Raman study of element doping effects on the superconductivity ofMgB2

Abstract

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...

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].…”

Improved Transport J c in MgB2 Tapes by Graphene Doping

“…4. Moreover, the grain growth and crystallinity improvement will assist the grain connectivity of the tapes sintered at high temperatures [28].…”

Superconducting Properties of Carbonaceous Chemical Doped MgB2

“…The effects of C substitution include an increase in impurity scattering and band filling, which reduces the density of states (DOS) and alters the shape of the Fermi surface. The E 2g phonon peak shifts to the higher energy side, and the peak is narrowed with increasing x in Mg(B 1−x C x ) 2 (Li et al, 2008). As a carbon source, nano-SiC shows a similar influence, due to its C atoms, on the J c , H irr , H c2 , and even Raman spectra in MgB 2 .…”