Thermal activation energy of 3D vortex matter in NaFe1-xCoxAs (x=0.01, 0.03 and 0.07) single crystals

We report on the thermally activated flux flow dependency on the doping dependent mixed state in NaFe1-xCoxAs (x=0.01, 0.03, and 0.07) crystals using the magnetoresistivity in the case of B//c-axis and B//ab-plane. It was found clearly that irrespective of the doping ratio, magnetoresistivity showed a distinct tail just above the Tc, offset associated with the thermally activated flux flow (TAFF) in our crystals. Furthermore, in TAFF region the temperature dependence of the activation energy follows the relation U(T, B)=U_0 (B) (1-T/T_c )^q with q=1.5 in all studied crystals. The magnetic field dependence of the activation energy follows a power law of U_0 (B)~B^(-{\alpha}) where the exponent {\alpha} is changed from a low value to a high value at a crossover field of B=~2T, indicating the transition from collective to plastic pinning in the crystals. Finally, it is suggested that the 3D vortex phase is the dominant phase in the low-temperature region as compared to the TAFF region in our series samples

Superconducting transition of a two-dimensional Josephson junction array in weak magnetic fields

The superconducting transition of a two-dimensional (2D) Josephson junction array exposed to weak magnetic fields has been studied experimentally. Resistance measurements reveal a superconducting-resistive phase boundary in serious disagreement with the theoretical and numerical expectations. Critical scaling analyses of the $IV$ characteristics indicate contrary to the expectations that the superconducting-to-resistive transition in weak magnetic fields is associated with a melting transition of magnetic-field-induced vortices directly from a pinned-solid phase to a liquid phase. The expected depinning transition of vortices from a pinned-solid phase to an intermediate floating-solid phase was not observed. We discuss effects of the disorder-induced random pinning potential on phase transitions of vortices in a 2D Josephson junction array.Comment: 9 pages, 7 figures (EPS+JPG format), RevTeX

Korringa ratio of ferromagnetically correlated impure metals

The Korringa ratio, $\cal K$, obtained by taking an appropriate combination of the Knight shift and nuclear spin-lattice relaxation time, is calculated at finite temperature, $T$, in the three-dimensional electron gas model, including the electron-electron interaction, $U$, and non-magnetic impurity scatterings. $\cal K$ varies in a simple way with respect to $U$ and $T$; it decreases as $U$ is increased but increases as $T$ is raised. However, $\cal K$ varies in a slightly more complicated way with respect to the impurity scatterings; as the scattering rate is increased, $\cal K$ increases for small $U$ and low $T$, but decreases for large $U$ or high $T$ regime. This calls for a more careful analysis when one attempts to estimate the Stoner factor from $\cal K$.Comment: 7 pages including 3 figures. To be published in Phys. Rev. B, Dec.

The Influence of Quantum Critical Fluctuations of Circulating Current Order Parameters on the Normal State Properties of Cuprates

We study a model of the quantum critical point of cuprates associated with the "circulating current" order parameter proposed by Varma. An effective action of the order parameter in the quantum disordered phase is derived using functional integral method, and the physical properties of the normal state are studied based on the action. The results derived within the ladder approximation indicate that the system is like Fermi liquid near the quantum critical point and in disordered regime up to minor corrections. This implies that the suggested marginal Fermi liquid behavior induced by the circulating current fluctuations will come in from beyond the ladder diagrams.Comment: 7pages, 1 figure included in RevTex file. To appear in Phys. Rev.

Formation of plasma induced surface damage in silica glass etching for optical waveguides

Ge, B, P-doped silicaglass films are widely used as optical waveguides because of their low losses and inherent compatibility with silica optical fibers. These films were etched by ICP(inductively coupled plasma) with chrome etch masks, which were patterned by reactive ion etching (RIE) using chlorine-based gases. In some cases, the etched surfaces of silicaglass were very rough (root-mean square roughness greater than 100 nm) and we call this phenomenon plasma induced surface damage (PISD). Rough surface cannot be used as a platform for hybrid integration because of difficulty in alignment and bonding of active devices. PISD reduces the etch rate of glass and it is very difficult to remove residues on a rough surface. The objective of this study is to elucidate the mechanism of PISD formation. To achieve this goal, PISD formation during different etching conditions of chrome etch mask and silicaglass was investigated. In most cases, PISD sources are formed on a glass surface after chrome etching, and metal compounds are identified in theses sources. Water rinse after chrome etching reduces the PISD, due to the water solubility of metal chlorides. PISD is decreased or even disappeared at high power and/or low pressure in glassetching, even if PISD sources were present on the glass surface before etching. In conclusion, PISD sources come from the chrome etching process, and polymer deposition on these sources during the silicaetching cause the PISD sources to grow. In the area close to the PISD source there is a higher ion flux, which causes an increase in the etch rate, and results in the formation of a pit

