[Paper Review] Modulation of superconducting properties by the charge density wave at the surface of 2H-NbSe2

T. Hanaguri|arXiv (Cornell University)|Jan 22, 2026

Organic and Molecular Conductors Research0 citations

TL;DR

Ultralow-temperature spectroscopic-imaging STM shows that 2H-NbSe2 has spatially uniform superconducting energy scales but CDW-modulated Bogoliubov quasiparticle weights aligned with surface-inversion symmetry breaking, possibly via Ising spin-orbit coupling.

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.

Motivation & Objective

• Investigate how charge density wave order influences superconductivity on the surface of 2H-NbSe2.
• Determine whether finite-momentum pairing is significant in the presence of CDW.
• Map energy scales and quasiparticle weights across the CDW unit cell at ultrahigh energy resolution.

Proposed method

• Perform ultralow-temperature spectroscopic-imaging STM on cleaved 2H-NbSe2 surfaces.
• Achieve an effective energy resolution of ~36 μeV to resolve intra-gap structures.
• Acquire dI/dV and d3I/dV3 spectra across CDW domains with high spatial detail.
• Map normalized conductance L = dI/dV / (I/V) to mitigate set-point effects.
• Analyze energies of characteristic spectral features via cubic polynomial fits to d3I/dV3 spectra.

Figure 1: (a) The crystal structure of 2 $H$ -NbSe 2 visualized using VESTA [ 22 ] . 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 o

Experimental results

Research questions

• RQ1 Do the characteristic superconducting gap energy scales vary spatially across the CDW unit cell?
• RQ2 How do superconducting-gap-related spectral weights (Bogoliubov quasiparticles) correlate with the CDW modulation?
• RQ3 Can surface-inversion-symmetry breaking and Ising spin-orbit coupling influence surface superconductivity in 2H-NbSe2?
• RQ4 What is the relationship between intra-gap fine structures and the multi-band, anisotropic gap of NbSe2?

Key findings

• The superconducting-gap energies are spatially uniform across the CDW domains (no detectable spatial variation within tens of μeV).
• Multiple fine structures in the gap reflect the multi-band, anisotropic superconducting gap.
• Spectral weight near coherence peaks modulates with the same periodicity as the CDW, not the gap amplitude.
• The maximum coherence-peak weight modulation centers on specific inequivalent CDW plaquettes, not at CDW maxima or peak bottom.
• Two-component quasiparticle-weight modulation model (components A and B) explains energy-dependent intensities and phase relationships with CDW, consistent with surface-inversion-symmetry breaking and possible Ising SOC.

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