INFRARED REFLECTANCE OF POLYPYRROLE - METAL WITH A GAP IN THE SPECTRUM OF CHARGED EXCITATIONS

We report reflectance (R) measurements of polypyrrole (PPy) films with d.c. conductivity of 350 S/cm at 300 K. The reflectance is metal-like in the IR at all temperatures, with R increasing in the far-IR to about 100% at temperatures below 50 K. The optical conductivity, sigma(omega), and the real part of the dielectric function, epsilon(1)(omega), are not typical of a Drude metal. At 10 K, epsilon(1)(omega)= -A/omega(2) for h omega<200 cm(-1) while sigma(omega) decreases in the same spectral range, implying a narrow peak in sigma(omega) at h omega well below 20 cm(-1) with oscillator strength corresponding to about 2% of the total free-carrier oscillator strength. 'Metallic' PPy is, therefore, a conductor with a gap in the spectrum of charged excitations; the d.c. conductivity comes from a narrow peak in sigma(omega) at or near omega=0.