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We report that the superconducting critical magnetic field can be nonreciprocal under a bias of an electric current in a superconducting [Nb/V/Ta] n superlattice without a center of inversion. The critical magnetic field showed a clear difference between positive and negative magnetic fields. Furthermore, the magnitude relation between the positive and negative critical magnetic fields is reversed when the direction of the electric current is reversed. Our findings indicate that the superconducting gap can be anisotropic by the application of an electric current.

Recently, an ultimate diode effect, a superconducting diode where an electric current shows the superconducting state in one direction and the normal state in the other direction, has been discovered in a noncentrosymmetric Nb/V/Ta superlattice. Here, we report that the polarity of the superconducting diode shows a sign reversal as a magnetic field is increased. Such a nonlinear behavior of the diode effect is beyond the phenomenology based on the Ginzburg-Landau theory. Based on a recent microscopic study, we propose the crossover and phase transitions of the finite-momentum pairing states as a possible origin of the sign reversals.

Artificially engineered superlattice offers highly flexible control of the quantized electronic state by manipulating its lattice structure. We fabricated artificially engineered superlattices [Nb/V/Ta] n as a new class of superconducting superlattices and investigated the upper critical field H c2 in terms of the structural difference. It is found that the out-of-plane upper critical field H c 2 ⊥ significantly increases compared with that of a Nb single layer film as the thickness of the constituent Nb, V, and Ta layers becomes thin. In the case of the superlattice [Nb/V/Ta] n , the H c 2 ⊥ is basically governed by the orbital pair-breaking effect rather than Pauli pair-breaking effect and increases only when the crystalline size of the superlattice decreases by the insertion of V layers. Thus, we conclude that the shortening of the coherence length of Cooper pairs due to the crystalline disorder predominantly contributes to the increase of the H c 2 ⊥ in the superlattice [Nb/V/Ta] n .