Brunetto Chiari

Controlling Luttinger liquid physics in spin ladders under a magnetic field.

We present a 14N nuclear magnetic resonance study of a single crystal of CuBr4(C5H12N)2 (BPCB) consisting of weakly coupled spin-1/2 Heisenberg antiferromagnetic ladders. Treating ladders in the gapless phase as Luttinger liquids, we are able to fully account for (i) the magnetic field dependence of the nuclear spin-lattice relaxation rate T1(-1) at 250 mK and for (ii) the phase transition to a 3D ordered phase occurring below 110 mK due to weak interladder exchange coupling. BPCB is thus an excellent model system where the possibility to control Luttinger liquid parameters in a continuous manner is demonstrated and the Luttinger liquid model tested in detail over the whole fermion band.

Quantum-critical spin dynamics in quasi-one-dimensional antiferromagnets.

By means of nuclear spin-lattice relaxation rate T(1)(-1), we follow the spin dynamics as a function of the applied magnetic field in two gapped quasi-one-dimensional quantum antiferromagnets: the anisotropic spin-chain system NiCl(2)-4SC(NH(2))(2) and the spin-ladder system (C(5)H(12)N)(2)CuBr(4). In both systems, spin excitations are confirmed to evolve from magnons in the gapped state to spinons in the gapless Tomonaga-Luttinger-liquid state. In between, T(1)(-1) exhibits a pronounced, continuous variation, which is shown to scale in accordance with quantum criticality. We extract the critical exponent for T(1)(-1), compare it to the theory, and show that this behavior is identical in both studied systems, thus demonstrating the universality of quantum-critical behavior.

New pentanuclear mixed valence Co(II)–Co(III) complexes of “short” salen homologues

We describe herein a series of novel pentanuclear Co(II)–Co(III) mixed-valence cationic complexes of formula [Co5L4(μ3-OH)2]+ {L2− = 2,2′-[1,1-methanediylbis(nitrilomethylidene)diphenate] or its arylmethane analogues}, obtained by controlled oxidation of [Co2L2] precursors. The [Co5(μ3-OH)2(salNO2ben)4]I·2(CH3)2CO and [Co5(μ3-OH)2(salClben)4]I·2DMF·CH2Cl2 compounds have been studied by X-ray methods and turn out to be isostructural. The structural data show that the cobalt atoms in the pentanuclear units are bridged by μ-O(phenolato) atoms and μ3-OH groups and approximate three types of co-ordination geometries: tetrahedral, octahedral and trigonal-bipyramidal. The X-ray structure of one proligand, namely 2,2′-[1,1-(4-nitrophenylmethanediyl)bis(nitrilomethylidene)diphenol] is also reported. The χT(T) behaviour of the complex with this ligand has been studied in the 4–300 K range. The data clearly show that ferromagnetic as well as (much weaker) antiferromagnetic exchange interactions occur within the cluster. A simple exchange model has been developed which nicely reproduces the experimental data.

Structure and Magnetism of a New 2-D Bimetallic Compound of Mn(II) and Cu(II)

Abstract We present the X-ray structure and the magnetic properties of the new compound {[Cu(en)2]3[Mn(NCS)6]} (NCS)2 (en═H2N-CH2-CH2-NH2).

Cobalt(II) complexes of ligands having several thiourea, or selenourea, groups

Abstract The ligand behaviour of polymethylenebis(phenylthiourea), L, compounds: PhNH·C-S·NH·CH 2 ·CH(Me)·NH·CS·NHPh (I); PhNH·CS·NH·(CH 2 ) n ·NH·CS·NHPh ( n = 2–10) (II)-(X) and triethylaminetris ((phenylthiourea), N(C 2 H 4 NH·CS·NHPh) 3 (XI), towards CoX 2 (XCl, Br and I)_and Co(ClO 4 ) 2 ·6H 2 O has been investigated. The halides all give tetrahedral complexes CoLX 2 , having chromophore CoS 2 X 2 . Complexes with L = (XI) probably contain a non-coordinating sulphur atom and have low magnetic moments (3·7-3·9 B.M.). Complexes of formula CoL 2 (ClO 4 ) 2 are formed with cobalt II perchlorate, although only those with L = (III)–(IV) could be separated as solids. These complexes are tetrahedral with a CoS 4 chromophore. Ligand (IV) also gives a complex of composition CoL 4 (ClO 4 ) 2 , which is probably pentacoordinate with a CoS 4 O chromophore, and is characterised by a rather high magnetic moment (5·25 B.M.). (IV) appears to coordinated as a monodentate ligand in this complex. The complex with L = (XI) is difficult to classify as tetrahedral. A comparison of these complexes with several analogues containing selenium in place of sulphur has been carried out.

