Quantum mechanics
School of Physics and Information Technology
Shaanxi Normal University
IDENTICAL PARTICLES
Chapter 5
5.1 Two-Particle Systems 2015.1 Two-Particle Systems 201
5.2 Atoms 210
5.3 Solids 218
5.4 Quantum Statistical Mechanics 230
5.1 Two-Particle System
For a single particle, is a function of the spatial coordinates r and the
time t (here now we ignore the spin). The wave function for a two-particle
system is a function of the coordinates of particle one (r1), the coordinates of
particle two (r2), and the time:
),( trΨ
).,,( 21 trrΨ
Its time evolution is determined by the Schrodinger equation:
Ψ∂,Ψ=
∂
Ψ∂H
tih
where H is the Hamiltonian for the whole system:
),,(22
21
2
2
2
22
1
1
2
tVmm
H rr+∇−∇−=hh
the subscript on ▽ indicates differentiation with respect to the coordinates
of particle 1 or particle 2).
is the probability of finding particle 1 in the volume d3r2 and particle 2 in the
volume d3r1; evidently must be normalized in such a way that
For time-independent potentials, we obtain a complete set of solutions by
The statistical interpretation carries over in the obvious way:
2
3
1
32
21 ),,( rrrr ddtΨ
),,( 21 trrΨ
.1),,( 2
3
1
32
21 =Ψ∫ rrrr ddt
For time-independent potentials, we obtain a complete set of solutions by
separation of variables:
h/
212121 ),()(),(),,( iEtett
−==Ψ rrrrrr ψϕψ
where the spatial wave function satisfies the time-independent Schrodinger
equation, with E the total energy of the system,
.),(22
21
2
2
2
22
1
1
2
ψψψψ EVmm
=+∇−∇− rrhh
5.1.1 Bosons and Fermions
Suppose particle 1 is in the (single-particle) state , and particle 2 is in the
state . In that case is a simple conduct
)(raψ)(rbψ ),( 21 rrψ
).()(),( 2121 rrrr ba ψψψ =
Of course, this assumes that we can tell the particles apart. In quantum
mechanics, all we could say is that one of the particle is in the state and
the other is in the state of . In classical mechanics, we can always
discriminate them from each other. In microscopic world, the fact is , all the
)(raψ)(rbψ
Quantum mechanics neatly accommodates the existence of particles that are
indistinguishable in principle: We simply construct a wave function that is
noncommittal as to which particle is in which state. There are actually two ways
to do it:
discriminate them from each other. In microscopic world, the fact is , all the
electrons are utterly identical, in a way that no two classical objects can ever be.
[ ].)()()()(),( 212121 rrrrrr abbaA ψψψψψ ±=±
Thus the theory admits two kinds of identical particles: bosons, for which we
use the plus sign, and fermions, for which we use the minus sign. Photons and
mesons are bosons; protons and electrons are fermions. It so happens that
all particles with integer spins are bosons, and
all particles with half-integer spins are fermions.
Further, bosons and fermions have quite different statistical properties. The
connection between spin and “statistics” can be proved in relativistic quantum connection between spin and “statistics” can be proved in relativistic quantum
mechanics; in non-relativistic theory it must be taken as an axiom.
It follows, in particular, that two identical fermions (for example, two electrons)
cannot occupy the same state. For if , then)()( rr ba ψψ =
[ ] .0)()()()(),( 212121 =−=− rrrrrr aaaaA ψψψψψ
and we have left with no wave function at all. This is the famous Pauli
exclusion principle.
Another general way to formulate this problem: bosons and fermions
),(),( 1221 rrrr ffP =
mmm == ).,(),( rrrr VV =
Let us define the exchange operator, P, which interchanges the two particles:
Clearly, P2=1 , and it follows that the eigenvalues of P are ±1.
