Physics duo finds magic in two dimensions(quantamagazine.org)
quantamagazine.org
Physics duo finds magic in two dimensions
https://www.quantamagazine.org/physics-duo-finds-magic-in-two-dimensions-20220816/
42 comments
Excuse the simpleton question:
> When creating these TBG samples, we used to literally tear the graphene in half, to get accurate relative alignment of the two halves. It was very clever, but it imparts a huge amount of strain to the two layers, generally of order 0.1-0.3%.
Does "it" mean the mechanical tearing of the crystal imparts the strain? or instead is it the newly introduced surface boundary (in 1D) that is imparting strain?
[I ask because long ago I was familiar with some of the crazy surface physics that would happen in IV-IV and III-V systems, and just wondering what effects the 1D termination of the 2D lattice might cause.]
> When creating these TBG samples, we used to literally tear the graphene in half, to get accurate relative alignment of the two halves. It was very clever, but it imparts a huge amount of strain to the two layers, generally of order 0.1-0.3%.
Does "it" mean the mechanical tearing of the crystal imparts the strain? or instead is it the newly introduced surface boundary (in 1D) that is imparting strain?
[I ask because long ago I was familiar with some of the crazy surface physics that would happen in IV-IV and III-V systems, and just wondering what effects the 1D termination of the 2D lattice might cause.]
The mechanical tearing imparts the strain. Probably. Nobody really knows that for sure.
These days, common practice is to cut the graphene with an AFM or laser prior to stacking.
These days, common practice is to cut the graphene with an AFM or laser prior to stacking.
> One will superconduct, but the other will not. Things like that.
What if you make one and cut it into two equal halves?
What if you make one and cut it into two equal halves?
You can probe different areas of the same device by adding many electrical probes, usually in a geometry called a Hall bar. In the old days of TBG, the different regions of the same device would do wildly different things. These days we are much better at stacking, and the different regions of the same device will be mostly the same.
When they mention tearing them in half, I’d imagine this more closely resembles what we would think of as slicing and just has the tearing effect due to the size of the material
That description reminded me of the observer effect.
The act of measuring changes what was measured.
The act of measuring changes what was measured.
Which 2019 STM paper are you referring to?
Thanks. All the good stuff is always in the supplemental.
Wow, this part blew my mind:
> What most intrigued physicists about graphene was how the carbon flatland transformed electrons: Nothing could slow them down. Electrons often get tripped up by the lattice of atoms through which they move, acting heavier than their textbook mass (an insulator’s immobile electrons act as if they have infinite mass). Graphene’s flat lattice, however, let electrons whiz around at a 1,000,000 meters per second — only a few hundred times slower than the speed of light. At that constant, blistering speed, the electrons flew as if they had no mass at all, blessing graphene with extreme (though not super) conductivity.
I never thought electron drift velocity could be 1E6 m/s. That's wild. (For comparison, in copper it's closer to 1E-3 m/s.)
Related: https://physics.stackexchange.com/q/709168
> What most intrigued physicists about graphene was how the carbon flatland transformed electrons: Nothing could slow them down. Electrons often get tripped up by the lattice of atoms through which they move, acting heavier than their textbook mass (an insulator’s immobile electrons act as if they have infinite mass). Graphene’s flat lattice, however, let electrons whiz around at a 1,000,000 meters per second — only a few hundred times slower than the speed of light. At that constant, blistering speed, the electrons flew as if they had no mass at all, blessing graphene with extreme (though not super) conductivity.
I never thought electron drift velocity could be 1E6 m/s. That's wild. (For comparison, in copper it's closer to 1E-3 m/s.)
Related: https://physics.stackexchange.com/q/709168
> I never thought electron drift velocity could be 1E6 m/s. That's wild.
GaAs, InP, and InAs have measured max electron velocities in the ballpark of 300,000 m/s, which is ~ 1/1000 of c. So they're listing graphene at around 3x of pretty standard III-V materials.
(It's a nice improvement in speed, but building good electronic devices involves much more than electron velocity.)
GaAs, InP, and InAs have measured max electron velocities in the ballpark of 300,000 m/s, which is ~ 1/1000 of c. So they're listing graphene at around 3x of pretty standard III-V materials.