Field-induced staggered magnetization and magnetic ordering in Cu2(C5H12N2)2Cl4.

We present a 2D NMR investigation of the gapped spin-1/2 compound Cu2(C5H10N2D2)2Cl4. Our measurements reveal the presence of a magnetic field-induced transverse staggered magnetization (TSM) which persists well below and above the field-induced 3D long-range magnetically ordered (FIMO) phase. The symmetry of this TSM is different from that of the TSM induced by the order parameter of the FIMO phase. Its origin, field dependence, and symmetry can be explained by an intradimer Dzyaloshinskii-Moriya interaction, as shown by DMRG calculations on a spin-1/2 ladder. This leads us to predict that the transition into the FIMO phase is not in the BEC universality class.

Transition metal complexes of sulfur-oxygen chelating ligands Synthesis and characterisation of polymethylen-1-(phenylthiourea)-n-(phenylurea) complexes of cobalt(II)

Abstract Cobalt(II) halides, CoX2 (X = Cl, Br, I), give complexes of composition CoLX2 and CoL2X2 with polymethylene-1-(phenylthiourea)-n-(phenylurea) (L = Ph·NH·CS·NH·(CH2)n·NH·CO·NH·Ph) ligands. In the former the ligand bonds via S + O or S + N atoms, i.e. acts as a chelating ligand; in the latter the ligand is, instead, monodentate and bonds via the S atom. All the complexes are non-electrolytes and all have structures of approximately Td symmetry, with bonding halide ions.

Binuclear Co(II)Co(II), Co(II)Co(III) and Co(III)Co(III) complexes of “short” salen homologues derived from the condensation of salicylaldehyde and methanediamine or phenylmethanediamines. Synthesis, structures and magnetism

Binuclear Co(II)Co(II), Co(II)Co(III) and Co(III)Co(III) complexes are reported of dianionic ligands, L2−, derived from the condensation of two mol of salicylaldehyde, or substituted salicylaldehydes, and one mol of methanediamine or phenyl substituted methanediamines. Direct evidence for the binuclear nature of the compounds is provided by X-ray structural and FAB mass spectroscopy data. Oxidation of the Co(II)Co(II) complexes, [Co2L2], with iodine, in the presence of pyridine, produces two types of compounds depending on the nature of L: mixed valence Co(II)Co(III) dimers of [Co2L2(py)2](I3) stoichiometry, in the case of the phenylmethane derivatives and fully oxidised Co(III)Co(III) dimers, [Co2L2(py)4](I3)2, obtained from the complexes with methanediamine. The crystal structures of [Co2(salben)2(py)2](I3) and [Co2(salmen)2(py)4](I3)2 (salben = N,N′-phenylmethanediyldiphenato, salmen = N,N′-methanediyldiphenato) have been determined. In both dimers the Co atoms are bridged by two bis-bidentate ligands. The coordination geometry of the Co(II) chromophore, CoN2O2, was found to be distorted tetrahedral, as preliminarily suggested by near infrared and room-temperature magnetic susceptibility data. The Co(III) environments, CoO2N4, involving two additional pyridine nitrogens, are regular octahedral. The magnetic behaviour of representative compounds has been determined and discussed.

Next-Nearest Neighbor Exchange Interactions in the Bimetallic Chain Compound Cu(en)2MnCl4

Abstract The magnetic properties of the title compound reveal antiferromagnetic coupling between the next-nearest neighbor Mn(II) ions and no discernible MnCu coupling.

Transition metal complexes of organo-selenium chelating ligands—nickel(II) complexes of polymethylenebis(phenylselenourea) compounds

Abstract Some polymethylenebis(phenylselenourea) (=L) complexes of nickel(II) halides have been prepared and characterised and compared with those of the sulphur analogues. The complexes, of formula NiL 2 X 2 , show magnetic moments which range between 0 and 3.0 B.M. depending on the temperature of preparation. At low temperature of preparation they tend to diamagnetism. The electronic spectra are not influenced by the halogen present. An interpretation of this behaviour is put forward based on the tetragonality of the NiSe 2 chromophore present and it is suggested that the singlet and triplet states are of virtually equal energy. The reaction of the ligands with nickel(II) perchlorate is described.