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Determine the transmission coefficient for a rectangular barrier (same as Equation 2.145, only with V(x)=+V0>0 in the regiona<x<a ). Treat separately the three casesE<V0,E=V0 , andE>V0 (note that the wave function inside the barrier is different in the three cases).

Short Answer

Expert verified

T1=1+V024E(V0E)sinh22ah2m(V0E)鈥夆赌夆赌夆赌夆赌夆赌夆赌夆赌xa

T1=1+2mEa2h2鈥夆赌夆赌夆赌夆赌夆赌夆赌夆赌夆夆a<x<a

T1=1+V024E(V0E)sin22ah2m(V0E)鈥夆赌夆赌夆赌夆赌夆赌夆赌夆赌夆夆夆xa

Step by step solution

01

 Define the Schrödinger equation

A differential equation describes matter in quantum mechanics in terms of the wave-like properties of particles in a field. Its answer is related to a particle's probability density in space and time.

02

Determine the transmission coefficient

V(x)=0 When localid="1656057126624" xa

V(x)=V0 When a<x<a

V(x)=0 When xa

Schrodinger equation

h22md2dx2(x)+V(x)(x)=贰唯(x)

We shall split into 3 regions

(x)=Aeikx+Beikx x<a

(x)=Feikxx>a

Aeika+Beika=Ce渭补+De渭补

ik(AeikaBeika)=(Ce渭补De渭补)

We have

Ce渭补+De渭补=Feika

ikFeika=(Ce渭补De渭补)

B=e2ika(k2+2)sinh(2渭补)2颈办渭肠辞蝉丑(2渭补)+(k22)sinh(2渭补)A

C=e补渭ika(k2+办渭颈)2颈办渭肠辞蝉丑(2渭补)+(k22)sinh(2渭补)A

D=e补渭ika(k2+办渭颈)2颈办渭肠辞蝉丑(2渭补)+(k22)sinh(2渭补)A

F=2e2ika办渭颈2颈办渭肠辞蝉丑(2渭补)+(k22)sinh(2渭补)A

T=FA2

T=2e2ika办渭颈2颈办渭肠辞蝉丑(2渭补)+(k22)sinh(2渭补)2

T=42k2(4+22k2+k4)[sinh2(2渭补)+42k2cosh2(2渭补)]

Putcosh2(2渭补)=1+sinh2(2渭补)

T=1(2+k2)2sinh2(2渭补)/42k2+1

T1=1+(2+k2)sinh2(2渭补)42k2

03

Plot the graph and value of constants

Put the value of constantandkwe get,

T1=1+V024E(V0E)sinh22ah2m(V0E)

When energy is equal to the potential but the barrier region we have

n=0

(x)=Cx+D

Aeika+Beika=Ca+D

ik(AeikaBeika)=C

At, x=a

Ca+D=Feika

C=ikFeika

B=kae2ikaka+iA

C=keikakaiA

D=eikaA

F=e2ika1ikaA

T=FA2

T=11+2mEa2h2

T1=1+2mEa2h2

h22md2dx2(x)+V(x)(x)=贰唯(x)

h22mn=(EV0)

n=2m(EV0)h2

n=2

=2m(EV0)h2

Aeika+Beika=Ce颈位补+De颈位补

ik(AeikaBeika)=颈位(Ce颈位补De颈位补)

We have

Ce颈位补+De颈位补=Feika

ikFeika=颈位(Ce颈位补De颈位补)

B=e2ika(k22)sin(2位补)2办位肠辞蝉(2位补)+(k2+2)sin(2位补)A

C=eia(+k)(+K)K2办位肠辞蝉(2位补)i(k2+2)sin(2位补)A

D=eia(k)(k)K2办位肠辞蝉(2位补)+i(k2+2)sin(2位补)A

F=2办位别2ika2办位肠辞蝉(2位补)i(k2+2)sin(2位补)A

T=FA2=2办位别2ika2办位肠辞蝉(2位补)i(k2+2)sin(2位补)2

T=42k2(422k2+k4)sin2(2位补)+42k2

T=11+(2k2)sin2(2位补)/42k2

T1=1+(2k2)sin2(2位补)42k2

T1=1+V024E(V0E)sin22ah2m(V0E)

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Most popular questions from this chapter

This is a strictly qualitative problem-no calculations allowed! Consider the "double square well" potential (Figure 2.21). Suppose the depth V0and the width a are fixed, and large enough so that several bound states occur.

(a) Sketch the ground state wave function 1and the first excited state localid="1658211858701" 2(i) for the case b = 0 (ii) forbaand (iii) for ba

(b) Qualitatively, how do the corresponding energies(E1andE2)and vary, as b goes from 0 to ? Sketch E1(b)and E2(b)on the same graph.

