Anisotropic optical conductivity of Sr4Ru3O10

The optical conductivity ?(?) of Sr4Ru3O10 has been studied by means of reflectivity measurements between 12 and 300 K both in the ab planes (from 40 to 28 000 cm-1) and along the c axis (from 40 to 19 000 cm-1). In the far and mid infrared, ?ab(?) results from a superposition of a Drude term, with a plasma frequency which increases below the ferromagnetic transition at Tc=105 K, and of two bands in the infrared. One is peaked at about 400 cm-1 for T< 300 K, but softens to 300 cm-1 at room temperature. The other one is peaked at 1300 cm-1 and, below Tc, provides spectral weight to the Drude term, suggesting that it has a magnetic origin. Along the c axis, ?c(?) is instead that of a good insulator, with phonon peaks whose frequencies ?ph regularly increase as T decreases. For the mode at the highest frequency, however, which can be assigned to the apical oxygens of the RuO6 octahedra, both ?ph and the intensity Iph have an anomalous behavior below Tc.