Doping Mott insulators can lead to the formation of a variety of emergent states such as high temperature superconductivity as well as charge, spin and orbital orders etc.  However, the underlying physics for understanding their evolution is rare.  In this Article, Yim et al. employed low temperature scanning tunnelling microscopy (STM), angle-resolved photoemission spectroscopy (ARPES) and density functional theory (DFT), to study the Mott insulating CrO₂ surface layer of delafossite PdCrO₂, whose instinct polar catastrophe provides a clean route for surface doping (see Fig. 1a).

Fig 1. a, Schematics of surface doping effect occurring on the two different surface terminations of PdCrO₂. b-d, STM (b) and ARPES (c-d) data obtained from the two surface terminations.

Through experiment, Yim et al. have found the evidence for the existence of the two different surface terminations of PdCrO₂ (Fig. 1, b-d), and unexpectedly, detected on the CrO₂ terminated surface an insulating gap with size of 500 mV centred around the Fermi level (Fig. 2).

Fig. 2. a-e, ARPES data obtained from the two surface terminations of PdCrO₂; f-g, ARPES (f) and STM/S (g) data revealing an energy gap of 500 meV on the CrO₂ terminated surface; h, STM topographic image of the CrO₂ terminated surface recorded at low bias voltage and extremely low tunnelling current. Inset of (h), that recorded at higher sample bias;  i, Fourier transformation of (h). 

To further explore the origin of the energy gap, Yim et al. performed STM measurement on the CrO₂ terminated surface at extremely low bias and tunnelling current condition, through which discovered that there exists a short-range ordered state (see Figure 2, h-i).  Based on the experimental observation, Dr Srdjan Stavrić (one of the collaborators) performed DFT calculations using a model with a √7×√7 spatial periodicity and comprising 4 Cr atoms with their spins pointing upward and 3 with spins pointing downward, to calculate the electronic structure of the CrO₂ surface.  The calculation results show that through charge disproportionation, the formation of the short-range ordered state leads to two different types of Cr atoms (Cr_A, Cr_B), both having different Bader charges, magnetic moments, and vertical displacements from the surface layer.

Fig. 3. a, Projected density of states (PDOS) of bulk PdCrO₂，calculated using a supercell having √7×√7 spatial periodicity and comprising 4 Cr atoms with spins pointing up and 3 with spins pointing down.  b, Results show that following surface relaxation, the vertical displacement of some Cr atoms can exceed ~0.1Å, meanwhile there exists two different types of Cr atoms on the CrO₂ surface.  c, Surface electronic structure of the CrO₂ terminated surface; inset of (c), PDOS of Cr_A and Cr_B in narrower energy range near E_F.

Furthermore, under weak perturbation (e.g., tens of meVs tip pulses), the short-range ordered state shows a temporal evolution, hence exhibiting a glassy dynamical behaviour (Fig. 4).

Fig. 4. a-d, time-dependent STM images showing temporal evolution of the short-range ordered state following a small perturbation (a 70 mV, 2 s tip pulse in this case); e-g，corresponding difference images；h，Variance versus time plot showing the glassy dynamics of the short-range ordered state.

Chi Ming Yim of Tsung Dao Lee Institute, Shanghai Jiao Tong University is the co-first author (STM experiment) and co-corresponding author of the paper.  Other co-first authors include Dr. Gesa -R. Siemann of University of St Andrews (ARPES) and Dr. Srdjan Stavrić of CNR-SPIN (theoretical calculations).  Other co-corresponding authors include Prof. Peter Wahl and Prof. Phil D. C. King of University of St Andrews, and Prof. Silvia Picozzi of CNR-SPIN, Italy.

Article link：https://www.nature.com/articles/s41467-024-52007-z