Spin–orbit effects in pentavalent iridates: models and materials

Abstract

Spin-orbit effects in heavy 5d transition metal oxides, in particular, iridates, have received enormous current interest due to the prediction as well as the realization of a plethora of exotic and unconventional magnetic properties. While a bulk of these works are based on tetravalent iridates (d 5 ), where the counter-intuitive insulating state of the rather extended 5d orbitals are explained by invoking strong spin-orbit coupling, the recent quest in iridate research has shifted to the other valencies of Ir, of which pentavalent iridates constitute a notable representative. In contrast to the tetravalent iridates, spin-orbit entangled electrons in d 4 systems are expected to be confined to the J = 0 singlet state without any resultant moment or magnetic response. However, it has been recently predicted that, magnetism in d 4 systems may occur via magnetic condensation of excitations across spin-orbit-coupled states. In reality, the magnetism in Ir 5+ systems are often quite debatable both from theoretical as well as experimental point of view. Here we provide a comprehensive overview of the spin-orbit coupled d 4 model systems and its implications in the studied pentavalent iridates. In particular, we review here the current experimental and theoretical understanding of the double perovskite (A 2 BYIrO 6 , A = Sr, Ba, B = Y, Sc, Gd), 6H-perovskite (Ba 3 MIr 2 O 9 , M = Zn, Mg, Sr, Ca), post-perovskite (NaIrO 3 ), and hexagonal (Sr 3 MIrO 6 ) iridates, along with a number of open questions that require future investigation.

On the electronic ground state of two non-magnetic pentavalent honeycomb iridates

“…There is a stark qualitative difference between the observed RIXS spectra for NaIrO 3 and that of Sr 3 CaIr 2 O 9 . In contrast to other typical Ir 5+ iridates that have well-defined ∆E ≈ 250 meV RIXS features from inter-t 2g split by 3/2 λ excitations [10,21,23,25,51], we observe a weak and broad inelastic signal that spans from ∆E of 0.5 eV up to 1.5 eV and can be fit to a minimum of six Gaussian peaks. Below 0.5 eV there is a nearly constant small intensity above background, but no discernible peak.…”

“…Depending on the electron filling, the j eff states form the basis for many novel quantum phases including Kitaev spin liquids [2], spin-orbital liquids [3][4][5], multipolar phases [3], and other unconventional ordered states [6][7][8]. The case of d 4 electron filling and octahedral coordination is of particular interest, as it realizes a nominally non-magnetic J eff = 0, m J eff = 0 single-ion singlet [9][10][11][12], as shown in Fig. 1 (a).…”

Emergent quantum phenomena in atomically engineered iridate heterostructures

“…Due to the strong spin-orbit coupling (SOC), compounds containing 5 d elements have gained increasing attention with the emergence of enormous exciting findings. Among the extensive collection of materials, iridates are particularly notable for being a rare playground that covers plenty of foremost research topics, such as frustrated magnetism, topological phases of matter, and superconductivity [1] , [2] , [3] , [4] , [5] , [6] , [7] , [8] . Pioneering works include those hunting for exotic magnetic states in complex lattice compounds, such as the hyper-kagome iridate Na 4 Ir 3 O 7 [9] .…”

Jahn-Teller states mixed by spin-orbit coupling in an electromagnetic field

“…On the other hand, spin orbit interactions are key to new developments in classical and quantum computation and lie beneath new discoveries in condensed matter physics related to topological matter, such as the quantum spin Hall effect or the realization of topological insulators and Weyl semimetals [15][16][17] and Kitaev physics in quantum spin-liquids [18][19][20][21]. At the same time, there is a highly nontrivial interplay between spin-orbit coupling and the Jahn-Teller effect when t 2g states are partially filled, where entangled quantum spin-orbital states may emerge [22][23][24].…”