Huiqian Luo

Critical fields and anisotropy of NdFeAsO0.82F0.18 single crystals

The newly discovered iron-based superconductors have stimulated enormous interests in the field of superconductivity. Since the new superconductor is a layered system, the anisotropy is a parameter with the first priority to know. Meanwhile any relevant message about the critical fields (upper critical field and irreversibility line) are essentially important. By using flux method, we have successfully grown the single crystals NdO0.82F0.18FeAs at ambient pressure. Resistive measurements reveal a surprising discovery that the anisotropy \Gamma = (mc/mab)^{1/2} is below 5, which is much smaller than the theoretically calculated results. The data measured up to 400 K show a continuing curved feature which prevents a conjectured linear behavior for an unconventional metal. The upper critical fields determined based on the Werthamer-Helfand-Hohenberg formula are H_{c2}^{H||ab}(T=0 K) = 304 T and H_{c2}^{H||c}(T=0 K)=62-70 T, indicating a very encouraging application of the new superconductors.

Superconductivity at 36 K in gadolinium-arsenide oxides GdO 1− x F x FeAs

In this paper we report the fabrication and superconducting properties of GdO1−xFxFeAs. It was found that when x is equal to 0.17, GdO0.83F0.17FeAs is a superconductor with the onset transition temperature Tcon ≈ 36.6 K. Resistivity anomaly near 130 K was observed for all samples up to x = 0.17, and such a phenomenon is similar to that of LaO1−xFxFeAs. Hall coefficient indicates that GdO1−xFxFeAs is conducted by electron-like charge carriers.

Roles of multiband effects and electron-hole asymmetry in the superconductivity and normal-state properties of Ba(Fe(1-x)Cox)(2)As-2

We report a systematic investigation, together with a theoretical analysis, of the resistivity and Hall effect in single crystals of Ba(Fe1-xCox)(2)As-2 over a wide doping range. We find a surprisingly great disparity between the relaxation rates of the holes and the electrons in excess of one order of magnitude in the low-doping, low-temperature regime. The ratio of the electron to hole mobilities diminishes with temperature and doping (away from the magnetically ordered state) and becomes more conventional. We also find a straightforward explanation of the large asymmetry (compared to cuprates) of the superconducting dome: in the underdoped regime the decisive factor is the competition between antiferromagnetism and superconductivity, while in the overdoped regime the main role is played by degradation of the nesting that weakens the pairing interaction. Our results indicate that spin fluctuations due to interband electron-hole scattering play a crucial role not only in the superconducting pairing but also in the normal transport.

Evidence for two energy gaps in superconducting Ba0.6K0.4Fe2As2 single crystals and the breakdown of the Uemura plot.

We report a detailed investigation on the lower critical field Hc1 of the superconducting Ba0.6K0.4Fe2As2 (FeAs-122) single crystals. A pronounced kink is observed on the Hc1(T ) curve, which is attributed to the existence of two superconducting gaps. By fitting the data Hc1(T ) to the two-gap BCS model in full temperature region, a small gap of ∆a(0) = 2.0 ± 0.3 meV and a large gap of ∆b(0) = 8.9± 0.4 meV are obtained. The in-plane penetration depth λab(0) is estimated to be 105 nm corresponding to a rather large superfluid density, which points to the breakdown of the Uemura plot in FeAs-122 superconductors.

Nematic spin correlations in the tetragonal state of uniaxial-strained BaFe2−xNixAs2

Scattering neutrons asymmetrically The crystal structure of solid materials often influences their properties. The more symmetric the structure, the less dependent these properties are on the spatial direction. The superconductors that derive from the compound BaFe2As2 are an exception: Their electronic transport properties can be anisotropic even in the phase where the crystal is symmetric. By scattering neutrons off their samples, Lu et al. found that the magnetic properties of these materials can also be anisotropic. The similar temperature and doping dependence of the anisotropies of both transport and magnetic properties suggests that they may have a common cause. Science, this issue p. 657 Inelastic neutron scattering detects an anisotropy in the spin fluctuations of a family of iron-based superconductors. Understanding the microscopic origins of electronic phases in high-transition temperature (high-Tc) superconductors is important for elucidating the mechanism of superconductivity. In the paramagnetic tetragonal phase of BaFe2−xTxAs2 (where T is Co or Ni) iron pnictides, an in-plane resistivity anisotropy has been observed. Here, we use inelastic neutron scattering to show that low-energy spin excitations in these materials change from fourfold symmetric to twofold symmetric at temperatures corresponding to the onset of the in-plane resistivity anisotropy. Because resistivity and spin excitation anisotropies both vanish near optimal superconductivity, we conclude that they are likely intimately connected.

