Paradoxical Crystal Baffles Physicists | Hacker News Reader

Paradoxical Crystal Baffles Physicists

quantamagazine.org

94 pointsandrewflnr31 comments

Hyperbolic journalism aside, this is certainly an interesting material. Surprisingly, from a cursory literature search, it appears that no one has attempted to perform electronic structure calculations on SmB6, although the compound seems to have only started gathering interest around 2013. DFT of course wouldn't work for something with high electron correlation, but considering the simplicity of the molecule, I imagine QMC could tackle this fairly readily to provide some insight as to what's going on.

Discrete Fourier transform and quantum Monte Carlo?

Delta Follicle Trapping and Quick Melting Carbon

"Density Functional Theory" and "Quantum Monte Carlo", two ways of calculating electronic structures (the former tends to be more approximate and the latter tends to be more computationally expensive)

Down for tacos; queso means cheese

Lanthanide (and Actinides) are difficult to handle, computationally. They tend to require relativistic methods. They also tend not to be single-determinant. (Source: I've tried and very much failed).

I'm not as familiar with QMC, so I don't know the status of QMC with relativistic methods, etc.

The opening sentence makes it sound like scientists have discovered "semiconductors" for the first time - yet they were never mentioned in the article. I feel like I would have gained a better understanding of samarium hexaboride's unexpected properties if the author had contrasted them with a concept that is conceptually (?) similar.

    > "In a deceptively drab black crystal, physicists have stumbled upon a baffling behavior, one that appears to blur the line between the properties of metals, in which electrons flow freely, and those of insulators, in which electrons are effectively stuck in place. The crystal exhibits hallmarks of both simultaneously."

> Calling to mind the famous wave-particle duality of quantum mechanics

Come on, quanta. You of all science news publications should know not to throw this misleading metaphor around willy-nilly. Not everything that seems contradictory ought to be compared to wave-particle duality, especially when so many people have the wrong idea about it.

It's not that misleading. Pure waves and pure particles are dual to each other in the mathematical formalism, though actual physical phenomena are neither.

> One promising approach models the material as a higher-dimensional black hole.

That's certainly not going to be misreported in the media.

But why? Isn't it big news that string theory has finally been proven?

Nothing about string theory is provable. If you throw a stone in a physics class you'll hit a new theory working with higher dimensions. A really simplified explanation might be "there's a qualitative attribute that means something special about how forces work with it".

Sure, I know.

What led you to believe that "string theory has finally been proven"?

What is the hypothesis that has been tested and demonstrated?

Wow, it's already being misreported in HN comments.

>Amazingly, the observed deviation from the Lifshitz-Kosevich formula was presaged in 2010 by Sean Hartnoll and Diego Hofman, both then at Harvard University, in a paper that recast strongly correlated materials as higher-dimensional black holes, those infinitely steep curves in space-time predicted by Albert Einstein.

This is boggling to me. Higher dimensional black holes!?

I think it is bad journalism, intended to boggle not enlighten.

The paper mentioned is http://arxiv.org/pdf/0912.0008, and does not itself make mention of black holes.

Presumably the "higher-dimensional" aspect is coming from Hartnoll and Hofman's "large number of degrees of freedom" analysis.

I don't know anything about this area to guess whether there is really a black hole analogy.

But I am confident it isn't a useful analogy to make anything more clear!

That paper does not directly mention black holes, but holographic correspondence (which is even in the title!) does seem to have strong connection to black holes and higher dimensions, which is further spelled out in the references. As far as I can tell (which admittedly isn't much), the black hole connection goes deeper than just an analogy. Overall, I don't feel like Quantas short reference there is bad journalism, the subject matter being truly quite mind-boggling.

Here is another article from the same reporter about this holographic stuff:

https://www.quantamagazine.org/20130701-signs-of-a-stranger-...

I think this is a reference to higher dimensional gravity, it's not coming from General Relativity but more of an attempt generalize or reconcile 4-dimensional GR gravity with more modern higher dimensional theories (usually string theory or one of it's offshoots).

To put it more "exactly" they might be referencing the topology of black holes vis-à-vis gauss–bonnet gravity or something similar which would produce non-spherical black holes.

Tho this is stretching my understanding of physics to it's limits, Quanta Mag is usually an excellent source as far scientific reporting goes, it usually holds a pretty solid footing which makes it accessible to popsci lovers and people in the know alike.

I've seen some oddities in Quanta articles before, I usually attribute them to using a very high end subject matter too liberally, but it could also be some misunderstanding by the reporter.

In the same paragraph where "Higher Dimensional Black Holes" is mentioned, the journalist hedges the outlandish suggestion with "But no theory yet captures the whole story. “I do not think that there is any remotely credible hypothesis proposed at this moment in time,” Zaanen said."

So... "Yea.. uh huh... interesting concept, not a chance" to paraphrase.

In what is known as the "AdS/CFT correspondence" or "holography", it is conjectured/believed (based on many reasonable physical arguments) that a condensed matter system will have an equivalent description in terms of a gravitational system in one higher dimension.

In those models, the temperature of the system in the lab typically corresponds to the Hawking temperature of the black hole in the higher dimensional gravitational system. (That is where my knowledge of the subject stops.)

For thos who seek technical details, Hartnoll has a short summary (2009) of the formalism on which the 2010 result is based: https://arxiv.org/abs/0909.3553

@dzdt: A little too hasty to judge don't you think? (ref comment quoted below)

> I think it is bad journalism, intended to boggle not enlighten [...] I don't know anything about this area to guess whether there is really a black hole analogy [...] But I am confident it isn't a useful analogy to make anything more clear!

> But while samarium hexaboride does conduct electricity at room temperature, things get strange as it cools.

So maybe the magnetic field undoes some of the effects of low temperatures? Or is that too un-sensational an explanation for modern physics?

When you suggest that "the magnetic field undoes some of the effects of low temperature", what do you mean? How could a magnetic field "undo" effects of low temperature? And which effects?

This sounds like a comment from the all too common armchair physicist that has no real rigorous background in physics.

Did you read the article? It's all there.

> Sebastian and her collaborators observed electrons traversing orbits millions of atoms in diameter inside the crystal in response to a magnetic field

If cooling the material down turns it from a conductor into an insulator, and then a magnetic field is observed to make it conduct again, then maybe the magnetic field reversed whatever effect the cooling had. Seems a lot more believable than "black holes".

I am mostly replying to your initial comment of

>Or is that too un-sensational an explanation for modern physics?

which has the frankly pretentious air of "I see an easy explanation you don't, why didn't you see it?"

This is a common phenomenon where someone reads a pop sci article and decides they know better than experts in the field. So I asked a couple basic questions to see whether you have any real idea what you're commenting on, or whether you're stringing together concepts you have no real understanding of and then being snide.

What is an insulator, and what is a conductor? What are the quantum mechanical properties of a material the determine whether it is one or the other, or potentially some sort of mixture? How would or could a magnetic field influence conductance of a material?

How does a material transition from a conductor to an insulator, and how does on recognize that transition?

For that matter, what are the known laws of magnetic interaction that one would have in mind when trying to model such an interaction?

Or have your comments had no real content?

Related video, Feynman can't explain magnets: https://www.youtube.com/watch?v=MO0r930Sn_8

It always amazes me how much we still have to learn about condensed matter physics.

It always amazes me how much we still have to learn about physics.

It always amazes me how much we still have to learn.

It always amazes me how much we have learned.

Please do not turn HN into Reddit. The signal:noise ratio is a very important thing to maintain. If it begins to slip too much, it might turn into a monotonically decreasing slide.

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