Researchers have directly visualized superconductivity confined to the surface of γ-PtBi₂, a topological semimetal composed of platinum and bismuth. The work demonstrates robust superconductivity with a critical temperature of 2.9 K and a critical field of approximately 1.8 T, revealed through very low temperature Scanning Tunneling Microscopy.

Observations of quantized superconducting vortices and the Josephson effect demonstrate two-dimensional macroscopic quantum phase coherence within the material’s surface state, a finding that addresses previous questions regarding the stability of this unusual superconducting behavior. This research involved a collaboration between scientists at Universidad Autónoma de Madrid and Iowa State University & Ames National Laboratory.

γ-PtBi₂ as a Weyl Semimetal with Fermi Arcs

The material exhibits properties linked to its Fermi arcs, conducting pathways on the surface connecting bulk Weyl points. Observing these arcs is crucial, as they appear to be the conduits for this unusual superconducting behavior, differing from proposals to induce superconductivity through proximity with other materials.

The material’s layered trigonal structure without inversion symmetry contributes to its unique electronic properties, including wide bands crossing the Fermi level and Weyl points approximately 50 meV above it. These Weyl points, previously identified through angle-resolved photoemission spectroscopy and quantum oscillations, are directly linked to the observed Fermi arcs on the surface.

Jose Antonio Moreno, Pablo García Talavera, Edwin Herrera, Sara López Valle, Zhuoqi Li, Lin-Lin Wang, Sergey Bud’ko, Alexander I. Buzdin, Isabel Guillamón, Paul C. Canfield, and Hermann Suderow detailed in their work that the observed superconducting gap size and temperature dependence closely align with predictions from BCS theory, a cornerstone of superconductivity. The authors state, “We find homogeneous superconductivity down to atomic scale with a critical temperature of T_c = 2.9 K,” emphasizing the consistency of their findings with established theory. The discovery of surface superconductivity in γ-PtBi₂ offers potential advantages for quantum device applications and complements ongoing research into two-dimensional superconductivity observed in materials like graphene.

Layered Structure and Electronic Band Properties of γ-PtBi₂

The layered material γ-PtBi₂, a topological semimetal, possesses a unique electronic structure without inversion symmetry, directly influencing its observed superconducting properties. Detailed analysis of γ-PtBi₂’s layered trigonal structure reveals a composition of platinum and bismuth atoms, resulting in surfaces predominantly terminated with bismuth following cleavage.

Atomic-resolution Scanning Tunneling Microscopy imaging of these surfaces demonstrates two distinct terminations, each exhibiting a hexagonal pattern formed by the bismuth atoms, though with differing atomic arrangements. These structural characteristics are crucial because the observed superconductivity is linked to these surface states, rather than being induced through proximity effects with another superconducting material.

Surface Superconductivity Initial Observations & Discrepancies

Unlike most known superconductors where this phenomenon occurs throughout the bulk material, the team’s findings demonstrate robust two-dimensional superconductivity specifically linked to the Fermi arcs present on the surface of γ-PtBi₂, a characteristic with potential implications for future quantum devices. Detailed analysis using very low temperature Scanning Tunneling Microscopy revealed not only the existence of this surface superconductivity, but also direct visual evidence of its quantum nature. Researchers observed quantized superconducting vortices, a repeating pattern of magnetic fields within the superconducting surface, and the Josephson effect, demonstrating two-dimensional macroscopic quantum phase coherence.

Low-Temperature STM Reveals Robust 2.9 K Superconductivity

The layered material gamma-Platinum Bismuth (γ-PtBi₂) exhibits superconductivity confined to its surface, a phenomenon confirmed through detailed scanning tunneling microscopy at extremely low temperatures. This surface-limited superconductivity presents a distinct pathway for exploring quantum phenomena and potential device applications.

Researchers from Universidad Autónoma de Madrid in Spain and Iowa State University in the United States directly visualized the superconducting state by observing quantized superconducting vortices within the γ-PtBi₂ surface. These vortices, arranged in a repeating lattice pattern, provide definitive evidence of macroscopic quantum phase coherence, a key characteristic of superconductivity, and were revealed using a dilution refrigerator STM under applied magnetic fields.

The team’s observations extend beyond simply detecting superconductivity; they demonstrate its robustness by showing these vortex lattices persist even with variations in temperature and magnetic field strength. Instead, the team’s work demonstrates that γ-PtBi₂ intrinsically supports a two-dimensional superconducting state on its surface. The observed critical field of approximately 1.8 T further characterizes the strength of the superconducting state.

Quantized Vortex Lattice Confirms Surface Superconducting Phase

The conventional understanding of superconductivity, where resistance vanishes at low temperatures throughout a material’s volume, faces a compelling challenge from γ-PtBi₂, a layered compound exhibiting this property exclusively at its surface. The team’s analysis of tunneling conductance confirms a superconducting gap size of 0.48 meV, aligning with theoretical predictions based on BCS theory, and demonstrating a temperature dependence consistent with this established model of superconductivity.

Critical Fields: Hc₂ ≈ 1.8 Tesla for γ-PtBi₂

The material exhibits a critical field of 1.8 Tesla, a characteristic markedly different from most known superconductors where the effect extends throughout the bulk material. These vortices, arranged in a predictable pattern, provide direct evidence of the two-dimensional superconducting state and its ability to support persistent currents despite the presence of an external magnetic field.

This is a crucial finding, as the absence of such vortices in previous studies cast doubt on the true superconducting nature of the surface state. This intrinsic nature, coupled with the observed critical field of approximately 1.8 T, is linked to the Fermi arcs.

Potential of γ-PtBi₂ for Two-Dimensional Quantum Devices

This visualization confirms the two-dimensional macroscopic quantum phase coherence necessary for potential applications in future quantum devices. The observation of these phenomena is crucial because previous studies lacked definitive proof of vortex formation, casting doubt on the robustness of superconductivity within γ-PtBi₂’s surface layers. These arcs, joining bulk Weyl points within the material, appear connected to the observed superconductivity, differing from approaches that induce surface superconductivity through proximity to other superconducting materials.

Researchers utilized low-temperature scanning tunneling microscopy to study γ-PtBi₂, revealing homogeneous superconductivity down to the atomic scale. Further investigation into the interplay between the Fermi arcs and superconductivity within γ-PtBi₂ could unlock even more sophisticated applications in the future.

👉 More information
🗞 Robust Two-Dimensional Surface Superconductivity and Vortex Lattice in the Weyl Semimetal γ−PtBi₂
✍️ Jose Antonio Moreno et al.
🧠 DOI: http://link.aps.org/doi/10.1103/9cyw-m5zr

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