Polarization Dynamics in La 5/3 Sr 1/3 NiO 4
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"**Department of Physics and
Astronomy, Rutgers University, Piscataway, NJ 08855.
ABSTRACT Dynamic susceptibility measurements at microwave frequencies (2 - 10GHz) are a sensitive probe of charge dynamcis in La 5 / 3 Sr113 Ni0 4 . Below the charge ordering temperature of 240K, a dielectric loss peak due to a relaxation mode with a large dielectric susceptibility is observed, and is associated with charge stripe formation. The dielectric response for HI[b (E, I_ b) is well represented by j(T) = e,/(1 - iw-r(T)), with Eo, - 50, and r(T) = 2 x 10- 9 (sec) exp(-T/37K). Parallel conductivity o(T) contributions dominate at higher temperatures and for HI Ic (E,_I c). The dielectric loss peak observed indicates that the charge relaxation rates lie in the GHz frequency ranges. INTRODUCTION The carrier doping in Mott insulators such as La2 MO4+6 (M = Ni, Cu) has attracted much attention bacause of the existence of the high T, superconductivity in hole doped La 2 Cu0 4 . These quasi-two dimensional electronic systems exhibit charge and spin correlations in which doped holes tend to undergo stripe like ordering on the domain walls of antiferromagnetic stripes[I, 2, 6]. The evidence of dynamic stripes (spin fluctuations) in superconducting La 2 _,Sr, Cu04_ 6 and static stripe correlations in nonsuperconducting La 2 _,Sr.,NiO 4 has generated compelling interest about the interplay of stripe dynamics and superconductivity. A variety of measurements have revealed three successive transitions associated with quasi-two dimensional commensurate charge ordering (at - 240K) and spin stripe ordering (, 190 - 160K) [1, 10, 9, 8, 2] in La5 / 3 Srj/ 3 Ni0 4 . The spin ordering at 190K is driven by charge ordering when the ordering between charge stripes takes place. The existence of stripe glass is proposed in the temperature regime, 240- 190K[2, 9, 8, 11], but the details of orientational order is missing probably due to the dominant effect of short stripes. A key issue that has arisen is the role of the measurement time-scale since stripes, and more generally magnetic and charge correlations, are now believed to have strong dynamical properties, and previous measurements have been principally carried out with probes having very different time scales such as Neutron scattering (_ 10-13 sec) and NMR ( 10-7 sec). In this paper, we present measurements of the dynamic ( microwave ) response of La5 / 3 Sr 1 / 3 Ni0 4 using a precision superconducting microwave cavity at 10GHz, supplemented by measurements at 2GHz using a normal Cu resonator. Our measurements probe short time (- 10-11 sec) or high frequency dynamics of charge correlations in this material. We find that charge ordering at - 240K suppresses the dia-electromagnetic contribution caused by eddy currents due to the conductivity. Instead, charge ordering is accompanied by the onset of a dynamic dielectric susceptibility, which freezes out (quasi-statically at the finite measuring frequency) as the temperature is lowered due to rapid increase of the 35 Mat. Res. Soc. Symp. Proc. Vol.
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