The use of experimental data in constraining the tight-binding band parameters of quasi-two-dimensional organic molecular metals: application to alpha-(BEDT-TTF)(2)KHg(SCN)(4)

Whilst tight-binding bandstructure calculations are very successful in desc ribing the Fermi-surface configuration in many quasi-two-dimensional organi c molecular metals, the detailed topology of the predicted Fermi surface of ten differs from that measured in experiments. This is very significant whe n, for example, the formation of a density-wave state depends critically on details of the nesting of Fermi-surface sheets. These differences between theory and experiment probably result from the limited accuracy to which th e pi-orbitals of the component molecules (which give rise to the transfer i ntegrals of the tight-binding bandstructure) are known. In order to surmoun t this problem, we have derived a method whereby the transfer integrals wit hin a tight-binding bandstructure model are adjusted until the detailed Fer mi-surface topology is in good agreement with a wide variety of experimenta l data. The method is applied to the charge-transfer salt alpha-(BEDT-TTF)( 2)KHg(SCN)(4), the Fermi surface of which has been the source of much specu lation in recent years. The Fermi surface obtained differs in detail from p revious bandstructure calculation findings. In particular, the quasi-one-di mensional component of the Fermi surface is more strongly warped; This impl ies that upon nesting of these sheets, significant pacts of the quasi-one-d imensional sheets remain, leading to a complicated Fermi-surface topology w ithin the low-temperature, low-magnetic-field phase. In contrast to previou s models of this phase, the model for the reconstructed Fermi surface in th is work can explain virtually all of the current experimental observations in a consistent manner.