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- Magnuson, Martin, et al.
(författare)
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Self-doping processes between planes and chains in the metal-to-superconductor transition of YBa2Cu3O6.9
- 2014
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Ingår i: Scientific Reports. - : Springer Science and Business Media LLC. - 2045-2322. ; 4, s. 7017-
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Tidskriftsartikel (refereegranskat)abstract
- The interplay between the quasi 1-dimensional CuO-chains and the 2-dimensional CuO2 planes of YBa2Cu3O61x (YBCO) has been in focus for a long time. Although the CuO-chains are known to be important as charge reservoirs that enable superconductivity for a range of oxygen doping levels in YBCO, the understanding of the dynamics of its temperature-driven metal-superconductor transition (MST) remains a challenge. We present a combined study using x-ray absorption spectroscopy and resonant inelastic x-ray scattering (RIXS) revealing how a reconstruction of the apical O(4)-derived interplanar orbitals during the MST of optimally doped YBCO leads to substantial hole-transfer from the chains into the planes, i.e. self-doping. Our ionic model calculations show that localized divalent charge-transfer configurations are expected to be abundant in the chains of YBCO. While these indeed appear in the RIXS spectra from YBCO in the normal, metallic, state, they are largely suppressed in the superconducting state and, instead, signatures of Cu trivalent charge-transfer configurations in the planes become enhanced. In the quest for understanding the fundamental mechanism for high-Tc-superconductivity (HTSC) in perovskite cuprate materials, the observation of such an interplanar self-doping process in YBCO opens a unique novel channel for studying the dynamics of HTSC.
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- Pfeiffer, Christoph, et al.
(författare)
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A 7-channel high-Tc SQUID-based on-scalp MEG system : A 7-channel high-$T_{\text{c}}$ SQUID-based on-scalp MEG system
- 2020
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Ingår i: IEEE Transactions on Biomedical Engineering. - 0018-9294. ; 67:5, s. 1483-1489
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Tidskriftsartikel (refereegranskat)abstract
- Objective: To present the technical design and demonstrate the feasibility of a multi-channel on-scalp magnetoencephalography (MEG) system based on high critical temperature (high- Tc ) superconducting quantum interference devices (SQUIDs). Methods: We built a liquid nitrogen-cooled cryostat that houses seven YBCO SQUID magnetometers arranged in a dense, head-aligned array with minimal distance to the room-temperature environment for all sensors. We characterize the performance of this 7-channel system in terms of on-scalp MEG utilization and present recordings of spontaneous and evoked brain activity. Results: The center-to-center spacing between adjacent SQUIDs is 12.0 and 13.4 mm and all SQUIDs are in the range of 1-3 mm of the head surface. The cryostat reaches a base temperature of ∼ 70 K and stays cold for > 16 h with a single 0.9 L filling. The white noise levels of the magnetometers is 50–130 fT/Hz 1/2 at 10 Hz and they show low sensor-to-sensor feedback flux crosstalk ( < 0.6%). We demonstrate evoked fields from auditory stimuli and single-shot sensitivity to alpha modulation from the visual cortex. Conclusion: All seven channels in the system sensitively sample neuromagnetic fields with mm-scale scalp standoff distances. The hold time of the cryostat furthermore is sufficient for a day of recordings. As such, our multi-channel high- Tc SQUID-based system meets the demands of on-scalp MEG. Significance: The system presented here marks the first high- Tc SQUID-based on-scalp MEG system with more than two channels. It enables us to further explore the benefits of on-scalp MEG in future recordings.
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