/*! This file is auto-generated */ .wp-block-button__link{color:#fff;background-color:#32373c;border-radius:9999px;box-shadow:none;text-decoration:none;padding:calc(.667em + 2px) calc(1.333em + 2px);font-size:1.125em}.wp-block-file__button{background:#32373c;color:#fff;text-decoration:none} Q. 7.74 Consider a Bose gas confined in ... [FREE SOLUTION] | 91Ó°ÊÓ

91Ó°ÊÓ

Consider a Bose gas confined in an isotropic harmonic trap, as in the previous problem. For this system, because the energy level structure is much simpler than that of a three-dimensional box, it is feasible to carry out the sum in equation 7.121 numerically, without approximating it as an integral.*

(a) Write equation 7.121 for this system as a sum over energy levels, taking degeneracy into account. Replace Tandμwith the dimensionless variables t=kT/hfandc=μ/hf.

(b) Program a computer to calculate this sum for any given values of tandc. Show that, for N=2000, equation 7.121 is satisfied at t=15provided that c=-10.534. (Hint: You'll need to include approximately the first 200 energy levels in the sum.)

(c) For the same parameters as in part (b), plot the number of particles in each energy level as a function of energy.

(d) Now reduce tto 14 , and adjust the value of cuntil the sum again equals 2000. Plot the number of particles as a function of energy.

(e) Repeat part (d) for t=13,12,11,and10. You should find that the required value of cincreases toward zero but never quite reaches it. Discuss the results in some detail.

Short Answer

Expert verified

(a) Bose-Einstein distribution function, N=∑0∞12(n+1)(n+2)dεe(ε-μ)kT-1

(b) Expression to calculate the sum for any given value of tandc, Listplot[occnum,plot range->{0,300}];(*plot occupation numbers*)

(c) The number of particles in each energy level as a function of energy is plotted.

(d) At this temperature tto14the occupancy in the ground state is small and and approximately less than 1.5.

(e) The value of cincreases towards zero but never reaches it.

Step by step solution

01

Step 1. Given information

Equation for Bose-Einstein condensation :

N=∑alls1eε1-μkT-1 (Equation-1)

Sound waves behave almost like light waves. Each made of oscillation has a set of equally spaced energy levels, with the limit of energy equal to:

ε=nhf

The relation between Fermi temperatures and Fermi energy is.

tεF=kT

Here, kis the Boltzmann constant:

The relation between the chemical potential and Fermi energy is,

μ=cεf

02

Step 2. (a) Substituting the value of ε=nhf,tεF=kT and μ=cεF in equation-1

we get,

N=∑alls1enhf-ccFttFhere,εF=hf

=∑allss1enep-crrtεrr-1

=∑allss1e(n-c)t-1

Degeneracy of a level'n'is

g(n)dn=12(n+1)(n+2)

Thus the Bose-Einstein distribution function:

N=∑0∞12(n+1)(n+2)dεe(ε-μ)kT-1

03

Step 3. (b) To find the expression to calculate the sum for any given value of t and c.

we have,

hÓ¬=1((hbar*omega *)

kt=15((*Boltzmann constant*temperature *)

N=2000(*number of atoms*)

normsumN-:=sumdist[c]*(n+1)*(n+2)2,{m,0,sumlim}

In an isotropic harmonic trap of 3-dimensional box, occupational number of particles can be attained by using this mathematic relation.

Listplot[occnum,plot range->{0,300}];plot occupation numbers**

04

Step 4. (c) The number of particles in each energy level as a function of energy is plotted using the following function

Listplot[Table{n,occ[n,-10.536,15]},{n,0,175}]

From the graph we can see that the occupancy peak is at height of. At this temperature the number of particles in the ground state is unity.

05

Step 5. (d) After reducing  t to 14 and adjusting the value of c until the sum again equals 2000.

The following mathematical instruction accustomed plot the occupancy graph for t=14. At this temperature the occupancy within the state is tiny and roughly less than 1.5.

06

Step 6. (e)The following table shows occupancy of ground state corresponding graphs at t=13,12,11,10.

t=kThfc=μhf
c=μhf
N
15
-10.51.0
13
-4.4
2.5
12
-1.86.1
11.6
-0.9
11.9
11
-0.1
105
10
-0.018
535
9
-0.009
900
8
-0.0066
1220

Unlock Step-by-Step Solutions & Ace Your Exams!

