Interfacial electron-phonon coupling and quantum confinement in ultrathin Yb films on graphite

Interfacial electron-phonon coupling in ultrathin films has attracted much interest recently. Here, by combining angle-resolved photoemission spectroscopy and scanning tunneling microscopy, we report quantized electronic states and strong interfacial electron-phonon coupling in ultrathin Yb films on graphite. We observed clear kinks in the energy-momentum dispersion of quantum well states, and the kink positions agree well with the energies of optical phonons of graphite. The extracted coupling strength $\lambda$ is largest for the thinnest film with a preferred (“magic”) thickness of four monolayers and exhibits a strong band dependence, which can be qualitatively accounted for by a simple model. The interfacial electron-phonon coupling also gives rise to characteristic steplike structures in the $dI/dV$ spectra, implying dominant coupling with the phonons with zero in-plane momentum. A Lifshitz transition occurs at higher coverage, where quantum well states derived mainly from $5d$ electrons dominate near the Fermi level and possess large effective mass (up to $\sim 19m_e$). Our results highlight the potentially important role of interfacial electron-phonon interaction for ultrathin films and provide spectroscopic insight to understand this cross-interface fermion-boson interaction.

Figure 1

Film growth and formation of magic-height islands. (a)–(d) STM images of clean graphite substrate (a) and Yb films with increasing nominal coverages: 2.5 MLs (b), 6 MLs (c), and 9.5 MLs (d). (e) The (1,0) peak intensity from RHEED as a function of film coverage, showing a slope change near three MLs. The inset is a line cut of RHEED intensity, implying possibly a small lattice change between 2 MLs and 15 MLs. (f) Height profile of a four-ML magic island along the dashed line shown in Fig. 1. (g) The island height distribution for a film with 2.5-ML coverage. (h) ARPES spectra for the substrate and for Yb films with one- and three-ML nominal coverages. (i) Cartoons illustrating different stages of film growth (stages 1–4).

Figure 2

Thickness-driven Lifshitz transition in Yb films. (a) and (b) ARPES spectra of a three-ML film (the magic-height islands) (a) and a ten-ML film (thick film) (b). In (a) the three-ML data are displayed, as the four-ML data contain a slight admixture from higher-thickness films due to inevitable roughness [27]. (c) and (d) The calculated band structures for four-ML (c) and ten-ML (d) film along two high-symmetry directions, obtained from slab calculations. (e) The $dI/dV$ curves of Yb films with different thicknesses, from STM measurements.

Figure 3

Band dispersion kinks and phonon-mediated tunneling in magic-thickness Yb films from interfacial EPC. (a) ARPES spectra near $E_F$ for the magic-thickness film. The extracted band dispersions (colored symbols) are overlaid on top of the experimental data. (b) Extracted band dispersions (colored pluses) and the extrapolated bare-band dispersion without EPC (dash-dotted curves). (c) Extracted real part of the self-energy and $\lambda$ for each QWS labeled by $(N,n)$. (d) A typical STM image of magic-thickness islands. (e) Calculated phonon dispersion relations for graphite. Dashed ovals and rectangular boxes indicate phonons contributing to inelastic tunneling for magic-thickness Yb films and the graphite substrate, respectively. (f) $dI/dV$ curves at different positions labeled in Fig. 3: Positions 1–3 are on bare graphite; positions 4–8 are on Yb islands. Vertical dashed lines indicate the steplike features due to the phonon-mediated inelastic tunneling.

Figure 4

The thickness ($N$) and subband ($n$) dependence of interfacial EPC coupling strength $\lambda$. (a) and (b) ARPES spectra for 10-ML (a) and 20-ML (b) film near $E_F$. The dispersion of the $n=0$ QWS subband is extracted and highlighted. A yellow arrow indicates a possible kink for the ten-ML film. A parabolic fitting to the heavy electron band right below $E_F$ is shown for the 20-ML film. (c) Simple drawings of the probability density of QWSs [labeled by $(N,n)]$ near the Yb/graphite interface. The probability density is shown as curves, with their zero positions offset vertically for clarity.