Ground states of the effective spin-1/2 XY pyrochlores: "Quantum Spin Ice" and "Order By Disorder"

Date: 
11/19/2013 - 3:30pm to 4:30pm
Location: 
CP179
Speaker(s) / Presenter(s): 
Kate Ross (Johns Hopkins/NIST)
Type of Event (for grouping events):
A knowledge of the spin-wave excitations in magnetic materials allows a robust determination of exchange parameters in suitable model Hamiltonians. We have used time-of-flight inelastic neutron scattering to measure the excitation spectra from field-polarized states of exotic frustrated magnets. We have taken this approach with two pyrochlores, Er2Ti2O7 and Yb2Ti2O7, whose magnetic properties have until this point been somewhat puzzling. Our strategy is to apply a magnetic field to push the systems to be in as classical a phase as possible, and then extract the exchange parameters using linear spin wave theory. Then, ramping the field down in the model, we reveal new information about the zero-field states of these magnets. The ground states of the frustrated spin-1/2 XY pyrochlores Er2Ti2O7 and Yb2Ti2O7 are thus revealed to be realizations of "quantum order by disorder" and "quantum spin ice", respectively. I will explain what these terms mean and show how the models lead to predictions, one of which we have confirmed (a small spin wave gap in Er2Ti2O7 opened by quantum fluctuations) and another which is still debated (does Yb2Ti2O7 support emergent electrodynamics?).
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