  In two recent articles (cond-mat/0606177 and arXiv:0804.1615), we have
suggested a unified theory of superconductivity based on the real-space
spin-parallel electron pairing and superconducting mechanism and have shown
that the stable hexagonal and tetragonal vortex lattices (the optimal doping
phases) can be expected in the newly discovered LaO{1-x}F{x}FeAs
(x0=1/7=0.1428) and SmO{1-x}F{x}FeAs (x0=1/6=0.1667), respectively. In this
paper, we present a theoretical study of the effects of hydrostatic and
anisotropic pressure on the superconducting transition temperature Tc of the
Fe-based layered superconductors based on the above mentioned theory. Our
results indicate a strong doping-dependent pressure effects on the Tc of this
compound system. Under high hydrostatic pressure, we find that dTc/dP is
negative when x>x0 (the so-called overdoped region) and is positive when x<x0
(the so-called underdoped region). Qualitatively, our finding is in good
agreement with the existing experimental data in LaO{1-x}F{x}FeAs (x=0.11<1/7)
(arXiv:0803.4266) and SmO{1-x}F{x}FeAs (x=0.13<1/6 and x=0.3>1/6)
(arXiv:0804.1582). Furthermore, Tc of both overdoped and underdoped samples
shows an increase with uniaxial pressure in the charge stripe direction and a
decrease with pressure in the direction perpendicular to the stripes. We
suggest that the mechanism responsible for the pressure effect is not specific
to the iron-based family and it may also be applicable to other superconducting
materials.
