Superconducting properties andc-axis superstructure ofMg1−xAlx

Jianyong Xiang¹,

D. N. Zheng²,

J. Q. Li³

et al.

Abstract

The superconducting and structural properties of a series of Mg 1-x Al x B 2 samples have been investigated. X-ray diffraction results confirmed the existence of a structural transition associated with the significant change in inter-boron layer distance as reported previously by Slusky et al. Moreover, transmission-electron-microscopy observations revealed the existence of a superstructure with doubled lattice constant along the c-axis direction. We propose that this superstructure is probably related to the structural transition. The modifications of superconducting transition temperature Tc, the normal state resistivity, and the upper critical field Bc2(0) by Al doping are discussed in terms of Al-substitution induced changes in the electronic structure at the Fermi energy.

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Interband scattering is weak in MgB 2 , 20 but this is not necessarily the case in samples in which Mg has been replaced by Al or B by C (Ref. 21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37) or which have been exposed to neutron irradiation. 38 Such samples exhibit considerably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.…”

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

“…13 Novel diffraction studies confirm a structural change around x=0.10, 15,16 and transmission-electronmicroscopy observations revealed for x>0.10 the existence of a superstructure, with doubled lattice constant along the c-axis. 15 The existence of three major ordered phases for x<0.10, at x=0.30 and at x=0.50, and an overall disordered phase for 0.10<x<0.30 have also been suggested. 16 As shown in Fig.…”

“…3a, where vertical lines at x=0.10 and at x=0.25 separate the three regions, ν 2 and ν 3 are nearly constant in the stable LD region, which is structurally characterized by small variations of the lattice parameters. 13,15 The onset of the HD phase in the intermediate x-region is marked by the appearance of the new spectral feature (ν 4 ). When the system enters in the pure HD phase, ν 3 and ν 4 strongly increase with x, while ν 2 remains nearly constant.…”

“…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 .…”

“…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.…”