Current Switching Boosts Superconducting Diode Efficiency

Superconducting diodes, devices allowing current to flow preferentially in one direction, represent a potentially revolutionary advancement in energy transmission and computing. Uddalok Nag from The Pennsylvania State University, Jonathan Schirmer from William & Mary, and Chao-Xing Liu, also of The Pennsylvania State University, alongside J. K. Jain, demonstrate a pathway to significantly enhance the efficiency of these diodes by exploiting the switching between different superconducting states induced by electrical current. Their research, a collaborative effort between The Pennsylvania State University and William & Mary, reveals that carefully tuning parameters to induce a transition between standard Bardeen-Cooper-Schrieffer (BCS) and orbital Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) superconducting orders can create a peak in diode efficiency. This finding is significant because it not only offers a novel design principle for high-performance superconducting diodes, but also suggests that measuring diode efficiency could provide valuable insight into the fundamental physics governing the BCS-FFLO transition under varying magnetic fields and supercurrents. For decades, building efficient electrical components that work differently depending on current direction has been a major engineering challenge. Now, calculations suggest a new way to control superconductivity and dramatically improve these ‘superconducting diodes’. This approach exploits the subtle changes in how materials conduct electricity, potentially leading to more effective devices. Scientists are increasingly focused on lossless current transmission, a defining characteristic of superconductivity with implications for diverse technologies. Beyond fundamental physics, superconductors promise advances in power grids, medical imaging, and quantum computing, all reliant on maintaining the supercurrent state. Yet, exceeding a critical current invariably forces a superconductor into a normal, resistive state. Recent investigations have revealed a phenomenon termed the “superconducting diode effect”, where supercurrent flows preferentially in one direction, mirroring the behaviour of semiconducting diodes and opening possibilities for novel electronic devices. This non-reciprocal supercurrent, however, typically exhibits low efficiency, hindering practical application. Researchers propose a method to substantially improve superconducting diode efficiency by exploiting the transition between different superconducting states. The core idea centres on inducing a change in the type of superconductivity, from one state to another, with current flow in only one direction, but not the other, before the material loses its superconducting properties entirely. This relies on the sensitivity of the critical current to the specific superconducting order present within the material. Detailed calculations performed on a layered superconductor subjected to an in-plane magnetic field demonstrate the feasibility of this approach. Specifically, the study examines a system exhibiting both conventional Bardeen-Cooper-Schrieffer (BCS) superconductivity, where electrons form pairs, and the orbital Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state, a more complex form of superconductivity with spatially varying pairing. Calculations predict a pronounced peak in diode efficiency when transitioning between these two states, arising from a complex interaction between the differing superconducting orders. Once a bilayer superconductor is exposed to an increasing supercurrent, the transition from the FFLO state to the BCS state can occur in one direction before reverting to a normal state, while the opposite direction transitions directly to the normal state. Measuring superconducting diode efficiency can provide valuable insight into the fundamental nature of the BCS-FFLO transition itself, both as a function of applied magnetic field and the magnitude of the supercurrent. Unlike previous approaches focusing on spin-based mechanisms, this research highlights the role of interlayer interactions and broken symmetry in enhancing the diode effect. Ginzburg-Landau modelling defines free energy and phase relationships in coupled superconducting layers A detailed examination of a superconducting bilayer system underpinned this work, employing the Ginzburg, Landau (GL) equations to map the phase diagram under applied supercurrent and in-plane magnetic fields. Specifically, the superconducting order parameters for each layer, denoted as Ψl, were defined as ψlei(ξ(x)−(−1)lφ(x)/2+qx), where ψl represents the spatially uniform amplitude, and ξ(x) and φ(x) describe the overall and relative phases between the layers, respectively. Incorporating the phase term eiqx allows control over current flow through the system. The GL free energy was formulated as Ω[Ψl] = 1 V0 ∫ d2r “X l αlψ2 l + β 2X l ψ4 l + ψ2 1 2m q −qB + ∇φ 2 2 + ψ2 2 2m q + qB −∇φ 2 2 −2Jψ1ψ2 cos φ, with αl governing the onset of superconductivity and changing sign at the transition temperature Tc,l. The fourth-order term β ensures system stability, while m signifies the cooper pair mass and J represents the interlayer Josephson coupling strength. The GL free energy was formulated as Ω[Ψl] = 1 V0 ∫ d2r “X l αlψ2 l + β 2X l ψ4 l + ψ2 1 2m q −qB + ∇φ 2 2 + ψ2 2 2m q + qB −∇φ 2 2 −2Jψ1ψ2 cos φ. This formulation allows for detailed analysis of the system’s energetic behaviour. To introduce asymmetry, α1 was set unequal to α2, breaking inversion symmetry between the layers. Since an in-plane magnetic field B = Bey was applied, the vector potential Al = (−1)l d 2B × ez = (−1)l dB 2 ex was included to account for its effects. A