HIGH-FIELD MAGNETOTRANSPORT OF ORGANIC CONDUCTORS (BEDT-TTF)2TLHG(XCN)4 WITH X = S AND SE

High field magnetoresistance of layered organic metals (BEDT-TTF)2TlHg (XCN)4 with X = S and Se has been studied using pulsed magnetic fields . Sharp negative slope and strong hysteresis of the magnetoresistance of (BEDT-TTF)2TlHg(SCN)4 found between about 11 and 27 T suggest a fir st-order phase transition to occur at these fields. Two different type s of angular magnetoresistance oscillations corresponding most likely to different electronic states are observed in the low- and high-field phases, respectively. In contrast, the classical magnetoresistance of the salt with X = Se is found to be smoothly growing with the field i ncrease. The obtained results are compared with those reported earlier on the (BEDT-TTF)2MHg(SCN)4 salts with M = K and NH4 and discussed in the framework of the model proposed recently for the low-temperature electronic state of (BEDT-TTF)2TlHg(SCN)4.