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

Abstract

Whilst tight-binding bandstructure calculations are very successful in describing the Fermi-surface configuration in many quasi-two-dimensional organic molecular metals, the detailed topology of the predicted Fermi surface often differs from that measured in experiments. This is very significant when, 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 the -orbitals of the component molecules (which give rise to the transfer integrals of the tight-binding bandstructure) are known. In order to surmount this problem, we have derived a method whereby the transfer integrals within a tight-binding bandstructure model are adjusted until the detailed Fermi-surface topology is in good agreement with a wide variety of experimental data. The method is applied to the charge-transfer salt -(BEDT-TTF)2KHg(SCN)4, the Fermi surface of which has been the source of much speculation in recent years. The Fermi surface obtained differs in detail from previous bandstructure calculation findings. In particular, the quasi-one-dimensional component of the Fermi surface is more strongly warped. This implies that upon nesting of these sheets, significant parts of the quasi-one-dimensional sheets remain, leading to a complicated Fermi-surface topology within the low-temperature, low-magnetic-field phase. In contrast to previous models of this phase, the model for the reconstructed Fermi surface in this work can explain virtually all of the current experimental observations in a consistent manner.

Angle dependent magnetothermopower of α-(ET)2KHg(SCN)4

“…The Harrison et al [33]. If, below T p , a modified closed orbit band still remains, which can give a metallic TEP contribution, its contribution is negligible small at lower angles.…”

Superconducting properties and Fermi-surface topology of the quasi-two-dimensional organic superconductor λ-(BETS)2GaCl4(BETS≡bis(ethylene-dithio)tetraselenafulvalene)

“…Figure 3(b) shows an elliptical-cross-section Q2D pocket of this size and orientation alongside the most recent calculation of the Fermi surface [15], based on structural studies carried out at 17 K. The Q2D pocket measured experimentally is smaller and somewhat more elongated than that suggested by the calculations; it occupies ≈ 16 % of the Brillouin zone, whereas the Q2D pocket of the calculation is 28 % of the Brillouinzone area. However, such discrepancies between calculation and experiment are not without precedent in crystalline organic metals [18,19,25].…”

“…Such a modulation of the lattice could occur in the event of a charge-density-wave (CDW) or spin-density-wave (SDW) instability; however, there is as yet no other evidence for the presence of such a groundstate (c.f. numerous other charge-transfer salts in which CDWs or SDWs cause extensive modification of the quantum-oscillation spectrum [1,25]). If such a modulation exists, it is too weak to be picked up by a careful X-ray study at 115 K (Bruker-AXS SMART6000 CCD, complete sphere of data, sixty-second frames, 0.3 degree scans) [26].…”

Quasi-two-dimensional organic superconductors: A review

“…the fact that there are quasi-one-dimensional and quasi-two-dimensional Fermi-surface sections), the details of the Fermi-surface topology are sometimes inadequately described (see e.g. [25]). This can be important when, for example, the detailed corrugations of a Fermi sheet govern the interactions which determine its low-temperature groundstate [5,25].…”