Luo Jian-Lin

Superconductivity in Hole-Doped (Sr1−xKx)Fe2As2

A series of layered (Sr1-xKx)Fe2As2 compounds with nominal x = 0-0.40 are synthesized by solid state reaction method. Similar to other parent compounds of iron-based pnictide superconductors, pure SrFe2As2 shows a strong resistivity anomaly near 210 K, which was ascribed to the spin-density-wave instability. The anomaly temperature is much higher than those observed in LaOFeAs and BaFe2As2, the two prototype parent compounds with ZrCuSiAs- and ThCr2Si2-type structures. K-doping strongly suppresses this anomaly and induces superconductivity. Like in the case of K-doped BaFe2As2, sharp superconducting transitions at Tc ~ 38 K is observed. We perform the Hall coefficient measurement, and confirm that the dominant carriers are hole-type. The carrier density is enhanced by a factor of 3 in comparison to F-doped LaOFeAs superconductor.

Element Substitution Effect in Transition Metal Oxypnictide Re(O1-xFx)TAs (Re = rare earth, T = transition metal)

Different element substitution effects in transition metal oxypnictide Re(O1-xFx)TAs, with Re = La, Ce, Nd, Eu, Gd, Tm, T = Fe, Ni, Ru, are studied. Similar to the La- or Ce-based systems, we find that the pure NdOFeAs shows a strong resistivity anomaly near 145 K, which is ascribed to the spin-density-wave instability. Electron doping by F increases Tc to about 50 K. While in the case of Gd, Tc is reduced below 10 K. The tetragonal ZrCuSiAs-type structure could not be formed for Eu or Tm substitution in our preparing process. For the Ni-based case, although both pure and F-doped LaONiAs are superconducting, no superconductivity is found when La is replaced by Ce in both the cases, instead a ferromagnetic ordering transition is likely to form at low temperature in the undoped sample. We also synthesize LaO1-xFxRuAs and CeO1-xFxRuAs compounds. The metallic behaviour is observed down to 4K.

Multiple Nodeless Superconducting Gaps in (Ba0.6K0.4)Fe2As2 Superconductor from Angle-Resolved Photoemission Spectroscopy

Lin Zhao, Haiyun Liu, Wentao Zhang, Jianqiao Meng, Xiaowen Jia, Guodong Liu, Xiaoli Dong, G. F. Chen, J. L. Luo, N. L. Wang, W. Lu, Guiling Wang, Yong Zhou, Yong Zhu, Xiaoyang Wang, Zuyan Xu, Chuangtian Chen, and X. J. Zhou National Laboratory for Superconductivity, Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China Key Laboratory for Optics, Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, China (Dated: July 13, 2008)

Superconductivity Tuned by the Iron Vacancy Order in KxFe2−ySe2

Combining in-depth neutron diffraction and systematic bulk studies, we discover that the √5 × √5 Fe vacancy order, with its associated block antiferromagnetic order, is the ground state with varying occupancy ratios of the iron 16i and vacancy 4d sites across the phase-diagram of KxFe2−ySe2. The orthorhombic order, with one of the four Fe sites vacant, appears only at intermediate temperatures as a competing phase. The material experiences an insulator to metal crossover when the √5 × √5 order is highly developed. Superconductivity occurs in such a metallic phase.

Anomalous Specific Heat Behavior of Nanocrystalline Fe at Low Temperatures

The specific heat of nanocrystalline iron with grain size of 40 nm has been measured in the low temperature range from 1.8 to 26 K. The anomalous specific heat behavior of nanocrystalline iron is compared to that of the standard bulk iron. It is found that the electronic specific heat coefficient γ obtained at low temperatures decreases by about 50%. A T2 term in the heat capacity associated with surface modes has been clearly observed. A large enhancement of the specific heat at temperatures above 16 K may be interpreted as the Einstein oscillator contributions.

Magnetoresistance in Parent Pnictide AFe2As2(A = Sr, Ba)

Magnetoresistances of SrFe2As2 and BaFe2As2 in the magnetic ordered state are studied. Positive magnetoresistance is observed in the magnetic fields H applied in the azimuthes of θ = 0° and 30° with respect to the c-axis. The magnetoresistance can reach 20% for SrFe2As2 and 12% for BaFe2As2 at H = 9 T with θ = 0° (H || c). Above the magnetic transition temperature, the magnetoresistance becomes negligible. The data in the magnetic ordered state could be described by a modified two-band galvanomagnetic model including the enhancement effect of the applied magnetic field on the spin-density-wave gap. The field enhanced spin-density-wave gaps for different types of carriers are different. Temperature dependencies of the fitting parameters are discussed.

Low-temperature specific heat of superconducting MgB2

The specific heat of the recently discovered superconductor MgB2 has been measured at temperatures ranging from 4.5 to 80 K. The superconducting anomaly ΔC at Tc is clearly observed. The total specific heat in the normal state can be well fitted by electronic and phonon contributions. The Debye temperature θD is found to be 737 K, much larger than other intermetallic superconductors. The normal-state electronic specific heat coefficient γ is found to be 2.48±0.5 mJ/molK2 and ΔC/γTc is between 1.41 and 2.15.

31P Nuclear Magnetic Resonance of Charge-Density-Wave Transition in a Single Crystal of RuP*

We perform 31P nuclear magnetic resonance (NMR) measurements on a single crystal of RuP. The anomalies in resistivity at about TA = 270 K and TB = 330 K indicate that two phase transitions occur. The line shape of 31P NMR spectra in different temperature ranges is attributed to the charge density distribution. The Knight shift and spin-lattice relaxation rate 1/T1T are measured from 10 K to 300 K. At about TA = 270 K, they both decrease abruptly with the temperature reduction, which reveals the gap-opening behavior. Well below TA, they act like the case of normal metal. Charge-density-wave phase transition is proposed to interpret the transition occurring at about TA.

75As Nuclear Magnetic Resonance Studies on Ba(Fe1?xNix)2As2 Single Crystals under High Pressure

75As nuclear magnetic resonance (NMR) was measured for Ba(Fe1−xNix)2As2 single crystals with x = 0.05 and x = 0.1 under 0 and 1.5 GPa, respectively. For the optimal doped sample with x = 0.05, the superconducting transition temperature, Tc, is strongly suppressed from 18 to 5 K, while for the over-doped sample with x = 0.1, it is turned from a superconducting ground state to a disordered paramagnetic state under 1.5 GPa. Our experimental results show that the antiferromagnetic spin fluctuations, as well as Tc, are suppressed. The experimental results can be explained with the two-band model. As a result, the electronic band is shifted downwards with an increase in pressure, and the electrons become the dominant carriers in the system.

Superconductivity in Mg-Doped Layered Intermetallic Compound NbB2

We have performed low temperature resistivity ?(T) and specific heat C(T) measurements on a superconducting polycrystalline Nb0.75Mg0.25B2 sample. The results indicate that the superconducting transition temperature is ~ 4.6 K. The zero temperature upper critical field determined from the resistivity and specific heat is 3123 Oe. The electronic coefficient of specific heat ?n = 4.51 mJmol?1K2 and the Debye temperature ?D = 419K are obtained by fitting the zero-field specific heat data in the normal state. At low temperatures, the electronic specific heat in the superconducting state follows Ces/?nTc = 2.84exp(-1.21Tc/T). This indicates that the superconducting pairing in Nb0.75Mg0.25B2 has s-wave symmetry.