orbital tails that hybridize strongly with Te p orbitals. By stabilizing surface magnetism, the QAH effect can be realized in the MnBi2Te4/CrI3 heterostructure. Moreover, the high–Chern number QAH state can be achieved by controlling external electric gates. In addition, we have derived a simple electron-counting rule to predict the magnetic exchange coupling between general vdW layers for both homo- and hetero-vdW junctions.
12:10 PM - *NM03.07.02
Chern Numbers and Nodal Points in Topological Semi Metals
Donostia International Physics Center1Show Abstract
Nonmagnetic topological materials have dominated the landscape of topological physics for the past two decades. These breakthroughs in nonmagnetic materials have not yet been matched by similar advances in magnetic compounds. Using magnetic band theory and topological indices obtained from Magnetic Topological Quantum Chemistry (MTQC), I will present a systematic way of identifying magnetic topological materials. I will then, focus on high order magnetic semimetals, and provide a topological classification of different fermions in these phases. Finally I will present new experimental realizations in materials. In particular I will focus on the pyrite compound CoS2, using complementary bulk- and surface-sensitive angleresolved photoelectron spectroscopy and ab-initio calculations we discovered Weyl-cones at the Fermi-level and we directly observed the topological Fermi-arc surface states that link the Weyl-nodes, which will influence the performance of CoS2 as a spin-injector by modifying its spin-polarization at interfaces.
12:35 PM - *NM03.07.03
Seeing the Topological Ground States Through Infrared Spectroscopy
University of Fribourg1Show Abstract
Nowadays we know of many gapless electronic phases with conical bands, such as graphene, Dirac semimetals, and Weyl semimetals. Their low-energy excitations resemble truly relativistic particles.
To see those excitations, we have to capture the physics at a milli-electron-volt scale.
In our experiments, we access electronic structures at these low energies by combining Landau level spectroscopy, infrared spectroscopy, and effective Hamiltonian models.
1:00 PM - NM03.07.04
Topological Engineering Through Nonlinear Phononics
Dominik Juraschek1,Nesta Joseph2,Prineha Narang1,Awadhesh Narayan2
Harvard University1,Indian Institute of Science2Show Abstract
Topological properties in quantum materials promise applications in lossless electronics and quantum information processing. Recently, dynamical control of the topological phase of the Weyl semimetal WTe2 has been demonstrated, in which a terahertz-field induced charge current leads to interlayer shear strain that dynamically changes the Weyl nodes of the system . Here, we theoretically describe an alternative route for topological phase control that is based on a nonlinear phonon coupling. The interlayer shear and breathing modes of layered topological materials couple to infrared-active phonon modes that can be resonantly excited with ultrashort terahertz pulses. The nonlinear coupling leads a transient distortion along the coordinate of the shear and breathing modes that in turn induce changes in the topological phase of the material. Using a combination of first-principles calculations and phenomenological modeling, we demonstrate that the Dirac and Weyl nodes in WTe2 and ZrTe5 can be modulated through this excitation. Our results suggest that nonlinear phononic rectification of the crystal lattice is a powerful tool for dynamical control of topology in layered quantum materials.
 E. J. Sie et al., Nature 565, 61 (2019)
 D. M. Juraschek, N. B. Joseph, P. Narang, and A. Narayan, in preparation
This work is supported by the Swiss National Science Foundation (SNSF) under Project No. 184259, the DARPA DSO under the Driven Nonequilibrium Quantum Systems (DRINQS) program, Grant No. D18AC00014, and by the Department of Energy ‘Photonics at Thermodynamic Limits’ Energy Frontier Research Center under Grant No. DE-SC0019140. This research used resources of the National Energy Research Scientific Computing Center (NERSC) under Contract No. DE-AC02-05CH11231.
NM03.08: Topological Materials III
Friday PM, April 23, 2021
2:15 PM - NM03.08.01
Late News: Filling Anomaly for General 2D and 3D $C_4$ Symmetric Lattices
Yuan Fang1,Jennifer Cano1
Stony Brook University, The State University of New York1Show Abstract
We derive symmetry indicator formulas for the filling anomaly on 2D square lattices with and without time reversal, inversion symmetry, or their product, in the presence of spin-orbit coupling. We go beyond previous work by considering lattices with atoms occupying multiple Wyckoff positions. We also provide an algorithm using the Smith normal form that systematizes the derivation.
