Conventional superconductivity in single-crystalline BiPt

TL;DR

BiPt is shown to host conventional, fully gapped $s$-wave superconductivity in the dirty limit with a bulk $T_c$ of about $T_c \approx 1.2\ \mathrm{K}$. A comprehensive set of bulk and microscopic probes (XRD, neutron pole figures, HAADF-STEM/EDS, magnetization, resistivity, heat capacity, $\mu$SR) demonstrates weak type-II behavior, hexagonal anisotropy, and time-reversal symmetry preservation. The data support a phonon-mediated, weakly coupled pairing mechanism with a hexagonal, multi-band electronic structure, providing a clean, topologically trivial benchmark for comparing Bi-based superconductors. Overall, BiPt serves as a crucial reference system for understanding pairing and topology in related Bi-Pd/Pt compounds and for disentangling conventional and topological superconductivity in this material family.

Abstract

Binary Bi-Pd/Pt systems have attracted a lot of interest because of their topologically non-trivial nature along with superconductivity. We report the structural and superconducting properties of high-quality single-crystalline BiPt using a comprehensive range of experimental techniques, including X-ray diffraction, electron microscopy, muon spin rotation/relaxation (μSR), magnetization, resistivity, and heat capacity. Our findings establish that BiPt is a weak type-II superconductor with a transition temperature (Tc) of 1.2 K which exhibits pronounced anisotropic superconducting characteristics attributed to its hexagonal crystal structure. Magnetization and electronic transport studies reveal that BiPt lies within the dirty limit, while μSR and heat capacity data indicate conventional s-wave superconductivity that maintains time-reversal symmetry. This work provides valuable insights into the pairing symmetry and superconducting mechanism of topologically trivial BiPt, a sound comparison system for other Bi-based topologically nontrivial superconductors.

Figures (5)

• Figure 1: Structural characterization of BiPt. (a) Powder XRD pattern confirms that BiPt crystallizes in a hexagonal crystal structure with space group P6$_3$/mmc (194) [Inset] Laue back reflection spectra of <001> plane clearly shows a six-fold symmetry as expected, confirming the high quality of the crystal. (b) Crystal structure of BiPt with Pt forming the hexagonal plane. (c) Plot of the neutron pole figure measurement performed on a 5 g piece of BiPt is shown. The data correspond to the (1,0,0)/(0,1,0) Bragg peak, which has a multiplicity of six. The three strong peaks shown here are consistent with one dominant grain, confirming the single crystallinity of the modified Bridgeman-grown sample. (d) Comparison of HAADF STEM with the EDX image which confirms single crystallinity and phase purity at the microscopic level. The HAADF STEM image of the <001> plane of BiPt shows a sharp contrast between Bi and Pt elements forming the hexagons.
• Figure 2: (a) ZFC and FC plot showing T$_c$ at 1.25(4) K (b) Temperature dependence of H$_{c1}$ and Ginzburg-Landau fits along and perpendicular to the c axis of a hexagonal crystal measured on a sphere. The inset shows a representative magnetization (M) versus field (H) plot, which is used to estimate the $H_{c1}$ (c) Temperature dependence of H$_{c2}$ along and perpendicular to the $c$-axis of BiPt crystal.
• Figure 3: Resistivity and heat capacity data. (a) Resistivity with current perpendicular and parallel to the $c$-axis is shown. Here, the dashed lines represent the Bloch-Grüneisen model fit of the resistivity.[inset] The expanded low-temperature region displays a transition from a resistive to a superconducting state at about 1.2 K (b) The resistivity deviates from quadratic behavior at around 35 K. (c) Electronic specific heat as a function of the temperature. [Inset] The specific heat in field of 0.1 T shows the absence of the superconducting transition.
• Figure 4: Results of the transverse field $\mu$SR measurements performed on single crystalline BiPt with 4.2 mT and 9.8 mT fields along the $c$-axis. (a) The temperature dependence of the inverse penetration depth square in the ab-plane and its $s$-wave fit when the fields were applied along the $c$-axis. The penetration depth was obtained from the fits of TF data with the iterative GL model. (b) The Fourier transform of the TF-SR data shows the comparison field distribution in the ab-plane when the out-of-plane field is applied below (blue) and above (dark yellow) the superconducting transition (c) The asymmetry spectra show increased relaxation upon cooling below the superconducting transition due to the formation of flux line lattice (FLL).
• Figure 5: (a) Zero-field $\mu$SR spectra above and below the superconducting transition, where the initial muon polarization was parallel to the $c$-axis. The spectra remain unchanged within the resolution of $\mu$SR, as evident from the relaxation rate parameters (b) $\Lambda$ and (c) $\Delta$, which do not show any systematic increase below T$_c$. This suggests that time-reversal symmetry is preserved in BiPt.