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Enhanced Charge-Density-Wave Order and Suppressed Superconductivity in Intercalated Bulk $\mathrm{Nb}{\mathrm{Se}}_{2}$

Huanhuan Shi, Qili Li, Antoine M. T. Baron, Marie-Aude Méasson, Sangjun Kang, Dirk Fuchs, Fabian Henssler, Alexander Haas, Paolo Battistoni, Nour Maraytta, Michael Merz, Amir-Abbas Haghighirad, Wulf Wulfhekel, Christian Kübel, Matthieu Le Tacon

Abstract

The electronic ground states of transition-metal dichalcogenides are strongly shaped by reduced dimensionality, yet the properties of atomically thin layers remain difficult to probe due to their small size and environmental sensitivity. Here we demonstrate that controlled electrochemical intercalation of organic cations provides a robust bulk platform for accessing monolayer-like physics in NbSe$_2$. Intercalation of tetrapropylammonium and tetrabutylammonium expands the interlayer spacing by nearly a factor of two, electronically decoupling the NbSe$_2$ layers while simultaneously introducing well-defined charge doping. Using a combination of Raman spectroscopy, scanning tunneling microscopy, X-ray diffraction, and photoemission, we uncover a pronounced enhancement of the charge-density-wave transition temperature to $\sim 130$ K together with a strong suppression of superconductivity, reproducing the phase diagram observed in exfoliated monolayers. The enhanced charge-density-wave order and reduced $T_c$ arise from the combined effects of dimensionality reduction and electron injection, and are accompanied by distinct dip-hump anomalies in the tunneling spectra suggestive of collective mode excitations. Our results establish molecular intercalation as a powerful and scalable route for engineering competing orders in layered quantum materials.

Enhanced Charge-Density-Wave Order and Suppressed Superconductivity in Intercalated Bulk $\mathrm{Nb}{\mathrm{Se}}_{2}$

Abstract

The electronic ground states of transition-metal dichalcogenides are strongly shaped by reduced dimensionality, yet the properties of atomically thin layers remain difficult to probe due to their small size and environmental sensitivity. Here we demonstrate that controlled electrochemical intercalation of organic cations provides a robust bulk platform for accessing monolayer-like physics in NbSe. Intercalation of tetrapropylammonium and tetrabutylammonium expands the interlayer spacing by nearly a factor of two, electronically decoupling the NbSe layers while simultaneously introducing well-defined charge doping. Using a combination of Raman spectroscopy, scanning tunneling microscopy, X-ray diffraction, and photoemission, we uncover a pronounced enhancement of the charge-density-wave transition temperature to K together with a strong suppression of superconductivity, reproducing the phase diagram observed in exfoliated monolayers. The enhanced charge-density-wave order and reduced arise from the combined effects of dimensionality reduction and electron injection, and are accompanied by distinct dip-hump anomalies in the tunneling spectra suggestive of collective mode excitations. Our results establish molecular intercalation as a powerful and scalable route for engineering competing orders in layered quantum materials.
Paper Structure (4 sections, 2 equations, 4 figures)

This paper contains 4 sections, 2 equations, 4 figures.

Table of Contents

  1. Synthesis of NbSe$_2$ single crystal
  2. Intercalation of (TAA)$_x$ molecules
  3. Low-temperature Raman spectroscopy
  4. Scanning tunneling microscopy

Figures (4)

  • Figure 1: Intercalant–temperature phase diagram of 2H-NbSe$_2$ based on Raman spectroscopy ($T_{\mathrm{CDW}}$) and STM (from $\Delta_{\text{SC}}$, see text); monolayer values are taken from Ref. Xi_2015.
  • Figure 2: (a) A$_{1g}$ mode of NbSe$_2$, (TPA)$_y$NbSe$_2$ and (TBA)$_x$NbSe$_2$; (b) XRD patterns of $\mathrm{Nb}{\mathrm{Se}}_{2}$, (TPA)$_y$NbSe$_2$ and (TBA)$_x$NbSe$_2$; (c) Cross-sectional HAADF-STEM images of pristine $\mathrm{Nb}{\mathrm{Se}}_{2}$ (top) and intercalated (TBA)$_x$NbSe$_2$ (bottom). Scale bars, 1nm.
  • Figure 3: (a,c) Temperature-dependent Raman spectra of pristine NbSe$_2$ (a) and intercalated (TBA)$_x$NbSe$_2$ (c) in the ab polarization configuration; (b,d) Temperature dependence of the phonon frequencies in pristine NbSe$_2$ (b) and intercalated (TBA)$_x$NbSe$_2$ (d).
  • Figure 4: (a) The STM image of the intercalated (TBA)$_x$NbSe$_2$. The tip was stabled at sample bias 20 mV with tunneling curren 20 pA; (b-c) CDW gap and superconductivity gap of NbSe$_2$ and intercalated (TBA)$_x$NbSe$_2$ measured at 44 mK. Measuring conditions: (a) the tip was stabled at sample bias $20\mV$ with tunneling current $20\pA$; (b) the tip was stabled at sample bias $100\mV$ and tunneling current $100\pA$, and the lock-in amplifier had a modulation amplitude $4\mV$; (c) the tip was stabled at sample bias $5\mV$ and tunneling current $100\pA$ with modulation amplitude $50\uV$ for NbSe$_2$, and sample bias $1\mV$ and tunneling curren $100\pA$ with modulation amplitude $15\uV$ for (TPA)$_y$NbSe$_2$ and (TBA)$_x$NbSe$_2$.