We have studied the variation of superconducting critical temperature Tc as a function of charge density and lattice parameters in Mg1-xAlxB2 superconducting samples. The AB2 heterostucture of metallic boron layers (intercalated by A = magnesium, aluminum layers, playing the role of spacers) is made by direct chemical reaction. The spacing between boron layers and their charge density are controlled by chemical substitution of Mg by Al atoms. We show that high Tc superconductivity is realized by tuning the chemical potential at a `shape resonance' according with the patent for `high-temperature superconductors made by metal heterostructures at the atomic limit'. The energy width of the superconducting shape resonance is found to be about 400 meV.

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...

Here we report synthesis and characterization of Mg1−xScxB2 (0.12Tc>6 K. We find that the Sc doping moves the chemical potential through the 2D/3D electronic topological transition (ETT) in the σ band where the "shape resonance" of interband pairing occurs. In the 3D regime beyond the ETT we observe a hardening of the E2g Raman mode with a significant line-width narrowing due to suppression of the Kohn anomaly over the range 0

The experimental determination of the scaling of the superconducting critical temperature (T-c) vs the Fermi temperature (T-f) of the holes in the boron sigma subband is presented. The Fermi level has been tuned near the "shape resonance," i.e., the two- to three-dimensional crossover of the Fermi surface of the boron sigma subband by changing the Al/Mg content in Al1-xMgxB2. The product k(f)xi(0) of the Fermi wave vector (k(f)) times the superconducting Pippard coherence length (xi(0)), that is a measure of the pairing strength, remains constant, k(f)xi(0)=90 for x>0.66. This high-T-c phase occurs in the boron superlattice under a tensile microstrain in the range 3%

The successful applications of magnesium-based alloys as biodegradable orthopedic implants are mainly inhibited due to their high degradation rates in physiological environment. This study examines the bio-corrosion behaviour of Mg-2Zn-0.2X (X = Ca, Mn, Si) alloys in Ringer's physiological solution that simulates bodily fluids, and compares it with that of AZ91 magnesium alloy. Potentiodynamic polarization and electrochemical impedance spectroscopy results showed a better corrosion behaviour of AZ91 alloy with respect to Mg-2Zn-0.2Ca and Mg-2Zn-0.2Si alloys. On the contrary, enhanced corrosion resistance was observed for Mg-2Zn-0.2Mn alloy compared to the AZ91 one: Mg-2Zn-0.2Mn alloy exhibited a four-fold increase in the polarization resistance than AZ91 alloy after 168 h exposure to the Ringer's physiological solution. The improved corrosion behaviour of the Mg-2Zn-0.2Mn alloy with respect to the AZ91 one can be ascribed to enhanced protective properties of the Mg(OH)(2) surface layer. The present study suggests the Mg-2Zn-0.2Mn alloy as a promising candidate for its applications in degradable orthopedic implants, and is worthwhile to further investigate the in vivo corrosion behaviour as well as assessed the mechanical properties of this alloy.

A comparative chemical bonding analysis for the germanides La2MGe6 (M=Li, Mg, Al, Zn, Cu, Ag, Pd) and Y2PdGe6 is presented, together with the crystal structure determination for M=Li, Mg, Cu, Ag. The studied compounds adopt the two closely related structure types oS72‐Ce2(Ga0.1Ge0.9)7 and mS36‐La2AlGe6, containing zigzag chains and corrugated layers of Ge atoms bridged by M species, with La/Y atoms located in the biggest cavities. Chemical bonding was studied by means of the quantum chemical position‐space techniques QTAIM (quantum theory of atoms in molecules), ELI‐D (electron localizability indicator), and their basin intersections. The new penultimate shell correction (PSC0) method was introduced to adapt the ELI‐D valence electron count to that expected from the periodic table of the elements. It plays a decisive role to balance the Ge−La polar‐covalent interactions against the Ge−M ones. In spite of covalently bonded Ge partial structures formally obeying the Zintl electron count for M=Mg2+, Zn2+, all the compounds reveal noticeable deviations from the conceptual 8−N picture due to significant polar‐covalent interactions of Ge with La and M ≠ Li, Mg atoms. For M=Li, Mg a formulation as a germanolanthanate M[La2Ge6] is appropriate. Moreover, the relative Laplacian of ELI‐D was discovered to reveal a chemically useful fine structure of the ELI‐D distribution being related to polyatomic bonding features. With the aid of this new tool, a consistent picture of La/Y−M interactions for the title compounds was extracted.

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Expansion of the superlattice of boron layers, with AB 2 structure, due to different intercalated A atoms has been studied to understand the emergence of high T c superconductivity in the diborides. The structure of these metal heterostructures at the atomic limit (MEHALs) (with A=Al, Mg, Ti, Hf, Zr) has been measured by synchrotron x-ray diffraction. The increasing atomic radius of the intercalated A ions induces an increase of 1) the separation between the boron layers and 2) the tensile micro-strain ε of the B-B distance within the boron layers. The results show that the superconductivity in these MEHAL appears in a critical region in a phase diagram controlled by two variables, the micro-strain and the charge density (ε, ρ). T c amplification in superconductors made of metal heterostructures at the atomic limit [MEHAL] was found in doped cuprate perovskites [1]. In the Bi 2 Sr 2 CaCu 2 O 8+δ system the one-dimensional ordering of both the intercalated oxygen ions between the CuO 2 planes and the polaron ordering in the CuO 2 plane produces a heterogeneous metal made of a superlattice of quantum stripes. The T c amplification occurs at the optimum doping by tuning the chemical potential at the "shape resonance" of the superlattice. This resonance occurs near the dimensional 1D-2D cross-over of the topology of the Fermi surface [2]. At beginning of the 2001 Jun Akimitsu reported evidence of superconductivity at 39K in a superlattice of quantum wells, boron layers intercalated by Mg ions, a system known as MgB 2 [3]. This system shows the superconducting phase [4] with a London penetration depth λ∼140 nm, a short Pippard coherence length ξ 0 ∼5.2 nm, and an isotope coefficient α∼0.25 [5]. It has been shown [6,7] that the high T c is obtained by tuning the chemical potential at the "shape resonance" of the superlattice of quantum wells. This is characterized by the dimensional 2D-3D cross-over of the topology of the Fermi surface, consistent with the patented process [8] of increasing the critical temperature in metal heterostructures at the atomic limit (MEHAL). According with this process the "shape resonance" can be reached by changing both the charge density and the structure of the superlattice. The fact that the critical temperature increases by increasing the separation between the metal units (and therefore the period of the superlattice) due to intercalation of larger ions, predicted in [2], has been recently verified experimentally in the superlattice of quantum dots: the metallic empty carbon spheres (C 60 buckyballs) separated by tribromomethane that reaches T c =117K [9].In this work we have studied the variation of the separation between the boron metal layers as a function of the atomic radius of the intercalated ion. In fact the superlattice expands with increasing the atomic radius of intercalated ions. This structural parameter is relevant since it controls the band dispersion along the c-axis direction and therefore it controls T c via both the 2D-3D cross-over of the Fermi surface and the in...