Giant phonon anomalies in the proximate Kitaev quantum spin liquid α-RuCl3

Haoxiang Li, Tiantian Zhang, Ayman Said +10 more

祥大 末次, S. Suetsugu, 柚希 鵜飼 +20 more

Steinn Ýmir Ágústsson, Vladimir Grigorev, Jure Demsar +2 more

A. Banerjee, Jian Yan, J. Knolle +7 more

The Kitaev quantum spin liquid (KQSL) is an exotic emergent state of matter exhibiting Majorana fermion and gauge flux excitations. The magnetic insulator alpha-RuCl3 is thought to realize a proximate KQSL. We used neutron scattering on single crystals of alpha-RuCl3 to reconstruct dynamical correlations in energy-momentum space. We discovered highly unusual signals, including a column of scattering over a large energy interval around the Brillouin zone center, which is very stable with temperature. This finding is consistent with scattering from the Majorana excitations of a KQSL. Other, more delicate experimental features can be transparently associated with perturbations to an ideal model. Our results encourage further study of this prototypical material and may open a window into investigating emergent magnetic Majorana fermions in correlated materials.

Kejing Ran, Jinghui Wang, Song Bao +12 more

It is known that α-RuCl3 has been studied extensively because of its proximity to the Kitaev quantum-spin-liquid (QSL) phase and the possibility of approaching it by tuning the competing interactions. Here we present the first polarized inelastic neutron scattering study on α-RuCl3 single crystals to explore the scattering continuum around the Γ point at the Brillouin zone center, which was hypothesized to be resulting from the Kitaev QSL state but without concrete evidence. With polarization analyses, we find that, while the spin-wave excitations around the M point vanish above the transition temperature T N, the pure magnetic continuous excitations around the Γ point are robust against temperature. Furthermore, by calculating the dynamical spin-spin correlation function using the cluster perturbation theory, we derive magnetic dispersion spectra based on the K–Γ model, which involves with a ferromagnetic Kitaev interaction of –7.2 meV and an off-diagonal interaction of 5.6 meV. We find this model can reproduce not only the spin-wave excitation spectra around the M point, but also the non-spin-wave continuous magnetic excitations around the Γ point. These results provide evidence for the existence of fractional excitations around the Γ point originating from the Kitaev QSL state, and further support the validity of the K–Γ model as the effective minimal spin model to describe α-RuCl3.

Sebastian Bachus, David A. S. Kaib, Y. Tokiwa +7 more

Measurements of the magnetic Gr\"uneisen parameter (${\mathrm{\ensuremath{\Gamma}}}_{B}$) and specific heat on the Kitaev material candidate $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{RuCl}}_{3}$ are used to access in-plane field and temperature dependence of the entropy up to 12 T and down to 1 K. No signatures corresponding to phase transitions are detected beyond the boundary of the magnetically ordered region, but only a shoulderlike anomaly in ${\mathrm{\ensuremath{\Gamma}}}_{B}$, involving an entropy increment as small as ${10}^{\ensuremath{-}5}R\mathrm{log}2$. These observations put into question the presence of a phase transition between the purported quantum spin liquid and the field-polarized state of $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{RuCl}}_{3}$. We show theoretically that at low temperatures ${\mathrm{\ensuremath{\Gamma}}}_{B}$ is sensitive to crossings in the lowest excitations within gapped phases, and identify the measured shoulderlike anomaly as being of such origin. Exact diagonalization calculations demonstrate that the shoulderlike anomaly can be reproduced in extended Kitaev models that gain proximity to an additional phase at finite field without entering it. We discuss manifestations of this proximity in other measurements.