Polar superconductors, in which paired electrons or quantum vortices move under the influence of electric polarization, are one of promising material platforms for searching exotic superconductivity and/or superconducting devices with new functions. Among them, the doped SrTiO3 is an ideal candidate of the polar superconductor [1], since it easily transforms to a ferroelectric and/or a superconductor with a little perturbation, leading to the coexistence of the ferroelectricity and superconductivity [2-4]. However, the distinct nature of the polar superconductors has been vague.
In this work, we have studied transport properties, containing the liner and nonreciprocal resistance, Rω and R2ω, of ion-gated SrTiO3 in an electric double layer transistor structure. In the normal state with carrier density 0.1 ~1 × 1014 cm-2, we found the sudden occurrence of finite nonreciprocal resistance R2ω at 30 K in the out-of-plane magnetic field B, implying the emergence of spontaneous in-plane polarization P in addition to out-of-plane P due to gating process. This leads to a unique polar superconducting state at low temperatures below 0.4 K in our systems. Indeed, in the superconducting transition region, we observed a variety of anomalous properties such as the strongly enhanced R2ω (B) and asymmetric Rω (B) in out-of-plane B, as well as the two-fold oscillation of Rω (ϕ) against the in-plane ϕ rotation of B, indicating the rachet motion of pancake vortices [1,5] in anisotropic two-dimensional (quasi one-dimensional) superconducting state. To explain these anomalies, we suggest a model for the polar superconductivity with the asymmetric stripe-like modulation of order parameter along P direction. We will also discuss that the pancake vortices can be induced by in-plane B due to the antisymmetric spin-orbit interaction originating from the peculiar electric polarization in ion-gated SrTiO3.
[1] Y. M. Itahashi, et al., Science Advances 6, eaay9120 (2020).
[2] C.W. Rischau et al., Nat. Phys. 13, 645 (2017).
[3] C. W. Rischau et al., Phys. Rev. Research 4, 013019 (2022).
[4] R. Russell et al., Phys. Rev. Mater 3, 091104 (2018).
[5] Y. M. Itahashi, Y. Saito, T. Ideue, T. Nojima, Y. Iwasa, Phys. Rev. Research 2, 023127 (2020).
This work was supported by the JSPS KAKENHI (No. 20H05145).
Keywords: Polar superconductivity, 2D vortex dynamics, Nonreciprocal transport, Spin-orbit interaction