Finite Temperature Density Matrix Calculation

Click For Summary
SUMMARY

The discussion focuses on calculating the finite temperature density matrix ##\rho(T)## for the Hamiltonian ##H=\begin{bmatrix} 0& \frac{-iw}{2}\\ \frac{iw}{2} & 0 \end{bmatrix}##. Participants identified the eigenvalues as ##\lambda=\pm\frac{w}{2}## with corresponding eigenvectors ##v_1=(-i, 1)## and ##v_2=(i, 1)##. The density operator is expressed as ##\rho=e^{-\beta E_1} | \psi_1 \rangle \langle \psi_1 |+e^{-\beta E_2} | \psi_2 \rangle \langle \psi_2 |##, where ##z=e^{-\beta E_1}+e^{-\beta E_2}## ensures the operator has unit trace. The final resolution clarified the normalization of eigenvectors and the correct form of the density matrix.

PREREQUISITES
  • Understanding of quantum mechanics concepts, specifically Hamiltonians.
  • Familiarity with eigenvalues and eigenvectors in linear algebra.
  • Knowledge of statistical mechanics, particularly the concept of temperature in quantum systems.
  • Proficiency in matrix exponentiation and normalization techniques.
NEXT STEPS
  • Study the derivation of the density matrix in quantum statistical mechanics.
  • Learn about the significance of eigenvalues and eigenvectors in quantum systems.
  • Explore the normalization process for quantum states and operators.
  • Investigate the role of the partition function in finite temperature calculations.
USEFUL FOR

Students and researchers in quantum mechanics, particularly those working on finite temperature systems, quantum statistical mechanics, and density matrix formalism.

bowlbase
Messages
145
Reaction score
2

Homework Statement


Consider the Hamiltonian ##H=\begin{bmatrix} 0& \frac{-iw}{2}\\ \frac{iw}{2} & 0 \end{bmatrix}##
Write the finite temperature density of the matrix ##\rho(T)##

Homework Equations


##\beta=\frac{1}{kT}##

The Attempt at a Solution


The initial part of the problem had me find the eigenvectors and eigenvalues. I got ##\lambda=\pm\frac{w}{2}## and eigenvectors ##v_1=(-i, 1)## and ##v_1=(i, 1)##

Not quite sure what to do with it from here. I know that ##\rho=e^{-\beta E_1} | \psi_1 \rangle \langle \psi_1 |+e^{-\beta E_2} | \psi_2 \rangle \langle \psi_2 |##

I think I remember that the eigenvalues are suppose to be ##E_{1,2}##. But I don't know what ##\psi_{1,2}## are suppose to be.

Further, my notes show that once I have the matrix I should have a fraction that looks something like ##\frac{1}{2z}## where ##z## is the sum ##z=e^{-\beta E_1}+e^{-\beta E_2}##
 
Physics news on Phys.org
[itex]|\psi_1 \rangle[/itex] and [itex]|\psi_2 \rangle[/itex] are normalized eigenvectors of the Hamiltonian. You have the eigenvectors, you just need to normalize them.
About the factor of [itex]\frac{1}{2z}[/itex], the density operator [itex]\rho[/itex] should have unit trace. But I don't think there should be a factor [itex]\frac 1 2[/itex] there!
 
I got it figured it out. I just couldn't find it in my textbook. Turns out in was in recommended reading I guess. Thanks for the help though.
 

Similar threads

  • · Replies 10 ·
Replies
10
Views
2K
Replies
9
Views
2K
  • · Replies 0 ·
Replies
0
Views
2K
  • · Replies 13 ·
Replies
13
Views
3K
  • · Replies 7 ·
Replies
7
Views
3K
Replies
2
Views
2K
  • · Replies 1 ·
Replies
1
Views
2K
  • · Replies 8 ·
Replies
8
Views
2K
  • · Replies 6 ·
Replies
6
Views
5K
  • · Replies 1 ·
Replies
1
Views
2K