Richard A. Mole

Hydrothermal synthesis and magnetic properties of novel Mn(II) and Zn(II) materials with thiolato-carboxylate donor ligand frameworks

The hydrothermal reaction of thiosalicylic acid, (C(6)H(4)(CO(2)H)(SH)-1,2) with manganese(III) acetate leads to formation of the coordination solid [Mn(5)((C(6)H(4)(CO(2))(S)-1,2)(2))(4)(mu3-OH)2] (1) via a redox reaction, where resulting manganese(II) centres are coordinated by oxygen donor atoms and S-S disulfide bridge formation is simultaneously observed. Reaction of the same ligand under similar conditions with zinc(II) chloride yields the layered coordination solid [Zn(C(6)H(4)(CO(2))(S)-1,2)] (2). Hydrothermal treatment of manganese(III) acetate with 2-mercaptonicotinic acid, (NC(5)H(3)(SH)(CO(2)H)-2,3) was found to produce the 1-dimensional chain structure [Mn(2)((NC(5)H(3)(S)(CO(2))-2,3)(2))(2)(OH(2))(4)].4H(2)O (3) which also exhibits disulfide bridge formation and oxygen-only metal interactions. Compound 3 has been studied by thermogravimetric analysis and indicates sequential loss of lattice and coordinated water, prior to more comprehensive ligand fragmentation at elevated temperatures. The magnetic behaviour of 1 and 3 has been investigated and both exhibit antiferromagnetic interactions. The magnetic behaviour of 1 has been modelled as two corner-sharing isosceles triangles whilst 3 has been modelled as a 1-dimensional chain.

Hidden magnetic excitation in the pseudogap phase of a high-Tc superconductor

The elucidation of the pseudogap phenomenon of the high-transition-temperature (high-Tc) copper oxides—a set of anomalous physical properties below the characteristic temperature T* and above Tc—has been a major challenge in condensed matter physics for the past two decades. Following initial indications of broken time-reversal symmetry in photoemission experiments, recent polarized neutron diffraction work demonstrated the universal existence of an unusual magnetic order below T* (refs 3, 4). These findings have the profound implication that the pseudogap regime constitutes a genuine new phase of matter rather than a mere crossover phenomenon. They are furthermore consistent with a particular type of order involving circulating orbital currents, and with the notion that the phase diagram is controlled by a quantum critical point. Here we report inelastic neutron scattering results for HgBa2CuO4+δ that reveal a fundamental collective magnetic mode associated with the unusual order, and which further support this picture. The mode’s intensity rises below the same temperature T* and its dispersion is weak, as expected for an Ising-like order parameter. Its energy of 52–56 meV renders it a new candidate for the hitherto unexplained ubiquitous electron–boson coupling features observed in spectroscopic studies.

Homoleptic 1-D iron selenolate complexes-synthesis, structure, magnetic and thermal behaviour of (1)(∞)[Fe(SeR)2] (R=Ph, Mes).

The first examples of polymeric homoleptic iron chalcogenolato complexes (1)(∞)[Fe(SePh)(2)] and (1)(∞)[Fe(SeMes)(2)] (Ph = phenyl = C(6)H(5), Mes = mesityl = C(6)H(2)-2,4,6-(CH(3))(3)) have been both prepared by reaction of [Fe(N(SiMe(3))(2))(2)] with two equivalents of HSeR (R = Ph, Mes) while (1)(∞)[Fe(SePh)(2)] was found to be also easily accessible through reactions of either FeCl(2), Fe(OOCCH(3))(2) or FeCl(3) with PhSeSiMe(3) in THF. In the crystal, the two compounds form one-dimensional chains with bridging selenolate ligands comprising distinctly different Fe-Se-Fe bridging angles, namely 71.15-72.57° in (1)(∞)[Fe(SePh)(2)] and 91.80° in (1)(∞)[Fe(SeMes)(2)]. Magnetic measurements supported by DFT calculations reveal that this geometrical change has a pronounced influence on the antiferromagnetic exchange interactions of the unpaired electrons along the chains in the two different compounds with a calculated magnetic exchange coupling constant of J = -137 cm(-1) in (1)(∞)[Fe(SePh)(2)] and J = -20 cm(-1) in (1)(∞)[Fe(SeMes)(2)]. In addition we were able to show that the ring molecule [Fe(SePh)(2)](12) which is a structural isomer of (1)(∞)[Fe(SePh)(2)] behaves magnetically similar to the latter one. Investigations by powder XRD reveal that the ring molecule is only a metastable intermediate which converts in THF completely to form (1)(∞)[Fe(SePh)(2)]. Thermal gravimetric analysis of (1)(∞)[Fe(SePh)(2)] under vacuum conditions shows that the compound is thermally labile and already starts to decompose above 30 °C in a two step process under cleavage of SePh(2) to finally form at 250 °C tetragonal PbO-type FeSe. The reaction of (1)(∞)[Fe(SePh)(2)] with the Lewis base 1,10-phenanthroline yielded, depending on the conditions, the octahedral monomeric complexes [Fe(SePh)(2)(1,10-phen)(2)] and [Fe(1,10-phen)(3)][Fe(SePh)(4)].

