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applsciletetrsa>vol-01>issue-06>Ubiquitous Superconducting Diode Effect in Superconductor Thin Films

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Letter

Ubiquitous Superconducting Diode Effect in Superconductor Thin Films

Yasen Hou
Fabrizio Nichele
Applied Science Letters

2022 ° 23(06) ° 01-05

https://www.wikipt.org/applscilettersa

DOI: 10.1490/6577036.842applsci

Abstract


A superconductor exhibiting a polarity-dependent critical current is of fundamental as well as technological interest because the superconducting (SC) layer can then admit a perfect dissipationless transmission along one direction while offering a large resistance along the opposite, leading to a phenomenon called the SC diode effect or rectification. Here we demonstrate that SC diode effects are ubiquitous in superconductors and observable in a large variety of settings. Controllable via an out-of-plane magnetic field, we observe an extremely sensitive SC diode effect (type A) in superconducting vanadium or niobium stripes with symmetry breaking between their two edges. Nonreciprocity of critical current results from an out-of-plane field as small as 1 Oe, while the diode efficiency and polarity are manipulated via the strength and direction of the field. With the out-of-plane field carefully eliminated, an in-plane field also creates a sizeable asymmetry between critical currents (type B), regardless of whether this field is perpendicular or parallel to the current flow direction. Finally, we demonstrate nonreciprocal critical currents with a giant diode efficiency reaching 65% when the SC V film couples to a ferromagnetic EuS layer with in-plane magnetization orthogonal to current flow (type C) and a clear diode rectification is seen. This is also realized at zero applied field, in the remnant magnetic state of EuS. Our observations show the ubiquity of the superconducting diode effect and pave the way for the development of versatile SC rectifiers employing simple structures using widely available materials.

Introduction

Similar to a traditional semiconductor diode, a superconductor with non-reciprocal current flow, an SC diode, may form the building block for, e.g., dissipationless SC digital logic. The recent observation of such an SC diode effect in a complex thin-film superconductor heterostructure subjected to an external magnetic field has stimulated vigorous activity towards understanding and replicating it1. Supercurrent rectification has also been demonstrated in multiple Josephson junction systems including Al-InGaAs/InAs-Al2, NbSe2/Nb3Br8/NbSe23 and Nb-NiTe2-Nb4, where the largest nonreciprocity is observed at large in-plane magnetic fields2,4. Furthermore, an intrinsic SC diode effect has been observed in few-layer NbSe25 and twisted trilayer graphene/WTe2 heterostructures6 in an out-of-plane magnetic field. Several theoretical mechanisms have been proposed to explain the SC diode effects in superconductors7-9 and in Josephson junctions10, with special emphasis on a potential role of the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state7,9,10. The breaking of time-reversal and inversion symmetries in a superconductor with Rashba spin-orbit coupling (SOC) supports a helical superconducting phase, with the order parameter modulated in a direction transverse to the Zeeman field: Δ(𝑟)=Δ𝑒𝑖𝑞0𝑟. In such an FFLO-like phase, Cooper pairs gain a finite momentum 𝑞0, and the depairing effect for supercurrents flowing parallel and anti-parallel to 𝑞0 is different, leading to a critical current nonreciprocity. On general grounds, interfaces cause inversion symmetry-breaking and Rashba SOC along the normal z-direction thereby admitting a polar and odd-under-time-reversal vector 𝑇 along 𝑧×ℎ, where ℎ is an applied magnetic field or an exchange field induced by an adjacent ferromagnet (FM)11. Thus, the application of an in-plane magnetic field perpendicular to the current flow direction should activate 𝑇, resulting in peculiar superfluid condensate properties and a critical current nonreciprocity. To quantify the diode effect, it is common to introduce an asymmetry parameter, called the diode efficiency, 𝜂=𝐼𝑐+−𝐼𝑐−𝐼𝑐++𝐼𝑐− where Ic+ and Ic- are the critical currents in the two directions. The value denotes the magnitude of the diode effect, while the sign defines the polarity. Up to now, reported values of 𝜂 range from a few percent to 30%1-6.

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