Competition between orbital effects, Pauli limiting, and Fulde-Ferrell-Larkin-Ovchinnikov states in 2D transition metal dichalcogenide superconductors

We compare the upper critical field of bulk single-crystalline samples of the two intrinsic transition metal dichalcogenide superconductors, 2H-NbSe2 and 2H-NbS2, in high magnetic fields where their layer structure is aligned strictly parallel and perpendicular to the field, using magnetic torque experiments and a high-precision piezo-rotary positioner. While both superconductors show that orbital effects still have a significant impact when the layer structure is aligned parallel to the field, the upper critical field of NbS2 rises above the Pauli limiting field and forms a Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state, while orbital effects suppress superconductivity in NbSe2 just below the Pauli limit, which excludes the formation of the FFLO state. From the out-of-plane anisotropies, the coherence length perpendicular to the layers of 31 angstrom in NbSe2 is much larger than the interlayer distance, leading to a significant orbital effect suppressing superconductivity before the Pauli limit is reached, in contrast to the more 2D NbS2.

Automated summary

This study compares the upper critical field behaviors of two superconductors, 2H-NbSe2 and 2H-NbS2, using magnetic torque experiments and a high-precision piezo-rotary positioner. It is found that when the layer structure is aligned parallel to the field, orbital effects significantly suppress superconductivity in NbSe2, while NbS2 forms a Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state above the Pauli limiting field. Additionally, the coherence length of NbSe2 perpendicular to the layers is much larger than the interlayer distance, leading to the suppression of superconductivity before reaching the Pauli limit, in contrast to NbS2.