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Local Tunneling Study of Three-Dimensional Order Parameter in the $π$-band of Al-doped MgB$_2$ Single Crystals

F. Giubileo·F. Bobba·A. Scarfato·A. M. Cucolo·A. Kohen·D. Roditchev·N. Zhigadlo·J. Karpinski
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Abstract

We have performed local tunneling spectroscopy on high quality Mg$_{1-x}$Al$_x$B$_2$ single crystals by means of Variable Temperature Scanning Tunneling Spectroscopy (STS) in magnetic field up to 3 Tesla. Single gap conductance spectra due to c-axis tunneling were extensively measured, probing different amplitudes of the three-dimensional $Δ_π$ as a function of Al content. Temperature and magnetic field dependences of the conductance spectra were studied in S-I-N configuration: the effect of the doping resulted in a monotonous reduction of the locally measured $T_C$ down to 24K for x=0.2. On the other hand, we have found that the gap amplitude shows a maximum value $Δ_π= 2.3$ meV for x=0.1, while the $Δ_π/ T_C$ ratio increases monotonously with doping. The locally measured upper critical field was found to be strongly related to the gap amplitude, showing the maximum value $H_{c2}\simeq3T$ for x=0.1 substituted samples. For this Al concentration the data revealed some spatial inhomogeneity in the distribution of $Δ_π$ on nanometer scale.

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F. Giubileo, F. Bobba, A. Scarfato, A. M. Cucolo, A. Kohen, D. Roditchev, N. Zhigadlo, J. Karpinski. 2006-04-13. Local Tunneling Study of Three-Dimensional Order Parameter in the $π$-band of Al-doped MgB$_2$ Single Crystals. https://doi.org/10.1103/physrevb.76.024507

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