Magnetic excitations in pure, lightly doped, and weakly metallic La2CuO4

B. Keimer, N. Belk, R. J. Birgeneau, A. Cassanho, C. Y. Chen, M. Greven, M. A. Kastner, A. Aharony, Y. Endoh, R. W. Erwin, G. Shirane

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We report a comprehensive neutron-scattering study of the evolution of the magnetic excitations in La2-xSrxCuO4 for 0≤x≤0.04. We first present accurate measurements of the magnetic correlation length and the sublattice magnetization of a carrier-free La2CuO4 crystal and analyze these in the context of recent theoretical predictions. We then systematically investigate the influence of different dopants on the magnetism: Our measurements indicate that static vacancies in the La2Cu1-yZnyO4 system affect the magnetic correlations in a similar manner as electrons in Pr2-xCexCuO4. The magnetic correlation length is much more rapidly suppressed as a function of x in La2-xSrxCuO4, and for x≤0.04 we find that it obeys the empirical relation ξ-1(x,T)=ξ-1(x,0)+ξ-1(0,T), where ξ(0,T) is the measured correlation length of the carrier-free sample. We also report an extensive set of measurements of the dynamical magnetic response function of a crystal of composition La1.96Sr0.04CuO4 for excitation energies 0.75≤ω≤45 meV and temperatures 1.5≤T≤500 K. The dc conductivity of this crystal exhibits three different regimes: metallic for T100 K, weakly localized for 100T10 K, and strongly localized below ∼1 K. Our neutron measurements show that the generalized susceptibility of this sample follows a surprisingly simple scaling function in the variable ω/T. This observation allows us to relate our data to a variety of normal-state properties of the layered copper oxides, in particular the dc and ac conductivities. Finally, at temperatures below ∼20 K a ''central peak'' with a characteristic energy scale of less than 0.1 meV becomes prominent. Its relation to the localization of the charge carriers at low temperatures remains speculative.

Original languageEnglish (US)
Pages (from-to)14034-14053
Number of pages20
JournalPhysical Review B
Issue number21
StatePublished - 1992


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