Growth and characterization of A1-xKxFe2As2 (A = Ba, Sr) single crystals with x = 0-0.4

Single crystals of A1−xKxFe2As2 (A = Ba, Sr) with high quality have been grown successfully by an FeAs self-flux method. The samples have sizes up to 4 mm with flat and shiny surfaces. The x-ray diffraction patterns suggest that they have high crystalline quality and c-axis orientation. The non-superconducting crystals show a spin-density-wave (SDW) instability at about 173 and 135 K for the Sr-based and Ba-based compound, respectively. After doping K as the hole dopant into the BaFe2As2 system, the SDW transition is smeared, and superconducting samples of the compound Ba1−xKxFe2As2 (0<x≤0.4) are obtained. The superconductors, characterized by AC susceptibility and resistivity measurements, exhibit very sharp superconducting transitions at about 36, 32, 27 and 23 K for x = 0.40,0.28,0.25 and 0.23, respectively.

Upper critical field, anisotropy, and superconducting properties of Ba1-xKxFe2As2 single crystals

The temperature-dependent resistivity of Ba1-xKxFe2As2 (x=0.23, 0.25, 0.28, and 0.4) single crystals and the angle-dependent resistivity of superconducting Ba0.6K0.4Fe2As2 single crystals were measured in magnetic fields up to 9 T. The data measured on samples with different doping levels revealed very high upper critical fields, which increase with the transition temperature, and a very low superconducting anisotropy ratio Gamma=H-c2(ab)/H-c2(c)approximate to 2. By scaling the resistivity within the framework of the anisotropic Ginzburg-Landau theory, the angle-dependent resistivity of the Ba0.6K0.4Fe2As2 single crystal measured with different magnetic fields at a certain temperature collapsed onto one curve. As the only scaling parameter, the anisotropy Gamma was alternatively determined for each temperature and was found to be between two and three.

Nature of magnetic excitations in superconducting BaFe1.9Ni0.1As2

An outstanding question about the iron-based superconductors has been whether or not their magnetic characteristics are dominated by itinerant or localized magnetic moments. Absolute measurements and calculations of the magnetic response of undoped and Ni-doped BaFe2As2 indicate the latter.

Doping dependence of spin excitations and its correlations with high-temperature superconductivity in iron pnictides

High-temperature superconductivity in iron pnictides occurs when electrons and holes are doped into their antiferromagnetic parent compounds. Since spin excitations may be responsible for electron pairing and superconductivity, it is important to determine their electron/hole-doping evolution and connection with superconductivity. Here we use inelastic neutron scattering to show that while electron doping to the antiferromagnetic BaFe2As2 parent compound modifies the low-energy spin excitations and their correlation with superconductivity (<50 meV) without affecting the high-energy spin excitations (>100 meV), hole-doping suppresses the high-energy spin excitations and shifts the magnetic spectral weight to low-energies. In addition, our absolute spin susceptibility measurements for the optimally hole-doped iron pnictide reveal that the change in magnetic exchange energy below and above Tc can account for the superconducting condensation energy. These results suggest that high-Tc superconductivity in iron pnictides is associated with both the presence of high-energy spin excitations and a coupling between low-energy spin excitations and itinerant electrons.Meng Wang∗,1 Chenglin Zhang∗,2 Xingye Lu∗,1, 2 Guotai Tan,2 Huiqian Luo,1 Yu Song,2 Miaoyin Wang,2 Xiaotian Zhang,1 E. A. Goremychkin,3 T. G. Perring,3 T. A. Maier,4 Zhiping Yin,5 Kristjan Haule,5 Gabriel Kotliar,5 and Pengcheng Dai2, 1 1Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China 2 Department of Physics and Astronomy, The University of Tennessee, Knoxville, Tennessee 37996-1200, USA 3 ISIS Facility, Rutherford Appleton Laboratory, Chilton, Didcot, Oxfordshire OX11 0QX, UK 4 Center for Nanophase Materials Sciences and Computer Science and Mathematics Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6494, USA 5 Department of Physics, Rutgers University, Piscataway, NJ 08854, USA

Coexistence and Competition of the Short-Range Incommensurate Antiferromagnetic Order with the Superconducting State of BaFe2-xNixAs2

Superconductivity in the iron pnictides develops near antiferromagnetism, and the antiferromagnetic (AF) phase appears to overlap with the superconducting phase in some materials such as BaFe(2-x)T(x)As2 (where T=Co or Ni). Here we use neutron scattering to demonstrate that genuine long-range AF order and superconductivity do not coexist in BaFe(2-x)Ni(x)As2 near optimal superconductivity. In addition, we find a first-order-like AF-to-superconductivity phase transition with no evidence for a magnetic quantum critical point. Instead, the data reveal that incommensurate short-range AF order coexists and competes with superconductivity, where the AF spin correlation length is comparable to the superconducting coherence length.