Electron tunneling spectroscopy on superconducting Al-dopedMgB2thin films:πenergy gap and Eliashberg function

Abstract

Superconducting thin films of composition Mg 1−x Al x B 2 with 0 Յ x Ͻ 0.5 were prepared in situ by sublimation of Mg combined with B and Al magnetron sputtering. The critical temperature T c decreased linearly with x up to 0.4. For 0.4Ͻ x Ͻ 0.5 the formation of a plateaulike feature at a T c Ϸ 12 K was observed. This effect is supposed to be due to the incipient formation of the superstructure MgAlB 4 with ordered alternating Mg and Al planes separated by B planes. To detailedly study the influence of Al doping on the electron-phonon coupling in the polycrystalline films with a preferred c-axis texture quasiparticle tunneling experiments were performed on planar tunnel junctions with natural thermal oxide or artificial aluminum oxide tunnel barriers. Differential conductance measurements at low-bias voltage and low temperature of superconductor-insulatorsuperconductor tunnel junctions allowed the direct determination of the energy gap of the Fermi surface sheet. The energy gap decreased linearly with decreasing T c of the films in agreement with the model of band filling. Whereas all the tunneling studies published so far mainly revealed features of the two energy gaps, the observation of phonon-induced structures in the differential conductance measurements at high bias voltage, i.e., in the phonon region, in this study enabled the important determination of the energy-dependent Eliashberg function ␣ 2 F of the Fermi surface sheet for various Al doping levels. Compared to the undoped MgB 2 , significant changes in ␣ 2 F could be observed that were confirmed by first-principles calculations.

Electron-phonon coupling and two-band superconductivity of Al- and C-dopedMgB2

“…On contrast, the σπ contribution shows an slight increase around 50 and 70 meV, and the ππ contribution at higher frequencies strengthens slightly with doping, but the position of its main peak is almost unaffected. In recent electron tunneling spectroscopy measurements on Al-doped thin films 35 , this general behavior of the Eliashberg function indeed has been observed, supporting our results. In Fig.…”

“…The gap values for T → 0 K are ∆ σ = 7.04 meV and ∆ π = 2.71 meV, respectively, in good agreement with experimental results 8,9,34 . (⊠) 18 , (•) 20 , ( ) 21 , (⋆) 30 , (▽) 32 , ( ) 33 , (⊞, ) 31,34 , ( * ) 35 ; (b): (×) 22 , ( * ) 23,24 , (•) 25 , ( ) 26 , (▽) 33 , (⊞, ) 31,34 ( ) 37 , (⊗) 38 .…”

“…In parallel with the reduction of T c also a decrease of the superconducting gaps ∆ σ and ∆ π has been observed. For single crystals as well as polycrystals Al doping was found to decrease both σ and π gaps monotonically, which, however, stay distinguishable even for T c as low as 10 K (x ≈ 0.32) [29][30][31][32][33][34][35] . These observations indicate that the interband scattering Γ σπ remains small even at high doping levels, and is insufficient to produce a merging of the σ and π gaps.…”

Tunneling spectroscopy of superconducting nanospheres

“…The off-diagonal matrix elements are correspondingly lowered to V 01 = V 10 = −1 meV. The BCS-like solution of the nonuniform pairing model (20) is distinct from the solution of the Richardson model (4). Also, for this model, the gate voltage gives us the freedom to tune the spectrum so that the peaks from both levels merge under thermal broadening; see the curve for gate voltage 25 meV in Fig.…”

“…The tunneling spectroscopy in superconductors is a sensitive tool traditionally used to measure the energy gap and the energy spectrum of bosons coupled to electrons, be it internal [1][2][3][4] or external [5]. Its modification to tunneling, through ultrasmall metallic grains, made it possible to observe size and even-odd effects predicted by non-BCS treatments [6][7][8][9] of the Richardson model.…”