Bulletin of the American Physical Society
APS March Meeting 2021
Volume 66, Number 1
Monday–Friday, March 15–19, 2021; Virtual; Time Zone: Central Daylight Time, USA
Session X52: Magnetic Topological Materials 8: New MaterialsLive
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Sponsoring Units: DMP GMAG Chair: Jonathan Gaudet, Johns Hopkins University |
Friday, March 19, 2021 8:00AM - 8:12AM Live |
X52.00001: Picosecond-long spin current generation along the low-symmetry axis of WTe2 Kyusup Lee, Mengji Chen, Jie Li, Liang Cheng, Qisheng Wang, Kaiming Cai, Ee Min Elbert Chia, Haixin Chang, Hyunsoo Yang Spin current generation and detection underlie modern spintronics. In particular, the spin current with an out-of-plane spin polarization can efficiently manipulate magnetic devices with perpendicular magnetic anisotropy. Weyl semimetal WTe2 is of great potential in generating spin currents with an out-of-plane spin polarization due to intrinsic inversion symmetry breaking. Here we use an optical-driven terahertz (THz) emission spectroscopy and show a picosecond-long spin current generation along the rotational asymmetry a-axis of WTe2 with the out-of-plane spin orientation. The underlying mechanism of the THz emission is the circular photogalvanic effect with a circularly polarized laser excitation, which is incident to the sample surface normal. The inversion symmetry breaking induces a spin-splitting band, enabling the manipulation of the spin current direction with opposite spin orientations depending on the helicity of the laser excitation. The temperature dependent study reveals that the CPGE vanishes at 175 K due to the carrier compensation. Our finding shows a great potential for the future development of THz spintronic devices with Weyl semimetals. |
Friday, March 19, 2021 8:12AM - 8:24AM Live |
X52.00002: Low-frequency ferromagnetic resonance in CeAlSi Yue Sun, Changmin Lee, Hung-Yu Yang, Pasha Reshetikhin, Ashok Ajoy, Darius Torchinsky, Fazel Tafti, Joseph Orenstein CeAlSi is a ferromagnetic Weyl semimetal with in-plane magnetization and a weak in-plane easy-axis magnetic anisotropy. The large domain size of CeAlSi makes it an ideal platform to study various in-plane magnetic textures. A singular angular magnetoresistance is observed in a similar compound CeAlGe recently [1], which makes CeAlSi a more interesting system to explore. We observe a ferromagnetic resonance (FMR) in CeAlSi with a novel time-resolved magneto-optical setup, which can resolve both amplitude and direction changes of magnetization based on polarimetry. The FMR frequency is as low as 200 MHz and drops with the amplitude of external out-of-plane magnetic field, which is also observed in other ferromagnets with small anisotropy in plane [2]. We also locally image the FMR induced by an AC magnetic field. |
Friday, March 19, 2021 8:24AM - 8:36AM Live |
X52.00003: Controlling magnetic order, magnetic anisotropy, and band topology in the semimetals Sr(Mn0.9Cu0.1)Sb2 and Sr(Mn0.9Zn0.1)Sb2 FARHAN ISLAM, Renu Choudhary, Yong Liu, Benjamin Ueland, Durga Paudyal, Thomas W Heitmann, Robert J McQueeney, David Vaknin Neutron diffraction and magnetic susceptibility studies show that orthorhombic single-crystals of the topological semimetals Sr(Mn0.9Cu0.1)Sb2 and Sr(Mn0.9Zn0.1)Sb2 undergo three-dimensional C-type AFM ordering at a significantly lower TN than that of the parent compound SrMnSb2. Magnetization versus magnetic field (applied perpendicular to the MnSb planes) measurements below TN exhibit dHvA oscillations for the Sr(Mn0.9Zn0.1)Sb2, but not for the Sr(Mn0.9Cu0.1)Sb2. This suggests that doping with Cu for Mn either substantially changes the electronic band structure; or that the Cu sites are strong scatterers of carriers, which significantly shortens the mean free path, and diminishes the quantum oscillations. DFT calculations including spin-orbit coupling find that doping with either Cu or Zn tunes the electronic bands near the Fermi level, resulting in different band topologies and semimetallicity. These calculations rationalize the experimental results by showing that the parent and Zn-doped compounds have coexistence of electron and hole pockets with an open Dirac cone around the Y-point, whereas the Cu-doped compound has dominant hole pockets around the Fermi level with a distorted Dirac cone. |
Friday, March 19, 2021 8:36AM - 8:48AM Live |
