Effects of the two-gap nature on the microwave conductivity of polycrystallineMgB2films with a critical temperature of39K

Abstract

The surface resistance (R S ) and the real part (σ 1 ) of the microwave complex conductivity of a ~380 nm-thick polycrystalline MgB 2 film with the critical temperature (T C ) of 39.3 K were investigated at ~8.5 GHz as a function of temperature. Two coherence peaks were observed in the σ 1 versus temperature curve at temperatures of ~0.5 T C and ~0.9 T C , respectively, providing a direct evidence for the two-gap nature of MgB 2 . The film appeared to have a π-band gap energy of 1.8 meV. For the MgB 2 film ion-milled down to the thickness of ~320 nm, two coherence peaks were still observed with the first conductivity peak at ~0.6 T C . Reduction of T C by 3 K and reduced normal-state conductivity at T C were observed along with an enhanced π-band gap energy of 2.1 meV and a reduced R S at temperatures below 15 K for the ion-milled film. Calculations based on the gap energies from the weak coupling Bardeen-Cooper-Schrieffer theory and the strong coupling theory suggest that both the σ-band and the π-band contribute to σ 1 of the polycrystalline MgB 2 films significantly. Our results are in contrast with the observation of single coherence peak at ~0.6 T C and dominant role of the π-band in the microwave conductivity of c-axis oriented MgB 2 films as reported by Jin et al. [Phys. Rev. Lett. 91, 127006 (2003)]. Variations in the inter-band coupling constants with the level of disorder can account for the different T C and σ 1 behavior for the as-grown and ion-milled films.Our results suggest that enhanced inter-band scattering can improve microwave properties of MgB 2 films at low temperatures due to the larger π-band gap despite the reduction of T C .

Effect of disorder on the far-infrared conductivity and on the microwave conductivity of two-band superconductors

“…In addition, the results obtained on the film with the higher T c showed a second, lower peak at about 0.53T c which appears to be sharper than the broad oval feature seen in [31] (see figures 3 and 4 in [32]). Upon ion-milling the second peak became more of a shoulder at about 0.6T c , the peak at 0.9T c became broadened, but increased in size, and the T c dropped from 39.4 to 36.3 K. The results for the normalized microwave conductivity obtained in [33] at 19 GHz (ν = 0.079 meV) on MgB 2 and Mg 0.95 Al 0.05 B 2 pellets resembled more those found in [32]. The Al-doped sample displayed a more pronounced shoulder at about 0.4-0.5T c and a more pronounced coherence peak just below the T c than the magnesium diboride sample.…”

“…The data of Lee et al [32] obtained at 8.5 GHz (ν = 0.035 meV) on polycrystalline MgB 2 films with T c s of 39.3 and 36.3 K showed rather sharp coherence peaks at about 0.9T c . In addition, the results obtained on the film with the higher T c showed a second, lower peak at about 0.53T c which appears to be sharper than the broad oval feature seen in [31] (see figures 3 and 4 in [32]). Upon ion-milling the second peak became more of a shoulder at about 0.6T c , the peak at 0.9T c became broadened, but increased in size, and the T c dropped from 39.4 to 36.3 K. The results for the normalized microwave conductivity obtained in [33] at 19 GHz (ν = 0.079 meV) on MgB 2 and Mg 0.95 Al 0.05 B 2 pellets resembled more those found in [32].…”

“…Section 3 contains our results for σ (ν, T ) obtained using realistic interaction parameters for MgB 2 and a comparison of experimental results [21] with our theoretical predictions. The same section also contains our results for the temperature dependence of the microwave conductivity and their comparison with experiments [31][32][33]. Section 4 contains a summary.…”

“…The microwave conductivity of MgB 2 was measured in [31][32][33] with different results. In [31], the normalized real part of the conductivity at 17.9 GHz (ν = 0.074 meV) measured on the c-axis oriented MgB 2 films had a broad maximum at a temperature of about 0.6T c , instead of the usual coherence peak seen in the dirty limit just below the T c [34].…”

“…In [31], the normalized real part of the conductivity at 17.9 GHz (ν = 0.074 meV) measured on the c-axis oriented MgB 2 films had a broad maximum at a temperature of about 0.6T c , instead of the usual coherence peak seen in the dirty limit just below the T c [34]. The data of Lee et al [32] obtained at 8.5 GHz (ν = 0.035 meV) on polycrystalline MgB 2 films with T c s of 39.3 and 36.3 K showed rather sharp coherence peaks at about 0.9T c . In addition, the results obtained on the film with the higher T c showed a second, lower peak at about 0.53T c which appears to be sharper than the broad oval feature seen in [31] (see figures 3 and 4 in [32]).…”