Bestimmung der Ladungsverteilung in YBa2Cu3O6+y unter hohem Druck mittels NMR
Zusammenfassung der Projektergebnisse
The understanding of high-temperature superconducting cuprates is still at the center of interest of modern condensed matter, given the many intriguing observations in terms of their electronic properties, as well as progress with theory. Nuclear magnetic resonance (NMR) as a bulk quantum probe of matter has been very influential in developing the early thinking about these complex materials that may provide key technological applications if a full understanding of the phenomena is reached, in particular, concerning the question of how one can achieve higher temperatures Tc for superconducting applications. Recently, we could show how one can measure with NMR the charge at copper (nCu ) and oxygen (nO) in the ubiquitous CuO2 plane of these superconducting cuprates. This enabled us to measure the total charge in the CuO2 plane (nCu + 2nO), which is typically altered by doping excess charges to obtain the highest Tc , known to depend on the particular family of materials, i.e. the chemistry surrounding the CuO2 plane. Since there are other means of estimating doping, this was not so much of interest. Surprisingly, however, we found that the atomic scale sharing of charge between Cu (nCu) and nO differs widely among materials, and that the following relation holds: Tc,max ≈ 200K ⋅ 2nO. This relation holds for all hole-doped cuprates indicating that it is more than just an interesting correlation, with predictive power. For decades, it is known that if one applies external, very high pressure to the cuprates, one can also increase Tc in excess to what one can achieve by ordinary doping. And it was stipulated based on experimental evidence that pressure has two effects: (1) it leads to additional doping of the materials (overall change of charge in the CuO2 plane); (2) it interferes with the mechanism for superconductivity to enhance the Tc even further. Since our group is pioneering the use of high-pressure NMR, it seemed natural for us to propose to investigate the charges in the cuprates by means of high-pressure NMR. Our main results are rather simple. First, there is charge ordering in YBa2 Cu3 O6+y (and other cuprates), and it was always present in the NMR data, but assumed to be caused by crystal symmetry. Second, we measure that high pressure causes doping of the CuO2 plane, as expected, but it also rearranges the charges in the plane to favor a higher Tc , according to our correlation discussed above. Thus, the old conundrum, how pressure can increase Tc , is solved. And it points indeed to a special role of planar O charge for the mechanism of superconductivity. Finally, while not in the center of the proposed project, in order to understand other NMR properties of the materials under pressure, we began with a greater survey of the literature data that had been collected by researchers all over the world until now. Much to our surprise, we discovered very different phenomenologies for the cuprates than what was and could only be deduced from the few families available for investigation early on. This concerns shift and relaxation, in particular for Cu and O. For example, while NMR discovered the so-called pseudogap - one of the most mysterious phenomena of the cuprates - in 1989, we now believe that it is a temperature independent, low-energy gap near the Fermi surface that depends on doping and pressure. We are certain that NMR will again contribute in a major way to the understanding of the cuprates, based on our experiments and analyses.
Projektbezogene Publikationen (Auswahl)
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Bulk charge ordering in the CuO2 plane of the cuprate superconductor YBa2 Cu3 O6 .9 by high-pressure NMR. Condens. Matter 3, 23 (2018)
Reichardt, S., Jurkutat, M., Guehne, R., Kohlrautz, J., Erb, A., Haase, J.
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Phenomenology of 63Cu Nuclear Relaxation in Cuprate Superconductors. J. Supercond. Nov. Magn. 155, 629–8 (2019)
Jurkutat, M., Avramovska, M., Williams, G. V. M., Dernbach, D., Pavićević, D., Haase, J.
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Tc and Other Cuprate Properties in Relation to Planar Charges as Measured by NMR. Condens. Matter 4, 67 (2019)
Jurkutat, M., Erb, A., Haase, J.
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Temperature-Independent Cuprate Pseudogap from Planar Oxygen NMR. Condens. Matter 5, 66–19 (2020)
Nachtigal, J., Avramovska, M., Haase, J.
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Moissanite anvil cell single crystal NMR at pressures of up to 4.4 Gpa. Rev. Sci. Instrum. (2021)
Kattinger, C., Guehne, R., Tsankov, S., Jurkutat, M., Erb, A., Haase, J.
