Quantum frustrations in quasi-2D antiferromagnets

An influence of localized holes on magnetic properties of the CuO₂ planes in quasi-2D antiferromagnets (La₂CuO₄, YBa₂Cu₃O_+x etc.) is studied. It is demonstrated that the hole localized on the bond connecting Cu²⁺ ions gives birth to frustration which in a wide range of parameters behaves as the two-level system with the energyEJ, whereJ is the exchange interaction of the neighbouring copper spins. As a result forTE the spin-wave energy may noticeably decrease even at a low concentration of the holesc if the parametercJ|E| is not small. When this parameter is greater than unity, the reentrant transition may occur in the system. Experimental data presently available are in a qualitative agreement with the theoretical predictions.