An important theme of modern condensed matter physics is the realization of novel excitations in materials (e.g. quasiparticles). Although they are not fundamental particles, such quasiparticles do constitute the most basic description of the excited states of the "vacuum" they reside in. In this regard the magnetic textures of the excited states of spin ices, magnetic pyrochlore oxides with dominant Ising interactions, can be modeled as effective magnetic monopoles. Utilizing the unique phase sensitive capabilities of time domain terahertz spectroscopy and microwave cavity techniques, we study the complex dynamic magnetic susceptibility of quantum spin ice Yb2Ti2O7. We find strong evidence of inertial effects in the monopoles dynamics. From the spectral weight, an effective mass of the monopoles is also obtained. Our results establish the magnetic monopoles as true coherently propagating quasiparticles in quantum spin ice.
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