Thickness-controlled order competition in few-layer superconducting NbSe2

The coexistence of multiple types of orders is a common theme in condensed matter physics and unconventional superconductors. Understanding the nature of superconducting orders can be achieved by examining local perturbations such as vortices. For thin films, the vortex magnetic profile is characterized by the Pearl length (Λ), which is inversely proportional to the 2D superfluid density and typically inversely proportional to the film thickness (d).

In Yonathan Anahory’s lab, scanning SQUID-on-tip microscopy has been employed to measure Λ in NbSe2 flakes with thicknesses ranging from 3 to 53 layers. For thicknesses greater than 10 layers, the expected relationship (Λ ∝ 1/d) is observed. However, in six-layer films, Λ exhibits a sharp increase, deviating by a factor of three from the anticipated value. Interestingly, this deviation remains constant for films with 3 to 6 layers. This anomalous behavior suggests the competition between two distinct orders: one present only on the first and last layers of the film, and the other existing throughout all layers.

In this talk, I will begin by reviewing the theory of Pearl vortices. Subsequently, I will present the experimental data and introduce a phenomenological model that explains these observations.