Chiral Superconductor Exhibits Half-Quantum Fluxoid Crossover, Demonstrating Potential for Majorana Fermion-Based Computers

The pursuit of topological superconductors represents a significant frontier in both fundamental physics and potential technological advancements, particularly in the development of quantum computers based on exotic particles called Majorana fermions. Masashi Tokuda, Fumiya Matsumoto, and Noriaki Maeda, along with colleagues at their institutions, now present compelling evidence for a crucial step towards realising these materials in practical devices. Their research focuses on a promising candidate, a thin film of bismuth and nickel, and demonstrates a distinct shift in electrical resistance within a ring-shaped sample when exposed to a magnetic field. This observation of a ‘half-quantum fluxoid’, a phenomenon linked to unconventional superconductivity, suggests the material possesses an internal property that allows for precise control of supercurrents, potentially enabling new types of electronic devices where current phase can be altered with minimal energy input.

Epitaxial Bismuth Nickel Crossover Oscillations Observed

This research investigates unusual resistance oscillations observed in thin films of bismuth (Bi) and nickel (Ni), comparing them to similar oscillations in films of niobium (Nb) and nickel, and polycrystalline Bi/Ni. The key finding is that epitaxial Bi/Ni films exhibit a transition between quantum and half-quantum resistance oscillations at a specific magnetic field, a behaviour not observed in polycrystalline or Nb/Ni films. Researchers believe this transition relates to the arrangement of electron spins within the Bi/Ni film and have developed a theoretical model to explain the observed behaviour. Resistance oscillations are periodic changes in a material’s electrical resistance as a function of magnetic field, stemming from the quantum mechanical behaviour of electrons.

Epitaxial films, with their highly ordered crystalline structure, differ from polycrystalline films, which consist of many small crystals with random orientations. The crossover between quantum and half-quantum oscillation is only observed in the epitaxial Bi/Ni films, suggesting the spin arrangement is crucial to understanding the behaviour. The team fabricated bilayer films of Bi/Ni, Nb/Ni, and polycrystalline Bi/Ni, growing epitaxial films on magnesium oxide substrates. Ring-shaped devices were created from these films, and researchers measured their resistance as a function of magnetic field at various temperatures.

Results show that the Bi/Ni device exhibits suppressed oscillations at certain magnetic fields, and the crossover between quantum and half-quantum oscillation occurs at a specific magnetic field. A theoretical model explains how the spin arrangement changes with magnetic field and relates this to the observed behaviour. This research provides new insights into the electronic and magnetic properties of Bi/Ni films. The observation of the half-quantum oscillation and the crossover between quantum and half-quantum oscillation is significant because it suggests a novel electronic structure and spin configuration in these films, potentially impacting future spintronic devices which utilize electron spin to store and process information.

Bi/Ni Ring Resistance and Superconductivity Measurement

Scientists engineered a ring-shaped device from an epitaxial Bi/Ni bilayer to investigate its superconducting properties and search for evidence of topological superconductivity, a state potentially useful for advanced computing. They pioneered a precise methodology to measure resistance oscillations within this ring, employing a combination of direct current and alternating current lock-in techniques. Researchers fabricated the ring device and characterized its superconducting transition, observing a sharp transition at approximately 4 K, indicating minimal damage occurred during the fabrication process. To accurately measure subtle changes in resistance, the team applied a relatively large current to suppress superconductivity at a low temperature of 2.

4 K, allowing them to observe resistance oscillations as a function of magnetic field. This approach enabled the detection of oscillations even at temperatures close to the critical temperature. The experiment employed a high-resolution electromagnet, capable of achieving a magnetic field resolution of approximately 0. 01 mT, coupled with a Hall sensor for precise field measurement. A niobium ring device, a conventional superconductor, was used as a reference for zero-field correction and comparison.

The alternating current lock-in technique was implemented with the external magnetic field perpendicular to the basal plane, enhancing the signal-to-noise ratio and enabling precise measurement of resistance changes. Results showed clear oscillations in resistance for both the Bi/Ni and Nb rings, with periods consistent with theoretical predictions based on the ring’s effective surface area. Notably, the Bi/Ni ring exhibited a suppression of oscillations at approximately ±8 mT, followed by recovery at higher fields, a behaviour not observed in the reference Nb ring, suggesting unique properties of the Bi/Ni bilayer.

Bi/Ni Rings Exhibit Topological Superconductivity

Scientists have demonstrated a significant advancement in topological superconductivity by observing a distinct phase shift in resistance oscillations within a fabricated ring-shaped device composed of epitaxial Bi/Ni bilayer material. This research centers on understanding and manipulating the superconducting properties of this bilayer system, potentially paving the way for novel quantum computing architectures. Experiments revealed a clear superconducting transition at approximately 4 K within the fabricated Bi/Ni rings, indicating minimal damage occurred during the device fabrication process. The team measured resistance oscillations as a function of magnetic field, confirming expectations from theoretical models predicting behaviour in such systems.

Precise measurements, conducted at a stable temperature of 2. 47 K, showed that the period of resistance oscillations for the Bi/Ni ring was 0. 96 mT, aligning with calculations based on the effective surface area of the ring and the vacuum permeability. Notably, the researchers observed a crossover in the oscillation phase at approximately ±8 mT, where the oscillation pattern shifted from a quarter-flux quantum type to a half-flux quantum type, indicating a unique characteristic of the superconducting order parameter within the Bi/Ni bilayer.

6 K, closer to the material’s critical temperature, showed only quarter-flux quantum-type oscillations, confirming the temperature dependence of this phase shift. Reproducibility was confirmed across multiple cooling runs and with different measurement currents, as well as across multiple devices with varying diameters. These results demonstrate the ability to manipulate the phase of supercurrent by π using a small magnetic field, a crucial step towards developing superconducting quantum circuits with enhanced computational capabilities. The observed phase shift, coupled with precise measurements of oscillation periods, provides strong evidence for the internal degree of freedom inherent in the topological superconductivity of the Bi/Ni bilayer system.

Spin-Triplet Superconductivity in Bismuth/Nickel Rings

This research demonstrates a clear phase shift in resistance oscillations within ring-shaped devices fabricated from epitaxial bismuth/nickel bilayers, induced by a relatively small magnetic field. The observed half-fluxoid behaviour provides evidence for unconventional superconductivity, specifically suggesting a spin-triplet component within the material’s superconducting state and an internal degree of freedom in the superconducting order parameter. This finding establishes a functional principle for manipulating supercurrent phase using magnetic fields, potentially enriching the capabilities of superconducting circuits at the nanoscale. The team attributes the observed crossover in resistance oscillations to the spin texture arising from the unique properties of the epitaxial bismuth/nickel bilayer, distinguishing it from polycrystalline bismuth/nickel and niobium/nickel samples.