Defects in Lattice
A perfect lattice is composed of N atoms on N sites. If M of these atoms are shifted
to interstitial sites (i.e. between regular positions) we have an imperfect lattice with
M defects. The number of available interstitial sites is N' and is of order N. The
energy needed to create a defect is . Assume that T<<, and show that the
leading order estimate for the typical number of defects is M= NN' exp(-/2T).
Evaluate also the contribution of the defects to the entropy, and to the specific heat
in the same level of approximation.
Virial theorem
Show that, for a statistical system in which the inter-particle interaction potential is
u(r) ~ r the virial v is given by v = 3PV U, where U is the mean potential
energy of the system. Hence, the mean kinetic energy K is
1
K = v/2 = (3PV + U)/2 =
(3PV +E) where E=K+U.
+2
What happens when = 2 ?
Occupations in a closed two level system
Consider N particles in a two level system. The one-particle energies of the two
levels are E1 and E2. Consider separately the two following cases: (i) The particles
can be distinguished; (ii) The particles are identical Bosons. Find the expectation
values n1 and n2 of the occupation numbers. Discuss the special limits N=1 and
N infinity. Explain the connection with Fermi/Bose occupation statistics.
Fluctuations in the grand canonical ensemble:
A fluid in a volume V is held (by a huge reservoir) at a temperature T and chemical
potential . Do not assume an ideal gas. Find the relation between <(E<E>)3>
and the heat capacity at constant fugacity , CV(T,).
Find the relation between <(N<N>)3> and the isothermal compressibility
T(V,)=(∂v/∂V,T where v=V/<N>.
[Hint: Evaluate 3rd derivatives of the grand canonical partition function.]
Find explicitly results in case of a classical ideal gas.
Sound velocity for Fermi gas
Determine the velocity of sound in a degenerate (T=0) Fermi gas and compare with
the Fermi velocity.
Entropy change due to mixing
Consider mixing of two gases with initial different temperatures, T1, T2. Evaluate
the mixing entropy. This is defined as the change of entropy upon mixing.
Consider two cases: (i) the gases are identical; (ii) the gases are distinct but their
molecules have equal masses.
Two level system, cooling process
Consider a system of N spins on a lattice at temperature T, each spin has a
magnetic moment . In presence of an external magnetic field each spin has two
energy levels, ±H.
a) Evaluate the changes in energy ∆E and in entropy ∆S as the magnetic field
increases from 0 to H. Derive the magnetization M(H) and show that
H
∆E=T∆S
M(H')dH'.
Interpret this result.
0
b) Show that the entropy S(E, N) can be written as S(M). Deduce the temperature
change when H is reduced to zero in an adiabatic process. Explain how can this
operate as a cooling machine to reach T≈10-4 K. (Note: below 10-4 K in realistic
systems spin-electron or spin-spin interactions reduce S(T, H=0) 0 as T0).
This method is known as cooling by adiabatic demagnetization.
Spin system, adiabatic process
Consider a solid with N non-magnetic atoms and Ni non-interacting magnetic
impurities with spin s. There is a weak spin-phonon interaction which allows
energy transfer between the impurities and the non-magnetic atoms.
a) A magnetic field is applied to the system at a constant temperature T. The field is
strong enough to line up the spins completely. What is the change in entropy of the
system due to the applied field? (neglect here the spin-phonon interaction).
b) Now the magnetic field is reduced to zero adiabatically. What is the qualitative
effect on the temperature of the solid? Why is the spin-phonon interaction relevant?
c) Assume that the heat capacity of the solid is CV=3NkB in the relevant
temperature range. What is the temperature change produced by the process (b)?
(assume the process is at constant volume).
Sudden versus adiabatic process
N atoms of mass m of an ideal classical gas are in a cylinder with insulating walls,
closed at one end by a piston. The initial volume and temperature are V0 and T0
respectively. a. If the piston is moving out rapidly the atoms cannot perform work,
i.e. their energy is constant. Find the condition on the velocity of the piston that
justifies this result. b. Find the change in temperature, pressure and entropy if the
volume increases from V0 to V1 in the condition of (a). c. Repeat (b) if the piston
is moving very slowly, i.e. an adiabatic process.
