Abstract
As described in Chap. 4, the non-magnetic delafossites PtCoO2 and PdCoO2 host bulk metallic states in their Pt and Pd layers, while the contribution of the CoO2 layer to the bands at the Fermi level is negligible. In this chapter I will concentrate on PdCrO2, the magnetic counterpart of PdCoO2. Its transition metal oxide layer is a correlated Mott insulator, hosting localised spins 3/2 on chromium sites, which at \(37.5 \,\mathrm {K}\) undergo a transition towards an antiferromagnetic state with a \(120^{\circ }\) order, as described in Sect. 1.1.4. The main question I will address in this chapter is how the coupling between the itinerant and antiferromagnetic Mott insulating subsystems affects the spectroscopic signatures, and what information can be obtained from the spectral function of such a coupled system. I will first show the measured spectra, and compare them to the bulk states of PtCoO2 and PdCoO2 (Sect. 5.1). In trying to understand the magnitude of the observed signal it became clear that our experimental observations cannot be explained in terms of simple models of electrons in a periodic potential. Reaching this conclusion required a general analysis of photoemission intensity in systems with periodic potentials of varying strength, given in Sect. 5.2. This discussion is not relevant only for PdCrO2, but for all systems with periodic potentials of varying strength, such as charge- or spin- density wave materials. A reader primarily interested in PdCrO2 may choose to read Sect. 5.2.4, in which the discrepancy between the measurement and the simple model becomes apparent, immediately after Sect. 5.1. The discrepancy motivated a many body calculation, done by our collaborators Sota Kitamura and Takashi Oka, which I outline in Sect. 5.3. This calculation in turn motivated additional experiments, described in Sect. 5.3.2, which both confirmed the theory and offered novel insight about the types of information accessible to angle resolved photoemission.
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Notes
- 1.
This is in fact a simplification, as the final state in the three step process is actually an unoccupied Bloch state. For high photon energies it can be approximated by a free-electron state, but even in these circumstances this is not the measured state, as the electron still needs to travel to the surface, and escape into vacuum (steps two and three of the three step process). These distinctions are however not important for the qualitative discussion outlined here.
- 2.
Calling this quantity the ‘simulated intensity’, as I did in the simple models above, is misleading if we wish to compare it to the experimental findings, as it does not account for the experimental geometry, or the symmetry of the underlying orbitals.
- 3.
The inter-layer coupling g here exhibits a weak momentum dependence, and is thus labelled as \(g_{\vec {k}}\). This is a consequence of the fact that in the real material each atom has more than one nearest neighbour in the neighbouring layer. The coupling therefore has a spatial structure, reflected in a momentum structure of its Fourier transform. For more details see Ref. [9].
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Sunko, V. (2019). Coupling of Metallic and Mott-Insulating States in PdCrO2. In: Angle Resolved Photoemission Spectroscopy of Delafossite Metals. Springer Theses. Springer, Cham. https://doi.org/10.1007/978-3-030-31087-5_5
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