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...

Superconductivity is a unique manifestation of quantum mechanics on a macroscopic scale, and one of the rare examples of many-body phenomena that can be explained by predictive, quantitative theories. The superconducting ground state is described as a condensate of Cooper pairs, and a major challenge has been to understand which mechanisms could lead to a bound state between two electrons, despite the large Coulomb repulsion. An even bigger challenge is to identify experimentally this pairing mechanism, notably in unconventional superconductors dominated by strong electronic correlations, like in high-Tc cuprates, iron pnictides or heavy-fermion compounds. Here we show that in the ferromagnetic superconductor UCoGe, the field dependence of the pairing strength influences dramatically its macroscopic properties like the superconducting upper critical field, in a way that can be quantitatively understood. This provides a simple demonstration of the dominant role of ferromagnetic spin fluctuations in the pairing mechanism.

The pressure-temperature phase diagram of the orthorhombic ferromagnetic superconductor UCoGe was determined by resistivity measurements up to 10.5 GPa. The Curie temperature TC is suppressed with pressure and vanishes at the critical pressure pc ≈ 1 GPa. Superconductivity is observed in both the ferromagnetic state at low pressure, and in the paramagnetic state above pc up to about 4 GPa. Non-Fermi liquid behavior appears in a large pressure range. The resistivity varies linearly with temperature around pc and evolves continuously with pressure to a T 2 Fermi-liquid behavior for p 5 GPa. The residual resistivity as a function of pressure shows a maximum far above pc at p ⋆ = 7.2 GPa and the amplitude of the inelastic scattering term of the resistivity decreases by more than one order in magnitude at p ⋆ , which appears to mark the entrance into a weakly correlated regime. The pressure dependence of the upper critical field for magnetic field applied along the b and c axis illustrates the drastic difference in the field dependence of the ferromagnetic superconducting pairing. While for H b axis Hc2(T ) is driven by the suppression of the ferromagnetic order, it is dominated by the strong initial suppression of the ferromagnetic fluctuations for a field applied in the easy magnetization axis c.

What are the lifetime and maximum length of a soap film pulled at a velocity V out of a bath of soapy solution? This is the question we explore in this article by performing systematic film rupture experiments. We show that the lifetime and maximal length of the films are fairly reproducible and controlled only by hydrodynamics. For surfactants with high surface elastic modulus, we argue that the rupture is triggered by the expansion of a thinning zone at the top of the film. The length ltz of this zone expands with time at a velocity equal to V/2, which can be obtained by a balance between gravity and viscous forces. The film lifetime is then found to decrease with the pulling velocity V, which implies that the surface tension gradient along the film increases with V. This surface tension gradient is found to be surprisingly small. Finally, the lifetime of films stabilised by solutions with small surface elastic modulus turns out to be much shorter than the ones for films with rigid interfaces.

Magnetic properties of the substitution series Ru1−xCrxCl3 were investigated to determine the evolution from the anisotropic Kitaev magnet α-RuCl3 with J eff = 1/2 magnetic Ru 3+ ions to the isotropic Heisenberg magnet CrCl3 with S = 3/2 magnetic Cr 3+ ions. Magnetization measurements on single crystals revealed a reversal of the magnetic anisotropy under doping, which we argue to arise from the competition between anisotropic Kitaev and off-diagonal interactions on the Ru-Ru links and approximately isotropic Cr-Ru and isotropic Cr-Cr interactions. In addition, combined magnetization, ac susceptibility and specific-heat measurements clearly show the destabilization of the long-range magnetic order of α-RuCl3 in favor of a spin-glass state of Ru1−xCrxCl3 for a low doping of x ⋍ 0.

Researchers found evidence for a quantum phase transition within a superconducting condensate.

Recently, several putative quantum spin liquid (QSL) states were discovered in {\tilde S} = 1/2S̃=1/2 rare-earth based triangular-lattice antiferromagnets (TLAF) with the delafossite structure. In order to elucidate the conditions for a QSL to arise, we report here the discovery of a long-range magnetic order in the Ce-based TLAF KCeS_22 below T_{\mathrm N} = 0.38TN=0.38 K, despite the same delafossite structure. Finally, combining various experimental and computational methods, we characterize the crystal electric field scheme, the magnetic anisotropy and the magnetic ground state of KCeS_22.

We report on a detailed neutron diffraction and 1 H-NMR study on the frustrated spin-1/2 chain material linarite, PbCuSO4(OH)2, where competing ferromagnetic nearest neighbor and antiferromagnetic next-nearest neighbor interactions lead to frustration. From the magnetic Bragg peak intensity studied down to 60 mK, the magnetic moment per Cu atom is obtained within the whole magnetic phase diagram for H b axis. Further, we establish the detailed configurations of the shift of the SDW propagation vector in phase V with field and temperature. Finally, combining our neutron diffraction results with those from a low-temperature/high-field NMR study we find an even more complex phase diagram close to the quasi-saturation field suggesting that bound twomagnon excitations are the lowest energy excitations close to and in the quasi-saturation regime. Qualitatively and semi-quantitatively, we relate such behavior to XY Z exchange anisotropy and contributions from the Dzyaloshinsky-Moriya interaction to affect the magnetic properties of linarite.