Where Superconductivity Meets Magnetism: A New Tunable Semiconductor Junction

The Current That Should Not Be Easy to Control

An electric current that encounters no resistance sounds like the end of an engineering problem. In a superconducting circuit, however, it can be the beginning of another: how do you regulate that current, route it, or make it respond to a decision? A perfectly lossless path is not automatically a useful switch. And a magnetic material, which might provide control, can interfere with the fragile pairing of electrons responsible for conventional superconductivity.

A team spanning the University of Tokyo and New York University has taken a significant step toward managing that contradiction. In a study published November 5, 2026, in the peer-reviewed journal Communications Materials, the scientists describe a layered semiconductor structure in which a superconducting current can be adjusted by an applied voltage. They also observe directional asymmetry in its critical current and a response that depends on the history of an applied magnetic field. This is a carefully measured laboratory device, not a commercial chip or a completed quantum computer.

The Experiment: Three Materials, One Channel

The device begins with indium arsenide (InAs), a semiconductor whose mobile electrons can respond effectively to an electric field. Below it sits iron-doped gallium antimonide, written (Ga,Fe)Sb, a ferromagnetic semiconductor. Above the InAs are aluminum layers and electrodes that become superconducting when sufficiently cold. The intention is to have two different influences—superconducting correlations from above and magnetic interactions from below—act on the same transport channel.

This is not a mixture in which every layer has suddenly become a bulk superconductor and ferromagnet. Instead, the architecture is designed around proximity effects: a neighboring material changes how electrons behave inside the channel. That distinction matters because superconductivity and ordinary ferromagnetism often compete. The achievement is a layered interface at which the interplay can be investigated and electrical transport can be controlled.

Building Clean Interfaces Atom by Atom

A superconducting device is unforgiving of rough or contaminated boundaries. Disorder can scatter electrons and weaken the coherence needed for supercurrent to traverse a semiconductor. The researchers therefore grew the layered structure in ultrahigh vacuum without exposing its interfaces to ordinary air between steps. Transmission electron microscopy revealed a clean, atomically abrupt boundary between aluminum and the semiconductor.

The team fabricated a Josephson junction with two aluminum electrodes separated by a semiconductor channel approximately 100 nanometers long and 5 micrometers wide. The dimension is important: a small bridge allows superconducting correlations from the two electrodes to communicate through the intervening material. Building such junctions repeatably and understanding how their interfaces affect transport remain central engineering challenges.

What Three Microamperes Actually Means

At 30 millikelvin—0.03 kelvin, only thirty thousandths of a degree above absolute zero—an ungated junction supported a superconducting current of approximately 3.00 microamperes. Up to a critical current, the measurement showed a zero-voltage branch associated with superconducting transport. Beyond the limit, a resistive response appears. That is not the same as saying an arbitrary amount of current can flow without losses.

In devices equipped with a gate, applying a more negative voltage depleted electrons in the InAs channel and reduced the critical supercurrent. This is the transistor-like element of the result. Voltage could modulate the maximum current the junction would carry in the superconducting state. The experiment did not demonstrate the room-temperature on-off control expected from everyday silicon electronics; it showed a form of control in a specialized, cryogenic quantum device.

The Strange Role of Magnetic History

A conventional Josephson junction can display oscillations of its critical current as a perpendicular magnetic field changes. Researchers refer to the resulting interference as a Fraunhofer-like pattern, borrowing a term from optics. Here, the pattern shifted depending on whether the researchers increased or decreased the magnetic field. That hysteresis was observed in multiple devices and points to a magnetic contribution to superconducting transport.

The Tokyo press statement connects the behavior with a possible magnetic proximity effect from the underlying ferromagnetic semiconductor into InAs. The measurement is meaningful evidence of magnetic influence, but it does not by itself provide a complete, direct microscopic map of the magnetization inside the channel. The team explicitly identifies improved magnetic-layer performance and clearer separation of the control mechanisms as future research priorities.

A Superconducting Diode—But Not a Household Rectifier

The researchers also found that the critical current could differ according to the direction in which current was driven. Under an applied magnetic field, the junction sometimes remained superconducting in one direction at a given current magnitude while becoming resistive in the reverse direction. This behavior is called the superconducting diode effect. The measured asymmetry reached a diode-efficiency metric of up to 20%, as defined by the difference between the two directional critical currents divided by their sum.

