By substituting Fe with the 5d-transition metal Pt in BaFe2As2, we have successfully synthesized the superconductors BaFe2−xPtxAs2. The systematic evolution of the lattice constants indicates that the Fe ions were successfully replaced by Pt ions. By increasing the doping content of Pt, the antiferromagnetic order and structural transition of the parent phase is suppressed and superconduc-tivity emerges at a doping level of about x = 0.02. At a doping level of x = 0.1, we get a maximum transition temperature Tc of about 25 K. While even for this optimally doped sample, the residual resistivity ratio (RRR) is only about 1.35, indicating a strong impurity scattering effect. We thus argue that the doping to the Fe-sites naturally leads to a high level impurity scattering, although the superconductivity can still survive at about 25 K. The synchrotron powder x-ray diffraction shows that the resistivity anomaly is in good agreement with the structural transition. The super-conducting transitions at different magnetic fields were also measured at the doping level of about x = 0.1, yielding a slope of-dHc2/dT = 5.4 T/K near Tc. Finally a phase diagram was established for the Pt doped 122 system. Our results suggest that superconductivity can also be easily induced in the FeAs family by substituting the Fe with Pt, with almost the similar maximum transition temperatures as doping Ni, Co, Rh and Ir.

We use inelastic neutron scattering to study magnetic excitations of the FeAs-based superconductor BaFe1.9Ni0.1As2 above and below its superconducting transition temperature Tc = 20 K. In addition to gradually open a spin gap at the in-plane antiferromagnetic ordering wavevector (1, 0, 0), the effect of superconductivity is to form a three dimensional resonance with clear dispersion along the c-axis direction. The intensity of the resonance develops like a superconducting order parameter, and the mode occurs at distinctively different energies at (1, 0, 0) and (1, 0, 1). If the resonance energy is directly associated with the superconducting gap energy ∆, then ∆ is dependent on the wavevector transfers along the c-axis. These results suggest that one must be careful in interpreting the superconducting gap energies obtained by surface sensitive probes such as scanning tunneling microscopy and angle resolved photoemission.PACS numbers: 74.25. Ha, 78.70.Nx Understanding the interplay between spin fluctuations and superconductivity in high-transition-temperature (high-T c ) superconductors is important because spin fluctuations may mediate electron pairing for superconductivity [1,2]. In the case of high-T c copper oxides, it is now well documented that the spin fluctuation spectrum is dominated by a collective excitation known as the resonance mode centered at the antiferromagnetic (AF) ordering wavevector Q = (1/2, 1/2) [3,4,5,6,7,8]. Although the intensity of the mode behaves like an order parameter below T c , the energy of the mode is dispersionless for wavevector transfers along the c-axis and directly tracks T c [4,5,6,7,8], thus suggesting that the mode is an intrinsic property of the two-dimensional (2D) CuO 2 planes and intimately associated with superconductivity. For FeAs-based superconductors [9,10,11,12], the presence of static AF ordering in their parent compounds (with spin structure of Fig. 1a) [13,14,15,16,17,18] and the remarkable similar doping dependent phase diagram to that of the high-T c copper oxides [15] suggest that AF spin fluctuations may also play an important role in the superconductivity of these materials. Indeed, recent neutron scattering measurements on spin fluctuations of powder samples of superconducting Ba 0.6 K 0.4 Fe 2 As 2 (T c = 38 K) [19] and crystalline electric field (CEF) excitations of Ce in CeFeAsO 0.84 F 0.16 (T c = 41 K) [20] found clear evidence for resonant-like magnetic intensity gain below T c athω ∼ 14 and 18.7 meV, respectively. However, the Ce CEF measurements give no information on the Q-dependence of the scattering [20]. Although the resonant-like scattering in Ba 0.6 K 0.4 Fe 2 As 2 occurs near the AF ordering wavevector, the powder nature of the experiment impedes to distinguish whether the resonant scattering is centered at the three-dimensional (3D) AF wavevector Q = (1, 0, 1) of its parent compound [16,17,18] or simply at a 2D AF in-plane wavevector Q = (1, 0, 0) [19].In this Letter, we report the results of inelastic neutron scattering studies of s...

