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Register a new userNeutron tomography of magnetic Majorana fermions in a proximate quantum spin liquid
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Quantum matter provides an effective vacuum out of which arise emergent particles not corresponding to any experimentally detected elementary particle. Topological quantum materials in particular have become a focus of intense research in part because of the remarkable possibility to realize Majorana fermions, with their potential for new, decoherence-free quantum computing architectures. In this paper we undertake a study on high-quality single crystal of $alpha-RuCl_3$ which has been identified as a material realizing a proximate Kitaev state, a topological quantum state with magnetic Majorana fermions. Four-dimensional tomographic reconstruction of dynamical correlations measured using neutrons is uniquely powerful for probing such magnetic states. We discover unusual signals, including an unprecedented column of scattering over a large energy interval around the Brillouin zone center which is remarkably stable with temperature. This is straightforwardly accounted for in terms of the Majorana excitations present in Kitaevs topological quantum spin liquid. Other, more delicate, features in the scattering can be transparently associated with perturbations to an ideal model. This opens a window on emergent magnetic Majorana fermions in correlated materials.
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A triangular lattice selenide series of rare earth (RE), CsRESe2, were synthesized as large single crystals using a flux growth method. This series stabilized in either trigonal (R-3m) or hexagonal (P63/mmc) crystal systems. Physical properties of CsRESe2 were explored by magnetic susceptibility and heat capacity measurements down to 0.4 K. Antiferromagnetic interaction was observed in all magnetic compounds, while no long-range magnetic order was found, indicating the frustrated magnetism. CsDySe2 presents spin freezing at 0.7 K, revealing a spin-glass state. CsCeSe2 and CsYbSe2 present broad peaks at 0.7 K and 1.5 K in the magnetization, respectively, suggesting the short-range interactions between magnetic rare earth ions. The lack of signature for long-range magnetic order and spin freezing down to 0.4 K in these compounds (RE = Ce, Yb) implies their candidacy for quantum spin liquid state.
The quasi two-dimensional Mott insulator $alpha$-RuCl$_3$ is proximate to the sought-after Kitaev quantum spin liquid (QSL). In a layer of $alpha$-RuCl$_3$ on graphene the dominant Kitaev exchange is further enhanced by strain. Recently, quantum oscillation (QO) measurements of such $alpha$-RuCl$_3$ / graphene heterostructures showed an anomalous temperature dependence beyond the standard Lifshitz-Kosevich (LK) description. Here, we develop a theory of anomalous QO in an effective Kitaev-Kondo lattice model in which the itinerant electrons of the graphene layer interact with the correlated magnetic layer via spin interactions. At low temperatures a heavy Fermi liquid emerges such that the neutral Majorana fermion excitations of the Kitaev QSL acquire charge by hybridising with the graphene Dirac band. Using ab-initio calculations to determine the parameters of our low energy model we provide a microscopic theory of anomalous QOs with a non-LK temperature dependence consistent with our measurements. We show how remnants of fractionalized spin excitations can give rise to characteristic signatures in QO experiments.
The Kitaev model on a honeycomb lattice predicts a paradigmatic quantum spin liquid (QSL) exhibiting Majorana Fermion excitations. The insight that Kitaev physics might be realized in practice has stimulated investigations of candidate materials, recently including alpha-RuCl3. In all the systems studied to date, non-Kitaev interactions induce magnetic order at low temperature. However, in-plane magnetic fields of roughly 8 Tesla suppress the long-range magnetic order in alpha-RuCl3 raising the intriguing possibility of a field-induced QSL exhibiting non-Abelian quasiparticle excitations. Here we present inelastic neutron scattering in alpha-RuCl3 in an applied magnetic field. At a field of 8 Tesla, the spin waves characteristic of the ordered state vanish throughout the Brillouin zone. The remaining single dominant feature of the response is a broad continuum centered at the Gamma point, previously identified as a signature of fractionalized excitations. This provides compelling evidence that a field-induced QSL state has been achieved.
The propagation of edge localized spin waves (E-SWs) in yttrium iron garnet (YIG) microstripes with/without the proximate magnetic microstructures is investigated by micromagnetic simulations. A splitting of the dispersion curve with the presence of permalloy (Py) stripe is also observed. The E-SWs on the two edges of YIG stripe have different wavelengths, group velocities, and decay lengths at the same frequencies. The role of the Py stripe was found to be the source of the inhomogeneous static dipolar field without dynamic coupling with YIG. This work opens new perspectives for the design of innovative SW interference-based logic devices.
The gapless Bogoliubov-de Gennes (BdG) quasiparticles of a clean three dimensional spinless $p_x+ip_y$ superconductor provide an intriguing example of a thermal Hall semimetal (ThSM) phase of Majorana-Weyl fermions in class D of the Altland-Zirnbauer symmetry classification; such a phase can support a large anomalous thermal Hall conductivity and protected surface Majorana-Fermi arcs at zero energy. We study the effect of quenched disorder on such a topological phase with both numerical and analytical methods. Using the kernel polynomial method, we compute the average and typical density of states for the BdG quasiparticles; based on this, we construct the disordered phase diagram. We show for infinitesimal disorder, the ThSM is converted into a diffusive thermal Hall metal (ThDM) due to rare statistical fluctuations. Consequently, the phase diagram of the disordered model only consists of ThDM and thermal insulating phases. Nonetheless, there is a cross-over at finite energies from a ThSM regime to a ThDM regime, and we establish the scaling properties of the avoided quantum critical point which marks this cross-over. Additionally, we show the existence of two types of thermal insulators: (i) a trivial thermal band insulator (ThBI) [or BEC phase] supporting only exponentially localized Lifshitz states (at low energy), and (ii) a thermal Anderson insulator (AI) at large disorder strengths. We determine the nature of the two distinct localization transitions between these two types of insulators and ThDM.We also discuss the experimental relevance of our results for three dimensional, time reversal symmetry breaking, triplet superconducting states.
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