Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session B59: Skyrmions and Topological Magnonics 
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Sponsoring Units: DMP Room: Mile High Ballroom 3C 
Monday, March 2, 2020 11:15AM  11:27AM 
B59.00001: THz spectroscopy of topological magnon candidate CoTiO_{3} Evan Jasper, Timothy DeLazzer, Kate Ross, Rolando Valdes Aguilar Recent advancements in topological band theory have predicted the existence of topologicallyprotected magnon excitations [1] with potential spintronic applications. CoTiO_{3} is an Ilmenitestructure antiferromagnet with a Néel temperature of 38 K and magnon energies in the THz range. Previous work has also identified CoTiO_{3} as a candidate to host topological magnons [2]. We follow up on that research by measuring the THz transmission and reflectivity of both single crystal and pressed powder samples of CoTiO_{3} and we report the results including a large absorptions near 1 and 1.6 THz. We propose that the modes broaden with decreasing temperature or a new absorption appears below the Néel temperature with energy coinciding with a zonecenter magnon excitation. We will discuss these possibilities together with the details of the THz polarization response. 
Monday, March 2, 2020 11:27AM  11:39AM 
B59.00002: Threedimensional topological magnon systems Hiroki Kondo, Yutaka Akagi, Hosho Katsura Recently, magnonic analogs of spin Hall insulators in two dimensions have been proposed theoretically [1]. In our previous work, we defined a Z_{2} topological invariant for them and proposed the first model of a magnonic analog of a spin Hall insulator without spin conservation [2]. 
Monday, March 2, 2020 11:39AM  11:51AM 
B59.00003: Chiral Magnonic Edge States in Ferromagnetic Skyrmion Crystals Controlled by Magnetic Fields Sebastian A Diaz, Tomoki Hirosawa, Jelena Klinovaja, Daniel Loss We show that an external magnetic field can drive a topological phase transition in the spin wave spectrum of a ferromagnetic skyrmion crystal. The topological phase transition is signaled by the closing of a lowenergy bulk magnon gap at a critical field. In the topological phase, below the critical field, two topologically protected chiral magnonic edge states lie within this gap, but they unravel in the trivial phase, above the critical field. Remarkably, the topological phase transition involves an inversion of two magnon bands that at the Γ point correspond to the breathing and anticlockwise modes of the skyrmions in the crystal. Our findings suggest that an external magnetic field could be used as a knob to switch on and off magnon spin currents carried by topologically protected chiral magnonic edge states. 
Monday, March 2, 2020 11:51AM  12:03PM 
B59.00004: Magnetic field control of topological magnonpolaron bands in twodimensional ferromagnets Pengtao Shen, Se Kwon Kim We theoretically study magnonphonon hybrid excitations in a square lattice ferromagnet subjected to a magnetic field by varying the field direction. The bulk bands of hybrid excitations, which are referred to as magnonpolarons, are investigated by considering all three phonon modes: vertical phonon, transverse phonon, and longitudinal phonon. We show that the topological proprieties of three hybridizations are different in terms of the Berry curvature and the Chern numbers. We also find that the topological properties of the bands can be controlled by changing the direction of the magnetic field, exhibiting one or more topological phase transitions. The dependence of thermal Hall conductivity as a function of magnetic field direction is proposed as an experiment probe of our theoretical results. 
Monday, March 2, 2020 12:03PM  12:15PM 
B59.00005: Magnonic SuSchrifferHeeger Model in Honeycomb Ferromagnets Yuhang Li, Ran Cheng We study a collinear honeycomb ferromagnet with variable nearestneighbor exchange interactions in the presence of the next nearestneighbor DzyaloshinskiiMoriya interaction. By examining the band topology and the associated chiral edge states, we find that varying the nearestneighbor exchange interactions can induce topological phase transitions between trivial insulators with vanishing Chern number C=0 and magnon Hall insulators with C=1. Such a phase transition from topological trivial phase into a topological nontrivial one can be characterized by a sharp increase of the thermal Hall conductivity, which is attributed to the emerge of the topologically protected chiral edge states of magnons. Moreover, the magnon Hall state is robust against magnonphoton interactions. Possible materials realization is also discussed. 
Monday, March 2, 2020 12:15PM  12:27PM 
B59.00006: Nonsymmorphicsymmetryprotected topological magnons in threedimensional Kitaev materials YongBaek Kim, Wonjune Choi, Tomonari Mizoguchi Topological phases in magnetic materials offer novel tunability of topological properties through varying the underlying magnetism. We show that fieldinduced canted zigzag order of threedimensional hyperhoneycomb Kitaev materials has gapless topological magnons protected by nonsymmorphic magnetic space group symmetries. The nonHermitian nature of the bosonic magnons leads to unique topological protection that is different from their fermionic counterparts. We investigate how such topological magnons can be controlled by varying the strength of the spinexchange interactions. 
