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New understanding of high temperature superconductivity

Left - Evolution of spectra taken at the impurity resonance center. For plots at higher temperatures, the experimental data (black circles) are overlaid with magenta lines, representing the 5.2 K spectrum thermally broadened to the respective temperatures. Even for temperatures above TC =15 K, the match is remarkable.
Right – Topography (top) shows none of the turmoil of the impurity resonance revealed in the Vsample = −2 meV conductance maps, which are remarkably similar at temperatures below and above TC = 15 K.

 

A thorough understanding of high temperature superconductivity (HTS) could potentially drive the development of new superconducting materials, leading to numerous industrial applications. At present, however, no theory of HTS has managed to unify the broad range of experimental observations into one coherent picture. One challenge is the necessity of understanding not only superconductivity but also other parts of the phase diagram, such as the pseudogap phase, which exists above the superconducting transition temperature (TC). Revealing new information about the pseudogap state and its relation to superconductivity is in the forefront of current research. In IRG –V of MIT MRSEC, Eric Hudson’s group uses scanning tunneling microscopy (STM) to observe the atomic scale electronic behavior of HTS materials over a wide range of temperatures, and in particular through the phase transition at TC. One aspect of their research is the study of local modifications to electronic behavior due to single atom impurities. As impurities significantly suppress TC in bulk samples, understanding their effects on electrons at the atomic scale, and particularly their deleterious effects on superconductivity, may lead to a understanding of how superconductivity arises in the first place.

By following isolated impurities in overdoped (TC=15K) Bi2Sr2CuO6+x (Bi-2201) as a function of temperature, Chatterjee et. al. (Chatterjee, K. et al. Nature Phys. 4, 108–111 (2008)) discovered that resonances associated with impurities are unchanged upon warming through TC, implying that these impurity resonances are caused not by interactions with superconductivity but rather with the pseudogap. Details of spatial variations of electronic spectra surrounding the impurities demonstrate an interesting interplay between the pseudogap and superconductivity in determining the nature of the impurity resonance and highlight the coexistence of superconductivity and the pseudogap below TC.

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