[Paper Review] Surface superconductivity in the topological Weyl semimetal t-PtBi$_2$

Sebastian Schimmel, Yanina Fasano+11 more|arXiv (Cornell University)|Feb 17, 2023

Topological Materials and Phenomena4 citations

TL;DR

This study demonstrates surface superconductivity in the topological Weyl semimetal t-PtBi₂ at elevated temperatures (5 K), using scanning tunneling microscopy and spectroscopy (STM/STS). It reports a spatially inhomogeneous superconducting gap spanning 0–20 meV, with the largest gap observed in topological surface states, making t-PtBi₂ a prime candidate for intrinsic topological superconductivity at technologically relevant conditions, including high critical fields up to 12 T.

ABSTRACT

The advancement of quantum computation is eager on generating fault tolerant qubits, and topological superconductivity is a very promising concept for reaching this goal. Early experimental achievements study hybrid systems as well as doped intrinsic topological or superconducting materials presenting the phenomena at very low temperatures. However, higher critical temperatures are indispensable for technological exploitation. Promising very recent angle-resolved photoemission spectroscopy results reveal that superconductivity of the type-I Weyl semimetal trigonal PtBi$_2$ (t-PtBi$_2$) is located at the Fermi arcs surface states which renders t-PtBi$_2$ a candidate for intrinsic topological superconductivity. Here we show, using scanning tunnelling microscopy and spectroscopy (STM/STS) that t-PtBi$_2$ presents surface superconductivity at elevated temperatures (5 K). The gap magnitude is elusive: it is spatially inhomogeneous and spans from 0 to 20 meV. In particular, the large gap value and the shape of the quasiparticle excitation spectrum resemble the phenomenology of high-Tc superconductors. To our knowledge, this is the largest superconducting gap so far measured in a topological material. Moreover, we show that the superconducting state at 5 K persists up to 12 T magnetic field. Thus, we show that t-PtBi2 is a prime candidate for intrinsic topological superconductivity at technologically relevant temperatures, fields and gap magnitudes.

Motivation & Objective

• To investigate the origin and nature of superconductivity in the topological Weyl semimetal t-PtBi₂.
• To determine whether superconductivity is intrinsic and localized on surface states, particularly Fermi arcs.
• To measure the superconducting gap magnitude and its spatial inhomogeneity at low temperatures.
• To assess the robustness of the superconducting state under high magnetic fields (up to 12 T).
• To evaluate the potential of t-PtBi₂ as a platform for topological quantum computation by identifying intrinsic topological superconductivity.

Proposed method

• Employed low-temperature scanning tunneling microscopy and spectroscopy (STM/STS) to probe local electronic structure and superconducting gap at atomic scale.
• Focused on surface regions of t-PtBi₂ crystals to isolate contributions from topological surface states, particularly Fermi arcs.
• Mapped spatial variations in the superconducting gap across the surface, revealing inhomogeneity from 0 to 20 meV.
• Measured the critical magnetic field by applying external fields up to 12 T to assess the stability of the superconducting state.
• Analyzed quasiparticle excitation spectra to compare with phenomenology of high-Tc superconductors.
• Correlated observed superconducting signatures with the known topological surface state structure of t-PtBi₂.

Experimental results

Research questions

• RQ1 Is superconductivity in t-PtBi₂ localized on topological surface states, particularly Fermi arcs?
• RQ2 What is the magnitude and spatial distribution of the superconducting gap in t-PtBi₂ at 5 K?
• RQ3 How robust is the superconducting state in t-PtBi₂ under high magnetic fields (up to 12 T)?
• RQ4 Does the observed superconducting gap magnitude and quasiparticle spectrum resemble those of high-Tc superconductors?
• RQ5 Can t-PtBi₂ host intrinsic topological superconductivity suitable for fault-tolerant quantum computing at higher temperatures?

Key findings

• Superconductivity in t-PtBi₂ is confined to surface states, with the highest superconducting gap reaching 20 meV, the largest reported in a topological material to date.
• The superconducting gap is spatially inhomogeneous, varying from 0 to 20 meV across the surface, indicating strong local variations in pairing strength.
• The quasiparticle excitation spectrum exhibits features reminiscent of high-Tc superconductors, suggesting unconventional pairing mechanisms.
• The superconducting state persists up to 12 T, indicating a robust pairing mechanism compatible with high-field applications.
• The observed surface superconductivity at 5 K, combined with topological surface states, positions t-PtBi₂ as a leading candidate for intrinsic topological superconductivity.
• These results establish t-PtBi₂ as a promising platform for realizing topological qubits at technologically relevant temperatures and magnetic fields.