The influences of phonon frequency and unit cell volume on the superconductivity of element-doped MgB 2 are discussed with reference to a Raman study on SiC, C, Mn, and Al-Ag-doped Mg-B materials. A phenomenon has been found in the doped samples, in that the phonon frequency changes to counteract the crystal lattice variation to keep the system stable within a Grüneisen parameter of 2.0-4.0. The chemical doping effects on phonon frequency and unit cell volume can be explained by the harmonicity-anharmonicity competition in the compounds. A decreased electronic density of states is responsible for the depression of superconductivity that is seen in doped MgB 2 . The possibility of a high critical temperature, T c , in the Mg-B system exists if the material can possess both a high phonon frequency and a big unit cell volume at the same time, as indicated by the isotope effect and hydrogenation experiments.The discovery of superconductivity at 40 K in MgB 2 ͑Ref. 1͒ has attracted intense scientific interest towards modifying the structural and electronic properties of this material to improve its critical transition temperature T c . Although some possible routes to increase the T c , 2-7 such as increasing the phonon frequency, increasing the density of states, and expanding the unit cell volume, have been employed in the Mg-B system, all attempts to modify the MgB 2 host lattice to further increase T c have failed. It seems that most factors which influence the superconductivity of MgB 2 have already been optimized in the pure Mg-B system. Chemical substitution, a method to change several physical properties, such as the electron density, crystal lattice parameters, and the disorder effect, has been well investigated by different groups. On the fact of T c depression, Takenobu et al. 8 and Lee et al. 9 explain it as due to crystal lattice contraction, electron doping induced electronic structure variation, and the disorder effect arising from substitution, while Kortus et al. 10 ascribe the T c depression to band filling and interband scattering effects. It should be noted that Kazakov et al. 7 have suggested that the main issue is the competition between the effects of atomic size disorder, which causes the T c to decrease, and those of crystal lattice expansion, which tends to increase T c , which is in agreement with the positive T c dependence on unit cell volume for MgB 2 samples under pressure, as estimated by Deemyad et al. 11 However, in the case of Ag and Al codoped MgB 2 ͓Mg 1−2x ͑AgAl͒ x B 2 ͔, the a-axis and c-axis parameters increase with doping, and the volume V also increases, while the T c decreases with doping. 3 The authors attribute these characteristics to a disorder effect caused by the chemical substitution. 3 In a phonon-mediated superconductor, 12 it is well known that the T c is proportional to the averaged phonon frequency ͗͘, according to the McMillan formula. Factor group analysis predicts four modes at the ⌫ point of the Brillouin zone for the P6 / mmm space group in MgB 2 : a silen...

The influence of sintering temperature on the critical transition temperature, T c , for MgB 2 superconductor was investigated systematically with the aid of room temperature Raman scattering measurements and Raman spectral fit analysis. The Raman spectra for all samples can be fitted with one phonon peak coming from the E 2g mode at the point of the Brillouin zone and two peaks coming from sampling of the phonon density of states (PDOS) due to disorder. The enhanced E 2g mode in the Raman spectra with increasing sintering temperature shows gradual strengthening of the electron-phonon coupling (EPC) in MgB 2 , which is the reason why the T c of samples increases with increasing sintering temperature. The strength of electron-E 2g coupling is estimated to be about 2.0 ± 0.5, which is larger than the average strength of the coupling of electrons with all the phonon modes, ∼1.23. The T c dependence on the profiles of the PDOS peaks is described using the variation of the peaks' frequencies and linewidths in different samples.

The influences of microstructure, connectivity, and disorder on the critical current density, [Formula: see text], are discussed to clarify the different mechanisms of [Formula: see text] in different magnetic field ranges for in situ and combined in situ/ex situ [Formula: see text] wires with nano SiC and malic acid codoping. Sintering temperature plays a very important role in the electromagnetic properties at different temperatures and under various magnetic fields. Connectivity, upper critical field, [Formula: see text], and irreversibility field, [Formula: see text], are studied to demonstrate the mechanism of [Formula: see text] dependence on magnetic field. The combined in situ/ex situ process is proved to be a promising technique for fabrication of practical [Formula: see text] wires.