Now, if the two particles are identical, the Hamiltonian must treat tem the same:
mmm == 21 ).,(),( 1221 rrrr VV =
),,(22
21
2
2
22
1
2
tVmm
H rr+∇−∇−=hh
HPH =
It follows that P and H are compatible observables,
,Ψ=Ψ∂
∂H
tih
Then we have, for any state
,Ψ=Ψ∂
∂HPP
tih
,Ψ=Ψ∂
∂PHPPP
tih
1=PP
HPHP =
0],[ =HP
and hence we can find a complete set of functions that are simultaneous
eigenstates of both. That is to say, we can find solutions to the Schrodinger
equation that are either symmetric or antisymmetric under exchange:
12 =P
),(),( 1221 rrrr ψψ ±=
1±=λ
Moreover, if the system starts out in such a state, it will remain in such a state.
),(),( 1221 rrrr ψψ +=
),(),( 1221 rrrr ψψ −=
If
If
for bosons
for fermions
This is the symmetrization requirement of identical particles, and this is a
general statement of bosons and fermions.
Example 5.1. On the book!
5.1.2 Exchange Forces
=),( 21 xxψ
To give you some sense of what the symmetrization requirement actually does.
Here we work out the simple one-dimensional example below. Suppose particle
is in the (single-particle) state , and the other particle is in the state ,
and these two states are orthogonal and normalized. If the two particles are
distinguishable, and number 1 is the one in state , then the combined wave
function is
)(xaψ )(xbψ
)(xaψ
)( 1xaψ )( 2xbψ
If they are identical bosons, the composite wave function is
and if they are identical fermions, it is
[ ];)()()()(2
1),( 212121 xxxxxx abba ψψψψψ +=+
[ ];)()()()(2
1),( 212121 xxxxxx abba ψψψψψ −=−
Let calculate the expectation value of the square of the separation distance
between the two particles,
21
2
2
2
1
2
21 2)( xxxxxx −+=−
(1) Distinguishable particles.
22
222 )()(),( dxdxxxxdxdxxxxx ∫∫ == ψψψ
For the wave function =),( 21 xxψ )( 1xaψ )( 2xbψ
2121
2
12121
2
1
2
1 )()(),( dxdxxxxdxdxxxxx ba∫∫ == ψψψ
,)()( 2
2
2
21
2
1
2
1a
ba xdxxdxxx =⋅= ∫∫ ψψ
.)()( 2
2
2
2
2
21
2
1
2
2b
ba xdxxxdxxx =⋅= ∫∫ ψψ
Similarly,
and
.)()( 2
2
221
2
1121 baba xxdxxxdxxxxx =⋅= ∫∫ ψψ
In this case, then
bababledistingusaxxxxxx 2)( 222
21 −+=−
If the states of two particles exchange, the answer will be the same.
(2) Identical particles (bosons and fermions).
For the wave function
[ ];)()()()(2
1),( 212121 xxxxxx abba ψψψψψ ±=±
(Bosons + and fermions -)
[ ] [ ];)()()()()()()()(2
1),( 2121
*
2
*
1
*
2
*
1
2
21 xxxxxxxxxx abbaabba ψψψψψψψψψ ±⋅±=±
[ 2
2
21
2
1
2
1
2
1 )()(2
1dxxdxxxx ba ∫∫ ⋅= ψψ
2
2
21
2
1
2
1 )()( dxxdxxx ab ∫∫ ⋅+ ψψ
22
*
211
*
1
2
1 )()()()( dxxxdxxxx abba ∫∫ ⋅± ψψψψ
22
*
211
*
1
2
1 )()()()( dxxxdxxxx baab ∫∫ ⋅± ψψψψ 2221111 )()()()( dxxxdxxxx baab ∫∫ ⋅± ψψψψ
[ ] ( )baba
xxxx2222
2
100
2
1+=±±+=
Similarly,
( )ba
xxx222
22
1+=
Naturally, , since we can’t tell them apart.2
2
2
1 xx =
But
[ ∫∫ ⋅= 2
2
221
2
1121 )()(2
1dxxxdxxxxx ba ψψ
2
2
221
2
11 )()( dxxxdxxx ab ∫∫ ⋅+ ψψ
22
*
2211
*
11 )()()()( dxxxxdxxxx abba ∫∫ ⋅± ψψψψ
** )()()()( dxxxxdxxxx ∫∫ ⋅± ψψψψ
where
22
*
2211
*
11 )()()()( dxxxxdxxxx baab ∫∫ ⋅± ψψψψ
[ ]abbabaababba
xxxxxxxx ±±+=2
1
,2
abbaxxx ±=
=ab
x dxxxx ba∫ )()( *ψψ
Evidently
Comparing the two cases, we see that the difference resides in the final term:
