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 determine the latter 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 TaS2layers. More specifically, we identify the collapse of inter-layer molecular orbital dimers, which are a characteristic feature of the insulating phase, as a key mechanism for the non-thermal IMT in 1T-TaS2, which indeed involves a collective transition between two truly long-range ordered electronic crystals.The layered transition metal dichalcogenides (TMDs) form a vast class of materials hosting diverse non-trivial quantum phenomena such as spin-valley polarization [1], Ising-superconductivity [2] or intertwined electronic orders [3,4]. All these intriguing electronic effects along with the natural suitability of TMDs for the preparation of quasi two-dimensional (2D) nano-sheets render them highly appealing for next-generation technologies [5][6][7][8].1T-TaS 2 is a particularly interesting and extensively studied TMD in which external tuning parameters like temperature, pressure or chemical substitution span a particularly complex electronic phase diagram. Apart from several charge density waves (CDWs) this phase diagram also features pressure-induced superconductivity and a so-called Mott-phase, which stands out due to its semiconducting electronic transport properties [3,9].Remarkably, besides the aforementioned states that can be reached in thermal equilibrium, femto to picosecond optical or electrical pulses can launch a nonequilibrium IMT into a previously hidden and persistent metallic CDW-state [7,10,11]. The discovery of this so-called hidden CDW (HCDW) has sparked wide excitement as it might provide a new platform for memory device applications. Accordingly, in recent years, a significant number of experimental and theoretical studies aimed at pinning down the microscopic mechanism of this non-equilibrium IMT that is believed to be connected to a reorganization of the CDW-order. However, despite significant efforts to determine the microscopic processes underlying this novel IMT have been made [12][13][14][15][16][17], a clear picture remains elusive.In this article we address this open issue directly by means of high-resolution synchrotron x-ray diffraction (XRD) in combination with laser pumping. Our experiments enable examination of the laser-driven transition and in-particular the HCDW-order in 1T-TaS 2 nanosheets wi...

Magnetization and high-resolution x-ray diffraction measurements of the Kitaev-Heisenberg material α-RuCl 3 reveal a pressure-induced crystallographic and magnetic phase transition at a hydrostatic pressure of p ∼ 0.2 GPa. This structural transition into a triclinic phase is characterized by a very strong dimerization of the Ru-Ru bonds, accompanied by a collapse of the magnetic susceptibility. Ab initio quantum-chemistry calculations disclose a pressure-induced enhancement of the direct 4d-4d bonding on particular Ru-Ru links, causing a sharp increase of the antiferromagnetic exchange interactions. These combined experimental and computational data show that the Kitaev spin-liquid phase in α-RuCl 3 strongly competes with the crystallization of spin singlets into a valence bond solid. DOI: 10.1103/PhysRevB.97.241108 The Kitaev model on a honeycomb lattice has grown into a hot topic in the last decade due to its exact solubility and its quantum spin-liquid ground state, which would be relevant for, e.g., quantum computing [1,2]. It implies a bonddependent compass-type coupling K and strong intrinsic spin frustration [3]. A crucial ingredient for realizing the Kitaev model in real materials is a strong spin-orbit coupling together with a honeycomb structure. Recently, Kitaev interactions were identified in α-RuCl 3 , from its unusual magnetic excitation spectrum [4,5], its strong magnetic anisotropy [6], and electronic-structure calculations [7,8], which render this material an ideal platform for exploring Kitaev magnetism experimentally.α-RuCl 3 is a j eff = 1/2 Mott insulator with a twodimensional (2D) layered structure of edge-sharing RuCl 6 octahedra forming a honeycomb lattice. At ambient pressure, * g.bastien@ifw-dresden.de Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.the honeycomb layers are arranged in a monoclinic (C2/m) structure at room temperature with one of the three nearestneighbor (NN) Ru-Ru bonds slightly shorter than the other two [9]. A structural phase transition was reported at T S 60 K under cooling and T S 166 K upon warming, but the low-temperature crystal structure is still under debate and could be either rhombohedral (R3) [10,11] or monoclinic (C2/m) [12,13]. The onset of long-range magnetic order at T N 7 K [9] in α-RuCl 3 implies that other magnetic interactions have to be considered in addition to the Kitaev interaction K: a NN Heisenberg J , an off-diagonal coupling , as well as next-NN interactions J 2 and J 3 [7,8,14,15]. While electronic-structure calculations indicate that K is ferromagnetic in α-RuCl 3 and indeed defines the largest exchange energy scale [7,8,14,15], the debate on the minimal effective spin model and precise magnitude of the different couplings is not fully settled yet. By applying a magnetic field in the basal plane, the magnetic zigzag ground sta...

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We report an optimized chemical vapor transport method, which allows growing FeP single crystals up to 500 mg in mass and 80 mm 3 in volume. The high quality of the crystals obtained by this method was confirmed by means of EDX, high-resolution TEM, low-temperature single crystal XRD and neutron diffraction experiments. We investigated the transport and magnetic properties of the single crystals and calculated the electronic band structure of FeP. We show both theoretically and experimentally, that the ground state of FeP is metallic. The examination of the magnetic data reveals antiferromagnetic order below TN =119 K while transport remains metallic in both the paramagnetic and the antiferromagnetic phase. The analysis of the neutron diffraction data shows an incommensurate magnetic structure with the propagation vector Q = (0, 0, ±δ), where δ ≈ 0.2.For the full understanding of the magnetic state, further experiments are needed. The successful growth of large high-quality single crystals opens the opportunity for further investigations of itinerant magnets with incommensurate spin structures using a wide range of experimental tools.

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 Cu 3 SO 4 (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.