Low Energy Electrodynamics of Novel Spin Excitations in the Quantum Spin Ice Yb$_2$Ti$_2$O$_7$
In condensed matter systems, the formation of long range order (LRO) with broken symmetry is often accompanied by new types of excitations. However, in many magnetic pyrochlore oxides, geometrical frustration suppresses conventional LRO while at the same time non-trivial spin correlations are observed. For such materials, a natural question to ask then is what is the nature of the excitations in this highly correlated state without broken symmetry? Frequently the application of a symmetry breaking field can stabilize excitations whose properties still reflect certain aspects of the anomalous state without long-range order. Here we report evidence of novel magnetic excitations in the quantum spin ice material Yb$_2$Ti$_2$O$_7$, obtained from time-domain terahertz spectroscopy (TDTS). At large fields, both magnon and two-magnon-like excitations are observed in a $<$001$>$ directed magnetic field illustrating the stabilization of a field induced LRO state. The unique ability of TDTS to measure complex response functions allows a direct study of magnetic responses in different polarization channels, revealing the existence of an unusual left-hand polarized magnon. The g-factors of these excitations are dramatically enhanced in the low-field limit, showing a cross-over of these one- and two-magnon states into features consistent with quantum string-like excitations proposed to exist in quantum spin ice in a small $<$001$>$ applied field.