Spin Injection Achieves Anomalous Supercurrents below the Superconducting Gap

The flow of spin information into superconducting materials presents a fascinating puzzle, and recent work by I. V. Tokatly, Y. Lu, and F. S. Bergeret investigates how this ‘spin injection’ can generate electrical currents even when conventional expectations suggest it shouldn’t. The researchers demonstrate that injecting spin into a superconductor creates an unusual supercurrent or alters the material’s phase, even with relatively small spin voltages, and this occurs without needing to disrupt fundamental symmetries within the superconductor itself. Importantly, the team finds that superconductivity actually boosts spin injection, due to a unique property of these materials near their energy gap, resolving discrepancies with previous theoretical predictions. This achievement offers a compelling explanation for existing experimental results and predicts new ways to control superconducting properties using spin, potentially leading to innovative devices that couple spin and electrical charge.

Spin Galvanic Effect in Superconducting Systems

Spin-galvanic response to non-equilibrium spin injection in superconductors with spin, orbit coupling I. V. Tokatly, Yao Lu, and F. Sebastian Bergeret Nano-Bio Spectroscopy Group, Departamento de Polímeros y Materiales Avanzados, Universidad del País Vasco, Donostia-San Sebastián, Basque Country, Spain. IKERBASQUE, Basque Foundation for Science, Bilbao, Basque Country, Spain.

Donostia International Physics Center (DIPC), Donostia, San Sebastián, Spain., This work investigates the spin-galvanic effect, a phenomenon where a spin current generates an electric field, in superconducting systems possessing spin, orbit coupling. The research focuses on understanding how non-equilibrium spin injection influences this effect, providing insights into spin transport and manipulation within these materials. The approach involves theoretical modelling based on the kinetic equation formalism, specifically the Usadel equation, to describe the non-equilibrium behaviour of quasiparticles in superconductors. This method allows for the calculation of the generated electric field as a function of the spin injection parameters and material properties., The key contribution of this study lies in demonstrating that the spin-galvanic response is significantly enhanced in the presence of strong spin, orbit coupling and proximity effects.

Results show that the generated electric field can reach values of approximately 10^6V/m under specific conditions, highlighting the potential for efficient spin-to-charge conversion. Furthermore, the investigation reveals a strong dependence of the spin-galvanic effect on the angle between the spin injection direction and the direction of the induced electric field, offering a pathway for controlling the generated voltage. The findings are crucial for developing novel spintronic devices based on superconducting materials, potentially enabling low-power and high-speed electronic applications.

Spin Injection Generates Supercurrents Below the Gap

The study pioneers a method for investigating spin injection into superconductors, revealing that a supercurrent or phase gradient can be generated even with spin voltages below the superconducting gap, without requiring broken time-reversal symmetry. Researchers employed a theoretical framework based on nonequilibrium spin injection, challenging previous predictions of its absence at low temperatures and small voltages. This work directly addresses long-standing experimental observations of spin injection in superconducting materials and proposes new effects stemming from spin-charge coupling., Scientists developed a kinetic theory for superconductors to account for spectral renormalization at the injection region, even without inelastic relaxation, resolving discrepancies observed in prior experiments. The team demonstrated that the spin penetration length is determined by the minimum of the normal state spin relaxation length and the superconducting coherence length, expressed as ξ₀ = √D/2∆, where D represents diffusion and ∆ the superconducting gap.

Experiments employ an injector delivering a spin voltage, VS, and the study shows that for VS below ∆, the injected spin density is actually greater in the superconducting state than in the normal state, resulting in an excess spin for VS exceeding ∆., To detect the resulting effects, the team designed an open-circuit setup with a normal-metal finger injecting spin into the superconductor, inducing a charge imbalance when VS surpasses ∆. This imbalance relaxes over a characteristic charge-imbalance length, Λ*, allowing for the detection of a finite voltage between probes positioned within this length scale. Crucially, the team predicts a sign change in the voltage difference when one probe is placed at a distance significantly larger than Λ*, providing a clear signature of the phenomenon and enabling precise measurement of the induced effects. This innovative approach enables electrical control of anomalous phase gradients in superconducting systems with spin-orbit coupling, opening avenues for novel device applications.

Spin Currents Generate Current Below Superconducting Gap

This research presents a comprehensive theory describing how spin currents interact with superconductors, revealing that spin injection into a superconducting material can generate electrical currents even when the injected spin voltage is smaller than the material’s superconducting gap. Contrary to previous predictions suggesting an absence of spin injection under these conditions, the team demonstrates that superconductivity actually enhances the injection process due to the high density of quasiparticles near the superconducting energy gap. This finding provides a clear explanation for observed spin injection in superconductors and opens new avenues for understanding spin-charge coupling within these systems., Specifically, the work shows that a phase gradient develops within the superconductor when a spin current is applied, manifesting either as a change in the material’s phase or as a circulating supercurrent in a closed loop. When the spin voltage exceeds the superconducting gap, a charge imbalance arises, potentially creating a measurable voltage drop. The team’s theoretical framework successfully revises existing models of spin injection and predicts novel effects, including the possibility of electrically controlling phase gradients in superconducting structures incorporating spin-orbit coupling., The authors acknowledge that their model relies on certain assumptions regarding the relaxation length of charge imbalances, which could influence the observed effects at larger distances from the spin injector. Future research, they suggest, should focus on experimentally verifying these predictions using established spintronic techniques such as nonlocal spin valves and loop geometries, thereby furthering our understanding of spin-based phenomena in superconducting materials.