Temperature dependence of single particle excitations in a S=1 chain: exact diagonalization calculations compared to neutron scattering experiments

Exact diagonalization calculations of finite antiferromagnetic spin-1 Heisenberg chains at finite temperatures are presented and compared to a recent inelastic neutron scattering experiment for temperatures T up to 7.5 times the intrachain exchange constant J. The calculations show that the excitations at the antiferromagnetic point q=1 and at q=0.5 remain resonant up to at least T=2J, confirming the recent experimental observation of resonant high-temperature domain wall excitations. The predicted first and second moments are in good agreement with experiment, except at temperatures where three-dimensional spin correlations are most important. The ratio of the structure factors at q=1 and at q=0.5 is well predicted for the paramagnetic infinite-temperature limit. For T=2J, however, we found that the experimentally observed intensity is considerably less than predicted. This suggests that domain wall excitations on different chains interact up to temperatures of the order of the spin band width.Comment: 9 pages revtex, submitted to PR

Substantially enhanced cloning efficiency of SAGE (Serial Analysis of Gene Expression) by adding a heating step to the original protocol

The efficiency of the original SAGE (Serial Analysis of Gene Expression) protocol was limited by a small average size of cloned concatemers. We describe a modification of the technique that overcomes this problem. Ligation of ditags yields concatemers of various sizes. Small concatemers may aggregate and migrate with large ones during gel electrophoresis. A heating step introduced before gel electrophoresis breaks such contaminating aggregates. This modification yields cloned concatemers with an average size of 67 tags as compared to 22 tags by the original protocol. It enhances the length of cloned concatemers substantially and reduces the costs of SAG

Candidate Quantum Spin Liquid in the Ce\textsuperscript{3+} Pyrochlore Stannate Ce2_2Sn2_2O7_7

We report the low temperature magnetic properties of Ce$_2$Sn$_2$O$_7$, a rare-earth pyrochlore. Our susceptibility and magnetization measurements show that due to the thermal isolation of a Kramers doublet ground state, Ce$_2$Sn$_2$O$_7$ has Ising-like magnetic moments of $\sim1.18$ $\mu_\mathrm{B}$. The magnetic moments are confined to the local trigonal axes, as in a spin ice, but the exchange interactions are antiferromagnetic. Below 1 K the system enters a regime with antiferromagnetic correlations. In contrast to predictions for classical $\langle 111 \rangle$-Ising spins on the pyrochlore lattice, there is no sign of long-range ordering down to 0.02 K. Our results suggest that Ce$_2$Sn$_2$O$_7$ features an antiferromagnetic liquid ground state with strong quantum fluctuations.Comment: 6 pages, 4 figure

Magnetically-induced electric polarization in an organo-metallic magnet

The coupling between magnetic order and ferroelectricity has been under intense investigation in a wide range of transition-metal oxides. The strongest coupling is obtained in so-called magnetically-induced multiferroics where ferroelectricity arises directly from magnetic order that breaks inversion symmetry. However, it has been difficult to find non-oxide based materials in which these effects occur. Here we present a study of copper dimethyl sulfoxide dichloride (CDC), an organo-metallic quantum magnet containing $S = 1/2$ Cu spins, in which electric polarization arises from non-collinear magnetic order. We show that the electric polarization can be switched in a stunning hysteretic fashion. Because the magnetic order in CDC is mediated by large organic molecules, our study shows that magnetoelectric interactions can exist in this important class of materials, opening the road to designing magnetoelectrics and multiferroics using large molecules as building blocks. Further, we demonstrate that CDC undergoes a magnetoelectric quantum phase transition where both ferroelectric and magnetic order emerge simultaneously as a function of magnetic field at very low temperatures

Single crystal growth, structure and magnetic properties of Pr2Hf2O7 pyrochlore

Large single crystals of the pyrochlore Pr2Hf2O7 have been successfully grown by the floating zone technique using an optical furnace equipped with high power Xenon arc lamps. Structural investigations have been carried out by both synchrotron X-ray and neutron powder diffraction to establish the crystallographic structure of the materials produced. The magnetic properties of the single crystals have been determined for magnetic fields applied along different crystallographic axes. The results reveal that Pr2Hf2O7 is an interesting material for further investigations as a frustrated magnet. The high quality of the crystals produced make them ideal for detailed investigations, especially those using neutron scattering techniques.Comment: Accepted for publication in J. Phys.: Condens. Matte

Magnetically driven ferroelectric order in Ni3_3V2_2O8_8

We show that for Ni$_3$V$_2$O$_8$ long-range ferroelectric and incommensurate magnetic order appear simultaneously in a single phase transition. The temperature and magnetic field dependence of the spontaneous polarization show a strong coupling between magnetic and ferroelectric orders. We determine the magnetic symmetry of this system by constraining the data to be consistent with Landau theory for continuous phase transitions. This phenomenological theory explains our observation the spontaneous polarization is restricted to lie along the crystal b axis and predicts that the magnitude should be proportional to a magnetic order parameter.Comment: 11 pages, 3 figure

Structural Evolution of One-dimensional Spin Ladder Compounds Sr14-xCaxCu24O41 with Ca doping and Related Hole Redistribution Evidence

Incommensurate crystal structures of spin ladder series Sr14-xCaxCu24O41 (x=3, 7, 11, 12.2) were characterized by powder neutron scattering method and refined using the superspace group Xmmm(00{\gamma})ss0 (equivalent to superspace group Fmmm(0,0,1+{\gamma})ss0); X stands for non-standard centering (0,0,0,0), (0,1/2,1/2,1/2), (1/2,1/2,0,0), (1/2,0,1/2,1/2)) with a modulated structure model. The Ca doping effects on the lattice parameters, atomic displacement, Cu-O distances, Cu-O bond angles and Cu bond valence sum were characterized. The refined results show that the CuO4 planar units in both chain and ladder sublattices become closer to square shape with an increase of Ca doping. The Cu bond valence sum calculation provided new evidence for the charge transfer from the chains to ladders (approximately 0.16 holes per Cu from x=0 to 12.2). The charge transfer was attributed to two different mechanisms: (a) the Cu-O bond distance shrinkage on the ladder; (b) increase of the interaction between two sublattices, resulting in Cu-O bonding between the chains and ladders. The low temperature structural refinement resulted in the similar conclusion, with a slight charge backflow to the chains.Comment: 29 pages, 16 figures, submitted to physics review b, accepte

Coupled SDW and Superconducting Order in FFLO State of CeCoIn5_5

The mechanism of incommensurate (IC) spin-density-wave (SDW) order observed in the Flude-Ferrell-Larkin-Ovchinnikov (FFLO) phase of CeCoIn$_5$ is discussed on the basis of new mode-coupling scheme among IC-SDW order, two superconducting orders of FFLO with B$_{1{\rm g}}$ ($d_{x^{2}-y^{2}}$) symmetry and $\pi$-pairing of odd-parity. Unlike the mode-coupling schemes proposed by Kenzelmann et al, Sciencexpress, 21 August (2008), that proposed in the present Letter can offer a simple explanation for why the IC-SDW order is observed only in FFLO phase and the IC wave vector is rather robust against the magnetic field.Comment: 3pages, 1 figure, accepted for publication in J. Phys. Soc. Jpn., Vol.77 (2008), No.1