News and posts

Kevin GEISHENDORF

Leibniz Institute Dresden

Weyl semimetals exhibit interesting electronic properties due to their topological band structure.
In particular, large anomalous Hall and anomalous Nernst signals are often reported, which allow for
a detailed and quantitative study of subtle features. We pattern single crystals of the magnetic Weyl
semimetal Co3Sn2S2 into nanoribbon devices using focused ion beam cutting and optical lithography.
This approach enables a very precise study of the galvano- and thermomagnetic transport properties.
Indeed, we found interesting features in the temperature dependency of the anomalous Hall and
Nernst effects. We present an analysis of the data based on the Mott relation and identify in the
Nernst response signatures of magnetic fluctuations enhancing the anomalous Nernst conductivity
at the magnetic phase transition.

K. YAMAMOTO(1), G. C. Thiang(2), P. Pirro(3), K.-W. Kim(4), K. Everschor-Sitte(5) and E. Saitoh(6,7,1)

1 Advanced Science Research Center, Japan Atomic Energy Agency
2 School of Mathematical Sciences, University of Adelaide
3 Fachbereich Physik and Landesforschungszentrum OPTIMAS, Technische Universität Kaiserslautern
4 Center for Spintronics, Korea Institute of Science and Technology
5 Institut für Physik, Johannes Gutenberg-Universität Mainz
6 Department of Applied Physics, University of Tokyo
7 Institute for Materials Research, Tohoku University

Magnetostatic surface spin waves (a.k.a Damon-Eshbach mode) have long been known to have the largest decay lengths of all available modes and be robust against surface shapes and disorders [1-3]. Combined with their chiral and unidirectional propagation with respect to the direction of the ground state magnetisation, these features remind one of topologically protected edge states of quantum Hall systems. We present a topological characterisation of the dipolar spin wave Hamiltonian, which predicts, via the bulk-edge correspondence, the presence of robust surface spin wave modes without explicitly calculating eigenmodes of a system with boundaries [4].

While the characterisation is based on the symmetry class CI of electronic topological band theory, it is reformulated for the particular dynamical structure of classical Hamiltonian systems in which symplectic, rather than unitary, structure plays an essential role. By suitably identifying the symplectic structure with the chiral symmetry of class CI, assuming a preferred metric tensor in the space of canonical coordinates, we show that the surface spin waves appear not in a gap of bulk frequency spectrum, consistent with the magnetostatic surface spin waves.

[1] A. V. Chumak et al., Appl. Phys. Lett. 94, 172511 (2009).
[2] M. Mohseni et al., Phys. Rev. Lett. 122, 197201 (2019).
[3] T. Yu et al., Phys. Rev. B 99, 174402 (2019).
[4] K. Yamamoto et al., Phys. Rev. Lett. 122, 217201 (2019).

Jacob Gayles, Yan Sun, and Claudia Felser

Max Planck Institute for Chemical Physics of Solids, D-01187 Dresden, Germany
(Dated: August 15, 2019)

Recently, the Heusler compounds Mn1:4PtSn and Mn1:4Pt0:9Pd0:1Sn were shown to stabilize an antiskyrmion
lattice above room temperature and with out an external magnetic field [1]. These Heusler compound forms in
a superstructure with the D2d symmetry, which allows for an anisotropic Dzyaloshinskii-Moriya interaction
(DMI) perpendicular to the tetragonal axis. Furthermore, many of these compounds show a spin reorientation
transition where the topological Hall effect is much larger below the transition than above in the known antiskyrmion
regime [2]. We use density functional theory calculations in combination with atomistic spin dynamic
calculation of MnxYZ compounds to extract the relevant exchange interactions that determine the rich phase
diagrams in these materials. The exchange interactions are between the large moments on the Mn atoms 4B,
which show magnetic states that are non-collinear ferrimagnetic up to the spin reorientation. The major role
of the spin-orbit driven DMI is due to the Z ion, either In, Ga,Sn or Sb where the Y ion (Ru,Rh,Pd,Ir, or Pt )
d-states lowered in energy due to the Jahn-Teller distortion. The content of Mn also plays a large role in the
stabilization of the magnetic textures. The Fermi level can be tuned by the Y ion, either In,Sn or Sb. We last
calculate the anomalous Hall effect and topological Hall effects in these regimes, to capture the influence of the
electronic structure on the Berry curvature.

