Physical properties of noncentrosymmetric tungsten and molybdenum aluminides

Abstract

A lack of spatial inversion symmetry gives rise to a variety of unconventional physics, from noncollinear order and Skyrmion lattice phases in magnetic materials to topologically-protected surface states in certain band insulators, to mixed-parity pairing states in superconductors. The search for exotic physics in such materials is largely limited by a lack of candidate materials, and often by difficulty in obtaining crystals. Here, we report the single crystal growth and physical properties of the noncentrosymmetric tungsten aluminide cage compounds Al4W and Al5W, alongside related molybdenum aluminides in which spin-orbit coupling should be significantly weaker. All compounds are nonmagnetic metals. Their high conductivities suggest the opportunity to find superconductivity at lower temperatures, while the limits we can place on their transition temperatures suggest that any superconductivity may be expected to exhibit significant parity mixing. arXiv:1807.03476v2 [cond-mat.mtrl-sci]

Type-I superconductivity in Al$_6$Re

“…The material crystallizes in the centrosymmetric orthorhombic space group Cmcm (No. 63), with lattice parameters a = 7.599(4) Å, b = 6.602(4) Å, and c = 9.040(5) Å at 203(2) K. As previously reported 31,32 , Re is contained in Al cages, and as in Al 4 W, Al 5 W, and Al 49 Mo 11 1 , the cages are too tight for rattling modes that would enhance superconductivity -Al-Re bond lengths are consistent with those expected based on the elements' atomic or covalent radii.…”

“…In particular, several closelyrelated tungsten and molybdenum aluminides have T 4 resistivity 1 . Such power laws have been attributed to an interplay of electron-electron, electron-phonon, electronimpurity, and electron-dislocation scattering 1,36,37 , but are seldom observed over such a wide range in temperature or to such high temperatures. In the W and Mo aluminides, the relatively weak electron-electron interactions were considered as one possible reason for the lack of superconductivity, but Al 6 Re superconducts despite its similarly high power law.…”

“…A vast array of exotic physics is possible in noncentrosymmetric superconductors, which prompted our prior project probing a family of tungsten and molybdenum aluminides 1 . Nearly all members of this family are noncentrosymmetric, most are straightforward to grow from aluminum flux, and all are excellent metals; however, none were found to superconduct down to 100-300 mK.…”