Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session W19: Spin Ice  From Freezing to Cerenkov RadiationInvited

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Sponsoring Units: GMAG Chair: Kate Ross, Colorado State University Room: 207 
Friday, March 6, 2020 8:00AM  8:36AM 
W19.00001: Tuning dipolar interactions in artificial spin ices  From Ising to Potts spins Invited Speaker: Francois Montaigne Since the proposal in 2006 [1] to use nanomagnets patterned by topdown techniques to mimic "artificial spins", the studies of artificial spin systems has attracted wide interest [2]. As a matter of facts, the possibility to design "upon request" arbitrary network thanks to lithography and the possibility to determine completely the "spin" configuration with magnetic imaging offer a wide playground for statistical physics. Up to now only Ising spin systems have been studied. However, beyond Ising spins, statistical physics and condensed matter physics have shown the interest of other spin models like qstate Potts models (q different spin orientation) or even XY model (isotropic in plane orientation). 
Friday, March 6, 2020 8:36AM  9:12AM 
W19.00002: Slow dynamics in classical and quantum spin ice systems Invited Speaker: Claudio Castelnovo The low temperature properties of magnetic systems with fractionalised pointlike excitations are determined by the interplay between a small density of quasiparticles and the underlying spin degrees of freedom out of which they are borne. Spin ice materials are a case in point. Even in their simplest classical modelling, a dramatic slowing down is expected due to the excitations becoming exponentially sparse. Once more realistic details are taken into account, and in particular quantum fluctuations are included, the situation becomes far more complex and interesting. The phenomenology uncovered thus far includes the formation of longlived metastable states, structural as well as correlationdriven disorder that alters the local quantum mechanical spin fluctuations, and a prominent freezing of the dynamics whose origin continues to elude our understanding. This talk presents a selection of results and observations that highlight the uniquely rich, tunable and experimentally accessible playground to study slow dynamics in frustrated magnetism offered by spin ice systems. 
Friday, March 6, 2020 9:12AM  9:48AM 
W19.00003: Machine learning assisted analysis of neutron scattering: new insights into spin ice Invited Speaker: David Tennant The macroscopic manifolds of ground states in highly frustrated magnets are responsible for their rich physical behavior. Thermal and quantum fluctuations within these manifolds gives rise to liquid states with remarkable properties which are receiving intense interest. An open question is, what is the fate of these liquid states as the system cools? To gain insight into this we have undertaken a comprehensive study of Dy_{2}Ti_{2}O_{7}. Dy_{2}Ti_{2}O_{7} is well known as a prototypical spin ice material that shows a U(1) gauge liquid behavior with magnetic monopole quasiparticles. From a combination of neutron scattering, magnetic noise, and thermodynamic measurements we have developed a model for the material using machine learning. The analysis involves use of autoencoders to identify phases and by optimizing in the network’s latent space highly accurate interaction parameters are extracted. A key part of this analysis is the ability to discriminate artifacts from physical signals in the neutron scattering data and to perform analysis on threedimensional diffuse data sets. The extracted model is shown to reproduce glass formation in the material and provides microscopic understanding of a range of observations including the arresting of order as freezing occurs and 1/f^{α} magnetic noise. High performance computer modeling has also been used to map the development of shortrange order and changes in dimensionality and topology in monopole pathways. Our results suggest ways in which spin glass formation could occur from simple interactions even without intrinsic disorder. 
Friday, March 6, 2020 9:48AM  10:24AM 
W19.00004: Dimensional transmutation of quantum monopole dynaimcs Invited Speaker: Masafumi Udagawa Fractionalization is one of the most remarkable phenomena in the systems of nontrivial topological character. Magnetic monopoles in quantum spin ice (QSI) give a typical example of fractional excitations, and a keen interest is focused on their role in dynamical and transport properties of QSI candidate materials. However, due to the fractional nature of excitations, magnetic monopoles behave quite differently from conventional excitations such as magnons, and their character still remains quite elusive. In this contribution, in order to understand the nature of magnetic monopoles, we address the anisotropic limit (J± << Jz) of spin1/2 quantum XXZ model defined on a pyrochlore lattice. In particular, we focus on the temperature region, J±^3/Jz^2 << T << J±, where the ground state spin ice manifold is still incoherent, while the monopole excitations already show coherent quantum motion. In this region, we obtained the local dynamical structure factor by the exact diagonalization of 32 site cluster [1]. The obtained spectrum shows a steep edge discontinuity at low energy attributed to the van Hove singularity of monopole spectrum, which results from the dimensional transmutation due to the coupling to background gauge field. We clarify the origin of this dimensional transmutation in terms of the state graph description, by mapping the monopole motion to a free particle Hamiltonian on a virtual Husimi cactus graph. 
Friday, March 6, 2020 10:24AM  11:00AM 
W19.00005: Spectroscopy of spinons in quantum spin ice: threshold enhancement and Cerenkov radiation Invited Speaker: Christopher Laumann Quantum spin liquids are low temperature phases of magnetic materials in which quantum fluctuations prevent the establishment of longrange magnetic order. These phases support exotic fractionalized spin excitations (spinons) and emergent gauge fields. In this talk, I will briefly review the basic theoretical picture of how a quantum Coulomb phase emerges in spin ice and then turn to recent results regarding the observable consequences in neutron scattering. The emergent Coulomb interaction modifies the threshold crosssection for spinon production dramatically, changing the weak turnon expected from a meanfield treatment to an abrupt onset reflecting the basic coupling parameters. The slow photon typical in existing lattice models and materials suppresses the intensity at finite momentum and allows profuse Cerenkov radiation beyond a critical momentum. These features are broadly consistent with recent numerical and experimental results. 
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