Superconductivity and role of pnictogen and Fe substitution in 112-${\mathrm{LaPd}}_{x}P{n}_2$ $(Pn=\mathrm{Sb},\mathrm{Bi})$

We report on the epitaxial growth of As-free and phase-pure thin films of the 112-pnictide compounds ${\mathrm{LaPd}}_{x}P{n}_2$ $(Pn=\mathrm{Sb},\mathrm{Bi})$ grown on (100) MgO substrates by molecular beam epitaxy. X-ray diffraction, reflection high-energy electron diffraction, and x-ray photoelectron spectroscopy confirm the ${\mathrm{HfCuSi}}_2$ structure of the material with a peculiar pnictogen square net layer. The superconducting transition temperature $T_{\text{c}}$ varies little with Pd concentration. ${\mathrm{LaPd}}_x{\mathrm{Sb}}_2$ has a higher $T_{\text{c}}$ (3.2 K) by about 20% compared with ${\mathrm{LaPd}}_x{\mathrm{Bi}}_2$ (2.7 K). Fe substitution of Pd leads to a rapid decay of superconductivity, suggesting that these superconductors are conventional type II.

Figure 1

Room-temperature RHEED pattern of ${\mathrm{LaPd}}_x{\mathrm{Sb}}_2$ with the beam azimuth in the $\langle 100\rangle$ direction of the MgO substrate. The streaks indicate atomically smooth and epitaxial growth. The inset shows a sketch of the crystal structure.

Figure 2

A $2\theta \text{−}\omega$ XRD scan of ${\mathrm{LaPd}}_x{\mathrm{Sb}}_2$. The intensities are shown on a logarithmic scale. The left inset shows an XRR measurement, and the right inset shows a zoom of the (003) reflection.

Figure 3

$\varphi$ scans of the ${\mathrm{LaPd}}_x{\mathrm{Sb}}_2$ (112) reflection and MgO (111) reflection. The drawings represent the two epitaxial relations. The dashed cube is the ${\mathrm{LaPd}}_x{\mathrm{Sb}}_2$ (112) unit cell; the solid line represents the MgO unit cell.

Figure 4

XPS spectra of the Sb $3d$ core level in ${\mathrm{LaPd}}_x{\mathrm{Sb}}_2$. The two larger peaks correspond to ${\mathrm{Sb}}^{-3}$; the smaller peaks correspond to ${\mathrm{Sb}}^{-1}$.

Figure 5

Temperature dependence of the resistivity of a ${\mathrm{LaPd}}_x{\mathrm{Sb}}_2$ thin film.

Figure 6

Superconducting transition of a ${\mathrm{LaPd}}_x{\mathrm{Sb}}_2$ thin film shown for different magnetic fields applied perpendicular to the film surface. The inset shows the extrapolation of the critical field vs transition temperature behavior.

Figure 7

Phase diagram of ${\mathrm{LaPd}}_x{\mathrm{Sb}}_2$ showing the critical temperature vs Pd content. Onset and zero-resistance values are shown. The line on the right corresponds to ${\mathrm{LaPdSb}}_2$ as calculated from ICP data. The dashed line is a guide to the eye.

Figure 8

RHEED pattern from the surface of a ${\mathrm{LaPd}}_x{\mathrm{Bi}}_2$ thin film with beam azimuth in the $\langle 110\rangle$ direction of MgO. The streaks indicate atomically smooth and epitaxial growth.

Figure 9

A $2\theta \text{−}\omega$ XRD scan of a ${\mathrm{LaPd}}_x{\mathrm{Bi}}_2$ thin film. The intensities are shown on a logarithmic scale. Only $\left(00l\right)$ reflections are observed. The inset shows the XRR measurement.

Figure 10

$\varphi$ scan around the ${\mathrm{LaPd}}_x{\mathrm{Bi}}_2$ (112) reflection and the MgO (111) reflection. The drawing indicates the epitaxial relation. The unit cell of ${\mathrm{LaPd}}_x{\mathrm{Bi}}_2$ is represented by the dashed line; MgO is represented by a solid line.

Figure 11

Temperature dependence of the resistivity of a ${\mathrm{LaPd}}_x{\mathrm{Bi}}_2$ thin film. The inset shows a zoom of the superconducting transition.

Figure 12

Phase diagram of ${\mathrm{LaPd}}_x{\mathrm{Bi}}_2$. Onset and zero-resistance values are shown.

Figure 13

Current-voltage characteristics of a ${\mathrm{LaPd}}_x{\mathrm{Sb}}_2$ bridge with a width of 400 $\mu \mathrm{m}$ (solid black line) and of a microbridge with a width of 1.7 $\mu \mathrm{m}$ (dashed red line) measured at 0.38 K.

Figure 14

Comparison of $2\theta \text{−}\omega$ XRD scans of ${\mathrm{LaPd}}_{0.9-y}{\mathrm{Fe}}_y{\mathrm{Sb}}_2$ with $0\le y\le 0.7$.

Figure 15

The $c$-axis lattice constant of ${\mathrm{Pd}}_{0.9-y}{\mathrm{Fe}}_y{\mathrm{Sb}}_2$ obtained using the Nelson-Riley method as a function of $y$.