Electric bond

Inside a Silicon Valley NASA research facility last month, a tuxedoed Charles Kane and Eugene Mele took the stage before a room of fellow scientists, celebrities (Pierce Brosnan hosted) and tech moguls (Mark Zuckerberg among them). They were greeted by Lucy Hawking, the daughter of Stephen Hawking, and Google co-founder Sergey Brin.

Kane, holding a delicate-looking, silver toroid trophy between his hands like a small basketball, approached the microphone first to pay homage to the things that had gotten the two Penn physicists to this place.

“I’d like to thank the electrons,” he deadpanned.

Laughter ensued.

Kane, assuming a decidedly more serious tone, went on to thank the broader community of scientists who had contributed to their field. He and Mele, two longtime collaborators, were being awarded the prestigious 2019 Breakthrough Prize in Fundamental Physics, cited for their work predicting “topological insulators”—a class of unique materials that could well forever shape the electronic landscape.

There was one more acknowledgement to make.

Mele told the room he was proud that their groundbreaking research in condensed matter physics had taken place at Penn—a university, it’s worth remembering, founded by a man known for casting a kite into the sky to study electricity.

“This is an institution that was established by Benjamin Franklin for, in his words, the pursuit of ‘those things that are likely to be most useful and most ornamental,’” Mele said. “And that is exactly the idea behind this subject.”

First forays

Mele remembers growing up tinkering with radios, taking them apart and putting them back together.

Even so, the West Philadelphia native said he wasn’t sure as a teenager what career path to choose; in high school, he vacillated as he admired the work of two teachers from very different areas—English and physics.

But to hear him to reminisce, it seems his aptitude for the latter was evident early on: Once, in the fifth grade, he challenged a textbook’s explanation of how speed works in rotating objects. His teacher compelled him to disprove the book in front of his class. (He now realizes the book discussed only one of several possible scenarios.)

“That was my first foray as a physicist I guess, was trying to explain the flow of matter in a rotating object,” he said.

Kane, too, demonstrated his ability at an early age. Raised in the college town of Iowa City, Iowa, the son of an engineering professor, Kane had a natural inclination toward math.

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“I loved mathematics when I was a kid,” Kane said. “I would sit by myself and think about it, spend hours.” By high school, he was taking college-level math courses.

Mele would go on to attend St. Joseph’s University and earn a Ph.D. from Massachusetts Institute of Technology. Likewise, Kane earned a Ph.D. from MIT after studying at the University of Chicago.

In 1984, after three years of working as a researcher at Xerox—conducting research intended to position the company as a thought leader—Mele was urged to apply for an open position in Penn’s Department of Physics and Astronomy. He thought he might be young to enter academia (he still does), but he applied, interviewing twice, and earned the job nonetheless.

Several years later, Mele invited Kane—then completing a post-doc at IBM—to speak on campus. The two maintained contact and, a year later, a job opened.

Kane would go on to secure the position, joining Mele at 33rd and Walnut streets.

Complementary colleagues

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From the slightly abridged door on the second floor of the David Rittenhouse Laboratory, a lick of light spilled into the otherwise dark suite.

Inside, Mele sat with his laptop, his back to a formula-scribbled chalkboard, amid stacks of books and binders and newspaper clippings. An unopened bottle of champagne sat on his desk on this late November morning, weeks after the Breakthrough ceremony in California.

Since 1991, it’s there, in 2N17A, that Mele and Kane have worked together, sharing a common area and occupying what Mele calls “complementary” offices. It’s a word that describes more than just their physical spaces.

Both share the title of being Penn’s Christopher H. Browne Distinguished Professor of Physics and have been awarded the Benjamin Franklin Medal in Physics.

Their collaboration truly began at the suggestion of the late Penn professor of material sciences Jack Fisher, an experimentalist who urged Kane and Mele to look at carbon nanotubes.

The year was 1996 and Kane and Mele, while collegial, had not yet conducted research together in an official capacity. Fisher figured the single planes of graphene—wrapped like a cylinder, not unlike a very thin straw—could be of interest for the two due to its interesting electronic properties.

 “That was the subject that brought Gene and I together,” Kane said.

It helped that Mele, thanks to research he conducted at Xerox, had become an expert on graphite—a material made of many sheets of graphene and used to make pencils, among other things.

The two proceeded to study nanotubes for several years, inadvertently accruing foundational knowledge that would prove helpful in much bigger matters.

Graphene and grit

In 2004, Kane and Mele got word that experimentalists had discovered that they could isolate single planes of graphene.

The development set off lightbulbs for the researchers.

The thing that’s interesting about graphene, Kane said, is that it straddles the line between two important classes of materials: conductors (which conduct electricity) and insulators (which don’t).

“We knew this was going to be big,” he added. “Not that many people were thinking about it at that time, but we knew once these experiments became widely known that this was going to turn into a huge subject.”

