Local characterization and engineering of proximitized correlated states in $\text{graphene}/\mathrm{Nb}{\mathrm{Se}}_2$ vertical heterostructures

Using a van der Waals (vdW) vertical heterostructure consisting of monolayer graphene, monolayer hBN and $\mathrm{Nb}{\mathrm{Se}}_2$, we have performed local characterization of induced correlated states in different configurations. At a temperature of 4.6 K, we have shown that both superconductivity and charge density waves can be induced in graphene from $\mathrm{Nb}{\mathrm{Se}}_2$ by proximity effects. By applying a vertical magnetic field, we imaged the Abrikosov vortex lattice and extracted the coherence length for the proximitized superconducting graphene. We further show that the induced correlated states can be completely blocked by adding a monolayer hBN between the graphene and the $\mathrm{Nb}{\mathrm{Se}}_2$, which demonstrates the importance of the tunnel barrier and surface conditions between the normal metal and superconductor for the proximity effect.

Figure 1

(a) Optical microscopy image of the measured device. Gray and blue dashed lines enclose the monolayer graphene and monolayer hBN flakes. (b) Schematic of the STM experimental setup, black and red arrows indicated the position where the $dI/dV$ curves in (c) and (d) were taken. (c) $dI/dV$ spectra acquired with $\mathrm{I}=100\mathrm{pA}$ and $V_{\mathrm{mod}}=5\mathrm{mV}$. (d) $dI/dV$ spectra acquired with $\mathrm{I}=500\mathrm{pA}$ and $V_{\mathrm{mod}}=0.4\mathrm{mV}$.

Figure 2

[(a), (c), and (e)] Topography images of the three different stacking configurations, acquired with ${\mathrm{V}}_{\mathrm{bias}}=0.3\mathrm{V}, \mathrm{I}=100\mathrm{pA}$. [(b), (d), and (f)] Symmetrized Fourier transform of (a), (c), and (e). Blue hexagons and orange rectangles mark the graphene and $\mathrm{Nb}{\mathrm{Se}}_2$ lattices; red circles mark the charge density waves; green, yellow, and purple triangles mark the graphene-$\mathrm{Nb}{\mathrm{Se}}_2$, graphene-hBN, and hBN-$\mathrm{Nb}{\mathrm{Se}}_2$ moiré, respectively. [(g)−(i)] Theory calculation of moiré wavelengths and θ for three different configurations, colored dots indicate the experimental values and dashed lines indicate the obtained twist angle.

Figure 3

(a) $dI/dV$ map showing the vortices in graphene/$\mathrm{Nb}{\mathrm{Se}}_2$ area, acquired with $V_{\mathrm{bias}}=-3\mathrm{mV}, I=200\mathrm{pA}$, and $V_{\mathrm{mod}}=0.4\mathrm{mV}$. Blue arrow indicates the position where the line cut spectroscopy were taken. (b) $dI/dV$ spectra at different distances from the center of a vortex, acquired with $I=500\mathrm{pA}$ and $V_{\mathrm{mod}}=0.4\mathrm{mV}$. (c) Extracted superconducting gap plotted against the distance from the vortex center, black curve indicates the fitting function. (Inset) Normalized zero bias conductance plotted against the distance from the vortex center, black curve indicates the fitting function. (d) Two terminal resistance measurement as a function of temperature, dashed line corresponding to the critical temperature.

Figure 4

(a) $dI/dV$ map under a perpendicular magnetic field around the monolayer hBN edge, acquired with $V_{\mathrm{bias}}=-2\mathrm{mV}, I=200\mathrm{pA}$, and $V_{\mathrm{mod}}=0.4\mathrm{mV}$. (b) Same image as (a) except acquired with $V_{\mathrm{bias}}=10\mathrm{mV}$.

Figure 5

(a) $dI/dV$ map near surface defects, acquired with $V_{\mathrm{bias}}=-40\mathrm{mV}, I=500\mathrm{pA}$, and $V_{\mathrm{mod}}=3\mathrm{mV}$. (b) $dI/dV$ map acquired with $V_{\mathrm{bias}}=-0.5\mathrm{mV}, I=50\mathrm{pA}$, and $V_{\mathrm{mod}}=0.4\mathrm{mV}$. (c) $dI/dV$ map acquired with $V_{\mathrm{bias}}=90\mathrm{mV}, I=40\mathrm{pA}$, and $V_{\mathrm{mod}}=5\mathrm{mV}$. [(b), (d), and (f)] Fourier transform of (a), (c), and (e). (g) Wave vector of the scattering wave as a function of bias voltage, solid black line indicates the fitting function.