Damping of dHvA oscillations and vortex-lattice disorder in the peak-effect region of strong type-II superconductors

The phenomenon of magnetic quantum oscillations in the superconducting state poses several questions that still defy satisfactory answers. A key controversial issue concerns the additional damping observed in the vortex state. Here, we show results of \mu SR, dHvA, and SQUID magnetization measurements on borocarbide superconductors, indicating that a sharp drop observed in the dHvA amplitude just below H_{c2} is correlated with enhanced disorder of the vortex lattice in the peak-effect region, which significantly enhances quasiparticle scattering by the pair potential.Comment: 4 pages 4 figure

Major Surge Activity of Super-Active Region NOAA 10484

We observed two surges in H-alpha from the super-active region NOAA 10484. The first surge was associated with an SF/C4.3 class flare. The second one was a major surge associated with a SF/C3.9 flare. This surge was also observed with SOHO/EIT in 195 angstrom and NoRh in 17 GHz, and showed similar evolution in these wavelengths. The major surge had an ejective funnel-shaped spray structure with fast expansion in linear (about 1.2 x 10^5 km) and angular (about 65 deg) size during its maximum phase. The mass motion of the surge was along open magnetic field lines, with average velocity about 100 km/s. The de-twisting motion of the surge reveals relaxation of sheared and twisted magnetic flux. The SOHO/MDI magnetograms reveal that the surges occurred at the site of companion sunspots where positive flux emerged, converged, and canceled against surrounding field of opposite polarity. Our observations support magnetic reconnection models for the surges and jets.Comment: 4 pages, 3 figures; To appear in "Magnetic Coupling between the Interior and the Atmosphere of the Sun", eds. S.S. Hasan and R.J. Rutten, Astrophysics and Space Science Series, Springer-Verlag, Heidelberg, Berlin, 200

Uniaxial strain control of spin-polarization in multicomponent nematic order of BaFe2_{2}As2_{2}

The iron-based high temperature superconductors exhibit a rich phase diagram reflecting a complex interplay between spin, lattice, and orbital degrees of freedom [1-4]. The nematic state observed in many of these compounds epitomizes this complexity, by entangling a real-space anisotropy in the spin fluctuation spectrum with ferro-orbital order and an orthorhombic lattice distortion [5-7]. A more subtle and much less explored facet of the interplay between these degrees of freedom arises from the sizable spin-orbit coupling present in these systems, which translates anisotropies in real space into anisotropies in spin space. Here, we present a new technique enabling nuclear magnetic resonance under precise tunable strain control, which reveals that upon application of a tetragonal symmetry-breaking strain field, the magnetic fluctuation spectrum in the paramagnetic phase of BaFe$_{2}$As$_{2}$ also acquires an anisotropic response in spin-space. Our results unveil a hitherto uncharted internal spin structure of the nematic order parameter, indicating that similar to liquid crystals, electronic nematic materials may offer a novel route to magneto-mechanical control.Comment: 11 pages, 5 figure

Systematic effects of carbon doping on the superconducting properties of Mg(B1−x_{1-x}Cx_x)2_2

The upper critical field, $H_{c2}$, of Mg(B$_{1-x}$C$_x$)$_2$ has been measured in order to probe the maximum magnetic field range for superconductivity that can be attained by C doping. Carbon doped boron filaments are prepared by CVD techniques, and then these fibers are then exposed to Mg vapor to form the superconducting compound. The transition temperatures are depressed about $1 K/$ C and $H_{c2}(T=0)$ rises at about $5 T/$ C. This means that 3.5% C will depress $T_c$ from $39.2 K$ to $36.2 K$ and raise $H_{c2}(T=0)$ from $16.0 T$ to $32.5 T$. Higher fields are probably attainable in the region of 5% C to 7% C. These rises in $H_{c2}$ are accompanied by a rise in resistivity at $40 K$ from about $0.5 \mu \Omega cm$ to about $10 \mu \Omega cm$. Given that the samples are polycrystalline wire segments, the experimentally determined $H_{c2}(T)$ curves represent the upper $H_{c2}(T)$ manifold associated with $H\perp c$

