Dichotomy of electron-phonon interactions in the delafossite PdCoO$_2$: From weak bulk to polaronic surface coupling

The metallic delafossites host ultra-high mobility carriers in the bulk, while at their polar surfaces, intrinsic electronic reconstructions stabilise markedly distinct electronic phases, from charge-disproportionated insulators, to Rashba-split heavy-hole gases and ferromagnetic metals. The understanding of these phases has been strongly informed by surface spectroscopic measurements, but previous studies have been complicated by the presence of spatially varying terminations of the material surface. Here, we demonstrate the potential of microscopic-area angle-resolved photoemission to overcome these challenges. Our measurements of the model compound PdCoO$_2$ yield extremely high-quality spectra of the electronic structure, which allows us to place new experimental constraints on the weak electron-phonon coupling in the bulk of PdCoO$_2$, while revealing much stronger interactions at its surfaces. While the CoO$_2$-terminated surface exhibits a conventional weak-coupling behavior, our measurements reveal surprising spectroscopic signatures of polaron formation at the Pd-terminated surface, despite its pronounced metallicity. Together, our findings reveal how mode and symmetry-selective couplings can markedly tune the electron-phonon interactions in a single host material, here opening routes to stabilise surprisingly persistent polaronic quasiparticles.

💡 Research Summary

The authors employ micro‑focused angle‑resolved photoemission spectroscopy (µ‑ARPES) to disentangle the electronic structures of the two polar terminations that naturally occur on cleaved PdCoO₂ surfaces: Pd‑terminated and CoO₂‑terminated. Conventional ARPES, with a spot size of tens of micrometres, inevitably mixes contributions from both terminations, obscuring the intrinsic properties of each. By using capillary‑focusing optics the authors achieve sub‑10 µm spatial resolution, allowing them to map the surface termination via the Co 3p : Pd 4p core‑level intensity ratio and then acquire high‑quality ARPES spectra from regions of pure termination.

In the bulk, the steep Pd‑derived band crossing the Fermi level exhibits a Fermi velocity of 8.1 × 10⁵ m s⁻¹. A linear bare‑band approximation yields an effective mass enhancement of only ~4 %, corresponding to an electron‑phonon coupling constant λ ≈ 0.04 ± 0.01. This value is consistent with first‑principles predictions (λ ≈ 0.06) and is even weaker than that of elemental copper (λ ≈ 0.13). Analysis of the imaginary part of the self‑energy shows no discernible kink or linewidth broadening associated with the known 20 meV bulk phonon, placing an upper bound of λ ≈ 0.1. The exceptionally weak coupling, together with the high crystalline quality, explains the record‑high in‑plane conductivity of PdCoO₂.

The CoO₂‑terminated surface hosts two spin‑polarised surface bands that disperse along Γ‑M and Γ‑K. By comparing the measured dispersions with density‑functional theory surface supercell calculations (energy‑scaled and shifted to match experiment), the authors construct a realistic bare‑band reference. Fitting the real and imaginary parts of the self‑energy with a Migdal‑Eliashberg model that includes two Einstein modes (≈20 meV and ≈45 meV) yields λ ≈ 0.05, indicating that the surface electron‑phonon interaction remains weak and can be described within conventional Migdal theory.

In stark contrast, the Pd‑terminated surface displays clear signatures of polaron formation. The ARPES spectra show a rapid loss of quasiparticle weight, multiple satellite peaks, and an asymmetric “waterfall‑like” dispersion indicative of strong coupling to a specific phonon. The authors attribute this to the out‑of‑plane Pd‑O stretch mode (~70 meV), whose coupling is enhanced because screening is reduced in the two‑dimensional surface electron gas. Importantly, the polaronic features are highly sensitive to surface adsorption; exposure to water dramatically suppresses the satellite intensity, demonstrating that the coupling strength can be tuned by the surface environment.

Overall, the work reveals a dichotomy of electron‑phonon interactions within a single material: bulk and CoO₂‑terminated surface states exhibit weak, Migdal‑type coupling, while the Pd‑terminated surface hosts robust, mode‑selective polaronic coupling despite its metallic character. This demonstrates that mode symmetry and surface charge reconstruction can be leveraged to engineer electron‑phonon coupling on the fly, opening pathways to simultaneously achieve ultra‑high conductivity and strong electron‑lattice interactions in layered oxides. Such control could be exploited for designing high‑performance thermoelectrics, polariton‑based devices, or surface‑catalytic systems where polaronic carriers play a functional role.