New Magnet Design Creates One-Way Electrical Flow

Researchers are increasingly focused on inducing non-reciprocal charge transport in superconducting circuits, and a recent study details a novel approach to achieving the Josephson diode effect. Lovy Sharma from the Department of Physics, Indian Institute of Technology Delhi, and Bimal Ghimire and Manisha Thakurathi from the Department of Physics, Indian Institute of Technology Hyderabad, present a Josephson junction constructed using a -wave magnet with proximity-induced superconductivity, in collaboration with colleagues at both the Indian Institute of Technology Delhi and the Indian Institute of Technology Hyderabad. This work demonstrates a field-free Josephson diode effect facilitated by an altermagnet barrier, crucially without relying on Rashba spin-orbit coupling or dissimilar superconductors, a significant departure from previous models. The robust performance of this system across a wide range of parameters suggests considerable potential for future applications in advanced circuit design and superconducting technologies.

Imagine building a one-way street for electricity, allowing current to flow preferentially in a single direction. This is the promise of the Josephson diode effect, now realised in a novel device using specially-aligned magnets, offering a potentially simpler route towards building more efficient electronic components. Scientists are increasingly focused on controlling supercurrent, the flow of electrical current with zero resistance, with directional precision.

Recent observations of the superconducting diode effect (SDE) demonstrate unequal critical currents depending on the direction of flow, a phenomenon initially predicted in theoretical models of both bulk superconductors and Josephson junctions (JJs). Researchers have constructed a Josephson junction utilising a recently identified unconventional magnet, termed the p-wave magnet (PM), to induce proximity superconductivity and, in doing so, have revealed a Josephson diode effect (JDE).

The barrier within this junction is formed from another unconventional magnet, specifically an altermagnet (AM). Investigations reveal that beyond the usual requirements of broken time-reversal and inversion symmetries, mirror symmetry emerges as a key constraint governing this effect. Unlike previous approaches to achieving the JDE with unconventional magnets, this new setup does not depend on Rashba spin-orbit coupling (SOC), nor does it necessitate differing superconducting materials on either side of the junction.

This system’s ability to function effectively across a broad range of parameters suggests considerable potential for integration into advanced circuits and computing technologies. Understanding the underlying physics of these unconventional magnetic materials is essential to unlocking their full potential. Altermagnets break time-reversal symmetry like ferromagnets, but exhibit zero net magnetisation, behaving similarly to antiferromagnets.

This unique characteristic arises from the arrangement of opposite spin sublattices, linked by rotational symmetry in both real and momentum space, resulting in alternating spin Fermi surfaces. Similarly, p-wave magnets possess zero net magnetisation but induce spin splitting in momentum space with p-wave symmetry, disrupting inversion symmetry. Once these unconventional magnets are combined, novel phenomena emerge when interfaced with superconductors, including spin-polarised transport and topological superconductivity.

By considering a junction where proximity-induced superconductivity in the PM forms the leads and the AM acts as the barrier, scientists have identified mirror symmetry as a further essential component for the appearance of the JDE. The system demonstrates nonreciprocal current even with identical superconductors on both sides of the junction and without the need for Rashba SOC or external magnetic fields, a departure from earlier proposals.

The ability to tune the magnitude and polarity of this nonreciprocity through the introduction of Rashba SOC in the AM offers additional control. A high degree of nonreciprocity is maintained across a wide range of exchange-field strengths and other system parameters, indicating that these planar Josephson junctions represent a promising advancement over previous designs. By requiring fewer constraints, this approach opens new avenues for developing more efficient and flexible superconducting devices.

Asymmetric supercurrents and harmonic generation in planar Josephson junctions

At a gate potential of zero, numerical calculations reveal a distinct asymmetry in current flow through the planar Josephson junction, establishing the Josephson diode effect. Current-phase relations, examined for crystallographic lobe angles of 0 and 0.1π, exhibit substantial contributions from higher harmonics beyond a simple sinusoidal component. These non-sinusoidal features are particularly apparent when the angle deviates from zero, indicating a complex interaction between the magnetic order and superconducting transport.

The magnitude of the diode effect varies with system parameters. Calculations show that the efficiency parameter, quantifying nonreciprocity, is strongly influenced by the strength of spin-dependent hopping in both the planar magnet and the altermagnet. The efficiency parameter reaches a maximum value of 0.42 when the exchange field in the planar magnet is 0.15t0 and the exchange field in the altermagnet is 0.2t0, where t0 represents the hopping amplitude.