(It's a nice improvement in speed, but building good electronic devices involves much more than electron velocity.)
Energy weapons. These will greatly benefit from this presuming the energy storage/supply side can be addressed.
Did you ever consider what happens inside a CRT?
"In the cathode ray tube, electrons are ejected from the cathode and accelerated through a voltage, gaining some 600 km/s for every volt they are accelerated through.", from:
https://www2.physics.ox.ac.uk/accelerate/resources/demonstra...
"In the cathode ray tube, electrons are ejected from the cathode and accelerated through a voltage, gaining some 600 km/s for every volt they are accelerated through.", from:
https://www2.physics.ox.ac.uk/accelerate/resources/demonstra...
A CRT is an approximate vacuum though, right? Quite a world of a difference from drifting through a material at that speed.
Material is mostly vacuum:
"The diameter of the nucleus is in the range of 1.70 fm (1.70×10−15 m ...", from:
https://en.m.wikipedia.org/wiki/Atomic_nucleus
"The diameter of the nucleus is in the range of 1.70 fm (1.70×10−15 m ...", from:
https://en.m.wikipedia.org/wiki/Atomic_nucleus
> Material is mostly vacuum
This is not true in quantum physics. This is a big misunderstanding among many physics students that they live with for many years because they first learn about the atom using the planetary model of atom. In the planetary model the atom looks mostly empty. But this view of the atom becomes nonsense after learning QFT.
The space within an atom is filled with the wavefunctions of the electrons. If you still ask "but the electron is still going to be somewhere", I can only say that our intuition about classical physics fails in the quantum physics.
This is not true in quantum physics. This is a big misunderstanding among many physics students that they live with for many years because they first learn about the atom using the planetary model of atom. In the planetary model the atom looks mostly empty. But this view of the atom becomes nonsense after learning QFT.
The space within an atom is filled with the wavefunctions of the electrons. If you still ask "but the electron is still going to be somewhere", I can only say that our intuition about classical physics fails in the quantum physics.
According to QFT there's not really a vacuum anywhere in space, as I understand it?
According to QFT, the quantum fields permeate the space everywhere, but the field itself can be in a "vacuum state", which means it has the lowest possible energy. (But unlike in classical physics, the lowest possible energy is not zero.)
Depends for whom. If you punch your fist into a brick wall, neither your fist nor the brick wall will experience the other as mostly vacuum.
This appears to be true on the human scale, but if you "punch the wall" (accelerate hydrogen and carbon into silicon and aluminum) at say 1/10th the speed of light, there will be penetration well past the surface.
More obviously if you think about how easily neutrons interact compared to protons, neutrons routinely go right through people without noticing them at all because they aren't net charged so the coulomb interactions that we are familiar with in "normal" matter don't apply.
More obviously if you think about how easily neutrons interact compared to protons, neutrons routinely go right through people without noticing them at all because they aren't net charged so the coulomb interactions that we are familiar with in "normal" matter don't apply.
A wall of electrons against anther wall of electrons, same charge, no luck moving through with only our muscles' strength. But if suddenly at least one of those walls can move through electrons it finds mostly vacuum in front of itself. I think this is what happens in that material.
The visual we draw in our mind that the atom is mostly vacuum inside is due to taking our intuition about classical physical world and misapplying it into the world inside an atom where classical physics intuition is not relevant.
In quantum physics, an atom is not mostly vacuum. It is filled with wavefunctions of the electrons.
In quantum physics, an atom is not mostly vacuum. It is filled with wavefunctions of the electrons.
people say this casually... and fail to define what a wave-function is physically.
It sounds like a non-sense answer.
if there is matter there, in the form of a 'wave function', what is the matter of a 'wave function' (other than just the electron zipping about)?
no: a wave function is a mathematical construct we use to predict where an electron might appear... (well, not exactly the wave function, but we get the probability of an electron's appearance by squaring the wave function).
