Fermi arcs carry superconductivity in PtBi₂ while the bulk remains normal

Surface‑Only Superconductivity in PtBi₂

PtBi₂, a non‑centrosymmetric Weyl semimetal, has revealed a striking departure from conventional superconductors: a superconducting gap opens on its surface Fermi‑arc states below ~14 K, yet the bulk remains metallic and non‑superconducting. Angle‑resolved photoemission spectroscopy (ARPES) and scanning‑tunneling microscopy (STM) have directly visualized this gap and identified nodes within it, pointing to an unconventional, momentum‑odd pairing symmetry. The phenomenon arises from a delicate balance between electron‑phonon attraction and screened Coulomb repulsion, which together select one of two competing pairing channels.

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1. Why the Fermi Arcs Carry the Pairing

The crystal structure of PtBi₂ produces multiple Weyl points in the bulk, each connected on the surface by open Fermi‑arc contours. Because the bulk density of states is suppressed near the Weyl nodes, the surface density of states dominates the low‑energy electronic landscape. This high surface DOS provides a fertile ground for a superconducting instability that is largely decoupled from the bulk. ARPES measurements show a clear gap opening on the arcs, while bulk‑sensitive probes detect no transition, confirming the spatial confinement of the superconducting order.

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2. Competing Intra‑Arc and Inter‑Arc Pairing Channels

Two distinct electron‑pairing processes can occur on the arcs:

• Intra‑arc pairing – electrons scatter within the same arc, preserving the arc’s momentum direction. This channel naturally favors a conventional, nodeless gap.
• Inter‑arc pairing – electrons hop between opposite arcs, involving a large momentum transfer. The resulting pair wavefunction changes sign across the arcs, producing a momentum‑odd (i‑wave) symmetry with nodes.

The relative strength of these channels is governed by how efficiently the surface screens Coulomb repulsion. Weak screening (small Thomas‑Fermi wavevector) suppresses low‑momentum intra‑arc scattering, effectively killing the nodeless solution and allowing the inter‑arc, nodal channel to dominate. Conversely, strong screening restores intra‑arc pairing, raising its critical temperature (T_c) and eliminating nodes.

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3. Role of Phonons and Bulk Degrees of Freedom

Even though superconductivity resides on the surface, bulk phonons still mediate part of the attractive interaction. Calculations show that lattice vibrations couple to surface electrons, enhancing the overall pairing strength. This hybrid electron‑phonon mechanism explains why the surface gap can reach several meV despite the bulk remaining normal. The model treats the electron‑phonon coupling as largely isotropic, while the Coulomb term retains a strong momentum dependence that distinguishes intra‑ and inter‑arc processes.

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4. Engineering the Surface Gap

Because the screening environment directly controls which pairing channel wins, external knobs can be used to tune the superconducting properties:

• Surface coatings – depositing a thin dielectric layer modifies the surface dielectric constant, altering the Thomas‑Fermi screening length.
• Electrostatic gating – shifting the surface chemical potential changes the arc’s carrier density and thus the screening efficiency.
• Strain or pressure – modest lattice distortions affect phonon frequencies, tweaking the electron‑phonon coupling strength.

By adjusting any of these parameters, researchers can potentially raise T_c, change the gap magnitude, or even switch between nodal and nodeless superconductivity. Such control is especially valuable for realizing topological superconductivity, where Majorana modes are expected to reside at the nodes of the surface gap.

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5. Outlook: A Platform for Intrinsic Topological Superconductivity

The coexistence of a robust, surface‑confined superconducting state with a normal bulk makes PtBi₂ a unique testbed for exploring intrinsic topological superconductivity. The nodal structure observed in ARPES aligns with theoretical predictions of Majorana‑type excitations bound to the arcs. Future experiments that combine surface gating with high‑resolution spectroscopy could map the evolution of these modes, paving the way for engineered platforms that host and manipulate Majorana quasiparticles without the need for proximity‑induced superconductivity.

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Takeaway: PtBi₂ demonstrates that superconductivity can be confined to topological surface states, governed by the interplay of electron‑phonon attraction and surface Coulomb screening. By mastering the screening environment, scientists can tailor the critical temperature and gap symmetry, opening a practical route toward controllable topological superconductors.