f[x] = Piecewise[{{Re[Exp[-I*x]], x<0}, {0, x>0}}]
g[x] = Piecewise[{{0.25*Re[Exp[I*2*x]], x<0}, {0, x>0}}]
h[x] = Piecewise[{{0.75*Re[Exp[-I*2*x]], x>0}, {0, x<0}}]

k = Sqrt[2*D]
R = 0.25
T = 0.75

0.75`
R = 1/(1+((2*D)/(a^2)))

(1/(1+((2 D)/(a^2))))

(1/(1+((2 D)/(a^2))))
R

(1/(1+((2 D)/(a^2))))

Manipulate[Plot[{Piecewise[{{Re[Exp[-I*Sqrt[2*Energy*m]*x]], x<0},{0, x>0}}],Piecewise[{{Re[(1/(1+((2*Energy)/(m*a^2))))*Exp[I*Sqrt[2*Energy*m]*x]], x<0},{0, x>0}}], Piecewise[{{Re[(1-(1/(1+((2*Energy)/(m*a^2)))))*Exp[-I*Sqrt[2*Energy*m]*x]], x>0},{0, x<0}}]} , {x, -10, 10}, PlotRange->{-1, 1}], {Energy, 0, 20, 0.1}, {a , 1, 7}, {m, 1, 10, 0.1}]
Manipulate[Plot[{Sqrt[m*a]*Exp[-m*a*Abs[x]]}, {x, -3, 3}, PlotRange-> {0, 3}], {a, 1, 7}, {m, 1, 10, 0.1}]