One-way supercurrent controlled by magnetic field

2021 Journal Club for Condensed Matter Physics  
Nonreciprocal charge transport in electrical conductors is characterized by the dependence of current-voltage characteristic on the direction of current, such that I(−V ) = −I(V ). This behavior is found in semiconductor p − n junctions: due to the built-in potential across the depletion region the resistance is low under forward bias and high under reverse bias. In recent years, another type of nonreciprocal transport has been observed in junction-free bulk crystals under magnetic field, which
more » ... takes the form of current-dependent resistance R = R 0 (1 + γBI) [1]. This so-called magnetochiral effect is allowed in crystals lacking inversion symmetry. It can arise from the combination of spin-orbit and Zeeman splitting that removes the degeneracy between left-and right-moving charge carriers. The resulting nonreciprocal resistance γ depends crucially on the relative orientation of the current with respect to the magnetic and spin-orbit fields, see for example Ref. [2]. Since both Zeeman and spin-orbit energies are much smaller than Fermi energy, the nonreciprocal effect is generally weak in normal conductors. Now, the highlighted work by Ando et al observed for the first time nonreciprocity in the critical current of a superconductor. The system is an artificially designed superconductor film under a small in-plane magnetic field. The film consists of thin alternating layers of three superconducting elements-niobium, vanadium and tantalum, and shows a sharp superconducting at T c = 4.41K. The authors performed direct-current measurements along the film's plane. Remarkably, they found that the critical current I x along the axis orthogonal to the magnetic field B y is direction dependent, i.e., I + c = I − c . As a result, a DC current in the range I − c < I x < I + c flows without resistance in one direction only, while passing the same current in the opposite direction causes finite resistance, see Fig.1 . Thus the junctionfree superconductor film operates as a supercurrent diode and achieves one-way transport of electrical charge without power consumption. Such supercurrent diode could be useful in superconducting devices and circuits that have practical applications. It may be integrated with SQUID for ultrasensitive magnetic field measurements with ultralow power consumption, or enable signal isolation in superconducting neural networks. Besides its potential for device applications, the highlighted work 1
doi:10.36471/jccm_april_2021_02 fatcat:vp2lomngefa63ifb6t2gmtk3wm