Evidence for a Nodal Energy Gap in the Iron-Pnictide Superconductor LaFePO from Penetration Depth Measurements by Scanning SQUID Susceptometry

We measure changes in the penetration depth $\lambda$ of the $T_c\approx 6\mathrm{K}$ superconductor LaFePO. In the process, scanning SQUID susceptometry is demonstrated as a technique for accurately measuring local temperature-dependent changes in $\lambda$, ideal for studying early or difficult-to-grow materials. $\lambda$ is found to vary linearly with temperatures from 0.36 to $\sim 2\mathrm{K}$, with a slope of $143\pm 15\AA /\mathrm{K}$, suggesting line nodes in the superconducting order parameter. The linear dependence up to $\sim T_c/3$, similar to the cuprate superconductors, indicates well-developed nodes.

Figure 1

Left: Field coil-pickup loop mutual inductance $M$ against height above the sample; $h_{\mathrm{contact}}$ is the height at which the corner of the susceptometer chip contacts the sample. The line is a fit of Eq. (1). Right: photograph of the front end of the susceptometer and schematic of the susceptometer-sample geometry. The dashed loop is the image field coil.

Figure 2

$\Delta \lambda (T)\equiv \lambda (T)-\lambda (0)$ of Pb. A drift has been subtracted as described in the text. Inset: Open symbols: measurement of Gasparovic and McLean [29]. Filled symbols: present data; the vertical error bars are the systematic $\pm 5\%$ error on all $\Delta \lambda$ data in this Letter.

Figure 3

Top: $\Delta \lambda$ of two LaFePO specimens, at the points indicated in Fig. 4. The lines were fit over $0.7<T<1.6\mathrm{K}$. Bottom: black lines are possible superfluid densities for LaFePO sample #1, point 2, with different $\lambda (0)$. Shaded area: superfluid density of ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{6.99}$ [$1/({\lambda }_a{\lambda }_b)$], from 30, 31; the width of the shaded area reflects uncertainty in $\lambda (0)$.

Figure 4

(a), (b) LaFePO specimens #1 and #2. (c) Susceptibility scan of #1 at $T=0.4\mathrm{K}$. Over the superconductor $M$ is reduced from its vacuum level by the Meissner response. (d) Change in $h_{\mathrm{eff}}=h+\lambda$ between 0.4 and 3 K over specimen #2. (e), (f) Maps of local $T_c$, over the same areas as in (c) and (d). The crosses indicate the points where $\Delta \lambda (T)$ data were collected.

Figure 5

Solid lines: superfluid density of LaFePO. Dashed lines: fits. Main panel: single-band $d$-wave model, with the indicated guesses for $\lambda (0)$ and resulting fitting parameter ${\Delta }_d(0)$. Inset: two-band $s$-wave model, taking $\lambda (0)=3000\AA$, with the fitting parameters.