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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, Z├╝rich 8093, Switzerland
3. P.N. Lebedev Physical Institute RAS, Moscow 119991, Russian Federation
4. LAMIA-INFM, Perrone 24, Genova 16152, Italy


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