Coulomb suppression of NMR coherence peak in fullerene superconductors

The suppressed NMR coherence peak in the fullerene superconductors is explained in terms of the dampings in the superconducting state induced by the Coulomb interaction between conduction electrons. The Coulomb interaction, modelled in terms of the onsite Hubbard repulsion, is incorporated into the Eliashberg theory of superconductivity with its frequency dependence considered self-consistently at all temperatures. The vertex correction is also included via the method of Nambu. The frequency dependent Coulomb interaction induces the substantial dampings in the superconducting state and, consequently, suppresses the anticipated NMR coherence peak of fullerene superconductors as found experimentally.Comment: 4 pages, Revtex, and 2 figures. Revised and final version to appear in Phys. Rev. Lett. (1998

Optical conductivity in the normal state fullerene superconductors

We calculate the optical conductivity, $\sigma(\omega)$, in the normal state fullerene superconductors by self-consistently including the impurity scatterings, the electron-phonon and electron-electron Coulomb interactions. The finite bandwidth of the fullerenes is explicitely considered, and the vertex corection is included $a$ $la$ Nambu in calculating the renormalized Green's function. $\sigma(\omega)$ is obtained by calculating the current-current correlation function with the renormalized Green's function in the Matsubara frequency and then performing analytic continuation to the real frequency at finite temperature. The Drude weight in $\sigma(\omega)$ is strongly suppressed due to the interactions and transfered to the mid-infrared region around and above 0.06 eV which is somewhat less pronounced and much broader compared with the expermental observation by DeGiorgi $et$ $al$.Comment: 6 pages, 4 figures. To be published in Physical Review B, July 1

Feasibility Study for Elimination of the Screening Current-Induced Fields in HTS Coil

Herein, we report a feasibility study for elimination of the screening current-induced field (SCIF) in fully insulated (INS) and no-insulation (NI) GdBCO coated conductor (CC) coils exposed to an external AC magnetic field generated by background solenoid copper coils. Prior to investigating the effects of external AC magnetic fields on SCIF in the GdBCO CC coils, the magnetic flux density (B[subscript z]) was calculated using the equivalent circuit model and compared to the B[subscript z] obtained empirically to quantify the SCIF in the INS and NI coils. The value of the SCIF in the NI coil was smaller than that in the INS coil without the use of an external AC magnetic field, due to the current paths originating from the turn-to-turn and layer-to-layer contacts, suggesting that the screening current dissipated by contact resistance during charging. When the INS and NI coils were exposed to the external AC magnetic field, the B[subscript z] of the coils increased gradually, and eventually saturating to the calculated B[subscript z] values, indicating full removal of the SCIF. In addition, the SCIF of the NI coil could be removed under subjection to a lower external AC magnetic field compared to the INS coil, due to the lower SCIF occurring in the NI coil after charging.Korea (South). Ministry of Education, Science and Technology (MEST). Mid-Career Researcher Program (Grant 2012-046999)Korea (South). Ministry of Knowledge Economy (MKE). International Collaborative R&D Program (KETEP Grant 20118520020020

Simultaneous multi-impairment monitoring of 640 Gb/s signals using photonic chip based RF spectrum analyzer

We report the first demonstration of simultaneous multiimpairment monitoring at ultrahigh bitrates using a THz bandwidth photonic-chip-based radio-frequency (RF) spectrum analyzer. Our approach employs a 7 cm long, highly nonlinear (γ ≈9900 /W/km), dispersion engineered chalcogenide planar waveguide to capture the RF spectrum of an ultrafast 640 Gb/s signal, based on cross-phase modulation, from which we numerically retrieve the autocorrelation waveform. The relationship between the retrieved autocorrelation trace and signal impairments is exploited to simultaneously monitor dispersion, in-band optical signal to noise ratio (OSNR) and timing jitter from a single measurement. This novel approach also offers very high OSNR measurement dynamic range (> 30 dB) and is scalable to terabit data rates