g-Factor-Enhanced Upper Critical Field in Superconducting PdTe2 due to Quantum Confinement.

The Pauli limiting field imposed by the Zeeman effect bounds the upper critical field of weak-coupling superconductivity. It is determined by setting the condensation energy equal to the paramagnetic energy and scales inversely with the effective g-factor. Here, we demonstrate that in a few-layer-thick van der Waals superconductor, PdTe2, quantum confinement can tune the effective g-factor causing the Pauli limit to become thickness-dependent. We experimentally probe the in-plane upper critical field, Hc2∥, of PdTe2 at multiple intermediate thicknesses down to 20 mK. We find that Hc2∥ is enhanced by more than an order of magnitude as the thickness is reduced from 50 nm down to 17 nm. We model the temperature- and thickness-dependent Hc2||, revealing a thickness-dependent spin Zeeman depairing mechanism impacting its value. Our findings reveal how quantum confinement drives a reduction in g that enhances the Pauli limiting field and allows the measured enhancement of Hc2∥. A violation of the Pauli limit is often associated with unconventional pairing symmetry in superconductors. Our work demonstrates that this simple association is difficult without knowledge of the g-factor, particularly in layered materials.