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.

We report the discovery of bulk superconductivity (SC) at 6.1 K in a quasi-one-dimensional (Q1D) chromium pnictide K2Cr3As3 which contains [(Cr3As3) 2− ]∞ double-walled subnano-tubes with face-sharing Cr 6/2 (As 6/2 ) octahedron linear chains in the inner (outer) wall. The material has a large electronic specific-heat coefficient of 70∼75 mJ K −2 mol −1 , indicating significantly strong electron correlations. Signature of non-Fermi liquid behavior is shown by the linear temperature dependence of resistivity in a broad temperature range from 7 to 300 K. Unconventional SC is preliminarily manifested by the estimated upper critical field exceeding the Pauli limit by a factor of three to four. The title compound represents a rare example that possibly unconventional SC emerges in a Q1D system with strong electron correlations.

Infrared reflectivity measurements on several 122 iron pnictides reveal the existence of two electronic subsystems. The one gapped due to the spin-density-wave transition in the parent materials, such as EuFe 2 As 2 , is responsible for superconductivity in the doped compounds, such as Ba͑Fe 0.92 Co 0.08 ͒ 2 As 2 and Ba͑Fe 0.95 Ni 0.05 ͒ 2 As 2 . Analyzing the dc resistivity and scattering rate of this contribution, a hidden T 2 dependence is found in the normal state. The second subsystem gives rise to incoherent background, present in all 122 compounds, which is basically temperature independent but affected by the superconducting transition.

We discover a robust coexistence of superconductivity and ferromagnetism in an iron arsenide RbEuFe4As4. The new material crystallizes in an intergrowth structure of RbFe2As2 and EuFe2As2, such that the Eu sublattice turns out to be primitive instead of being body-centered in EuFe2As2. The FeAs layers, featured by asymmetric As coordinations, are hole doped due to charge homogenization. Our combined measurements of electrical transport, magnetization and heat capacity unambiguously and consistently indicate bulk superconductivity at 36.5 K in the FeAs layers and ferromagnetism at 15 K in the Eu sublattice. Interestingly, the Eu-spin ferromagnetic ordering belongs to a rare third-order transition, according to the Ehrenfest classification of phase transition. We also identify an additional anomaly at ∼ 5 K, which is possibly associated with the interplay between superconductivity and ferromagnetism.further revised the electronic phase diagram because of the discovery of a reentrant spin glass state. Recent x-ray resonant magnetic scattering[3] and neutron scattering[4] experiments however indicated long-range ferromagnetic orderings for Eu spins in superconducting EuFe 2 (As 1−x P x ) 2 with x = 0.19 and 0.15, respectively. It was demonstrated that the Eu spins align exactly along the c axis, in contradiction to the spin-canting scenario. So far, this discrepancy remains unresolved. Note that the spin-tilting angle (∼20 • from the c axis, as detected by Mössbauer measurements[2]) coincides with the direction that connects the interlayer next-nearest (NN) Eu atoms because of the body-centered Eu sublattice. To clarify whether the Eu-sublattice type is relevant to Eu spin orientations, it is desirable to study a related material system in which Eu atoms form a primitive tetragonal lattice.Local-moment FM and spin-singlet SC are known to be mutually incompatible [20][21][22], which makes their coexistence (hereafter abbreviated as FM+SC) very rare [23]. The FM+SC phenomenon observed in FeSCs has been ascribed to the multi-orbital character as well as the robustness of superconductivity against magnetic fields [10,24]. On the one hand, the zero-temperature upper critical magnetic field, H c2 (0), of FeSCs is typically higher than 50 T [25,26], which is large enough to fight the internal exchange field that is comparable to the hyperfine field on the Eu nucleus (∼ 25 T) [2]. On the other hand, the Eu-spin FM can be satisfied even in the presence of SC, because the Fe-3d multi-orbitals enable both superconducting pairing (dominated by the d yz and d zx electrons [27]) and the Ruderman-Kittel-Kasuya-Yosida (RKKY) exchange interaction between Eu local moments. The RKKY interaction can be mediated arXiv:1605.04396v3 [cond-mat.supr-con]

We report ^{75}As NMR measurements on the new quasi-one-dimensional superconductor K_{2}Cr_{3}As_{3} (T_{c}∼6.1  K) [J. K. Bao et al., Phys. Rev. X 5, 011013 (2015)]. We found evidence for strong enhancement of Cr spin fluctuations above T_{c} in the [Cr_{3}As_{3}]_{∞} double-walled subnanotubes based on the nuclear spin-lattice relaxation rate 1/T_{1}. The power-law temperature dependence, 1/T_{1}T∼T^{-γ} (γ∼0.25), is consistent with the Tomonaga-Luttinger liquid. Moreover, absence of the Hebel-Slichter coherence peak of 1/T_{1} just below T_{c} suggests an unconventional nature of superconductivity.