doi:10.1016/j.physc.2008.05.051 
Copyright © 2008 Elsevier B.V. All rights reserved.
Hole superconductivity in arsenic–iron compounds
F. Marsiglioa, 
, 
and J.E. Hirschb
aDepartment of Physics, University of Alberta, Edmonton, Alberta, Canada T6G 2G7
bDepartment of Physics, University of California, San Diego, La Jolla, CA 92093-0319, USA
Received 1 April 2008;
accepted 15 May 2008.
Available online 22 May 2008.
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Abstract
Superconductivity above 25 K, and possibly above 40 K, has recently been discovered in LaO1−xFxFeAs and related compounds. We propose that this is another example of the mechanism of hole superconductivity at play. This requires the existence of hole carriers at the Fermi energy, which appears to contradict current observations. We propose that two-band conduction is taking place in these materials, that the negative ion As−3 plays a key role, and that superconductivity is non-phononic and driven by pairing and undressing of heavily dressed hole carriers to lower their kinetic energy. We make several predictions of future observations based on our theory.
Keywords: Hole superconductivity; Arsenic–iron compounds; Two-band model; Tunneling asymmetry
PACS classification codes: 74.10.+v; 74.20.−z; 74.70.−b
Fig. 1. Tc versus hole concentration in the arsenic band for the two-band model (solid line; red line in the web version) and the single-band model (dashed line; green line in the web version). For simplicity we used equal bandwidths (1 eV) for both the arsenic-based and iron-based bands, with the center of the Fe band shifted with respect to the center of the As band. We used interaction parameters Ua = 5 eV and Ka = 1.86 eV, and Ud = Kd = 0. The interband coupling was chosen to be a constant, Vad = 0.2 eV. For the single-band result we set Vad = 0.
Fig. 2. Schematic depiction of how holes are created by electron doping. The electron added to Fe2+ repels an electron from As3− to the neighboring Fe2+, leaving behind a hole in arsenic (As2−).
Fig. 3. Schematic depiction of As and Fe bands. In the undoped case (a), the Fermi level (horizontal full line) is very close to the top of the As band since Tc is very low. Upon hole doping (b), the Fermi level moves down (dashed horizontal line in b) and the bands don’t move. Upon electron doping (c), the Fermi level moves up (dashed horizontal line), the As band moves up and the Fe band moves down, as indicated by the dashed bands in (c).
Fig. 4. The As-based gap, Δ0a, and the Fe-based gap, Δ0d versus temperature. The temperature dependence is essentially identical, and very BCS-like. For these parameters, the gap ratios at zero temperature are 3.9 and 1.1 for the large and small gap, respectively. The ratio between the two is Δ0a/Δ0d = 3.7. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 5. dI/dV versus sample voltage, for the parameters used in Fig. 4, for a variety of temperatures below Tc. In (a) we use identical electronic densities of states in the As and Fe bands. In (b) we increase the degeneracy in the Fe band by a factor of 3. In both cases the two gap structure in the tunneling curves is most evident at low temperatures, and a tunneling asymmetry exists – large gap coherence peak higher on the left than on the right. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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