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arXiv:1905.13590 · Physical Review Research

Fingerprints of Kitaev physics in the magnetic excitations of honeycomb iridates

Alessandro Revelli, M. Moretti Sala, G. Monaco, Ciarán Hickey, P. Becker, F. Freund, Anton Jesche, P. Gegenwart, Tim Eschmann, Finn Lasse Buessen, Simon Trebst, Paul H. M. van Loosdrecht, Jeroen van den Brink, M. Grüninger

Published Oct 16, 2020• 86 citations
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Abstract

The authors report on a proximate Kitaev spin liquid regime in two different honeycomb iridates in a broad temperature and energy range. The common phenomenology with $\ensuremath{\alpha}$-RuCl${}_{3}$ defines what constitutes a ``Kitaev material'' beyond the existence of a dominant bond-directional exchange.

DOI
10.1103/physrevresearch.2.043094

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The authors report on a proximate Kitaev spin liquid regime in two different honeycomb iridates in a broad temperature and energy range. The common phenomenology with \ensuremathα\ensuremath{\alpha}\ensuremathα-RuCl3{}_{3}3​ defines what constitutes a ``Kitaev material'' beyond the existence of a dominant bond-directional exchange.
Quantum spin liquids have fascinated condensed matter physicists for decades because of their unusual properties such as spin fractionalization and long-range entanglement. Unlike conventional symmetry breaking, the topological order underlying quantum spin liquids is hard to detect experimentally. Even theoretical models are scarce for which the ground state is established to be a quantum spin liquid. The Kitaev honeycomb model and its generalizations to other tricoordinated lattices are chief counterexamples—they are exactly solvable, harbor a variety of quantum spin liquid phases, and are also relevant for certain transition metal compounds including the polymorphs of (Na,Li) 2 IrO 3 iridates and RuCl 3 . In this review, we give an overview of the rich physics of the Kitaev model, including two-dimensional and three-dimensional fractionalization as well as dynamic correlations and behavior at finite temperatures. We discuss the different materials and argue how the Kitaev model physics can be relevant even though most materials show magnetic ordering at low temperatures.
In Kitaev spin liquids, SSS = \textonehalf{} quantum spins on a honeycomb lattice with bond-dependent interactions fractionalize into Majorana fermions. Fingerprints of these fractionalized excitations can be detected via thermodynamic measurements. Here, the authors perform detailed heat capacity experiments on the spin-liquid candidate $\ensuremath{\alpha}$-RuCl3{}_{3}3​ as function of temperature and magnetic field. To estimate the intrinsic phonon contribution, they measure the nonmagnetic reference compound RhCl3{}_{3}3​ and perform $a\phantom{\rule{0}{0ex}}b$ $i\phantom{\rule{0}{0ex}}n\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}o$ phonon calculations. Measurements and analysis provide experimental evidence of fractionalized excitations in good agreement with theoretical predictions.
The Kitaev quantum spin liquid has been among the most intensively studied states of matter but there has been no clear-cut materials realization of it. This absence is largely because all existing honeycomb lattices are inherently distorted and disordered, which inevitably amplifies the competing Heisenberg interaction in the Kitaev-Heisenberg model, leading to an unwanted antiferromagnetic state. This work reports a new, undistorted honeycomb iridate NaxIrO3{}_{3}3​, where all evidence indicates that the elusive Kitaev spin liquid may finally be within reach.
Much interest has emerged in the realization of materials with quantum spin liquid ground states, which would provide fertile ground for realizing exotic excitations. Three ``Kitaev'' spin liquid candidate materials have been proposed -- Na2{}_{2}2​IrO3{}_{3}3​, $\ensuremath{\alpha}$-RuCl3{}_{3}3​, and $\ensuremath{\alpha}$-Li2{}_{2}2​IrO3{}_{3}3​ -- and there has been many diverging proposals for the most relevant interactions. Here, the authors re-evaluate carefully the magnetic interactions in these materials, using nonperturbative exact diagonalization methods. The results clarify misconceptions in the literature and help estimate the potential for realizing the spin liquid state in real materials.
Abstract Large anisotropic exchange in 5 d and 4 d oxides and halides open the door to new types of magnetic ground states and excitations, inconceivable a decade ago. A prominent case is the Kitaev spin liquid, host of remarkable properties such as protection of quantum information and the emergence of Majorana fermions. Here we discuss the promise for spin-liquid behavior in the 4 d 5 honeycomb halide α -RuCl 3 . From advanced electronic-structure calculations, we find that the Kitaev interaction is ferromagnetic, as in 5 d 5 iridium honeycomb oxides, and indeed defines the largest superexchange energy scale. A ferromagnetic Kitaev coupling is also supported by a detailed analysis of the field-dependent magnetization. Using exact diagonalization and density-matrix renormalization group techniques for extended Kitaev-Heisenberg spin Hamiltonians, we find indications for a transition from zigzag order to a gapped spin liquid when applying magnetic field. Our results offer a unified picture on recent magnetic and spectroscopic measurements on this material and open new perspectives on the prospect of realizing quantum spin liquids in d 5 halides and oxides in general.
$\ensuremath{\alpha}$-RuCl3{}_{3}3​ promises to harbor an exotic Kitaev spin liquid that could shape the fortunes of the fields of quantum materials and quantum computing. Here, the authors offer a Rosetta Stone for this and other anisotropic-exchange Kitaev materials. The key message is the approach for constraining parameters of their low-energy spin models using experimental observables. This work clears the path to a consistent interpretation of experiments and gives important new insights into the fundamental properties of quantum magnets with spin-orbit coupling.