UNIVERSALITY OF CHARGE AND SPIN RESPONSE IN DOPED ANTIFERROMAGNETS

Recent results for finite-temperature static and dynamical properties of the planar t - J model, obtained by a novel numerical method for sm all correlated system, are reviewed. Of particular interest are T > 0 charge and spin response functions: optical conductivity sigma(omega) and spin susceptibility chi((q) over right arrow omega), which show ve ry universal features at intermediate doping. In spin dynamics the uni versality is best seen in the local spin correlation function, which a ppears to be T-independent and only weakly doping dependent. Results a pply directly to the inelastic neutron scattering and to the NMR relax ation experiments in cuprates. Analysing sigma(omega) we find that the current correlation function C(omega) is nearly T- and moreover omega -independent in the regime T < J, leading to the sigma(omega) behavior , very close to the marginal Fermi liquid concept and experiments. The se properties are interpreted in connection with the large entropy, re lated to the large degeneracy of the low-lying states and quantum frus tration in the doped antiferromagnets. On the other hand, results for the uniform susceptibility chi(o)(T), as well as for the sigma(omega), reveal qualitative changes on entering the underdoped regime.