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March 27, 2023• Physics 16, 47

A quantum phase of matter detected in an iron-based superconductor could host Majorana zero modesquasiparticles that may serve as building blocks for future quantum computers.

Building a quantum computer is challenging, not least due to computational errors that arise from the interaction of the quantum system with its environment. In principle, this error problem can be mitigated in a fault-tolerant approach called topological quantum computing, which relies on non-Abelian anyonsexotic quasiparticles that can exist only in two dimensions. However, realizing a material system that can host such quasiparticles typically requires a strong magnetic field, which makes device integration tricky. Now Yishi Lin of Fudan University in China and colleagues have detected and manipulated structures called quantum anomalous vortices (QAVs) in the iron-based superconductor Fe(Se,Te) [1]. Remarkably, these structures form in the absence of a magnetic field and could theoretically support non-Abelian anyons known as Majorana zero modes [2].

To understand QAVs, it is helpful to consider the conventional behavior of a superconductor in a magnetic field. Famously, the field will be expelled from the materials interior through a phenomenon called the Meissner effect if the field strength is below a critical value. A type-II superconductor retains superconductivity to higher field strengths than this value by channeling the field through nonsuperconducting regions known as vortex cores. These regions are surrounded by circulating superconducting currents that shield the field at the cores, forming so-called Abrikosov vortices (Fig. 1, top left).

Rather than applying a magnetic field to a superconductor, isolated magnetic impurities can be inserted into the superconductor. Such impurities break the materials time-reversal symmetry and locally suppress the strength of the electron-pairing interaction responsible for superconductivity, defined by the magnitude of a key quantity known as the order parameter. The result is a collection of localized states called Yu-Shiba-Rusinov states (Fig. 1, top right). The energies of these states lie in the superconducting gapa range of energies that are forbidden to single electrons in a superconductor. This picture is modified in the presence of spin-orbit coupling, which couples the magnetic moment of each impurity to the angular momentum of superconducting quasiparticles. In that case, there is a quantized twist of the order parameter around each impurity. This twist forms QAVs (Fig. 1, bottom).

The spontaneous creation of QAVs in the absence of an external magnetic field has an interesting analogy. In 1980, physicists observed the quantum Hall effectthe quantization of the transverse electrical conductance of a two-dimensional electron gas in a strong magnetic field [3]. A longstanding question had been whether a similar phenomenon could exist in the absence of a field. In 2013, scientists detected such a phenomenon, dubbed the quantum anomalous Hall effect [4].

Lin and colleagues have now directly observed QAVs in Fe(Se,Te), a superconductor that has strong spin-orbit coupling and spin-polarized states associated with particular Fe atoms that act as isolated magnetic impurities. The team cooled crystalline flakes of Fe(Se,Te) through their superconducting transition, which occurs at about 14 K. The researchers then used a highly sensitive instrument called a scanning superconducting quantum interference device (sSQUID) microscope to sense and image the magnetic flux emerging from the flakes.

The team detected random patterns of vortices paired with antivorticesstructures that differ from vortices only in the orientation of their circulating currents. These patterns were spotted at an applied magnetic field weaker than that corresponding to a single flux quantum and even in the absence of such a field. In this magnetic-field regime, vortices are not expected.

In Lin and colleagues experiments, a field coil of the sSQUID microscope generated a weak magnetic field. This field produced a synchronous hysteretic switching of the vorticitythe curl of the flow velocityassociated with each vortex and antivortex. Such behavior is similar to the magnetization switching of a ferromagnet. Furthermore, the superconducting current induced by this weak field drove a rotation of the flux lines threading pairs of impurity magnetic moments. This effect is analogous to the current-induced torque observed in ferromagnets that have spin-orbit coupling [5], and it provides a way to manipulate these vortices.

Surface states in Fe(Se,Te) have been shown to have a nontrivial topological band structure with accompanying superconductivity [6]. Under these circumstances, Majorana zero modes can theoretically form inside the vortex cores of QAVs [7]. Furthermore, the members of a QAV-antivortex pair have opposite vorticities such that they do not repel each other, unlike the Abrikosov vortices seen in conventional superconductors. Consequently, it might be possible to use QAVs to exchange Majorana zero modes in a process known as braiding, a key requirement for topological quantum computing. A potential next step, therefore, is to obtain evidence for Majorana zero modes in these systems and then to explore the conditions needed to manipulate QAVs, albeit slowly to preserve adiabaticityanother important requirement for this type of computing.

Niladri Banerjee is a senior lecturer at Blackett Laboratory at Imperial College London and a steering committee board member of the Atoms to Devices Research Area at the Henry Royce Institute. His experimental research focuses on understanding and exploiting novel emergent electronic and magnetic phases in low-dimensional materials for quantum technologies. His contributions include the first demonstration of a controllable Josephson junction based on unconventional triplet superconductivity.

Jason W. A. Robinson has a professorial chair in materials physics at the University of Cambridge, UK, where he is a joint head of the Department of Materials Science & Metallurgy, director of the Quantum Materials & Devices Group, and codirector of the Centre for Materials Physics. His experimental research focuses on developing multifunctional materials and nanoelectronic devices, approaching key problems in the fields of spintronics, superconductivity, and quantum technologies. His contributions to these fields include pioneering studies on triplet proximity effects at superconductor-magnet interfaces and helping to establish the subfield of superconducting spintronics.

Y.S. Lin, S.Y. Wang, X. Zhang, Y. Feng, Y.P. Pan, H. Ru, J.J. Zhu, B.K. Xiang, K. Liu, C.L. Zheng, L.Y. Wei, M.X. Wang, Z.K. Liu, L. Chen, K. Jiang, Y.F. Guo, Ziqiang Wang, and Y.H. Wang

Phys. Rev. X 13, 011046 (2023)

Published March 27, 2023

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