The discovery of high-temperature superconductivity in iron pnictides raised the possibility of an unconventional superconducting mechanism in multiband materials. The observation of Fermisurface (FS)-dependent nodeless superconducting gaps suggested that inter-FS interactions may play a crucial role in superconducting pairing. In the optimally hole-doped Ba0.6K0.4Fe2As2, the pairing strength is enhanced simultaneously (2⌬/TcϷ7) on the nearly nested FS pockets, i.e., the inner hole-like (␣) FS and the 2 hybridized electron-like FSs, whereas the pairing remains weak (2⌬/ TcϷ3.6) in the poorly nested outer hole-like (␤) FS. Here, we report that in the electron-doped BaFe1.85Co0.15As2, the FS nesting condition switches from the ␣ to the ␤ FS due to the opposite size changes for hole-and electron-like FSs upon electron doping. The strong pairing strength (2⌬/TcϷ6) is also found to switch to the nested ␤ FS, indicating an intimate connection between FS nesting and superconducting pairing, and strongly supporting the inter-FS pairing mechanism in the iron-based superconductors.angle-resolved photoemission ͉ band structure ͉ iron pnictide ͉ superconductivity I n charge-doped superconductors, such as copper oxides (cuprates), electron or hole doping may influence the superconducting (SC) properties differently (1, 2). As an example, angle-resolved photoemission spectroscopy (3) (ARPES) and Raman scattering (4) revealed a nonmonotonic behavior in the SC gap function of the electron-doped cuprates that is different from the simple dx 2 -y 2 -wave function observed in the hole-doped cuprates (5). On the other hand, in the new Fe-based superconductors (6-9), no direct comparison of the SC order parameter has been made between hole-and electron-doped systems. ARPES studies on hole-doped Ba 1-x K x Fe 2 As 2 have observed isotropic gaps that have different values on different Fermi surfaces (FSs) with strong pairing occurring on the nearly nested FS pockets (10-13). Thus, it is particularly important to conduct a comparison of the SC gaps and their FS dependence of an electron-doped pnictide. We have chosen BaFe 1.85 Co 0.15 As 2 , which is optimally electron doped (14) with the same crystal structure as the Ba 1-x K x Fe 2 As 2 system (9). ResultsFig . 1A and B show ARPES intensity plots of BaFe 1.85 Co 0.15 As 2 (T c ϭ 25.5 K) as a function of binding energy and momentum (k) along 2 high-symmetry lines in the Brillouin zone (BZ). We observe a hole-like dispersion centered at the ⌫ point and 2 electron-like FSs near the M point. Even though a reasonable agreement is found between experiment and renormalized band calculations (15), some experimental features such as the energy position of the 0.2 eV band at the ⌫ point and the bottom of the electron band at the M point, are not well reproduced by band calculations. This suggests a possible orbital and k dependence of the mass-renormalization factor. Fig. 1C shows the ARPES intensity at the Fermi level (E F ) plotted as a function of the in-plane wave vector. A circular and an...

Following the discovery of superconductivity in quasi-one-dimensional K2Cr3As3 containing [(Cr3As3) 2− ]∞ chains [J. K. Bao et al., arXiv: 1412.0067 (2014], we succeeded in synthesizing an analogous compound, Rb2Cr3As3, which also crystallizes in a hexagonal lattice. The replacement of K by Rb results in an expansion of a axis by 3%, indicating a weaker interchain coupling in Rb2Cr3As3. Bulk superconductivity emerges at 4.8 K, above which the normal-state resistivity shows a linear temperature dependence up to 35 K. The estimated upper critical field at zero temperature exceeds the Pauli paramagnetic limit by a factor of two. Furthermore, the electronic specific-heat coefficient extrapolated to zero temperature in the mixed state increases with √ H, suggesting existence of nodes in the superconducting energy gap. Hence Rb2Cr3As3 manifests itself as another example of unconventional superconductor in the Cr3As3-chain based system.