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Trends in Applied Spectroscopy   Volumes    Volume 4 
Abstract
Neutron crystal-field spectroscopy in rare-earth based high-temperature superconductors
Joel Mesot, Daniel Rubio Temprano, Albert Furrer
Pages: 75 - 100
Number of pages: 26
Trends in Applied Spectroscopy
Volume 4 

Copyright © 2002 Research Trends. All rights reserved

ABSTRACT

Neutron spectroscopy is a powerful tool to determine unambiguously the crystal-field potential in rare-earth based high-temperature superconducting materials. This technique provides detailed information on the electronic ground state of the rare-earth ions which is important to understand the thermodynamic magnetic properties as well as the coexistence between superconductivity and long-range magnetic ordering of the rare-earth ion sublattice at low temperatures. Moreover, the decay of the antiferromagnetic state of the parent compound as well as the evolution of the superconducting state upon doping can be directly and quantitatively monitored. It is found that the observed crystal-field spectra separate into different local components whose spectral weights distinctly depend on the doping level, i.e., there is clear evidence for cluster formation. The onset of superconductivity can be shown to result from percolation which means that the superconductivity is an inhomogeneous materials property. Moreover, the linewidths of crystal-field transitions directly probe the electronic density-of-states at the Fermi energy through relaxation, i.e., the crystal-field transitions exhibit line broadening due to the interaction with the charge carriers according to the well-known Korringa law. If an energy gap is present due to pairing, the line broadening of the crystal-field transitions is suppressed. This effect was clearly observed in the slightly underdoped high-temperature superconductor HoBa2Cu4O8 in which the opening of the gap is observed at the pseudogap temperature T* far above the superconducting transition temperature Tc. The pseudogap temperature T* is found to exhibit huge shifts upon both oxygen and copper isotope substitution, which suggests that phonons or lattice fluctuations involving both the oxygen and copper ions are important for the pairing mechanism in high- temperature superconducting materials.

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