Ising Superconductivity and Quantum Phase Transition in Macro-Size Monolayer NbSe2

TL;DR

This study reports the successful fabrication of macro-size monolayer NbSe2 films with enhanced superconducting properties and evidence of Ising superconductivity and quantum phase transition, advancing 2D material applications.

Contribution

The paper demonstrates the growth of large-area monolayer NbSe2 with higher Tc and reveals quantum Griffiths singularity and Ising superconductivity in these films.

Findings

Superconducting transition temperature above 6 K, twice that of exfoliated flakes.

Parallel critical field exceeds paramagnetic limit by at least 4.5 times.

Observation of quantum Griffiths singularity near quantum critical point.

Abstract

Two-dimensional (2D) transition metal dichalcogenides (TMDs) have a range of unique physics properties and could be used in the development of electronics, photonics, spintronics and quantum computing devices. The mechanical exfoliation technique of micro-size TMD flakes has attracted particular interest due to its simplicity and cost effectiveness. However, for most applications, large area and high quality films are preferred. Furthermore, when the thickness of crystalline films is down to the 2D limit (monolayer), exotic properties can be expected due to the quantum confinement and symmetry breaking. In this paper, we have successfully prepared macro-size atomically flat monolayer NbSe2 films on bilayer graphene terminated surface of 6H-SiC(0001) substrates by molecular beam epitaxy (MBE) method. The films exhibit an onset superconducting critical transition temperature above 6 K, 2 times higher than that of mechanical exfoliated NbSe2 flakes. Simultaneously, the transport measurements at high magnetic fields reveal that the parallel characteristic field Bc// is at least 4.5 times higher than the paramagnetic limiting field, consistent with Zeeman-protected Ising superconductivity mechanism. Besides, by ultralow temperature electrical transport measurements, the monolayer NbSe2 film shows the signature of quantum Griffiths singularity when approaching the zero-temperature quantum critical point.