Quantum spin liquids have been long hypothesized to demonstrate macroscopic entanglement of
emergent degrees of freedom. While verification of such quantum mechanical nature remains
experimentally challenging, advanced measurement schemes can be developed to exemplify spin
fractionalization using classical model systems. Here, I will discuss our recent work in classical
spin ice Ho2Ti2O7, where we parse through the dynamical process of magnetic relaxation to
experimentally distinguish emergent magnetic monopoles from individual spin dipoles. With the
development of a time-resolved neutron scattering technique, we are able to directly visualize the
motion of monopole quasiparticles in the momentum space under the perturbation of a magnetic
field. Further corroborated by the broad-band magnetic susceptibility to extend our probed time
scale to over ten decades, we clearly discover a thermal crossover between two distinct relaxation
processes, namely, monopole motion through the spin-ice vacuum at low temperatures, and
reorientation of spin dipoles at elevated temperatures. These relaxation processes are highly
sensitive to disorder, which suggests a prospective controlling method to switch between the
classical and quantum regimes. I will also relate to other experimental scenarios that potentially
advance our search of emergent phenomena in quantum spin systems.
Yishu Wang, T. Reeder, Y. Karaki, J. Kindervater, T. Halloran, N. Maliszewskyj, Yiming Qiu, J. A.
Rodriguez, S. Gladchenko, S. M. Koohpayeh, S. Nakatsuji, C. Broholm, Monopolar and dipolar relaxation
in spin ice Ho2Ti2O7, Sci. Adv. 7, ea0908 (2021).
Yishu Wang is currently a postdoctoral fellow of the Institute for Quantum Matter at The Johns
Hopkins University. She obtained the Ph.D. degree in Physics from California Institute of
Technology in 2018, M.S. degree in Physics from The University of Chicago in 2014, and a B.S.
degree in Engineering Physics from Tsinghua University in 2013. Her research interest focuses on
experimental explorations of correlated electron systems, covering an expanding range of topics
including quantum magnetism, quantum phase transition, magnetoresistance, and
superconductivity. She has utilized and developed a broad sort of experimental techniques at both
university research labs and international user facilities at neutron and synchrotron sources,
covering physical parameter spaces of cryogenic temperature, magnetic field, and pressure.
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