Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session B36: Exotic Dynamics of Pyrochlore MagnetsInvited
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Sponsoring Units: GMAG Chair: Claudio Castelnovo, University of Cambridge Room: BCEC 205C |
Monday, March 4, 2019 11:15AM - 11:51AM |
B36.00001: Excitation Continuum in a Pyrochlore S=1 Heisenberg Magnet Invited Speaker: Kemp Plumb The existence of a spin-liquid for Heisenberg spins on the pyrochlore lattice was first speculated by Jacques Villain nearly 40 years ago and it is now recognized that the ground state of this model is a Coulomb spin liquid. However, experimental realizations are scarce. In this talk, I will discuss a new pyrochlore lattice magnet, NaCaNi2F7, which realizes the isotropic Coulomb spin liquid with S=1 spins and the complication of random Na+ - Ca2+ charge disorder in the crystal structure. I will present neutron scattering and calorimetric measurements that were used to uncover the magnetic correlations in this material and determine the magnetic Hamiltonian. The ionic disorder creates a rugged energy landscape that acts to freeze a small fraction of the magnetic degrees of freedom. However, the energy scale set by this disorder is small, and the Heisenberg interactions prevail. Only a small fraction of the available moment is frozen, and the magnetism in NaCaNi2F7 is dominated by a persistently fluctuating component that appears as a broad continuum of magnetic signal in inelastic neutron scattering measurements. These measurements demonstrate a beautiful realization of the Coulomb spin liquid and provide new insight into the interplay between disorder and magnetic exchange interactions in highly frustrated magnets. |
Monday, March 4, 2019 11:51AM - 12:27PM |
B36.00002: Novel Photogalvanic Effects in Weyl Semimetals Invited Speaker: Naoto Nagaosa Photovoltaic effect is attracting growing interest from the viewpoint of the geometrical quantal phase of Bloch wavefunctions, i.e., Berry phase. The interband transitions between bands causes the shift of the electrons due to the difference in the intra-cell coordinates encoded as the Berry phase connection, resulting in the shift current without the external bias voltage. On the other hand, it has been known that the Weyl fermion is the source or sink of the Berry curvature, i.e., monopole or anti-monopole in momentum space, and leads to the enhanced Berry phase phenomena. Therefore, it is natural to expect the novel photovoltaic effect there. In this talk, I will describe the physical mechanism of shift current, and its features such as I-V characteristics, dependence on the mean free time, and noise, and discuss its realizations in Weyl semimetals such as the surface state of topological insulator, and TaAs. This talk is based on the works with collaborators T. Morimoto, K.W. Kim, H.Ishizuka, Y.Zhang, J.v. d.Brink, C. Felser, B. Yan, N. Ogawa, T. Hayata, M. Ueda, M. Sotome, M. Nakamura, J. Fujioka, M. Ogino, Y. Kaneko, M. Kawasaki, and Y.Tokura. |
Monday, March 4, 2019 12:27PM - 1:03PM |
B36.00003: Metamagnetism, Criticality and Dynamics in the Quantum Spin Ice Pr2Zr2O7 Invited Speaker: Nan Tang Geometrical frustration may prevent the formation of long-range magnetic order, giving rise to a diverse range of novel ground states. One of such an unusual state is called quantum spin liquid (QSL) possibly realized in quantum spin ice systems. Its excitations behave like charged particles interacting with linearly dispersive “photon”, protected by hidden topological order parameter. Yet, the properties of quantum phase transition between such topological phases remain unknown and are waiting to be established. |
Monday, March 4, 2019 1:03PM - 1:39PM |
B36.00004: Magnetic moment fragmentation and dynamical spin ice features in Neodymuim pyrochlores Invited Speaker: Bella Lake Neodymium pyrochlores show rich, complex and exotic physical phenomena. The Nd3+ ions have a well-isolated ground state doublet with octupolar-dipolar symmetry and they interact with each other via highly anisotropic exchange interactions. We have studied two members of this family Nd2Zr2O7 and Nd2Hf2O7. Both show long-range all-in-all-out antiferromagnetic order revealing broken gauge symmetry in the ground state. The lowest energy excitations form a flat, gapped mode with a pinch-point structure factor suggesting a proximate gauge symmetry leading to dynamical spin ice. At higher energies there are dispersive spin-wave modes which touch the flat mode, at the pinch points. They have a pinch point pattern of their own which is rotated with respect to that of the flat mode and which give rise to half-moon features in constant energy slices. The coexistence of long-range magnetic order, the pinch-point mode and half-moon features originate from fragmentation of the Nd3+ magnetic moment into divergence-free and divergence-full parts. Finally at temperatures just above the Neel temperature, the pinch point mode becomes gapless and the spin-waves broaden suggesting a spinon continuum. |
Monday, March 4, 2019 1:39PM - 2:15PM |
B36.00005: Neutron scattering signatures of pyrochlore spin liquids and nematic phases Invited Speaker: Ludovic Jaubert Research on spin liquids has gradually evolved from the search of systems that do not order, to the quest of exotic quasi-particles and emergent gauge fields. But how to measure a gauge field in experiments ? Neutron scattering certainly offers a helpful microscopic probe, e.g. with Coulomb gauge fields famously identified by pinch points in spin ice materials. But there is no spin-liquid analog of a Bragg peak; the very diversity of spin liquids makes them both fascinating and evasive. |
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