The formulas determine the corner charge in 2D atomic or fragile topological insulators, as well as in 3D insulators and semimetals by studying their 2D slices. We apply our results to a 3D tight-binding model on a body-centered tetragonal lattice, whose projection into the 2D plane has two atoms in the unit cell. We apply these results to several antiperovskites as material examples.
2:30 PM - *NM03.08.02
Topological Magnets in 2D and 3D
M. Zahid Hasan1
Princeton University1Show Abstract
Electrons organize in ways to give rise to distinct phases of matter such as insulators, metals, magnets or superconductors. In the last ten years or so, it has become increasingly clear that in addition to the symmetry-based classification of matter, topological consideration of wavefunctions plays a key role in determining distinct or new quantum phases of matter [see, for an introduction, Hasan & Kane, Reviews of Modern Physics 82, 3045 (2010)].
In this talk, I briefly introduce these new topological concepts in the context of their experimental realizations in magnetic materials. As examples, I present how tuning a topological insulator whose surface hosts an unpaired Dirac fermion can give rise to Weyl fermion (massless charged fermion) in nonmagnetic and magnetic semimetals with “fractional” Fermi surfaces, and in strongly correlated magnets such as the Heusler, kagome and related materials. These exotic topological matter harbor novel and unprecedented properties that may lead to the development of next generation quantum technologies including novel qubits.
2:55 PM - *NM03.08.03
New Insights into Topological Semimetals
Stony Brook University, The State University of New York1,Flatiron Institute2Show Abstract
The field of topological semimetals continues to reveal new insights. I will discuss recent developments starting with the classification of nodal fermions in both magnetic and non-magnetic space groups. I will then introduce higher order Fermi arcs as a bulk-edge correspondence for Dirac fermions, and discuss a refinement of the symmetry indicators that predict these hinge states. Finally, I will discuss some material predictions.
NM03.09: Topological Films
Friday PM, April 23, 2021
5:15 PM - *NM03.09.01
Topological States of Cadmium Arsenide Films Grown by Molecular Beam Epitaxy
University of California, Santa Barbara1Show Abstract
Interfaces and heterostructures with topological semimetals offer new opportunities to control and manipulate their unique electronic states and a wealth of associated phenomena, for example, via electric field effect, strain, or symmetry breaking. In this presentation, we will discuss recent progress in heterostructures of a prototype three-dimensional Dirac semimetal, cadmium arsenide (Cd3As2), which are grown by molecular beam epitaxy. We show that high-mobility, epitaxial Cd3As2 films can be grown in different crystallographic orientations. We discuss the nature of the quantum Hall effect of thin Cd3As2 films grown in different orientations and using different measurement geometries that uniquely identify the quantum Hall effect from single Dirac fermions on the surfaces. We will discuss other remarkable phenomena in the quantum limit of (001) films. We will also discuss pathway towards realizing novel topological systems.
5:40 PM - *NM03.09.02
Quantized Transport on Topological Semimetal Fermi Arcs
The University of Tokyo1Show Abstract
Cd3As2 is an ideal system for investigating quantum transport in topological semimetals. In addition to its high electron mobility and long mean free path, its natural growth orientation is different from the rotational axis connecting the two Dirac nodes. This allows us to detect orbital motions in topological semimetal surfaces. We have successfully developed a growth technique realizing high mobility Cd3As2 films with excellent surface flatness, and first observed quantum Hall states induced by quantum confinement . Related film techniques such as electric gating and chemical doping of Zn also enable systematic transport studies of the Dirac semimetal films, with controlling the bulk dimensionality [1, 2], Fermi energy [3, 4], and band topology [3, 4].
By carefully fabricating (Cd1-xZnx)3As2 films with three-dimensional and uniform thickness, we have observed surface quantum oscillations and their evolution into quantized states in films [5, 6]. This is evidenced by distinct differences in oscillation frequency, field angle dependence, and temperature change from the bulk ones. Moreover, we have found intrinsic coupling between two spatially-separated surface states in Weyl orbits by measuring dual-gate devices . Independent scans of top- and back-gate voltages reveal concomitant modulation of doubly-degenerate quantum Hall states, which is not possible in conventional surface orbits as in topological insulators. Our results evidencing the unique spatial distribution of Weyl orbits provide new opportunities for controlling the novel quantized transport. In particular, fabrication of heterointerfaces for proximitizing the surface Fermi-arcs with ferromagnets  and superconductors will be promising, as well as another chemical doping of Sb for increasing band-inversion energy .
