FROM CHEMICAL BONDS TO TOPOLOGY
Leslie Schoop1
1Department of Chemistry, Princeton University, Princeton, NJ, USA
E-Mail: lschoop@princeton.edu

In the discipline of chemistry, it is common to have guidelines and heuristics that help topredict how chemical reactions will proceed. We are interested to expand these heuristics
to understand if we can predict topological materials. In this talk, I will show how
delocalized chemical bonds in certain structural networks allow us to define chemical
descriptors that predict band inversions. Using these descriptors, we found a layered,antiferromagnetic van der Waals material with very high mobility. These properties havepreviously not coexisted in a material that can be mechanically exfoliated. We further
implemented our heuristics to discover novel complex topological phases, including
magnetic ones, and phases that are in competition with complex structural distortions. I
will show how structural distortions can have a positive effect on topological bandstructures.

ELECTRICAL MANIPULATION OF AN ANTIFERROMAGNETICWEYL SEMIMETAL STATE
Satoru Nakatsuji1,2,31

Department of Physics, School of Science, The University of Tokyo, Tokyo 113-0033,
Japan
2
Institute for Solid State Physics, The University of Tokyo, Kashiwa, Chiba 277-8581,
Japan
3

Department of Physics and Astronomy, Johns Hopkins University, Baltimore, MD
21218, USA
E-Mail: satoru@scphys.s.u-tokyo.ac.jp

Electrical manipulation of emergent phenomena due to nontrivial band topology is a key
to realize next-generation technology using topological protection. Recent discovery ofthe magnetic Weyl fermions in the antiferromagnet Mn3Sn has attracted significant
attention [1], as the magnetic Weyl semimetal exhibits various exotic phenomena such aschiral anomaly [1], large anomalous Hall effect (AHE) [2], anomalous Nernst effect [3,4],
magneto-optical effects [5], and magnetic spin Hall effect [6], which have robust
properties due to the topologically protected Weyl nodes. Given the prospects of
antiferromagnetic (AF) spintronics for realizing high-density devices with ultrafast
operation, it would be ideal if one could electrically manipulate an AF Weyl semimetal.
Here we demonstrate the electrical switching of a topological AF state and its detection
by AHE at room temperature [7]. In particular, we employ a polycrystalline thin film of
the AF Weyl metal Mn3Sn, which exhibits zero-field AHE. Using the bilayer device ofMn3Sn and nonmagnetic metals (NMs), we find that an electrical current density of ~10101011 A/m2 in NMs induces the magnetic switching with a large change in Hall voltage,and besides, the current polarity along a bias field and the sign of the spin Hall angle ofNMs determines the sign of the Hall voltage. Notably, the electrical switching in theantiferromagnet is made using the same protocol as the one used for ferromagnetic metals.
Our observation may well lead to another leap in science and technology for topologicalmagnetism and AF spintronics.
This is the work in collaboration with Hanshen Tsai, Tomoya Higo, Kouta
Kondou, Takuya Nomoto, Akito Sakai, Ayuko Kobayashi, Takafumi Nakano, Kay
Yakushiji, T. Koretsune, M. Suzuki, Ryotaro Arita, Shinji Miwa, YoshiChika Otani,
Muhammad Ikhlas, Tahahiro Tomita, Hua Chen, and Allan MacDonald.

[1] K. Kuroda, T. Tomita et al., Nature Materials 16, 1090-1095 (2017)[2] S. Nakatsuji, N. Kiyohara, T. Higo, Nature 527, 212-215 (2015)[3] M. Ikhlas, T. Tomita, T. Koretsune, M.-T. Suzuki, D. Nishio-Hamane, R. Arita, Y.
Otani and S. Nakatsuji, Nature Physics 13, 1085-1090 (2017)[4] A. Sakai, Y.P. Mizuta, A.A. Nugroho, R. Sihombing, T. Koretsune, M.-T. Suzuki, N.
Takemori, R. Ishii, D.N. Hamane, R. Arita, P. Goswami and S. Nakatsuji, Nature Physics,14, 1119–1124 (2018)[5] T. Higo et al., Nature Photonics 12, 73-78 (2018); T. Matsuda et al., NatureCommunications 11, 2863 (2020)[6] M. Kimata et al., Nature 565, 627–630 (2019)[7] H. Tsai, T. Higo, K. Kondou, T. Nomoto, A. Sakai, A. Kobayashi, T. Nakano, K.
Yakushiji, R. Arita, S. Miwa, Y. Otani, and S. Nakatsuji, Nature 580, 608–613 (2020)

