Department of Physics and Astronomy, the Johns Hopkins University, Baltimore, Maryland, 21218

Spin ice is a highly frustrated ferromagnet displaying rich emergent phenomena. In its ground states, magnetization of spin ice satisfies a zero-divergence constraint leading to an effective cancellation of the internal magnetic field. Spin excitations violate this constraint and thus behave as magnetic monopoles. Strong quantum fluctuations in spin-ice materials such as Tb₂Ti₂O₇ [1] and Tb₂Ti₂O₇ [2] may produce qualitatively new physics. I will discuss the nature of low-energy excitations in quantum spin ice in the presence of spontaneous magnetization or an external magnetic field along the <100> direction [3,4]. When monopoles are confined by the magnetic field or the effective molecular field, elementary excitations are Dirac strings connecting pairs of monopoles [5]. In contrast to classical spin ice, where string dynamics are driven by thermal fluctuations, strings in quantum spin ice possess inherent quantum dynamics that can be described by an emergent quantum string theory. When quantum fluctuations are weak, the vibrational modes of quantum string are manifested as multiple resonances in the dynamical structure factor. Strong fluctuations make the string tension negative and destabilize the polarized ground state. As the external field is increased, the strings gradually evolve to magnons, and the effective theory reduces to conventional spin wave theory [6].

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