Ubiquitous Superconducting Diode Effect in Superconductor Thin Films

The macroscopic coherence in superconductors supports dissipationless supercurrents that could play a central role in emerging quantum technologies. Accomplishing unequal supercurrents in the forward and backward directions would enable unprecedented functionalities. This nonreciprocity of critical supercurrents is called the superconducting (SC) diode effect. We demonstrate the strong SC diode effect in conventional SC thin films, such as niobium and vanadium, employing external magnetic fields as small as 1 Oe. Interfacing the SC layer with a ferromagnetic semiconductor EuS, we further accomplish the nonvolatile SC diode effect reaching a giant efficiency of 65%. By careful control experiments and theoretical modeling, we demonstrate that the critical supercurrent nonreciprocity in SC thin films could be easily accomplished with asymmetrical vortex edge and surface barriers and the universal Meissner screening current governing the critical currents. Our engineering of the SC diode effect in simple systems opens the door for novel technologies while revealing the ubiquity of the Meissner screening effect induced SC diode effect in superconducting films, and it should be eliminated with great care in the search for exotic superconducting states harboring finite-momentum Cooper pairing.

Figure 1

Demonstration of out-of-plane field induced diode effect in a SC film at 1.8 K. (a) Top: schematic drawing of the vanadium thin film strip. Bottom: optical microscope image of the Hall bar strip of V film. Scale bar denotes $8\mathrm{\mu }\mathrm{m}$. (b) $I\text{−}V$ scans of the device at 2.8 Oe out-of-plane field along $\pm z$ direction as indicated by the red and blue lines. Black arrows indicate the scan direction. (c) Critical currents and diode efficiency as a function of the magnetic field. Black dashed lines show the calculated critical current values based on our model. (d) Schematic depiction of Meissner screening currents and the asymmetry between the critical current densities at the two edges.

Figure 2

SC diode effect in $\mathrm{Pt}/\mathrm{V}/\mathrm{EuS}$ trilayers. (a) $I\text{−}V$ scans of a $\mathrm{Pt}/\mathrm{V}/\mathrm{EuS}$ device showing giant critical current rectification effect at 1.8 K. Inset shows a schematic of the $\mathrm{Pt}/\mathrm{V}/\mathrm{EuS}$ stack. (b) Temperature dependence of the critical current at $B_y=200\mathrm{Oe}$. (c) Magnetic field dependence of the critical current at 1 K. Solid (dashed) lines were obtained when scanning the magnetic field up (down). (d) Angle ($\varphi$) dependence of $\eta$ at $T=1\mathrm{K}$ and $B=200\mathrm{Oe}$. Data in (a) and (b)–(d) are from two different $\mathrm{Pt}/\mathrm{V}/\mathrm{EuS}$ devices.

Figure 3

SC diode effect in $\mathrm{SC}/\mathrm{FM}$ bilayers. (a) $I\text{−}V$ scans of a $\mathrm{V}/\mathrm{EuS}$ device showing a similarly large SC diode effect as in $\mathrm{Pt}/\mathrm{V}/\mathrm{EuS}$. Inset shows a schematic of the $\mathrm{V}/\mathrm{EuS}$ stack. (b) Temperature dependence of the critical currents for a second $\mathrm{V}/\mathrm{EuS}$ at $B_y=30\mathrm{Oe}$. (c) $I\text{−}V$ scans of a $\mathrm{Nb}/\mathrm{EuS}$ device exhibiting nonreciprocity. Inset shows a schematic of the $\mathrm{Nb}/\mathrm{EuS}$ stack. (d) Temperature dependence of the critical currents at $B_y=30\mathrm{Oe}$ for the same $\mathrm{Nb}/\mathrm{EuS}$ device.

Figure 4

(a) Resistance vs temperature shows the superconducting transition for Nb film in SC/FM bilayer with and without a 3 nm ${\mathrm{Al}}_2{\mathrm{O}}_3$ spacer layer. (b) $I\text{−}V$ scans of the Nb/EuS device measured at 1.8 K showing type C SC diode effects. Inset shows the schematic of Nb/EuS stack. (c) $I\text{−}V$ scans of the $\mathrm{Nb}/{\mathrm{Al}}_2{\mathrm{O}}_3/\mathrm{EuS}$ device at 1.8 K showing similar diode effects as in (b). Inset shows the $\mathrm{Nb}/{\mathrm{Al}}_2{\mathrm{O}}_3/\mathrm{EuS}$ stack structure. (d)  Screening current mechanism for the SC diode effect in SC/FM bilayers. Schematic of the screening currents (IS) induced by the out-of-plane edge magnetic fields due to the EuS layer.