Field-induced magnon excitation and in-gap absorption in the Kitaev candidate RuCl3

Abstract

We use time-domain terahertz spectroscopy to measure the low energy conductivity and magnons in RuCl 3 under external magnetic field. At zero field, an oscillation with a frequency of 0.62 THz is clearly observed in time-domain spectrum below T N , which is identified as a magnon excitation in the magnetic order state. The magnon excitation is not affected by the external magnetic field H DC when it is applied along the c-axis, but is clearly suppressed when H DC is applied within ab plane. More interestingly, when the magnetic component of THz wave h(t) is perpendicular to the applied in-plane magnetic field, we observe another coherent oscillation at slightly higher energy scale at the field above 2 T, which is eventually suppressed for H DC >5 T. The measurement seems to indicate that the in-plane magnetic field can lift the degeneracy of two branches of low energy magnons at Γ point. The low energy optical conductivity calculated from the measured transmission spectrum is dominated by a broad continuum contribution, which is not affected by changing either temperature or external magnetic field. The continuum is likely to be related to the fractional spin excitation due to dominated Kitaev interaction in the material.

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

“…Consistent with the hypothesis of a magnetic origin of these resonances, we note that mode II was recently observed in independent TDTS [31][32][33]56] and EPR [34] experiments and assigned to a zone-center magnon of the zigzag ordered phase. On the other hand, while signatures of mode I have also been seen in previous measurements [28,31], this resonance has never been discussed.…”

“…1(b)] [23,24]. Nevertheless, spectroscopic probes, including inelastic neutron scattering (INS) [25][26][27][28], spontaneous Raman scattering [29,30], time-domain terahertz spectroscopy (TDTS) [31][32][33], and electron paramagnetic resonance (EPR) [34], have discovered signatures of a field-induced QSL state above 7.5 T in the form of a broad continuum at the 2D magnetic Brillouin zone center. Yet a complete understanding of the origin of these excitations as well as of the spin dynamics is still lacking.…”

Magnetic anisotropy of the alkali iridate Na2IrO3 at high magnetic fields: Evidence for strong ferromagnetic Kitaev correlations

“…Intriguingly, a similar feature in the anisotropic magnetisation has also been observed in α−RuCl 3 , but not explained [36,41]. Here we show the likely universality of such a feature in the magnetic torque, as a signature of the field-induced spin liquid (also revealed in the Kitaev system α−RuCl 3 at lower energy scales [46][47][48]), thus shedding light on the relevance of Kitaev materials for realising a quantum spin liquid ground state. Na 2 IrO 3 is a layered Mott insulator with an energy gap E g = 340 meV [19] and spin-orbit coupling λ ≈ 0.5 eV [6].…”

Field evolution of low-energy excitations in the hyperhoneycomb magnet β−Li2IrO3

“…The pure Kitaev model features a spin-liquid ground state characterized by fractionalized excitations [2], but the majority of materials reported to date are magnetically ordered in zero field, because additional interactions beyond the Kitaev term are present [3]. Nevertheless, at least one of these materials, α-RuCl 3 , reveals not only magnon excitations at low energies [4][5][6][7], but also a peculiar excitation continuum at higher energies [8][9][10][11]. This continuum is sometimes interpreted as a vestige of spin-liquid physics of the pure Kitaev model [12], although a more mundane explanation in terms of magnon breakdown caused by anisotropic terms in the spin Hamiltonian appears equally plausible and even better justified microscopically [13,14].…”