Magnetic Excitations in Polyoxotungstate-Supported Lanthanoid Single-Molecule Magnets: An Inelastic Neutron Scattering and ab Initio Study

Inelastic neutron scattering (INS) has been used to investigate the crystal field (CF) magnetic excitations of the analogs of the most representative lanthanoid-polyoxometalate single-molecule magnet family: Na9[Ln(W5O18)2] (Ln = Nd, Tb, Ho, Er). Ab initio complete active space self-consistent field/restricted active space state interaction calculations, extended also to the Dy analog, show good agreement with the experimentally determined low-lying CF levels, with accuracy better in most cases than that reported for approaches based only on simultaneous fitting to CF models of magnetic or spectroscopic data for isostructural Ln families. In this work we demonstrate the power of a combined spectroscopic and computational approach. Inelastic neutron scattering has provided direct access to CF levels, which together with the magnetometry data, were employed to benchmark the ab initio results. The ab initio determined wave functions corresponding to the CF levels were in turn employed to assign the INS transitions allowed by selection rules and interpret the observed relative intensities of the INS peaks. Ultimately, we have been able to establish the relationship between the wave function composition of the CF split LnIII ground multiplets and the experimentally measured magnetic and spectroscopic properties for the various analogs of the Na9[Ln(W5O18)2] family.

Switchable magnetism: neutron diffraction studies of the desolvated coordination polymer Co3(OH)2(C4O4)2.

We report the magnetic structure of the two magnetically ordered phases of Co3(OH)2(C4O4)2, a coordination polymer that consists of a triangular framework decorated with anisotropic Co(II) ions. Neutron diffraction experiments allow us to confirm that the magnetic behavior changes upon dehydration and reveal the complex phase behavior of this system, relative to the hydrated compound Co3(OH)2(C4O4)2·3H2O. One phase is shown to display spin idle behavior, where only a fraction of the moments order at intermediate temperatures, while at the lowest temperatures the system orders fully, in this case with a net magnetic moment. This novel magnetic behavior is discussed within the framework of a simple Hamiltonian and representational analysis and rationalizes this multiphase behavior by considering the combination of frustration and anisotropy. The change in behavior on dehydration is also rationalized with respect to the changes in the single-ion anisotropy of the cobalt.

Two stage magnetic ordering and spin idle behavior of the coordination polymer Co3(OH)2(C4O4)2·3H2O determined using neutron diffraction.

We report the magnetic structure of two of the magnetically ordered phases of Co(3)(OH)(2)(C(4)O(4))(2)·3H(2)O, a coordination polymer that consists of a triangular framework decorated with anisotropic Co(II) ions. The combination of neutron diffraction experiments and magnetic susceptibility data allows us to identify one phase as displaying spin idle behavior, where only a fraction of the moments order at intermediate temperatures, while at the lowest temperatures the system orders fully. This novel magnetic behavior is discussed within the framework of a simple Hamiltonian and representational analysis and rationalizes this multiphase behavior by considering the combination of frustration and anisotropy.

