Electron-phonon coupling and the coexistence of superconductivity and charge-density wave in monolayer NbSe2

Abstract

Monolayer 2H-NbSe2 has recently been shown to be a 2-dimensional superconductor, with a coexisting charge-density wave (CDW). As both phenomena are intimately related to electron-lattice interaction, a natural question is how superconductivity and CDW are interrelated through electronphonon coupling (EPC), which is important to the understanding of 2-dimensional superconductivity. This work investigates the superconductivity of monolayer NbSe2 in CDW phase using the anisotropic Migdal-Eliashberg formalism based on first principles calculations. The mechanism of the competition between and coexistence of the superconductivity and CDW is studied in detail by analyzing EPC. It is found that the intra-pocket scattering is related to superconductivity, leading to almost constant value of superconducting gaps on parts of the Fermi surface. The inter-pocket scattering is found to be responsible for CDW, leading to partial or full bandgap on the remaining Fermi surface. Recent experiment indicates that there is transitioning from regular superconductivity in thin-film NbSe2 to two-gap superconductivity in the bulk, which is shown here to have its origin in the extent of Fermi surface gapping of K and K pockets induced by CDW. Overall blue shifts of the phonons and sharp decrease of Eliashberg spectrum are found when the CDW forms. arXiv:1904.03672v2 [cond-mat.str-el]

Electronic and hyperbolic dielectric properties of ZrS2/HfS2 heterostructures

“…2D monolayer TMDs display a rich physics and hold promise for a wide range of applications in flexible electronics, optoelectronics, energy storage, catalysis or spintronics [1,4,6]. At low temperature, some monolayer TMDs can be a 2D superconductor with coexisting charge-density wave [7]. In recent years, a variety of heterostructures have also been fabricated through the vertical or lateral combination of single layer or multilayer TMDs with other TMDs or with other 2D materials such as h-BN or graphene [2,[8][9][10][11].…”

Emergent superconductivity in two-dimensional NiTe2 crystals

“…[3][4][5][6][7] In particular, monolayer transition metal dichalcogenides, NbSe 2 and TaS 2 , thinned down to the monolayer limit display coexisting superconductivity and charge-density wave driven by electron-phonon coupling. [1,7,[9][10][11] These twodimensional crystals can be transferred from one substrate to another, and the superconductivity persists subject to perhaps but mild tuning by the substrate in proximity. Thus, this class of highly crystalline superconductors harbor truly two-dimensional and freestanding superconductivity, offering an ideal platform for exploring the interplay of novel superconductivity, quantum criticality and electron correlation effects, as well as novel superconducting device systems.…”

“…Thus, this class of highly crystalline superconductors harbor truly two-dimensional and freestanding superconductivity, offering an ideal platform for exploring the interplay of novel superconductivity, quantum criticality and electron correlation effects, as well as novel superconducting device systems. [1][2][3][5][6][7][8][9][10][11][12]…”

“…Clearly, the availability of superconducting phase in the true two-dimensional limit will help shed light on the decades-old puzzle regarding the coexistence and interplay of these competing orders, [1] in addition to the exploration of novel physics in two-dimensional superconducting phases. There also appears to be an imminent spin-density wave when NbSe 2 is thinned to monolayer limit in addition to the coexistent superconductivity and charge-density wave, [10,11] which may be suggesting a highly curious possibility that two-dimensional crystalline superconductors may have a phase diagram resembling those in the high-T c class. The lack of such materials hinders the exploration of such phase diagrams.…”

Monopole charge density wave states in Weyl semimetals

“…It is possible to only have the CDW instability without superconductivity or, if their energy scales are comparable, to have both instabilities coexist. Analogous to the coexistence of the CDW and superconductivity seen in the transition-metal dichalcogenide NbSe 2 [54][55][56][57][58], competition between monopole harmonic orders is possible, and this interesting problem will be studied in future work.…”