  • Full Textbook Solutions

    Get detailed explanations and key concepts

  • Unlimited Al creation

    Al flashcards, explanations, exams and more...

  • Ads-free access

    To over 500 millions flashcards

  • Money-back guarantee

    We refund you if you fail your exam.

Over 30 million students worldwide already upgrade their learning with 91Ó°ÊÓ!

One App. One Place for Learning.

All the tools & learning materials you need for study success - in one app.

Get started for free

Most popular questions from this chapter

Consider a system consisting of a single impurity atom/ion in a semiconductor. Suppose that the impurity atom has one "extra" electron compared to the neighboring atoms, as would a phosphorus atom occupying a lattice site in a silicon crystal. The extra electron is then easily removed, leaving behind a positively charged ion. The ionized electron is called a conduction electron because it is free to move through the material; the impurity atom is called a donor, because it can "donate" a conduction electron. This system is analogous to the hydrogen atom considered in the previous two problems except that the ionization energy is much less, mainly due to the screening of the ionic charge by the dielectric behavior of the medium.

(a) Write down a formula for the probability of a single donor atom being ionized. Do not neglect the fact that the electron, if present, can have two independent spin states. Express your formula in terms of the temperature, the ionization energy I, and the chemical potential of the "gas" of ionized electrons.

(b) Assuming that the conduction electrons behave like an ordinary ideal gas (with two spin states per particle), write their chemical potential in terms of the number of conduction electrons per unit volume,NcV.

(c) Now assume that every conduction electron comes from an ionized donor atom. In this case the number of conduction electrons is equal to the number of donors that are ionized. Use this condition to derive a quadratic equation for Ncin terms of the number of donor atoms Nd, eliminatingµ. Solve for Ncusing the quadratic formula. (Hint: It's helpful to introduce some abbreviations for dimensionless quantities. Tryx=NcNd,t=kTland so on.)

(d) For phosphorus in silicon, the ionization energy is localid="1650039340485" 0.044eV. Suppose that there are 1017patoms per cubic centimeter. Using these numbers, calculate and plot the fraction of ionized donors as a function of temperature. Discuss the results.

Carry out the Sommerfeld expansion for the energy integral (7.54), to obtain equation 7.67. Then plug in the expansion for μto obtain the final answer, equation 7.68.

Consider a gas of nidentical spin-0 bosons confined by an isotropic three-dimensional harmonic oscillator potential. (In the rubidium experiment discussed above, the confining potential was actually harmonic, though not isotropic.) The energy levels in this potential are ε=nhf, where nis any nonnegative integer and fis the classical oscillation frequency. The degeneracy of level nis(n+1)(n+2)/2.

(a) Find a formula for the density of states, g(ε), for an atom confined by this potential. (You may assume n>>1.)

(b) Find a formula for the condensation temperature of this system, in terms of the oscillation frequency f.

(c) This potential effectively confines particles inside a volume of roughly the cube of the oscillation amplitude. The oscillation amplitude, in turn, can be estimated by setting the particle's total energy (of order kT) equal to the potential energy of the "spring." Making these associations, and neglecting all factors of 2 and πand so on, show that your answer to part (b) is roughly equivalent to the formula derived in the text for the condensation temperature of bosons confined inside a box with rigid walls.

If you have a computer system that can do numerical integrals, it's not particularly difficult to evaluate μfor T>Tc.

(a) As usual when solving a problem on a computer, it's best to start by putting everything in terms dimensionless variables. So define t=T/Tc,c=μ/kTc,andx=ϵ/kTc. Express the integral that defines μ, equation N=∫0∞g(ϵ)1e(ϵ-μ)/kT-1dϵ, in terms of these variables, you should obtain the equation

2.315=∫0∞xdxe(x-c)/t-1

(b) According to given figure , the correct value of cwhen T=2Tc, is approximately -0.8. Plug in these values and check that the equation above is approximately satisfied.

(c) Now vary μ, holding Tfixed, to find the precise value of μfor T=2Tc. Repeat for values ofT/Tcranging from 1.2up to 3.0, in increments of 0.2. Plot a graph of μas a function of temperature.

The speed of sound in copper is 3560m/s. Use this value to calculate its theoretical Debye temperature. Then determine the experimental Debye temperature from Figure 7.28, and compare.

See all solutions

Recommended explanations on Physics Textbooks

View all explanations

What do you think about this solution?

We value your feedback to improve our textbook solutions.

Study anywhere. Anytime. Across all devices.