AC-CONDUCTIVITY MEASUREMENTS ON LA2NIO4+DELTA

We have carried out ac-conductivity sigma(omega) measurements on La2Ni O4+delta,0<delta less than or equal to 0.08, in the frequency (omega) and temperature ranges 5 Hz-13 MHz and 75-300 K. The ac conductivity f ollows a set of basic characteristics: (i) sigma(omega) obeys the omeg a(s) power law (s similar to 0.38-0.85). (ii) At higher frequencies, s igma(omega) saturates. To our knowledge, saturation of sigma(omega) wi th omega in the radio frequency range has not been previously reported for an electronic system. (iii) sigma(omega) is weakly temperature de pendent in the dispersive region. (iv) Below the saturation region, fo r a fixed delta, the reduced conductivity sigma(omega)/a(0), at variou s values of T can be scaled to a generalized reduced frequency omega(n )=omega/sigma(0)T-n. For delta=0.01-0.08. n is = 1/3 or 1/4. (v) Furth ermore, at a fixed T, the reduced conductivities for various delta lie on a universal curve; n in this case varies from -1/4 to -1/2. (vi) G ood agreement of data with the Summerfield extended pair approximation theory, sigma(omega)/sigma(0) = 1 + (A omega alpha e2/ sigma(0)k(B)T( n))(b), is found; b similar to 0.64-1. (vii) Our data analysis reveals two loss peaks; the first is located near the critical frequency omeg a(c) below which the imaginary part of conductivity sigma(2)(omega) is proportional to omega. (viii) The relation sigma(0)proportional to om ega(c) is obeyed, with omega(c)(proportional to)exp[-(T-2/T)(1/3)]. Th e second loss peak lies near the onset of saturation. These characteri stic signatures in the ac conductivity point to a transport mechanism involving localized states near the Fermi level.