HighT_csuperconductivity in a critical range of micro-strain and charge density in diborides

Abstract

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...

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.…”

Anomalous Thermal Expansion in Superconducting Mg1− x Al x B2 System

“…In fact it is actually a composite intermetallic formed by a multilayer of first planes of a bulk low T c superconductor, boron that shows T c =11K under pressure [14]: graphene B monolayers, intercalated by second metallic planes of a non superconducting element, magnesium [14]: hcp Mg monolayers. Therefore following the discovery of Nagamatsu et al [13] it was first proposed by the Rome group [15][16][17] that MgB 2 shows mutigap superconductivity. The multigap superconductivity in MgB 2 was quickly confirmed by experiments on the upper critical field H c2 (T) [18], tunneling [19] and angular resolved photoemission (ARPES) [20] and many other measurements [2].…”

Substitution ofScforMginMgB2Agrestini1, Metallo2, Filippi3 et al. 2004Phys. Rev. B833720View full textAdd to dashboardCite

“…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<q<2kF . Following the discovery of superconductivity in MgB 2 with a T c of 40 K 1 large attention has been paid to chemical substitutions 2,3,4,5,6,7,8 aiming at enhancement of T c and H c2 , and to manipulate the electronic structure for the understanding of the high T c superconductivity. As a matter of fact, chemical substitution in the MgB 2 is difficult and only Al replacing Mg 4,5,6,7 and C replacing B 8 had been successful.…”

“…The samples were characterized for their superconducting properties by the temperature dependence of complex conductivity using the single-coil inductance method 5,6 . The temperature dependent radio-frequency complex conductivity for representative Sc contents is shown in Fig.…”