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Coexistence of two order parameters and a pseudogap in the iron-based superconductor LaFeAsO1-xFx | | R.S. Gonnelli
; D. Daghero
; M. Tortello
; G.A. Ummarino
; V.A. Stepanov
; J.S. Kim
; R.K. Kremer
; | | Date: |
20 Jul 2008 | | Abstract: | The determination of the order parameter (OP) and its symmetry has always
been of crucial importance in the study of superconducting materials. The
recent discovery of high-temperature superconductivity in XFeAsO1-xFx with Tc
ranging from 26 K (X=La) to 43-55 K (X= Sm, Nd) poses important questions on
the origin of the electron pairing in these compounds. While the standard
electron-phonon mechanism seems ruled out, an unconventional, spin-fluctuation
mediated pairing has been proposed. However, the nature and the precise value
of the order parameters in these materials are still a matter of debate. Here
we show that point-contact Andreev-reflection experiments on LaFeAsO1-xFx give
direct evidence of three energy scales in the superconducting state: i) a
nodeless superconducting OP, Delta1 = 2.8-4.6 meV, scaling with the local Tc of
the contact; ii) a larger unconventional OP (possibly of non-superconducting
origin) that gives conductance peaks at 9.8-12 meV, apparently closes below Tc
and disappears for Tc ~ 31 K; iii) a pseudogap surviving in the normal state up
to T* ~ 140 K (close to the Neel temperature of the undoped compound), which we
associate to the presence of antiferromagnetic spin fluctuations (AF SF). This
last finding indicates that AF SF coexist with superconductivity in
LaFeAsO1-xFx. These results and their analogy with old and recent findings in
superconductors like Ba0.6K0.4Fe2As2, high-Tc cuprates and URu2Si2 prove the
complex, strongly unconventional, nature of superconductivity in Fe-As-based
superconductors. | | Source: | arXiv, 0807.3149 | | Services: | Forum | Review | PDF | Favorites | |
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