Bulletin of the American Physical Society
APS March Meeting 2023
Volume 68, Number 3
Las Vegas, Nevada (March 510)
Virtual (March 2022); Time Zone: Pacific Time
Session S24: Pyrochlore Coulomb Spin Liquids: From Fragmentation to AxionsInvited

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Sponsoring Units: DCMP Chair: Gaurav Chaudhary, University of Cambridge Room: Room 237 
Thursday, March 9, 2023 8:00AM  8:36AM 
S24.00001: Magnetic Fragmentation in Spin Ice Systems Invited Speaker: Peter Holdsworth The apparent fragmentation [1,2] of magnetic moments in spin icelike systems into coexisting longitudinal, transverse and harmonic parts is a direct consequence of emergent electromagnetism in these systems. At the microscopic level, the moments act as elements of this emergent lattice field from which topological defects, monopoles [3] can be excited. By construction, the left over is made up of a divergence free, or transverse component and a harmonic component controlled by the boundaries. The fragmentation process therefore provides a continuous fractionalisation of the moments allowing for novel magnetic phases and topological phase transitions. In this seminar I will review some of these including the partially ordered monopole crystal phase in which a Coulomb spin liquid coexists with antiferromagnetic order, as well as two fluid phases and topological aspects related to the harmonic component. I will also show how fragmentation leads naturally to the emergence of classical Dirac strings and how it can be used as a tool for the analysis of neutron scattering data in the presence of quantum spin fluctuations [4]. Several experimental examples of fragmented systems will be discussed. 
Thursday, March 9, 2023 8:36AM  9:12AM 
S24.00002: Thermal conductivity of square ice Invited Speaker: Jeffrey G Rau We investigate thermal transport in square ice, a twodimensional analog of spin ice, exploring the role played by emergent magnetic monopoles in transporting energy. Using kinetic Monte Carlo simulations based on energypreserving extensions of singlespinflip dynamics, we explicitly compute the (longitudinal) thermal conductivity κ over a broad range of temperatures. We use two methods to determine κ: a measurement of the energy current between thermal baths at the boundaries, and the GreenKubo formula, yielding quantitatively consistent values for the thermal conductivity. We interpret these results in terms of transport of energy by diffusion of magnetic monopoles. We relate the thermal diffusivity κ/C, where C is the heat capacity, to the diffusion constant of an isolated monopole, showing that the subdiffusive motion of the monopole implies κ/C vanishes at zero temperature. Finally, we discuss the implications of these results for thermal transport in threedimensional spin ice, in spinice materials such as Dy_{2}Ti_{2}O_{7} and Ho_{2}Ti_{2}O_{7}, and outline some open questions for thermal transport in highly frustrated magnets. 
Thursday, March 9, 2023 9:12AM  9:48AM 
S24.00003: Strongcoupling QED in quantum spin ice Invited Speaker: Roderich Moessner Condensed matter systems provide alternative `vacua' exhibiting emergent lowenergy properties drastically different from those of the standard model. A case in point is the emergent quantum electrodynamics (QED) in the fractionalized topological magnet known as quantum spin ice, whose magnetic monopoles set it apart from the familiar QED of the world we live in. Here, we show that the two greatly differ in their finestructure constant α, which parametrizes how strongly matter couples to light: α_{QSI} is more than an order of magnitude greater than α_{QED}≈1/137. Furthermore, α_{QSI}, the emergent speed of light, and all other parameters of the emergent QED, are tunable by engineering the microscopic Hamiltonian. We find that α_{QSI} can be tuned all the way from zero up to what is believed to be the strongest possible coupling beyond which QED confines. In view of the small size of its constrained Hilbert space, this marks out quantum spin ice as an ideal platform for studying exotic quantum field theories and a target for quantum simulation. The large α_{QSI} implies that experiments probing candidate condensedmatter realizations of quantum spin ice should expect to observe phenomena arising due to strong interactions. 
Thursday, March 9, 2023 9:48AM  10:24AM 
S24.00004: Sleuthing out Quantum Spin Liquidity in Cerium Based Pyrochlore Magnets Invited Speaker: Andriy H Nevidomskyy The search for quantum spin liquids – topological magnets with fractionalized excitations – has been a central theme in condensed matter and materials physics. While theories are no longer in short supply, tracking down materials has turned out to be remarkably tricky. Pyrochlore systems have proven particularly promising, hosting a classical Coulomb phase in the spin ices, with subsequent proposals of candidate quantum spin liquids in other pyrochlores. Here, we focus on the strongly spinorbit coupled effective spin1/2 pyrochlores cerium zirconate (Ce_{2}Zr_{2}O_{7}) and cerium stannate (Ce_{2}Sn_{2}O_{7}), analyzing recent thermodynamic and neutron scattering experiments, to identify a microscopic effective Hamiltonian through a combination of finite temperature Lanczos, Monte Carlo and analytical spin dynamics calculations. The parameter values we deduced for Ce_{2}Zr_{2}O_{7} suggest a previously unobserved exotic phase, a piflux U(1) quantum spin liquid. Intriguingly, the octupolar nature of the magnetic moments makes them less prone to be affected by crystal imperfections or magnetic impurities, while also hiding some otherwise characteristic signatures from neutrons. The sister compound, Ce_{2}Sn_{2}O_{7} appears to also lie in the quantum spin liquid phase, with a more pronounced octupolar signature of Ce moments. Recent neutron backscattering measurements on this material, with extremely high energy resolution, offer an unprecedented glimpse into the spectrum of the fractionalized excitations, spinons. The theoretical analysis corroborates the evidence of strong interaction between these particles and the emergent “photons” in the theory that mimics the quantum electrodynamics. 
Thursday, March 9, 2023 10:24AM  11:00AM 
S24.00005: Dynamical Axions in U(1) Quantum Spin Liquids Invited Speaker: Christopher R Laumann Since their proposal nearly half a century ago, physicists have sought axions in both high energy and condensed matter settings. Despite intense and growing efforts, to date experimental success has been limited, with the most prominent results arising in the context of topological insulators. Here we propose a novel mechanism whereby axions can be realized in quantum spin liquids. We discuss the necessary symmetry requirements and identify possible experimental realizations in candidate pyrochlore materials. In this context, the axions couple both to the external and to the emergent electromagnetic fields. We show that the interaction between the axion and the emergent photon leads to a characteristic dynamical response, which can be measured experimentally in inelastic neutron scattering. 
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