Abstract-Sub-gap optical response across the structural phase transition in van der Waals layered α-RuCl3

S. Reschke, F. Mayr, S. Widmann, H.-A. Krug von Nidda, V. Tsurkan, M. V. Eremin, Seung-Hwan Do, K.-Y. Choi, Zhe Wang, A. Loidl

https://arxiv.org/abs/1803.04887

We report on magnetic, thermodynamic, thermal expansion, and optical experiments on the layered compound \alpha-RuCl3 focusing on the sub-gap optical response across the structural phase transition from the monoclinic high-temperature to the rhombohedral low-temperature structure, where mainly the stacking sequence of the molecular layers is changed. The temperature dependence of the complex dielectric response, including symmetry changes via the phase transition should allow the identification of the microscopic origin of the recorded phonon and spin-orbital excitations. In addition, this type of phase transition seems to be characteristic for a variety of tri-halides crystallizing in a layered honeycomb-type structure and so far is unique, as the low-temperature phase has the higher symmetry. We document a number of highly unusual findings: A characteristic two-step hysteresis of the structural phase transition, accompanied by a dramatic change of the reflectivity. An electronic excitation, which only appears in a narrow temperature range just across the structural phase transition, and a complex dielectric loss spectrum in the THz regime, which could indicate some remnants of Kitaev physics. Despite the significant structural effects across the monoclinic to rhombohedral phase transition, phonon eigenfrequencies and the majority of spin-orbital excitations are not strongly influenced. Obviously, symmetry and binding forces of the single molecular layers only determine the eigenfrequencies of most of these excitations. Finally, from a detailed and combined terahertz, far- and mid-infrared study we try to shed some light on the so far unsolved low energy (< 1eV) electronic structure of the ruthenium 4d5 electrons in \alpha-RuCl3.

Abstract-Excitations and relaxation dynamics in multiferroic GeV 4 S 8 studied by terahertz and dielectric spectroscopy

S. Reschke, Zhe Wang, F. Mayr, E. Ruff, P. Lunkenheimer, V. Tsurkan, and A. Loidl

https://journals.aps.org/prb/abstract/10.1103/PhysRevB.96.144418

We report on THz time-domain spectroscopy on multiferroic ${\mathrm{GeV}}_4{S}_8$, which undergoes orbital ordering at a Jahn-Teller transition at 30.5 K and exhibits antiferromagnetic order below 14.6 K. The THz experiments are complemented by dielectric experiments at audio and radio frequencies. We identify a low-lying excitation close to 0.5 THz, which is only weakly temperature dependent and probably corresponds to a molecular excitation within the electronic level scheme of the $V_4$ clusters. In addition, we detect complex temperature-dependent behavior of a low-lying phononic excitation, closely linked to the onset of orbitally driven ferroelectricity. In the high-temperature cubic phase, which is paramagnetic and orbitally disordered, this excitation is of relaxational character becomes an overdamped Lorentzian mode in the orbitally ordered phase below the Jahn-Teller transition, and finally appears as well-defined phonon excitation in the antiferromagnetic state. Abrupt changes in the real and imaginary parts of the complex dielectric permittivity show that orbital ordering appears via a structural phase transition with strong first-order character and that the onset of antiferromagnetic order is accompanied by significant structural changes, which are of first-order character, too. Dielectric spectroscopy documents that at low frequencies, significant dipolar relaxations are present in the orbitally ordered, paramagnetic phase only. In contrast to the closely related ${\mathrm{GaV}}_4{S}_8$, this relaxation dynamics that most likely mirrors coupled orbital and polar fluctuations does not seem to be related to the dynamic processes detected in the THz regime.

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Abstract-Electronic and phonon excitations in α − RuC l 3

S. Reschke, F. Mayr, Zhe Wang, Seung-Hwan Do, K.-Y. Choi, and A. Loi

https://journals.aps.org/prb/abstract/10.1103/PhysRevB.96.165120

We report on terahertz (THz), infrared reflectivity, and transmission experiments for wavenumbers from 10 to $8000c{m}^{-1}$ ($\sim 1\mathrm{meV}-1\mathrm{eV}$) and for temperatures from 5 to 295 K on the Kitaev candidate material $\alpha -\mathrm{RuC}{l}_3$. 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 to 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 on the order of 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.

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Abstract-Spin-orbiton and quantum criticality in FeSc2S4

L. Mittelstädt, M. Schmidt, Zhe Wang, F. Mayr, V. Tsurkan, P. Lunkenheimer, D. Ish, L. Balents, J. Deisenhofer, and A. Loidl

http://journals.aps.org/prb/abstract/10.1103/PhysRevB.91.125112

In FeSc2S4 spin-orbital exchange competes with strong spin-orbit coupling, suppressing long-range spin and orbital order and, hence, this material represents one of the rare examples of a spin-orbital liquid ground state. Moreover, it is close to a quantum-critical point separating the ordered and disordered regimes. Using terahertz and far-infrared spectroscopy we study low-lying excitations in FeSc2S4 and provide clear evidence for a spin-orbiton, an excitation of strongly entangled spins and orbitals. It becomes particularly well pronounced upon cooling, when advancing deep into the quantum-critical regime. Moreover, indications of an underlying structureless excitation continuum are found, a possible signature of quantum criticality.

DOI: http://dx.doi.org/10.1103/PhysRevB.91.125112

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