Low-energy electrodynamics of novel spin excitations in the quantum spin ice Yb2Ti2O7

In condensed matter systems, formation of long-range order (LRO) is often accompanied by new excitations. However, in many geometrically frustrated magnetic systems, conventional LRO is suppressed, while non-trivial spin correlations are still observed. A natural question to ask is then what is the nature of the excitations in this highly correlated state without broken symmetry? Frequently, applying a symmetry breaking field stabilizes excitations whose properties reflect certain aspects of the anomalous state without LRO. Here we report a THz spectroscopy study of novel excitations in quantum spin ice Yb 2 Ti 2 O 7 under a directed magnetic field. At large positive fields, both right- and left-handed magnon and two-magnon-like excitations are observed. The g-factors of these excitations are dramatically enhanced in the low-field limit, showing a crossover of these states into features consistent with the quantum string-like excitations proposed to exist in quantum spin ice in small fields. In geometrically frustrated magnets, long-range magnetic order is typically suppressed, whereas at the same time non-trivial spin correlations are observed. Using time-domain terahertz spectroscopy, the authors find evidence for extended quantum string-like excitations in the quantum spin ice material Yb 2 Ti 2 O 7 .