Proc. "Local and Nanoscale Structure in Complex Systems", Santa Fe, 2002(subm. to Journal of Nanoscience and Nanotechnology 2002)Spatial and temporal probes in inhomogeneous systems: Theory and experimentDragan Mihailovic"Jozef Stefan" Institute, Jamova 39, 1000 Ljubljana, Slovenia,Tel. +386 1 477 3388, Fax: +386 1 425 1077dragan.mihailovic@ijs.siKeywords: Superconductivity, Inhomogeneity, Jahn-Teller Effect, INS, XAFS, PDF, STM, ARPES, Femtosecond spectroscopy.AbstractThe experimental and theoretical challenges posed by the study of dynamically inhomogeneous systems are outlined in the context of cuprates and other oxides. Considering the pitfalls in the single-component approach to the analysis of inhomogeneous systems, the effect of either temporal or spatial averaging by different experiments is discussed. A group-theoretical symmetry analysis of the observed inhomogeneities in real space and k-space observed in the cuprates is shown to lead to a quantitatively verifiable description of the inhomogeneous state, comprising of bound singlet pairs in the ground state and unbound fermions in the excited state. The predicted symmetry breaking associated with pairing is shown to be verifiable experimentally.IntroductionDynamic mesoscopic inhomogeneities seem to be a ubiquitous feature of “complex matter”. The origin of these inhomogeneities may be anything from the formation of local superconducting pairs in high-T c superconductors to migrating photoexcited charges on DNA molecules or proteins. The experimental challenge of the last decade has been to invent and perfect new techniques for the investigation of dynamic inhomogeneities on time-scales sufficiently short to freeze the motion of the relevant excitations and give information on the microscopic origin of the dominant interactions leading to the observed complexity.Theoretical progress in accurately describing inhomogeneities occurring on various length scales depends crucially on high-quality experimental data and a detailed microscopic understanding of the relevant interactions. This challenge is gradually being met by the development of new techniques such as time-resolved X-ray diffraction and the extension of various established techniques to the study of inhomogeneous systems.Cuprate superconductors is a good case example, where the importance of inhomogeneity was not obvious. Although very soon after the discovery of superconductivity in cuprates, the possible existence of charge inhomogeneity was recognized by Gorkov and Sokol1 and possibly in the form of spin stripes by Zannen and Gunnarson2 and others, it has taken quite some time to show experimentally that these systems are indeed inhomogeneous and that these inhomogeneities are relevant3.Most experimental techniques discussed in this volume involve some kind of averaging, either spatial or temporal. Not surprisingly, the conclusions reached on the basis of different techniques are sometimes in direct and fundamental disagreement. The origin of these disagreements is believed to be in the interpretation of data, which has been averaged in different ways. As one example, time-resolved optical techniques, which have been discussed in this section give accurate time-domain information on lifetimes and energies of elementary excitations in cuprate superconductors, but involve spatial averaging, and cannot give any direct information on nano-scale spatial structure. However, they show no evidence for a strongly anisotropic superconducting gap (or pseudogap). This is inconsistent with the interpretation
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