Non-oxy delafossites: unconventional spin orbit entangled magnetism and electron transport
Layered chalcogenide (Ch) delafossites ABCh2, where A is a monovalent and B is a trivalent transition metal/rare earth ion, feature a wide variety of interesting phenomena, including superconductivity (NaxCoO2) , large thermoelectricity (Cu[Rh,Mg]O2) , multiferroicity (CuFeO2) , hydrodynamic electron flow (PdCoO2) [4,5] and spin-orbit driven frustration on a perfect triangular lattice (NaYbCh2) [6-9].
Most of the research done so far was on oxygen-based delafossites (DFs) whereas non-oxygen DFs (with S, Se or Te) are less investigated yet. The variation of the ionic radii of the mono- and trivalent- ions has only a little effect on the band structure and mainly tunes the lattice parameters whereas the choice of the chalcogenide ion itself has a strong impact on the band structure. For example, the oxy delafossite PdCoO2 has a quasi two-dimensional conductivity and the density of states at the Fermi level show minor chalcogenide 2p- electron admixture. In contrast to that in AgCrS2 or AgCrSe2 the larger spatial extended 3(4)p- states of S and Se promote the emergence of hybridized p- states at the Fermi level which finally leads to three-dimensional metallic or semi-metallic conductivity . This promotes correlation effects in general and together with the emergence of strong spin polarization unconventional magnetism and (magneto-)transport is expected.
AgCrSe2 is a promising primer among non oxy delafossites, which exhibit an unusual liquid-like thermal conduction originating from the complex interplay of magnetic and charge degrees of freedom [11-13]. In particular the spin polarization of the charge carriers (Rashba effect) originate strong correlations and unconventional thermoelectricity . We were able to successfully grow large AgCrSe2 single crystals. A first joint study with the Rossendorf High Field Laboratory and the ISIS Neutron and Muon Facility (Oxford, UK) has demonstrated anisotropic cycloidal magnetic ordering with unusual extended two-dimensional fluctuations  We have developed a model for the magnetic exchange that consistently describes the determined magnetic phase diagram. Currently, we are extending the studies to AgCrS2.
This project combines the expertise in the field of oxy-delafossite at the MPI CPfS  with the search for new correlated triangular lattice, delafossite-like, systems. The aim is (i) to synthesize new materials (chemical vapor transport & solid state reaction - M. Schmidt, S. Khim) accompanied by structural characterization, (ii) to determine the thermodynamic (heat conductivity, specific heat) properties and the electronic (and magneto-) transport (M. Baenitz) and (iii) to apply microscopic spectroscopy like electron spin resonance (ESR, J. Sichelschmidt) and nuclear magnetic resonance (NMR, M. Baenitz). In addition, in-house theory support on band structure (H. Rosner) and magnetic exchange models (B. Schmidt) is available.