Futile? Nope, Just Zero

“Megabar superconductivity.”

“Whoa, Susan. Too much information, too few words. Could you unpack that, please?”

“No problem, Sy. A bar is the barometric pressure (get it?) at sea level. A megabar is—”

“A million atmospheres, right?”

“Right, Al. So Ranga Dias and his crew were using their Diamond Anvil Cells to put their chemical samples under million-atmosphere pressures while they tested for superconductivity—”

“Like Superman uses?”

“Is he always like this, Sy?”

“Just when he gets excited, Susan. The guy loves Science, what can I say?”

“Sorry, Susan. So what makes conductivity into superconductivity?”

“Excellent question, Al. Answering it generated several Nobel Prizes and we still don’t have a complete explanation. I can tell you the what but I can’t give you a firm why. Mmm… what do you know about electrical resistance?”

“Just what we got in High School General Science. We built a circuit with a battery and a switch and an unknown resistor and a meter to measure the current. We figured the resistance from the voltage divided by the current. Or maybe the other way around.”

“You got it right the first try. The voltage drop across a resistor is the current times the resistance, V=IR so V/I=R. That’s for ordinary materials under ordinary conditions. But early last century researchers found that for many materials, if you get them cold enough the resistance is zero.”

“Zero? But … if you put any voltage across something like that it could swallow an infinite amount of current.”

“Whoa, Al, what’s my motto about infinities?”

“Oh yeah, Sy. ‘If your theory contains an infinity, you’ve left out physics that would stop that.’ So what’d stop an infinite current here?”

“The resistor wasn’t the only element in your experimental circuit. Internal resistance within the battery and meter would limit the current. Those 20th-century researchers had to use some clever techniques to measure what they had. Back to you, Susan.”

“Thanks, Sy. I’m going to remember that motto. Bottom line, Al, superconductors have zero resistance but only under the right conditions. You start with your test material, with a reasonable resistance at some reasonable temperature, and then keep measuring its resistance as you slowly chill it. If it’s willing to superconduct, at some critical temperature you see the resistance abruptly drop straight down to zero. The critical temperature varies with different materials. The weird thing is, once the materials are below their personal critical temperature all superconductors behave the same way. It’s seems to be all about the electrons and they don’t care what kind of atom they rode in on.”

“Wouldn’t copper superconduct better than iron?”

“Oddly enough, pure copper doesn’t superconduct at all. Iron and lead both superconduct and so do some weird copper-containing oxides. Oh, and superconductivity has another funny dependency — it’s blocked by strong magnetic fields, but on the other hand it blocks out weaker ones. Under normal conditions, a magnetic field can penetrate deep into most materials. However, a superconducting piece of material completely repels the field, forces the magnetic lines to go around it. That’s called the Meissner effect and it’s quantum and—”

“How’s it work?”

“Even though we’ve got a good theory for the materials with low critical temperature, the copper oxides and such are still a puzzle. Here’s a diagram I built for one of my classes…”

“The top half is the ordinary situation, like in a copper wire. Most of the current is carried by electrons near the surface, but there’s a lot of random motion there, electrons bouncing off of impurities and crystal defects and boundaries. That’s where ordinary conduction’s resistance comes from. Compare that with the diagram’s bottom half, a seriously simplified view of superconduction. Here the electrons act like soldiers on parade, all quantum‑entangled with each other and moving as one big unit.”

“The green spirals?”

“They represent an imposed magnetic field. See the red bits diving into the ordinary conductor? But the superconducting parade doesn’t make space for the circular motion that magnetism tries to impose. The force lines just bounce off. Fun fact — the supercurrent itself generates a huge magnetic field but only outside the superconductor.”

“How ’bout that? So how is megabar superconductivity different?”

~~ Rich Olcott

Diamonds in The Tough

“Excuse me, they said there’s a coffee shop over here somewhere. Could you please point me to it?”

“Sure. Al’s place is right around the next corner, behind the Physics building. I’ll walk you over there.”

“Oh, I don’t want to bother you.”

“No bother, it’s my coffee time anyway. Hi, Al, new customer for you.”

“Hi, Sy. What’ll it be, Ms … ?”

“I’m Susan, Susan Kim. A mocha latte, please, Al. And you’re Sy …?”

“Moire. Sy Moire, Consulting Physicist. Who’s the ‘they’ that told you about Al’s?”

“An office staffer in the Chemistry Department. I just joined the research faculty over there.”

Al’s ears perk up. “A chemist, at last! For some reason they don’t show up over here very much.”

“Hah, I bet it’s because they’re used to drinking lab coffee from beakers.”

“As a matter of fact, Sy, I do have a coffee beaker. A glass‑worker friend added a very nice handle to a 500‑milliliter beaker for me. It’s not unpacked yet which is why I was looking for a coffee shop. This latte is very good, Al, better than lab coffee any day.”

“Thanks. So what’s the news in Science, guys?”

“Mmm… On Mars, the Insight mission‘s ‘mole’ thermal probe has finally buried itself completely, on its way down we hope to its targeted 5‑meter depth. And the OSIRIS‑REx mission to Asteroid Bennu successfully collected maybe a little too much asteroid sample. One rock fragment blocked the sampler’s lid like a bit of souvenir sticking out of a tourist’s carry‑on bag. Fortunately the engineers figured out how to stow the stuff more neatly for the two‑year trip back home. How about in the Chemistry world, Susan?”

“Hmm… Ranga Dias and his team at the University of Rochester used a diamond anvil cell to—”

“Wait — a diamond anvil? Like the Village Blacksmith but made of diamond?”

“No, Al, nothing like that. Diamond is the hardest substance we know of, right? A DAC uses a pair of quarter‑carat gem‑quality diamonds pushing against each other to create a small volume of crazy high pressure in the space between them, up into the million‑atmosphere range. Here, I’ve got a gorgeous photo of one on my phone…

Diamond anvil cell, photo by J. Adam Fenster / University of Rochester

“To give you an idea of the scale, that square black gasket between the two diamonds is a piece of rhenium metal foil that’s a quarter of a millimeter thick. The reaction vessel itself is a hole they spark-drilled through the gasket. This is teeny, nanoliter chemistry.”

“OK, they’re small diamonds, but .. DIAMONDS! I bet they crack some of them. That’s got to be ex‑PENsive, our tax dollars going CRUNCH!.”

“Not really. You’re right, some do crack, up around the seven million atmosphere mark. But here’s the fun part — the researchers don’t pay market price for those diamonds. They come from the government’s stock of smuggled goods that Customs agents have confiscated at the border.”

“Why go to all that trouble? What’s wrong with test tubes and beakers?”

“Because not all chemistry takes place at atmospheric pressure, Sy. High pressure crams molecules closer together. They get in each other’s way, maybe deform each other enough to react in ways that they wouldn’t under conditions we’d call ‘normal.’ Even water has something like 17 different forms of ice under different pressure‑temperature conditions. The whole discipline of high‑pressure chemistry got started because the seismologists needed to know how minerals transform, melt, flow and react under stress. The thing about diamond is that it doesn’t transform, melt, flow or react.”

“Oo, oo, you can see through a diamond, sorta. I’ll betcha people pipe laser beams down them, right?”

“Absolutely, Al. Before lasers came along researchers were using regular light and optics to track events in a pressurized DAC. Lasers and fiber optics completely changed the game. Not just for observation — you can use intense light to heat things up, get them even closer to deep‑Earth conditions.”

“I suppose chemists are like physicists — once a new tool becomes available everybody dives in to play.”

“You know it. There’s thousands of papers out there detailing work that used a DAC.”

“So what did Dias report on?”

~~ Rich Olcott