X52.00004: Phenomenology of anomalous Kerr effect in SrRuO3 thin films F. Michael Bartram, Sopheak Sorn, Zhuolu Li, Kyle Hwangbo, Shengchun Shen, Felix Frontini, Liqun He, Pu Yu, Arun Paramekanti, Luyi Yang Recent Hall experiments on ultrathin films of SrRuO3 ferromagnet with several-unit-cell thickness have observed unusual hysteresis loops which display bump features near the coercive mangetic fields. This resembles the topological Hall effect induced by skyrmions which can nucleate during the magnetization reversal process and can arise from the interfacial Dzyaloshinskii-Moriya (IDM) interation at the film-substrate interface. A recent experiment on thicker films with more than 100 unit cells, where the IDM interation and skyrmions are not expected to be important, has also found the unusual hysteresis loops. Intriguingly, such unusual hystersis loops are also discovered in the optical Kerr effect. We formulate a phenomenological theory for the unusual Kerr effect which is shown to arise from the interplay between two non-skyrmion sources: (1) magnetic domains which are naturally present during the magnetization reversal and (2) an unsual nonmonotonic relation between the Kerr angle and the magnetization. Our work suggests that magnetic domains, instead of skyrmions, may provide a common origin for the unusual hysteresis loops in both Kerr and Hall effects in these thin films. |
Friday, March 19, 2021 8:48AM - 9:00AM Live |
X52.00005: Magneto-terahertz studies on the Kondo-Weyl Mn3+xSn1-x films Xingyue Han, Durga Khadka, Tilak Ram Thapaliya, Sunxiang Huang, Liang Wu Mn3Sn has attracted a lot of interests because it is an antiferromagnetic Weyl metal that shows a large anomalous Hall effect with vanishing net magnetization at room temperature. In this work, we study the doping dependece in epitaxial Mn3+xSn1-x films by time domain terahertz spectroscopy under a magnetic field up to 7T. We observe resonance enhanced Faraday rotation in samples with excess Mn doping, which provides a strong evidence of a gap opening due to the Kondo effect. We also measure the anomalous terahertz Faraday rotation as a function of temperature and obtain a phase diagram for this series of samples. |
Friday, March 19, 2021 9:00AM - 9:12AM Live |
X52.00006: Breaking Lorentz Reciprocity in the Weyl Semimetal Co3Sn2S2 to Enable Time-Asymmetric Photonics Arun Nagpal, Dennis Nenno, Christina Garcia, Chandra Shekhar, Claudia Felser, Prineha Narang, Harry Atwater Weyl semimetals that break time-reversal symmetry are predicted to exhibit giant magneto-optical effects, the optical response from which can be described using an axion electrodynamics formalism. The formalism produces a dielectric tensor with non-vanishing off-diagonal components even without external magnetic fields; thus, these systems are interesting examples of violations of Lorentz reciprocity and Kirchhoff’s Law in small form factors. |
Friday, March 19, 2021 9:12AM - 9:24AM Live |
X52.00007: Magnetic Dipole Transitions in the Second Harmonic Generation Response of the LnAlSi (Ln = La, Ce, Pr, Nd) Series of Weyl Semimetals Sujan Subedi, Baozhu Lu, Hung-Yu Yang, Fazel Tafti, Darius Torchinsky Optical transitions are usually considered in the electric dipole (ED) approximation since magnetic dipole (MD) and higher order multipole transitions are typically orders of magnitude weaker. Although some compounds can host MD transitions whose strength rivals that of the ED, teasing them apart can be difficult at linear order because the form of the linear suscepibility tensors for the two responses χij is typically the same. In contrast, separating ED and MD responses is simplified in second harmonic generation (SHG) because the higher rank nature of the second order optical susceptibility tensor χijk can result in quite different placement of their respective nonzero components, allowing each response to be selectively probed. To date, SHG MD emission has only been observed in the presence of magnetic order. In this talk, we present evidence of strong MD transitions that occur in the non-magnetically ordered states of the LnAlSi (Ln = La, Ce, Pr, Nd) series of Weyl semimetals and we discuss this observation in the context of the physics of these materials. |
Friday, March 19, 2021 9:24AM - 9:36AM Live |
