Synthesis, properties and possible applications of MgB2
Paul C. Canfield
Ames Laboratory and Department of Physics and Astronomy, Iowa State University, Ames, Iowa, 50011 U.S.A.
Over the past year there has been a great deal of excitement about the intermetallic superconductor MgB2. MgB2 has a superconducting transition temperature Tc ~ 40 K, can be synthesized as single phase powders, wire segments, and thin films with remarkably low normal state resistivity, and manifests a promising critical current density in the superconducting state. In this lecture I will review recent experimental work on MgB2 and try to show how MgB2 fits into the basic gestalt of superconducting, intermetallic compounds. This will include a review of such topics as: sample preparation, isotope effect, critical current and irreversibility field, and Hc2 and its anisotropy. During the lecture I will try to indicate possible applications of this remarkable material.
de Haas-van Alphen effect in single crystal MgB2
A. Carringtona, J.R. Cooperb, N.E. Husseya, P.J. Meesona, E.A. Yellandb, S. Leec, A. Yamamotoc, S. Tajimac
aH.H. Wills Physics Laboratory, University of Bristol, U.K. bIRC in Superconductivity, University of Cambridge, U.K. cSuperconductivity Research Laboratory, ISTEC, Tokyo, Japan.
We report observations of quantum oscillations in single crystals of the 39 K superconductor MgB2. Three de Haas-van Alphen frequencies are clearly resolved. Comparison with band structure calculations strongly suggests that two of these come from a single warped Fermi surface tube along the c direction, and that the third arises from cylindrical sections of an in-plane honeycomb network. The measured values of the effective mass range from 0.44-0.68me. By comparing these with band masses calculated recently by three groups, we find that the electron-phonon coupling strength l, is a factor ~ 3 larger for the c-axis tube orbits than for the in-plane network orbit, in accord with recent microscopic calculations.
Experimental Study of Electron-phonon Coupling in MgB2
Setsuko Tajima, Sergey Lee, Takahiko Masui, James Quilty, Ayako Yamamoto
Superconductivity Research Laboratory, ISTEC, Tokyo 135-0062, Japan
We have investigated various physical properties of MgB2, using high quality single crystals. In the Raman scattering spectrum, the peak of E2g boron vibrational mode shows anomalous broadening, softening and asymmetry, indicating strong coupling with the electronic system. From the temperature dependence of resitivity, it has been deduced that the high-frequency optical phonon makes a major contribution to the carrier scattering. The pressure dependence of resistivity and critical temperature reveals a big contribution of this phonon to the superconductivity. Combined with the result of de Haas van Alphen effect, these results demonstrate that the strong coupling of the boron E2g mode with the boron s-band gives rise to the high-Tc value of this superconductor. This work was supported by the NEDO, Japan.
Scanning Tunneling Spectroscopy in MgB2
Goran Karapetrov, Maria Iavarone, Alex E. Koshelev, W.K. Kwok, G.W. Crabtree, D.G. Hinks
Materials Science Division, Argonne National Laboratory, Argonne, IL 60439, USA
We present study of the anisotropic superconductor MgB2 using a combination of scanning tunneling microscopy and spectroscopy. The results reveal two distinct energy gaps at D1=2.3 meV and D2=7.1 meV. Different spectral weights of the partial superconducting density of states are a reflection of different tunneling directions in this multi-band system. Our experimental observations are consistent with the existence of two-band superconductivity in the presence of interband superconducting pair interaction and quasiparticle scattering. Temperature evolution of the tunneling spectra follows the BCS scenario with both gaps vanishing at the bulk Tc. The data confirm the importance of Fermi-surface sheet dependent superconductivity in MgB2 proposed in the multigap model by Liu et al.
Parasitic Superconductivity in Magnesium Diboride
Morten R. Eskildsena, Martin Kuglera, Shukichi Tanakaa, Jan Junb, Serguei M. Kazakovb, Janusz Karpinskib, Ø ystein Fischera
aDPMC, University of Geneva, CH-1211 Geneve 4, Switzerland
bSolid State Physics Laboratory, ETH, CH-8093 Zurich, Switzerland
We report results of scanning tunneling spectroscopy on a single crystal of MgB2. The measurements were made on the surface of an as grown crystal, with the tunnel current parallel to the c-axis. In this geometry one only couples to the p-band, and we observe a single gap with D = 2.2 meV. Vortex imaging was performed at a range of applied fields, H ||c, going from 0.05 T to 0.5 T, and revealed giant vortices with an extreme degree of vortex core overlap already at low fields. These results are consistent with superconductivity in the p-band being a parasite of the s-band. Furthermore, using a single measured vortex profile, we are able to explain the anomalous field dependence of the elecronic specific heat.