Critical field ofMgB2: Crossover from clean to dirty regimes

Abstract

We have studied the upper critical field, B c2 , in poly-crystalline MgB 2 samples in which disorder was varied in a controlled way to carry selectively π and σ bands from clean to dirty limit. We have found that the clean regime survives when π bands are dirty and σ bands are midway between clean and dirty. In this framework we can explain the anomalous behaviour of Al doped samples, in which B c2 decreases as doping increases.The two gap superconductivity, studied since the fifties, has found a very nice paradigm in MgB 2 which shows two well separated gaps associated with the two band sets which cross the Fermi level.The larger gap ∆ σ (≈7 meV) is related to the two-dimensional p-type σ bands, strongly coupled with optical boron modes, whereas the smaller gap ∆ π (≈2 meV) is related with the three-dimensional π bands.1,2 π and σ bands have nearly equal density of states, so that both contribute significantly to the physical properties.The upper critical field B c2 was one of the first properties that emphasized the existence of two 6-8

Magnetic field dependence of the density of states in the multiband superconductorβ−Bi2Pd

“…Previous theoretical work remarked that a decrease in H c2 might be observed by producing strong enough interband scattering [10,50]. The experiments show, however, a decrease of T c with interband scattering, concomitant with a more isotropic superconducting gap and increased H c2 [51][52][53]60]. In β-Bi 2 Pd, T c is practically not modified by scattering and the gap is already nearly isotropic, but H c2 (T ) behavior shows the influence of multiband Fermi surface properties and interband scattering.…”

“…8), and compared results with available data in a sample with a larger mean free path [17]. A smaller mean free path leads to a shorter coherence length and hence to an increased H c2 (T), also in multigap superconductors, see for instance MgB 2 [10,[50][51][52][53]. Here, however, we do not observe such an increase in H c2 and instead we find slightly smaller H c2 (T ) than in previous measurements on samples with a larger mean free path.…”

Effect of processing temperature on high field critical current density and upper critical field of nanocarbon doped MgB2

“…16,26 All of this can be understood in terms of carbon substitution being the dominant defect changing the H c2 for nanocarbon doped samples and hence increasing the charge carrier scattering rates. [27][28][29] On the other hand, the drop of H c2 and higher RRR for both undoped and SiC doped MgB 2 sintered at high temperature suggests a reduction of the impurity scattering of charge carriers due to improved crystallnity with high sintering temperature. Impurity scattering of charge carriers in nano-SiC doped MgB 2 is contributed by both C substitution and large crystal disorder due to crystal growth at low temperature and small crystal size.…”

Superconducting Properties of Carbonaceous Chemical Doped MgB2

“…H c2 (0) of pure MgB 2 increased from 16.0 to 32.5 T in a carbon doped MgB 2 filament with slight depression of T c from 39.2 to 36.2 K for 3.8% C substitution, using the chemical vapor deposition (CVD) method to co-deposit B together with carbon (Wilke et al, 2004). The carbon substitution effects on H c2 have shown an encouraging enhancement, with a range of enhanced H c2 values from 25 to 40 T at temperatures of 4.2 K and below Ohmichi et al, 2004;Putti et al, 2004). Furthermore, H c2 with a value of 52-55 T has been commonly observed for carbon alloyed thin films at temperatures around 1.5-4.2 K Ferrando et al, 2005).…”