X52.00008: Long-wavelength incommensurate spin structure in the non-centrosymmetric Weyl semimetal NdAlSi Jonathan Gaudet, Hung-Yu Yang, Santu Baidya, Baozhu Lu, Guangyong Xu, Yang Zhao, Jose A. Rodriguez-Rivera, Christina Hoffmann, David E Graf, Darius Torchinsky, David Vanderbilt, Fazel Tafti, Collin Leslie Broholm Weyl fermions can be stabilized in crystals that break inversion or time-reversal symmetry. NdAlSi has a non-centrosymmetric crystal structure and is also magnetic, so it breaks both of these symmetries. NdAlSi is thus an excellent template to study the connection between magnetism, electronic transport, and the Weyl fermions. To do so, we characterized the magnetism of NdAlSi using neutron diffraction and found a ferrimagnetic state at low temperature, which is followed by a long-wavelength incommensurate order at finite temperature. A combination of quantum oscillations measurements and DFT calculations were then performed to determine the Fermi surface of NdAlSi. We found that the periodicity of the magnetic order in NdAlSi matches the momentum space vector that connects two of its Fermi pockets. The Fermi pockets involved in this interaction contain Weyl fermions, which raises the interesting possibility for magnetic order driven by relativistic electrons in NdAlSi. |
Friday, March 19, 2021 9:36AM - 9:48AM Live |
X52.00009: Topology and Magnetism in Monolayers and Bilayers of ReX3 (X = Br, I) Sharad Mahatara, Boris Kiefer Two-dimensional (2D) van der Waals (vdW) magnets such as transition metals (TM) halides exhibiting topological states have provided a fertile ground for spintronic and quantum computing applications. In TM halides the presence of topologically protected states depends on the delicate balance of near degenerate interactions: (1) magnetic exchange interaction, (2) interlayer vdW interactions, and (3) amplified spin-orbit coupling (SOC). In this contribution, we have computed electronic, magnetic and topological properties of 2D (mono- and bi-layers) of ReX3 (X = Br, I) with density functional theory (DFT) including vdW interactions and SOC. We report for the first-time layer-dependent magnetism in ReX3 (X = Br, I), with intralayer ferromagnetism and interlayer antiferromagnetism. Moreover, we find that emerging topological states in these materials depend strongly on the fidelity of the electronic structure description. Our analysis shows that additional biases may be needed to induce topologically protected states in these materials, that allow for improved decoherence times with possible application to quantum computers. |
Friday, March 19, 2021 9:48AM - 10:00AM Live |
X52.00010: Machine-learning-assisted growth of ultrahigh-quality epitaxial 4d ferromagnetic Weyl semimetal SrRuO3 Yuki Wakabayashi, Takuma Otsuka, Yoshiharu Krockenberger, Hiroshi Sawada, yoshitaka taniyasu, Hideki Yamamoto Although Weyl fermions have been predicted to exist in various oxides [1], evidence for their existence remains elusive. SrRuO3, a 4dferromagnetic metal often serves as an epitaxial conducting layer in oxide heterostructures, provides a promising opportunity to seek their existence. State-of-the-art oxide thin film growth technologies, augmented by machine learning techniques, may allow access to such topological matter, which we have actually accomplished [2] using machine-learning-assisted molecular beam epitaxy (ML-MBE)[3]. To simplify the intricate search space of three entangled growth conditions (growth temperature, Ru/Sr ratio, oxidation strength), we ran the Bayesian optimization for a single growth condition while keeping the other growth conditions fixed. As a result, an ultrahigh-crystalline-quality SrRuO3film exhibiting the highest residual resistivity ratio (RRR) of 84 was developed. With the highest RRR ever reported, our SrRuO3thin films are superior to those by any other method and have enabled to probe the intrinsic quantum transport properties of Weyl fermions [2]. |
Friday, March 19, 2021 10:00AM - 10:12AM Live |
X52.00011: Observation of magnetic Weyl fermion excitation in antiferromagnetic kagome Mn3Pt thin films Joynarayan Mukherjee, Subhadeep Bandyopadhyay, Indra Dasgupta, Tanusri Saha-Dasgupta, Karthik Raman Kagome metals have triggered immense interest in the research community due to their interesting spin-lattice structure giving rise to finite Berry curvature, linearly dispersive band crossings, flat bands, and exotic magnetotransport phenomena [1,2]. Here, we report the observation of linearly dispersive Weyl nodes in strongly correlated kagome Mn3Pt epitaxial films using magnetotransport measurements and density functional theory (DFT) calculations. We observe a large, non-saturating, positive, transverse magnetoresistance (MR) which changes sign in parallel electric and magnetic fields, confirming the chiral anomaly arises in Weyl semimetals [3]. The chiral anomaly induced negative MR is very sensitive to the angle between electric and magnetic fields. Additionally, we provide evidence of planar Hall signal supporting the presence of chiral anomaly. Moreover, we also find a large anomalous Hall conductivity at low-temperature regime. These results pave the possibilities of new Weyl semimetals in thin films for topo-electronic devices. |
