PTCDA Molecular Monolayer on Pb Thin Films: An Unusual $\pi$-Electron Kondo System and Its Interplay with a Quantum-Confined Superconductor

The hybridization of magnetism and superconductivity has been an intriguing playground for correlated electron systems, hosting various novel physical phenomena. Usually, localized $d$ or $f$ electrons are central to magnetism. In this study, by placing a PTCDA (3,4,9,10-perylene tetracarboxylic dianhydride) molecular monolayer on ultrathin Pb films, we built a hybrid magnetism/superconductivity ($\mathrm{M}/\mathrm{SC}$) system consisting of only $sp$ electronic levels. The magnetic moments reside in the unpaired molecular orbital originating from interfacial charge transfers. We reported distinctive tunneling spectroscopic features of such a Kondo screened $\pi$ electron impurity lattice on a superconductor in the regime of $T_K\gg \mathrm{\Delta }$, suggesting the formation of a two-dimensional bound states band. Moreover, moiré superlattices with tunable twist angle and the quantum confinement in the ultrathin Pb films provide easy and flexible implementations to tune the interplay between the Kondo physics and the superconductivity, which are rarely present in $\mathrm{M}/\mathrm{SC}$ hybrid systems.

Figure 1

(a) STM image of ML $\mathrm{PTCDA}/\mathrm{Pb}$ film. The red rectangle represents a unit cell of the herringbone structure. The inset shows the atomically resolved image of the nearby Pb surface ($4\times 4{\mathrm{nm}}^2$). The twist angle $\theta$ is defined as labeled. (b) $dI/dV$ spectra for the ML $\mathrm{PTCDA}/\mathrm{Pb}$, $T_{\mathrm{S}}=0.4$ (gray) and 6.5 K (blue), a single molecule on Pb at 0.4 K (green), and the ML PTCDA on Ag(111) at 4.2 K (red). Spectra are shifted vertically for clarity. (c) The charge transfer and the magnetic moment as a function of the $d$ for all three situations. The equilibrium distances are marked by vertical dashed lines. (d) Partial DOS at various $d$ for ML $\mathrm{PTCDA}/\mathrm{Pb}$. The dashed lines indicate the Fermi levels (a) Sample bias $V_s=1.0\mathrm{V}$, setpoint current $I=25\mathrm{pA}$. Setpoint in (b): $V_s=95\mathrm{mV}$, $I=200\mathrm{pA}$, and the lock-in modulation $V_{\mathrm{rms}}=0.8\mathrm{mV}$.

Figure 2

(a)–(b) Typical Moiré superlattice for $\theta =11°$, $\theta =13°$, and $\theta =17°$. The red parallelogram in (a) represents the moiré supercell. $\lambda$ is the periodicity along ${\overrightarrow{\mathit{a}}}_1$ direction. The $\mathrm{Pb}<1\stackrel{\rightharpoonup }{1}0>$ direction is marked by the yellow dashed arrow. Inset in (a) is an enlarged image at the black dashed rectangle with molecular models overlaid. (c) The upper panel is a falsecolor image of $dI/dV$ spectra taken along the bright row marked in (a) by the red arrow. The lower panel shows the moiré modulations of $\mathrm{\Gamma }$ (wine) and ${\mathrm{\Delta }}^{*}$ (gray). A small magnetic field (0.5 T) was applied to quench superconductivity. (d) The corresponding oscillations of $N_F|J|$ (upper panel) and $K_U/|J|$ (lower panel). Spectra in (c) were taken at 0.4 K with $V_s=95\mathrm{mV}$, $I=100\mathrm{pA}$, $V_{\mathrm{rms}}=2.5\mathrm{mV}$. (a)–(b) $V_s=2.0\mathrm{V}$, $I=20\mathrm{pA}$; inset in (a) $V_s=1.0\mathrm{V}$. Scale bars are 5.0 nm.

Figure 3

(a) Pairing gap measurements for the molecule #5, #6, and #10 in the bright row in Fig. 2. (b) Sample DOSs obtained by numerical deconvolution of the superconducting tip DOS. Results of bare 23 ML Pb are displayed for comparison. The peak area $S_1$, gap area $S_2$, and bound state energy level ${\mathrm{\Delta }}^{*}$ are labeled as shown. (c) False-color image of the bound state measurements along the bright row in Fig 2. (d) Ensembles of data points on $\mathrm{PTCDA}/23\mathrm{ML}$ Pb (orange) and $\mathrm{PTCDA}/22\mathrm{ML}$ Pb (green), plotted in terms of the ${\mathrm{\Delta }}^{*}$ versus $\mathrm{\Gamma }$. The solid lines are fittings based on Matsuura’s theory. The asymptotic limits of the SC order parameters at $\mathrm{\Gamma }=\infty$ are labeled. All spectra were taken by Nb tips at 0.4 K. $V_s=4.0\mathrm{mV}$, $I=100\mathrm{pA}$, $V_{\mathrm{rms}}=50\mu \mathrm{V}$.

Figure 4

(a) $dI/dV$ spectra from $-0.6$ to $+2.0\mathrm{V}$ taken on the ML $\mathrm{PTCDA}/22\mathrm{ML}$ Pb, bare 22 ML, ML $\mathrm{PTCDA}/23\mathrm{ML}$ Pb, and 23 ML Pb surfaces. (b) and (c) Measurements of Kondo resonances and pairing gaps on ML $\mathrm{PTCDA}/22\mathrm{ML}$ Pb. Two typical spots are chosen: the morphologically highest (red) and lowest (blue) molecules within one moiré periodicity [labeled in the inset of (b)]. The black curve in (c) is taken on bare 22 ML Pb with its SC gap ${\mathrm{\Delta }}_0^{22ML}=1.35\mathrm{meV}$ [21]. All spectra were taken at 0.4 K with a normal tip. (b) $V_s=95\mathrm{mV}$, $I=100\mathrm{pA}$, and $V_{\mathrm{rms}}=2.5\mathrm{mV}$; (c) $V_s=3.5\mathrm{mV}$, $I=100\mathrm{pA}$, and ${\mathrm{V}}_{\mathrm{rms}}=50\mu \mathrm{V}$.