Possible observation of photon excitations in the quantum spin-ice Pr$_{\mathrm{2}}$Zr$_{\mathrm{2}}$O$_{\mathrm{7}}$

It has been theoretically shown that the ground state of spin-ice system with quantum fluctuations can be quantum spin liquid, where new elementary excitations, photon, emerge [1]. In the rare-earth pyrochlore, Pr$_{\mathrm{2}}$Zr$_{\mathrm{2}}$O$_{\mathrm{7}}$, which contains spin-ice correlations with significant quantum fluctuations, the absence of magnetic ordering even at very low temperature suggests formation of quantum spin liquid state [2]. In order to examine the emergence of new exotic excitations, we have performed low-temperature thermal conductivity ($\kappa )$ measurements of Pr$_{\mathrm{2}}$Zr$_{\mathrm{2}}$O$_{\mathrm{7}}$. Interestingly, our data of $\kappa $/T shows a steep increase with decreasing temperature below 0.2K. Since the monopole density is negligibly small at such low temperature, the steep increase possibly indicates emergence of new elementary excitations. Anomalous magnetic-field dependence of $\kappa $/T observed below 0.2K further supports this possibility. [1] M. Hermele et al., Phys. Rev. B 69, 064404 (2004). [2] K. Kimura et al., Nature Commun. 4, 1934 (2013).