.22)(2
222
21 abbabaxxxxxxx m−+=−
±
bababledistingusaxxxxxx 2)( 222
21 −+=−
=∆±
2)( xbledistingusa
x2)(∆ .2
2
abxm
(Bosons - and fermions +)
=∆=−±±
22
21 )()( xxx
=∆±
2)( xbledistingusa
x2)(∆ .2
2
abxm
(Bosons - and fermions + )
Discussion:
(1) Identical bosons tend to be somewhat closer together, and identical
fermions somewhat father apart, than distinguishable particles in the
same two states.
(2) Exchange force.(2) Exchange force.
The system behaves as though there were a “force of attraction” between
identical bosons, pulling them closer together, and a “force of repulsion”
between identical fermions, pushing them apart ( remember that we are
for the moment ignoring spin). We call it an exchange force, although it’s
not really a force at all-----no physical agency is pushing on the particles;
rather, it is a purely geometrical consequence of the symmetrization
requirement. It is also a strictly quantum mechanical phenomenon, with
no classical counterpart.
(3) Notice that <x>ab vanishes unless the two wave functions overlap.
=ab
x dxxxx ba∫ )()( *ψψ
=∆±
2)( xbledistingusa
x2)(∆ .2
2
abxm
(Bosons + and fermions - )
∫That is, if two particles are too far away, the difference between
±∆ 2)( x
bledistingusax
2)(∆ , no matter bosons or fermions of the two particles is.
That is natural that, when two particles are very far away, the two particles
become classical particles that can be distinguished.
(4) Effect: The symmetrization requirement on Hydrogen molecule (H2)
e-
e-e- e-
bosons fermions
We have been ignoring spin. The complete state of the electron includes
not only its position wave function, but also a spinor, describing the
?Covalent bond 共价键
)()( sr χψ
not only its position wave function, but also a spinor, describing the
orientation of its spin:
)(sχ
)(sχ Singlet state: antisymmetric spin state
Triplet states: symmetric spin state
)(rψ
)(rψ
antisymmetricAntisymmetric spatial state
Symmetric spatial statebonding
antibonding
5.2 Atoms
The term in curly brackets represents the kinetic plus potential energy of the
A neutral atom, of atomic number Z, consists of a heavy nucleus, with
electric charge Ze, surrounded by Z electrons (mass m and charge –e). The
Hamiltonian for this system is
.4
1
2
1
4
1
2
2
01
2
0
22
∑∑≠= −
+
−∇−=
Z
kj kj
Z
j j
j
e
εr
Ze
εmH
rrππ
h[5.24]
The term in curly brackets represents the kinetic plus potential energy of the
jth electron, in the electric field of the nucleus; the second sum (which runs
over all values of j and k except j=k) is the potential energy associated with
the mutual repulsion of the electrons (the factor of 1/2 in front corrects for
the fact that summation counts each pair twice).
The problem is to solve Schrödinger’s equation,
,H Eψ ψ=
for the wave function .1 2 3( , , , , )
Zψ r r r rL
Because the electrons are identical fermions, however, not all solutions are
acceptable: only those for which the complete state (position and spin),
is antisymmetric with respect to interchange of any two electrons. In particular,
no two electrons can occupy the same state.
