The superconducting gaps of C-substituted and Al-substituted MgB2 single crystals by point-contact spectroscopy

Dario Daghero 1,4Renato S. Gonnelli 1,4Andrea Calzolari 1Giovanni A. Ummarino 1,4Valeria Dellarocca 1Valeri A. Stepanov 3Nikolai D. Zhigadlo 2S. M. Kazakov 2Janusz Karpinski 2

1. INFM and Dipartimento di Fisica, Politecnico di Torino, Corso Duca degli Abruzzi, 29, Torino 10129, Italy
2. Laboratory for Solid State Physics ETH (ETH), Schafmatstr. 16, Zürich 8093, Switzerland
3. P.N. Lebedev Physical Institute RAS, Moscow 119991, Russian Federation
4. LAMIA-INFM, C.so Perrone 24, Genova 16152, Italy

Abstract

We report the results of the first directional point-contact measurements in Mg(B1-xCx)2 single crystals with 0 <= x <= 0.132 and in Mg1-yAlyB2 crystals with 0 <= y <= 0.21, obtained at ETH (Zurich) by means of a high-pressure technique in a cubic anvil. We made the point contacts by using the "soft" technique described elsewhere [1]. The Andreev-reflection features of our conductance curves were measured in magnetic fields up to 9 T, applied either parallel to the c axis or to the ab plane of the crystals. By fitting the normalized conductance with the Blonder-Tinkham-Klapwijk (BTK) model generalized to the two-band case we obtained the dependence of the gaps Δπ and Δσ on the C or Al content (x or y). This dependence was confirmed by applying to the junctions a suitable magnetic field B* able to remove the contribution of the π-band gap to the total conductance, without appreciably affecting the σ-band one [2]. This allowed the separate determination of the gaps via a single-band three-parameter BTK fit.
In C-substituted crystals up to x ~ 0.10, the two-gap nature of superconductivity characteristic of pure MgB2 is preserved. At x = 0.132 we clearly and reproducibly observed for the first time the theoretically predicted merging of Δπ and Δσ into a single gap Δ ~ 3 meV with a gap ratio 2Δ/kBTc close to the standard BCS value [3].
In Al-substituted crystals, we found no evidence of gap merging: Δπ reaches the value 0.4 meV at y=0.21, where Δσ saturates at about 4 meV. The behaviour of Δπ(y) and Δσ(y) cannot be simply explained by the two-band model and can be due to the presence in these crystals of a phase segregation of Al-rich layers.

[1] R.S. Gonnelli et al., Phys. Rev. Lett. 89, 247004 (2002).
[2] R.S. Gonnelli et al., Phys. Rev. B 69, 100504(R) (2004).
[3] R.S. Gonnelli et al., cond-mat/0407265.

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Presentation: oral at E-MRS Fall Meeting 2004, Symposium E, by Dario Daghero
See On-line Journal of E-MRS Fall Meeting 2004

Submitted: 2004-07-22 14:44
Revised:   2009-06-08 12:55
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