Thermopower is systematically studied for quasi-one-dimensional (Q1D) organic conductors $(\mathrm{DMET}{)}_{2}X$ $[X=\mathrm{Au}(\mathrm{CN}{)}_2,$ ${\mathrm{AuI}}_2,$ ${\mathrm{AuCl}}_2,$ ${\mathrm{IBr}}_2,$ ${\mathrm{I}}_3,$ ${\mathrm{AuBr}}_2$ $\mathrm{}(Z=1\mathrm{}),$ and SCN]. The field-orientation dependence of magnetoresistance of $(\mathrm{DMET}{)}_2{\mathrm{AuCl}}_2$ was measured by rotating magnetic field within its most conducting plane and the third angular effect (TAE) was observed at and below 4.2 K. Bandwidth estimated from the thermopower and its anisotropy (or dimensionality) from TAE are compared among $(\mathrm{DMET}{)}_{2}X$ to discuss chemical pressure effect on the low temperature state of Q1D conductors. It was found that bandwidth varies drastically (25 %) by changing the counter anions, while the dimensionality is almost the same for $(\mathrm{DMET}{)}_2{\mathrm{AuBr}}_2,$ $(\mathrm{DMET}{)}_2{\mathrm{AuCl}}_2,$ and $(\mathrm{DMET}{)}_2{\mathrm{I}}_3,$ which show different temperature dependence of the electric resistivity at low temperature from one another. An analytical formula is presented for the thermopower of dimerized one-dimensional (1D) metals within the tight-binding and the relaxation time approximations. It is shown that the effect of the dimerization is small on magnitude and sign of the thermopower of 1/4-filled 1D metals, while large deviation from the uniform case is predicted if band filling is changed. An approximate simple expression is also given for the angular width of the TAE anomaly as a function of the anisotropy of the transfer integrals and the lattice constants of a triclinic crystal for easy application of this new method.

• Received 15 July 2002

DOI:https://doi.org/10.1103/PhysRevB.67.035111