Talk:Van Hove singularity

All the edits on this page (so far) are by me; I didn't realize that I had been logged out. Alison Chaiken 04:55, 8 January 2006 (UTC)

Mathematical Errors

I don't know the theory, but the mathematics in the article are a disaster.

• Division by ${\vec {\nabla }}E$ is improper
• Density and total number seem to be mixed up. There are not $V/2\pi ^{3}$ possible k vectors
• $E=E_{0}+{\vec {\nabla }}E\cdot d{\vec {k}}$ does not imply $d{\vec {k}}=dE/{\vec {\nabla }}E$
• I assume E is energy, but it is not defined
• I assume m is mass, but it is not defined
• Assuming E is energy, m is mass, then $E=\hbar ^{2}k^{2}/2m$ is dimensionally incorrect. $\hbar$ has units of action $ML^{2}/T$, E has units $ML^{2}/T^{2}$ and k has units of $1/L$.

Thank you for taking the time to check the article. If you don't like division by ${\vec {\nabla }}E$, you won't like van Hove's original paper. While I've cribbed together several versions of the derivation from textbooks, the answer is correct!

The equation $E=\hbar ^{2}k^{2}/2m$ is one of the best-known equations in all of physics and is not dimensionally incorrect. $\hbar$ has units as you suggest, of energy*time = action. $\hbar k=p$ so the expression just says that $E=p^{2}/2m=1/2mv^{2}$. Let's check: $\hbar k=ML/T$ so $\hbar ^{2}k^{2}/2m$ has units $ML^{2}/T^{2}=E$. Where's the mistake? By the way, I do say "in energy space" when I introduce the symbol $E$ although I agree that strictly speaking I should define $m$.

The idea that "Density and total number seem to be mixed up" worries me too. That part of the article was changed by someone else and I don't have time to think it through before running off to a real-world event that will rudely interrupt my time thinking about math. Alison Chaiken 03:17, 10 January 2006 (UTC)

You are right about the dimensons - I was asleep I guess. Anyway, I have done some more with the derivation. I have defined what amounts to the differential density of states as g(k) - I think its clear that this is what is being discussed. Also, we can use the chain rule to go directly to $dE{\vec {\nabla }}E\cdot d{\vec {k}}$ instead of making a linear approximation. Finally, I am quite sure that the ${\vec {\nabla }}E$ in the denominators should be $|{\vec {\nabla }}E|$ but I will do that later. PAR 05:35, 10 January 2006 (UTC)</math>

You're right about $|{\vec {\nabla }}E|$; I've made that change. It still bothers me that density of states (as opposed to *number* of states) should be extensive (proportional to volume) but I don't see an error in the derivation. But now it's that time of day when I stop messing with WP and do research! Alison Chaiken 15:50, 10 January 2006 (UTC)

I have changed the derivation. I think the $dE/|\nabla E|$ expression only holds for one dimension. It's more complicated in 2 and 3 dimensions, but the conclusions are the same. PAR 03:03, 11 January 2006 (UTC)

Is the first sentence wrong?

There are no consequences, except logical rigor. But the first sentence in the Theory section reads:

Consider a one-dimensional lattice of N particles, with each particle separated by distance a, for a total length of L = Na. A standing wave in this lattice will have a wave number k of the form...

But standing wave obey the condition that $kL=n\pi$ and not $kL=2n\pi$ as your calculation suggests. There is nothing wrong with your equation,

$k={\frac {2\pi }{\lambda }}=n{\frac {2\pi }{L}}$

The problem is your reference to "standing waves". You are actually using periodic boundary conditions, where L is the period.

Consider a one-dimensional lattice of N particles, with each particle separated by distance a, for a total length of L = Na. Instead of using standing waves, it is more convenient to adopt periodic waves, $\psi (x+L)=\psi (x)$. Each such periodic wave will have a wave number k of the form...--guyvan52 (talk) 23:19, 20 September 2014 (UTC)

Found a reference. See equation 2.9 in http://www2.physics.ox.ac.uk/sites/default/files/BandMT_02.pdf --guyvan52 (talk) 23:23, 20 September 2014 (UTC)

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Image "File:NewvanHove.png"

If I correctly understood the text, the black arrows in the image are supposed to highlight points where $|{\vec {\nabla }}E|=0$, e.g. the extrema.
But the first as well as the last arrow are pointing to general roots, which by looking at the steepness of the curve should not at all be extrema i.e. in those cases minima.

If the text describes Van Hove singularities adequately, the image got it wrong and should be corrected acordingly.
But if Van Hove singularities emerges from roots in the DOS function too, than the text should reflect this fact as well!

--DakiwipieRuse (talk) 17:56, 22 February 2022 (UTC)

Mention of Van Hove singularity in articles about oscillating superconductivity

The Van Hove singularity is mentioned in these articles.

• Nield, David (2023-08-20). "Physicists Identify a Strange New Form of Superconductivity". ScienceAlert. Retrieved 2023-08-21.
• Clark, Carol (2023-08-07). "Physicists open new path to an exotic form of superconductivity". News. Retrieved 2023-08-21.

Peaceray (talk) 05:55, 21 August 2023 (UTC)

Another mention:

• Turner, Ben (2023-08-23). "Scientists discover strange 'singularities' responsible for exotic type of superconductivity". livescience.com. Retrieved 2023-08-24.

Peaceray (talk) 17:44, 24 August 2023 (UTC)

Searching for ”Van Hove singularity” and ”superconductivity” gives many more papers in which these two concepts overlap. Most of these papers are not that notable, but one could cover a subset of them by citing some review paper which discussed the effect of VHS on superconductivity. Jähmefyysikko (talk) 18:57, 24 August 2023 (UTC)