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) [1], large thermoelectricity (Cu[Rh,Mg]O2) [2], multiferroicity (CuFeO2) [3], 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 [10]. 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 [14]. 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 [15] 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.
More detailed studies show an unusual anomalous Hall effect [16] and interesting surface states in AgCrSe2 [17]. There is also evidence for a Cr based Kondo effect [18]
This project combines the expertise in the field of oxy-delafossite at the MPI CPfS [19] 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.
Figure: Crystal structure of AgCrSe2. CrSe6 octahedral layers are separated by AgSe4 equivalent tetrahedral layers. Cr3+ ions form triangular layers in the ab plane
Figure: Crystal structure of AgCrSe2. CrSe6 octahedral layers are separated by AgSe4 equivalent tetrahedral layers. Cr3+ ions form triangular layers in the ab plane
References
[1] K. Takada, H. Sakurai, E. Takayama-Muromachi, F. Izumi, R. A. Dilanian, and T. Sasaki
[5] N. Nandi, T. Scaffidi, P. Kushwaha, S. Khim, M.E. Barber, V. Sunko, F. Mazzola, P.D.C. King, H. Rosner, P.J.W. Moll, M. König, J.E. Moore, S. Hartnoll, and A.P. Mackenzie
Unconventional magneto-transport in ultrapure PdCoO2 and PtCoO2
[6] M. Baenitz, Ph. Schlender, J. Sichelschmidt, Y.A. Onykiienko, Z. Zangeneh, K. M. Ranjith, R. Sarkar, L. Hozoi, H. C. Walker, J.-C. Orain, H. Yasuoka, J. van den Brink, H.H. Klauss, D.S. Inosov, and Th. Doert
NaYbS2: A planar spin-1/2 triangular-lattice magnet and putative spin liquid
[8] K.M. Ranjith, S. Luther, T. Reimann, B. Schmidt, Ph. Schlender, J. Sichelschmidt, H. Yasuoka, A.M. Strydom, Y. Skourski, J. Wosnitza, H. Kühne, Th. Doert, and M. Baenitz
Anisotropic field-induced ordering in the triangular-lattice quantum spin liquid NaYbSe2
[11] B. Li, H. Wang, Y. Kawakita, Q. Zhang, M. Feygenson, H.L. Yu, D. Wu, K. Ohara, T. Kikuchi, K. Shibata, T. Yamada, X.K. Ning, Y. Chen, J.Q. He, D. Vaknin, R.Q. Wu, K. Nakajima, and M.G. Kanatzidis
Liquid-like thermal conduction in intercalated layered crystalline solids
[15] M. Baenitz, M.M. Piva, S. Luther, J. Sichelschmidt, K.M. Ranjith, H. Dawczak-Dȩbicki, M.O. Ajeesh, S.-J. Kim, G. Siemann, C. Bigi, P. Manuel, D. Khalyavin, D.A. Sokolov, P. Mokhtari, H. Zhang, H. Yasuoka, P.D.C. King, G. Vinai, V. Polewczyk, P. Torelli, J. Wosnitza, U. Burkhardt, B. Schmidt, H. Rosner, S. Wirth, H. Kühne, M. Nicklas, and M. Schmidt
Planar triangular S=3/2 magnet AgCrSe2: Magnetic frustration, short range correlations, and field-tuned anisotropic cycloidal magnetic order
[16] S. Kim, J. Zhu, M. M. Piva, M. Schmidt, D. Fartab, A. P. Mackenzie, M. Baenitz, M. Nicklas, H. Rosner, A. M. Cook, R. González-Hernández, L. Šmejkal, H. Zhang
Observation of the Anomalous Hall Effect in a Layered Polar Semiconductor
[17] G. R. Siemann, S. J. Kim, E. A. Morales, P. A. E. Murgatroyd, A. Zivanovic, B. Edwards, Marković, F. Mazzola, L. Trzaska, O. J. Clark, C. Bigi, H. Zhang, B. Achinuq, Th. Hesjedal, M. D. Watson, T. K. Kim, P. Bencok, G. van der Laan, C. M. Polley, M. Leandersson, H. Fedderwitz, K. Ali, B.Thiagarajan , M. Schmidt , M. Baenitz, H. Rosner, P. D. C. King
Spin-orbit coupled spin-polarised hole gas at the CrSe2-terminated surface of AgCrSe2