The word diode can mislead readers accustomed to a familiar semiconductor rectifier. This experiment does not establish a zero-field, room-temperature, standalone replacement for an ordinary diode, nor does it show perfect one-way conduction. It characterizes an unequal superconducting critical-current threshold under particular cryogenic and magnetic conditions. The precise definition of efficiency is important: 20% is not a measured 20% reduction in electricity consumption.

From 1911 to the Josephson Junction

The history behind the result stretches more than a century. In 1911, Heike Kamerlingh Onnes observed superconductivity in mercury at very low temperature. The theoretical description developed dramatically in 1957 with the Bardeen–Cooper–Schrieffer account of paired electrons. In 1962, Brian Josephson predicted that superconducting current could tunnel between separated superconductors, a phenomenon that became the foundation of numerous precision instruments and superconducting quantum circuits.

Researchers subsequently learned how superconductors could induce superconducting correlations in neighboring metals and semiconductors. That proximity-effect approach expanded what could serve as the weak link of a Josephson junction. The new Japan–U.S. device belongs to this lineage, but adds a magnetic semiconductor beside an electrically adjustable channel, bringing another control variable into the problem.

Why Superconductivity and Ferromagnetism Are Uneasy Neighbors

In an ordinary singlet superconductor, electrons form correlated pairs with opposite spin configurations. Ferromagnetic ordering, by contrast, favors aligned electronic spins and can act as a pair-breaking influence. The simple intuition that adding a magnet should improve a superconducting switch therefore fails. Much of the relevant materials science involves carefully choosing geometry, interfaces, spin interactions and the distance over which each effect persists.

The Tokyo–NYU architecture does not make that antagonism disappear. It spatially organizes the competing ingredients, putting aluminum, InAs and (Ga,Fe)Sb into contact so that the transport channel can experience both. Whether related arrangements will eventually deliver stable, programmable phase control or useful memory functions depends on further measurements and materials development.

A Trans-Pacific Research Collaboration

The paper lists Hirotaka Hara and Lukas Baker as equal first authors. The Japanese team includes Shingen Miura, Keita Ishihara, Professor Masaaki Tanaka and Associate Professor Le Duc Anh, working with researchers at New York University including Professor Javad Shabani. The Japanese announcement identifies Hara as a doctoral student in the University of Tokyo Graduate School of Engineering; the study combines materials growth, microscopic characterization, nanofabrication and cryogenic measurements.

According to the paper's acknowledgments, support included Japanese research grants, JST programs and research networks, while Hara's visit and experimental work at NYU received additional foundation and university support. The collaboration illustrates why quantum-device research increasingly depends on teams that can manufacture an unusual material stack, fabricate nanoscale structures, measure them under demanding conditions, and evaluate alternative explanations for the signals.

What It Could Mean for Quantum Electronics

Superconducting Josephson junctions already sit at the heart of many quantum circuits. A controllable junction whose response remembers magnetic history could someday contribute to circuit elements for switching, phase bias, nonvolatile operation or other functions. Semiconductor fabrication methods also offer opportunities for electrical gating. The researchers describe their result as a materials foundation for future low-energy electronics and more robust quantum devices.

But several gaps separate a promising junction from a scalable processor. The experiment used 30-millikelvin conditions, a tiny current scale and an applied magnetic field for the directional effect. The publication does not report a working qubit, benchmark quantum error rates, show nonvolatile memory-cell endurance or establish a whole-system energy saving after refrigeration costs. Those are development questions, not achievements already demonstrated by the study.

The Honest Meaning of a Breakthrough

The central achievement is narrower—and stronger—than the claim that magnetism and superconductivity have somehow been made universally compatible. The researchers produced an electrically tunable semiconductor Josephson junction next to a ferromagnetic layer, established superconducting transport, and measured signatures of magnetic influence and directional critical-current asymmetry. Their results offer an experimental platform on which to test ideas that have long been difficult to combine.

Future work must determine how the magnetic state can be manipulated reliably, which physical mechanisms dominate the hysteresis and diode response, how consistently the device can be fabricated, and whether operation can be improved beyond its current cryogenic limits. The remarkable part is not that a quantum computer has been built; it is that engineers have gained one more controllable way to explore the boundary between two competing quantum behaviors.