Anomalous Nernst effect of the heavy-fermion superconductor URu$_{2}$Si$_{2}$

The heavy-fermion material URu$_{2}$Si$_{2}$ exhibits the ``hidden order'' and superconducting phase transitions at T$_{\rm{HO}}$ = 17.5 K and T$_{\rm{SC}}$ = 1.4 K, respectively. Below T$_{\rm{HO}}$ a significant decrease of carrier density has been observed, and the remaining carriers condense into the superconducting state below T$_{\rm{SC}}$. The superconducting symmetry is suggested to be chiral d-wave with time reversal symmetry breaking. We have recently measured the Nernst coefficient ${\nu}$(T) in an ultraclean single crystal of URu$_{2}$Si$_{2}$ with RRR $\sim$ 700, which is much larger than the previous report [1]. We observed an increase of in $\nu$(T) below T$_{\rm{HO}}$ which shows an additional steep increase below $\sim$ 3T$_{\rm{SC}}$. The magnitude of $\nu$(T) is much larger than the previous report and reaches $\sim$ 200 $\mu$V/KT at 1 T. We show that such a giant Nernst effect in an ultraclean sample cannot be explained by conventional Gaussian superconducting fluctuations. Possible origins including fluctuations of exotic chiral superconductivity will be discussed.\\[4pt] [1] R. Bel $\it{et}$ $\it{al}$., Phys. Rev. B $\bf{70}$, 220501 (2004).