Unfortunately, the Schrodinger equation with Hamiltonian in Equation 5.24
cannot be solved exactly (at any rate, it hasn’t been), except for the very
1 2 3 1 2 3( , , , , ) ( , , , , ),Z Zψ χr r r r s s s sL L
cannot be solved exactly (at any rate, it hasn’t been), except for the very
simplest case, Z=1 (hydrogen). In practice, one must resort to elaborate
approximation methods. Some of these we shall explore in Part II. Now we
plan only to sketch some qualitative features of the solutions, obtained by
neglecting the electron repulsion term altogether.
In section 5.2.1 we study the ground state and excited states of helium.
In section 5.2.2 we will examine the ground states of higher atoms.
After hydrogen, the simplest atom is helium (Z=1). The Hamiltonian,
consists of two hydrogenic Hamiltonians (with nuclear charge 2e), one for
electron 1 and one for electron 2, together with a final term describing the
repulsion of the two electrons. It is the last term that causes the trouble. If we
5.2.1 Helium
.4
12
4
1
2
2
4
1
2 21
2
02
2
0
2
2
2
1
2
0
2
1
2
rr −+
−∇−+
−∇−=e
εr
e
εmr
e
εmH
πππ
hh
with half the Bohr radius,
repulsion of the two electrons. It is the last term that causes the trouble. If we
simply ignore it, the Schrodinger equation separates, and the solutions can be
written as products of hydrogen wave functions:
( ) ( ) ( ), , 2121 rrrr mlnnlm ′′′= ψψψ
mme
a10
2
2
0 10529.04 −×==
hπε( )2
2
0
2
4
emaHe
hπε=
The total energy would be
and four times the Bohr energies
,11
42122
2
0
2
2E
nn
emEn =
−=
πεh
.41
4
2
2 2
2
0
2
2 nEn
emE =
−=
πεh
eV.-E 6131 =
The total energy would be
).(4 nn EEE ′+=
(1) In particular, the ground state would be
( ) ( ) ( ) ,8
,/)(2
32100110021021 arr
ea
+−==π
ψψψ rrrr
and its energy would be . -109)6.136.13(4 eVE =−−=
Because the ground state is a symmetric function, the spin state
has to be antisymmetric, so the ground state of helium should be a singlet
configuration, which the spins “oppositely aligned”. The actual ground state of
helium is indeed a singlet, but the experimentally determined energy is –78.975
eV, so the agreement is not very good.
ψψ
( )210 , rrψ
The excited states of helium consist of one electron in the hydrogenic ground
state, and the other in an excited state:
(2) The excited state
In this case, we can construct both symmetric and antisymmetric combinations,
in the usual way;
nlmψψ 100
( ) ( ) ( ) ( ) ( )210012110021 , rrrrrr ψψψψψ nlmnlm ±∝
The former go with the antisymmetric spin configuration (the singlet), and
they are called parahelium, while the latter require a symmetric spin
configuration (the triplet), and they are known as orthohelium.
The ground state is
necessarily parahelium;
the excited states come
in both forms. Because
the symmetric spatial
state brings the
electrons closer together,
we expect a higher
interaction energy in
parahelium, and indeed,parahelium, and indeed,
it is experimentally
confirmed that the
parahelium states have
somewhat higher
energy than their
orthohelium
counterparts.
-
Ignoring the mutual
repulsion between electrons.
The individual electrons
occupy one-particle
hydrogenic state—orbitals
The heavier atom: Z > 2
5.2.2 The Periodic Table
nlmψ ),,( mln
Pauli exclusion principle:
),,( mlnOnly two electron can
occupy one orbital
22nnth shell
Ze
e−
),,( mln
(P152 4.85)
2
8
8
18
2
8
18
32
1850
2 n2neon
repulsion between electrons
three electrons The n=1 shell is filled with two,
the other electron must be put into the n=2 shell.
