MAGNETORESISTANCE STUDIES IN THE LOW-DIMENSIONAL ORGANIC METAL ALPHA-(ET)(2)TLHG(SECN)(4) UNDER PRESSURE - EXPERIMENTS AND SIMULATION BY TIGHT-BINDING BAND-STRUCTURE CALCULATIONS

Magnetotransport measurements have been carried out in layered organic metal alpha-(ET)(2)TlHg(SeCN)(4) at temperatures down to 0.4 K and un der hydrostatic pressure. Only one series of Shubnikov-de Haas oscilla tions is observed in the applied pressure range from 3.5 to 11 kbar. T he cyclotron mass decreases slowly with increasing pressure and the os cillation frequency increases rapidly from its ambient pressure Value of (653 +/- 3)T to (790 +/- 3)T at 11 kbar. A model tight binding stud y suggests that the pressure dependence of the area of the closed part of the Fermi surface is consistent with a pressure induced sliding mo tion of the molecules in the donor chains containing only one type of donors. Slow oscillations with frequency (47 +/- 3)T have also been ob served at ambient pressure in contrast to its superconducting sister c ompound alpha-(ET)(2)NH4Hg(SCN)(4). They are not observed in the appli ed pressure range and might be connected with some nesting of the open orbits at ambient pressure which may result in a destruction of super conductivity in the Se-containing compound.