Positron annihilation as a probe of electronic density in the two-dimensional conductor 1T-TAS2

2014 Tantalum disulfide is a layered compound with quasi-two-dimensional electronic conduction. The particular topology of the 2D-Fermi surface causes the onset of charge density waves which drive a metal-to-insulator phase transition in two steps. Three standard positron annihilation techniques have been used to sample the electronic density and momentum distributions in this compound as a function of temperature from 4 K to 400 K. The positron lifetime is 261(2)ps at 300 K (single component) and does not change very much with temperature. It increases to 293(2)ps in a sample containing 1 % of irradiation induced defects. The anisotropy of the momentum distribution is clearly visible both in angular correlation and Doppler broadening experiments. This anisotropy is reduced but remains when positrons are trapped by irradiation defects. The momentum distributions change significantly with temperature, especially when the charge density wave phase transitions are crossed. An interpretation taking into account the possible trapping of positrons by charge density waves is proposed. The remarkable opposite temperature dependences of the positron data obtained in the in-layer and transverse directions are attributed to electron localization in a preferential orientation. Finally, an attempt is made to have access to the topology of the Fermi surface of this material by mean of the projected density of states along different axes. This procedure demonstrates that even with standard positron techniques, insight on an unusual Fermi surface can be obtained. J. Physique 44 (1983) 1157-1166 OCTOIBRE 1983,