Disorder-induced collapse of the electron-phonon coupling inMgB2observed by Raman spectroscopy

Abstract

The Raman spectrum of the superconductor MgB 2 has been measured as a function of the T c of the film. A striking correlation is observed between the T c onset and the frequency of the E 2g mode. Analysis of the data with the McMillan formula provides clear experimental evidence for the collapse of the electron-phonon coupling at the temperature predicted for the convergence of two superconducting gaps into one observable gap. This gives indirect evidence of the convergence of the two gaps and direct evidence of a transition to an Since the discovery of superconductivity in MgB 2 ͑Ref. 1͒, a large volume of work has been directed towards developing and improving the quality of MgB 2 thin films. 2 The temperature of the onset of superconductivity (T c ) in these films has varied considerably, although there is no consensus on the mechanism behind the T c suppression. Nonetheless, theoretical models suggest that a two-band superconductivity model is needed to explain the high T c in MgB 2 . 3-6 As interband scattering increases perhaps due to impurities in the boron layer, the two gaps in MgB 2 are predicted to converge into one gap with ⌬ ϭ⌬ ϭ4.1 meV; 6 this transition is predicted to occur as the T c is suppressed below 25 K. 6 Measurements of the phonon energies in YBa 2 Cu 3 O 7Ϫ␦ helped the understanding of the relation between the crystal structure and the T c in this material. 7 Therefore, similar measurements made on MgB 2 films with different T c 's may help to elucidate the mechanisms behind the suppression of the critical temperatures in MgB 2 , while casting light on the nature of the two-band superconductivity.Numerical calculations indicate that in MgB 2 there are two phonon modes involving vibrations along the c axis (A 2u and B 1g ) and two doubly degenerate modes with vibrations only in the ab plane (E 1u and E 2g ); 8 for the P 6 mmm space group to which MgB 2 belongs, only the E 2g mode is Raman active. 9 Predictions for the frequency of the E 2g mode vary between 515 cm Ϫ1 ͑64 meV͒ ͑Ref. 3͒ and 665 cm Ϫ1 ͑82 meV͒, 10 while experimental studies have shown a broad Raman mode between 600 cm Ϫ1 and 630 cm Ϫ1 . 9,11-18 The consensus is that this mode is the E 2g mode, however, there remains controversy about this assignment, 9,14 with Kunc et al. 9 concluding that this mode was due to a ''contaminant phase at the sample surface.'' In this communication, Raman spectroscopy has been used to characterize the quality of MgB 2 films both in terms of impurity phases and disorder. The effects of a suppressed T c on the phonon mode at ϳ600 cm Ϫ1 were studied. Both magnesium stoichiometry and oxygen contamination may play a role in determining the T c . 19,20 It has been shown previously that as-grown films of MgB 2 can have significant magnesium concentration gradients through the thickness of the film. 21This effect is equally likely where a precursor boron film has been annealed in magnesium vapor. Therefore, measurement of the superconducting properties of a post annealed boron film through the thickness...

Temperature‐dependent Raman study of PrFeO3thin film

“…[34] Due to the existing strain, the collapse of the electron-phonon coupling and the consequent the appearance of the 703 cm −1 mode may be the one of the explanations. [35] Further, in these films Fe is in the 3+ oxidation state, which was confirmed by the X-ray Absorption spectroscopy (XAS) study performed on these films (not discussed here). This is similar to what we observed in the bulk target that was used to prepare the film.…”

New class of photocatalytic materials and a novel principle for efficient water splitting under infrared and visible light: MgB_2 as unexpected example

“…The higher frequency Raman band (centred at 800 cm −1 in Fig. 3(a)) has been observed earlier in MgB 2 and was attributed to phonon density of states (PDOS) due to disorder [17,18]. It is interesting to note that despite the peak located at about 800 cm −1 is not observed for powder MgB 2 it becomes the prominent feature in the Raman spectrum for layered samples.…”

“…Two broadband features centred at about 590 and 800 cm −1 were observed for layered materials. In powder MgB 2 , the most prominent phonon peak is located at lower frequency (590 cm −1 ) and assigned to the E 2g mode [16][17][18]. The half-width of the E 2g peak in powder MgB 2 exceeds 200 cm −1 , which is attributed to the presence of strong electron-phonon coupling and phononphonon interaction [16].…”

Vibrational thermodynamics of materials

“…Furthermore, the adiabatic approximation itself has been questioned because the electrons may not respond instantly to nuclear displacements [390]. A limitation of the previous work on phonon anharmonicity in MgB 2 is that the measurements were at temperatures of 300 K and below, such as the extensive Raman spectrometry work that has been performed on MgB 2 and related materials [388,391,392,393,394].…”