Negative magnetoresistance in some dimethyltrimethylene-tetraselenafulvalenium salts: A signature of weak-localization effects

The transverse magnetoresistance of the organic series (DMtTSF${)}_2$X (with X=${\mathrm{AsF}}_6$, ${\mathrm{PF}}_6$, ${\mathrm{BF}}_4$, ${\mathrm{ClO}}_4$, and ${\mathrm{ReO}}_4$) has been investigated in a wide range of temperature and magnetic field, down to 2 K and up to 40 T. The angular dependence of the magnetoresistance on the field orientation has also been systematically studied. A negative, low-field and low-temperature magnetoresistance appears, roughly under 10 T and 20 K, only for the salts whose anion X has a tetrahedral geometry (X=${\mathrm{BF}}_4$, ${\mathrm{ClO}}_4$, and ${\mathrm{ReO}}_4$). This magnetoresistance vanishes when the field or the temperature is increased. This new phenomenon is interpreted in terms of two-dimensional weak localization possibly due to some disorder in the anion lattice.