Adiabatic expansion of the universe
The universe is pervaded by a black body radiation corresponding to a temperature
of 3 K. In a simple view, this radiation was produced from the adiabatic expansion
of a much hotter photon cloud which was produced during the big bang.
a) Why is the recent expansion adiabatic rather than, for example, isothermal?
b) If in the next 1010 years the volume of the universe increases by a factor of two,
what then will be the temperature of the black body radiation?
c) Estimate the number of photons per cm3 and the energy density in erg/cm3 at
present.
Adiabatic cooling of liquid He3
If liquid 3He is pressurized adiabatically, it becomes a solid and the temperature
drops. This is a method of cooling by pressurization. Develop the theory of this
process in the following steps: a) Calculate the low temperature (T<<TF) entropy
of 3He, assuming an ideal fermion gas with a Fermi temperature of TF ≈ 5 ˚K. (Use
the low temperature form of E(T) as derived in class). b) At low temperatures the
entropy of solid 3He comes almost entirely from the spins while below 10-3 ˚K the
spins become antiferromagnetically ordered. Draw the entropy of solid 3He as
function of temperature, assuming independent spins for T>10-3 ˚K; below 10-3
˚K draw a qualitative form. Plot the result (a) on the same diagram. c) From the
diagram above, explain the method of cooling by pressurization. Below what
temperature T* (approximately) must the initial temperature be for the method to
work? d) What is the significance of T* for the equilibrium solid-liquid P(T) line?
Boltzman: theory
consider an ideal gas in an external potential (r)
Let
H=
d3v
d3r f(r,v,t)logf(r,v,t))
where f(r,v,t) is arbitrary except for the conditions on density n and energy E

d3r
d3v f(r,v,t) = n
1

d3r
d3v [2 mv2+(r)] f(r,v,t)) = E
Find f(r,v) (i.e. t independent) which minimizes H.
(Note: do not assume binary collisions, i.e. the Boltzmann equation).
Use Boltzmann's equation to find the general form of the equilibrium distribution
of the ideal gas (i.e. no collision term). Determine the solution by allowing for
collisions and requiring that the collision term vanishes. Find also the average
density n(r).
Boltzman: conductivity
Electrons in a metal can be described by a spectrum (k), where k is the crystal
momentum, and a Fermi distribution f0() at temeparature T.
(a) Find the correction to the Fermi distribution distribution due to a weak electric
field E using the Boltzmann equation and assuming that the collision term can be
replaced by [f(k)f0(k)]/. Note that dk/dt=eE/h- and vk=/h- , i.e. in general
dvk/dt is k dependent. (The parameter  is the relaxation time.)
(b) Find the conductivity tensor , where J=E. In what situation would  be nondiagonal?
(c) Find  explicitely for =h- 2k2/2m* in terms of the electron density n. (m* is an
effective mass).
Langevin and FD for spring system
A balance for measuring weight consists of a sensitive spring which hangs from a
fixed point. The spring constant is is K, i.e. the force opposing a length change x is
Kx. The balance is at a temperature T and gravity accelaration is g. A small mass
m hangs at the end of the spring.
a) Write the partition function and evaluate the average <x> and the fluctuation
<(x<x>)2>. What is the minimal m which can be meaningfully measured?
b). Write a Langevin equation for x(t) with friction  and a random force A(t).
~() |2 where ~
Assuming <A(t)A(0)> = C(t) evaluate the spectrum |x
x() is the
~ 2(t)> and from (a) find the
Fourier transform of ~
x = x<x>. Evaluate <x
coefficient C.
c). Evaluate the dissipation function Im() (response to an external field) and
show that the fluctuation dissipation tehorem holds.
Langevig and Milikan experiment
Consider a Millikan type experiment to measure the charge e of a particle with
mass m. The particle is in an electric field E in the z direction, produced by a
capacitor whose plates are distance d apart. The experiment is at temperature T and
in a poor vacuum, i.e. col is short. (col is the average time between collisions of
the air molecules and the charged particle). The field is opposite to the gravity
force and the experiment attempts to find the exact field E* where eE*=mg by
monitoring the charge arriving at the plates.
a). Write a Langevin equation for the velocity v with a friction coefficient 
describing the particle dynamics. If E=E* find the time TD (assuming TD>>1)
after which a current noise due to diffusion is observed. What is the condition on
col for the validity of this equation?
b). When E≠E* the equation has a steady state solution <vz>=vd. Find the drift
velocity vd. Rewrite the equation in terms of ~
v z=vzvd and find the long time
limit of <z2>. From the condition that observation time<<TD deduce a limit on the
accuracy in measuring E*.
c). If the vacuum is improved (i.e. air density is lowered) but T is maintained, will
the accuracy be improved?
