The normal-state charge transport is studied systematically in high-quality single crystals of BaFe2(As1−xPx)2 (0 ≤ x ≤ 0.71). By substituting isovalent P for As, the spin-density-wave (SDW) state is suppressed and the dome-shaped superconducting phase (Tc 31 K) appears. Near the SDW end point (x ≈ 0.3), we observe striking linear temperature (T ) dependence of resistivity in a wide T -range, and remarkable low-T enhancement of Hall coefficient magnitude from the carrier number estimates. We also find that the magnetoresistance apparently violates the Kohler's rule and is well scaled by the Hall angle ΘH as ∆ρxx/ρxx ∝ tan 2 ΘH . These non-Fermi liquid transport anomalies cannot be attributed to the simple multiband effects. These results capture universal features of correlated electron systems in the presence of strong antiferromagnetic fluctuations.

A second-order phase transition is characterized by spontaneous symmetry breaking. The nature of the broken symmetry in the so-called "hidden-order" phase transition in the heavy-fermion compound URu(2)Si(2), at transition temperature T(h) = 17.5 K, has posed a long-standing mystery. We report the emergence of an in-plane anisotropy of the magnetic susceptibility below T(h), which breaks the four-fold rotational symmetry of the tetragonal URu(2)Si(2). Two-fold oscillations in the magnetic torque under in-plane field rotation were sensitively detected in small pure crystals. Our findings suggest that the hidden-order phase is an electronic "nematic" phase, a translationally invariant metallic phase with spontaneous breaking of rotational symmetry.

Among the iron-based pnictide superconductors the material KFe2As2 is unusual in that its Fermi surface does not consist of quasi-nested electron and hole pockets. Here we report measurements of the temperature dependent London penetration depth of very clean crystals of this compound with residual resistivity ratio > 1200. We show that the superfluid density at low temperatures exhibits a strong linear-in-temperature dependence which implies that there are line nodes in the energy gap on the large zone-centered hole sheets. The results indicate that KFe2As2 is an unconventional superconductor with strong electron correlations.

In-plane microwave penetration depth λ ab and quaiparticle conductivity at 28 GHz are measured in underdoped single crystals of the Fe-based superconductor PrFeAsO1−y (Tc ≈ 35 K) by using a sensitive superconducting cavity resonator. λ ab (T ) shows flat dependence at low temperatures, which is incompatible with the presence of nodes in the superconducting gap ∆(k). The temperature dependence of the superfluid density demonstrates that the gap is non-zero (∆/kBTc 1.6) all over the Fermi surface. The microwave conductivity below Tc exhibits an enhancement larger than the coherence peak, reminiscent of high-Tc cuprate superconductors.PACS numbers: 74.25. Nf, 74.20.Rp, 74.25.Fy Since the discovery of superconductivity in, high transition temperatures (T c ) up to 56 K have been reported in the doped Fe-based oxypnictides [2,3,4,5,6,7,8,9]. The nature of superconductivity and the pairing mechanism in this system are fundamental physical problem of crucial importance. The first experimental task to this problem is to elucidate the superconducting pairing symmetry, which is intimately related to the pairing interaction.The NMR Knight-shift measurements appear to indicate the spin-singlet pairing [10,11]. However, the superconducting gap structure, particulary the presence or absence of nodes in the gap, is highly controversial. The specific heat shows a nonlinear magnetic field dependence [12]. The NMR relaxation rate shows the absence of the coherence peak and the T 3 -dependence below T c [10,11,13,14]. The lower critical field exhibits a T -linear dependence at low temperatures [15]. The µSR experiments report an unusual field-dependence of the penetration depth [16]. In the point-contact spectroscopy, a zero-bias conductance peak is reported [17,18]. These results have been interpretated as an indication of unconvensional superconductivity with line nodes. On the other hand, the Andreev reflection data are found to be consistent with an isotropic gap [19]. All of these experiments have been performed by using polycrystalline samples. Definitely, measurements using single crystals are highly desired to obtain unambiguous conclusions on the superconducting gap structure.In this paper, we report on the measurements of the complex surface impedance in underdoped single crystals of the oxypnictide superconductor PrFeAsO 1−y (T c ≈ 35 K), from which properties of thermally excited quasiparticles can be directly deduced. Since the recent NMR experiments of the Pr-based iron oxypnictide suggest the non-magnetic state in the superconducting samples [11], PrFeAsO 1−y seems suitable for the penetration depth study [20,21]. Moreover, PrFeAsO 1−y has a higher T c than that of La-compounds, which enables the measurements in a wider temperature range. We observe flat temperature dependence of the in-plane penetration depth λ ab (T ) at low temperatures, indicating exponentially small quasiparticle excitations, which clearly contradicts the presence of nodes in the gap. The quasiparticle conductivity is enhanced compared with ...

Treatment of an overcrowded triselenatribismane, 2,4,6-tris(bis(trimethylsilyl)methyl)phenyl-1,3,5-triselena-2,4,6-tribismane, with hexamethylphosphorous triamide in toluene at 100°C resulted in the quantitative formation of a stable dibismuthene [TbtBi=BiTbt, where Tbt is 2,4,6-tris(bis(trimethylsilyl)methyl)phenyl], a compound containing a double bond formed between two bismuth atoms. The compound formed as deep purple crystals upon cooling. Ultraviolet-visible and Raman spectra, x-ray crystallographic structural analysis, and theoretical calculations provided evidence for the double bond character of the Bi–Bi bond.

We report high-sensitivity microwave measurements of the in-plane penetration depth λ ab and quasiparticle scattering rate 1/τ in several single crystals of hole-doped Fe-based superconductor Ba1−xKxFe2As2 (x ≈ 0.55). While power-law temperature dependence of λ ab with the power ∼ 2 is found in crystals with large 1/τ , we observe exponential temperature dependence of superfluid density consistent with the existence of fully opened two gaps in the cleanest crystal we studied. The difference may be a consequence of different level of disorder inherent in the crystals. We also find a linear relation between the low-temperature scattering rate and the density of quasiparticles, which shows a clear contrast to the case of d-wave cuprate superconductors with nodes in the gap. These results demonstrate intrinsically nodeless order parameters in the Fe-arsenides. The discovery of high-T c superconductivity in Fepnictides [1] has attracted tremendous interests both experimentally and theoretically. The 'mother' materials have antiferromagnetic spin-density-wave order [2] and the superconductivity appears by doping charge carriers, either electrons or holes. Such carrier doping induced superconductivity resembles high-T c cuprates, but one of the most significant differences is the multiband electronic structure having electron and hole pockets in the Fe-based superconductors. Unconventional superconducting pairings, most notably the sign-reversing s ± state, have been suggested by several theories [3] featuring the importance of the nesting between the hole and electron bands. This is also in sharp contrast to other multiband superconductors such as MgB 2 , where the coupling between the different bands is very weak. Thus the most crucial is to clarify the novel multiband nature of superconductivity in this new class of materials.

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