We present initial Raman spectroscopy experiments on exfoliated flakes of α-RuCl 3 , from tens of nm thick down to single layers. Besides unexpectedly finding this material to be air stable, in the thinnest layers we observe the appearance with decreasing temperature of a symmetry-forbidden mode in crossed polarization, along with an anomalous broadening of a mode at 164 cm −1 that is known to couple to a continuum of magnetic excitations. This may be due to an enhancement of magnetic fluctuations and evidence for a distorted honeycomb lattice in single-and bi-layer samples. arXiv:1709.00431v1 [cond-mat.str-el] 1 Sep 2017 the spin couple along different bonds (see Figure 1a). This model can be realized in materials under the right conditions of crystal electric field, spinorbit coupling and on-site Coulomb repulsion that produce an insulator with J ef f = 1/2 moments. In systems where the honeycomb lattice is formed by placing the magnetic atom inside edge-sharing octahedra, one can realize the necessary bond-dependent exchange due to the impact of strong spin-orbit coupling on the hopping (see Figure 1) [3,4,5]. A key difficulty with this proposal is that additional interaction terms may arise and produce long range order [6,7,8,9]. While some of these terms are enabled simply by symmetry, they are strongly enhanced by lattice distortions that mix the J ef f = 1/2 and J ef f = 3/2 states, altering the hopping terms. Recently, α-RuCl 3 has emerged as a potential candidate to realize a Kitaev quantum spin liquid state [10,11,2,12,13,14,15,16,17,18,19,20].IR, Raman and photo-emission spectroscopy combined with DFT calculations strongly suggest the system is close to the J ef f = 1/2 limit, with octahedra that are nearly undistorted at low temperatures. Perhaps due to the smaller spin-orbit coupling expected in a 4d system, α-RuCl 3 reveals an extremely narrow spin-orbit exciton (2 meV wide) well separated from charge excitations [14]. Thus the low-energy model of α-RuCl 3 does not contain any charge fluctuations, unlike the 5d Ir systems where the spin-orbit and onsite d−d excitations are overlapped in energy [9]. Perhaps most promising is the observed continuum of magnetic excitations, where the Raman temperature dependence and the excitation dispersion seen by neutrons is consistent with fractional particles expected from the pure Kitaev model [2,11,21,20].Despite its importance to the formation of an ordered state, the structure of α-RuCl 3 remains controversial. In particular, the exact structure appears to be sensitive to atomic disorder and stacking faults, which are not uncommon in van der Waals crystals such as α-RuCl 3 [15,22,23,21]. Surprisingly, the addition of stacking faults leads to an enhanced onset of antiferromagnetic order (higher T N ) [15]. This rather counterintuitive observation may result from additional tunneling pathways opened by the stacking disorder that boost the Heisenberg terms. If correct, this suggests that exfoliating α-RuCl 3 down to single layers could suppress the long range ord...

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We study the infrared cyclotron resonance of high-mobility monolayer graphene encapsulated in hexagonal boron nitride, and simultaneously observe several narrow resonance lines due to interband Landau-level transitions. By holding the magnetic field strength B constant while tuning the carrier density n, we find the transition energies show a pronounced nonmonotonic dependence on the Landau-level filling factor, ν∝n/B. This constitutes direct evidence that electron-electron interactions contribute to the Landau-level transition energies in graphene, beyond the single-particle picture. Additionally, a splitting occurs in transitions to or from the lowest Landau level, which is interpreted as a Dirac mass arising from coupling of the graphene and boron nitride lattices.

Quantum reactive scattering calculations on the vibrational quenching of HD due to collisions with H were carried out employing an accurate potential energy surface. The state-to-state cross sections for the chemical reaction HD­(v = 1, j = 0) + H → D + H2(v′ = 0, j′) at collision energies between 1 and 10 000 cm–1 are presented, and a Feshbach resonance in the low-energy regime, below the reaction barrier, is observed for the first time. The resonance is attributed to coupling with the vibrationally adiabatic potential correlating to the v = 1, j = 1 level of the HD molecule, and it is dominated by the contribution from a single partial wave. The properties of the resonance, such as its dynamic behavior, phase behavior, and lifetime, are discussed.