Anisotropic optical conductivity of Sr4Ru3O10

The optical conductivity sigma(omega) 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, sigma(ab)(omega) results from a superposition of a Drude term, with a plasma frequency which increases below the ferromagnetic transition at T-c = 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 T-c, provides spectral weight to the Drude term, suggesting that it has a magnetic origin. Along the c axis, sigma(c)(omega) is instead that of a good insulator, with phonon peaks whose frequencies omega(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 omega(ph) and the intensity I-ph have an anomalous behavior below T-c.