The Dendritic magnetic avalanches in carbon-free MgB2_2 thin films with and without a deposited Au layer

From the magneto optics images (MOI), the dendritic magnetic avalanche is known to appear dominantly for thin films of the newly discovered MgB$_2$. To clarify the origin of this phenomenon, we studied in detail the MOI of carbon-free MgB$_2$ thin films with and without a deposited gold layer. The MOI indicated carbon contamination was not the main source of the avalanche. The MOI clearly showed that the deposition of metallic gold deposition on top of a MgB$_2$ thin film improved its thermal stability and suppressed the sudden appearance of the dendritic flux avalanche. This is consistent with the previous observation of flux noise in the magnetization.Comment: 9 pages, 4 figeure

Reversible magnetization of MgB2 single crystals with a two-gap nature

We present reversible magnetization measurements on MgB2 single crystals in magnetic fields up to 2.5 T applied parallel to the crystal's c-axis. This magnetization is analyzed in terms of the Hao-Clem model, and various superconducting parameters, such as the critical fields [Hc(0) and Hc2(0)], the characteristic lengths [xi(0) and lambda(0)], and the Ginzburg-Landau parameter, kappa, are derived. The temperature dependence of the magnetic penetration depth, lambda(T), obtained from the Hao-Clem analysis could not be explained by theories assuming a single gap. Our data are well described by using a two-gap model.Comment: 20 pages, 1 table, 4 figures, will be published in Phys. Rev.

Thermally activated flux flow and fluctuation conductivity in LiFeAs single crystal

The thermally activated flux flow (TAFF) of a LiFeAs single crystal was studied in magnetic fields up to 9 T. The thermally activated energy (TAE) was analyzed using a conventional Arrhenius relation and a modified TAFF theory. The modified TAFF method results are in better agreement with experimental data than the Arrhenius ones. Through the analysis of the modified TAFF method, we found that the LiFeAs superconductor is regarded as a three-dimensional (3D) system in the TAFF region. In addition, we obtained the vortex-glass temperature Tg from the linear region of the (dln ρ/dT)− 1. And the fluctuation conductivity was analyzed for the LiFeAs superconductor using Aslamazov-Larkin (AL) theory and the lowest Landau level (LLL) scaling, in zero and nonzero magnetic fields. With the determined values of Tc(H), the fluctuation conductivity was scaled within a 3D-LLL approach. Compared with the NMR result of the 2D antiferromagnetic spin fluctuation, these results imply that LiFeAs exhibited a 2D-3D crossover of the fluctuation conductivity (FC) with decreasing temperature

Comparative Transport Studies of '1212' Superconductors

HgBa2CaCu2O6+δ (Hg -1212) thin films were fabricated by exchanging the T1 cations in TlBa2CaCu2O7-δ (Tl-1212) thin films with Hg cations, causing a 30-K increase in Tc. To determine how this exchange effects such a Tc increase, the irreversibility lines, temperature dependence of critical current density, and temperature dependence of Hall angle ofHg-1212 and TI-1212 thin films were measured and then compared. The results strongly suggest that the Tc shift is caused by a doubling of charge carrier density

Plasmonic polymer nanoantenna arrays for electrically tunable and electrode-free metasurfaces

Electrically tunable metasurfaces and interrelated nanofabrication techniques are essential for metasurface-based optoelectronic applications. We present a nanofabrication method suitable for various types of plasmonic polymer metasurfaces including inverted arrays of nanoantennas. Inverted metasurfaces are of particular interest since the metasurface itself can work as an electrode due to its interconnected nature, which enables electrical control without adopting an additional electrode. In comparison with inverted nanodisk arrays that support relatively weak resonance features, we show that inverted nanorod arrays can possess stronger resonances, even comparable with those of nanorod arrays. The origin of plasmon resonances in inverted arrays is systematically investigated using finite-difference time-domain (FDTD) simulations. Further, we demonstrate electrically tunable electrode-free metasurface devices using polymer inverted nanorod arrays, which can operate in the full spectral range of the material including the mid-infrared region

Comparative transport studies of ``1212'' superconductors

HgBa{sub 2}CaCu{sub 2}O{sub 6+{delta}} (Hg-1212) thin films were fabricated by exchanging the TI cations in TlBa{sub 2}CaCu{sub 2}O{sub 7-{delta}} (Tl-1212) thin films with Hg cations, causing a 30-K increase in Tc. To determine how this exchange effects such a Tc increase, the irreversibility lines, temperature dependence of critical current density, and temperature dependence of Hall angle of Hg-1212 and T1-1212 thin films were measured and then compared. The results strongly suggest that the Tc shift is caused by a doubling of charge carrier density

Study on the Efficient Readout Scheme of a Single Superconducting Transmon Qubit in Dispersive Regime Circuit QED

We present analyses and measurements of the readout scheme in a dispersive circuit QED system by varying detuning between the qubit frequency and the cavity resonant frequency. With larger detuning, the ground state frequency and the bright state frequency get closer and the transition power between themincreases in the cavity transmission. This implies that the relatively strong microwave power is allowed for far-detuned qubit readout. Additional number splitting peaks are also observed in qubit spectroscopy. We will discuss the relation between the difference of the ground state and the bright state frequencies and the difference of the qubit and the cavity frequencies