Molecular beam epitaxy of a half-Heusler topological superconductor candidate YPtBi

The search for topological superconductivity has motivated investigations into materials that combine topological and superconducting properties. The half-Heusler compound YPtBi appears to be such a material; however, experiments have thus far been limited to bulk single crystals, drastically limiting the scope of available experiments. This has made it impossible to investigate the potential topological nature of the superconductivity in this material. Experiments to access details about the superconducting state require sophisticated lithographic structures, typically based on thin films. Here we report on the establishment of high-crystalline-quality epitaxial thin films of YPtBi(111), grown using molecular beam epitaxy on ${\mathrm{Al}}_2{\mathrm{O}}_3$(0001) substrates. A robust superconducting state is observed, with both critical temperature and critical field consistent with that previously reported for bulk crystals. Moreover, we find that $\mathrm{Al}{\mathrm{O}}_x$ capping sufficiently protects the sample surface from degradation to allow for proper lithography. Our results pave a path towards the development of advanced lithographic structures, which will allow the exploration of the potentially topological nature of superconductivity in YPtBi.

Figure 1

Growth temperature dependence of the structural properties of YPtBi thin films. (a) RHEED patterns taken in situ immediately after growth for each YPtBi thin film. The layers were grown with substrate temperatures of 300, 350, 400, and 450 ${}^{\circ }\mathrm{C}$. (b) $2\theta \text{−}\theta$ XRD diffractograms for each film. The expected positions of the (111) peaks of the YPtBi layer ($F\overline{4}$$3m$) are shown as dotted lines. The diffraction peaks of the ${\mathrm{Al}}_2{\mathrm{O}}_3$(0001) substrate, as well as those of monoclinic Bi, hexagonal Bi, and ${\mathrm{Pt}}_3\mathrm{Y}$ are indicated by asterisks, circles, squares, and diamonds, respectively. The curves are offset for clarity. The unit of intensity is counts per second (cps). (c) Scanning electron microscope images of the films.

Figure 2

Phase space for YPtBi growth based on the XRD analysis. The YPtBi(111) thin films with elemental Bi phase and ${\mathrm{Pt}}_3\mathrm{Y}$ phase are depicted as black solid squares and open squares, respectively. Two YPtBi(111) films with no detectable secondary phases are shown as blue circles.

Figure 3

Crystal structure and surface morphology of a YPtBi(111) thin film grown under optimum growth conditions, as confirmed by transport experiments. (a) $2\theta \text{−}\theta$ XRD diffractogram of the film. The indices label diffraction peaks of YPtBi. Diffraction peaks from the ${\mathrm{Al}}_2{\mathrm{O}}_3$(0001) substrate are marked with asterisks. (b) Rocking curve of the YPtBi(111) diffraction peak. The peak is deconvoluted into two pseudo-Voigt curves. (c) Azimuthal rotation $\varphi$ scans of the asymmetric (022) plane of YPtBi and the (012) plane of ${\mathrm{Al}}_2{\mathrm{O}}_3$. (d) SEM image of the film's surface morphology. Distinct grain boundaries are highlighted with white dashed lines, illustrating the ${60}^{\circ }$ angles due to twinned growth of (111)-oriented YPtBi crystals. (e) Schematic of the atomic configurations of (111)-oriented YPtBi crystals on the Al-terminated ${\mathrm{Al}}_2{\mathrm{O}}_3$(0001) surface.

Figure 4

(a) Temperature dependence (cooldown curves) of the four-terminal resistance of sample S1, with an out-of-plane external magnetic field of 0 or 0.5 T. Au wires bonded on the sample surface and used for the four-terminal measurement are indicated as numbers on the inset image. (b) Magnetic field dependence of resistance of the YPtBi(111) film at various temperatures. As a benchmark for the critical field $H_c$ we took the magnetic field where the resistance is half of the normal state resistance and plotted it in the inset. The zero-temperature value of $H_c$ is estimated to be 1.4 T based on the linear extrapolation. This value is compatible with the one previously reported for bulk YPtBi (1.5 T [8]). (c) Influence of $\mathrm{Al}{\mathrm{O}}_x$ capping on the YPtBi(111) thin film transport properties. A 5-nm-thick $\mathrm{Al}{\mathrm{O}}_x$ layer was deposited using atomic layer deposition (ALD), without exposing the YPtBi film surface to air. Layers S1 and S2 were grown under nominally identical growth conditions to the optimized layer presented in Fig. 3. The four-terminal resistance ($R=V_{3-4}/I_{1-2}$) for each sample is normalized to the normal state resistance ($R_n$) at 1 K.