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Quantum fluctuation in frustrated magnets and quantum criticality at the transition between different quantum phases of matter are two of the cornerstones in condensed matter physics. Here we demonstrate the nontrivial interplay between them in the spin-1/2 coupled two-leg ladder antiferromagnet C9H18N2CuBr4. Employing the high-resolution neutron spectroscopy, we unambiguously identify a weakly first-order hydrostatic pressure-driven quantum phase transition, which arises from fluctuations enhanced by the frustrating interlayer coupling. An exotic pressure-induced quantum disordered state is evidenced by the broad spectral linewidth observed near the phase transition. Interestingly, we find that the gapped transverse excitations in the Neel-ordered phase at ambient pressure cannot be described by the conventional S=1 magnons, i.e., the spin wave quanta, associated with explicit symmetry breaking, and thus the three-dimensional magnetic order ought to emerge in an unconventional way. We further apply the quantum Fisher information to show the presence of bipartite entanglement at criticality at least up to 1.1 K in the same material., Comment: 10 pages and 6 figures. We call for theoretical understanding of the nontrivial interplay observed in this material

Reaction of CuX2·nH2O (X = Cl, n = 2; X = Br, n = 0) with 2-amino-5-methylpyridine (5MAP) in acetonitrile yielded [(5MAP)2CuX2]2 (1, Cl; 2, Br). The compounds are isomorphous and crystallize in the monoclinic space group P21/n as bihalide bridged dimers. Variable temperature magnetic measurements show very weak exchange in the two compounds (-2.1(1) K) which may correlate with previously proposed magnetostructural correlations for bihalide bridged Cu(II) dimers. In attempts to prepare 2 in 2-butanone as solvent, [(3-Br-2-Et-5-Mepyrpy)(CH3CO2)4Cu2] (3) was serendipitously isolated (3-Br-2-Et-5-Mepyrpy = 3-bromo-2-ethyl-5-methyl-7H-pyrrolido[2,3-b]pyridinium). The bicyclic ligand was synthesized in situ by condensation of 5MAP with the solvent; the acetate ions were generated by copper(II) catalyzed Baeyer-Villiger oxidation of the solvent. A mechanism for the synthesis is proposed. The resulting Cu-paddlewheel compound exhibits strong (-400 K) antiferromagnetic interactions.

Three new methoxide-bridged Cu(II) dimers are reported: tetrakis(2-methylpyrazine)di(μ-methoxy)diperchloratodicopper(II) (1), tetrakis(2-amino-5-iodopyridine)dibromide-di-μ-methoxydicopper(II) tetramethanol (2) and catena[(2-amino-5-methylpyridine)μ-bromido-μ-methoxycopper(II) (3). The compounds were characterized by single-crystal X-ray diffraction and variable temperature magnetic measurements. All three compounds crystallize in the triclinic space group P-1, and the dimers lie across a crystallographic inversion center. The dimers in 3 are further bridged into an alternating chain motif via semi-coordinated Cu(II)/Br interactions. All three compounds exhibit strong antiferromagnetic interactions with J/kB = −348(8) K for 1, and the exchange in 2 and 3 being too strong to estimate from the available data. The magnetic data for 1-3 are dominated by trace paramagnetic impurities at temperatures below 200 K. Correlation of the structures with the observed magnetic behavior is proposed.

Quantum phase transitions in quantum matter occur at zero temperature between distinct ground states by tuning a nonthermal control parameter. Often, they can be accurately described within the Landau theory of phase transitions, similarly to conventional thermal phase transitions. However, this picture can break down under certain circumstances. Here, we present a comprehensive study of the effect of hydrostatic pressure on the magnetic structure and spin dynamics of the spin-1/2 ladder compound C$_9$H$_{18}$N$_2$CuBr$_4$. Single-crystal heat capacity and neutron diffraction measurements reveal that the N$\rm \acute{e}$el-ordered phase breaks down beyond a critical pressure of $P_{\rm c}$$\sim$1.0 GPa through a continuous quantum phase transition. Estimates of the critical exponents suggest that this transition may fall outside the traditional Landau paradigm. The inelastic neutron scattering spectra at 1.3 GPa are characterized by two well-separated gapped modes, including one continuum-like and another resolution-limited excitation in distinct scattering channels, which further indicates an exotic quantum-disordered phase above $P_{\rm c}$., The revised version includes 10 pages and 5 figures in the main text. To appear in Nature Communications. Any comments are welcome

