Abstract
We present a scanning tunneling microscopy (STM) and ab-initio study of the anisotropic superconductivity of 2H-NbSe2 in the charge-density-wave (CDW) phase. Differential-conductance spectra show a clear double-peak structure, which is well reproduced by density functional theory simulations enabling full k- and real-space resolution of the superconducting gap. The hollow-centered (HC) and chalcogen-centered (CC) CDW patterns observed in the experiment are mapped onto separate van der Waals layers with different electronic properties. We identify the CC layer as the high-gap region responsible for the main STM peak. Remarkably, this region belongs to the same Fermi surface sheet that is broken by the CDW gap opening. Simulations reveal a highly anisotropic distribution of the superconducting gap within single Fermi sheets, setting aside the proposed scenario of a two-gap superconductivity. Our results point to a spatially localized competition between superconductivity and CDW involving the HC regions of the crystal.
Highlights
We present clear evidence of anisotropic superconductivity in NbSe2 by measuring the tunneling density of states (DOS) with unprecedented resolution combined with state-of-the-art first-principles simulations of the superconducting state based on density functional theory for superconductors (SCDFT)
Besides the atomic corrugation of the terminating Se layer, the image reveals the modulation of the local DOS imposed by the CDW
The improved SCDFT estimacuts on the Nb planes of L1 and L2
Summary
Collective phenomena such as superconductivity and charge- or spin-density modulations have been found to co-exist in many materials, the interplay among these correlations being discussed in terms of their competition or mutual enhancement[1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22]. The layered transition-metal dichalcogenide 2H-NbSe2 is a prominent example undergoing a transition into a chargeordered phase at T ≤33 K, and into the superconducting phase at Tc ≤ 7.2 K. Both orderings are driven by electron-phonon coupling[23,24,25,26,27,28,29,30,31] with a strong momentum dependence[14,17,20,32]. Accessing the momentum-dependent structure of the superconducting gap in the CDW phase would allow one to unveil the link between superconducting condensation and CDW ordering in this material, resolving the long-standing debate on their competition
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