Multiple superconducting states induced by pressure in Mo_3Sb_7

Tuning competing ordering mechanisms with hydrostatic pressure in the 4d intermetallic compound Mo_3Sb_7 reveals an intricate interplay of structure, magnetism, and superconductivity. Synchrotron x-ray diffraction and magnetic susceptibility measurements, both employing diamond anvil cell technologies, link a first-order structural phase transition to a doubling of the superconducting transition temperature. In contrast to the spin-dimer picture for Mo_3Sb_7, we deduce from x-ray absorption near-edge structure and dc magnetization measurements at ambient pressure that Mo_3Sb_7 should possess only very small, itinerant magnetic moments. The pressure evolution of the superconducting transition temperature strongly suggests its enhancement is due to a difference in the phonon density-of-states with changed crystal symmetry.