How Does Dimensionality Affect Phonon Polarizations?

  • Thread starter Thread starter Kyuubi
  • Start date Start date
  • Tags Tags
    Dimensions
AI Thread Summary
When transitioning from three to two dimensions, the number of phonon polarizations decreases, with the expectation that it reduces from three in 3D to two in 2D. The integration factors change accordingly, with the 1/8 factor in 3D becoming 1/4 in 2D, and the spherical coordinates transforming to polar coordinates. Despite the reduction in polarizations, electromagnetic modes retain two polarizations regardless of dimensionality. This discussion clarifies the relationship between dimensionality and phonon polarizations in elastic waves. The topic highlights the complexities of phonon behavior in different dimensions.
Kyuubi
Messages
18
Reaction score
8
Homework Statement
Consider a crystal made up of two-dimensional layers of atoms, with rigid coupling between the layers. You may assume the motion of atoms is restricted to the plane of the layer. Calculate the phonon heat capacity in the Debye approximation, and show that the low temperature limit is proportional to ##T^2##.
Relevant Equations
In 3D,
##\sum_n{(...)} = \frac{3}{8}\int4\pi n^2dn(...)##

In 2D,
##\sum_n{(...)} = \frac{a}{4}\int2\pi ndn(...)##

What is ##a##?
When going from 3 to 2 dimensions, I am unsure about how the number of polarizations will be affected.

I know the following though:
The 1/8 factor becomes a 1/4 since we are now integrating over the positive quadrant in 2d rather than the positive octant in 3d.
The ##4\pi n^2## becomes a ##2\pi n## because we moved from spherical coordinates to just polar coordinates.

I am told that in 3D, an elastic wave has three polarizations: two transverse and one longitudinal. Do they remain unchanged in lower dimensions? My guess is that they should become 2. But if we lose a polarization every time we lower dimensions, then a photon gas in one dimension shouldn't exist, but it does since the book has a question about it :)
 
Physics news on Phys.org
Kyuubi said:
Homework Statement: Consider a crystal made up of two-dimensional layers of atoms, with rigid coupling between the layers. You may assume the motion of atoms is restricted to the plane of the layer. Calculate the phonon heat capacity in the Debye approximation, and show that the low temperature limit is proportional to ##T^2##.
Relevant Equations: In 3D,
##\sum_n{(...)} = \frac{3}{8}\int4\pi n^2dn(...)##

In 2D,
##\sum_n{(...)} = \frac{a}{4}\int2\pi ndn(...)##

What is ##a##?

When going from 3 to 2 dimensions, I am unsure about how the number of polarizations will be affected.

I know the following though:
The 1/8 factor becomes a 1/4 since we are now integrating over the positive quadrant in 2d rather than the positive octant in 3d.
The ##4\pi n^2## becomes a ##2\pi n## because we moved from spherical coordinates to just polar coordinates.

I am told that in 3D, an elastic wave has three polarizations: two transverse and one longitudinal. Do they remain unchanged in lower dimensions? My guess is that they should become 2. But if we lose a polarization every time we lower dimensions, then a photon gas in one dimension shouldn't exist, but it does since the book has a question about it :)
It's been answered for me. Indeed the number of polarizations goes down. I suppose then that EM modes retain their 2 polarizations regardless of dimension (?)
 
Thread 'Collision of a bullet on a rod-string system: query'
In this question, I have a question. I am NOT trying to solve it, but it is just a conceptual question. Consider the point on the rod, which connects the string and the rod. My question: just before and after the collision, is ANGULAR momentum CONSERVED about this point? Lets call the point which connects the string and rod as P. Why am I asking this? : it is clear from the scenario that the point of concern, which connects the string and the rod, moves in a circular path due to the string...
Back
Top