Lower critical fields of ultraclean URu$_{2}$Si$_{2}$ single crystal

We study the lower critical fields $H_{c1}$ on extremely clean single crystals of URu$_2$Si$_2$ ($T_c = 1.4$ K) by using a miniature Hall-sensor array magnetometry which can precisely determine the first flux penetration above $H_{c1}$ [1]. For H // $c$, $H_{c1}(T)$ shows highly unusual behavior and is strongly suppressed below 1.2 K with a distinct kink anomaly, while $H_{c1}(T)$ for H // $a$ is well described by the previously proposed multiband nature. We suggest that this anomalous $H_{c1}(T)$ behavior for H // $c$ derives from a peculiar flux penetration through the domain walls in the multicomponent superconductor with broken time reversal symmetry. [1] R. Okazaki et al., arXiv:0909.1075.