Plasmon Dispersions of Superconducting BiPt

Bi-based binary systems exhibit non-trivial electronic band structure [1] and they represent unique platforms for investigating exotic properties such as topology, Dirac-like band dispersion, surface states, and anisotropic plasmons. In this work we focus on studying the dispersion properties of collective plasmon excitations sustained in pure BiPt crystals, Single crystals of BiPt were grown using the modified Bridgman method. A thin specimen for electron microscopy experiments was created using a focused ion beam microscope. We used diffraction and STEM imaging methodologies to obtain structural information, EDS to access relevant chemical composition information, and momentum-resolved EELS to investigate the dispersive behavior of plasmons. The HAADF STEM images were acquired on a Nion-UltraSTEM microscope operating at 60 kV with an electron probe of ∼ 1 Å resolution. The EDS maps were obtained from Spectra Ultra STEM microscope equipped with an Ultra-X detector for X-ray collection. It was operated at 200 kV with an electron beam current around 100 pA and probe size resolution of ∼ 0.5 Å. Atomically resolved elemental maps were extracted with selected x-ray lines for each element: Bi (M-line of 2.58 keV) and Pt (M-line of 2.12 keV). The momentum-resolved EELS acquisition was conducted using an electron beam with a convergence semi-angle of 2 mrad (∼2 nm probe size). The scattering signal was selected along specific crystallographic directions using a rectangular slot entrance aperture with the electron beam impinging the crystal along the [0001] direction.​ Figure 1 illustrates the structural and chemical characterization results. Fig 1a displays a HAADF-STEM image of BiPt imaged along the [0001] direction. The image exhibits a hexagonal contrast pattern with bright features representing Pt columns, while the dark ones represent Bi atomic columns. This experimental observation is underscored by the atomic-resolution EDS maps (Fig. 1b) demonstrating that BiPt crystallizes in a hexagonal structure. Figure 2 shows the plasmon dispersion findings. Measurements of the dispersive plasmon behavior along the ΓKMKΓ direction is shown in Fig. 2b. The bulk plasmon peak appears at 18.6 eV at the Γ point dispersing towards 19.6 eV around the K point. The plasmon dispersion takes a shape that is dissimilar to the traditional parabolic dispersion of free electron solids [2]. To show the dispersion, the energy of the plasmon at symmetry points was extracted and plotted with momentum transfer values ranging from 0 to 2.2 Å – 1 (Fig. 2c). Further experiments were performed along the ΓM direction revealing a different dispersion that deviates from the observed results above. This suggests an anisotropic behavior operating in the dispersion of bulk plasmons. Our investigation into the structural and plasmonic properties of superconducting BiPt shows that it is a monocrystalline HCP crystal and sustains anisotropic bulk plasmons. This work represents contributions towards our understanding of the nature of quantum materials and their collective electronic behavior [3]. (a) ADF-STEM image of BiPt along the [0001] direction. Bi and Pt atomic column positions are indicated by purple and gray discs, respectively. (b) EDS elemental maps of BiPt with red discs representing Pt atomic columns and green representing Bi atomic columns. (a) The Brillouin zone of BiPt. (b) Momentum-resolved EELS map along the ΓKMKΓ’ direction showing dispersion of bulk plasmon. (c) EELS spectra indicating the plasmon peaks at different symmetry points extending beyond the first BZ.

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