True charge-transfer gap in parent insulating cuprates

摘要: A large body of experimental data points toward a charge-transfer (CT) instability parent insulating cuprates to be their unique property. It is argued that the true CT gap in these compounds as small 0.4--0.5 eV rather than 1.5--2.0 usually derived from optical measurements. In fact we deal with competition conventional (3${d}^{9}$) ground state and formation electron-hole dimers which evolves under doping an unconventional bosonic system. My conjecture does provide unified standpoint on main findings for including linear nonlinear optical, Raman, photoemission, photoabsorption, transport properties related excitations. addition I suggest scenario evolution CuO${}_{2}$ planes unstable nonisovalent doping.