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It is well documented that SDW in (TMTSF)2PF6 undergoes another phase transition at T* ≈ 4 K , though the nature of the new phase has not been identified. In particular the angular dependence of the magnetoresistance for B ‖( a, b ′) plane below 4.2 K is dramatically different from the one above 4.2 K. We propose that below T* the new phase unconventional SDW (USDW) appears which modifies drastically the quasiparticle spectrum. The present model describes the observed angular dependence of the magnetoresistance reasonably well, which cannot be interpreted in terms of the conventional SDW.

We have calculated the electron-electron scattering contribution to the electrical resistivity ( rho perpendicular to ) in the two transverse directions for an anisotropic tight-binding cosine band. This type of band structure is thought to apply to organic metals such as the Bechgaard salts. Simple expressions for rho perpendicular to were obtained using the standard variational method. In all cases we found rho perpendicular to = alpha T+ beta T^2. For the b direction alpha approximately t_b^-2 and beta approximately t_b^-3, and for the c axis alpha approximately t_c^-2 while the expression for beta is more complicated. The above formulas are compared with experimental data and the effective Coulomb interaction for the Bechgaard salts is found to be U approximately=0.64 eV.

The magnetic penetration depth λ (T) has been studied for single crystals of yttrium barium copper oxide with the magnetic field parallel to the ab plane. λ(T)-λ(0) obeys a T2 law as found recently from measurements on powder samples.