FD for galvanometer
A galvanometer at temperature T has a deflection spring with an oscillation period
 and a damping resistance R.
a). Evaluate the dissipation rate and identify R by equating the dissipation with
I2R/2 where I is a current with frequency ~2/
b). What is the lower limit on a current which can be safely recorded?
Onsager?
Consider a fluid in two compartments connected with a small hole. Although
particles can pass easily through the hole, it is small enough so that within each
compartment the fluid is in thermodynamic equilibrium. The compartments have
pressure, temperature, volume and particle number P1 T1 V1 N1 and P2 T2 V2 N2
respectively. There is an energy transfer rate dE/dt and particle transfer rate dN/dt
throught the hole.
a. Identify the kinetic coefficients for dE/dt and dN/dt driven by temperature and
chemical poptential differences. Rewrite the equations in terms of T=T1-T2 and
p=p1-p2 to first order in T and p.
b. If T=0 one measures 1= (dE/dt)/(dN/dt). One can also adjust the ratio 2= p/
T so that dN/dt=0. Show the relation 2=[(E/V) + P  (N/V)1]/T. (E/V or P for
either compartment).
c. Assume that the work done during the transfer is due to the pressure (reducing
the effective volume to zero within the hole). Evaluate 1 and show that 2=0.
Shot noise
The discreteness of the electron charge e implies that the current is not uniform in
time and is a source of noise. Consider a vacuum tube in which electrons are
emitted from the negative electrode and flow to the positive electrode; the
probability of emitting any one electron is independent of when other electrons are
emitted. Suppose that the current meter has a response time . The average current
is <I> so that the number n of electrons during a measurement period is on average
<n> = <I>/e.
a) Show that the fluctuations in n are <n2> = <n>. (Hint: Divide  into
microscopic time intervals so that in each interval ni=0 or ni=1.)
b) Consider the meter response to be in the range 0<||<2/. Show that the
fluctuations in a frequency interval d are d<I2> = e<I>d/2. At what
frequencies does this noise dominate over the Johnson noise in the circuit?
Master Eq: damping
Consider the derivation of Liouville's theorem for the ensemble density (p, q, t) in
phase space (p, q). Consider the motion of a particle of mass m with friction 
.
.
q = p/m,
p = p
a. Show that Liouville's theorem is replaced by d/dt = .
b. Assume that the initial (p, q, t=0) is uniform in a volume 0 in phase space and
<p<p
 , q
<q<q
.
zero outside of this volume. Find (p, q, t) if 0 is a rectangle p
Find implicitely (p, q, t) for a general 0.
c. what happens to the occupied volume 0 as time evolves? (assume a general
shape of 0). Explain at what t this description breaks down due to quantization.
d. Find the Boltzmann entropy as function of time for case (b). Discuss the
meaning of the result.
Bloch Eq: Spin Resonance
Consider a spin 1/2 particle with magnetic moment  in a constant magnetic field
B0 in the z direction and a perpendicular rotating magnetic field with frequency 
and amplitude B1; the Hamiltonian is
^ =H
^ 0 + 1h- 1[xcos(t) + ysin(t)]
H
2
^
where H
10=2h
1
0z, 2h-
1
0=B0, 2h- 1=B1 and x, y, z are the Pauli matrices.
^ 0)/Tr[exp(H
^ 0)], so that the heat
^ eq=exp(H
The equilibrium density matrix is 
^ 0 while the weak field B1
bath drives the system towards equilibrium with H
^ (t) is
opposes this tendency. Assume that the time evolution of the density matrix 
^ 
^ eq

^ ,
^ /dt =  i [H
^]
determined by
d

ha. Show that this equation has a stationary solution of the form 11=22=a,
^ =
^ 
^ eq.
12=21*= beit where 
^ bath ,
^ bath is the
^ 
^ eq]/represents (i/h- ) [H
^ ] where H
b. The term 
interaction Hamiltonianm with a heat bath. Show that the power absorption is
^
d
^ +H
^ bath) 
^ ] = Tr[dH 
^
Tr[(H
dt
dt ]
c. Determine b to first order in B1 (for which a=0 can be assumed), derive the
power absorption and show that it has a maximum at =0, i.e. a resonance
^ )=0, i.e. the absorption is dissipation into the
phenomena. Show that (d/dt) Tr(^ H
heat bath.