Optical probe of anisotropic and incoherent charge dynamics in a layered ferromagnet:La1.2Sr1.8Mn2O7

Highly anisotropic optical conductivity spectra and their temperature variation have been investigated for a layered ferromagnetic ${(T}_{\mathrm{c}}$=121 K) crystal of La${}_{2-2x}$Sr${}_{1+2x}$Mn${}_2$O${}_7$ $(x$=0.4). The in-plane spectra show conspicuous spectral-weight transfer over an energy region of 0–3 eV with evolution of spin polarization below $T_{\mathrm{c}}$ and in the ferromagnetic, highly conducting ground state form a prominent peak structure around 0.4 eV with minimal Drude weight. By contrast, the $c$-axis spectra show little change with temperature and remain insulatorlike. The results indicate highly incoherent dynamics of the fully quasi-two dimensional spin-polarized carriers.