Abstract
We investigated a high-quality MgB2 thin film with a thickness of ~1000 nm on an Al2O3 substrate using optical spectroscopy. We measured the reflectance spectra of the film at various temperatures both below, and above, the superconducting transition temperature, Tc simeq 40 K. An earlier study showed that when the sample surface is exposed to air the optical properties of the surface change immediately, however, the saturated change is negligibly small in the far-infrared region. The optical conductivity spectrum in the normal state shows two (narrow and broad) Drude modes, with the narrow Drude mode being dominant in the low frequency region below 1000 cm−1. Our study, which uses a good-quality sample, provides more reliable data on the optical properties of MgB2, in a similar spectral range. The optical data is analyzed further using an extended Drude model, and the electron-phonon spectral density function, α2F(ω), is extracted. The spectral density function α2F(ω) features two peaks: a small one near 114 cm−1, and a strong peak around the 550 cm−1 where the B-B bond stretching phonon exists. Our data in the superconducting state does not show the expected energy shift of the onset of scattering associated with the α2F(ω) peaks.
Highlights
Superconductivity in magnesium diboride (MgB2), which has a superconducting transition temperature of Tc = 39 K, was discovered in 20011
Our report concludes with a further analysis of the measured reflectance spectra and comparison of other optical quantities with those reported by Tu et al Results and Discussion In Fig. 1(a), we display the measured reflectivity spectra from our MgB2 thin film/Al2O3 sample, alongside the reproduced reflectivity spectra of Tu et al It is evident from the figure that our reflectance spectra are higher than Tu et al.’s, at the three selected temperatures
We find that the four phonon peaks that are visible in Tu et al.’s results are greatly suppressed in our data, due to our sample having a stronger charge carrier screening effect than Tu et al.’s sample
Summary
We present new optical data obtained from a high-quality MgB2 thin film sample, over a wide spectral range that includes the far-infrared (FIR) regime (60–8000 cm−1). Our sample shows one-tenth of the residual resistivity compared to previously studied samples in the FIR region by optical spectroscopy[6, 8]. It is noteworthy that the surface of a single crystal MgB2 is very sensitive to air; one study showed that when the sample surface is exposed to air the ellipsometric parameters are affected immediately, due to a formation of a contamination[10]. Our reflectance spectra yield higher values than those reported by Tu et al in the FIR region. Our report concludes with a further analysis of the measured reflectance spectra and comparison of other optical quantities with those reported by Tu et al
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