arXiv subjects

Y. Su

Publications and source records attributed to Y. Su.

97 records · Page 6Linked to original sources

arXiv2008

Neutron diffraction on antiferromagnetic ordering in single-crystal BaFe2As2

Neutron diffraction experiments have been carried out on a Sn-flux grown BaFe2As2 single crystal, the parent compound of the A-122 family of FeAs-based high-Tc superconductors. A tetragonal to orthorhombic structural phase transition and a three dimensional long-range antiferromagnetic ordering of the iron moment, with a unique magnetic propagation wavevector k = (1, 0, 1), have been found to take place at ~90 K. The magnetic moments of iron are aligned along the long a axis in the low temperature orthorhombic phase (Fmmm with b<a<c). Our results thus demonstrate that the magnetic structure of BaFe2As2 single crystal is the same as those in other A-122 iron pnictides compounds. We argue that the tin incorporation in the lattice is responsible for a smaller orthorhombic splitting and lower Neel temperature T_N observed in the experiment.

cond-mat.supr-con↗

arXiv2008

Magnetic order in CaFe1-xCoxAsF (x = 0, 0.06, 0.12) superconductor compounds

A Neutron Powder Diffraction (NPD) experiment has been performed to investigate the structural phase transition and magnetic order in CaFe1-xCoxAsF superconductor compounds (x = 0, 0.06, 0.12). The parent compound CaFeAsF undergoes a tetragonal to orthorhombic phase transition at 134(3) K, while the magnetic order in form of a spin-density wave (SDW) sets in at 114(3) K. The antiferromagnetic structure of the parent compound has been determined with a unique propagation vector k = (1,0,1) and the Fe saturation moment of 0.49(5)uB aligned along the long a-axis. With increasing Co doping, the long range antiferromagnetic order has been observed to coexist with superconductivity in the orthorhombic phase of the underdoped CaFe0.94Co0.06AsF with a reduced Fe moment (0.15(5)uB). Magnetic order is completely suppressed in optimally doped CaFe0.88Co0.12AsF. We argue that the coexistence of SDW and superconductivity might be related to mesoscopic phase separation.

cond-mat.supr-con↗

arXiv2008

Phonon dynamics in Sr0.6K0.4Fe2As2 and Ca0.6Na0.4Fe2As2

We report inelastic neutron scattering measurements of the phonon density-of-states in superconducting Sr0.6K0.4Fe2As2 (Tc=30 K) and Ca0.6Na0.4Fe2As2 (Tc=18 K). Compared with the parent compound BaFe2As2 doping affects mainly the lower and intermediate frequency part of the vibrations. Mass effects and lattice contraction cannot explain these changes. A lattice dynamical model has been used to identify the character of the various phonon bands. Softening of phonon modes below 10 meV has been observed in both samples on cooling from 300 K to 140 K. In the Ca doped compound the softening amounts to about 1 meV while for the Sr doped compound the softening is about 0.5 meV. There is no appreciable change in the phonon density of states when crossing Tc.

cond-mat.supr-con↗

arXiv2008

Inelastic neutron scattering and lattice dynamical calculations in BaFe2As2

We report here first extensive measurements of the temperature dependence of phonon density of states of BaFe2As2, the parent compound of the newly discovered FeAs-based superconductors, using inelastic neutron scattering. The experiments were carried out on the thermal time-of-flight neutron spectrometer IN4 at the ILL on a polycrystalline sample. There is no appreciable change in the spectra between T = 10 K and 200 K, although the sample undergoes a magnetic as well as a tetragonal-to-orthorhombic structural phase transition at 140 K. This indicates a rather harmonic phonon system. Shell model lattice dynamical calculations based on interatomic potentials are carried out to characterize the phonon data. The calculations predict a shift of the Ba-phonons to higher energies at 4 GPa. The average energy of the phonons of the Ba-sublattice is also predicted to increase on partial substitution of Ba by K to Ba0.6K0.4. The calculations show good agreement with the experimental phonon spectra, and also with the specific heat data from the literature.

cond-mat.supr-con↗

arXiv2008

Lattice dynamical calculations of infinite layer iron oxides SrFeO2 and CaFeO2

We report extensive lattice dynamical calculations of the newly discovered infinite-layer iron oxides SrFeO2 and CaFeO2. For SrFeO2, the parameters of the interatomic potential have been determined to reproduce the zone-centre phonon frequencies reported using ab-initio calculations. Further we have extended the potential model for calculations of CaFeO2. The potential parameters are found to be transferable between the two compounds, and are used to calculate the phonon spectra in the whole Brillouin zone and several thermodynamic properties for these compounds. The calculations show fair agreement with the available experimental data of structure, thermal expansion, and mean-squared amplitudes of the atoms.

cond-mat.mtrl-sci↗

arXiv2004

Linear Accelerating Superluminal Motion Model

Accelerating superluminal motions were detected recently by multi-epoch Very Long Baseline Interferometry (VLBI) observations. Here, a Linear Accelerating Superluminal Motion (LASM) model is proposed to interpret the observed phenomena. The model provides a direct and accurate way to estimate the viewing angle of a relativistic jet. It also predicts that both Doppler boosting and deboosting effects may take place in an accelerating forward jet. The LASM model is applied to the data of the quasar 3C 273, and the initial velocity, acceleration and viewing angle of its three components are derived through model fits. The variations of the viewing angle suggest that a supermassive black hole binary system may exist in the center of 3C273. The gap between the inner and outer jet in some radio loud AGNs my be explained in terms of Doppler deboosting effects when the components accelerate to ultra-relativistic speed.

astro-ph↗

arXiv2001

Ge(001)-(2 1 0 3)-Pb <-> (2 1 0 6)-Pb: low-temperature two-dimensional phase transition

The Ge(001)-(2 1 0 3)-Pb surface reconstruction with a lead coverage of 5/3 monolayer is on the borderline between the low-coverage covalently-bonded and high-coverage metallic lead overlayers. This gives rise to an unusual low-temperature phase transition with concomitant changes in the bonding configuration. Both the room-temperature and low-temperature phases of this system were investigated by surface x-ray diffraction using synchrotron radiation. The room-temperature Ge(001)-(2 1 0 3) phase is best described by a model with dynamically flipping germanium dimers underneath a distorted Pb(111) overlayer with predominantly metallic properties. In the low-temperature Ge(001)-(2 1 0 6) phase the dimers are static and the interaction between adsorbate and substrate and within the adsorbate is stronger than at room temperature. These results suggest that the phase transition is of order-disorder type.

cond-mat.mtrl-sci↗