In-plane charge dynamics in La 1.6 − x Nd 0.4 Sr x CuO ₄ : Absence of a charge gap in the spin/charge ordered state

We have investigated the optical conductivity of the spin/charge "stripe" order phase in La1.6 − xNd0.4SrxCuO4 in the microwave and the infrared regions. The in-plane conductivity is appreciably suppressed with respect to the spectrum for the Nd-free crystal over a wide frequency range of up to 8000 cm−1. The suppression persists even at room temperature, indicating a strong fluctuation of this order. Nevertheless, the spectrum is basically metallic without showing a gap associated with charge order. It was found that the magnetic penetration depth is extremely long in the superconducting state which coexists with the static spin/charge order.