Abstract
Magnetic moments arranged at the corners of a honeycomb lattice are predicted to form a novel state of matter, the Kitaev quantum spin liquid, under the influence of frustration effects between bond-dependent Ising interactions. Some layered honeycomb iridates and related materials, such as and , are proximate to the Kitaev quantum spin liquid, but bosonic spin-wave excitations associated with undesirable antiferromagnetic long-range order mask the inherent properties of the Kitaev Hamiltonian. Here, we use nuclear quadrupole resonance to uncover the low-energy spin excitations in the nearly ideal honeycomb lattice of effective spin at the sites in . We demonstrate that, unlike , Ir spin fluctuations exhibit no evidence for critical slowing-down toward magnetic long-range order in zero external magnetic field. Moreover, the low-energy spin excitation spectrum is dominated by a mode that has a large excitation gap comparable to the Ising interactions, a signature expected for Majorana fermions of the Kitaev quantum spin liquid.
11 More- Received 9 May 2019
- Revised 22 July 2019
DOI:https://doi.org/10.1103/PhysRevX.9.031047
Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.
Published by the American Physical Society
Physics Subject Headings (PhySH)
Popular Summary
Heat a magnetic material to high temperature, and the spins of its electrons, which behave like tiny bar magnets, point in random directions. Upon cooling, the spin orientations form a regular pattern. But do magnetic materials whose spins never undergo such a change exist? In a 2006 paper, Kitaev showed that such materials could theoretically exist, if the spins are arranged in a 2D honeycomb pattern, and the interactions between spins satisfy certain rules. In such a “Kitaev spin liquid,” the electron spins keep randomly fluctuating even at absolute zero. Here, we unravel the unusual properties of spins in a new candidate for a Kitaev spin liquid——using nuclear magnetic resonance techniques, a powerful microscopic probe of magnets.
In essence, we use copper nuclear spins to “spy on” nearby electrons in the iridium ions. We first flip the copper nuclear spins by applying radio-frequency pulses. Then we wait. The fluctuating spins of the iridium electrons help to ease the copper nuclei back to equilibrium. By monitoring this “relaxation rate,” we probe how strongly the iridium spins fluctuate. Analysis of the data reveal that the relaxation depends on a thermal activation—most likely because the iridium spins can fluctuate only if they are given some minimum thermal energy. While there are caveats, this behavior is consistent with what is expected from a Kitaev spin liquid.
We expect that our approach to analyzing the relaxation rate will shed new light on various exotic quantum materials, such as the candidate materials of the Kitaev spin liquid and beyond.