Physica B 180 & 181 ( 1992) 15-20 North-Holland zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA PHYSICA ,’ Static and dynamic spin fluctuations in the spin glass doping regime in La2_,SrxCu04+6 R.J. Birgeneau”, N. Belk”, Y. Endohb, R.W. Erwin’, M.A. Kastner”, B. Keimef and G. Shiraned zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA “Department of Physics. Massachusetts Institute of Technology. Cambridge. MA 021.39, USA “Department of Physics, Tohoku University, Sendai 980, Japan ‘National Institute of Standards and Technology. Gaithershurg, MD 20899. USA “Brookhaven National Laboratory, Upton, NY 11973. USA We review the results of neutron scattering studies of the static and dynamic spin fluctuations in crystals of La, I Sr,CuO,,, in the doping regime intermediate between the N&l and superconducting regions. In this regime the in-plane resistance is linear in temperature down to -80 K with a crossover due to logarithmic conductance effects at lower temperatures. The static spin correlations are well described by a simple model in which the inverse correlation length K(X, 7‘) = K(X, 0) + ~(0, T). The most dramatic new result is the discovery by Keimer et al. that the dynamic spin fluctuations exhibit a temperature dependence which is a simple function of w/T for temperatures IO KG T G 500 K for a wide range of energies. This scaling leads to a natural explanation of a variety of normal state properties of the copper oxides. zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA 1. Introduction In virtually all cases, highly correlated electronic systems exhibit unusual and, most often, dramatic magnetic properties. This is true for mixed valent materials, heavy fermion systems and many organic conductors. in particular, those with competing spin density wave states. Without ambiguity, the most re- markable highly correlated electronic systems are the lamellar copper oxides [l]. As a function of doping, these materials evolve from antiferromagnetic Mott insulators to two-dimensional weakly localized spin glasses to novel metals exhibiting superconductivity at extraordinarily high temperatures [2]. Because of its unique properties as a probe of microscopic static and dynamic spin fluctuations, neutron scattering is playing an essential role in elucidating the fundamental prop- erties of the copper oxide materials. However, the physics of the copper oxides is sufficiently complicated that neutron scattering alone has not been able to solve this problem. Rather. information from neutron experiments must be combined with data from other spectroscopic probes as well as data from transport, thermodynamic and magnetic measurements to pro- duce a complete empirical picture. In general, the neutron experiments themselves are very difficult so our characterization of the spin fluc- tuations in the lamellar copper oxides is still quite incomplete. So far, reliable experimental data are available only in the La,_,Sr,CuO, and YBa,Cu,O (,+R systems. At this conference, Rossat- Mignod et al. (31 will discuss their work on the latter materials. In this review, therefore, we will largely confine our discussion to measurements in the La> ,Sr,CuO, materials. Our approach to this prob- lem is that the antiferromagnetic Mott insulating, weakly localized spin glass and superconducting re- gimes each represent quite interesting and challenging basic physics problems in their own right. Further, to understand the high temperature superconductivity one will almost certainly have to understand the man- ner in which the properties of the CuOZ sheets evolve between these three states. In this brief review we will not attempt to describe in detail all of the results in the La,_, Sr,CuO, system. Instead, we will focus on the behavior in the intermediate spin glass regime where Keimer et al. [4] have recently obtained par- ticularly interesting results. 2. Transport measurements Samples of La,_ zyxwvutsrqponmlkjihgfedcbaZYXWV ,Sr,CuO, , B, which order magneti- cally, typically exhibit the transport behavior of lightly doped semiconductors [S]. The in-plane room tem- perature resistivity decreases rapidly with increasing x or 6 and there is substantial evidence for an insulator to metal transition within the CuO, sheets at a hole concentration of about 1.5% [6]. This is suggestively close to the hole concentration at which the N&e1 order vanishes although a direct causal relationship between these two phenomena has not yet been estab- lished. In the spin glass regime, the between-plane conduc- tivity is semiconducting for all temperatures below 500 K. On the other hand, the in-plane behavior is quite striking. We show in fig. 1 results of measure- OY2l-~S26/Y2i$OS.O0 0 lY92 - Elsevier Science Publishers B.V. All rights reserved