Spin dynamics in the ordered spin ice Tb2_2Sn2_2O7_7

Geometrical frustration is a central challenge in contemporary condensed matter physics, a crucible favourable to the emergence of novel physics. The pyrochlore magnets, with rare earth magnetic moments localized at the vertices of corner-sharing tetrahedra, play a prominent role in this field, with a rich variety of exotic ground states ranging from the "spin ices" \hoti\ and \dyti\ to the "spin liquid" and "ordered spin ice" ground states in \tbti\ and \tbsn. Inelastic neutron scattering provides valuable information for understanding the nature of these ground states, shedding light on the crystal electric field (CEF) level scheme and on the interactions between magnetic moments. We have performed such measurements with unprecedented neutron flux and energy resolution, in the "ordered spin ice" \tbsn. We argue that a new interaction, which involves the spin lattice coupling through a low temperature distortion of the trigonal crystal field, is necessary to account for the data

Transverse Magnetic Anisotropy in Mn12-acetate: Direct Determination by Inelastic Neutron Scattering

A high resolution inelastic neutron scattering (INS) study of fully deuterated Mn$_{12}$-acetate provides the most accurate spin Hamiltonian parameters for this prototype single molecule magnet so far. The Mn$_{12}$-clusters deviate from axial symmetry, a non-zero rhombic term in the model Hamiltonian leading to excellent agreement with observed positions and intensities of the INS peaks. The following parameter set provides the best agreement with the experimental data: $D=-0.0570(1)$ meV, $B_{4}^0=-2.78(7)\cdot 10^{-6}$ meV, $B_{4}^4=-3.2(6)\cdot 10^{-6}$ meV and $\mid$\textit{E}$\mid =6.8(15)\cdot 10^{-4}$ meV. Crystal dislocations are not the likely cause of the symmetry lowering. Rather, this study lends strong support to a recently proposed model, which is based on the presence of several molecular isomers with distinct spin Hamiltonian parameters.Comment: 4 pages, 4 figure

Metamagnetism and soliton excitations in the modulated ferromagnetic Ising chain CoV2O6

We report a combination of physical property and neutron scattering measurements for polycrystalline samples of the one-dimensional spin chain compound CoV2O6. Heat capacity measurements show that an effective S = 1/2 state is found at low temperatures and that magnetic fluctuations persist up to 6.Tn. Above Tn = 6.3 K, measurements of the magnetic susceptibility as a function of T and H show that the nearest neighbour exchange is ferromagnetic. In the ordered state, we have discovered a crossover from a metamagnet with strong fluctuations between 5 K and Tn to a state with a 1/3 magnetisation plateau at 2 < T < 5 K. We use neutron powder diffraction measurements to show that the AFM state has incommensurate long range order and inelastic time of flight neutron scattering to examine the magnetic fluctuations as a function of temperature. Above Tn, we find two broad bands between 3.5 and 5 meV and thermally activated low energy features which correspond to transitions within these bands. These features show that the excitations are deconfined solitons rather than the static spin reversals predicted for a uniform FM Ising spin chain. Below Tn, we find a ladder of states due to the confining effect of the internal field. A region of weak confinement below Tn, but above 5 K, is identified which may correspond to a crossover between 2D and 3D magnetic ordering.Comment: Expanded version, includes results from arXiv:0804.2966 and neutron powder diffraction. To appear in PR

Pressure Dependence of the Magnetic Anisotropy in the "Single-Molecule Magnet" [Mn4O3Br(OAc)3(dbm)3]

The anisotropy splitting in the ground state of the single-molecule magnet [Mn4O3Br(OAc)3(dbm)3] is studied by inelastic neutron scattering as a function of hydrostatic pressure. This allows a tuning of the anisotropy and thus the energy barrier for slow magnetisation relaxation at low temperatures. The value of the negative axial anisotropy parameter $D_{\rm cluster}$ changes from -0.0627(1) meV at ambient to -0.0603(3) meV at 12 kbar pressure, and in the same pressure range the height of the energy barrier between up and down spins is reduced from 1.260(5) meV to 1.213(9) meV. Since the $\rm Mn-Br$ bond is significantly softer and thus more compressible than the $\rm Mn-O$ bonds, pressure induces a tilt of the single ion Mn$^{3+}$ anisotropy axes, resulting in the net reduction of the axial cluster anisotropy.Comment: 4 pages, 3 figure

Antiferro-quadrupolar correlations in the quantum spin ice candidate Pr2Zr2O7

We present an experimental study of the quantum spin ice candidate pyrochlore coumpound \przr\ by means of magnetization measurements, specific heat and neutron scattering up to 12 T and down to 60 mK. When the field is applied along the $[111]$ and $[1\bar{1}0]$ directions, ${\bf k}=0$ field induced structures settle in. We find that the ordered moment rises slowly, even at very low temperature, in agreement with macroscopic magnetization. Interestingly, for $H \parallel [1\bar{1}0]$, the ordered moment appears on the so called $\alpha$ chains only. The spin excitation spectrum is essentially {\it inelastic} and consists in a broad flat mode centered at about 0.4 meV with a magnetic structure factor which resembles the spin ice pattern. For $H \parallel [1\bar{1}0]$ (at least up to 2.5 T), we find that a well defined mode forms from this broad response, whose energy increases with $H$, in the same way as the temperature of the specific heat anomaly. We finally discuss these results in the light of mean field calculations and propose a new interpretation where quadrupolar interactions play a major role, overcoming the magnetic exchange. In this picture, the spin ice pattern appears shifted up to finite energy because of those new interactions. We then propose a range of acceptable parameters for \przr\, that allow to reproduce several experimental features observed under field. With these parameters, the actual ground state of this material would be an antiferroquadrupolar liquid with spin-ice like excitations

Expression of 6 Biomarkers in Liver Grafts After Pediatric Liver Transplantation : Correlations with Histology, Biochemistry, and Outcome

Background: Subclinical graft inflammation and fibrosis after pediatric liver transplantation (LT) are common. Biomarkers are needed that precede and are associated with these changes and graft outcome. Material/Methods: We evaluated immunohistochemical expression of 6 biomarkers [alpha-smooth muscle actin (alpha-SMA), collagen I, decorin, vimentin, P-selectin glycoprotein ligand-1 (PSGL-1), and CD34] in biopsies taken intraoperatively at LT (baseline) (n=29) and at 11.3 years after LT (first follow-up) (n=51). Liver biochemistry and graft histology were assessed at the first follow-up and at final assessment (19.6 years after LT) (n=48). Second follow-up biopsies for histology were available from 24 patients. The immunostainings were correlated with liver histology, biochemistry, and outcome at these time-points. Results: Baseline levels of the biomarkers were unrelated to presence of fibrosis at follow-up. Increased a-SMA, collagen I levels, decorin, and vimentin were associated with simultaneous fibrosis at the first follow-up (p=0.001-0.027). Increased SMA, collagen I, decorin, vimentin, PSGL-1, and CD34 expression at first follow-up were associated with simultaneous portal inflammation (p=0.001-0.025). alpha-SMA, decorin, and vimentin expression were increased in patients without fibrosis at the first follow-up but who developed fibrosis in second follow-up (p=0.014 p=0.024 and p=0.024). Significant fibrosis (F2) and markedly increased alpha-SMA, collagen I, decorin, and vimentin levels at first follow-up were associated with suboptimal liver status at the final assessment (p=0.002-0.042). Conclusions: The expression of the biomarkers at LT was unrelated to later development of graft fibrosis. alpha-SMA, decorin, and vimentin were associated with later graft fibrosis and suboptimal liver status.Peer reviewe