Having taught low-temperature condensed matter labs, a big part of the grade is figuring out what went wrong, and either correcting for it, or at least acknowledging that it went wrong. The student needed to give more information about the experimental setup (what instruments did they use? four point or two point resistance? resistivity vs resistance? what is R_0?) and why they think the experiment didn't work. It looks to me like they had something miswired, so they only measured noise.
Superconductivity in TBG was originally sold as "unconventional". This article reaffirms that claim by showing how it cannot be a BCS superconductor, and more. Very interesting.
It's worth reiterating that while graphene can have some niche uses in "the real world", the main reason that it is so highly prized within academia is that it is a superb platform for studying fundamental physics, as in this work. Maybe in the future this will lead to room-temperature superconductors or something along those lines. Maybe not. Nobody jokes about how the Higgs boson has failed to leave the lab.
The other problem with this approach is that it is limited to 50% efficiency long-term. They claim "up to 90%", but it is only 90% efficient immediately after switching directions. The efficiency then drops to 0% before switching modes again.
I was measuring a delicate electronic device at MagLab in Tallahassee. They warned me that the best data would be had at night, because of reduced noise from a nearby radio station. Precisely at 8 PM every night my data became noticably sharper.
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.
> 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.
I'm probably misreading this, but I see "In this study, age ≥65 years, immunosuppression, diabetes, and chronic kidney, cardiac, pulmonary, neurologic, and liver disease were associated with higher odds for severe COVID-19 outcomes;" listed as the eight risk factors. Where are you seeing the ones you listed?
I have comcast internet with my own modem/router. Comcast will not give me a static IP unless I use their rental modem/router, which costs quite a bit monthly. I use ddns to work around this issue, and it mostly works fine.
It's sad that a great scientist got caught up in politics here. I suspect that if he had received money from any other country then he would have received a mere slap on the wrist.
Of course what he did was mega illegal and he knew it, but it's still sad.
I felt like this book was incredibly clever, but at times it felt like the pacing was infuriatingly slow. Especially near the start, it feels like there are too many chapters of world building per chapter of story.
> The new particle contains two charm quarks and an up and a down antiquark. Several tetraquarks have been discovered in recent years (including one with two charm quarks and two charm antiquarks), but this is the first one that contains two charm quarks, without charm antiquarks to balance them.
This is not the first time a tetraquark has been measured, but instead it's the first time a tetraquark with two charm quarks and no charm antiquarks. That's still nice work, but I was initially confused by the headline ("didn't they discover those already?").