S. L. Bud'ko

Boron Isotope Effect in Superconducting MgB2

We report the preparation method of and boron isotope effect for MgB2, a new binary intermetallic superconductor with a remarkably high superconducting transition temperature T(c)(10B) = 40.2 K. Measurements of both temperature dependent magnetization and specific heat reveal a 1.0 K shift in T(c) between Mg11B2 and Mg10B2. Whereas such a high transition temperature might imply exotic coupling mechanisms, the boron isotope effect in MgB2 is consistent with the material being a phonon-mediated BCS superconductor.

Superconductivity in Dense MgB2 Wires

MgB2 becomes superconducting just below 40 K. Whereas porous polycrystalline samples of MgB2 can be synthesized from boron powders, in this Letter we demonstrate that dense wires of MgB2 can be prepared by exposing boron filaments to Mg vapor. The resulting wires have a diameter of 160 microm, are better than 80% dense, and manifest the full chi = -1/4pi shielding in the superconducting state. Temperature-dependent resistivity measurements indicate that MgB2 is a highly conducting metal in the normal state with rho(40 K) = 0.38 microOmega cm. By using this value, an electronic mean-free path, l approximately 600 A can be estimated, indicating that MgB2 wires are well within the clean limit. Tc, Hc2(T), and Jc data indicate that MgB2 manifests comparable or better superconducting properties in dense wire form than it manifests as a sintered pellet.

Superconductivity in Dense MgB{sub 2} Wires

MgB2 becomes superconducting just below 40 K. Whereas porous polycrystalline samples of MgB2 can be synthesized from boron powders, in this Letter we demonstrate that dense wires of MgB2 can be prepared by exposing boron filaments to Mg vapor. The resulting wires have a diameter of 160 microm, are better than 80% dense, and manifest the full chi = -1/4pi shielding in the superconducting state. Temperature-dependent resistivity measurements indicate that MgB2 is a highly conducting metal in the normal state with rho(40 K) = 0.38 microOmega cm. By using this value, an electronic mean-free path, l approximately 600 A can be estimated, indicating that MgB2 wires are well within the clean limit. Tc, Hc2(T), and Jc data indicate that MgB2 manifests comparable or better superconducting properties in dense wire form than it manifests as a sintered pellet.

Systematic effects of carbon doping on the superconducting properties of Mg(B1-xCx)(2).

The upper critical field, H(c2), of Mg(B1-xCx)(2) has been measured in order to probe the maximum magnetic field range for superconductivity that can be attained by C doping. Carbon doped MgB2 filaments were prepared, and for carbon levels below 4% the transition temperatures are depressed by about 1 K/% C and H(c2)(T=0) rises by about 5 T/% C. This means that 3.8% C substitution will depress T(c) from 39.2 to 36.2 K and raise H(c2)(T=0) from 16.0 to 32.5 T. These rises in H(c2) are accompanied by a rise in resistivity at 40 K from about 0.5 to about 10 microOmega cm.

Anomalous Suppression of the Orthorhombic Lattice Distortion in Superconducting Ba(Fe1-xCox)2As2 Single Crystals

High-resolution x-ray diffraction measurements reveal an unusually strong response of the lattice to superconductivity in Ba(Fe1-xCox)2As2. The orthorhombic distortion of the lattice is suppressed and, for Co doping near x=0.063, the orthorhombic structure evolves smoothly back to a tetragonal structure. We propose that the coupling between orthorhombicity and superconductivity is indirect and arises due to the magnetoelastic coupling, in the form of emergent nematic order, and the strong competition between magnetism and superconductivity.

Momentum Dependence of the Superconducting Gap in NdFeAsO0.9F0.1 Single Crystals Measured by Angle Resolved Photoemission Spectroscopy

We use angle resolved photoemission spectroscopy to study the momentum dependence of the superconducting gap in NdFeAsO0.9F0.1 single crystals. We find that the Gamma hole pocket is fully gapped below the superconducting transition temperature. The value of the superconducting gap is 15+/-1.5 meV and its anisotropy around the hole pocket is smaller than 20% of this value-consistent with an isotropic or anisotropic s-wave symmetry of the order parameter. This is a significant departure from the situation in the cuprates, pointing to the possibility that the superconductivity in the iron arsenic based system arises from a different mechanism.

K-Doping Dependence of the Fermi Surface of the Iron-Arsenic Ba1-xKxFe2As2 Superconductor Using Angle-Resolved Photoemission Spectroscopy

C. Liu, G. D. Samolyuk, Y. Lee, N. Ni, T. Kondo, A. F. Santander-Syro, 3 S. L. Bud’ko, J. L. McChesney, E. Rotenberg, T. Valla, A. V. Fedorov, P. C. Canfield, B. N. Harmon, and A. Kaminski Ames Laboratory and Department of Physics and Astronomy, Iowa State University, Ames, Iowa 50011, USA Laboratoire Photons Et Matière, UPR-5 CNRS, ESPCI, 10 rue Vauquelin, 75231 Paris cedex 5, France Labratoire de Physique des Solides, UMR-8502 CNRS, Université Paris-Sud, Bât. 510, 91405 Orsay, France Advanced Light Source, Berkeley National Laboratory, Berkeley, California 94720, USA Condensed Matter Physics and Materials Science Dept., Brookhaven National Laboratory, Upton, New York 11973, USA (Dated: June 20, 2008)

Observation of Fermi arcs in the type-II Weyl semimetal candidate WTe2

In recently predicted type-II Weyl semimetals, the Weyl states connect hole and electron bands that would otherwise be separated by an indirect gap. The set of points in the momentum space at which both bands touch are called Weyl points and are connected by Fermi arcs at the surface of the sample. Experimental evidence confirmed existence of these exotic fermions in MoTe

Itinerant magnetic excitations in antiferromagnetic CaFe2As2.

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Coexistence of competing antiferromagnetic and superconducting phases in the underdoped Ba(Fe0.953Co0.047)2As2 compound using x-ray and neutron scattering techniques.

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