In a Kitaev spin liquid, electron spins can break into fractional particles known as Majorana fermions and Z
2 fluxes. Recent experiments have indicated the existence of such fractional particles in a two-dimensional Kitaev material candidate,
α-RuCl
3. These exotic particles can be used in topological quantum computations when braided within their lifetimes. However, the lifetimes of these particles, critical for applications in topological quantum computing, have not been reported. Here we study ultrafast dynamics of photoinduced excitations in single crystals of
α-RuCl
3 using pump-probe transient grating spectroscopy. We observe intriguing photoexcited nonequilibrium states in the Kitaev paramagnetic regime between
TN~7 K and
TH~100 K, where
TN is the N\'eel temperature and
TH is set by the Kitaev interaction. Two distinct lifetimes are detected: a longer lifetime of ~50 ps, independent of temperature; a shorter lifetime of 1-20 ps, with a strong temperature dependence,
T−1.40. We analyze the transient grating signals using coupled differential equations and propose that the long and short lifetimes are associated with fractional particles in the Kitaev paramagnetic regime, Z
2 fluxes and Majorana fermions, respectively.
We introduce a matrix-product state based method to efficiently obtain dynamical response functions for two-dimensional microscopic Hamiltonians. We apply this method to different phases of the Kitaev-Heisenberg model and identify characteristic dynamical features. In the ordered phases proximate to the spin liquid, we find significant broad high-energy features beyond spin-wave theory, which resemble those of the Kitaev model. This establishes the concept of a proximate spin liquid, which was recently invoked in the context of inelastic neutron scattering experiments on α-RuCl_{3}. Our results provide an example of a natural path for proximate spin liquid features to arise at high energies above a conventionally ordered state, as the diffuse remnants of spin-wave bands intersect to yield a broad peak at the Brillouin zone center.
Abstract We present comprehensive inelastic neutron spectroscopic maps of the magnetic field-induced disordered phase of the Kitaev quantum spin liquid candidate material α-RuCl 3 . For fields along both in-plane high symmetry directions we observe that the spin excitation spectrum at and above a magnetic field of 8 T is gapped. Excitation modes then sharpen for increasing field but are consistently broader than experimental resolution even at 13.5 T. The out-of-plane dispersion diminishes in the 7-10 T regime, signifying enhanced two-dimensional behavior as the in-plane liquid correlations are established. In this regime, excitations are very broad and largely flat for all accessible energy-momenta, which is kinematically at odds with a magnon-decay picture. By contrast, a continuum of fractionalized excitations naturally yields a broad continuum response, which crucially may be accompanied by sharper modes of bound states of fractionalized excitations. Their damping by the continuum accounts for the observed spectral broadening and field dependence. Our results provide strong evidence for the existence of fractionalized excitations in α-RuCl 3 in a magnetic field.
Excitation by light pulses enables the manipulation of phases of quantum condensed matter. Here, we photoexcite high-energy holon-doublon pairs as a way to alter the magnetic free energy landscape of the Kitaev-Heisenberg magnet
α-RuCl
3, with the aim to dynamically stabilize a proximate spin liquid phase. The holon-doublon pair recombination through multimagnon emission is tracked through the time-evolution of the magnetic linear dichroism originating from the competing zigzag spin ordered ground state. A small holon-doublon density suffices to reach a spin disordered state. The phase transition is described within a dynamic Ginzburg-Landau framework, corroborating the quasistationary nature of the transient spin disordered phase. Our work provides insight into the coupling between the electronic and magnetic degrees of freedom in
α-RuCl
3 and suggests a new route to reach a proximate spin liquid phase in Kitaev-Heisenberg magnets.
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.
We report on terahertz spectroscopy of quantum spin dynamics in $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{RuCl}}_{3}$, a system proximate to the Kitaev honeycomb model, as a function of temperature and magnetic field. We follow the evolution of an extended magnetic continuum below the structural phase transition at
Ts2=62 K. With the onset of a long-range magnetic order at
TN=6.5 K, spectral weight is transferred to a well-defined magnetic excitation at $\ensuremath{\hbar}{\ensuremath{\omega}}_{1}=2.48\text{ }\text{ }\mathrm{meV}$, which is accompanied by a higher-energy band at $\ensuremath{\hbar}{\ensuremath{\omega}}_{2}=6.48\text{ }\text{ }\mathrm{meV}$. Both excitations soften in a magnetic field, signaling a quantum phase transition close to
Bc=7 T, where a broad continuum dominates the dynamical response. Above
Bc, the long-range order is suppressed, and on top of the continuum, emergent magnetic excitations evolve. These excitations follow clear selection rules and exhibit distinct field dependencies, characterizing the dynamical properties of a possibly field-induced quantum spin liquid.