lecture15 – chapter. 6 unbound statesslee/2402/2010_fall/f10_2402_lecture15.pdf · lecture15 –...

Lecture15 – Chapter. 6

Unbound States

Outline:

•! The Potential Step

•! The Potential Barrier & Tunneling

•! Alpha Decay & Other Applications

•! Particle-Wave Propagation

1)'(

)'(

)'(

'2''4

)'(

)'(

)'(

'4

?

2

2

2

22

2

2

2

=+

+=

=+

!++=

=+

!+

+=+

=+

kk

kk

kk

kkkkkk

kk

kk

kk

kkRT

RT

T + R = 1

C = [2k/(k+k’)]A

B = [(k-k’_/(k+k’)]A

Like in

Optics

Finally, we express the Prob. in terms of U0 & E using the definitions of k, k’

5 eV

2 eV

3 eV

Potential Wall

E > U

Case (2) 6.2 The Potential Barrier

&

Tunneling

•!Tunneling is one of the most important & startling"

ideas in QM. "

•!The simplest solution is a potential barrier, a PE "

jumps that is only temporary."

•! If a particle#s E < Barrier#s height, (i.e. E < U0) "

it should not get through - classically"

Potential Barrier

R(L)

T(L)

(!)

Resonant Transmission

Is there any “L” that would give

T = 1, R = 0

? Like i

n OPTIC

S

Resonant Transmission

Is there any “L” that would give

T = 1, R = 0

? Like i

n OPTIC

S

= 0

Resonant Transmission

T = 1, R = 0

for:

Potential Wall (KE<U) – “Potential Step”

< 0

The solution is not oscillatory !

(exponentially damped wave function)

C = 0

> 0

100% Reflected“Penetration

depth”Section 5.6

Reflection (R) &transmission (T)

probabilities for a potential step

If E < U0! wave is totally reflected.

When E > U0! R falls rapidly with increasing E

Region

II

Would a

particle

penetrate

all the way

through?

Barrier Penetration – “Tunneling”

Barrier Penetration – “Tunneling” Barrier Penetration – “Tunneling”

Reflection (R)

&transmission (T) probabilities for a potential

barrier

1st

Transmission

resonance

tunneling