Experimental evidence that zinc impurities pin pair-density-wave order in ${\mathrm{La}}_{2\text{−}x}{\mathrm{Ba}}_x{\mathrm{CuO}}_4$

Both Zn doping and $c$-axis magnetic fields have been observed to increase the spin stripe order in ${\mathrm{La}}_{2\text{−}x}{\mathrm{Ba}}_x{\mathrm{CuO}}_4$ with $x$ close to 1/8. For $x=0.095$, the applied magnetic field also causes superconducting layers to decouple, presumably by favoring pair-density-wave order that consequently frustrates interlayer Josephson coupling. Here we show that introducing 1% Zn also leads to an initial onset of two-dimensional (2D) superconductivity, followed by 3D superconductivity at lower temperatures, even in zero field. We infer that the Zn pins pair-density-wave order locally, establishing the generality of such behavior.

Figure 1

(a) Temperature dependence of the resistivity (in zero applied magnetic field) for current along the $c$ axis (blue squares) and parallel to the $ab$ planes (red circles). (b) Temperature dependence of the volume magnetic susceptibility with a field of ${\mu }_{0}H=0.2$ mT applied parallel to the planes, ${\chi }_c$ (blue squares), and perpendicular to the planes, ${\chi }_{ab}$ (red circles). The inset shows the susceptibilities on a linear scale near the initial transitions; linear fits to the normal state, and extrapolated to low $T$, are indicated by dashed lines. Subtraction of these fits enables the logarithmic-scale plots of the main panel. Characteristic temperatures discussed in the text are indicated by vertical dashed lines with labels at the top edge.

Figure 2

Semilog plots of the resistivity vs temperature for two field directions: ${\rho }_{ab}$ with (a) $\mathbf{H}\parallel \mathbf{c}$ and (b) $\mathbf{H}\perp \mathbf{c}$; ${\rho }_c$ with (c) $\mathbf{H}\parallel \mathbf{c}$ and (d) $\mathbf{H}\perp \mathbf{c}$. The value of ${\mu }_{0}H$ for each color-coded curve is indicated in the legend on the right.

Figure 3

Plots of the temperature dependence of ${\chi }_{ab}$ and ${\chi }_c$ under various magnetic fields applied either along or perpendicular to the $c$ axis. The magnetization measurement was taken after zero-field cooling the samples first. Robust shielding was observed for $H\parallel c$, indicating strong supercurrent flow in the $ab$ plane. In sharp contrast, the shielding breaks down for mere 1 mT of external magnetic field applied perpendicular to the $c$ axis, indicating a weak-link behavior between the superconducting planes ($ab$ planes), a characteristic of Josephson junctions.

Figure 4

$H$ vs $T$ phase diagram with (a) logarithmic and (b) linear scales for the field. Symbols are defined in the legends; the transition lines for the Zn-free sample are from Ref. [23].