 M. Uchida et al., Nat. Commun. 8, 2274 (2017).
 Y. Nakazawa, M. Uchida et al., Sci. Rep. 8, 2244 (2018).
 S. Nishihaya, M. Uchida et al., Sci. Adv. 4, eaar5668 (2018).
 S. Nishihaya, M. Uchida et al., Phys. Rev. B 97, 245103 (2018).
 S. Nishihaya, M. Uchida et al., Nat. Commun. 10, 2564 (2019).
 Y. Nakazawa, M. Uchida et al., APL Mater. 7, 071109 (2019).
 S. Nishihaya, M. Uchida et al., submitted.
 M. Uchida et al., Phys. Rev. B 100, 245148 (2019).
 Y. Nakazawa, M. Uchida et al., Phys. Rev. B 103, 045109 (2021).
6:05 PM - *NM03.09.03
Investigations of Topological and Quantum Phenomena in Heusler and Rare-Earth Monopnictide Epitaxial Thin Films
University of California, Santa Barbara1Show Abstract
Controlling electronic properties via bandstructure engineering is at the heart of modern semiconductor devices. We have extended this concept to semimetals utilizing confined thin film geometries and hetero-epitaxial interfaces to engineer electronic structure in rare-earth monopnictide and half-Heusler semimetallic systems. In the case of the rare-earth monopnictide, LuSb, quantum confinement changes the carrier compensation and differentially affects the mobility of the electron and hole-like carriers resulting in a strong modification in its large, non-saturating magnetoresistance behavior. Bonding mismatch at the heteroepitaxial interface of the semi-metal (LuSb) and a semiconductor (GaSb) leads to the emergence of a novel, two-dimensional, interfacial hole gas and is accompanied by a charge transfer across the interface that provides additional avenues to modify the electronic structure and magnetotransport properties in the ultra-thin limit.
The prediction and observation of topological surface states in half-Heusler compounds raises exciting possibilities to realize exotic electronic states and novel devices by exploiting their multifunctional nature. However, their position with respect to the Fermi level and the high density of bulk carriers has made it difficult to detect them through transport measurements. Here, we introduce compensation doping in epitaxial PtLuSb thin films as an effective route to tune the chemical potential and simultaneously reduce the bulk carrier concentration by more than two orders of magnitude compared to the parent compound. Linear magnetoresistance is shown to appear as a precursor phase that transmutes into a quantum Hall phase arising from the topological surface states on further reduction of the coupling between the surface states and the bulk carriers. Our approach paves the way to both reveal and manipulate exotic properties of topological phases in Heusler compounds.
6:30 PM - NM03.09.04
Magnetic Properties on Bulk MnBi2Te4 Samples Grown by MBE
Seul-Ki Bac1,Logan Riney1,Jiashu Wang1,Kerrie Koller2,1,Xinyu Liu1,Maksym Zhukovskyi1,Tatyana Orlova1,Malgorzata Dobrowolska1,Jacek Furdyna1,Badih Assaf1
University of Notre Dame1,Saint Mary's College2Show Abstract
Intrinsic magnetic topological insulator MnBi2Te4 has been extensively studied due to the theoretical prediction that it will provide a platform for novel topological phases, such as the quantum anomalous Hall effects (QAHEs), the axion insulators (AIs), and the type-II magnetic Weyl semimetals depending on thickness. Recently, the QAHE and AI have been observed on MnBi2Te4 flakes with odd and even number of layers, respectively. However, further progress on this novel material is hindered by the difficulty in preparing its high-quality thin films with well-controlled composition and thickness. Here we report on the magnetic properties of MnBi2Te4 samples grown by molecular beam epitaxy (MBE) with various thicknesses. We observed two main results: magnetic phase transitions and the linear dependence between the Hall conductance and the magnetization. Both transport and SQUID measurements clearly show magnetic phase transitions between a ferromagnetic to an antiferromagnetic state and vice versa, which brings the possibility of hosting those new topological phases on MBE grown samples. In addition, we verify that the origin of the anomalous Hall effect of MnBi2Te4 is intrinsic, from its linear dependence of magnetization. Our results enable the realization of quantum anomalous Hall effects in the large area MnBi2Te4 films with thickness precisely controlled by MBE.
