Effect of the Al content on the optical phonon spectrum inMg1−xAlx

Abstract

Raman and infrared absorption spectra of Mg1−xAlxB2 have been collected for 0 ≤ x ≤ 0.5 in the spectral range of optical phonons. The x-dependence of the peak frequency, the width and the intensity of the observed Raman lines has been carefully analized. A peculiar x-dependence of the optical modes is pointed out for two different Al doping ranges. In particular the onset of the high-doping structural phase previously observed in diffraction measurements is marked by the appearence of new spectral components at high frequencies. A connection between the whole of our results and the observed suppression of superconductivity in the high doping region is established.The recent discovery 1 of superconductivity below 39 K in MgB 2 has stimulated a great deal of effort among the scientific community and a large number of theoretical and experimental papers have been published within few months. The debate on the origin of this unexpected superconductivity is still open, although both experimental 2-4 and theoretical 5-7 works indicate that MgB 2 is a BCS-like system. In this framework, the obvious relevant interaction in the superconducting transition is the electron-phonon (e-ph) coupling. Owing to the simple hexagonal structure (space group P 6 mmm), four zone-center optical modes are predicted for MgB 2 : a silent B 1g mode, the E 2g Raman mode, and the infrared active E 2u and A 2u modes. While the doubly-degenerate E 2u and E 2g modes are ascribed to in-plane stretching modes of the boron atoms, both non-degenerate A 2u and B 1g modes involve vibrations along the perpendicular direction (c axis). It is quite a general statement that the E 2g mode is expected to allow for the strongest e-ph coupling 5-7 and then to play a relevant role in superconductivity. Raman experiments 8-12 carried out on MgB 2 have shown that the spectrum is dominated by a quite large and asymmetric band around 600 cm −1 , ascribed to the E 2g mode. The anomalous width of this phonon peak has been interpreted as a signature of the e-ph coupling.Up to now, no other isostructural boride (XB 2 ) has shown the peculiar high temperature superconductivity of MgB 2 . In particular, MgAl 2 is not superconducting. Indeed, several studies on the Mg 1−x Al x B 2 compounds have shown that superconductivity is progressively suppressed for increasing x and vanishes for x>0.5. [13][14][15] In order to achieve a deeper understanding of the effects of Al doping, we have studied the evolution of the phonon spectrum of Mg 1−x Al x B 2 in the 0 ≤ x ≤ 0.5 range by means of both Raman and infrared spectroscopy.Pure MgB 2 and Al doped polycrystalline samples have been synthesized at high temperature by direct reaction of the elements in a tantalum crucible under argon atmosphere. The samples, which show an average grain dimension around 1-2µm, have been characterized by xray diffraction and by resistivity measurements, in order to determine, in particular, the x-dependence of the superconductivity transition temperature T c . 14,16 .The Raman spectra were measure...

Structural and Superconducting Property Variations with Nominal Mg Non‐Stoichiometry in MgxB2 and Its Enhancement of Upper Critical Field

“…These features have been observed in Al-doped samples and have been associated with structural disorder. [27][28][29] The appearance of more than one phonon peak further implies the violation of Raman selection rules induced by disorder resulting in a modified …”

Thermodynamics of two-band superconductors: The case ofMgB2

“…16,17,18,19 Interband scattering is weak in MgB 2 , 20 but this is not necessarily the case in samples in which Mg has been replaced by Al or B by C (Ref. 21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37) or which have been exposed to neutron irradiation. 38 Such samples exhibit considerably reduced T c , while the two gaps persist even at very low critical temperatures.…”

Raising the diboride superconductor transition temperature using quantum interference effects

“…Going from x=1 to x=0.5 a dramatic increase in the E 2g phonon mode energy ω E2g takes place [29][30][31] increasing from 70 meV to 115 meV as shown in Fig. 2a (from the data in ref.…”

“…It has been found that the Al 1-x Mg x B 2 alloys [20][21][22][23][24][25][26][27][28][29][30][31] show a continuos evolution through a complicated mixed phase from MgB 2 (x=1) to the end member AlMgB 4 (x=0.5) where a well ordered superlattice structure of boron layers intercalated by alternating layers of Al and Mg is formed [20][21][22][23][24]. In spite of the fact that the alloys with intermediate x are rather disordered, the superconducting transition temperature is still well defined and it drops systematically with decreasing x as shown in Fig.…”