TOPOLOGICAL PHYSICS IN THE 2D KAGOME NETWORK
Riccardo Comin1
1

Department of Physics, Massachusetts Institute of Technology, Cambridge, 02139,
Massachusetts, USA
E-Mail: rcomin@mit.edu

The kagome network is one of the possible tilings of two-dimensional space, onewith the same point symmetries as the hexagonal lattice of graphene. Recent theoreticaldevelopments suggest that the combination of unusual magnetism, spin-orbit coupling,
and geometric frustration in kagome metals may lead to a wide range of novel topologicalphysics, such as fractional quantum Hall effect and intrinsic anomalous Hall effect. In
these phenomena, a major role is played by the topologically nontrivial flat bands andmassive Dirac cones, both of which are predicted to exist from the unique geometrical
hopping pathways of kagome lattice. Despite these predictions, the experimental bandstructure of kagome compounds has long remained unreported.
In this talk, I will report on the experimental band structure of various kagome
compounds belonging to the family of transition metal stannides, and in particular Fe3Sn2,FeSn, and CoSn. In these systems which intertwine robust magnetism and electronic
topology, we observed various manifestations of topological physics. These include therealization of the Kane-Mele model for 2D Dirac fermions with a spin-orbit-induced
topological gap, as well as the discovery of the elusive flat bands and its nontrivial
topology.
In sum, transition metal-based kagome lattices have emerged a new platform forintriguing phenomena arising from the combination of topology, magnetism, andcorrelations. I will discuss the prospects and outlook for further exploration of novel
topological physics in this materials family.

ORBITAL MAGNETISM AND ISOSPIN POMERANCHUKEFFECT IN MOIRE HETEROSTRUCTURES
Andrea F. Young1
1Department of Physics, University of California, Santa Barbara 93106, USA
E-Mail: andrea@physics.ucsb.edu

Moire flat bands host a wide range of correlated states at low temperatures. My talkwill focus on the role of orbital magnetism in these systems, in which the electron systemspontaneously polarizes into one or more spin- and valley-isospin flavors.
In the first part of the talk, I will focus on the observation of quantized anomalous
Hall effects in a variety of moire systems. In this spectacular manifestation of orbital
magnetism, the ground state at certain integer filling factors is spontaneously polarized
into a single valley-projected moire miniband, leading to robust magnetic hysteresis anda quantized Hall effect at zero magnetic field. We observe a variety of novel phenomenain this regime, including ultra-low power current induced switching and gate-tuned
reversal of magnetic order that can be tied to the magnetization of the topological edge
states.
In the second part of the talk, I will discuss the more general role orbital magnetismplays in the phase diagram of these materials at high temperatures. In particular, we findthat even when the ground state is isospin unpolarized, a finite polarization can obtain athigh temperatures. We ascribe this effect—observed generically in twisted bilayer
graphene—to an isospin analogue of the Pomeranchuk effect long studied in 3He, inwhich the high entropy associated with isospin excitations of the orbital magnets favors
fluctuating magnetic order at high temperatures.
References:Serlin et al., arXiv:1907.00261
Polshyn et al., arXiv:2004.11353
Tschirhart et al., arXiv:2006.08053
Saito et al., arXiv:2008.10830

QUANTUM TRANSPORT IN THIN FILM HETEROSTRUCTURESOF TOPOLOGICAL INSULATORS
Ryutaro Yoshimi1
1

RIKEN Center for Emergent Matter Science (CEMS) 2-1 Hirosawa, Wako-shi,Saitama, 351-0198 Japan
E-Mail: ryutaro.yoshimi@riken.jp

Topological insulators are a new class of semiconductors displaying charge-gapped
insulating behavior in the bulk but hosting a spin-polarized massless Dirac electron stateat the surface. Quantum transport in surface Dirac electron systems has been attracting
much attention for the half-integer quantum Hall effect (QHE) and quantum anomalous
Hall effect (QAHE) [1]. Fabricating layered structures of thin film is one of the usefultechniques to efficiently control the surface states of a topological insulator. We havefabricated topological insulator (Bi1−xSbx)2Te3 (BST) thin films by molecular beam
epitaxy (MBE), in which the carrier density can be widely controlled by Bi/Sb ratio.
Precise tuning of the Fermi level by field-effect transistor (FET) enables us to observe theQHE [2] and QAHE [3] in BST and Cr-doped compound Crx(Bi1−ySby)2-xTe3 (CBST).
Furthermore,
wehavedesignedsandwich
heterostructuresof(Zn,Cr)Te/(Bi,Sb)2Te3/(Zn,Cr)Te in which a sizable exchange gap at the TI surface stateis induced by the proximity coupling. The proximity coupling realized in the all-telluride
based heterostructure will enable a realistic design of versatile tailor-made topologicalmaterials coupled with ferromagnetism, ferroelectricity, superconductivity, and so on.
[1] C.-Z. Chang et al., Science 340, 167 (2013)[2] R. Yoshimi et al., Nature Communications 6, 6627 (2015)[3] J. G. Checkelsky, R. Yoshimi et al., Nature Physics 10, 731 (2014)[4] M. Mogi, R. Yoshimi et al., Appl. Phys. Lett. 107, 182401 (2015)[5] R. Watanabe, R. Yoshimi et al., Appl. Phys. Lett. 115, 102403 (2019)