self-consistent approach was adopted to determine the phase diagram, allowing for the interaction between the superconducting orders to be accurately modelled. Previous calculations indicated a minimal spatial dependence of the order parameter in the orbital Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state, simplifying the analysis. By solving these equations, researchers aimed to predict a sharp peak in superconducting diode (SD) efficiency near the transition between the Bardeen-Cooper-Schrieffer (BCS) and FFLO states, a phenomenon reliant on the sensitivity of the critical current to the superconducting order. The study focused on a bilayer superconductor, known to exhibit both BCS and FFLO orders depending on the magnetic field strength. At low magnetic fields, the phases of the order parameters in both layers are equal, defining the BCS state. However, increasing the magnetic field induces a transition to the FFLO state, characterised by a spatially varying phase difference between the layers. The work neglects the Zeeman coupling of the magnetic field to electron spin, a valid simplification for non-centrosymmetric layered superconductors exhibiting strong Ising spin-orbit coupling. By carefully controlling the magnetic field and supercurrent, the research sought to demonstrate a scenario where increasing current in one direction drives a transition from FFLO to BCS before reaching the normal state, while the opposite direction leads directly from FFLO to normal. Superconducting diode efficiency peaks near FFLO-BCS state transitions Calculations reveal a peak superconducting diode (SD) efficiency arising from a complex interaction between superconducting orders. Specifically, the research predicts a sharp increase in SD efficiency within the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state, occurring close to the transition point between the FFLO and Bardeen-Cooper-Schrieffer (BCS) states. This enhancement is linked to the coupling between interlayer vortices and the net supercurrent when symmetry between layers is disrupted. Initial analysis of the Ginzburg-Landau (GL) free energy demonstrates that the ground state energy, Econ, varies with momentum, q, and magnetic field, qB. At a magnetic field of qB, the supercurrent, Ix, is calculated as 2e multiplied by the derivative of Econ with respect to q. Phase diagrams constructed from these calculations delineate the boundaries between BCS, FFLO, and normal states. In a symmetric bilayer, the phase diagram shows second-order transitions separating the BCS and FFLO states, alongside first-order transitions to the normal state at higher currents. In particular, an asymmetric bilayer, where the superconducting order parameter differs between layers, exhibits asymmetric phase boundaries around Ix = 0. This asymmetry results in unequal critical currents, I+c and I−c, and as a result, a non-zero superconducting diode efficiency. The density of magnetic flux quanta, given by Φ/(LxΦ0) = qB/(2π), presents a important parameter in understanding the system’s behaviour. Unlike previous attempts to create superconducting diodes, this approach doesn’t rely on breaking time-reversal symmetry, a complex and often unstable process. Instead, it offers a pathway based on manipulating inherent material properties. For the broader effort to realise practical superconductivity, this work represents a step forward, suggesting that the key to unlocking its potential may lie not in overcoming resistance, but in intelligently directing the current itself. Specifically, calculations suggest a dramatic increase in ‘superconducting diode’ efficiency when the material transitions between conventional and more exotic forms of superconductivity. The difficulty lies in precisely controlling this transition, demanding materials and conditions where the change in superconducting order is sensitive enough to influence current flow. Calculations involving layered materials subjected to magnetic fields demonstrate a peak in diode efficiency near this transition point, a result stemming from the competition between two distinct superconducting arrangements. The reliance on specific magnetic field strengths and material properties presents a practical hurdle, as maintaining these conditions in a real-world device could prove challenging. The significance extends beyond simply improving current control. Measuring this diode efficiency offers a new way to probe the fundamental physics governing the shift between these superconducting states, providing insights into a long-studied phenomenon. Future research could explore different material combinations or alternative methods to induce this transition, perhaps using strain or other external stimuli. A key question remains: can these effects be scaled up and maintained reliably enough to create practical, energy-saving devices, or will they remain confined to the laboratory. Once the limitations of material purity and fabrication are addressed, the potential applications are broad, ranging from more efficient power transmission to advanced computing technologies. Unlike previous attempts to create superconducting diodes, this approach doesn’t rely on breaking time-reversal symmetry, a complex and often unstable process. Instead, it offers a pathway based on manipulating inherent material properties. For the broader effort to realise practical superconductivity, this work represents a step forward, suggesting that the key to unlocking its potential may lie not in overcoming resistance, but in intelligently directing the current itself. 👉 More information 🗞 Current Induced Switching of Superconducting Order and Enhancement of Superconducting Diode Efficiency 🧠 ArXiv: https://arxiv.org/abs/2602.16648 Tags: