In the pursuit of developing routes to enhance magnetic Kitaev interactions in α-RuCl3, as well as probing doping effects, we investigate the electronic properties of α-RuCl3 in proximity to graphene. We study α-RuCl3/graphene heterostructures via ab initio density functional theory calculations, Wannier projection and non-perturbative exact diagonalization methods. We show that α-RuCl3 becomes strained when placed on graphene and charge transfer occurs between the two layers, making α-RuCl3 (graphene) lightly electron-doped (hole-doped). This gives rise to an insulator to metal transition in α-RuCl3 with the Fermi energy located close to the bottom of the upper Hubbard band of the t2g manifold. These results suggest the possibility of realizing metallic and even exotic superconducting states. Moreover, we show that in the strained α-RuCl3 monolayer the Kitaev interactions are enhanced by more than 50% compared to the unstrained bulk structure. Finally, we discuss scenarios related to transport experiments in α-RuCl3/graphene heterostructures. arXiv:1908.04793v2 [cond-mat.str-el]

The honeycomb Mott insulator α-RuCl3 loses its low-temperature magnetic order by pressure. We report clear evidence for a dimerized structure at P > 1 GPa and observe the breakdown of the relativistic j eff picture in this regime strongly affecting the electronic properties. A pressure-induced Kitaev quantum spin liquid cannot occur in this broken symmetry state. We shed light on the new phase by broad-band infrared spectroscopy of the low-temperature properties of α-RuCl3 and ab initio density functional theory calculations, both under hydrostatic pressure.

No abstract

No abstract

STEM study of Cu kagome lattice in claringbullite.

The experimental study of the CO2 phase diagram is hampered by strong kinetic effects leading to wide regions of metastability and to large uncertainties in the location of some phase boundaries.Here we determine CO2's thermodynamic phase boundaries by means of ab initio calculations of the Gibbs free energy of several solid phases of CO2 up to 50 Gigapascals. Temperature effects are included in the quasi-harmonic approximation. Contrary to previous suggestions, we find that the boundary between molecular forms and the non-molecular phase V has indeed a positive slope and starts at 21.5 GPa at T = 0 K. A triple point between phase IV, V, and the liquid phase is found at 35 GPa and 1600 K, indicating a broader region of stability for the non-molecular form, than previously thought. The experimentally determined boundary line between CO2-II and CO2-IV phases is reproduced by our calculations, indicating that kinetic effects do not play a major role in that particular transition. Our results also show that CO2-III is stabilized at high temperature and its stability region coincides with the P − T conditions where phase VII has been reported experimentally; instead, phase II is the most stable molecular phase at low temperatures, extending its region of stability to every P − T condition where phase III is reported experimentally.

We present first-principles results on the magnetoelastic coupling in α-RuCl3 and uncover a striking dependence of the magnetic coupling constants on strain effects. Different magnetic interactions are found to respond very unequally to variations in the lattice, with the Kitaev interaction being the most sensitive. Exact diagonalization results on our magnetoelastic model reproduce recent measurements of the structural Grüneisen parameter and explain the origin of the negative magnetostriction of α-RuCl3, disentangling contributions related to different anisotropic interactions and g factors. Uniaxial strain perpendicular to the honeycomb planes is predicted to reorganize the relative coupling strengths, strongly enhancing the Kitaev interaction while simultaneously weakening the other anisotropic exchanges under compression. Uniaxial strain may therefore pose a fruitful route to experimentally tune α-RuCl3 nearer to the Kitaev limit.

We use powder x-ray diffraction to study the effect of pressure on the crystal structure of the honeycomb rhodate Li2RhO3. We observe low-pressure (P Pc2 = 14 GPa) regions corresponding to the monoclinic C2/m symmetry, while a phase mixture is observed at intermediate pressures. At P >Pc2, the honeycomb structure becomes distorted and features short Rh-Rh bonds forming zigzag chains stretched along the crystallographic a direction. This is in contrast to dimerized patterns observed in triclinic high-pressure polymorphs of α-Li2IrO3 and α-RuCl3. Density-functional theory calculations at various pressure conditions reveal that the observed rhodium zigzag-chain pattern is not expected under hydrostatic pressure but can be reproduced by assuming anisotropic pressure conditions.