Observation of half-integer level shift of vortex bound states in an iron-based superconductor (1901.02293v1)

Abstract: Vortices in topological superconductors host Majorana zero modes (MZMs), which are proposed to be building blocks of fault-tolerant topological quantum computers. Recently, a new single-material platform for realizing MZM has been discovered in iron-based superconductors, without involving hybrid semiconductor-superconductor structures. Here we report on a detailed scanning tunneling spectroscopy study of a FeTe0.55Se0.45 single crystal, revealing two distinct classes of vortices present in this system which differ by a half-integer level shift in the energy spectra of the vortex bound states. This level shift is directly tied with the presence or absence of zero-bias peak and also alters the ratios of higher energy levels from integer to half-odd-integer. Our model calculations fully reproduce the spectra of these two types of vortex bound states, suggesting the presence of topological and conventional superconducting regions that coexist within the same crystal. Our findings provide strong evidence for the topological nature of superconductivity in FeTe0.55Se0.45 and establish it as an excellent platform for further studies on MZMs.

Observation of Half-Integer Level Shift of Vortex Bound States in an Iron-Based Superconductor

This paper presents a sophisticated exploration into the topological properties of iron-based superconductors. Through a meticulous scanning tunneling spectroscopy paper on FeTe$_{0.55}$Se$_{0.45}$, the authors reported the observation of two distinct classes of vortices, characterized by an intriguing half-integer level shift in the energy spectra of vortex bound states. The phenomenon provides concrete evidence supporting the topological nature of superconductivity in FeTe$_{0.55}$Se$_{0.45}$.

The investigation focuses on Majorana zero modes (MZMs), which are anticipated as vital components for fault-tolerant topological quantum computing due to their unique non-Abelian statistics. MZMs were observed in certain vortices within the superconducting FeTe$_{0.55}$Se$_{0.45}$, marked by zero-bias conductance peaks (ZBCPs). Nevertheless, the absence of ZBCP splitting alone doesn't conclusively indicate the presence of MZMs, necessitating additional evidence related to the underlying topological features of the system being investigated.

The analysis divulges the coexistence of topological and conventional superconducting regions in the same crystal, permitting a direct comparison of vortex types for the first time within a singular material. Employing model calculations, the authors reproduced the spectral characteristics of these vortex bound states, highlighting the half-integer level spacing transition from integer-spaced levels, tied to topological surface states, to half-odd-integer-spaced levels in ordinary vortices without MZMs. This significant insight into the spectral mechanics substantiates the identification of a pure Majorana mode in this superconducting framework.

The paper reveals that integer quantization of Caroli-de Gennes-Matricon bound states (CBSs) becomes observable under the quantum limit conditions allowed by the particular Δ/E$_F$ ratio characteristic of FeTe$_{0.55}$Se$_{0.45}$. For topological vortices, integer quantization emerges due to the intrinsic spin carried by surface Dirac fermions contributing to angular momentum. Conversely, the energy values in an ordinary vortex are proportionate to half-odd-integers—indicative of the trivial band topology underneath.

Practically speaking, these findings pave the way for broader applications in quantum computation, as the presence of MZMs within this material system offers a promising avenue for future experimental verifications of non-Abelian statistics. Additionally, in-depth exploration of these systems could catalyze the development of quantum devices where robust topological protection mechanisms are crucial for operation.

Theoretically, the research progresses our understanding of superconducting behaviors in iron-based materials and elucidates the topological versus conventional superconductivity dichotomy. As researchers continue to manipulate and probe these superconductors, examining their behavior under diverse conditions and external influences will offer profound implications for both condensed matter physics and quantum technology fields.

Future research could focus on refining the spectroscopic techniques to unravel more nuanced interactions within the quantum limit or exploring broader classes of materials that harbor similar Majorana-carrying properties. Additionally, the investigation into mechanisms that aid or inhibit the distinction between topological and trivial vortices provides a fertile ground for understanding topological phase transitions within complex superconductor environments.