Friday, March 19, 2021 10:12AM - 10:24AM Live |
X52.00012: Epitaxial growth of Kagome semimetal Mn3Ge Deshun Hong, Changjiang Liu, Haw-Wen Hsiao, Jian-Min Zuo, Haihua Liu, Ilke Arslan, John Pearson, J Samuel Jiang, Anand Bhattacharya Materials with layered Kagome lattice allow the interplay of topological properties and interactions in flat bands, and can give rise to spin liquids, skyrmions and other phases. Mn3Ge as noncollinear antiferromagnet is Kagome semimetal, which hosts large anomaly in transverse charge transport and spin Hall angle. As an ideal material candidate for topological physics as well as antiferromagnetic spintronics, synthesis of high quality Mn3Ge epitaxial thin film is highly in demand and has been of great interest among experimentalists. Here, high quality epitaxial Mn3Ge films have been synthesized by two different techniques: magnetron co-sputtering [1] as well as molecular beam epitaxy. Both in-situ and ex-situ characterizations indicate these films are highly crystalline and c-axis oriented. Highly ordered atomic layers are captured by scanning transmission electron microscopy. Large anomalous Nernst effect as well as spin to charge conversion by FMR have been measured, which can be related to non-vanishing Berry curvature in reciprocal space. This work makes tunability in topological semimetal possible and opens a pathway for both the Kagome materials related physics and devices. |
Friday, March 19, 2021 10:24AM - 10:36AM Live |
X52.00013: The origin of complex magnetic phases in Fe based binary Fe3Ga4 compound Mahdi Afshar, Igor Mazin
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Friday, March 19, 2021 10:36AM - 10:48AM Live |
X52.00014: Imaging the motion of individual domain walls in the magnetic Weyl semimetal Co3Sn2S2 Changmin Lee, Praveen Vir, Kaustuv Manna, Ella Lachman, Chandra Shekhar, James Analytis, Claudia Felser, Joseph Orenstein The recently discovered magnetic Weyl semimetal Co3Sn2S2 is a uniaxial ferromagnet with a large c-axis anisotropy and a Tc of 175 K. Previous measurements of the temperature dependence of magnetization and ac susceptibility have revealed anomalous changes in slope near 130 K. Here we use scanning magneto-optic Kerr microscopy to measure the motion of individual domain walls in response to oscillating magnetic fields in the frequency range from 10 Hz to 10 kHz. As the temperature reaches 130 K from below, the amplitude of domain wall motion abruptly decreases by an order of magnitude, indicating that the effective magnetic torque exerted on the wall is significantly reduced. As the temperature is further increased toward the Curie temperature, the magnitude of wall motion becomes larger again and the magnetic domains shrink in size. We comment on how various scenarios of magnetic ordering can generate striking anomalies in the amplitude of domain wall motion vs. temperature. |
Friday, March 19, 2021 10:48AM - 11:00AM On Demand |
X52.00015: Ferromagnetic Weyl semimetals and their electromagnetic response in RAlX materials class (R= Ce or Pr and X = Si or Ge) Cheng-Yi Huang, Bahadur Singh, Hung-Yu Yang, Baozhu Lu, Faranak Bahrami, Jonathan Gaudet, Bochao Xu, Jacob D Franklin, Ilya Sochnikov, David E Graf, Guangyong Xu, Yang Zhao, Christina Hoffmann, Collin Leslie Broholm, Wei-Chi Chiu, Shin-Ming Huang, Baokai Wang, Hsin Lin, Darius Torchinsky, Arun Bansil, Fazel Tafti Ferromagnetic Weyl semimetals (FMWSMs) are exotic topological materials in which nontrivial band topology and magnetism drive novel electromagnetic responses such as the anomalous Hall effect (AHE). Currently available FMWSMs are limited to materials that preserve the inversion symmetry and generate the Weyl nodes by breaking the time-reversal symmetry (TRS). Here we present RAlX materials (R= Ce, Pr; X = Si, Ge) as new FMWSMs where the Weyl nodes are stabilized by breaking the inversion symmetry and their separation is controlled by breaking TRS. They show an unconventional AHE that can be tuned continuously by controlling the dispersion of the Weyl nodes. Based on our systematic first-principles calculations and transport measurements, we show that PrAlGe1-xSix (x = 0-1) is a type II FMWSM with an AHE that shows a transition from an intrinsic mechanism driven by the Berry curvature to an extrinsic scattering mechanism [1]. In contrast, CeAlSi is a noncollinear FMWSM with type I Weyl nodes [2]. It shows an AHE that appears (disappear) as the Fermi level is tuned to lie close to (away from) the Weyl nodes. Our study uncovers RAlX as a new, tunable FMWSM family hosting unconventional AHE. |
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