(1) Helium, Z=2: two electrons the n=1 shell is filled.
electron repulsion
Within a given shell, the state with lowest energy is l=0,
Occupation-filling rules:
(2) Lithium, Z=3:
n=2 l=1, or l=0. l=0?
),,( mln
n=2
l=1
Within a given shell, the state with lowest energy is l=0,
and the energy increases with increasing l.
The third electron )0,0,2(
(3) Beryllium, Z=4:
The fourth electron )0,0,2(
The third electron Spin up
Spin down
(4) Boron, Z=5: l=1The fifth electron ),1,2( m
n=1
n=2
l=0
)0,0,2(
Aluminum Z=13— Argon Z=18
(5) Neon, Z=10: n=1and n=2 shells are
all filled.
(6) Sodium and magnesium: Z=11,12
)0,0,3(Sodium
magnesium )0,0,3(
),2,3( m
n=4
l=2
)0,0,4(
),1,3( m
l=1 ),1,4( m
n=5l=2 ),2,4( m
)0,0,5(l=0
l=3 ),3,4( ml=1
Potassium Z=19
)0,0,3(
n=3
l=0
l=1 ),1,3( m
10
11 12
Calcium Z=20
l=2
l=0 )0,0,4(
)0,0,4(
)0,0,4(
Scandium Z=21— Zinc Z=30 ),2,3( m
n=1,2
Gallium Z=31— Krypton Z=36 ),1,4( m
18CaK
Rubidium Z=37
Strontium Z=38
)0,0,5(
)0,0,5(
Nomenclature for atomic states:
l=0 is called s (for “sharp”)
l=1 is called p (for “principle”)
l=2 is called d (for “diffuse”)
l=4 is called g (out of imagination)
l=5 is called h
n=1 shell is called K
n=2 shell is called L
n=3 shell is called M
shell angular
l=3 is called f (for “fundamental”)n=4 shell is called N
magnetic
not listed
l=5 is called h
l=6 is called i
l=7 is called k
The state of a particular electron is represented by the pair n,l, with n (the
number) giving the shell and l (the letter) specifying the orbital angular
momentum; the magnetic quantum number m is not listed, but an exponent is
used to indicate the number of electrons that occupy the state in question.
(3d)2for example
The configuration, for example, for the ground state of carbon,
222 )2()2()1( pss
Tells us that there are two electrons in the orbital (1,0,0), two in the orbital
(2,0,0), and two in some combination of the orbitals (2,1,1), (2,1,0) and
(2,1,-1).
For total atom:
The total atom state can be presented by a the following label
S, P, D, F,
2S+1LJ
The total orbital angular momentum number
Grand total angular momentum number
J=S+LTotal spin momentum number
222 )2()2()1( pss
For the ground state of carbon
0,1,2=L
0,1=S0,1,2,3=J ?
How to determine these
total quantum number
L,S,J ?
Hund’s Rules: See book problem 5.13. homework
Ground state of carbon
2=L
1=S
0=J0
3P
See book, the Table 5.1
5.3 Solids
In the solid state, outermost valence electron become detached, and roam
throughout the material. Two primitive models:
(1) The electron gas theory: 松模费德(Sommerfeld)
Which ignores all forces Free particles in a box.
(2) Bloch’s theory: Periodic potential
Which introduces a regularly spaced periodic potential.
