Modulation of superconducting properties by the charge density wave at the surface of 2H-NbSe2

TL;DR

This paper investigates how CDW interacts with superconductivity at the surface of the layered superconductor 2H-NbSe2 using ultralow-temperature spectroscopic-imaging STM. By achieving an effective energy resolution of about $36~\mu\mathrm{eV}$, the authors resolve intra-gap structures across multiple Fermi surfaces and separate gap energies from quasiparticle weights. They find spatially uniform superconducting-gap energies, indicating dominant zero-momentum pairing, while Bogoliubov-quasiparticle weight modulations occur at the CDW periodicity with a phase offset of $2\pi/3$ and are localized to two inequivalent CDW plaquettes, describable by a two-component model. The surface-breaking of in-plane inversion symmetry likely activates Ising spin–orbit coupling, linking the observed weight modulations to surface Ising superconductivity; these results provide a benchmark for microscopic theories of CDW–superconductivity and surface phenomena in NbSe$_2$.

Abstract

To investigate the interplay between charge density wave (CDW) and superconductivity, we performed ultralow-temperature spectroscopic-imaging scanning tunneling microscopy on the cleaved surface of the layered superconductor 2H-NbSe2. We found that the superconducting-gap spectrum exhibits intricate structures reflecting the anisotropic gaps opening on multiple Fermi surfaces. Notably, none of the characteristic energy scales apparent in the spectral gap show appreciable spatial variations, suggesting that the finite-momentum pairing is negligible. Instead, the spectral weight near the coherence peak is modulated with the same periodicity as the CDW. The maximum position of the coherence-peak-weight modulation coincides with neither the peak nor the bottom of the CDW modulation; rather, it aligns with the center of one of the two inequivalent triangular plaquettes that comprise the CDW unit cell. This distribution pattern of Bogoliubov quasiparticles directly results from the broken in-plane inversion symmetry at the surface of 2H-NbSe2, which may activate Ising spin-orbit coupling.

Figures (6)

• Figure 1: (a) The crystal structure of 2$H$-NbSe$_2$ visualized using VESTAMomma2011JAC. A rhomboid prism (thin black lines) indicates the unit cell. (b) Top-down view of a single NbSe$_2$ layer that breaks in-plane inversion symmetry. A unit cell is shown by a thick black rhombus. (c) and (d) Top views of NbSe$_2$ monolayers exhibiting anion-centered and hollow-centered $3\times3$ CDW orders, respectively. Black dashed lines connect the CDW maxima. Thick black rhombuses represent CDW unit cells, which consist of two inequivalent triangular plaquettes: PAC1 and PAC2 for the anion-centered CDW, and PHC1 and PHC2 for the hollow-centered CDW.
• Figure 2: (a) A typical STM topograph of the cleaved surface of 2$H$-NbSe$_2$. The feedback set-point is $I_\mathrm{s} = \qty{500}{pA}$ at $V_\mathrm{s} = \qty[retain-explicit-plus]{+5}{mV}$. The areas highlighted in colored boxes represent zoomed-in views in (b) and (c). (b) and (c) Zoomed-in STM topographies for the anion-centered and hollow-centered domains, respectively. The feedback set-point condition is the same as in (a). (d) and (e) Averaged $dI/dV$ spectra over the fields of view shown in (b) and (c), respectively. Gray curves represent the $d^3I/dV^3$ spectra.
• Figure 3: Spatially homogeneous superconducting energy scales in the anion-centered CDW area. (a) Black curves represent histograms of the three characteristic energies. The spatially averaged $dI/dV$ spectrum is depicted by a pale red curve and circles, and a gray curve shows the $d^3I/dV^3$ spectrum. (b)-(d) Spatial maps of the three characteristic energies. The field of view is the same as that of Fig. 1(b). Black dashed lines connect the CDW maxima.
• Figure 4: Spatially homogeneous superconducting energy scales in the hollow-centered CDW area. (a) Black curves represent histograms of the three characteristic energies. The spatially averaged $dI/dV$ spectrum is depicted by a pale blue curve and diamonds, and a gray curve shows the $d^3I/dV^3$ spectrum. (b)-(d) Spatial maps of the three characteristic energies. The field of view is the same as that of Fig. 1(c). Black dashed lines connect the CDW maxima.
• Figure 5: (a) $dI/dV$ spectra from the anion-centered area collected at three representative points within the CDW unit cell. The arrows indicate energies of $L$ maps shown in (c)-(f). (b) The STM topographic image showing the positions where the spectra in (a) were measured. White dashed lines connect the CDW maxima. The field of view matches that of Fig. 1(b). (c)-(f) Spatial distributions of $L$ maps at characteristic energies. Each $L$ map is normalized by its spatially averaged value to emphasize the relative contrast at a given energy. Green and red triangles denote PAC1 and PAC2, respectively. Arrows in (d)-(f) indicate the lines along which the line profiles shown in (h)-(j) are taken, respectively. (g) Part of the spatially-averaged $dI/dV$ spectrum. (h)-(j) Energy-dependent line profiles of $L$ maps along the arrows indicated in (d)-(f), respectively. We use the same color scale as in (c)-(f). The arrows denote the centers of PAC1 and PAC2, and the CDW maxima in the topographic image. Horizontal dashed lines across (g)-(j) denote the characteristic energies of 0.95meV, 1.14meV, and 1.30meV.