This signifies a considerable degree of current rectification, meaning nearly half of the current is preferentially directed in one direction. The crystallographic orientation of the altermagnet also plays a key role. By altering the lobe angle, researchers observed a tunable current-phase relation, even achieving a reversal in the diode’s polarity. At an angle of 0.1π, the critical current is 0.08, while at an angle of 0, the critical current is 0.12, demonstrating a clear dependence on the crystallographic orientation.

This diode effect arises without the need for Rashba spin-orbit coupling or dissimilar superconductors, simplifying the requirements for practical implementation. A short junction regime was employed with the altermagnet barrier extending 5 lattice spacings, ensuring that the current is primarily determined by the properties of the barrier itself. The current is calculated using a recursive algorithm applied to the lesser Green’s function, allowing for precise determination of the current-phase relation. Analysing the calculated current confirms that the observed non-reciprocity is a genuine diode effect, stemming from the unique symmetry properties of the system.

P-wave Magnet and Altermagnet Josephson Junctions for Symmetry-Constrained Diode Effect Investigation

A Josephson junction incorporating a p-wave magnet (PM) and an altermagnet (AM) forms the basis of this research into the Josephson diode effect (JDE). The team fabricated a junction where proximity-induced superconductivity developed within the PM acted as the superconducting leads, and the AM functioned as the barrier. This configuration allows investigation of non-reciprocal supercurrents without relying on external magnetic fields or differing superconductors, simplifying potential applications.

Detailed analysis focused on symmetry constraints governing the JDE within this heterostructure. Simulations revealed that mirror symmetry plays a key role in enabling the diode effect, beyond the established requirements of broken time-reversal symmetry and inversion symmetry. The researchers carefully considered the magnetic order within both the PM and AM materials, exploiting their unique spin configurations to establish these symmetries.

This work avoids the need for Rashba spin-orbit coupling, a property that can complicate device fabrication and performance. Characterising the JDE necessitated a precise understanding of the materials’ properties. The PM, a recently discovered unconventional coplanar magnet, exhibits zero net magnetization but possesses p-wave symmetry in its spin splitting.

The AM, another unconventional magnet, breaks time-reversal symmetry while also maintaining zero net magnetization. By combining these materials, the study aimed to create a system where the JDE arises from intrinsic material properties rather than externally imposed conditions. The anisotropic spin splitting within the PM was expected to contribute to the observed non-reciprocity.

To model the behaviour of this junction, the researchers employed theoretical calculations to map the interaction between symmetry, spin configuration, and supercurrent flow. These calculations were performed to confirm the emergence of the JDE and to identify the parameter regimes where the effect is most pronounced. Variations in material properties and junction geometry were investigated to determine how they influence the diode ratio, which quantifies the asymmetry in critical current.

Superconductivity and unconventional magnetism enable directional current control

Scientists have long sought to build electronic components that behave differently depending on the direction of current flow, a concept mirroring the one-way valve of fluid dynamics. Achieving this in a compact, solid-state device has proven remarkably difficult, largely because of the fundamental symmetries governing electron behaviour in most materials.

Research detailing a novel Josephson junction constructed from specifically engineered magnetic materials offers a potential pathway towards realising this goal, demonstrating a pronounced ‘superconducting diode effect’. Unlike previous attempts relying on complex material combinations or finely tuned conditions, this approach appears surprisingly resilient and broadly applicable.

This work hinges on exploiting the interaction between superconductivity and unconventional magnetism, specifically utilising a ‘wave magnet’ and an ‘altermagnet’ to create a junction where current flows more easily in one direction than the other. The significance lies in the simplicity of the design, bypassing the need for precise control of spin-orbit coupling, a quantum mechanical effect sensitive to material imperfections.

Translating this laboratory demonstration into widespread technological use will not be immediate. While the effect is shown to persist across a range of parameters, scaling up production and integrating these junctions into complex circuits presents considerable engineering challenges. A deeper understanding of the underlying physics, particularly the role of mirror symmetry identified by the researchers, remains an open question.

Once these hurdles are addressed, however, the potential is considerable. This work represents a shift away from chasing increasingly complex materials and towards exploiting fundamental symmetry breaking. At a time when energy efficiency is central, directional current control could revolutionise everything from power transmission to data storage. Beyond this specific junction design, the principles demonstrated here may inspire new approaches to building asymmetric electronic components using a wider range of materials and magnetic configurations. The convergence of superconductivity and unconventional magnetism is rapidly becoming a fertile ground for discovery, promising a future where electron flow is not simply a matter of resistance, but of direction.