# Why does the velocity of 2D plasmon diverge at small q?

1. Dec 17, 2014

### hiyok

Now plasma represents collective wave-like motions of charged particles. In 3D, their frequency is well known to be almost a constant, $\omega^{3D}_p \approx \sqrt{4\pi ne^2/m}$ with n=charge density, m=particle mass. However, in 2D, one can show that it becomes $\omega^{2D}_p \sim \sqrt{q}$, where q= wave number. It gets dispersion. The group velocity is then $v^{2D}_g \sim 1/\sqrt{q}$, which diverges as q→0. How could this be possible ? How could $v^{2D}_g$ exceed the speed of light ? This sounds ridiculous !

2. Dec 21, 2014

### DrDu

Could you provide a reference? There are no truely 2d systems in nature, so this may depend on details of the system.

3. Dec 21, 2014

### hiyok

4. Dec 22, 2014

### DrDu

In that derivation, they assumed that the charges only interact via Coulombic forces. This is strictly true in the 3D case, as the charge density is longitudinal and the Coulomb field is the only longitudinal electric field. In the 2D case this is no longer true. You can calculate the field of a time dependent charge distribution locatet in the xy-plane and travelling in the x-direction: $\rho(t,r)=\rho_0 \delta(z) \exp(iqx-i\omega t)$ and you will find that it's electric field is not purely longitudinal. Hence the interaction is not purely Coulombic. I suppose this will change the dispersion relation, especially for small q.

5. Dec 22, 2014

### hiyok

Thanks for your suggestion. But that kind of dispersion had been experimentally verified: see e.g. Phys. Rev. Lett. 36: 145(1976).

6. Dec 22, 2014

### DrDu

I must say that I don't understand this paper: In eq (5) they express div E in terms of first the potential and then again in terms of the charge density, but $\nabla\cdot E=\rho$ by Maxwells equation, so there must be something fishy.

7. Dec 22, 2014

### DrDu

Ok, I suppose he means the nabla in reduced density. Using the ansatz for the charge distribution given above, I find for the electric potential $E=(\kappa/(iq) e_x+\mathrm{sgn}(z)e_z)\rho/2 \exp(-\kappa |z|) \exp(iqx-i\omega t)$, where $\kappa=\sqrt(|\omega^2-q^2|)$.
Hence the driving force in equation 5 does not depend linearly on q but on kappa. For large values of q, kappa goes like q but for small values like omega. Hence for small values of q there should the dispersion relation changes.

Last edited: Dec 22, 2014
8. Dec 24, 2014

### hiyok

9. Dec 25, 2014

### DrDu

there might be some small errors, but the expression for E is a correct solution of the wave eqation for z not equal zero while forming the divergence yields the correct charge density. Forming the 2d-divergence in eq. 5 ammounts to multiplication of the x component with iq which yields kappa.
Maybe you could post your solution?

Last edited: Dec 25, 2014
10. Dec 25, 2014

### hiyok

I found the electric potential to be $V(\vec{r},z) = \frac{\rho_0}{2}\kappa^{-1}e^{i(\vec{q}\vec{r}-\omega t)}e^{-\kappa|z|}$, where $\vec{r}$ denotes the in-plane components. The electric field strength in the x-direction is $E_x = \frac{\rho_0}{2}(-iq_x\kappa^{-1})e^{i(\vec{q}\vec{r}-\omega t)}e^{-\kappa|z|}$. Here I keep the speed of light so that $\kappa = \sqrt{|q^2-\omega^2/c^2|}$. Now the plasmon dispersion is determined by this equation, $\omega^4|q^2-\omega^2/c^2| = \frac{2\pi n_0 e^2}{m}q^4$. Indeed the initial problem is solved !

11. Dec 25, 2014

### hiyok

I also found that, if both scalar and vector potentials are included, the plasmon dispersion is determined by an even simple equation: $\left(2\pi n_0 e^2/m\right)^2 |q^2-\omega^2/c^2| = \omega^4$. Obviously, the dispersion becomes linear for small $q$.

12. Dec 26, 2014

### hiyok

This is what looks like:

#### Attached Files:

• ###### mod2D.pdf
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13. Dec 26, 2014

### DrDu

You can't derive a retarded field from only a potential. Namely your potential gives $\nabla^2 V=\omega^2 V\ne 0$ for $z \ne 0$, however, it should be the charge density which vanishes for nonzero z.

14. Dec 26, 2014

### hiyok

The retarded potential I gave is a solution to this equation: $(\partial^2_{\vec{x}}-\partial^2_t/c^2) V(\vec{x},t)= -\rho_0e^{i(\vec{q}\vec{r}-\omega t)}\delta(z)$.

15. Dec 26, 2014

### DrDu

But the electric field is cannot be expressed in terms of a retarded potential alone, you will also have to specify the retarded magnetic vector potential.

16. Dec 26, 2014

### hiyok

That is exactly what I did. I have worked out the vector potential as well. The final result was shown in the figure attached in previous post.

17. Dec 27, 2014

### DrDu

So what is the z-component of the electric field you found?

18. Dec 27, 2014

### hiyok

The z-component is obtained as $E_z = -\partial_z V(\vec{x},t) - c^{-1}\partial_tA_z(\vec{x},t)$, where $A_z$ is the z-component of the vector potential. The final expressions are a little complicated, because now the vector potential depends on the current density, which further depends on the conductivity of the system.

19. Dec 27, 2014

### DrDu

Current density is completely determined by continuity.

20. Dec 27, 2014

### hiyok

Dear DrDu,

I'm much grateful for your comments which have led me to a good understanding of my problem. As this understanding is significant in improving one of my recent manuscripts, which I'll submit for publication, I'm thinking formally acknowledging you or adding you as a coauthor. So, if possible, I hope to discuss more by private emails, instead here. I definitely have to know you real profile before I can do this.

Thanks.

hiyok