6:45 PM - NM03.09.05
Characterization of the Sr Impurity in Bi2Se3 Thin Films by Molecular Beam Epitaxy
Logan Riney1,Xinyu Liu1,Christian Bunker1,Dominic Battaglia1,Seul-Ki Bac1,Jiashu Wang1,Yum Chang Park2,Malgorzata Dobrowolska1,Jacek Furdyna1,Badih Assaf1
University of Notre Dame1,National Nanofab Center2Show Abstract
The Bi-chalcogenides are the most prominent topological materials to be studied to date. Alloying transition and alkali metals into the Bi-chalcogenides has enabled the discovery of topological superconductor candidates Sr-, Nb, and Cu- doped Bi2Se3. In this work, we synthesize SrxBi2Se3 thin film by molecular beam epitaxy on GaAs(111) substrates. While we do not succeed in obtaining superconductivity, our results provide insight on the mechanism by which Sr is alloyed into Bi2Se3. First, X-ray diffraction measurements indicate an increasing c-axis lattice parameter suggesting the formation of some Sr interstitials. Second, the mobility extracted from transport measurements and phonon linewidths extracted from Raman spectroscopy indicate that Sr likely alloys into the structure. Third, an increasing n-doping is seen with increasing Sr content. And lastly, a reduction of the electron coherence length with increasing Sr suggests that inelastic charged impurity or electron-electron scattering is enhanced. It is thus clear that Sr leads donor type impurities and more n-doping in Bi2Se3 despite. The thermodynamics of the MBE codeposition process seem to be generally unfavorable to the formation of Sr interstitials, thus making the synthesis of superconducting films highly challenging.
7:00 PM - NM03.09.06
Late News: Two-Dimensional Quantum Oscillations Observed in Magnetic Topological Semimetal EuSb2 Films
Mizuki Ohno1,Masaki Uchida1,2,3,Ryosuke Kurihara4,5,Susumu Minami6,Yusuke Nakazawa1,Shin Sato1,Markus Kriener5,Motoaki Hirayama1,5,Atsushi Miyake4,Yasujiro Taguchi5,Ryotaro Arita1,5,Masashi Tokunaga4,5,Masashi Kawasaki1,5
Dept. of Appl. Phys., the Univ. of Tokyo1,PRESTO, Japan Science and Technology Agency (JST)2,Department of Physics, Tokyo Institute of Technology3,ISSP, the Univ. of Tokyo4,RIKEN CEMS5,Dept. of Phys., the Univ. of Tokyo6Show Abstract
Topological nodal-line semimetals (TNLSMs) have attracted burgeoning attention because unprecedented quantum transport and magnetoelectric response originating from the Zak phase have been theoretically expected. Moreover, recent computational exploration has accelerated efforts to find magnetic TNLSM materials for controlling these topological phenomena by magnetic fields. In general, however, time-reversal symmetry breaking by magnetic ordering destroys robustness of the nodal lines, and thus only few magnetic TNLSM materials have been reported so far. In addition, quantum oscillations have never been observed in these magnetic topological nodal-line semimetals. Namely, discovery of a new magnetic TNLSM and fabrication of its high-mobility films have been strongly desired.
In order to introduce magnetic moments into typical nonmagnetic TNLSM CaSb2, we prepare isostructural EuSb2 single-crystalline films by molecular beam epitaxy (MBE). Our first-principles calculations demonstrate that EuSb2 hosts topological nodal lines protected by nonsymmorphic symmetry even under antiferromagnetic ordering of the Eu2+ spins. An X-ray diffraction rocking curve is very sharp, ensuring high crystallinity of the obtained film. Observed Shubnikov-de Haas oscillations with multiple frequency components exhibit small effective masses and two-dimensional field-angle dependence even in a 250 nm thick film, suggesting possible contributions of surface states.
Our demonstration of the new magnetic TNLSM and first observation of quantum magnetotransport in its films will stimulate further investigation and control of exotic transport phenomena proposed for magnetic topological semimetals.