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Register a new userAntiferromagnetic resonance and terahertz continuum in $alpha-$RuCl$_3$
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We report measurements of optical absorption in the zig-zag antiferromagnet $alpha$-RuCl$_3$ as a function of temperature, $T$, magnetic field, $B$, and photon energy, $hbaromega$ in the range $sim$ 0.3 to 8.3 meV, using time-domain terahertz spectroscopy. Polarized measurements show that 3-fold rotational symmetry is broken in the honeycomb plane from 2 K to 300 K. We find a sharp absorption peak at 2.56 meV upon cooling below the Neel temperature of 7 K at $B=0$ that we identify as magnetic-dipole excitation of a zero-wavevector magnon, or antiferromagnetic resonance (AFMR). With application of $B$, the AFMR broadens and shifts to lower frequency as long-range magnetic order is lost in a manner consistent with transitioning to a spin-disordered phase. From direct, internally calibrated measurement of the AFMR spectral weight, we place an upper bound on the contribution to the $dc$ susceptibility from a magnetic excitation continuum.
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The combination of electronic correlation and spin-orbit coupling is thought to precipitate a variety of highly unusual electronic phases in solids, including topological and quantum spin liquid states. We report a Raman scattering study that provides evidence for unconventional excitations in $alpha$-RuCl$_3$, a spin-orbit coupled Mott insulator on the honeycomb lattice. In particular, our measurements reveal unusual magnetic scattering, typified by a broad continuum. The temperature dependence of this continuum is evident over a large scale compared to the magnetic ordering temperature, suggestive of frustrated magnetic interactions. This is confirmed through an analysis of the phonon linewidths, which show a related anomaly due to spin-phonon coupling. These observations are in line with theoretical expectations for the Heisenberg-Kitaev model and suggest that $alpha$-RuCl$_3$ may be close to a quantum spin liquid ground state.
We report on THz, infrared reflectivity and transmission experiments for wave numbers from 10 to 8000 cm$^{-1}$ ($sim$ 1 meV - 1 eV) and for temperatures from 5 to 295 K on the Kitaev candidate material {alpha}-RuCl$_3$. As reported earlier, the compound under investigation passes through a first-order structural phase transition, from a monoclinic high-temperature to a rhombohedral low-temperature phase. The phase transition shows an extreme and unusual hysteretic behavior, which extends from 60 to 166 K. In passing this phase transition, in the complete frequency range investigated we found a significant reflectance change, which amounts almost a factor of two. We provide a broadband spectrum of dielectric constant, dielectric loss and optical conductivity from the THz to the mid infrared regime and study in detail the phonon response and the low-lying electronic density of states. We provide evidence for the onset of an optical energy gap, which is of order 200 meV, in good agreement with the gap derived from measurements of the DC electrical resistivity. Remarkably, the onset of the gap exhibits a strong blue shift on increasing temperatures.
The Kitaev quantum spin liquid (KSL) is a theoretically predicted state of matter whose fractionalized quasiparticles are distinct from bosonic magnons, the fundamental excitation in ordered magnets. The layered honeycomb antiferromagnet $alpha$-RuCl$_3$ is a KSL candidate material, as it can be driven to a magnetically disordered phase by application of an in-plane magnetic field, with $H_c sim 7$ T. Here we report a detailed characterization of the magnetic excitation spectrum of this material by high-resolution time-domain terahertz (THz) spectroscopy. We observe two sharp magnon resonances whose frequencies and amplitudes exhibit a discontinuity as a function of applied magnetic field, as well as two broader peaks at higher energy. Below the Neel temperature, we find that linear spin wave theory can account for all of these essential features of the spectra when a $C_3$-breaking distortion of the honeycomb lattice and the presence of structural domains are taken into account.
The layered honeycomb magnet alpha-RuCl3 has been proposed as a candidate to realize a Kitaev spin model with strongly frustrated, bond-dependent, anisotropic interactions between spin-orbit entangled jeff=1/2 Ru4+ magnetic moments. Here we report a detailed study of the three-dimensional crystal structure using x-ray diffraction on untwinned crystals combined with structural relaxation calculations. We consider several models for the stacking of honeycomb layers and find evidence for a crystal structure with a monoclinic unit cell corresponding to a stacking of layers with a unidirectional in-plane offset, with occasional in-plane sliding stacking faults, in contrast with the currently-assumed trigonal 3-layer stacking periodicity. We report electronic band structure calculations for the monoclinic structure, which find support for the applicability of the jeff=1/2 picture once spin orbit coupling and electron correlations are included. We propose that differences in the magnitude of anisotropic exchange along symmetry inequivalent bonds in the monoclinic cell could provide a natural mechanism to explain the spin gap observed in powder inelastic neutron scattering, in contrast to spin models based on the three-fold symmetric trigonal structure, which predict a gapless spectrum within linear spin wave theory. Our susceptibility measurements on both powders and stacked crystals, as well as neutron powder diffraction show a single magnetic transition at TN ~ 13K. The analysis of the neutron data provides evidence for zigzag magnetic order in the honeycomb layers with an antiferromagnetic stacking between layers. Magnetization measurements on stacked single crystals in pulsed field up to 60T show a single transition around 8T for in-plane fields followed by a gradual, asymptotic approach to magnetization saturation, as characteristic of strongly anisotropic exchange interactions.
Thermodynamics of the Kitaev honeycomb magnet $alpha$-RuCl$_3$ is studied for different directions of in-plane magnetic field using measurements of the magnetic Gruneisen parameter $Gamma_B$ and specific heat $C$. We identify two critical fields $B_c^{rm AF1}$ and $B_c^{rm AF2}$ corresponding, respectively, to a transition between two magnetically ordered states and the loss of magnetic order toward a quantum paramagnetic state. The $B_c^{AF2}$ phase boundary reveals a narrow region of magnetic fields where inverse melting of the ordered phase may occur. No additional transitions are detected above $B_c^{rm AF2}$ for any direction of the in-plane field, although a shoulder anomaly in $Gamma_B$ is observed systematically at $8-10$ T. Large field-induced entropy effects imply additional low-energy excitations at low fields and/or strongly field-dependent phonon entropies. Our results establish universal features of $alpha$-RuCl$_3$ in high magnetic fields and challenge the presence of a field-induced Kitaev spin liquid in this material.
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