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!