Fermi surface of y-tin studied by positron 2D-ACAR

We present the electron-position momentum distribution measured in single crystals of 7-tin (Sno.~Ino. 2) by 2D angular correlation of positron annihilation radiation. The Fermi sphere and its Umklapp components up to the 3rd order are easily distinguishable. The deviation of the Fermi surface from a sphere is studied in detail, and compares nicely with theory based on Linear Muffin Tin Orbital band structure calculations. The measured Fermi momentum is estimated to be 1.63 + 0.01 A 1 along (1 00) (F-M), 1.60 +_ 0.01 ~,1 along (1 10) (F-K), and 1.62 +_ 0.01 ~,1 along (00 1) (F-A), close to the calculated values. Compared to neutron scattering results, these values are 3-5% larger. This deviation may find its origin in the different nature of these two experimental techniques.