Monday, March 2, 2020 12:27PM  12:39PM 
B59.00007: Quantum field theory of topological spin dynamics Predrag Nikolic Topological magnetic materials are promising hosts for novel strongly correlated states. I will present a unifying quantum field theory of a wide range of such materials, which captures their universal and topological dynamics. All topological features arise from the continuumlimit gauge fields that one can calculate starting with a microscopic lattice model. Noncollinear incommensurate magnets are shaped by nonAbelian vector gauge fields, including the DzyaloshinskiiMoriya interaction. Chiral spin couplings related to topological defects give rise to rank2 antisymmetric tensor gauge fields. The spinorbit coupling of mobile electrons is similarly described by nonAbelian gauge fields, including rank2 tensors related to the 3D Berry flux. All gauge fluxes are generally exchanged between electrons and local moments through Kondotype interactions. I will discuss applications of this theory to (i) spinwave dynamics in the presence of Weyl electrons, (ii) instabilities of Weyl semimetals, (iii) fluctuations and temperaturedependence of the anomalous/topological Hall effect, and (iv) topological phase transitions involving monopoles and hedgehogs (which can fractionalize electrons). 
Monday, March 2, 2020 12:39PM  12:51PM 
B59.00008: Polarizationresolved Raman spectroscopy of αRuCl3 and evidence of roomtemperature twodimensional magnetic scattering Angela Hight Walker, Thuc Mai, Amber McCreary, Paula LampenKelley, Nicholas Butch, Jeffrey Simpson, Jiaqiang Yan, Stephen E Nagler, David Mandrus, Rolando Valdes Aguilar Polarizationresolved Raman spectroscopy was performed and analyzed from large, highquality, monodomain single crystal of αRuCl3, a proximate Kitaev quantum spin liquid^{1}. Spectra were collected with laser polarizations parallel and perpendicular to the honeycomb plane. Pairs of nearly degenerate phonons were discovered and show either a fourfold or twofold polarization angle dependence in their Raman intensity, thereby providing evidence to definitively assign the bulk crystal point group as C2h. The lowfrequency continuum that is often attributed to scattering from pairs of Majorana fermions was also examined and found to disappear when the laser excitation and scattered photon polarizations were perpendicular to the honeycomb plane. This disappearance, along with the behavior of the phonon spectrum in the same polarization configuration, strongly suggests that the scattering continuum is twodimensional. We argue that this scattering continuum originates from the Kitaev magnetic interactions that survives up to room temperature, a scale larger than the bare Kitaev exchange energy of approximately 50 K. 
Monday, March 2, 2020 12:51PM  1:03PM 
B59.00009: Dimension transcendence and anomalous charge transport in magnets with moving multipleQ spin textures Ying Su, Satoru Hayami, Shizeng Lin MultipleQ spin textures, such as magnetic bubble and skyrmion lattices, have been observed in a large family of magnets. These spin textures can be driven into motion by external stimuli and affects the electronic states. Here we show that to describe correctly the electronic dynamics, the momentum space needs to be transcended to higher dimensions by including the ancillary dimensions associated with phason modes of the translational motion of the spin textures. The electronic states have nontrivial topology characterized by the first and second Chern numbers in the high dimensional hybrid momentum space. This gives rise to an anomalous electric charge transport due to the motion of spin textures. By deforming the spin textures, a nonlinear response associated with the second Chern number can be induced. The charge transport is derived from the semiclassical equation of motion of electrons that depends on the Berry curvature in the hybrid momentum space. Our results suggest that the motion of multipleQ spin textures has significant effects on the electronic dynamics and provides a new platform to explore high dimensional topological physics. 
Monday, March 2, 2020 1:03PM  1:15PM 
B59.00010: Thermaltransport properties of chiral antiferromagnet CsCuCl_{3} Masatoshi Akazawa, Masaaki Shimozawa, Minoru Yamashita, Yusuke Kousaka, Jun Akimitsu, Naoto Tsuchiya, Katsuya Inoue, Julien Zaccaro, Isabelle GautierLuneau, Dominique Luneau Recently, topological effects related to a Berry curvature dipole has been extensively studied in metals without inversion symmetry. On the other hand, these topological effects on chargeneutral bosonic excitations have yet to be clarified. To investigate this issue, we measured the longitudinal and transverse thermal conductivity of the chiral antiferromagnet CsCuCl_{3,} a highquality homochiral single crystal of which becomes available very recently. We find a large enhancement of the longitudinal thermal conductivity in the ordered phase. This result suggests a large spin thermal conduction that could not be observed clearly in multidomain crystals. We further find a sizable thermal Hall effect with a nonlinear heater dependence even at zero magnetic field. We will discuss a possible origin for this nonlinear thermal Hall effect. 
