M. V. Twigg

Bio: M. V. Twigg is an academic researcher. The author has contributed to research in topics: Flux pumping & High-temperature superconductivity. The author has an hindex of 1, co-authored 1 publications receiving 163 citations.

Conductive dense hydrogen

Abstract: Molecular hydrogen is expected to display metallic properties under high pressures, but so far experiments performed at low temperatures (100 K) have showed that hydrogen remains insulating up to 300 GPa. A transformation of normal molecular hydrogen to a conductive and metallic state at room temperature is now observed above 220 GPa.

High-temperature superconductivity in iron-based layered compounds

Abstract: Basic experimental data are presented for a new class of high-temperature superconductors — iron-based layered compounds of the types REOFeAs (RE = La, Ce, Nd, Pr, Sm, ...), AFe2As2 (A = Ba, Sr, ...), AFeAs (A = Li, ...), and FeSe(Te). The structure of electronic spectra in these compounds is discussed, including the correlation effects, as is the spectrum and role of collective excitations (phonons and spin waves). Basic models for describing various types of magnetic ordering and Cooper pairing are reviewed.

Crystal structures of high- T c oxides

Abstract: A comprehensive review of structure work on high-Tc oxides as reported during the years 1987 and 1988 is given. Thirteen structures are refined from X-ray single-crystal and/or neutron powder diffraction data:I. (Ba1−x,Kx)BiO3 (Tc=30 K),II. (La2−x, Srx)CuO4 (Tc=40 K),III. (Nd, Ce, Sr)2CuO4 (Tc=28 K),IV. (Nd2−x, Cex)CuO4 (Tc=24 K),V. YBa2Cu3O7 (Tc=90 K),VI. YBa2Cu4O8 (Tc=80 K),VII. Y2Ba4Cu7O14 (Tc=40 K),VIII. Pb2Sr2NdCu3O8 (Tc=70 K),IX. TlBa2CaCu2O7 (Tc=103 K),X. TlBa2Ca2Cu3O9 (Tc=120 K),XI. Tl2Ba2CuO6 (Tc=90 K),XII. Tl2Ba2CaCu2O8 (Tc=112 K),XIII. Tl2Ba2Ca2Cu3O10 (Tc=125 K). Except forI (perovskite type),II (K2NiF4 type) andIV (Nd2CuO4 type) they represent new structure types. Structure data, bond distances, structure drawings and calculated X-ray powder diffraction patterns are given for each compound. Structural features and correlations with superconductivity are discussed. The review contains 301 citations.

Feshbach shape resonance in multiband superconductivity in heterostructures

Abstract: We report experimental data showing the Feshbach shape resonance in the electron doped MgB2 where the chemical potential is tuned by Al, Sc, and C substitutions. The scaling of the critical temperature Tc as a function of the Lifshitz parameter z = EΓ−EF, where EF is the chemical potential and EΓ is the energy of the Γ critical point where the σ Fermi surface changes from the 3D to a 2D topology, is reported. The resonant amplification of Tc(z) driven by the interband pairing is assigned to a Feshbach shape resonance characterized by quantum superposition of pairs in states corresponding to different spatial location and different parity. It is centered at z = 0 where the chemical potential is tuned to a Van Hove-Lifshits feature for the change of Fermi surface dimensionality in the electronic energy spectrum in one of the subbands. In this heterostructure at atomic limit the multiband superconductivity is in the clean limit because the disparity and negligible overlap between electron wavefunctions in different subbands suppresses the single electron interband impurity scattering rate. The emerging scenario from these experimental data suggests that the Feshbach shape resonance could be the mechanism for high Tc in particular nanostructured architectures.

Nonaqueous chemical etch for YBa2Cu3O7−x

Abstract: A nonaqueous chemical etch, with Br as the active ingredient, is described which removes the insulating hydroxides and carbonates that form on high-temperature superconductor surfaces as a result of atmospheric exposure. X-ray photoemission spectra have been recorded before and after etching YBa2Cu3O(7-x) films. It is found that, after the etch, the high binding energy O 1s and Ba 3d peaks associated with surface contaminants are greatly reduced, the Y:Ba:Cu ratio is close to the expected 1:2:3, and the oxidation state of the Cu(2+) is not affected. The resistance of an etched film reaches zero at 78 K, compared to 81 K for a similar unetched film. The suitability of other nonaqueous halogen-based etches is discussed, as is the applicability of this etch to other high-Tc superconductors.