Physics duo finds magic in two dimensions
quantamagazine.org
quantamagazine.org
> 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.
What if you make one and cut it into two equal halves?
> 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.]
These days, common practice is to cut the graphene with an AFM or laser prior to stacking.
The act of measuring changes what was measured.
> 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.)
"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...
"The diameter of the nucleus is in the range of 1.70 fm (1.70×10−15 m ...", from:
In quantum physics, an atom is not mostly vacuum. It is filled with wavefunctions of the electrons.
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
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.
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.
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.
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.
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.
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.)
Wow, basically they created a "virtual machine" of atoms. Crazy.
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.