Band
5.3.1 The Free Electron Gas Theory
Suppose the object in question is a rectangular solid, with dimensions lx, ly, lz,
and imagine that an electron inside experiences no forces at all, except at the
impenetrable walls:
otherwise. ,
;0,0,0 if 0,),,(
∞
<<<<<<=
zyx lzlylxzyxV
l
zThe Schrodinger equation,
lxly
zThe Schrodinger equation,
,2
22
ψψ Em
=∇−h
separates, in cartesian coordinates: ),()()(),,( zZyYxXzyx =ψ with
;2 2
22
XEdx
Xd
mx=−
h;
2 2
22
YEdy
Yd
my=−
h,
2 2
22
ZEdz
Zd
mz=−
h
and .zyx EEEE ++= Letting
,2
h
x
x
mEk ≡ ,
2
h
y
y
mEk ≡ ,
2
h
z
z
mEk ≡
we obtain the general solutions
),cos()sin()( xkBxkAxX xxxx +=;2
2
2
Xkdx
Xdx−=
2Yd
),cos()sin()( ykBykAyY yyyy +=;2
2
2
Ykdy
Ydy−=
).cos()sin()( zkBzkAzZ zzzz +=;2
2
2
Zkdz
Zdz−=
The boundary conditions require that X(0)=Y(0)=Z(0)=0, so Bx=By=Bz=0, and
X(lx)=Y(ly)=Z(lz)=0, so that
,πxxx nlk = ,πyyy nlk = ,πzzz nlk =
where each n is a positive integer:
,,3,2,1 L=xn ,,3,2,1 L=yn ,,3,2,1 L=zn
The normalized wave functions are
),()()(),,( zZyYxXzyx =ψ
8 nnn πππ),sin()sin()sin(
8),,( z
l
ny
l
nx
l
n
lllzyx
z
z
y
y
x
x
zyx
nnn zyx
πππψ =
and the allowed energies are
,2
)(22
22222
2
2
2
2
2
2
222
m
kkkk
ml
n
l
n
l
n
mE zyx
z
z
y
y
x
xnnn zyx
hhh=++=
++=
π
where k is the magnitude of the wave vector, k=(kx,ky,kz).
If you imagine a three-dimensional space, with axes kx,ky,kz, and planes
drawn in at
:/ xxx lnk π= L),/3(),/2(),/( xxxx lllk πππ=
:/ yyy lnk π= L),/3(),/2(),/( yyyy lllk πππ=
:/ zzz lnk π= L),/3(),/2(),/( zzzz lllk πππ=
(kx,ky,kz) One stationary state
Each block in this grid, and hence each
state, occupies a volume in k-space:
Vlll zyx
33 ππ=
Block: one state for every block
lxly
l
z
Suppose our sample contains N atoms, and each atom contributes q free electrons.
q is small number: 1 or 2
Avogadro's Constant = 6.0221415 × 1023 mol-1
If electrons were bosons ( or distinguishable particles), they would all settle down
to the ground state, . But electrons are in fact identical fermions, subject to
the Pauli exclusion principle, so only two of them can occupy any given state.
They will fill up one octant of a sphere in k-space, whose radius, kF, is determined
111ψ
They will fill up one octant of a sphere in k-space, whose radius, kF, is determined
by the fact that each pair of electrons requires a volume :V/3π
, 3
4
8
1 3
FkπVolume
,2
1 3
VNq
π
.2
3
4
8
1 33
=
V
NqkF
ππ
Fk
Fk
Fk
Thus
where
is the free electron density (the number of free electrons per unit volume).
=
V
NqkF
33
2
3
4
8
1 ππ 23 3 π
V
NqkF = ( ) 3/123ρπ=Fk
,V
Nq≡ρ
is the free electron density (the number of free electrons per unit volume).
