Giant microwave absorption in fine powders of superconductors.

Enhanced microwave absorption, larger than that in the normal state, is observed in fine grains of type-II superconductors (MgB2 and K3C60) for magnetic fields as small as a few % of the upper critical field. The effect is predicted by the theory of vortex motion in type-II superconductors, however its direct observation has been elusive due to skin-depth limitations; conventional microwave absorption studies employ larger samples where the microwave magnetic field exclusion significantly lowers the absorption. We show that the enhancement is observable in grains smaller than the penetration depth. A quantitative analysis on K3C60 in the framework of the Coffey-Clem (CC) theory explains well the temperature dependence of the microwave absorption and also allows to determine the vortex pinning force constant.

Orbital upper critical field and its anisotropy of clean one- and two-band superconductors.

The Helfand-Werthamer (HW) scheme (Helfand and Werthamer 1966 Phys. Rev. 147 288; another part of this work published as a separate paper by Werthamer et al 1966 Phys. Rev. 147 295) of evaluating the orbital upper critical field is generalized to anisotropic superconductors in general, and to two-band clean materials, in particular. Our formal procedure differs from those in the literature; it reproduces not only the isotropic HW limit but also the results of calculations for the two-band superconducting MgB(2) (Miranovic et al 2003 J. Phys. Soc. Japan 72 221, Dahm and Schopohl 2003 Phys. Rev. Lett. 91 017001) along with the existing data on H(c2)(T) and its anisotropy γ(T) = H(c2,ab)(T)/H(c2,c)(T) (a, c are the principal directions of a uniaxial crystal). Using rotational ellipsoids as model Fermi surfaces we apply the formalism developed to study γ(T) for a few different anisotropies of the Fermi surface and of the order parameters. We find that even for a single band d-wave order parameter γ(T) decreases on warming; however, relatively weakly. For order parameters of the form Δ(k(z)) = Δ(0)(1 + η cos k(z)a) (Xu et al 2011 Nature Phys. 7 198), according to our simulations γ(T) may either increase or decrease on warming even for a single band depending on the sign of η. Hence, the common belief that the multi-band Fermi surface is responsible for the temperature variation of γ is proven incorrect. For two s-wave gaps, γ decreases on warming for all Fermi shapes examined. For two order parameters of the form Δ(k(z)) = Δ(0)(1 + η cos k(z)a), presumably relevant for pnictides, we obtain γ(T) increasing on warming provided both η(1) and η(2) are negative, whereas for η > 0, γ(T) decreases. We study the ratio of the two order parameters at H(c2)(T) and find that the ratio of the small gap to the large one does not vanish at any temperature, even at H(c2)(T), an indication that this does not happen at lower fields.

Nonexponential London penetration depth of FeAs-based superconducting RFeAsO(0.9)F(0.1) (R = La, Nd) single crystals.

The superconducting penetration depth lambda(T) has been measured in RFeAsO(0.9)F(0.1) (R = La, Nd) single crystals (R-1111). In Nd-1111, we find an upturn in lambda(T) upon cooling and attribute it to the paramagnetism of the Nd ions, similar to the case of the electron-doped cuprate Nd-Ce-Cu-O. After the correction for paramagnetism, the London penetration depth variation is found to follow a power-law behavior, Deltalambda_{L}(T) proportional, variantT;{2} at low temperatures. The same T2 variation of lambda(T) was found in nonmagnetic La-1111 crystals. Analysis of the superfluid density and of penetration depth anisotropy over the full temperature range is consistent with two-gap superconductivity. Based on this and on our previous work, we conclude that both the RFeAsO (1111) and BaFe(2)As(2) (122) families of pnictide superconductors exhibit unconventional two-gap superconductivity.

Superfluid density in gapless superconductor CeCoIn(5).

Temperature dependence of the London penetration depth λ measured in single crystals of CeCoIn(5) is interpreted as being caused by a strong pair-breaking scattering that makes the superconductivity in this compound gapless. For a gapless d-wave superconductor, we derive λ = λ(0)(1-T(2)/T(c)(2))(-1/2) caused by the combined effect of magnetic and non-magnetic scattering, in excellent agreement with the data in the full temperature range and with the gapless s-wave case of Abrikosov and Gor'kov. We also obtain the slope of the upper critical field at T(c) that compares well with the measured slope.

