Fermi surface study of CeRu2Si2 above the Kondo temperature

The 2-dimensional angular correlation of the positron annihilation radiation (2D-ACAR) of the heavy fermion system CeRu2Si2 was measured above the Kondo temperature TK. The 3-dimensional electron-positron momentum density rho (p) was reconstructed from five projections via the Cormack [1] and a modified Fourier-transform-based method [2]. After the 3D Lock-Crisp-West transformation [3], the resulting k-space density rho (k) was tested against band structure calculations treating the f-electrons as itinerant (f-band) and localised (f-core). Our preliminary results show unoccupied regions centred at the Z point of the Brillouin zone (BZ), predicted by both models and consistent with the de Haas van Alphen experiments. Furthermore, we observe large and connected electron-like columnar structures along the X-P direction. The f-band calculation predicts that these structures are much more prominent than in the f-core calculation and more similar to those reported by our measurements. Therefore, we suggest that in CeRu2Si2 the itinerant character of the f-electrons persists above TK.