Retention of two-band superconductivity in highly carbon-dopedMgB2

Abstract

Tunneling data on MgB1.8C0.2 show a reduction in the energy gap of the π-bands by a factor of two from undoped MgB2 that is consistent with the Tc reduction, but inconsistent with the expectations of the dirty limit. Dirty-limit theory for undoped MgB2 predicts a single gap about three times larger than measured and a reduced Tc comparable to that measured. Our heavilydoped samples exhibit a uniform dispersion of C suggestive of significantly enhanced scattering, and we conclude that the retention of two-band superconductivity in these samples is caused by a selective suppression of interband scattering. The simple binary compound, MgB 2 , 1 appears to be an elegant example of a two-band superconductor with a high T c of 39 K, in which superconductivity due to strong electron-phonon coupling in the two-dimensional (2D) σ-bands is induced into the 3D π-bands by a combination of interband electron-phonon coupling and weak interband quasiparticle scattering. Several key signatures of this include the closing of two energy gaps 2,3,4,5,6,7 at the same T c , anomalous features in the specific heat and its field dependence 8 and the direct observation of interband scattering in the tunneling spectrum. 2

Raman scattering investigation of electron–phonon coupling in carbon substituted MgB2

“…From these experimental results it is now clear that the T C of MgB 2 with ~10 at.% of the B substituted with carbon is 22 K. This decrease in T C to 22 K, is much smaller than that expected from rigid band behavior. Understanding the T C dependence and robustness of two gap superconductivity in carbon doped MgB 2 [12,13] has motivated several band structure calculations [14 -17]; in particular, the effect of electron doping on the σ band has been investigated in [16]. These calculations indicate that for x=0.2 in MgB 2-x C x (corresponding to10 at.% C substitution), the σ band holes are still present and that for x=0.4 they completely vanish at E F .…”

Magnesium Diboride MgB2: A Simple Compound with Important Physical Properties

“…These bands are closely related to the two gaps predicted from the theoretical calculations and realised in the experimental results, particularly in specific heat measurements and tunnelling data. [149][150][151][152][153][154][155][156][157][158][159][160] Figure 5.19(a) depicts the scanning tunnelling spectrum with tunnel current along the c-axis. The spectrum consists of a single gap feature with coherence peaks at -2:9 meV which has been attributed to the p-band and to the weak shoulders at -6 meV arising mainly from the s-band.…”

Influence of aluminum substitution on the vortex matter properties ofMgB2

“…Until now, carbon [8][9][10][11][12][13][14][15][16][17][18][19] and aluminum 20-29 substitutions have been studied in appreciable detail. Since carbon substitutes boron, it is expected that it induces large interband scattering.…”