Kharzeev’s theoretical idea now a reality

Kharzeev’s theoretical idea now a reality

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Dmitri Kharzeev stands with Qiang Li, a physicist and head of the Advanced Energy Materials Group at BNL, Genda Gu, a senior physicist from the Condensed Matter Physics & Materials Sciences Department and Tonica Valla, a BNL physicist. Photo from BNL

More than a decade ago, Dmitri Kharzeev came up with an idea he thought he should find in nature. Many such concepts come and go, with some, like the Higgs boson particle, taking over 50 years to discover.

After working with numerous collaborators over the years, the professor of physics and astronomy at Stony Brook University and a senior scientist at Brookhaven National Laboratory found proof.

“This was absolutely amazing,” said Kharzeev. “You think an idea in your head, but whether or not it’s realized in the real world is not at all clear. When you find it in the laboratory on a table top experiment, it’s pretty exciting.”

The discovery triggered a champagne party in Kharzeev’s Port Jefferson home, which included collaborators such as Qiang Li, a physicist and head of the Advanced Energy Materials Group at Brookhaven, and Tonica Valla, a physicist at BNL, among others. “There was a feeling that something new is about to begin,” Kharzeev said.

Kharzeev’s idea was that an imbalance in particles moving with different projections of spin on momentum generates an electric current that flows with resistance. That resistance drops in a magnetic field that the scientists hope can reach zero, which would give their material superconducting properties.

A particle’s projection of spin on momentum is its chirality. The magnetic field aligns the spins of the positive and negative particles in opposite directions. When the scientists applied an electric field, the positive particles moved with it and the negative ones moved against it. This allows the particles to move in a direction consistent with their spin, which creates an imbalance in chirality.

The chiral magnetic effect can enable ultra-fast magnetic switches, sensors, quantum electricity generators and conventional and quantum computers.

Kharzeev had expected this kind of separation for particles at the Relativistic Heavy Ion Collider at BNL, where he figured he might observe the separation for quarks in the quark-gluon plasma.

Instead, he and his colleagues, including co-author Li, discovered this phenomenon with zirconium pentatelluride, which is in a relatively new class of materials called Dirac semimetals, which were created in 2014. Their paper was published in Nature Physics earlier this year.

Dmitri Kharzeev at the control center of the Relativistic Heavy Ion Collider at BNL. Photo from BNL
Dmitri Kharzeev at the control center of the Relativistic Heavy Ion Collider at BNL. Photo from BNL

The particles had to be nearly massless to allow them to move through any obstacles in their path. Particles that collided with something else and changed their direction or chirality would create resistance, which would reduce conductivity.

Genda Gu, who is in the Condensed Matter Physics & Materials Sciences Department at BNL, grew the zirconium pentatelluride crystals in his laboratory. Gu “is one of the best crystal growers in the world and he has managed to grow the cleanest crystals of zirconium pentatelluride currently available,” said Kharzeev.

Gu said he collaborates regularly with Li. This, however, was the first time he worked with Kharzeev. He called the work “fruitful and productive” and said the crystals had “generated a number of exciting scientific results.”

The materials they worked with have a wide range of potential applications. The semimetals strongly interact with light in the terahertz frequency range, which is a useful and unique property, Kharzeev suggested. Terahertz electromagnetic radiation, which is called T-rays, can be used for nondamaging medical imaging, including the diagnosis of cancer and high-speed wireless communications.

To be sure, there are limitations to zirconium pentatelluride. For starters, it only displays this chiral magnetic effect at temperatures below 100 degrees Kelvin, or minus 280 degrees Fahrenheit, which is on par with the best high-temperature semiconductors, but still well below room temperature. Its chirality is also only approximately conserved, so the resistance does not drop all the way to zero.

Another hurdle is that scientists have to improve the technique for growing thin films of this material. While it is possible, it will take considerable research and development, Kharzeev said. He hopes to find a material that will exhibit chiral magnet effects at room temperature.

Kharzeev has received interest from companies and other researchers but said “we have a lot of work to do before we can create practical devices” based on this effect. He hopes scientists will create such products within the next five to ten years.

There are numerous potential uses for zirconium pentatelluride and other similar materials, including in space, where temperatures remain low enough for these quasi-particles.

“You could envision this on space stations to generate electricity from sunlight,” Kharzeev said. When he saw the movie “The Martian,” Kharzeev said he thought about how thermoelectrics could power a station on the Red Planet.

“If we managed to increase the temperature at which the chiral magnetic effect is present just a little, by about 70 degrees Fahrenheit, our thermoelectric would be even more efficient,” he said.

Kharzeev, who grew up in Russia and moved to Long Island in 1997, appreciates the beauty and comforts of the area.

“The combination of Stony Brook, BNL and Cold Spring Harbor Lab makes Long Island one of the best places in the world to do science,” he said. He also loves the beaches and the ocean and plays tennis at the Port Jefferson Country Club.

As for his collaborations, Kharzeev is excited by the work ahead with a material he didn’t envision demonstrating these superconducting properties when he came up with this concept in 2004.

When he learned of the work Li was doing with zirconium pentatelluride, Kharzeev “rushed” into his lab. “It appeared that even though he and his group were not thinking about the chiral magnetic effect at the time, they had already set up an experiment that was perfect for this purpose,” Kharzeev said. They “even had a preliminary result that literally made my heart jump.”