A wave function (probably), doesn't 'physically' exist. it's just a useful model to predict probabilities...
waves exist only in a medium of expressive material - if that material is a single electron, there isn't magically more material there once we use a wave function to predict it's location - it's still only one electron - it's position predicted by the square of the wave function - but that doesn't mean the wave function is a physical entity.
And it's not like the electron is 'going faster than light' and 'blurring frames in reality'... it's just the electron there - the squaring wavefunction is just a way to predict it's location: just because it (seems to) works doesn't mean it physically exists.
but if it DOES exist physically (in a concrete way) - I would love an explanation or link to that proof - as that would be news to me.
fundamentally this might just be a semantics issue on the word physical
It sounds like a non-sense answer.
if there is matter there, in the form of a 'wave function', what is the matter of a 'wave function' (other than just the electron zipping about)?
no: a wave function is a mathematical construct we use to predict where an electron might appear... (well, not exactly the wave function, but we get the probability of an electron's appearance by squaring the wave function).
A wave function (probably), doesn't 'physically' exist. it's just a useful model to predict probabilities...
waves exist only in a medium of expressive material - if that material is a single electron, there isn't magically more material there once we use a wave function to predict it's location - it's still only one electron - it's position predicted by the square of the wave function - but that doesn't mean the wave function is a physical entity.
And it's not like the electron is 'going faster than light' and 'blurring frames in reality'... it's just the electron there - the squaring wavefunction is just a way to predict it's location: just because it (seems to) works doesn't mean it physically exists.
but if it DOES exist physically (in a concrete way) - I would love an explanation or link to that proof - as that would be news to me.
fundamentally this might just be a semantics issue on the word physical
What you're asking for doesn't currently exist. At least not any proven ones. There's a dozen new philosophical interpretations of quantum mechanics every year but they usually make no new predictions, so there is no way to test if the interpretation is correct.
Most (all?*) interpretations only differ in defining the "wavefunction collapse" and what happens before it. But since measuring anything involves collapsing the wavefunction, it's impossible to measure what happens before that.
We do know, from real-world experiments, that particles cannot actually be moving point-like objects. The most famous example is the double slit experiment where a single particle can cause wavelike interference with itself.
But also we know it isn't exactly a classical wave. We can only measure it as a single point, and it arrives in discrete events, not a continuous transfer of energy like a wave.
So "wavefunction" and the rest of QM lingo is what we have. We don't fully know what those are, but we also know that being just a point or wave in the style of classical physics cannot be correct.
* If an interpretation does make a new prediction that is measurable, I'm not sure if it's considered just an interpretation anymore.
Most (all?*) interpretations only differ in defining the "wavefunction collapse" and what happens before it. But since measuring anything involves collapsing the wavefunction, it's impossible to measure what happens before that.
We do know, from real-world experiments, that particles cannot actually be moving point-like objects. The most famous example is the double slit experiment where a single particle can cause wavelike interference with itself.
But also we know it isn't exactly a classical wave. We can only measure it as a single point, and it arrives in discrete events, not a continuous transfer of energy like a wave.
So "wavefunction" and the rest of QM lingo is what we have. We don't fully know what those are, but we also know that being just a point or wave in the style of classical physics cannot be correct.
* If an interpretation does make a new prediction that is measurable, I'm not sure if it's considered just an interpretation anymore.
Note that you are demanding something impossible: you are demanding a "physical" explanation, but what you seem mean by "physical" is "something that I can intuitively understand with my preconceived notions". But nature doesn't care whether the reality fits your intuition or not!
The wave-function – a model that fits the data – is defined mathematically, and by Occam's razor, anything "fluff" added to it that makes it "easier to grok", makes it _further_ away from an actual explanation of reality.
The wave-function – a model that fits the data – is defined mathematically, and by Occam's razor, anything "fluff" added to it that makes it "easier to grok", makes it _further_ away from an actual explanation of reality.
But we are not talking about just nuclei here, we are also talking about the electron shells around it! And if electron shells are "mostly vacuum", then what is electron degeneracy pressure? https://en.wikipedia.org/wiki/Electron_degeneracy_pressure And why Prof. Phil Moriarty gets irked about the meme "things never truly touch each other"? https://www.youtube.com/watch?v=P0TNJrTlbBQ
Right. There's no such thing as "solid", it's just point particles and fields all the way down.