TOPOLOGICAL PROTECTION OF WEYL FERMIONSVISUALIZED ON THE ATOMIC SCALE
Haim Beidenkopf1, Nurit Avraham1, Noam Morali1, Rajib Batabyal1, AbhayKumar Nayak1, Pranab Kumar Nag1, Binghai Yan1, Yan Sun2, Claudia
Felser2
1

Department of Condensed Matter Physics, Weizmann Institute of Science, Rehovot
7610001, Israel.

2

Max Planck Institute for Chemical Physics of Solids, D-01187 Dresden, Germany.
E-Mail: haim.beidenkopf@weizmann.ac.il

Topological electronic materials host exotic boundary modes, that cannot be realizedas standalone states, but only at the boundaries of a topologically classified bulk.
Topological Weyl semimetals, whose bulk electrons exhibit chiral Weyl-like dispersion,
host Fermi-arc states on their surfaces. The Fermi-arc surface bands disperse along open
momentum contours terminating at the surface projections of bulk Weyl nodes withopposite chirality. Such reduction of the surface degrees of freedom by their segregation
to opposite surfaces of the sample, that reoccurs in all topological states of matter andeven exhibited by topological defects [1], provides topological protection from theirsurface elimination. We have confirmed the Weyl topological classification of both theinversion symmetry broken compound TaAs [2] and the time reversal symmetry broken
Co3Sn2S2 [3] by spectroscopic visualization of their Fermi-arc surface states through theinterference patterns those electrons embed in the local density of states. This has allowed
us to examine their unique nature and level of protection against perturbations. In TaAs
the Fermi arc bands are found to be much less affected by the surface potential compared
to trivial bands that also exist on its surfaces. In contrast, in Co3Sn2S2 the dispersion of
the topological Fermi-arc bands, and even their inter-Weyl node connectivity, are foundto vary with the surface termination. A possible resolution of this discrepancy will bediscussed.
[1] Abhay Kumar Nayak et al., “Resolving the Topological Classification of Bismuth withTopological Defects” Science Advances 5, eaax6996 (2019)[2] Rajib Batabyal et al., “Visualizing weakly bound surface Fermi arcs and theircorrespondence to bulk Weyl fermions” Science Advances 2, e1600709 (2016)[3] Noam Morali et al., “Fermi-arc diversity on surface terminations of the magnetic Weyl
semimetal Co3Sn2S2”, Science 365, 1286 (2019)

BEYOND TOPOLOGICAL QUANTUM CHEMISTRY
Maia G. Vergniory1
1

Donostia International Physics Center, Donostia-San Sebastian, Spain Ikerbasque,
Basque Foundation for Science, Bilbao, Spain
E-Mail: maiagvergniory@dipc.org

Topological quantum chemistry (TQC) framework has provided a complete description of
the universal properties of all possible atomic band insulators in all space groups considering
the crystalline unitary symmetries. It links the chemical and symmetry structure of a given
material with its topological properties. While this formalism filled the gap between themathematical classification and the practical diagnosis of topological materials, an obvious
limitation is that it only applies to weakly interacting systems. It is an open question to which
extent this formalism can be generalized to strongly correlated system that can exhibit
symmetry protected topological Mott insulators. In this talk I will first introduce TQC and its
application and then I will address this question by combining cluster perturbation theory andtopological Hamiltonians within TQC. This simple formalism will be applied to calculate to
the phase diagram of the Hubbard model for a diamond chain. The results are compared tonumerically exact calculations from density matrix renormalization group and variational
Monte Carlo simulations together with many-body topological invariants.

[1] M.G. Vergniory, L. Elcoro, C. Felser, N. Regnault, B.A. Bernevig, Z. Wang, “Acomplete catalogue of High-Quality Topological Materials “, Nature 566, 480-485 (2019)[2] B. Bradlyn, L. Elcoro, J. Cano, M.G. Vergniory, Z. Wang, C. Felser, M.I. Aroyo, B.A.
Bernevig, “Topological quantum chemistry”, Nature 547, 298-305 (2017).
[3] M. Iraola et al., in preparation