Unconventional spin density wave in $(\mathrm{TMTSF}{)}_2{\mathrm{PF}}_6$ below $T^{\star }\approx 4\hspace{0.5em}\mathrm{K}$

The presence of subphases in the spin-density wave (SDW) phase of $(\mathrm{TMTSF}{)}_2{\mathrm{PF}}_6$ below $T^{\star }\approx 4\hspace{0.5em}\mathrm{K}$ has been suggested by several experiments but the nature of the new phase is still controversial. We have investigated the temperature dependence of the angular dependence of the magnetoresistance in the SDW phase which shows different features for temperatures above and below $T^{\star }\approx 4\hspace{0.5em}\mathrm{K}.$ For $T>\mathrm{}4\mathrm{}\hspace{0.5em}\mathrm{K}$ the magnetoresistance can be understood in terms of the Landau quantization of the quasiparticle spectrum in a magnetic field, where the imperfect nesting plays the crucial role. We propose that below $T^{\star }\approx 4\hspace{0.5em}\mathrm{K}$ the new unconventional SDW (USDW) appears modifying dramatically the quasiparticle spectrum. Unlike conventional SDW the order parameter of USDW depends on the quasiparticle momentum ${\Delta }_1\left(\mathbf{k}\right)\propto \cos {2bk}_y.$ The present model describes many features of the angular dependence of magnetoresistance reasonably well. Therefore, we may conclude that the subphase in $(\mathrm{TMTSF}{)}_2{\mathrm{PF}}_6$ below $T^{\star }\approx 4\hspace{0.5em}\mathrm{K}$ is described as SDW plus USDW.