Effects of Neutron Irradiation on Carbon Doped MgB2 Wire Segments

We have studied the evolution of superconducting and normal state properties of neutron irradiated Mg(B$_{.962}$C$_{.038}$)$_2$ wire segments as a function of post exposure annealing time and temperature. The initial fluence fully suppressed superconductivity and resulted in an anisotropic expansion of the unit cell. Superconductivity was restored by post-exposure annealing. The upper critical field, H$_{c2}$(T=0), approximately scales with T$_c$ starting with an undamaged T$_c$ near 37 K and H$_{c2}$(T=0) near 32 T. Up to an annealing temperature of 400 $^ o$C the recovery of T$_c$ tends to coincide with a decrease in the normal state resistivity and a systematic recovery of the lattice parameters. Above 400 $^ o$C a decrease in order along the c- direction coincides with an increase in resistivity, but no apparent change in the evolution of T$_c$ and H$_{c2}$. To first order, it appears that carbon doping and neutron damaging effect the superconducting properties of MgB$_2$ independently

NMR evidence for inhomogeneous glassy behavior driven by nematic fluctuations in iron arsenide superconductors

We present $^{75}$As nuclear magnetic resonance spin-lattice and spin-spin relaxation rate data in Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ and Ba(Fe$_{1-x}$Cu$_x$)$_2$As$_2$ as a function of temperature, doping and magnetic field. The relaxation curves exhibit a broad distribution of relaxation rates, consistent with inhomogeneous glassy behavior up to 100 K. The doping and temperature response of the width of the dynamical heterogeneity is similar to that of the nematic susceptibility measured by elastoresistance measurements. We argue that quenched random fields which couple to the nematic order give rise to a nematic glass that is reflected in the spin dynamics.Comment: Accepted to Physical Review

Optical spectroscopy of superconducting Ba{0.55}K{0.45}Fe2As2: evidence for strong coupling to low energy bosons

Optical spectroscopy on single crystals of the new iron arsenide superconductor Ba{0.55}K{0.45}Fe2As2 shows that the infrared spectrum consists of two major components: a strong metallic Drude band and a well separated mid infrared absorption centered at 0.7 eV. It is difficult to separate the two components unambiguously but several fits of Lorentzian peaks suggest a model with a Drude peak having a plasma frequency of 1.8 to 2.1 eV and a midinfrared peak with a plasma frequency of 2.5 eV. In contrast to the cuprate superconductors the scattering rate obtained from the extended Drude model saturates at 150 meV as compared to 500 meV for a typical cuprate. Detailed analysis of the frequency dependent scattering rate shows that the charge carriers interact with broad bosonic spectrum with a peak at 25 meV and a coupling constant lambda =approx 2 at low temperature. As the temperature increases this coupling weakens to lambda=0.6 at ambient temperature. This suggests a bosonic spectrum that is similar to what is seen in the lower Tc cuprates.Comment: 4 pages, 4 figure

Magnetotransport and the upper critical magnetic field in MgB2

Magnetotransport measurements are presented on polycrystalline MgB2 samples. The resistive upper critical magnetic field reveals a temperature dependence with a positive curvature from Tc = 39.3 K down to about 20 K, then changes to a slightly negative curvature reaching 25 T at 1.5 K. The 25- Tesla upper critical field is much higher than what is known so far on polycrystals of MgB2 but it is in agreement with recent data obtained on epitaxial MgB2 films. The deviation of Bc2(T) from standard BCS might be due to the proposed two-gap superconductivity in this compound. The observed quadratic normal-state magnetoresistance with validity of Kohler's rule can be ascribed to classical trajectory effects in the low-field limit.Comment: 6 pages, incl. 3 figure

Effects of Co substitution on thermodynamic and transport properties and anisotropic Hc2H_{c2} in Ba(Fe1−x_{1-x}Cox_x)2_2As2_2 single crystals

Single crystalline samples of Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ with $x < 0.12$ have been grown and characterized via microscopic, thermodynamic and transport measurements. With increasing Co substitution, the thermodynamic and transport signatures of the structural (high temperature tetragonal to low temperature orthorhombic) and magnetic (high temperature non magnetic to low temperature antiferromagnetic) transitions are suppressed at a rate of roughly 15 K per percent Co. In addition, for $x \ge 0.038$ superconductivity is stabilized, rising to a maximum $T_c$ of approximately 23 K for $x \approx 0.07$ and decreasing for higher $x$ values. The $T - x$ phase diagram for Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ indicates that either superconductivity can exist in both low temperature crystallographic phases or that there is a structural phase separation. Anisotropic, superconducting, upper critical field data ($H_{c2}(T)$) show a significant and clear change in anisotropy between samples that have higher temperature structural phase transitions and those that do not. These data show that the superconductivity is sensitive to the suppression of the higher temperature phase transition