1/4/2016 EECS 277B Adv. Semiconductor Devices © 2013 P. Burke Lecture 11, p. 1

Lecture 12: Quantum point contact

Resistance is independent of length.

d ~ lFermi

212.5

2quantum

hR k

e

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Readings this lecture covers

Ferry pp. 124-139

Van Wees PRL (reading packet)

Marcus APL (reading packet)

Zhou APL (reading packet)

1/4/2016 EECS 277B Adv. Semiconductor Devices © 2013 P. Burke Lecture 11, p. 3

Ballistic vs. diffusive transport

W

W

l

l

L

Ballistic

Diffusive

?R

2

LR

W

1/4/2016 EECS 277B Adv. Semiconductor Devices © 2013 P. Burke Lecture 11, p. 4

1d system:22 22 2 2 2 2

2 2 2( )

2 2

x y zn n n

x y z

x y z

k k kE n n n

m m L L L

xL 0yL 0zL

E

kx=nx/Lx

22 22

2

2x

x y z

E nm L L L

22 22

2 222

x

x y z

E nm L L L

22 22

2 232

x

x y z

E nm L L L

Fermi energy

1/4/2016 EECS 277B Adv. Semiconductor Devices © 2013 P. Burke Lecture 11, p. 5

Resistance quantumBallistic conductor

Right

contact

Left

contact

E

kx=2nx/Lx

eV

Fermi energy

225 quantum

hR k

e

212.5

2quantum

hR k

e

With spin:

22quantum

eG

h

22eG T

h

If injection from leads is not perfect:

T is the transmission probability.

1/4/2016 EECS 277B Adv. Semiconductor Devices © 2013 P. Burke Lecture 11, p. 6

Variable width wire:

E

kx=nx/Lx

Fermi energy

1/4/2016 EECS 277B Adv. Semiconductor Devices © 2013 P. Burke Lecture 11, p. 7

Landauer formula:

22eG n

h

If the leads are not perfect injectors into each “channel” then:

22n

eG T

h

1/4/2016 EECS 277B Adv. Semiconductor Devices © 2013 P. Burke Lecture 11, p. 8

Experimental realizations:

Pinch-off gate in semiconductor 2DEG (QPC)

Break junction

Electrochemical addition of atoms

Scanning tunneling microscope

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Quantum point contact

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Quantum point contact

B.J. van Wees et al. (1988), Phys. Rev. Lett., 60, 848.

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0.7 anomaly

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Break junction

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Zhou, et al, Applied Physics Letters 67, 8 (1995) p. 1160.

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Electroplating

A.F.Morpurgo, C.M.Marcus and D. B. Robinson,

Controlled Fabrication of Metallic Electrodes with Atomic Separation, Appl. Phys. Lett. 74, 2084 (1999).