Fabrication of microstructured devices of the unconventional superconductor CeCoIn5 for investigations of isolated grain boundaries

TL;DR

This study shows how to extract and study isolated grain-boundary microstructures from bulk CeCoIn$_5$, an unconventional superconductor with $T_c=2.3$ K and $d_{x^2-y^2}$ pairing, by combining EBSD imaging, EDS composition mapping, and FIB milling to fabricate bridge devices across single grain boundaries. EBSD reveals a strong bias toward $90^deg$ misorientations about $[100]$, informing nucleation growth on CeIn$_3$ sites and guiding GB device fabrication. Electrical transport demonstrates phase-coherent superconductivity across grain boundaries, with evidence of weak-link/Josephson-like behavior and a measurable critical current ($I_c \\sim 100~$ A at 1.8 K), supported by the Ambegaokar-Baratoff relation, though GB resistance remains a small component of the total device resistance. The work provides a concrete fabrication recipe and establishes the feasibility of bulk-material Josephson-junction-like devices in CeCoIn$_5$, enabling future quantum-device architectures assembled from polycrystalline unconventional superconductors.

Abstract

Grain boundaries are critical for determining the functionality of polycrystalline materials. Here we present on the structural $\&$ transport properties of grain boundaries in the unconventional superconductor CeCoIn$_5$. We provide a detailed recipe for the fabrication of isolated grain boundary devices from of as-grown polycrystalline samples of CeCoIn$_5$. Electron backscattered diffraction imaging of polycrystalline CeCoIn$_5$ samples reveals an abundance of $90^\circ$ misorientation grain boundaries suggesting a preferential nucleation of CeCoIn$_5$ grains with 90$^\circ$ misorientation over a random distribution of grain orientations. Transport measurements across grain boundary devices establish coherence of superconductivity and allows us to establish a lower bound on the critical current density for the grain boundaries. Our work opens new possibilities for fabrication of quantum devices such as Josephson-junctions out of bulk unconventional superconducting materials.

Figures (12)

• Figure 1: Grain and grain boundary imaging of a polycrystaline CeCoIn$_5$ sample using EBSD. (a) Schematic diagram of the EBSD imaging setup. (b) Image quality (IQ) map demonstrating the quality of collected diffraction patterns in grayscale as discussed in the text. (c) Inverse pole figure (IPF) map for the sample normal along the $[001]$ crystal lattice direction representing different grain orientations specified by the color-coded label on the left. (d) IPF map overlaid on an grayscale IQ map. (e) Enlarged view of a small region of IPF in (c) depicting the grain boundaries between neighboring grains as well as orientation of individual grains through an overlaid tetragonal unit cell.
• Figure 2: Identification of grain boundaries between neighboring grains in a polycrystalline sample of CeCoIn$_5$. (a) Inverse pole figure depicting grain boundaries between the neighboring grains misoriented by angles 90$^\circ$ about $[100]$ tetragonal direction (solid blue line), 70$^\circ$ about $[110]$ direction (solid red line) and 60$^\circ$ about $[3\bar{3}5]$ direction (solid green line), respectively. (b) Histogram illustrating the fractional distribution of grain boundaries with misorientation angles between 0$^\circ$ to 90$^\circ$, binned in interval size of 10$^\circ$. Inset shows the fractional distribution of grain boundaries projected on an inverse pole figure for the misorientation angles 10$^\circ$, 70$^\circ$ and 90$^\circ$.
• Figure 3: Illustration of 90$^\circ$ grain boundary in polycrystalline CeCoIn$_5$ sample. (a) Cartoon depicting two tetragonal CeCoIn$_5$ grains growing at adjacent orthogonal faces of a CeIn$_3$ nucleation site, leading to a 90$^\circ$ misorientation grain boundary. Yellow, cyan and gray spheres are cerium, cobalt, and indium atoms, respectively. (b) $d_{x^2-y^2}$ SC order parameter overlaid on the tetragonal CeCoIn$_5$ lattice across the 90$^\circ$ grain boundary about $[100]$. (c) 90$^\circ$ grain boundary in (b) oriented at an angle $\phi$ with respect to the tetragonal $c$ axis as discussed in the text.
• Figure 4: Fabrication of microstructured devices across an isolated grain boundary for the samples (a) $S1$ and (b) $S2$. (Left panels) Inverse pole figure maps of a polycrystalline CeCoIn$_5$ sample with a 90$^\circ$ misorientation grain boundary about $[100]$ depicted by the solid blue line. (Middle panel) SEM images of the fabricated grain boundary devices. (Right panel) Enlarged view of the 90$^\circ$ misorientation grain boundaries with an overlaid SEM image of the corresponding devices.
• Figure 5: Temperature-dependent resistivity $\rho (T)$ of CeCoIn$_5$ (a) single crystal, single grain devices and (b) grain boundary devices. $\rho_c (T)$ data is taken from Ref. Malinowski. Dashed lines in (a) corresponds to $\rho_{eff}$ as discussed in the text.