G. Xu

Improvement of the phase formation and superconductivity of the (Bi, Pb) 2Sr2Ca2Cu3Ox silver-sheathed tapes with B2O3 addition

In (Bi,Pb)2Sr2Ca2Cu3Ox silver-sheathed (Bi2223/Ag) tapes, melted liquid phase plays an important role to form the Bi2223 phase. We have added a small amount of B2O3 into Bi2223/Ag tapes to assist in inducing melted phase because B2O3 has a melting point (460°C) much lower than the general sintering temperature of the tapes, and studied the influence of B2O3 doping on the microstructure and critical current density (Jc) of Bi2223/Ag tapes. The results show that B2O3 doping is really effective to result in the faster growth and better alignment of the Bi2223 grains in the superconducting core and improve the magnetic field dependence of the Jc value.

Electron-hole asymmetry in the superconductivity of doped BaFe2As2 seen via the rigid chemical-potential shift in photoemission

M. Neupane, P. Richard, Y.-M. Xu, K. Nakayama, T. Sato, T. Takahashi, 3 A. V. Federov, G. Xu, X. Dai, Z. Fang, Z. Wang, G.-F. Chen, N.-L. Wang, H.-H. Wen, and H. Ding Department of Physics, Boston College, Chestnut Hill, MA 02467, USA WPI Research Center, Advanced Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan Department of Physics, Tohoku University, Sendai 980-8578, Japan Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, CA 94720 Beijing National Laboratory for Condensed Matter Physics, and Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China Department of Physics, Renmin University of China, Beijing 100872, China (Dated: May 18, 2010)

Infrared spectrum and its implications for the electronic structure of the semiconducting iron selenide K0.83Fe1.53Se2

We report an infrared spectroscopy study on K0.83Fe1.53Se2, a semiconducting parent compound of the 245 iron selenide superconductors. The major spectral features are found to be distinctly different from all other Fe-based superconducting systems. Our measurement revealed two peculiar spectral structures: a double-peak structure between 4000 and 6000 cm(-1) and abundant phonon modes-much more than expected for a 122 structure. We elaborate that those features could be naturally explained from the blocked antiferromagnetism due to the presence of Fe vacancy ordering as determined by recent neutron-diffraction experiments. The double peaks reflect the coexistence of ferromagnetic and antiferromagnetic couplings between the neighboring Fe sites.

Effects of Ru substitution on electron correlations and Fermi-surface dimensionality in Ba(Fe1-xRux)(2)As-2

N. Xu, T. Qian, P. Richard, Y.-B. Shi, X.-P. Wang, P. Zhang, Y.-B. Huang, Y.-M. Xu, H. Miao, G. Xu, G.-F. Xuan, W.-H. Jiao, Z.-A. Xu, G.-H. Cao, H. Ding,1∗ Beijing National Laboratory for Condensed Matter Physics, and Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA and State Key Lab of Silicon Materials and Department of Physics, Zhejiang University, Hangzhou 310027, China (Dated: May 5, 2014)

Field-induced spin-flop transitions in single-crystalline CaCo2As2

CaCo2As2, a ThCr2Si2-structure compound, undergoes an antiferromagnetic transition at T-N = 76 K with the magnetic moments being aligned parallel to the c axis. Electronic transport measurement reveals that the coupling between conducting carriers and magnetic order in CaCo2As2 is much weaker compared to the parent compounds of iron pnictide. Applying magnetic field along the c axis induces two successive spin-flop transitions in its magnetic state. The magnetization saturation behaviors with H parallel to c and H parallel to ab at 10 K indicate that the antiferromagnetic coupling along the c direction is very weak. The interlayer antiferromagntic coupling constant J(c) is estimated to be about 2 meV.

The resonant tunneling through a graphene multiquantum well system

The transport properties of a graphene multiquantum well system are investigated numerically using transfer-matrix method. There are transmission gaps for electrons and holes in the transmission spectra at tilted incidence. In the transmission gaps, a few resonant tunneling peaks appear, defined as transmission windows, which are related with the bound states in the quantum wells. Unlike conventional semiconductor nanostructures, the location and the width of the transmission windows are sensitive not only to the quantum well width but also the incident angle. The number of the quantum wells determines the fine structure of the transmission windows. The anisotropic property is affected in the following way: the increase in well width makes the nonzero-transmission incident angle range decrease, and the interference effect is enhanced as the well number increases. Tiny oscillation of the conductance and fine structures in the middle energy range are due to the resonant tunneling induced by the multiquantum...