Hikaru WATANABE

Kyoto University

The physics of multipole moment has been discussed in broad systems such as
strongly-correlated electron systems [1], multiferroic materials [2], and so on. Multipole
moments arise from the coupling between spin, orbital, sublattice degrees of freedom,
and lead to various phenomena in condensed matter: e.g. unconventional phases and
peculiar responses to external fields.
Furthermore, recent studies suggest that the uniform alignment of parityviolating
multipole moments, namely, odd-parity multipole order induces exotic
quantum phases and cross-correlated responses in metals [3]. An important feature of
odd-parity multipole ordered systems is unusual itinerant properties. In fact,
spontaneous emergence of spin-momentum coupling or asymmetric band structure is
closely related to intriguing quantum phenomenon.
In our work, we classified the even-/odd-parity multipole from the viewpoint of
point-group classification [4]. Our results systematically clarify the physical properties
of odd-parity multipole ordered systems and cross-correlated responses both in metals
and insulators, while previous studies have been limited to case studies [3]. We further
identified a novel transport phenomenon, magneto-piezoelectric effect, which is caused
by a coupling between elasticity and electricity in “conductors” [5]. This response may
suggest a topic in a recently developed field, antiferromagnetic spintronics [6], and will
promote functionalities of antiferromagnets.

[1] Y. Kuramoto, H. Kusunose, and A. Kiss, J. Phys. Soc. Japan 78, 072001 (2009); P.
Santini, S. Carretta, G. Amoretti, R. Caciuffo, N. Magnani, and G. H. Lander, Rev. Mod.
Phys. 81, 807 (2009).
[2] N. A. Spaldin, M. Fiebig, and M. Mostovoy, J. Phys. Condens. Matter 20, 434203
(2008).
[3] Y. Yanase, J. Phys. Soc. Japan 83 , 014703 ( 2014); S. Hayami, H. Kusunose, and Y.
Motome, Phys. Rev. B 90 , 024432 (2014); S. Sumita and Y. Yanase, Phys. Rev. B 93 ,
224507 (2016).
[4] H. Watanabe and Y. Yanase, Phys. Rev. B 98 , 2451 29 (201 8)
[5] H. Watanabe and Y. Yanase, Phys. Rev. B
96 , 0 64432 (2017) 2017); Y. Shiomi, H. Watanabe,
H. Masuda, H. Takahashi, Y. Yanase, and S. Ishiwata, Phys. Rev. Lett. 122 , 127207
(
[6] T. Jungwirth, X. Marti, P. Wadley, and J. Wunderlich, Nat. Nanotechnol. 11 , 231
( 2016); V. Baltz, A. Manchon, M. Tsoi, T. Moriyama, T. Ono, and Y. Tserkovnyak, Rev.
Mod. Phys. 90 , 015005 (

Haruki Watanabe

University of Tokyo

The interplay between symmetry and topology has been the central subject of condensed matter physics over decades. In particular, the systematic diagnosis of band topology enabled by the method of “symmetry indicators” underlies the recent advances in the search for new materials realizing topological crystalline insulators.
In this talk, we review the symmetry indicators for band insulators first and then discuss the extension of the formalism to topological superconductors.

Frank SCHINDLER

University of Zürich

Crystal defects in topological insulators are known to bind anomalous electronic states with two fewer dimensions than the bulk; the most commonly cited examples are the helical modes bound to screw dislocations in time-reversal invariant weak topological insulators. In my talk, I will explain how one can extend the classification of topological electronic defect states, in particular to time-reversal symmetry breaking magnetic systems. By mapping the Hamiltonians of planes in momentum space to the real-space surfaces between screw or edge dislocations with integer Burgers vectors, I show that crystalline defects can bind higher-order end states with fractional charge. I will present extensive numerical calculations that support these findings.

