arXiv subjects

C. E. Cunningham

Publications and source records attributed to C. E. Cunningham.

4 recordsLinked to original sources

arXiv2001

Synthesis and Processing of $MgB_2$ powders and wires

Sintered powders and wires of superconducting $MgB_2$ have been fabricated under a variety of conditions in order to determine details of the diffusion of the $Mg$ into $B$ and to study the types of defects that arise during growth. For samples prepared by exposure of boron to $Mg$ vapor at $950^{\circ}C$, the conversion of particles of less than $100μm$ size particles to $MgB_2$ is complete in about $2 h$. The lattice parameters of the $MgB_2$ phase determined from X-ray are independent of the starting stoichiometry and the time of reaction. Wire segments of $MgB_2$ with very little porosity have been produced by reacting $141 μm$ diameter boron fibers in an atmosphere of excess $Mg$ vapor at $950^{\circ}C$. Defects in the reacted fibers are predominantly the voids left as the boron is converted to $MgB_2$.

cond-mat.supr-con↗

arXiv2001

Magnetoresistivity and Complete $H_{c2}(T)$ in $MgB_2$

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 $κ$, clean limit, boron-bearing intermetallics: $YNi_2B_2C$ and $LuNi_2B_2C$.

cond-mat.supr-con↗

arXiv2001

Superconductivity in Dense $MgB_2$ 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 $μm$, are better than 80% dense and manifest the full $χ= -1/4π$ shielding in the superconducting state. Temperature-dependent resistivity measurements indicate that $MgB_2$ is a highly conducting metal in the normal state with $ρ(40 K)$ = 0.38 $μOhm$-$cm$. Using this value, an electronic mean free path, $l \approx 600~Å$ 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.

cond-mat.supr-con↗

arXiv2001

Boron Isotope Effect in Superconducting MgB$_2$

We report the preparation method of, and boron isotope effect for MgB$_2$, a new binary intermetallic superconductor with a remarkably high superconducting transition temperature $T_c$($^{10}$B) = 40.2 K. Measurements of both temperature dependent magnetization and specific heat reveal a 1.0 K shift in $T_c$ between Mg$^{11}$B$_2$ and Mg$^{10}$B$_2$. Whereas such a high transition temperature might imply exotic coupling mechanisms, the boron isotope effect in MgB$_2$ is consistent with the material being a phonon-mediated BCS superconductor.

cond-mat.supr-con↗