We report magnetotransmission measurements on a series of ${\text{La}}_{2-x}{\text{Sr}}_x{\text{CuO}}_4$ thin films. The measurements were performed in magnetic fields of 18 T, on films with doping levels of $x=0$, 0.01, 0.03, 0.045, 0.06, 0.08, and 0.10. In addition, an optimally doped film $\left(x=0.16\right)$ was studied in magnetic fields up to 33 T, both above and below its superconducting critical temperature $T_c=41\text{K}$. A combination of Gaussian and wavelet filtering was employed to improve the signal-to-noise ratio of the data. However, even after this procedure, we could not detect any field-induced changes of transmission in any of the studied samples. Our results therefore rule out a direct relation between intensity changes in mid-infrared charge excitations and a bosonic mode in the far infrared. We discuss these observations in the context of existing proposals regarding the nature of medium energy range excitations in the cuprates.

• Received 4 December 2008

DOI:https://doi.org/10.1103/PhysRevB.79.134503