Daniel Gosálbez-Martínez

Institute of Physics, École Polytechnique Fédérale de Lausanne (EPFL), CH-1015 Lausanne, Switzerland
National Centre for Computational Design and Discovery of Novel Materials MARVEL, École Polytechnique Fédérale de Lausanne (EPFL), CH-1015 Lausanne, Switzerland

Weyl nodes with chiral charge +/-1 are classified as types I and type II according to the tilting of
the conical band dispersion at the band degeneracy. Their Fermi surface, described by a quadratic
form, is different for these two types. We extend this classifcation to the case of composite Weyl
nodes with chiral charge larger than one. When the C4 and C6 rotation symmetries forbid the linear
band dispersion on the plane perpendicular to the symmetry axis, new terms with quadratic and
cubic momentum dependence must be included. Consequently, the Fermi surface produced by these
band degeneracies are described by a 4 or 6 order algebraic surfaces. In this more complex situation,
instead of classifying Fermi surfaces, we study numerically the possible Lifshitz transitions of the
Fermi surface produced by a composite Weyl node as the chemical potential is varied. We use this
methodology to classify the different types the composite Weyl fermions. In particular, for the case
of quadratic Weyl nodes generated by the C4 rotation symmetry. we find four different types band
dispersion morphologies, two analogous to the type-I and type-II case of the conical Weyl nodes,
and two new distinct morphologies within the type-II class. We illustrate the existence of these new
types of band degeneracies in real materials such as ferromagnetic iron.

Christina PSAROUDAKI

Department of Physics, University of Basel, Klingelbergstrasse 82, 4056 Basel, Switzerland

We provide a comprehensive picture of the quantum propagation of skyrmions in
chiral magnets, focusing on the microscopic description of dissipation at zero and
finite temperatures, originating from the interaction of the skyrmion with quantum
fluctuations. The most interesting feature is that the effect of this damping is reduced
to a mass term that is predicted for the first time [1]. We demonstrate that a skyrmion
in a confined geometry behaves as a massive particle, a discovery with great impact
on the technologically important case of linear tracks relevant for magnetic memory
devices [1]. An additional quantum mass term is predicted with an explicit
temperature dependence which remains finite even at zero temperature.
In the presence of time-dependent oscillating magnetic field gradients, the
unavoidable coupling of the external field to the magnons gives rise to timedependent
dissipation for the skyrmion, with measurable consequences on the
skyrmion’s path [2]. These ac fields act as a net driving force on the skyrmion via its
own intrinsic magnetic excitations. We generalize the standard quantum theory of
dissipation to include the effects of the driven bath on the skyrmion dynamics. We
address the stochastic effects of the quantum driven bath on the skyrmion propagation
[3], and provide a generalized version of the nonequilibrium fluctuation-dissipation
relation for externally driven reservoirs.
Our work initiates studies towards the possibility of observing a quantum mechanical
behavior at a mesoscopic scale. I will briefly talk about the observability of tunneling
events, in particular quantum depinning of a magnetic skyrmion out of a pinning
center [4].

[1] C. Psaroudaki, S. Hoffman, J. Klinovaja, and D. Loss, Quantum Dynamics of
Skyrmions in Chiral Magnets, Phys. Rev. X 7, 041045 (2017).
[2] C. Psaroudaki and D. Loss, Skyrmions Driven by Intrinsic Magnons, Phys. Rev.
Lett. 120, 237203 (2018).
[3] C. Psaroudaki, P. Aseev, and D. Loss, "Quantum Brownian Motion of a Magnetic
Skyrmion", arXiv:1904.09215
[4] C. Psaroudaki and D. Loss, "Quantum Deppining of a Magnetic Skyrmion",
manuscript in preparation (2019).

Alexander MOOK

MPI Halle

The interplay of spin-orbit coupling (SOC) and magnetism gives rise to a plethora of unconventional charge/heat-to-spin conversion phenomena in solids that harbor great potential for spintronic applications. Accounting for both the metallic and insulating states of matter, electrons as well as magnons emerge as two prominent spin carriers.

In this talk, I focus on selected aspects of SOC-driven phenomena in magnets with ferromagnetic or antiferromagnetic textures. Among those are (i) the thermal Hall effect of magnons in magnon Chern phases, (ii) spin-polarized magnon currents in fully compensated antiferromagnets, and (iii) spin current vortices that give rise to a magnetic spin Hall effect, the time-reversal odd cousin of the usual spin Hall effect.