Abstract
Recent studies on superconductivity in NbSe$_2$ have demonstrated a large anisotropy in the superconducting critical field when the material is reduced to a single monolayer. Motivated by this recent discovery, we use density functional theory (DFT) calculations to quantitatively address the superconducting properties of bulk and monolayer NbSe$_2$. We demonstrate that NbSe$_2$ is close to a ferromagnetic instability, and analyze our results in the context of experimental measurements of the spin susceptibility in NbSe$_2$. We show how this magnetic instability, which is pronounced in a single monolayer, can enable sizeable singlet-triplet mixing of the superconducting order parameter, contrary to contemporary considerations of the pairing symmetry in monolayer NbSe$_2$, and discuss approaches as to how this degree of mixing can be addressed quantitatively within our DFT framework. Our calculations also enable a quantitative description of the large anisotropy of the superconducting critical field, using DFT calculations of monolayer NbSe$_2$ in the normal state
Highlights
The transition metal dichalcogenides (TMD) exhibit an astonishing variety of phenomena and phase transitions, which includes charge-density waves (CDWs) [1,2,3,4,5], superconductivity [6], and magnetism [7]
As a function of increasing disorder introduced by electron irradiation [14]
Theoretical analyses of the superconducting pairing mechanism in monolayer NbSe2 [26,27] have relied on model descriptions of superconductivity in materials that lack inversion symmetry [28,29,30], loosely based on the band structure calculated from first principles [27]
Summary
The transition metal dichalcogenides (TMD) exhibit an astonishing variety of phenomena and phase transitions, which includes charge-density waves (CDWs) [1,2,3,4,5], superconductivity [6], and magnetism [7]. Theoretical analyses of the superconducting pairing mechanism in monolayer NbSe2 [26,27] have relied on model descriptions of superconductivity in materials that lack inversion symmetry [28,29,30], loosely based on the band structure calculated from first principles [27]. We use the insights obtained from our calculations to discuss possible ramifications on the superconducting order parameter in NbSe2, in particular, the factors that control the scale of the triplet admixture to the order parameter This knowledge is the first step toward a comprehensive quantitative theory of Ising superconductivity in monolayer NbSe2
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