Electronic properties of 3d transitional metal pnictides: A comparative study by optical spectroscopy

Single-crystalline KFe2As2 and CaT2As2 (T = Fe, Co, Ni, and Cu) are synthesized and investigated by resistivity, susceptibility, and optical spectroscopy. It is found that CaCu2As2 exhibits a similar transition to the lattice abrupt collapse transitions discovered in CaFe2(As1-xPx)(2) and Ca1-xRx Fe2As2 (R = rare-earth element). The resistivity of KFe2As2 and CaT2As2 (T = Fe, Co, Ni, and Cu) approximately follows the similar T-2 dependence at low temperature, but the magnetic behaviors vary with different samples. Optical measurement reveals that the optical response of CaCu2As2 is not sensitive to the transition at 50 K, with no indication of development of a new energy gap below the transition temperature. Using Drude-Lorentz model, we find that two Drude terms, a coherent one and an incoherent one, can fit the low-energy optical conductivity of KFe2As2 and CaT2As2 (T = Fe, Co, and Ni) very well. However, in CaCu2As2, which is a sp-band metal, the low-energy optical conductivity can be well described by a coherent Drude term. Lack of the incoherent Drude term in CaCu2As2 may be attributed to a weaker electronic correlation than in KFe2As2 and CaT2As2 (T = Fe, Co, and Ni). Spectral weight analysis of these samples indicates that the unconventional spectral weight transfer, which is related to Hunds coupling energy J(H,) is only observed in iron pnictides, supporting the viewpoint that J(H) may be a key clue in the search for the mechanism of magnetism and superconductivity in pnictides.

Single-crystal growth and optical conductivity of SrPt2As2 superconductors

SrPt2As2 single crystals with CaBe2Ge2-type structure were synthesized by self-melting technique. X-ray diffraction, transmission electron microscopy, electrical resistivity, magnetic susceptibility, specific heat, and optical spectroscopy measurements were conducted to elucidate the properties of SrPt2As2. SrPt2As2 single crystals undergo a structural phase transition well above room temperature (about 450 K) and become superconducting at 5.18 K. The superconducting and structural phase transition temperatures are reduced by 6% iridium doping. Both the pure SrPt2As2 and the Ir-doped single crystals are demonstrated to be highly metallic with rather high plasma frequencies. In particular, the optical spectroscopy measurement revealed two gaplike suppression structures: a stronger one at high energy near 12 000 cm(-1) (similar to 1.5 eV), and a less prominent one at lower energy near 3200 cm(-1) (similar to 0.4 eV) for the pure compound. We elaborate that the former is related to the correlation effect, while the latter could be attributed to the partial energy gap formation associated with structural phase transition.

Continuous magnetic phase transition in half-frustrated Ca2Os2O7

We present the specific heat, magnetization, optical spectroscopy measurements, and the first-principles calculations on the Weberite structure Ca2Os2O7 single-crystal/polycrystalline sample. The Ca2Os2O7 shows a Curie-Weiss nature at high temperature and goes into a ferrimagnetic insulating state at 327 K on cooling. A lambda-like peak is observed at 327 K in the specific heat implying a second-order phase transition. The vanishing electronic specific heat at low temperature suggests a full energy gap. At high temperature above the transition, a small amount of itinerant carriers with short life time tau is observed, which is gapped at 20 K with a direct gap of 0.24 eV. Our first-principles calculations indicate that the antiferromagnetic (AFM) correlation with intermediate Coulomb repulsion U could effectively split Os(4b) t(2g) bands and push them away from Fermi level (E-F). On the other hand, a noncollinear magnetic interaction is needed to push the Os(4c) bands away from E-F, which could be induced by Os(4c)-Os(4c) frustration. Therefore, AFM correlation, Coulomb repulsion U and noncollinear interaction all play important roles for the insulating ground state in Ca2Os2O7.

Three-dimensionality of band structure and a large residual quasiparticle population in Ba 0 . 67 K 0 . 33 Fe 2 As 2 as revealed by c -axis polarized optical measurements

We report on a c-axis polarized optical measurement on a Ba0.67K0.33Fe2As2 single crystal. We find that the c-axis optical response is significantly different from that of high-T-c cuprates. The experiments reveal an anisotropic three-dimensional (3D) optical response with the absence of the Josephson plasma edge in R(omega) in the superconducting state. Furthermore, different from the ab-plane optical response, a large residual quasiparticle population down to T similar to 1/5T(c) was observed in the c-axis polarized reflectance measurement. We elaborate that there exist horizontal nodes for the superconducting gap in regions of the 3D Fermi surface that contribute dominantly to the c-axis optical conductivity.