1. Department of Electronics
Nanoelectronics 13
Atsufumi Hirohata
Department of Electronics
13:00 Monday, 27/February/ (P/T 006)

2. Quick Review over the Last Lecture
Input
Output
Gate Voltage
( Spin-transfer torque )
FM / SC hybrid Structures
Magnetic tunnel junctions (MTJ)
All metal and
spin valve structures
Interface
Ohmic / Schottky barriers
Tunnel barriers
Ohmic (no barrier)
Spin carriers SC
Non-magnetic metals
Device applications
FM / 2DEG
Schottky diodes
Spin FET
Spin LED
Spin RTD
MOS junctions
Coulomb blockade structures
SP-STM
Supercond. point contacts
Johnson transistors
Spin valve transistors
* After M. Johnson, IEEE Spectrum 37, 33 (2000).

3. Contents of Nanoelectonics
I. Introduction to Nanoelectronics (01)
01 Micro- or nano-electronics ?
II. Electromagnetism (02 & 03)
02 Maxwell equations
03 Scholar and vector potentials
III. Basics of quantum mechanics (04 ~ 06)
04 History of quantum mechanics 1
05 History of quantum mechanics 2
06 Schrödinger equation
IV. Applications of quantum mechanics (07, 10, 11, 13 & 14)
07 Quantum well
10 Harmonic oscillator
11 Magnetic spin
13 Quantum statistics 1
V. Nanodevices (08, 09, 12, 15 ~ 18)
08 Tunnelling nanodevices
09 Nanomeasurements
12 Spintronic nanodevices

4. 13 Quantum Statistics 1 Classical distribution
Fermi-Dirac distribution
Bose-Einstein distribution

5. Classical Distribution
Maxwell-Boltzmann distibution :
In the classical theory, the same types of particles can be distinguished.
 Every particle independently fills each quantum state.
In order to distribute N particles to the states 1, 2, ... with numbers of N1, N2, ...
To distribute Ni particles to the states Gi is GiNi, and hence the whole states are
Microscopic states in the whole system W is
By taking logarithm for both sides and assuming Ni and Gi are large, we apply
the Stirling formula.

6. Classical Distribution (Cont'd)
In the equilibrium state,
Again, by applying the Stirling formula,
Here, with using
By using and
Now, the partition function is

7. Classical Distribution (Cont'd)
Therefore,
By substituting this relationship into
Here,  = 1/kBT
Gi : degree of degeneracy of eigen enrgy Ei *

8. Fermi-Dirac / Maxwell-Boltzmann Distribution
Electron number density :
Fermi sphere :
sphere with the radius kF
Fermi surface :
surface of the Fermi sphere
Increase number density
classical
quantum mechanical
Maxwell-Boltzmann distribution
(small electron number density)
Fermi-Dirac distribution
(large electron number density)
* M. Sakata, Solid State Physics (Baifukan, Tokyo, 1989).

9. Fermi Energy Fermi-Dirac distribution : E T = 0 T ≠ 0 EF
Pauli exclusion principle
At temperature T, probability that one energy state E is occupied by an electron :
f(E) E
T = 0 1
 : chemical potential
(= Fermi energy EF at T = 0)
kB : Boltzmann constant
T1 ≠ 0
1/2
T2 > T1 

10. Fermi velocity and Mean Free Path
Fermi wave number kF represents EF :
Fermi velocity :
Under an electrical field :
Electrons, which can travel, has an energy of ~ EF with velocity of vF
For collision time , average length of electrons path without collision is
Mean free path
g(E) E
Density of states :
Number of quantum states at a certain
energy in a unit volume

11. Density of States (DOS) and Fermi Distribution
Carrier number density n is defined as : E
T = 0
f(E)
g(E) EF E
T ≠ 0
f(E)
g(E)
n(E) EF

12. Fermi-Dirac Distribution
In the Fermi-Dirac distribution, 1 quantum state can be filled by 1 particle.
 Fermi particle (spin 1/2)
For the entire wavefunction :
 Particle exchange induces a sign change of the wavefunction.
For a 2-Fermi-particle system, due to the exchange characteristics,
the wavefunction is expressed as
If the 2 particles satisfy the same wavefunction,
 Pauli exclusion principle

13. Bose-Einstein Distribution
In the Bose-Einstein distribution, 1 quantum state can be filled by many particles.
 Bose particle (spin 1,2,...)
For the entire wavefunction :
 Particle exchange does not induce a sign change of the wavefunction.
For a 2-Bose-particle system, due to the exchange characteristics,
the wavefunction is expressed as
If the 2 particles satisfy the same wavefunction,
f(E) E

14. Bose-Einstein Distribution (Cont'd)
Difference between Bose-Einstein and Maxwell-Boltzmann distributions : *

15. Quantum Statistics
In quantum mechanics, the same types of particles cannot be distinguished :
By exchanging coordinate systems, Hamiltonian (operator) does not change :
P : coordinate exchange operator
 Diagonalization between H and P
By assuming the eigenvalue for P to be ,
Since twice exchange of the coordinate systems recover the original state,
Therefore,
(symmetric exchange)  Bose-Einstein statistics
Bosons  integer spins
(asymmetric exchange)  Fermi-Dirac statistics
Fermions  half-integer spins