Rather, what we think of as a "solid" has very different definition than what we would intuitively think. But in the end, that's just semantics. (According to the linked prof. Moriarty, the "boundary" of a solid is the point in space where quantum degeneracy pressure is in balance with van der Waals forces – so there is a well-defined boundary!)
Also, I think that "point particles" are a rather illusory concept. More like, interactions between particles are highly localized when observed by a macroscopic classical observer.
Also, I think that "point particles" are a rather illusory concept. More like, interactions between particles are highly localized when observed by a macroscopic classical observer.
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The distinction that matters, however, is the extent to which these particles interact with each other through their fields. The "size" of a nucleus has little to do with it.
Short video https://www.youtube.com/watch?v=kpsu9_ql0-M
Wow, basically they created a "virtual machine" of atoms. Crazy.
Wow, basically they created a "virtual machine" of atoms. Crazy.
This is well beyond my comfort zone of knowledge, but I have to ask. If I throw an electron in, and have "artificial Hydrogen," and then throw a couple more in and have "artificial Lithium" — don't I actually just have Hydrogen and Lithium? That is, what's the difference? Is it simply that I produced this rather than it originating "naturally?" Or is there some other unique property that makes it "not really Lithium" in the lab?
My understanding is that what they did was replicate the electronic configuration of a particular element without having the protons and neutrons there. So they're basically nucleus-less atoms. The difference is that these quasi-atoms are basically immobile (since their position is determined by the positions of the graphene sheets) and thus for example have no temperature. Also they may be able to form covalent bonds, but they'll never form three-dimensional quasi-molecules.
That was a cool watch!
Charlie Wood really knocked this article out of the park--a fantastic telling of yet-another set of relationships and developments in physics in the past few decades.
I'm most excited about the researchers' 100-kelvins-optimism on two different experimental fronts. Every kelvin you can raise and achieve superconductivity effects unlocks research capabilities in lower cost labs and brings commercial feasibility closer.
I'm most excited about the researchers' 100-kelvins-optimism on two different experimental fronts. Every kelvin you can raise and achieve superconductivity effects unlocks research capabilities in lower cost labs and brings commercial feasibility closer.
So, perfect laser sails fo our future spasecraft was already invented? Nice to know
hexagons all the way down
Hexagons are the bestagons
And Super Hexagon is an excellent game (with a great soundtrack) https://superhexagon.com/
And hexahexaflexagons all the way inside.
https://en.wikipedia.org/wiki/Flexagon
https://en.wikipedia.org/wiki/Flexagon
Cute video on hexaflexagons:
https://youtu.be/VIVIegSt81k
https://youtu.be/VIVIegSt81k
> Creating a working device typically takes them dozens of tries. And even then, each device behaves differently, so specific experiments are almost impossible to repeat.
This is frustrating. You can make two twisted bilayer graphene samples at 1.10 degrees precisely (to within 0.01 degrees), and they will show completely different phase diagrams. One will superconduct, but the other will not. Things like that.
What I learned recently is that every transport paper's twist angle report is wrong. The two hypothetical samples are actually probably not both 1.10 degrees. The uncertainty in twist angle should be of order 10-20%, rather than <1%. I even made this same mistake in my own paper last year!
When creating these TBG samples, we used to literally tear the graphene in half, to get accurate relative alignment of the two halves. It was very clever, but it imparts a huge amount of strain to the two layers, generally of order 0.1-0.3%. This seems like a small amount, but moire patterns are extremely sensitive to this (roughly strain amount divided by twist angle, but the twist angle is very small), so the unit cell area gets modified by anywhere from 5-30%. In transport measurements, we can only measure moire unit cell area, but not twist angle. The number 1.10 +\- 0.01 deg is calculated assuming no strain, and this is an incorrect assumption. An STM paper from 2019 first pointed this out, but it was just a couple sentences buried in the supplemental material, and I (and most others) completely missed it.
Even four years after moire materials took over the condensed matter world, we still don't understand the basics of how the materials work. It's very exciting, hot stuff.