Yixi Su

Publications of Yixi Su

• Higgs transition from magnetic Coulomb liquid to ferromagnet in Yb_2Ti_2O_7

  Authors: Lieh-Jeng Chang, Shigeki Onoda, Yixi Su, Ying-Jer Kao, Ku-Ding Tsuei, Yukio Yasui, Kazuhisa Kakurai, Martin Richard Lees

  11/2011;

  Emergent magnetic monopoles have recently been identified in many condensed-matter systems. In particular, experiments on dipolar spin ice have established a magnetic Coulomb phase where monopolesEmergent magnetic monopoles have recently been identified in many condensed-matter systems. In particular, experiments on dipolar spin ice have established a magnetic Coulomb phase where monopoles interact through a magnetic Coulomb interaction. With quantum-mechanical interactions, these magnetic charges can be carried by fractionalized bosonic quasi-particles, spinons, which can undergo Bose-Einstein condensation via the Higgs mechanismHere, we report evidence of a transition from a magnetic Coulomb liquid to a ferromagnet in single-crystal Yb_2Ti_2O_7. Polarised neutron-scattering experiments show that the diffuse [111]-rod scattering whose intensity grows with decreasing temperature is suddenly suppressed below T_C ~ 0.21 K, where magnetic Bragg peaks and a full depolarisation of neutron spins are observed with thermal hysteresis, indicating a first-order ferromagnetic transition. Our results are explained from a quantum spin-ice modelwhose high-temperature phase belongs to the magnetic Coulomb phase, while the ground state shows a nearly collinear ferromagnetism with gapped spin excitations.

• Suppression of superconductivity by V-doping and possible magnetic order in Sr2VO3FeAs

  Authors: Marcus Tegel, Tanja Schmid, Tobias Stürzer, Masamitsu Egawa, Yixi Su, Anatoliy Senyshyn, Dirk Johrendt

  08/2010;

  Superconductivity at 33 K in Sr2VO3FeAs is completely suppressed by small amounts of V-doping in Sr2VO3[Fe0.93(+/-0.01)V0.07(+/-0.01)]As. The crystal structures and exact stoichiometries areSuperconductivity at 33 K in Sr2VO3FeAs is completely suppressed by small amounts of V-doping in Sr2VO3[Fe0.93(+/-0.01)V0.07(+/-0.01)]As. The crystal structures and exact stoichiometries are determined by combined neutron- and x-ray powder diffraction. Sr2VO3FeAs is shown to be very sensitive to Fe/V mixing, which interferes with or even suppresses superconductivity. This inhomogeneity may be intrinsic and explains scattered reports regarding Tc and reduced superconducting phase fractions in Sr2VO3FeAs. Neutron diffraction data collected at 4 K indicates incommensurate mag- netic ordering of the V-sublattice with a propagation vector q = (0,0,0.306). This suggests strongly correlated vanadium, which does not contribute significantly to the Fermi surface of Sr2VO3FeAs. Comment: text revised, magnetic q-vector added, one reference added 4 pages, 4 figures

• Magnetic lattice dynamics of the oxygen-free FeAs pnictides: how sensitive are phonons to magnetic ordering?

  Authors: Mohamed Zbiri, Ranjan Mittal, Stéphane Rols, Yixi Su, Yinguo Xiao, Helmut Schober, Samrath L Chaplot, Mark R Johnson, Tapan Chatterji, Yasunori Inoue, Satoru Matsuishi, Hideo Hosono, Thomas Brueckel

  Journal of physics. Condensed matter : an Institute of Physics journal. 08/2010; 22(31):315701.

  To shed light on the role of magnetism on the superconducting mechanism of the oxygen-free FeAs pnictides, we investigate the effect of magnetic ordering on phonon dynamics in the low-temperatureTo shed light on the role of magnetism on the superconducting mechanism of the oxygen-free FeAs pnictides, we investigate the effect of magnetic ordering on phonon dynamics in the low-temperature orthorhombic parent compounds, which present a spin density wave. The study covers both the 122 (AFe(2)As(2); A = Ca, Sr, Ba) and 1111 (AFeAsF; A = Ca, Sr) phases. We extend our recent work on the Ca (122 and 1111) and Ba (122) cases by treating, computationally and experimentally, the 122 and 1111 Sr compounds. The effect of magnetic ordering is investigated through detailed non-magnetic and magnetic lattice dynamical calculations. The comparison of the experimental and calculated phonon spectra shows that the magnetic interactions/ordering have to be included in order to reproduce well the measured density of states. This highlights a spin-correlated phonon behavior which is more pronounced than the apparently weak electron-phonon coupling estimated in these materials. Furthermore, there is no noticeable difference between phonon spectra of the 122 Ba and Sr, whereas there are substantial differences when comparing these to CaFe(2)As(2) originating from different aspects of structure and bonding.

• Non-stoichiometry and the magnetic structure of Sr2CrO3FeAs

  Authors: Marcus Tegel, Franziska Hummel, Yixi Su, Tapan Chatterji, Michela Brunelli, Dirk Johrendt

  11/2009;

  The iron arsenide Sr2CrO3FeAs with the tetragonal Sr2GaO3CuS-type structure was synthesized and its crystal structure re-determined by neutron powder diffraction. In contrast to previous X-rayThe iron arsenide Sr2CrO3FeAs with the tetragonal Sr2GaO3CuS-type structure was synthesized and its crystal structure re-determined by neutron powder diffraction. In contrast to previous X-ray crystallographic studies, a mixed occupancy of chromium and iron was found within the FeAs4/4 layer (93+/-1% Fe : 7+/-1% Cr). We suggest that the partial Cr-doping at the Fe site is the reason for the absence of a spin-density wave anomaly and superconductivity in this compound. Additional experiments via neutron polarization analysis revealed short-range spin correlations below ~100 K and long-range antiferromagnetic ordering below T_N = 36 K with a magnetic propagation vector of q = (1/2, 1/2, 0). The Cr3+ ions form a collinear magnetic structure of the C-type in the magnetic space group C_Pmma' (a' = a-b, b'=a+b, c'=c), where Cr3+-ions occupy the 4g (0, 1/4, z) Wyckoff position. The magnetic moments are aligned along the orthorhombic a'-axis. At 3.5 K, an ordered magnetic moment of 2.75+/-0.05 mu_B for the Cr3+-sublattice was refined. Comment: 6 pages, 6 figures

• EuFe_2As_2 magnetic structure determination

  Authors: Javier Herrero-Martin, Valerio Scagnoli, Claudio Mazzoli, Yixi Su, Ranjal Mittal, Y Xiao, Th. Brückel, Neeraj Kumar, S. K. Dhar, A. Thamizhavel, Luigi Paolasini

  06/2009;

  Magnetic resonant X ray scattering experiments have been performed on a single crystal of EuFe_2As_2 at the Eu L_3 absorption edge. This has permitted us to determine for the first time theMagnetic resonant X ray scattering experiments have been performed on a single crystal of EuFe_2As_2 at the Eu L_3 absorption edge. This has permitted us to determine for the first time the orientation of Eu magnetic moments in the antiferromagnetic ordered phase below T_N=19 K as parallel to the a-axis. In addition, non-resonant magnetic X ray measurements indicate that Fe magnetic moments align along the same direction in the spin density wave phase (T<T_S~190 K). Temperature dependences indicate that the onset of the Eu AFM phase barely affects the Fe magnetic sublattice. Comment: accepted for publication in Phys. Rev. B