(c) The double well is a very primitive one-dimensional model for the potential experienced by an electron in a diatomic molecule (the two wells represent the attractive force of the nuclei). If the nuclei are free to move, they will adopt the configuration of minimum energy. In view of your conclusions in (b), does the electron tend to draw the nuclei together, or push them apart? (Of course, there is also the internuclear repulsion to consider, but that's a separate problem.)

A particle of mass m is in the potential

V(x)={,(x<0)-32h2ma2,(0xa)0,(x>a)

How many bound states are there?

In the highest-energy bound state, what is the probability that the particle would be found outside the well (x>a)? Answer: 0.542, so even though it is 鈥渂ound鈥 by the well, it is more likely to be found outside than inside!

The gaussian wave packet. A free particle has the initial wave function

Y(x,0)=Ae-ax2

whereAand are constants ( is real and positive).

(a) NormalizeY(x,0)

(b) Find Y(x,t). Hint: Integrals of the form

-+e-(ax2+bx)dx

Can be handled by 鈥渃ompleting the square鈥: Lety=a[x+bl2a], and note that(ax2+bx)=y2-(b2l4a). Answer:

localid="1658297483210" Y(x,t)=(2a)1/4e-ex2l[1+(2ihatlm)]1+(2ihatlm)

(c) Find . Express your answer in terms of the quantity

localid="1658297497509" =a1+(2ihatlm)2

Sketchlocalid="1658124147567" |Y|2(as a function of x) at t=0, and again for some very large t. Qualitatively, what happens to |Y|2, as time goes on?

(d) Find <x>,<p>,<x2>,<p2>,xand P. Partial answer:localid="1658297458579" <p2>=ah2, but it may take some algebra to reduce it to this simple form.

(e) Does the uncertainty principle hold? At what time tdoes the system come

closest to the uncertainty limit?

The scattering matrix. The theory of scattering generalizes in a pretty obvious way to arbitrary localized potentials to the left (Region I),V(x)=0so(x)=Aeikx+Be-ikx,wherek2mE

To the right (Region III),V(x)is again zero, so(x)=Feikx+Ge-ikxIn between (Region II), of course, I can't tell you what is until you specify the potential, but because the Schr枚dinger equation is a linear, second-order differential equation, the general solution has got to be of the form

where f(x)and g(x)are two linearly independent particular solutions. 48 There will be four boundary conditions (two joining Regions I and II, and two joining Regions II and III). Two of these can be used to eliminate C and D, and the other two can be "solved" for B and F in terms of \(A\) and G

B=S11A+S12G.F=S21A+S22G

The four coefficientsSijwhich depend on k (and hence on E), constitute a22matrix s called the scattering matrix (or S-matrix, for short). The S-matrix tells you the outgoing amplitudes (B and F) in terms of the incoming amplitudes (A and G):

(BF)=(S11S21S21S22)(AG)

In the typical case of scattering from the left, G=0so the reflection and transmission coefficients are

Rl=|B|2|A|2|G=0=|S11|2.TI=|F|2|A|2|G=0=|S2|2.

For scattering from the right, and

Rr=|F|2|G|2A=0=|S22|2.Tr=|B|2|G|2A=0=|S12|2.

(a) Construct the S-matrix for scattering from a delta-function well (Equation 2.114). (b) Construct the S-matrix for the finite square well (Equation 2.145). Hint: This requires no new work, if you carefully exploit the symmetry of the problem.

The transfer matrix. The S- matrix (Problem 2.52) tells you the outgoing amplitudes (B and F)in terms of the incoming amplitudes (A and G) -Equation 2.175For some purposes it is more convenient to work with the transfer matrix, M, which gives you the amplitudes to the right of the potential (F and G)in terms of those to the left (A and b):

(FG)=(M11M12M21M22)(AB)[2.178]

(a) Find the four elements of the M-matrix, in terms of the elements of theS-matrix, and vice versa. ExpressRI,TI,RrandTr(Equations 2.176and 2.177) in terms of elements of the M-matrix.,

(b) Suppose you have a potential consisting of two isolated pieces (Figure 2.23 ). Show that the M-matrix for the combination is the product of the twoM-matrices for each section separately: M=M2M1[2.179]

(This obviously generalizes to any number of pieces, and accounts for the usefulness of the M-matrix.)

FIGURE : A potential consisting of two isolated pieces (Problem 2.53 ).

(c) Construct the -matrix for scattering from a single delta-function potential at point V(x)=-(x-a) :

(d) By the method of part , find the M-matrix for scattering from the double delta functionV(x)=-[(x+a)+(X-a)] .What is the transmission coefficient for this potential?

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