Structure of the superconducting gap in MgB 2 from point-contact spectroscopy (original) (raw)
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Measurement of the superconducting gap of MgB2 by point contact spectroscopy
Physica C: Superconductivity, 2002
We measured the superconducting gap of MgB 2 thin films and pellets by utilizing point contact Andreev reflection method with a gold tip. From more than 100 measurements, we obtained the distribution of the superconducting gap of the material. This distribution shows two peaks, one at 2-3 meV and the other around 6-8 meV while BCS weak coupling limit predicts 5.9 meV. From temperature dependence measurements, we observed energy gap to nearly follow BCS-like behavior. These results also seem to agree well with the recent anisotropic energy gap estimation based on the ab initio calculation from the band structure of MgB 2 .
physica status solidi (c), 2005
We studied the effects of carbon and aluminum substitutions on the gaps of the two-band superconductor MgB2 by means of point-contact measurements in Mg(B1−xCx)2 and Mg1−yAlyB2 single crystals with 0 ≤ x ≤ 0.132 and 0 ≤ y ≤ 0.21. The gap amplitudes, ∆σ and ∆π, were determined by fitting the conductance curves of the point contacts with the standard Blonder-Tinkham-Klapwijk (BTK) model generalized to the two-band case. Whenever possible, their values were confirmed by the independent fit (with a single-band BTK model) of the partial contribution of the two bands to the conductance, separated by means of a suitable magnetic field B * . In C-substituted crystals, the two gaps remain clearly distinct up to x ∼ 0.10, but at x = 0.132 we observed for the first time their merging into a single gap ∆ 3 meV with a gap ratio 2∆/kBTc close to the standard BCS value. In Al-substituted crystals, we found no evidence of this gap merging. Instead, ∆π reaches the value 0.4 meV at y=0.21, where ∆σ saturates at about 4 meV. These results are compared with other recent experimental findings in polycrystals and with the predictions of the models for multiband superconductivity.
Point-contact spectroscopy in MgB 2 : from fundamental physics to thin-film characterization
Superconductor Science and Technology, 2004
In this paper we highlight the advantages of using point-contact spectroscopy (PCS) in multigap superconductors like MgB 2 , both as a fundamental research tool and as a non-destructive diagnostic technique for the optimization of thin-film characteristics. We first present some results of crucial fundamental interest obtained by directional PCS in MgB 2 single crystals, for example the temperature dependence of the gaps and of the critical fields and the effect of a magnetic field on the gap amplitudes. Then, we show how PCS can provide useful information about the surface properties of MgB 2 thin films (e.g. T c , gap amplitude(s), clean or dirty-limit conditions) in view of their optimization for the fabrication of tunnel and Josephson junctions for applications in superconducting electronics.
Superconducting gap parameters of MgB 2 obtained on MgB 2 /Ag and MgB 2 /In junctions
MgB2 superconducting wires with critical temperature Tc approaching 40 K were used for preparation of MgB2/Ag and MgB2/In junctions. The differential conductance vs. voltage characteristics of N-S junctions exhibit clear contribution of Andreev reflection. Using modified BTK theory for s-wave superconductors two order parameters ∆ dirty ≈ 4 meV and ∆3D ≈ 2.6 meV have been determined from temperature dependencies. Surprisingly, larger order parameter ∆ dirty vanishes at lower temperature T cdirty ≈ 20 K than smaller one ∆3D with Tc ≈ 38 K. Both the magnitudes of the order parameters and their critical temperatures are in good agreement with theoretical calculations of electron-phonon coupling in MgB2 carried out by Liu et al.
Journal of Physics and Chemistry of Solids, 2006
We present the results of directional point-contact spectroscopy in state-of-the-art Mg 1Kx Al x B 2 and Mg(B 1Ky C y ) 2 single crystals produced at ETHZ. Fitting the conductance curves of our point contacts, that always feature Andreev reflection structures, we obtained the doping dependence of the gap amplitudes. The results are discussed in comparison with other experimental findings and relevant theoretical predictions. We conclude that the physics of Al-substituted crystals at xR0.09 might be governed by phase segregation, while C-substituted crystals unexpectedly show a doping-induced transition to single-gap superconductivity at yZ0.132. q
Physical Review Letters, 2002
We present the results of the first directional point-contact spectroscopy experiments in highquality MgB2 single crystals. Due to the directionality of the current injection into the samples, the application of a magnetic field allowed us to separate the contributions of the σ and π bands to the total conductance of our point contacts. By using this technique, we were able to obtain the temperature dependency of each gap independent of the other. The consequent, strong reduction of the error on the value of the gap amplitude as function of temperature allows a stricter test of the predictions of the two-band model for MgB2.
The origin of the anomalous superconducting properties of MgB2
Nature, 2002
Magnesium diboride 1 differs from ordinary metallic superconductors in several important ways, including the failure of conventional models 2 to predict accurately its unusually high transition temperature, the effects of isotope substitution on the critical transition temperature, and its anomalous specific heat . A detailed examination of the energy associated with the formation of charge-carrying pairs, referred to as the 'superconducting energy gap', should clarify why MgB 2 is different. Some early experimental studies have indicated that MgB 2 has multiple gaps 3-9 , but past theoretical studies 10-16 have not explained from first principles the origin of these gaps and their effects. Here we report an ab initio calculation of the superconducting gaps in MgB 2 and their effects on measurable quantities. An important feature is that the electronic states dominated by orbitals in the boron plane couple strongly to specific phonon modes, making pair formation favourable. This explains the high transition temperature, the anomalous structure in the specific heat, and the existence of multiple gaps in this material. Our analysis suggests comparable or higher transition temperatures may result in layered materials based on B, C and N with partially filled planar orbitals.
Physical Review B, 2004
We present the results of directional point-contact measurements in MgB2 single crystals, in magnetic fields up to 9 T parallel to the c axis. By fitting the conductance curves of our point contacts -showing clear Andreev-reflection features -with a generalized Blonder-Tinkham-Klapwijk model, we were able to extract the values of the two gaps ∆σ and ∆π. The comparison of the resulting ∆σ(B) and ∆π(B) curves to the theoretical predictions of the two-band model in dirty limit, recently appeared in literature, allows the first direct test of this model and gives a clear and quantitative proof that the π band, even in the best single crystals, is in the moderate dirty limit.
Two-gap interplay in MgB2: a tunneling spectroscopy study
Physica C: Superconductivity, 2004
Tunneling spectroscopy on various samples of MgB 2 was performed. The first direct evidence for the two-gap superconductivity was given with an inverted junction setup, in which a small crystal of MgB 2 was used as the STM tip and 2H-NbSe 2 as the sample. This technique allowed to show that both gaps close at the critical temperature of the bulk material and thus are intimately related to the superconductivity. The experiments performed in the standard N-IS geometry evidenced for two strongly coupled gaps D L ¼ 7:0 AE 1:0 meV and D S ¼ 3:0 AE 1:0 meV at 4.2 K. STS on asgrown c-axis oriented thin films yielded only small gap which confirmed the identification of this gap as originating from 3D-like p-band and, by exclusion, that of the large gap from 2D-like r-band. The low gap values D S ¼ 2:2 AE 0:3 meV were attributed to the degraded film surface. After chemical etching, the gap increased to D S ¼ 2:8 AE 0:3 meV. The c-axis tunneling spectra are better fitted considering anisotropic superconductivity inside the p-band. The issues of our findings are discussed in terms of two-band superconductivity.