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Dichotomy of electron-phonon interactions in the delafossite PdCoO2: From weak bulk to polaronic surface coupling

Gesa-R. Siemann1, Philip A. E. Murgatroyd1, Tommaso Antonelli1, Edgar Abarca Morales1,2, Seunghyun Khim2, Helge Rosner2, Matthew D. Watson3, Andrew P. Mackenzie2,1, and Phil D. C. King1,*

  • *Contact author: pdk6@st-andrews.ac.uk
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Phys. Rev. B 112, 085113 – Published 8 August, 2025

DOI: https://doi.org/10.1103/5w81-7jtk

Abstract

The metallic delafossites host ultrahigh-mobility carriers in the bulk, while at their polar surfaces, intrinsic electronic reconstructions stabilize 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 PdCoO2 yield extremely high quality spectra of the electronic structure, which allows us to place stringent experimental constraints on the weak electron-phonon coupling in the bulk of PdCoO2, while revealing much stronger interactions at its surfaces. While the CoO2-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 stabilize surprisingly persistent polaronic quasiparticles.

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Phys. Rev. B 112, 085113– Published 8 August, 2025
  • Received 20 December 2024
  • Revised 18 July 2025
  • Accepted 23 July 2025
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