Josephson diode effect from Cooper pair momentum in a topological semimetal

Abstract: In the presence of an external magnetic field Cooper pairs in noncentrosymmetric superconductors can acquire finite momentum. Recent theory predicts that such finite-momentum pairing can lead to an asymmetric critical current, where a dissipationless supercurrent can flow along one direction but not the opposite. However, to date this has not been observed. Here we report the discovery of a giant Josephson diode effect (JDE) in Josephson junctions formed from a type II Dirac semimetal, NiTe2. A distinguishing feature is that the asymmetry in the critical current depends sensitively on the magnitude and direction of an applied magnetic field and achieves its maximum value of ~60% when the magnetic field is perpendicular to the current and is of the order of just 10 mT. Moreover the asymmetry changes sign several times with increasing field. These characteristic features are accounted for in a theoretical model based on finite-momentum Cooper pairing derived from spin-helical topological surface states, in an otherwise centrosymmetric system. The finite pairing momentum is further established, and its value determined, from the evolution of the interference pattern under an in-plane magnetic field. The observed giant magnitude of the asymmetry in critical current and the clear exposition of its underlying mechanism paves the way to building novel superconducting computing devices using the Josephson diode effect.

Josephson Diode Effect from Cooper Pair Momentum in a Topological Semimetal

The study presents a thorough exploration of the Josephson diode effect (JDE) exhibited in Josephson junctions (JJs) formed from a type-II Dirac semimetal, specifically NiTe₂. The paper articulates both the experimental observations and theoretical model underpinning their findings, emphasizing the sensitivity of the asymmetric critical current to applied magnetic fields and their orientations.

A key aspect of this research is the identification of an asymmetric critical current, responsible for the directional flow of dissipationless supercurrent, which manifests when the applied magnetic field is perpendicular to the current direction. The researchers report a significant JDE with an asymmetry in critical current reaching approximately 60% under a small magnetic field of 10 mT. Importantly, the asymmetry exhibits multiple sign reversals as the magnetic field strength increases. These results are noteworthy for their implications in designing novel superconducting devices with rectification capabilities.

The paper advances a theoretical framework predicated on finite-momentum Cooper pairing, particularly derived from spin-helical topological surface states in otherwise centrosymmetric systems, such as NiTe₂. The finite Cooper pair momentum (FCPM) emerges as a pivotal factor contributing to the non-reciprocal phenomena inherent to the JDE. The study leverages both ARPES measurements and density functional theory (DFT) calculations to establish the presence of surface states ideally positioned for inducing substantial Cooper pair momentum.

The experimental setup involves mechanically exfoliated NiTe₂ flakes with superconducting contacts formed from Ti/Nb/Au layers, allowing precise control over inter-electrode spacing. The authors conduct comprehensive I-V measurements under various conditions—temperature, magnetic field magnitude, and orientation—to observe the JDE. Their observations are supported by the presence of higher harmonics and a non-sinusoidal current-phase relation in the superconductors due to broken inversion symmetry, contributing to the non-reciprocal critical current.

Furthermore, the researchers speculate on future implications with respect to superconductivity and quantum computing, suggesting that the demonstrated JDE could enhance the efficiency and functionality of superconducting devices used in quantum sensing and computation. The finiteness and directionality of Cooper pair momentum can lead to advancements in this realm, possibly lending a new dimension to quantum device architecture design.

In conclusion, this comprehensive study provides substantial evidence for the Josephson diode effect, propelled by finite momentum Cooper pairing, presenting a forward path in superconductivity research. The synergy between experimental results and theoretical modeling contributes to the understanding of this effect, setting a foundation for novel developments in superconducting technology and advancing applications where directional superconducting currents provide distinct advantages.