Nature | Vol 584 | 20 August 2020 | 373Article
Observation of superconducting diode effect
Fuyuki Ando^1 , Yuta Miyasaka^1 , Tian Li^1 , Jun Ishizuka^2 , Tomonori Arakawa3,4, Yoichi Shiota^1 ,
Takahiro Moriyama^1 , Youichi Yanase^2 & Teruo Ono1,4 ✉Nonlinear optical and electrical effects associated with a lack of spatial inversion
symmetry allow direction-selective propagation and transport of quantum particles,
such as photons^1 and electrons^2 –^9. The most common example of such nonreciprocal
phenomena is a semiconductor diode with a p–n junction, with a low resistance in one
direction and a high resistance in the other. Although the diode effect forms the basis
of numerous electronic components, such as rectifiers, alternating–direct-current
converters and photodetectors, it introduces an inevitable energy loss due to the
finite resistance. Therefore, a worthwhile goal is to realize a superconducting diode
that has zero resistance in only one direction. Here we demonstrate a magnetically
controllable superconducting diode in an artificial superlattice [Nb/V/Ta]n without a
centre of inversion. The nonreciprocal resistance versus current curve at the
superconducting-to-normal transition was clearly observed by a direct-current
measurement, and the difference of the critical current is considered to be related
to the magnetochiral anisotropy caused by breaking of the spatial-inversion and
time-reversal symmetries^10 –^13. Owing to the nonreciprocal critical current, the
[Nb/V/Ta]n superlattice exhibits zero resistance in only one direction. This
superconducting diode effect enables phase-coherent and direction-selective charge
transport, paving the way for the construction of non-dissipative electronic circuits.Nonreciprocal charge transport is important for the wide use of elec-
tronic components, such as rectifiers, alternating–direct-current
converters and photodetectors. In 1874, Braun discovered rectification
in a metal–semiconductor contact^2 , heavily influencing the develop-
ment of semiconductor devices. In modern condensed matter physics,
Rikken et al.^1 ,^3 –^5 demonstrated that nonlinear optical and electrical
responses can generally be achieved when both spatial inversion and
time-reversal symmetry are broken in a system, a phenomenon
described as magnetochiral anisotropy^1 –^9. In a Rashba system, where
the spatial inversion is uniaxially broken along the z axis, the spin–orbit
interaction causes spin-dependent band splitting^14 –^16. The spin σ and
the wavevector k are required to be orthogonal in the x–y plane
and the electrons with +k and −k have opposite spin directions
(σ(+kσ)=−(−)k). For example, if we apply a magnetic field By to break
time-reversal symmetry along the y axis, the electrons with wavevec-
tors +kx and −kx come to have non-equivalent energy depending on
whether the spin σ(±kx) is parallel or antiparallel to the magnetic field
By. This results in a nonlinear response of current Ix proportional to the
square of the electric field along the x axis under magnetic field By. This
nonreciprocal effect, induced by the Rashba spin–orbit interaction
and magnetic field, is expressed in the form of current-dependent
resistance as shown in equation ( 1 ).
RR=( 0 1+γ(×Bz)⋅I) (1)where γ is the coefficient of magnetochiral anisotropy depending on the
Rashba spin–orbit interaction. The nonlinear resistance is considered
to be the perturbation to the linear resistance R 0 that is generally scaled
by the kinetic energy of the electrons. Therefore, the magnitude of γ is
typically very tiny in normal conductors because the Rashba spin–orbit
interaction and magnetic energy are much smaller than the kinetic
energy of the electrons, that is, the Fermi energy EF.
Recently, nonreciprocal charge transport in superconductors has
attracted considerable interest because the nonlinear resistance was
found to be remarkably enhanced in the superconducting fluctuation
region compared to the normal conducting state^10 –^13 ,^17. This trend can be
explained by the replacement of the energy denominator in the second
term of equation ( 1 ) from EF (in electronvolts) to the energy gap (in
millielectronvolts) in the superconductors^10 ,^13. The discovery strongly
suggests the potential for directional transport of superconducting
current. However, for low-dimensional superconductors such as MoS 2
(ref.^10 ), WS 2 (ref.^11 ) and Bi 2 Te 3 /FeTe (ref.^12 ), the rectification ratio is not
sufficient for implementation in devices because the linear resistance
R 0 gradually decreases during the superconducting transition and is
orders of magnitude larger than the nonlinear one. Consequently, there
still remains the need to realize a superconducting diode that has zero
resistance in only one direction.
Here we fabricate a noncentrosymmetric superlattice^18 ,^19 by stacking
three kinds of superconducting elements—niobium, vanadium and
tantalum—repeatedly, and observe a superconducting diode effect con-
trolled by a magnetic field (Fig. 1 ). We clearly observed nonreciprocity
in the resistance–current (R–I) curve, specifically the critical current, by
direct-current (d.c.) measurement (Fig. 2 ). Furthermore, the nonlinear
resistance unique to the Rashba superconductor was also observed
in the resistive superconducting fluctuation region. This means that
the Rashba superconductivity can be accessed using an artificiallyhttps://doi.org/10.1038/s41586-020-2590-4
Received: 14 March 2020
Accepted: 23 June 2020
Published online: 19 August 2020
Check for updates(^1) Institute for Chemical Research, Kyoto University, Kyoto, Japan. (^2) Department of Physics, Graduate School of Science, Kyoto University, Kyoto, Japan. (^3) Graduate School of Science, Osaka
University, Osaka, Japan.^4 Center for Spintronics Research Network, Graduate School of Engineering Science, Osaka University, Osaka, Japan. ✉e-mail: [email protected]