Abstract
The quasiparticles in the normal state of iron-based superconductors have been shown to behave universally as a 3-dimensional Fermi liquid. Because of interactions and the presence of sharp Fermi surfaces, the quasiparticle energy contains, as a function of the momentum
$$\varvec{p}$$
p
, a term of the form
$$( p - p_0)^3 \ln {( |p-p_0|/p_0)} $$
(
p
-
p
0
)
3
ln
(
|
p
-
p
0
|
/
p
0
)
, where
$$p = | \varvec{p} |$$
p
=
|
p
|
and
$$p_0$$
p
0
is the Fermi momentum. The electronic specific heat coefficient, magnetic susceptibility (Knight shift), electrical resistivity, Hall coefficient and thermoelectric power divided by temperature follow, as functions of temperature T, the logarithmic formula
$$a-b T^2 \ln {(T/T^*)}$$
a
-
b
T
2
ln
(
T
/
T
∗
)
,
$$a, \, b$$
a
,
b
and
$$T^*$$
T
∗
being constant; these formulae have been shown to explain the observed data for all iron-based superconductors. It is shown that the concept of non-Fermi liquids or anomalous metals which appears in the literature is not needed for descriptions of the present systems. When the superconducting transition temperature
$$T_{\mathrm {C}}$$
T
C
and the b / a value for the resistivity are plotted as functions of the doping content x, there appear various characteristic diagrams in which regions of positive correlation and those of negative correlation between
$$T_{\mathrm {C}}$$
T
C
and b / a are interconnected; from these diagrams, we may make speculations about the types of superconductivity and the crossover between them.
Unconventional pairing originating from disconnected Fermi surfaces in the iron-based superconductor