The boundary separating occupied and unoccupied states, in k-space, is
called the Fermi surface (hence the subscript F). The corresponding energy
is called the Fermi energy, EF; for a free electron gas,
( ) .322
3/22222
ρπmm
kE F
F
hh==
The total energy of the electron gas can be calculated as follows: A shell of
thickness dk contains a volume
( ) , 48
1 2 dkkd πτ =
so the number of electron states in the
shell is( )
,/
48
1
2/
23
2
3dk
V
k
V
ddn
π
π
π
τ==
τd
// VV ππ
.2
2dkk
Vdn
π=
Each of these states carries an energy so the energy of the
shell is,
2
22
m
kE
zyx nnn
h=
,2
2
2
22
dkkV
m
kdE
π
h=
and hence the total energy is
,22 0
4
2
2
0
2
2
22
∫∫∫ ===FF kk
total dkkm
Vdkk
V
m
kdEE
ππ
hh
.10
)3(
5
1
2
3/2
2
3/5225
2
2−== V
m
Nqk
m
VF
π
π
π
hh
This quantum mechanical energy plays a role rather analogous to the internal This quantum mechanical energy plays a role rather analogous to the internal
thermal energy (U ) of an ordinary gas. In particular, it exerts a pressure on the
walls, for if the box expands by an amount dV, the total energy decreases:
,3
2
10
)3(
3
2 3/5
2
3/522
PdVV
dVEV
m
NqdE totaltotal −=−=−= −
π
πh
and this shows up as work done on the outside (dW=PdV) by the quantum
pressure P. Evidently
Quantum pressure P:
Collapse:
( ).
5
3
103
2
3
2 3/523/22
2
52
ρπ
π mm
k
V
EP Ftotal hh
===
Quantum reason: internal quantum pressure
Quantum essential reason: antisymmetrization requirement of
the wave function of identical fermions.
Degeneracy pressure Exclusion pressure
We are now going to improve on the free electron model by including the
forces exerted on the electrons by the regularly spaced, positively charged,
essentially stationary nuclei. The periodic potential here determines the
qualitative behavior of solids. Now we develop the simplest possible model:
a one-dimensional Dirac comb, consisting of evenly spaced delta function
spikes.
5.3.2 The Bloch’s theory: Band Structure
1. Theorem of periodic potential: Bloch theorem
A periodic potential is one that repeats itself after some fixed distance a:
).()( xVaxV =+
Bloch theorem tells us that for such a potential the solutions to the Schrodinger
equation,
,)(2 2
22
ψψψ
ExVdx
d
m=+−
h
can be taken to satisfy the condition
),()( xeax iKaψψ =+ ),()( xeax ψψ =+
for some constant K, which is independent on x.
Proof: Let D be the “displacement” operator:
).()( axfxDf +=
Then, for a periodic potential, we have
HxVm
paxV
m
pxV
m
pDDH =+=++=
+= )(
2)(
2)(
2
222
Therefore D commutes with the Hamiltonian H :
[ ] 0, =HD
and hence we are free to choose eigenfunctions of H that are simultaneously
eigenfunctions of D:
),()( xExH ψψ =
),()( xxD λψψ = ),()( xax λψψ =+
2222)()()( xxax ψψλψ ==+As ,1
2=λ
Now, λ is a nonzero constant complex number, and can be expressed as an
exponential:
.iKaeλ =
2. Energy analysis: Band structure
Of course, no real solid satisfies the
condition of periodic potential.
