Pressure-tuning of α\alpha-RuCl3_3 towards the ideal Kitaev-limit

We report the discovery of an intriguing pressure-driven phase transformation in the layered Kitaev-material $\alpha$-RuCl$_3$. By analyzing both the Bragg scattering as well as the diffuse scattering of high-quality single crystals, we reveal a collective reorganization of the layer stacking throughout the crystal. Importantly, this transformation also effects the structure of the RuCl$_3$ honeycomb layers, which acquire a high trigonal symmetry with a single Ru--Ru distance of 3.41\r{A} and a single Ru--Cl--Ru bond angle of 92.8{\deg}. Hydrostatic pressure therefore allows to tune the structure of $\alpha$-RuCl$_3$ much closer to the ideal Kitaev-limit. The high-symmetry phase can also be stabilized by biaxial stress, which can explain conflicting results reported earlier and, more importantly, makes the high-symmetry phase accessible to a variety of experiments

Collapse of layer dimerization in the photo-induced hidden state of 1T-TaS2

Photo-induced switching between collective quantum states of matter is a fascinating rising field with exciting opportunities for novel technologies. Presently, very intensively studied examples in this regard are nanometer-thick single crystals of the layered material 1T-TaS2, where picosecond laser pulses can trigger a fully reversible insulator-to-metal transition (IMT). This IMT is believed to be connected to the switching between metastable collective quantum states, but the microscopic nature of this so-called hidden quantum state remained largely elusive up to now. Here, we characterize the hidden quantum state of 1T-TaS2 by means of state-of-the-art x-ray diffraction and show that the laser-driven IMT involves a marked rearrangement of the charge and orbital order in the direction perpendicular to the TaS2-layers. More specifically, we identify the collapse of interlayer molecular orbital dimers as a key mechanism for this non-thermal collective transition between two truly long-range ordered electronic crystals

Stabilization mechanism of molecular orbital crystals in IrTe2

Doped IrTe2 is considered a platform for topological superconductivity and therefore receives currently a lot of interest. In addition, the superconductivity in these materials exists in close vicinity to electronic order and the formation of molecular orbital crystals, which we explore here by means of high-pressure single crystal x-ray diffraction in combination with density functional theory. Our crystallographic refinements provide detailed information about the structural evolution as a function of applied pressure up to 42 GPa. Using this structural information for density functional theory calculations, we show that the local multicenter bonding in IrTe2 is driven by changes in the Ir-Te-Ir bond angle. When the electronic order sets in, this bond angle decreases drastically, leading to a stabilization of a multicenter molecular orbital bond. This unusual local mechanism of bond formation in an itinerant material provides a natural explanation for the different electronic orders in IrTe2. It further illustrates the strong coupling of the electrons with the lattice and is most likely relevant for the superconductivity in this material

Coupled frustrated ferromagnetic and antiferromagnetic quantum spin chains in the quasi-one-dimensional mineral antlerite Cu3 SO4 (OH) 4

Magnetic frustration, the competition among exchange interactions, often leads to novel magnetic ground states with unique physical properties which can hinge on details of interactions that are otherwise difficult to observe. Such states are particularly interesting when it is possible to tune the balance among the interactions to access multiple types of magnetic order. We present antlerite Cu3SO4(OH)4 as a potential platform for tuning frustration. Contrary to previous reports, the low-temperature magnetic state of its three-leg zigzag ladders is a quasi-one-dimensional analog of the magnetic state recently proposed to exhibit spinon-magnon mixing in botallackite. Density functional theory calculations indicate that antlerite's magnetic ground state is exquisitely sensitive to fine details of the atomic positions, with each chain independently on the cusp of a phase transition, indicating an excellent potential for tunability.This project was funded by the German Research Foundation (DFG) via the projects A05, C01, C03, and C06 of the Collaborative Research Center SFB 1143 (project-id 247310070); GRK 1621 (project-id 129760637); the Würzburg-Dresden Cluster of Excellence on Complexity and Topology in Quantum Matter—ct.qmat (EXC 2147, project-id 390858490); through individual research grants, Grants No. IN 209/9-1 and No. PE 3318/2-1; and through project-id 422219907. D.K. and O.J. were supported by the Leibniz Association through the Leibniz Competition.Peer reviewe

Physics and Chemistry of Minerals / Structural and spectroscopic characterization of the brownmillerite-type Ca2Fe2xGaxO5 solid solution series

Here, we present a comprehensive study that encompasses changes within the crystal and magnetic structure in the brownmillerite-type phase Ca2Fe2O5 induced by the substitution of Fe3+ with Ga3+. 61 synthetic single-crystal samples of Ca2Fe2xGaxO5 0.00x1.328 have been investigated by single-crystal X-ray diffraction at 25 C. We find that pure Ca2Fe2O5 and samples up to x1.0 have space group Pnma, Z=4, whereas samples with x>1.0 show I2mb symmetry, Z=4. The Raman spectroscopic measurements exhibit that the change from Pnma to I2mb space group symmetry is reflected by a significant shift of two Raman modes below 150 cm1. These Raman modes are obviously linked to changes in the CaO bond lengths at the phase transition. 57Fe Mössbauer spectroscopy was used to characterize the cation distribution and magnetic structure as a function of composition and temperature. Thereby, the strong preference of Ga3+ for the tetrahedral site is verified, as an independent method besides XRD. At room-temperature, Ca2Fe2xGaxO5 solid solution compounds with 0x1.0 are antiferromagnetic ordered, as revealed by the appearance of magnetically split sextets in the Mössbauer spectra; samples with higher Ga3+ contents are paramagnetic. Over and above, the substitution of Fe3+ by Ga3+ results in the appearance of sharp, additional magnetic hyperfine split sextets, which can be attributed to cluster configurations within the individual tetrahedral chains. The temperature-dependent (20720 K) Mössbauer study reveals a transition from the magnetically ordered to the paramagnetic state at a temperature of about 710 K for the Ca2Fe2O5 end-member.(VLID)295667

Collapse of layer dimerization in the photo-induced hidden state of 1T−TaS21T-TaS_{2}

Photo-induced switching between collective quantum states of matter is a fascinating rising field with exciting opportunities for novel technologies. Presently, very intensively studied examples in this regard are nanometer-thick single crystals of the layered material 1T-TaS2, where picosecond laser pulses can trigger a fully reversible insulator-to-metal transition (IMT). This IMT is believed to be connected to the switching between metastable collective quantum states, but the microscopic nature of this so-called hidden quantum state remained largely elusive up to now. Here, we characterize the hidden quantum state of 1T-TaS2 by means of state-of-the-art x-ray diffraction and show that the laser-driven IMT involves a marked rearrangement of the charge and orbital order in the direction perpendicular to the TaS2-layers. More specifically, we identify the collapse of interlayer molecular orbital dimers as a key mechanism for this non-thermal collective transition between two truly long-range ordered electronic crystals