Much about the basic electronic structure was known, but what more could they add? It was a question that prompted further inquiries: Why was it precisely on the border of being a conductor and insulator? What explained it?

Kane said their original thinking in trying to understand the quality that gave graphene its unique distinction—that its energy gap seemed to be exactly equal to zero—had to do with its particular symmetries in the crystal lattice formed by the atoms in graphene (think of it like chicken wire, he said). Yet further probing showed that wasn’t the case—it wasn’t the symmetry protecting the energy gap.

“There’s a voice in my head saying, this is stupid, no one’s going to care about this puny energy gap,” Kane said. “OK, it’s not equal to zero, but why would anyone care? But it was kind of cool. It seemed fundamental. I couldn’t put it down.”

They didn’t relent.

The researchers would go on to incorporate other aspects of physics to make sense of it, including a concept called spin-orbit interaction.

What they began to realize was that the energy gap in graphene acted just like a phenomenon known as the quantum Hall effect, on which Kane had conducted research years earlier.

Tracing a finger along the edge of the circular wooden coffee table in his office to demonstrate, Kane said it essentially meant that two sets of electrons—spinning in opposite directions—were operating in a sort of divided highway.

The unique properties—not affected by disorder, as one might have suspected, and protected by a principle known as time-reversal symmetry—dictated that graphene, despite being only one material, actually acted as an insulator in the interior (think rubber) and conductor on the edge (like metal).

‘The big win’

The discovery was uncommon, in part because such findings often result from experiment, and then are understood theoretically. In this case, that process was reversed.

What Kane and Mele predicted was a new class of materials called “topological insulators.” But they didn’t call it that at first, and they didn’t know it would be such a watershed moment.

Nor, it turned out, did others.

When Kane and Mele submitted their first paper to the peer-reviewed journal Physical Review Letters, they went through two rounds of reviews. And then they got rejected.

“We had to fight in order to get it published,” Kane said.

After an appeal, the paper was finally published, but not until after their follow-up paper was published first, ironically.

“Quantum Spin Hall Effect in Graphene”—received Nov. 29, 2004—officially published a year later, on Nov. 23, 2005. Meanwhile their next paper, introducing the notion of topology and submitted in June 2005, published Sept. 28 of that year.

Those seminal papers have become key underpinnings for a whole industry of thought and trial.

In the years since, peers at other institutions have added to their theory and proven it in experiment, and have identified other topological insulator materials that will be of more practical use than graphene. One recent study, Mele said, suggests that thousands of inorganic solid compounds constitute this wider class of unconventional materials.

“People often ask, so what’s the application? The point to make is that there are two applications,” Mele said.

The materials could be used to improve existing architectures, like small scale electronics—or pave the way to new ones.

“It licenses you to create new architectures,” Mele said. “That’s where the big win will be. Things you haven’t thought of yet but that are now possible based on the novel properties of these new materials.”

One oft-cited example: The materials could play a role in the persisting quest to develop quantum computers, whose capabilities and efficiencies would far surpass computers as we think of them today.

Classes and collaboration

When they are not making game-changing research advances, Mele and Kane have other responsibilities, including writing and reviewing funding proposals and mentoring Ph.D. students.

But both cite their time in the classroom as being core to their love for the job. This semester, Kane taught a freshman honors physics course, and Mele instructed a junior-level advanced electrodynamics class.

“I think it’s wonderful that this transformative work they’ve done, of creating this entirely new area of condensed matter physics, has been carried out by two people who are also among our most successful classroom teachers,” said Mark Trodden, chair of the Physics and Astronomy Department. “A lot of people don’t get the opportunity to learn the subject from those kinds of people—and our students do, and that’s something remarkable.”

Mele said he appreciates that his students are “happy when they’re being challenged.”

It’s something he no doubt enjoys himself.

“I really feel energized and invigorated when I’ve got a new calculation I’m able to work through,” he said.

Kane, too, describes a sort of relentless attraction to pursuing research of unknown or little-understood aspects of matter: “I’m at a point where if I think it’s cool, that’s sort of motivation enough for me.”

The pair balances one another—Kane gets glued on abstract ideas, Mele is the more pragmatic of the two.

Colleagues see it as a bond that works.

“This kind of work is very mathematical, it’s very demanding and it involves a great deal of staying power—to have an idea and to see it through with this quality and impact requires many many years of different tries, different attempts,” Trodden said. “One nice thing for them is that their collaboration has been so long-term that that’s a path they’re very comfortable traveling together.”

Homepage photo: Behind the discovery of a new class of electronic materials is a 20-year collaboration between two Penn physicists, Charles Kane (left) and Eugene Mele, winners of the 2019 Breakthrough Prize in Fundamental Physics. Photo by Eric Sucar, University Communications.