In the ground state of a Bardeen-Cooper-Schrieffer superconductor, current is carried by Cooper pairs with zero total spin and momentum. Through magnetic influence on spins in a superconductor, a new ground state called Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) [1,2] with finite pair-momentum may appear. This state is characterized by a spatially oscillating order parameter. In this work we show for the first time that such a state can be induced by proximity effect when coupling a conventional superconductor to a two-dimensional dilute magnetic topological insulator [3]. In our experiment, the existence of the state is confirmed by the observation of superconducting reentrance effects as a function of both magnetic field and temperature.

The device comprises of superconducting MoRe leads side-contacting a Mn doped HgTe quantum well. The incorporation of Mn atoms creates a giant Zeeman effect allowing great tunability of the magnetic state [4]. Dilute magnetic topological insulators combined with superconductors are thus a promising platform to explore emergent spin-correlated supercurrent transport in two dimensions and edge modes. Our architecture, with a semiconductor weak link, allows access to the mesoscopic transport regime and easy external tunability of the effect. This provides the experimenter tools to control the phase difference across the superconductors.

References:

[1] P. Fulde and R. A. Ferrell, Physical Review 135, (1964) A550.

[2] A. Larkin and Y. N. Ovchinnikov, Soviet Physics-JETP 20, (1965) 762. [Zh.Eksp.Teor.Fiz. 47, 1136 (1964)].

[3] P. Mandal, S. Mondal, M. P. Stehno, S. Ilić, F. S. Bergeret, T. M. Klapwijk, C. Gould, L. W. Molenkamp, Nature Physics, (2024). DOI: https://doi.org/10.1038/s41567-024-02477-1

[4] J. K. Furdyna, Journal of Applied Physics 64, (1988) R29