Investigation of the Pauli paramagnetic effect in systematically tuned NbN thin films

TL;DR

This study investigates the Pauli paramagnetic effect in NbN thin films by analyzing their upper critical fields across various orientations and temperatures, revealing the influence of spin paramagnetism on superconductivity.

Contribution

It provides systematic measurements of upper critical fields in NbN films with varying $T_c$ and resistivity, highlighting the role of Pauli paramagnetism and two-dimensional effects near $T_c$.

Findings

Upper critical fields exceed the paramagnetic limit.

Temperature dependence of $B_{c2}^{ot}$ fits extended WHH model.

Evidence of two-dimensional superconductivity near $T_c$.

Abstract

Superconductivity and the normal-state properties of NbN films can be tuned in a wide range, supplying a suitable platform to investigate the systematical evolution of the superconducting performances. Herein, we report the upper critical field of NbN films in both the vertical ($B\perp$ film) and parallel ($B\parallel$ film) orientations over a wide temperature range. Eight samples with the superconducting critical temperature $T_c$ ranging from 2.5 K to 9.8 K are studied. Meanwhile, the normal-state resistivity is tuned by more than six times by changing the conditions of the film growth. It is found that the magnitudes of the upper critical field in both field directions ($B_{c2}^{\perp}$ and $B_{c2}^{\parallel}$) exceed the paramagnetic limiting field $B_p$. The temperature dependent $B_{c2}^{\perp}$ can be described by the extended Werthamer--Helfand--Hohenberg (WHH) model considering the Pauli spin paramagnetism. Meanwhile, the $B_{c2}^{\parallel}$-$T$ data shows the feature of two-dimensional superconductivity in the temperature range near $T_c$. The evolution of the obtained Maki parameter with other parameters, such as the slope of the upper critical field near $T_c$ and the normal-state resistivity, are discussed to reveal the characteristics of the Pauli paramagnetic effect in this system.