Carbonate-Bridged Lanthanoid Triangles: Single-Molecule Magnet Behavior, Inelastic Neutron Scattering, and Ab Initio Studies

Optimization of literature synthetic procedures has afforded, in moderate yield, homogeneous and crystalline samples of the five analogues Na11[{RE(OH2)}3CO3(PW9O34)2] (1-RE; RE = Y, Tb, Dy, Ho, and Er). Phase-transfer methods have allowed isolation of the mixed salts (Et4N)9Na2[{RE(OH2)}3CO3(PW9O34)2] (2-RE; RE = Y and Er). The isostructural polyanions in these compounds are comprised of a triangular arrangement of trivalent rare-earth ions bridged by a μ3-carbonate ligand and sandwiched between two trilacunary Keggin {PW9O34} polyoxometalate ligands. Alternating-current (ac) magnetic susceptibility studies of 1-Dy, 1-Er, and 2-Er reveal the onset of frequency dependence for the out-of-phase susceptibility in the presence of an applied magnetic field at the lowest measured temperatures. Inelastic neutron scattering (INS) spectra of 1-Ho and 1-Er exhibit transitions between the lowest-lying crystal-field (CF) split states of the respective J = 8 and (15)/2 ground-state spin-orbit multiplets of the Ho(III) and Er(III) ions. Complementary ab initio calculations performed for these two analogues allow excellent reproduction of the experimental magnetic susceptibility and low-temperature magnetization data and are in reasonable agreement with the experimental INS data. The ab initio calculations reveal that the slight difference in coordination environments of the three Ln(III) ions in each complex gives rise to differences in the CF splitting that are not insignificant. This theoretical result is consistent with the observation of multiple relaxation processes by ac magnetic susceptibility and the broadness of the measured INS peaks. The ab initio calculations also indicate substantial mixing of the MJ contributions to the CF split energy levels of each Ln(III) ion. Calculations indicate that the CF ground states of the Ho(III) centers in 1-Ho are predominantly comprised of contributions from small MJ, while those of the Er(III) centers in 1-Er are predominantly comprised of contributions from large MJ, giving rise to slow magnetic relaxation. Although no direct evidence for intramolecular RE···RE magnetic coupling is observed in either magnetic or INS studies, on the basis of the ab initio calculations, we find noncollinear magnetic axes in 1-Er that are coplanar with the erbium triangle and radially arranged with respect to the triangles centroid; thus, we argue that the absence of magnetic coupling in this system arises from dipolar and antiferromagnetic superexchange interactions that cancel each other out.

Proline induced disruption of the structure and dynamics of water

We use quasi-elastic neutron scattering spectroscopy to study the diffusive motion of water molecules at ambient temperature as a function of the solute molar fraction of the amino acid, proline. We validate molecular dynamics simulations against experimental quasielastic neutron scattering data and then use the simulations to reveal, and understand, a strong dependence of the translational self-diffusion coefficient of water on the distance to the amino acid molecule. An analysis based on the juxtaposition of water molecules in the simulation shows that the rigidity of proline imposes itself on the local water structure, which disrupts the hydrogen-bond network of water leading to an increase in the mean lifetime of hydrogen bonds. The net effect is some distortion of the proline molecule and a slowing down of the water mobility.

Low-energy excitations and ground-state selection in the quantum breathing pyrochlore antiferromagnet Ba3Yb2Zn5 O11

UTokyo Research掲載「新しいスピン状態を絶対零度付近にて発見」 URI: http://www.u-tokyo.ac.jp/ja/utokyo-research/research-news/new-spin-state-discovered-near-absolute-zero.html

Dynamics of linarite: Observations of magnetic excitations

Here we present inelastic neutron scattering measurements from the frustrated, quantum spin-1/2 chain material linarite, PbCuSO_4(OH)_2. Time of flight data, taken at 0.5K and zero applied magnetic field reveals low-energy dispersive spin wave excitations below 1.5meV both parallel and perpendicular to the Cu-chain direction. From this we confirm that the interchain couplings within linarite are around 10% of the nearest neighbour intrachain interactions. We analyse the data within both linear spin-wave theory and density matrix renormalisation group theories and establish the main magnetic exchange interactions and the simplest realistic Hamiltonian for this material.