The Splitting of SDW State into Commensurable and Incommensurables Ones and the Peculiarities of the Behavior of Thermodynamic Quantities in a Magnetic Field Arbitrarily Oriented to Magnetization in Quasi Two-Dimensional Systems

摘要

The quasi-two-dimensional system in which magnetism is caused by spin density wave (SDW) with an anisotropic energy spectrum (with defined impurity concentration x) is examined. The wave vector Q is supposed to be different from 2k_F and the umklapp scattering (U-processes) is taken into account. The system is placed in a magnetic field arbitrarily oriented with respect to the vector M_Q. The basic equations for order parameters M_Q~z, M'_Q, M_z, M~σ are obtained and the system of these equations is transformed taking into account the U-processes. The particular cases (H‖M_Q) and (H⊥M_Q) and the case of small arbitrarily oriented magnetic fields H are examined in detail. The conditions of the system transition to commensurable and incommensurable SDW state are analyzed. The phase diagram (T, x) at H = 0 is traced. The influence of the magnetic field H on the temperature of magnetic transition is researched and the aspect of the phase diagram in magnetic field in the cases H~z H~σ = 0 is presented. The longitudinal magnetic susceptibility X‖ which demonstrates that at x < x_c the temperature behavior is similar to the case when the system has a gap, and at x > x_c to a gapless case. At x ～ x_c in the dependence X_(‖)(T) a local maximum appears. The influence of the energy spectrum anisotropy on the system's properties is researched. Also the angular anisotropy of the quantity χ‖ at different values of T and x is determined.