- #1
Felicity
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Homework Statement
consider the scattering matrix for the potential
2m/hbar2 V(x) = λ/a δ(x-b)
show that it has the form
(2ika/(2ika-λ) , (e-2kib) λ/(2ika-λ)
(e2kib) λ/(2ika-λ) , 2ika/(2ika-λ)
(I've used commas just to separate terms in the matrix)
prove that it is unitary and that it will yield the condition for bound states when the elements of that matrix becoe infinite (this will only occur for λ < 0)
Homework Equations
suppose the matrix is expressed as
S11 S12
S21 S22
where S11 = (2ika/(2ika-λ)
S12 = (e-2kib) λ/(2ika-λ)
S21 = (e2kib) λ/(2ika-λ)
S22 = 2ika/(2ika-λ)
The Attempt at a Solution
I see that this is a delta potential well at x=b
ok so I know that S11 = T S21= R S22 = T and S12 = R where T and R are the reflection and transmission coefficients so I figure that if I can find those then I show the s-matrix in the above form so here it goes...
take
u(x) = Arekx +Bre-kx x < b
= Ale-kx +Blekx x > b
the boundary condition is (du/dx at x = b+) - (du/dx at x = b-) = λ/a u(b)
so
k(Arekb -Bre-kb+Ale-kb -Blekb)= λ/a u(b)
then for an incoming particle that can be either reflected or transmitted I make Ar= 1 Al = r Bl=0 and Br=t
where r2 = R (reflection coefficient and t2= t (transmission coefficient)
to get
ekb-te-kb + re-kb= λ/a u(b)
so how do I solve for r and t separately and how do I get rid of the u(b)?
Thank you
Felicity