Amplitude excitation and giant spin-lattice fluctuations in a pyrochlore ruthenate single crystal

We uncover a wide temperature regime of enhanced spin-lattice fluctuations in single crystals of the pyrochlore ruthenate Nd$_2$Ru$_2$O$_7$. These fluctuations, prominent in Raman spectroscopy, yet elusive to thermodynamic probes, emerge below the N\'{e}el temperature and culminate in a new low-energy amplitude excitation with an order-parameter-like behavior at 3 meV. Concomitantly, an enormous phonon energy renormalization of $\Delta \omega_{\mathrm{ph}} > 3$ meV -- an order of magnitude larger than those in related pyrochlore oxides -- is observed. The unique two-fold symmetry of the low-energy mode, incompatible with the underlying crystal structure, raises the possibility of nematic order. Our study shines a light on instabilities in pyrochlore systems and on resulting symmetries and amplitude excitations that can be described within the Higgs mechanism.