Quadrupole order of Tb$_{2+x}$Ti$_{2-x}$O$_{7+y}$
We argue that the frustrated pyrochlore magnet Tb2+xTi2-xO7+y (TTO) has a quantum phase transition between a quantum spin liquid (QSL) state and a hidden order -- probably quadrupolar -- state, as a function of the off-stoichiometry x. The QSL state, referred to as "quantum spin ice", is characterized by an emergent U(1) field and excitations in form of gapped bosonic spinons and gapless photons. This state was recently measured on IN5 using a high-quality single crystal assembly, showing short-range spin-fluctuations that characterize the QSL state. We now propose to study the mixed spinwave-quadrupolar excitations expected in the hidden-order phase using another high-quality crystal assembly on IN5. The data will be analyzed using a mean-field random-phase approximation incorporating planar (anti-)ferropseudospin states. The existence of such excitations would be compelling evidence for quadrupole ordering, and hence provide strong evidence for our conjecture.
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FAK Bjorn; KADOWAKI Hiroaki and OLLIVIER Jacques. (2016). Quadrupole order of Tb$_{2+x}$Ti$_{2-x}$O$_{7+y}$. Institut Laue-Langevin (ILL) doi:10.5291/ILL-DATA.4-05-635
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