Monday, March 2, 2020 1:15PM  1:27PM 
B59.00011: Intrinsic Quantized Anomalous Hall Effect in a Moiré Heterostructure, Part II: Temperature Dependence and Current Switching Marec Serlin, Charles Tschirhart, Hryhoriy Polshyn, Yuxuan Zhang, Jiacheng Zhu, Martin E Huber, Leon Balents, Kenji Watanabe, Takashi Taniguchi, Andrea Young This is the second of three talks describing the observation and characterization of a ferromagnetic moiré heterostructure based on twisted bilayer graphene aligned to hexagonal boron nitride. I will compare the qualitative and quantitative features of this observed quantum anomalous Hall state to traditional systems engineered from thin film (Bi,Sb)2Te3 topological insulators. In particular, we find that the measured electronic energy gap of ~30K is several times higher than the Curie temperature, consistent with a lack of disorder associated with magnetic dopants. In this system, the quantization arises from spontaneous ferromagnetic polarization into a single spin and valley moiré subband, which is topological despite the lack of spin orbit coupling. I will also discuss the observation of current induced switching, which allows the magnetic state of the heterostructure to be controllably reversed with currents as small as a few nanoamperes. 
Monday, March 2, 2020 1:27PM  1:39PM 
B59.00012: Intrinsic Quantized Anomalous Hall Effect in a Moiré Heterostructure, Part I: Device Fabrication and Transport Yuxuan Zhang, Marec Serlin, Charles Tschirhart, Hryhoriy Polshyn, Jiacheng Zhu, Leon Balents, Martin E Huber, Takashi Taniguchi, Kenji Watanabe, Andrea Young We report the observation of a quantized anomalous Hall effect in a moiré heterostructure consisting of twisted bilayer graphene aligned to an encapsulating hBN substrate. The effect occurs at a density of 3 electrons per superlattice unit cell, where we observe magnetic hysteresis and a Hall resistance quantized to within 0.1% of the resistance quantum at temperatures as high as 3K. In this first of 3 talks, I will describe the fabrication procedure for our device as well as basic transport characterization measurements. I will introduce the phenomenology of twisted bilayer graphene and present evidence for hBN alignment as manifested in the hierarchy of symmetrybreaking gaps and anomalous magnetoresistance. 
Monday, March 2, 2020 1:39PM  1:51PM 
B59.00013: Intrinsic Quantized Anomalous Hall Effect in a Moiré Heterostructure, Part III: Scanning Probe Magnetometry Charles Tschirhart, Marec Serlin, Hryhoriy Polshyn, Yuxuan Zhang, Jiacheng Zhu, Leon Balents, Martin E Huber, Kenji Watanabe, Takashi Taniguchi, Andrea Young This is the third of three talks describing the observation and characterization of a ferromagnetic moiré heterostructure based on twisted bilayer graphene aligned to hexagonal boron nitride. In this segment I will present scanning probe magnetometry data acquired using a nanoSQUIDontip microscope, which provides ~150 nm spatial resolution and a field sensitivity of ~10 nT/rtHz. We study the distribution of magnetic domains within the device as a function of density, magnetic field training, and DC current. Our data allow us to constrain the magnitude of the orbital magnetic moment of the electrons in the QAH state. Comparison with simultaneously acquired transport data allows us to precisely correlate single domain dynamics with discrete jumps in the observed anomalous Hall signal. 
Monday, March 2, 2020 1:51PM  2:03PM 
B59.00014: The RKKY Interaction in Graphene Proximity Coupled to an Antiferromagnetic Insulator JiaJi Zhu, Jin Yang We study theoretically the longrange RudermanKittelKasuyaYosida (RKKY) interaction mediated by itinerant electrons of graphene between magnetic moments induced by proximity effect of an antiferromagnetic insulator. The antiferromagnetic insulators provides both broken timereversal symmetry and spinorbit coupling [1]. We find more spinspin interaction terms which had not reported previously in other spinorbit systems[2]. The additional terms could induce a spatially inhomogeneous spin order. We also find a rich texture of the total RKKY range function. With the Fermi energy residing in the band gap induced by the spinorbit coupling, the RKKY interaction degenerates to the BloembergenRowland (BR) interaction. The BR interaction is a short range exponentiallydecay interaction mediated by virtual excitations. We show a possible transformation of anomalous Hall effect to quantum anomalous Hall effect while the RKKY interaction degenerating to the BR interaction. 

B59.00015: NonSpinWave type of Magnetic Excitations in a Wellcharacterized NearestNeighbor Triangular Antiferromagnet Wei Bao

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