Superconducting energy gap in aMgB2film observed by infrared reflectance

Abstract

Far-infrared reflectance of a MgB2 film has been measured by Fourier-transform spectroscopy for frequencies 10 cm −1 < ν < 4000 cm −1 above and below the superconducting transition. The data provide clear experimental evidence for the onset of a superconducting gap at 24 cm −1 at T = 5 K. On increasing temperature the gap energy increases, contrary to what is expected in isotropic BCS superconductors. The small zero-temperature gap value and its unconventional increases on increasing temperature can only be explained by a highly anisotropic or multiple gap function.

Terahertz conductivity at the Verwey transition in magnetite

“…In addition, the reflectance for the frequency range 4 cm −1 Ͻ Ͻ 40 cm −1 has been calculated using the complex conductivity data, obtained by the transmittance technique described above. The combination of the results from two experimental techniques 36 substantially expands the low-frequency limit of the available spectrum and the quality of the subsequent Kramers-Kronig transformation. Figure 1 shows the complex reflectance and transmittance of magnetite in the vicinity of the Verwey transition.…”

Anisotropic s -wave superconductivity: Comparison with experiments on MgB 2 single crystals

“…It has recently been noted that the gap seen in optical conductivity appears to be very small [24,25]. We calculated the anisotropic analog of the Mattis-Bardeen conductivity [26] averaged over all directions, which should correspond to the response of a polycrystalline thin film sample as has been used in Refs.…”

Multiband and impurity effects in infrared and optical spectra of MgB2

“…The spectra were taken down to 3 meV while changing temperature down to 1.6 K and applying magnetic fields up to 7 T, which allowed a fully optical determination of the temperature dependence of the c-axis critical field H c2 (∼ 5 T at lowest 1.6 K). The authors [46] observed the absorption threshold at about ∼ 4 meV, which is close to the observation on thin films [52,49]. Di Castro et al [24] measured the ratio R S /R N on separate ab-oriented single crystals of MgB 2 (T c =38.5 K) using the synchrotron radiation between 3 and 12 meV to compensate the loss of signal due to the small sample size.…”

“…Further experiments on thin films [49,50,51,52,53,54], thick films [18], single crystals [46,24] and dense polycrystals [19,55] provided much more detailed and quantitative spectral information that in general supported such assignment ( Table 1). Note that this gap value gives an anomalously small ratio 2∆/k B T c ∼ 1 − 2 as compared to the weak-coupling BCS result of 3.52.…”

“…A strong depletion of absorption was observed below ∼ 5 meV at 6 K, although the data do not exclude that the full conductivity suppression is at somewhat lower energy (Fig.11). Pimenov et al [49] obtained the reflectance of a thin MgB 2 film with a similar T c (32 K) (Fig.12). The spectral range was extended down to 0.5 meV by using a quasioptical backward wave oscillator technique which gives directly σ 1 and σ 2 .…”