Possible structural transformation and enhanced magnetic fluctuations in exfoliated α-RuCl3

Abstract

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...

Polarization-resolved Raman spectroscopy of α−RuCl3 and evidence of room-temperature two-dimensional magnetic scattering

“…By comparing our result to other Raman studies 10,16,33,34 , we noticed that the pairs of phonons A 1 g and B 1 g , A 2 g and B 2 g , A 4 g and B 4 g , A 5 g and B 5 g were labeled as E g modes. Factor group analysis reveals that the doubly degenerate E g phonons of D 3d , the often assumed symmetry of α-RuCl 3 crystals, would split into the A g and B g modes of C 2h , the point group symmetry of bulk α-RuCl 3 24,25 .…”

“…Because of the exact analytical solution of its ground state and excitations, the Kitaev model 2 is one of the most studied theoretical models of a QSL. Recent experimental and theoretical studies have converged onto a few real systems that could host this model [7][8][9][10][11][12][13][14][15][16][17] , including α-RuCl 3 and the iridate family of A 2 IrO 3 . In these systems, strong spin-orbit coupling, which leads to J ef f = 1/2, and the honeycomb lattice of the edge-sharing IrO 6 and RuCl 6 octahedra give rise to effective bond-dependent exchange interaction of the Kitaev model 18,19 .…”

Magnetic field-dependent low-energy magnon dynamics in α–RuCl3

“…First, we consider phonons. The first-order transition from a monoclinic to a rhombohedral structure that takes place in the temperature range from 60 to 150 K in α-RuCl 3 [44,50,53] has been interpreted as evidence of a magnetoelastic coupling scheme and a natural explanation for the observed phonon anomalies in this material [53,54]. This raises the question of whether a similar mechanism could explain the unconventional temperature response of mode I, invoking a phonon picture for the ob- served resonances.…”

The range of non-Kitaev terms and fractional particles in α-RuCl3

“…The absence of hysteresis effects was confirmed by taking numerous spectra at the same temperature after different thermal cycles (100 K in the middle of the hysteresis region). In addition, recent studies of the Raman spectra of RuCl 3 suggest an effect of the surface structure upon exposure to air 49,69 . To minimize this, crystals were freshly cleaved and immediately placed in vacuum within three minutes.…”

“…Raman scattering is a powerful probe of magnetic materials, revealing the presence of long range order, symmetry and statistics of the excitations, as well as the strength and nature of the exchange, even in single 2D atomic layers 3,[46][47][48][49][50][51][52][53] . Indeed, Raman scattering was the first to reveal the continuum from magnetic excitations in α-RuCl 3 11 .…”