  Quantum spin liquid (QSL) is a disordered state of quantum-mechanically
entangled spins commonly arising from frustrated magnetic dipolar interactions.
However, QSL in some pyrochlore magnets can also come from frustrated magnetic
octupolar interactions. Although the key signature for both dipolar and
octupolar interaction-driven QSL is the presence of a spin excitation continuum
(spinons) arising from the spin quantum number fractionalization, an external
magnetic field-induced ferromagnetic order will transform the spinons into
conventional spin waves in a dipolar QSL. By contrast, in an octupole QSL, the
spin waves carry octupole moments that do not couple, in the leading order, to
the external magnetic field or to neutron moments but will contribute to the
field dependence of the heat capacity. Here we use neutron scattering to show
that the application of a large external magnetic field to Ce2Zr2O7, an
octupolar QSL candidate, induces an Anderson-Higgs transition by condensing the
spinons into a static ferromagnetic ordered state with octupolar spin waves
invisible to neutrons but contributing to the heat capacity. Our theoretical
calculations also provide a microscopic, qualitative understanding for the
presence of octupole scattering at large wavevectors in Ce2Sn2O7 pyrochlore,
and its absence in Ce2Zr2O7. Therefore, our results identify Ce2Zr2O7 as a
strong candidate for an octupolar U (1) QSL, establishing that frustrated
magnetic octupolar interactions are responsible for QSL properties in Ce-based
pyrochlore magnets.