2310≈N
aa− 0
2310≈N
a2x
0 1−N
)()( xVaxV =+
0
1
2
1−N
1
However, for large number of atoms containing in the solid, we can wrap the
x-axis around a circle to meet the periodic condition and, finally, we impose
the following boundary condition
),()( xNax ψψ =+
It follows that
),()( xxeiKNa ψψ =
)()( xVaxV =+
),()( xeaxiKaψψ =+
).,3,2,1,0( ,22
L±±±=⋅== nN
n
aNa
nK
ππ
Now, suppose the potential consists of a long string of delta-function spikes:
so ,1=iKNae or ,2 nNKa π= and hence
.)()(1
0
∑−
=
−=N
j
jaxδαxV0=j
According to Bloch’s theorem, the wave function in the cell immediately to the
The general solution is
In region 0<x<a the potential is zero, so
,2
2
2
ψψ
kdx
d−=,
2 2
22
ψψ
Edx
d
m=−
h,
2
h
mEk ≡
).0( ),cos()sin()( axkxBkxAx <<+=ψ
According to Bloch’s theorem, the wave function in the cell immediately to the
left of the origin is)0a( <′<− x
axx −=′ axx +′=
).0a( )],(cos[)](sin[)( <′<−+′++′=+′ xaxkBaxkAaxψ
),()( xeax iKaψψ =+
[ ] ).0a( ,)(cos)(sin)( <′<−+′++′=′ −xaxkBaxkAex
iKaψ
At x=0, ψ must be continuous, so
).0( ),cos()sin()( axkxBkxAx <<+=ψ
[ ] ).0a( ,)(cos)(sin)( <′<−+′++′=′ −xaxkBaxkAex
iKaψ
[ ];)cos()sin( kaBkaAeBiKa += − (Equation 5.61)
)(xψ )(x′ψ
0=x
At x=0, ψ’ (its derivative) is not continuous, satisfying [2.125]
)0(2
2ψ
αψ
h
m
dx
d=
∆
[ ])sin()cos( kaBkaAkekAiKa −− − B
m2
2
h
α=
Solving Equation 5.61 for Asin(ka) yields
(Equation 5.62)
Substituting this into Equation 5.62, and cancelling kB, we find
which simplifies to
[ ] .)cos()sin( BkaekaA iKa −=
[ ] [ ] ),sin(2
)(sin)cos(1)cos(2
2 kak
mkaekaekae iKaiKaiKa
h
α=+−⋅− −−
mα).sin()cos()cos(
2ka
k
mkaKa
h
α+=
This is the fundamental result, from which
all else follows. For any other potential,
Kronig-Penney for example, the above
formula is more complicated, but is shares
the qualitative features we are about to
explore.
(1) Above equation determines the possible values of k, and hence the allowed
energies.
Discussion:
(2) The allowed energy and energy band.
To simplify the notation, let
).sin()cos()cos(2
kak
mkaKa
h
α+= 5.64
,kaz ≡ ,2
h
amαβ ≡
so the right set of equation 5.64 can be written as
.)sin(
)cos()(z
zzzf β+=
The constant β is a dimensionless measure of the “strength” of the delta
function.
Graph of f(z): )()cos( zfKa =
1st 2nd
gap
bandband
).,3,2,1,0( ,2
0 L±±±=== nnKN
nKaπ
01 K⋅
02 K⋅
⋅−
)cos()( 0nKzf =)(zf
00 K⋅
Figure 5.6 Graph of f(z) for β=10, showing allowed bands (shaded) separated by
forbidden gaps (where |f(z)|>1).
0)1( KN ⋅−
N lines
In practice there will be Nq of them, where q is again the number of “free”
electrons per atom. Because of Pauli exclusion principle, only two electrons
can occupy a given spatial state, so if q=1, they will half fill the first band, if
q=2 they will completely fill the first band, if q=3 they half fill the second
band, and so on——in the ground state.
(1) Electrons occupation of the band:
(2) Conductors, insulators and semiconductors
Band structure is the signature of periodic potential.Band structure is the signature of periodic potential.
If a band is entirely filled, it takes a relatively large energy to excite an
electron, since it has to jump across the forbidden zone. Such materials will be
electrical insulators. On the other hand, if a band is only partly filled, it takes
very little energy to excite an electron, and such materials are typically
conductors. If you dope an insulator with a few atoms of larger or smaller q,
this puts some “extra” electrons into the next higher band, or creates some
holes in the previously filled one, allowing in either case for weak electric
currents to flow; such materials are called semiconductors.
5.4 Quantum Statistical Mechanics
If we have a large number of N particles, in thermal equilibrium at
temperature T, what is the probability that a particle would be found to
have the specific energy, Ej?
The fundamental assumption of statistical mechanics is that in thermal
equilibrium every distinct state with the same total energy, E, is equally
probable.
The temperature, T, is a measure of the total energy of a system in thermal
equilibrium in classical mechanics. What is the new in quantum mechanics?
How to count the distinct states!
Why? Give a example to demonstrate!