Charge Order, Metallic Behavior, and Superconductivity in ${\mathrm{La}}_{2-x}{\mathrm{Ba}}_x{\mathrm{CuO}}_4$ with $x=1/8$

The $ab$-plane optical properties of a cleaved single crystal of ${\mathrm{La}}_{2-x}{\mathrm{Ba}}_x{\mathrm{CuO}}_4$ for $x=1/8$ ($T_c\simeq 2.4\mathrm{K}$) have been measured over a wide frequency and temperature range. The low-frequency conductivity is Drude-like and shows a metallic response with decreasing temperature. However, below $\simeq 60\mathrm{K}$, corresponding to the onset of charge-stripe order, there is a rapid loss of spectral weight below about 40 meV. The behavior is quite different from that typically associated with the pseudogap in the normal state of the cuprates. Instead, the gapping of the normal-state single-particle excitations looks surprisingly similar to that observed in superconducting ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x{\mathrm{CuO}}_4$, including the presence of a residual Drude peak with reduced weight.

Figure 1

The reflectance at a near-normal angle of incidence for several temperatures over a wide spectral range for a cleaved surface of ${\mathrm{La}}_{1.875}{\mathrm{Ba}}_{0.125}{\mathrm{CuO}}_4$ ($T_c\simeq 2.4\mathrm{K}$) for light polarized in the $ab$ plane. The infrared reflectance increases with decreasing temperature until about 60 K, below which it is suppressed in the $200–2000{\mathrm{cm}}^{-1}$ region. Inset: The detailed temperature dependence of the far-infrared reflectance at and below 60 K. (The resolution is $2{\mathrm{cm}}^{-1}$.)

Figure 2

The $ab$-plane optical conductivity of ${\mathrm{La}}_{1.875}{\mathrm{Ba}}_{0.125}{\mathrm{CuO}}_4$ ($T_c\simeq 2.4\mathrm{K}$). (a) The conductivity in the infrared region between 295 and 60 K, showing a steady narrowing of the Drude-like component. The sharp features in the conductivity are the normally allowed infrared active vibrations; a feature at $\simeq 125{\mathrm{cm}}^{-1}$ at 295 K is not visible at 60 K. (b) The conductivity for several temperatures below 60 K. A new vibrational feature appears at $\simeq 205{\mathrm{cm}}^{-1}$ below about 40 K. Inset: The $ab$-plane resistivity of ${\mathrm{La}}_{2-x}{\mathrm{Ba}}_x{\mathrm{CuO}}_4$ for $x\simeq 1/8$ (solid curve) and in a 9 T magnetic field (dotted curve). The open circles are the estimated values for the dc resistivity calculated from the Drude parameters in Table .

Figure 3

The $ab$-plane spectral weight $N(\omega )$ of ${\mathrm{La}}_{1.875}{\mathrm{Ba}}_{0.125}{\mathrm{CuO}}_4$ for several different temperatures above and below 60 K. To simplify the units, the conductivity has been expressed in ${\mathrm{cm}}^{-1}$. Inset: The temperature dependence of the spectral weight for cutoff frequencies ${\omega }_c=40$, 100, and $300{\mathrm{cm}}^{-1}$.