5 ms·
Physics duo finds magic in two dimensions
- reality_inspctr 4y agohexagons all the way down
- WesSouza 4y agoHexagons are the bestagons
- asgeir 4y agoAnd Super Hexagon is an excellent game (with a great soundtrack) https://superhexagon.com/ https://superhexagon.com/
- pmontra 4y agoAnd hexahexaflexagons all the way inside. https://en.wikipedia.org/wiki/Flexagon https://en.wikipedia.org/wiki/Flexagon
- jcims 4y agoCute video on hexaflexagons: https://youtu.be/VIVIegSt81k https://youtu.be/VIVIegSt81k
- spxtr 4y agoLovely article. > 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.
- amelius 4y ago> One will superconduct, but the other will not. Things like that. What if you make one and cut it into two equal halves?
- hulahoof 4y agoWhen 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
- spxtr 4y agoYou 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.
- 11101010001100 4y agoWhich 2019 STM paper are you referring to?
- spxtr 4y agohttps://www.nature.com/articles/s41586-019-1431-9 https://www.nature.com/articles/s41586-019-1431-9 https://arxiv.org/abs/1812.08776 https://arxiv.org/abs/1812.08776
- 11101010001100 4y agoThanks. All the good stuff is always in the supplemental.
- jpmattia 4y agoExcuse 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.]
- dataflow 4y agoWow, 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 https://physics.stackexchange.com/q/709168
- jonsen 4y agoDid 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/demonstrations/cathode-ray-tube https://www2.physics.ox.ac.uk/accelerate/resources/demonstra...
- dataflow 4y agoA CRT is an approximate vacuum though, right? Quite a world of a difference from drifting through a material at that speed.
- jonsen 4y agoMaterial 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 https://en.m.wikipedia.org/wiki/Atomic_nucleus
- deleted 4y ago[deleted]
- est 4y agoShort video https://www.youtube.com/watch?v=kpsu9_ql0-M https://www.youtube.com/watch?v=kpsu9_ql0-M Wow, basically they created a "virtual machine" of atoms. Crazy.
- asciii 4y agoThat was a cool watch!
- mrcwinn 4y agoThis 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?
- fluoridation 4y agoMy 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.
- hamilyon2 4y agoSo, perfect laser sails fo our future spasecraft was already invented? Nice to know
- xbar 4y agoCharlie 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.