Current in narrow channels of anisotropic superconductors.

We argue that, in channels cut out of anisotropic single crystal superconductors and narrow on the scale of London penetration depth, the persistent current must cause the transverse phase difference provided the current does not point in any of the principal crystal directions. The difference is proportional to the current value and depends on the anisotropy parameter, on the current direction relative to the crystal, and on the transverse channel dimension. An idea on how to measure the transverse phase is proposed.

Free energy and torque for superconductors with different anisotropies of H(c2) and lambda.

The free energy is evaluated for a uniaxial superconductor with the anisotropy of the upper critical field, gamma(H)=H(c2,a)/H(c2,c), different from the anisotropy of the penetration depth gamma(lambda)=lambda(c)/lambda(a). With increasing difference between gamma(H) and gamma(lambda), the equilibrium orientation of the crystal relative to the applied field may shift from theta=pi/2 (theta is the angle between the field and the c axis) to lower angles and reach theta=0 for large enough gamma(H). These effects are expected to take place in MgB2.

Effect of fluctuations on vortex lattice structural transitions in superconductors.

The rhombic-to-square transition field H(square)(T) for cubic and tetragonal materials in fields along [001] is evaluated using the nonlocal London theory with the account of thermal vortex fluctuations. Unlike extended Ginzburg-Landau models, our approach shows that the line H(square)(T) and the upper critical field H(c2)(T) do not cross due to strong fluctuations near H(c2)(T) which suppress the square anisotropy induced by the nonlocality. In increasing fields, fluctuations cause a reentrance of the rhombic vortex lattice, in agreement with recent neutron scattering data on borocarbides.

Elastic moduli of vortex lattices within nonlocal London model.

Vortex lattice (VL) elastic response is analyzed within the nonlocal London model which holds for high-kappa clean superconductors. The squash modulus vanishes at the field H( square) where VL undergoes a square-to-rhombus transition. For H>H( square), where the square VL is stable, the rotation modulus turns zero at H = H(r), indicating VL instability to rotations. The shear modulus depends on the shear direction; the dependence is strong in the vicinity of H( square) where the square VL is soft with respect to the shear along [110]. The H dependences of the moduli are evaluated for LuNi(2)B(2)C.

Anisotropy of superconducting MgB2 as seen in electron spin resonance and magnetization data.

We observed the conduction electron spin resonance (CESR) in fine powders of MgB2 both in the superconducting and normal states. The Pauli susceptibility is chi(s) = 2.0 x 10(-5) emu/mole in the temperature range of 450 to 600 K. The spin relaxation rate has an anomalous temperature dependence. The CESR measured below T(c) at several frequencies suggests that MgB2 is a strongly anisotropic superconductor with the upper critical field, H(c2), ranging between 2 and 16 T. The high-field reversible magnetization data of a randomly oriented powder sample are well described assuming that MgB2 is an anisotropic superconductor with H(ab)(c2)/H(c)(c2) approximately 6-9.

Temperature dependence of the flux line lattice transition into square symmetry in superconducting LuNi2B2C.

We have investigated the temperature dependence of the H parallel to c flux line lattice structural phase transition from square to hexagonal symmetry, in the tetragonal superconductor LuNi2B2C ( T(c) = 16.6 K). At temperatures below 10 K the transition onset field, H2(T), is only weakly temperature dependent. Above 10 K, H2(T) rises sharply, bending away from the upper critical field. This contradicts theoretical predictions of H2(T) merging with the upper critical field and suggests that just below the H(c2)(T) curve the flux line lattice might be hexagonal.

An immunofluorescence diagnostic test for avian infectious laryngotracheitis.

An antiserum to a recent field isolate of infectious laryngotracheitis virus was conjugated with fluorescein isothiocyanate and used to detect viral antigen to infected chorioallantoic membranes and trachael epithelium by the direct fluorescent antibody technique. In experimentally infected birds, viral antigen was detected with the fluorescent antibody technique from 2 to 14 days post-inoculation but histological evidence of tracheitis was only observed from day three to day ten. The fluorescent antibody test detected 22 of 23 histologically confirmed cases of ILT and was more accurate than virus isolation when used in the diagnosis of respiratory disease from field outbreaks. It was concluded that the speed, accuracy and sensitivity of the fluorescent antibody technique make it a useful tool in the diagnosis of infectious laryngotracheitis.