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Register a new userDimerization in honeycomb Na2RuO3 under pressure: a DFT study
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The structural properties of Na2RuO3 under pressure are studied using density functional theory within the generalized gradient approximation (GGA). We found that one may expect a structural transition at ~ 3 GPa. This structure at the high-pressure phase is exactly the same as the low-temperature structure of Li2RuO3 (at ambient pressure) and is characterized by the P21/m space group. Ru ions form dimers in this phase and one may expect strong modification of the electronic and magnetic properties in Na2RuO3 at pressure higher than 3 GPa.
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Single-crystal x-ray diffraction studies with synchrotron radiation on the honeycomb iridate $alpha$-Li$_{2}$IrO$_{3}$ reveal a pressure-induced structural phase transition with symmetry lowering from monoclinic to triclinic at a critical pressure of $P_{c}$ = 3.8 GPa. According to the evolution of the lattice parameters with pressure, the transition mainly affects the $ab$ plane and thereby the Ir hexagon network, leading to the formation of Ir--Ir dimers. These observations are independently predicted and corroborated by our textit{ab initio} density functional theory calculations where we find that the appearance of Ir--Ir dimers at finite pressure is a consequence of a subtle interplay between magnetism, correlation, spin-orbit coupling, and covalent bonding. Our results further suggest that at $P_{c}$ the system undergoes a magnetic collapse. Finally we provide a general picture of competing interactions for the honeycomb lattices $A_{2}$$M$O$_{3}$ with $A$= Li, Na and $M$ = Ir, Ru.
We report high-pressure x-ray diffraction and magnetization measurements combined with ab-initio calculations to demonstrate that the high-pressure optical and transport transitions recently reported in TiOCl, correspond in fact to an enhanced Ti3+-Ti3+ dimerization existing already at room temperature. Our results confirm the formation of a metal-metal bond between Ti3+ ions along the b-axis of TiOCl, accompanied by a strong reduction of the electronic gap. The evolution of the dimerization with pressure suggests a crossover from the spin-Peierls to a conventional Peierls situation at high pressures.
We studied the effect of external pressure on the electrodynamic properties of $alpha$-Li$_2$IrO$_3$ single crystals in the frequency range of the phonon modes and the Ir $d$-$d$ transitions. The abrupt hardening of several phonon modes under pressure supports the onset of the dimerized phase at the critical pressure $P_c$=3.8 GPa. With increasing pressure an overall decrease in spectral weight of the Ir $d$-$d$ transitions is found up to $P_c$. Above $P_c$, the local (on-site) $d$-$d$ excitations gain spectral weight with increasing pressure, which hints at a pressure-induced increase in the octahedral distortions. The non-local (intersite) Ir $d$-$d$ transitions show a monotonic blue-shift and decrease in spectral weight. The changes observed for the non-local excitations are most prominent well above $P_c$, namely for pressures $geq$12 GPa, and only small changes occur for pressures close to $P_c$. The profile of the optical conductivity at high pressures ($sim$20 GPa) appears to be indicative for the dimerized state in iridates.
The honeycomb antiferromagnet under a triaxial strain is studied using the quantum Monte Carlo simulation. The strain dimerizes the exchange couplings near the corners, thus destructs the antiferromagnetic order therein. The antiferromagnetic region is continuously reduced by the strain. For the same strain strength, the exact numerical results give a much smaller antiferromagnetic region than the linear spin-wave theory. We then study the strained $XY$ antiferromagnet, where the magnon pseudo-magnetic field behaves quite differently. The $0$th Landau level appears in the middle of the spectrum, and the quantized energies above (below) it are proportional to $n^{frac{1}{3}} (n^{frac{2}{3}})$, which is in great contrast to the equally-spaced ones in the Heisenberg case. Besides, we find the antiferromagnetic order of the $XY$ model is much more robust to the dimerization than the Heisenberg one. The local susceptibility of the Heisenberg case is extracted by the numerical analytical continuation, and no sign of the pseudo-Landau levels is resolved. It is still not sure whether the result is due to the intrinsic problem of the numerical analytical continuation. Thus the existence of the magnon pseudo-Landau levels in the spin-$frac{1}{2}$ strained Heisenberg Hamiltonian remains an open question. Our results are closely related to the two-dimensional van der Waals quantum antiferromagnets and may be realized experimentally.
We report $^{115}$In nuclear-quadrupole-resonance (NQR) measurements of the pressure($P$)-induced superconductor CeRhIn$_5$ in the antiferromagnetic (AF) and superconducting (SC) states. In the AF region, the internal field $H_{int}$ at the In site is substantially reduced from $H_{int}=1.75$ kOe at P=0 to 0.39 kOe at $P=1.23$ GPa, while the Neel temperature slightly changes with increasing $P$. This suggests that either the size in the ordered moment $M_{Q}(P)$ or the angle $theta (P)$ between the direction of $M_{Q}(P)$ and the tetragonal $c$ axis is extrapolated to zero at $P^*=1.6 pm 0.1$ GPa at which a bulk SC transition is no longer emergent. In the SC state at $P=2.1$ GPa, the nuclear spin-lattice relaxation rate $^{115}(1/T_1)$ has revealed a $T^3$ dependence without the coherence peak just below $T_c$, giving evidence for the unconventional superconductivity. The dimensionality of the magnetic flutuations in the normal state are also discussed.
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