Origin of Magnetoresistance Anomalies in Antiferromagnetic   YBa_{2}Cu_{3}O_{6+x}
  Specific d-wave angular dependence of the in-plane magnetoresistence in antiferromagnetic tetragonal YBa_{2}Cu_{3}O_{6+x} (x~0.3) on orientation of the external magnetic field in (a,b) plane is assigned to the effective hole transport through low lying excited purely oxygen doublet O2pe_u state, not the ground b_{1g}(d_{x^2-y^2}) state. External magnetic field determines the orientation of the strong exchange field for the spin-triplet b_{1g}e_u:{}^{3}E_u state of the hole CuO_4 center and due to the spin-orbital coupling results in either orbital polarization of the E_u doublet giving rise to the spatial anisotropy of the hole transport. Experimental data allow to estimate parameter of the effective spin Hamiltonian. 
A. S. Moskvin, Yu. D. Panov (Ural State University, Ekaterinburg,   Russia)