Au Fe vs Cu thermocouples

A calibration of gold iron thermocouples is given

Superconductivity in Dense MgB2MgB_2 Wires

$MgB_2$ becomes superconducting just below 40 K. Whereas porous polycrystalline samples of $MgB_2$ can be synthesized from boron powders, in this letter we demonstrate that dense wires of $MgB_2$ can be prepared by exposing boron filaments to $Mg$ vapor. The resulting wires have a diameter of 160 ${\mu}m$, are better than 80% dense and manifest the full $\chi = -1/4{\pi}$ shielding in the superconducting state. Temperature-dependent resistivity measurements indicate that $MgB_2$ is a highly conducting metal in the normal state with $\rho (40 K)$ = 0.38 $\mu Ohm$-$cm$. Using this value, an electronic mean free path, $l \approx 600~\AA$ can be estimated, indicating that $MgB_2$ wires are well within the clean limit. $T_c$, $H_{c2}(T)$, and $J_c$ data indicate that $MgB_2$ manifests comparable or better superconducting properties in dense wire form than it manifests as a sintered pellet.Comment: Figures' layout fixe

Superconducting MgB2 thin films by pulsed laser deposition

Growth of MgB2 thin films by pulsed laser deposition is examined under ex situ and in situ processing conditions. For the ex situ process, Boron films grown by PLD were annealed at 900 C with excess Mg. For the in situ process, different approaches involving ablation from a stoichiometric target under different growth conditions, as well as multilayer deposition involving interposed Mg layers were examined and analyzed. Magnetic measurements on ex situ processed films show TC of ~39 K, while the current best in situ films show a susceptibility transition at ~ 22 K.Comment: 3 pages, PD

Method of increasing magnetostrictive response of rare earth-iron alloy rods

This invention comprises a method of increasing the magnetostrictive response of rare earth iron (RFe) magnetostrictive alloy rods by a thermal-magnetic treatment. The rod is heated to a temperature above its Curie temperature, viz. from 400 rod is at that temperature, a magnetic field is directionally applied and maintained while the rod is cooled, at least below its Curie temperature

Phonon dispersion and electron-phonon coupling in MgB_2 and AlB_2

We present a first principles investigation of the lattice dynamics and electron-phonon coupling of the superconductor MgB_2 and the isostructural AlB_2 within the framework of density functional perturbation theory using a mixed-basis pseudopotential method. Complete phonon dispersion curves and Eliashberg functions \alpha^2F are calculated for both systems. We also report on Raman measurements, which support the theoretical findings. The calculated generalized density-of-states for MgB_2 is in excellent agreement with recent neutron-scattering experiments. The main differences in the calculated phonon spectra and \alpha^2F are related to high frequency in-plane boron vibrations. As compared to AlB_2, they are strongly softened in MgB_2 and exhibit an exceptionally strong coupling to electronic states at the Fermi energy. The total coupling constants are \lambda_{MgB_2}=0.73 and \lambda_{AlB_2}=0.43. Implications for the superconducting transition temperature are briefly discussed.Comment: 10 pages, 4 figures, to appear in Phys. Rev. Let

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 $H\perp c$

Magnetoresistivity and Complete Hc2(T)H_{c2}(T) in MgB2MgB_2

Detailed magneto-transport data on dense wires of $MgB_2$ are reported for applied magnetic fields up to 18 T. The temperature and field dependencies of the electrical resistivity are consistent with $MgB_2$ behaving like a simple metal and following a generalized form of Kohler's rule. In addition, given the generally high $T_c$ values and narrow resistive transition widths associated with $MgB_2$ synthesized in this manner, combined with applied magnetic fields of up to 18 T, an accurate and complete $H_{c2}(T)$ curve could be determined. This curve agrees well with curves determined from lower field measurements on sintered pellets and wires of $MgB_2$. $H_{c2}(T)$ is linear in $T$ over a wide range of temperature (7 K $\le~T~\le$ 32 K) and has an upward curvature for $T$ close to $T_c$. These features are similar to other high $\kappa$, clean limit, boron-bearing intermetallics: $YNi_2B_2C$ and $LuNi_2B_2C$.Comment: minor changes in styl

Penetration Depth and Anisotropy in MgB2

The penetration depth lambda of MgB2 was deduced from both the ac susceptibility chi and the magnetization M(H) of sorted powders. The good agreement between the two sets of data without geometric correction for the grain orientation suggests that MgB2 is an isotropic superconductor.Comment: 9 pages, 5 figures; submitted to Physical Review B (February 28, 2001; revised June 28, 2001); reference list update