Supravodivosť
Superconductivity

MgB₂ is a two-gap superconductor in which the quasi two dimensional s band and the isotropic p band almost separately contributes to the superconductivity. The influence of impurity scattering and/or chemical doping on the physical properties of such a two-gap system is widely studied. The only on site substitutions by non magnetic elements which are known so fare are C for B and Al for Mg. Both these elements act not only as the scattering centers but they also dope MgB₂ with one extra electron filling the s band with strongly coupled holes, thus decreasing their density of states and T_c. Recently, Kortus et al. [1] proposed a model incorporating the interband scattering and at same time the band filling in MgB₂. The former effect leads to approaching of the two gaps by increasing of D_p and decreasing D_s while the latter suppresses both gaps proportionally with T_c. This model is general with no prediction on the strength of band filling and interband scattering effects in a particular doped material. The published experiments bring rather controversial picture as far as the strength of both effects and the development of two gaps in the Al and C-doped MgB₂ is concerned.
    We have studied superconductivity in both Al and C-doped MgB₂ systems with T_c’s varying between ~ 20 K and 39 K. We have investigated different types of samples: polycrystalline samples from both dopings and Al-doped single crystals with about the same Al content. Point-contact (PC) spectroscopy measurements have been performed for the systematic study of the two-gap superconductivity in both doped series. The values of the energy gaps have been determined from the PC spectra by fitting for the two-gap theoretical model. Experimental data, obtained on polycrystalline C- and Al-doped samples have been presented in [2,3]. In the paper [4] we report on specific heat, Hall probe magnetization and PC spectroscopy measurements on Mg_(1-x)Al_xB₂ single crystals. In this paper except of energy gap studies the influence of Al-doping for the critical magnetic fields and their temperature dependent anisotropies have been investigated too. The summarization of our main results is displayed in Figure 1, where the values of the energy gaps as a function of T_c of Al-doped and C-doped samples are shown. The large gap D_s is essentially decreased linearly with the respective T_c. The behavior of D_p is more complicated. For both kinds of substitution the small gap is almost unchanged at smaller dopings. In Mg_(1-x)Al_xB₂ D_p of the 10 % Al-doped sample seems to even slightly increase.

Figure 1:
The s band and the p band energy gaps as a function of T_c obtained from the point-contact spectroscopy on: C-doped polycrystals (solid symbols), Al-doped polycrystals (large open circles), Al-doped single crystals (small open circles). The open diamonds are the gaps from the specific heat data on Al-doped single crystals.

    But for the highest dopings D_p decreases in both cases. The solid lines are theoretical calculations for the case of a pure band filling effect and no interband scattering. The dashed lines show the calculations including also the interband scattering with the rate IB = 1000 x cm-1 (or 2000 y cm-1). Comparing all here presented gap values obtained on different types of polycrystals and single crystals it is evident, that the the decreased Tc and the evolution of the two gaps upon both substitutions are mainly the consequence of the band filling effect. However, the energy gaps in Al-doped samples reveal stronger tendency to approach each other and their evolution cannot be accounted without an interband scattering but it is smaller than in the presented calculations.

[1]J.Kortus et al., Phys.Rev.Lett.94 (2005) 027002.
[2]P.Samuely et al., Physica C, in Press.
[3]P.Samuely et al.,Phys.Rev.Lett.95 (2005) 099701.
[4]T.Klein,... P.Szabó et al., Phys.Rev.B73 (2006) 224528.

P. Szabó, Hoľanová, P. Samuely (Inst. Exp. Physics, Košice, Slovakia)
T. Klein, J. Marcus, L. Lyard (CNRS, Grenoble),
C.Marcenat (CEA-DRFMC, Grenoble),
S-I.Lee (Pohang University, Korea)