Reaction of copper(II) chloride or bromide with 2-chloro-3-bromopyridine or 2,3-dichloropyridine generates a family of compounds of the general formula L2CuX2 (1–4). X-ray crystallography shows that the bromide complexes (3-bromo-2-chloropyridine)dibromidocopper(II) (1) and (2,3-dichloropyridine)dibromidocopper(II) (3) are particularly unusual in that they crystallize with both the syn- and anti-conformation structures in the same crystal. A review of the literature on complexes of the formula (substituted-pyridine)2CuX2 suggests that these are the first examples of such complexes. The members of the family show a variety of magnetic behaviors and variable temperature magnetic susceptibility data indicate that 1 is essentially paramagnetic (θ = −0.9(1) K) while 3 is weakly ferromagnetic (J = 2.9(1) K). Compound 2 [(3-bromo-2-chloropyridine)dichloridocopper(II)] is fit by the uniform 1-D antiferromagnetic model (J = −19.6(1) K), while 4 [(2,3-dichloropyridine)dichloridocopper(II)] exhibits weak anti-ferromagnetic interactions (J = −3.68(3) K).

A family of compounds of general formula (QuinH)2MX4·2H2O has been prepared and characterized [Quin = quinoline; M, X = Co,Cl (1); M, X = Co,Br (2); M, X = Zn,Br (3); M, X = Mn,Cl (4)]. The complexes crystallize in the monoclinic space group C2/c as well-isolated layers containing the MX4−2 anions and water molecules, separated by the quinolinium cations. The bromide analogue of 4, compound 5 (QuinH)2[MnBr2(H2O)2](Br)2, also crystallizes in the C2/c space group, but comprises a co-crystal of manganese bromide dihydrate and quinolinium bromide. The temperature dependent magnetic properties of the complexes are described, along with the tetrachlorocuprate analogue (7). All compounds show weak antiferromagnetic interactions (J/kB ∼ 0.06–1.4 K) and good one- or two-dimensional isolation. In addition, the crystal structure of the mixed quinoline/quinolinium complex (QuinH)[Cu(Quin)Br3] (triclinic, P-1) is reported.

Reaction of 2-amino-5-halopyridine molecules with Cu(II) halides in the solid state via mechanochemical techniques produced four coordination complexes, (5BAP)2CuBr2 (2a), (5BAP)2CuCl2 (3), (5CAP)2CuCl2 (4), (5CAP)2CuBr2 (5) [5BAP = 2-amino-5-bromopyridine; 5CAP = 2-amino-5-chloropyridine]. In all compounds, the 5BAP or 5CAP ligands are coordinated to the Cu(II) ion through the pyridine N atom along with two halide ions. Compounds 2 and 3 crystallized in the P-1 and P21/c space groups, respectively. Temperature dependent magnetic susceptibility data for 2 were fit to the antiferromagnetic rectangle model (J = –4.5(2) K; α = 0.89(17)), while the data for 3 were fit to the uniform chain model (J = –4.20(6) K), indicating a significant difference in the interchain interactions. Both 4 and 5 crystallize with the 5CAP ligands in the syn-conformation in the space groups I2/a and P21/n, respectively. Compound 4 formed the expected dimeric structure via bridging chloride ions and the magnetic data were successfully fit to the dimer model (J = –2.57(5) K), while 5 unexpectedly formed a structural chain via Br…Br contacts which exhibits very weak antiferromagnetic interactions. An intermediate structure, (5BAP)(H2O)CuBr2 (1), was prepared and characterized via mechanochemical methods along with an isomer of (5BAP)2CuBr2 (2b).

Two isostructural double halide-bridged polymers, consisting of stacked Cu2L2X4 dimers, with L = benzamide (BA) and X = Cl- in CuClBA and X = Br- in CuBrBA, were synthesised. The experimental magnetic data of both compounds were recorded and it was found that the chi(m) data fitted a 1D alternating FM/AFM model. A computational First-Principles Bottom-Up computational study was conducted to understand the micro- and bulk magnetic properties. It was determined that a strong ferromagnetic (FM) interaction occurs within the Cu(2)BA(2)X(4) dimer, with an anti-ferromagnetic (AFM) interaction diagonally connecting Cu(2)BA(2)X(4) dimeric moieties. For CuClBA, a 1D alternating FM/AFM chain topology was found, whereas CuBrBA shows additional competing interactions within the 1D alternating FM/AFM chain. Strikingly, the magnetic topology for both compounds is different than that expected from crystal packing analyses. Tuning the weaker competing interactions within the double halide-bridged chains might be a good strategy to extend the bulk FM character of the compound. (C) 2020 Elsevier Ltd. All rights reserved.