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Center for Nonlinear Studies |
Revealing the nature of new exotic excitations is one of the grand challenges in condensed matter physics. Magnetic monopoles, which are hypothetical elementary particles in particle physics, are now effectively realized by thermal fluctuations in the so-called classical spin ice systems. A fundamental question is how quantum fluctuations affect the nature of magnetic monopole and whether new exotic excitation emerges in "quantum" spin ice. We address these issues in the key materials, Yb2Ti2O7, Pr2Zr2O7 and Pr2Ir2O7, which contain spin-ice correlations with significant quantum fluctuations, by measuring thermal conductivity and specific heat. We find that the elementary excitations “quantum monopoles†transport heat by propagating coherently and almost ballistically in the spin fluid state of 3D pyrochlore lattice. Such a ballistic propagation is in a striking contrast to diffusive propagation of classical monopoles, and bears a striking resemblance to the case of 1D and 2D quantum spin liquids. There, the excitations are fractional spinons, which obey semion and likely fermion statistics, respectively. On the other hand, our results show that the quantum monopoles are likely bosonic in 3D spin liquid. We also found additional anomalously large enhancement of thermal conductivity and specific heat at very low temperature below monopole excitation energy. This is naturally explained by the emergence of "artificial photon", which is the characteristic elementary excitation of the 3D quantum spin liquid state of spin ice.
Host: Filip Ronning