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 ...

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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In order to study the cation dependence of the ionic dynamics in molten nitrates, molecular dynamics simulations including vibrational degrees of freedom were carried out for molten LiNO3, NaNO3, and RbNO3. Coulomb pair potential with Born-type repulsion was adopted for the interionic interaction. The simulated diffusion coefficient was smaller for a larger cation, and that of nitrate ions did not change with changing cation species. The mean squared charge displacements showed that the static conductivity decreased considerably as the cation size increased from Li+ to Rb+. The simulated orientational correlation function of nitrate ions decayed more quickly as the cation size increased. Far infrared absorption spectrum simulated from the time evolution of the dipole moment (or the current) of the system showed that the peak shifted to the low energy side and the intensity decreased as the cation size increased. Results of the simulation were compared with the experimental diffusion constants, static and dynamic conductivities, and rotational behavior revealed by Raman spectroscopy. The simulated vibrational correlation functions and the power spectra of NO3− could reproduce the observed cation dependence of the peak frequencies of the ν1(A′1) and ν2(A″2) modes. However, the assumed interionic potentials in the present simulation were found to result in too slow vibrational dephasing of the ν1 mode and too fast dephasing of the ν2 mode as compared with the infrared and isotropic Raman spectra. Strong correlation between radial and angular distributions of cations was found in the first coordination spheres of nitrate ions in the simulated molten nitrates.

Microwave penetration depth lambda and surface resistance at 27 GHz are measured in high quality crystals of KOs(2)O(6). Firm evidence for fully gapped superconductivity is provided from lambda(T). Below the second transition at T(p) approximately 8 K, the superfluid density shows a steplike change with a suppression of effective critical temperature T(c). Concurrently, the extracted quasiparticle scattering time shows a steep enhancement, indicating a strong coupling between the anomalous rattling motion of K ions and quasiparticles. The results imply that the rattling phonons help to enhance superconductivity, and that K sites freeze to an ordered state with long quasiparticle mean free path below T(p).