THE LOW-TEMPERATURE PHASE OF ALPHA-(BEDT-TTF)(2)KHG(SCN)(4) .2. PRESSURE-DEPENDENCE OF THE SHUBNIKOV-DEHAAS OSCILLATIONS

The magnetoresistance of alpha-(BEDT-TTF)(2)MHg(SCN)(4) (where BEDT-TT F is bis(ethylenedithio)tetrathiafulvalene and M = NH4 or K) has been studied under pressures of up to 14.8 kbar and for temperatures down t o 0.7 K. The similar to 671 T (alpha) and similar to 4270 T (beta) Shu bnikov-de Haas oscillations observed in the ambient pressure magnetore sistance of alpha-(BEDT-TTF)(2)KHg(SCN)(4) persist to the highest pres sure while the other quantum oscillatory frequencies are removed under pressure. A strong second-harmonic component of the alpha-frequency o scillations is observed on the raw data at 1 bar and is initially supp ressed by pressure but returns above similar to 9 kbar. Furthermore, i n the salt alpha-(BEDT-TTF)(2)NH4Hg(SCN)(4) a similar pronounced secon d harmonic of the quantum oscillations has been observed at the highes t pressures. The origins of these features of the data are discussed i n the context of current models of the Fermi surfaces of these materia ls.