Coexistence of static and dynamic magnetism in the Kitaev spin liquid material Cu2IrO3

Abstract

Anyonic excitations emerging from a Kitaev spin liquid can form a basis for quantum computers 1, 2 . Searching for such excitations motivated intense research on the honeycomb iridate materials 3-17 . However, access to a spin liquid ground state has been hindered by magnetic ordering 5 . Cu 2 IrO 3 is a new honeycomb iridate without thermodynamic signatures of a long-range order 18 . Here, we use muon spin relaxation to uncover the magnetic ground state of Cu 2 IrO 3 . We find a two-component depolarization with slow and fast relaxation rates corresponding to distinct regions with dynamic and static magnetism, respectively. X-ray absorption spectroscopy and first principles calculations identify a mixed copper valence as the origin of this behavior. Our results suggest that a minority of Cu 2+ ions nucleate regions of static magnetism whereas the majority of Cu + /Ir 4+ on the honeycomb lattice give rise to a Kitaev spin liquid.

Spin Excitations of a Proximate Kitaev Quantum Spin Liquid Realized in Cu2IrO3

“…On the other hand, Cu 2 IrO 3 has its own complications arising from stacking faults, as is often the case for previously identified Kitaev candidate materials, and from the aforementioned defect spins [25]. The defect-induced enhanced susceptibility below ∼30 K is known to obey a peculiar scaling law [23], reminiscent of analogous scaling behavior reported earlier for other spin liquid candidate materials with disorder [26].…”

“…In Fig. 5(a), we summarize the temperature dependence of the lost fraction f NQR of the NQR spin echo intensity, measured at a constant pulse separation time τ ¼ 10 μs, down to 1.5 K. f NQR shows qualitatively the same behavior as the volume fraction f μSR estimated by μSR experiments [25], in which muons exhibit fast depolarization due to slow spin fluctuations in their vicinity [23,25]. Note that f μSR saturates at 50% below 1 K. (f NQR overestimates the volume fraction with slowed spin fluctuations, because the distribution of the spin-spin relaxation time T 2 prevented us from accurately taking into account the contributions of nuclear spins with fast T 2 .)…”

“…Unlike Na 2 IrO 3 , however, Cu 2 IrO 3 shows no evidence for antiferromagnetic long-range order [22], and recent μSR [23,25] as well as our nuclear quadrupole resonance (NQR) measurements presented below establish that only ∼50% of the sample volume exhibits spin freezing below the onset temperature of T f ∼ 10 K. Moreover, muons exhibit no spin precession about static hyperfine field, and Ir spins remain dynamic even below 1 K [23,25]. This means that the low-energy sector of the spin excitation spectrum in Cu 2 IrO 3 is largely immune from the spin waves, as schematically shown in the right-hand panel of Fig.…”

“…The origin and nature of the defect spins in Cu 2 IrO 3 as well as in other related iridates [15,[27][28][29] are a matter of intense research and are yet to be clarified. In the case of Cu 2 IrO 3 , x-ray absorption nearedge spectroscopy (XANES) measurements showed that up to ∼1=3 of 63 CuðHÞ sites are occupied by Cu 2þ ions rather than Cu þ ions [25]. To maintain charge neutrality, the charge state of an Ir 4þ ion(s) in their vicinity may change, possibly to Ir 3þ with null spin.…”

Magnetism of Ir5+ -based double perovskites: Unraveling its nature and the influence of structure

“…However, the exact relationship between noncubic distortions and the J eff = 1/2 state is currently unknown. While even a small tetragonal distortion, as observed in Sr 2 IrO 4 and Sr 3 Ir 2 O 7 , can lead to some orbital mixing [52], a dominant J eff = 1/2 character is preserved even within the distorted octahedra of the A 2 IrO 3 (A = Li,Na,Cu) family [53,54]. Nevertheless, it is well established that, in transition-metal oxides, the local structure ∼1 nm around a dopant can be highly distorted [41].…”