Science & Technology

Quantum Melting of Wigner Crystals: Creating a System for Studying Quantum Phase Transitions

A schematic of a quantum part transition from an electron liquid to a bilayer Wigner crystal. Every ball represents a single electron. Credit score: Ella Maru Studio in collaboration with Hongkun Park and You Zhou

The examine marks a main step towards creating a system for learning quantum part transitions.

In 1934, physicist Eugene Wigner made a theoretical prediction primarily based on quantum mechanics that for 87 years went unseen.

The idea urged how a metallic that usually conducts electrical energy might flip into a nonconducting insulator when the density of electrons is decreased. Wigner theorized that when electrons in metals are delivered to ultracold temperatures, these electrons can be frozen of their tracks and type a inflexible, non-electricity conducting construction — a crystal — as an alternative of zipping round at 1000’s of kilometers per second and creating an electrical present. Since he found it, the construction was coined a Wigner Crystal and was noticed for the primary time in 1979.

What’s remained stubbornly elusive to physicists, nonetheless, has been the melting of the crystal state into a liquid in response to quantum fluctuations. At the very least, it was: Now, nearly 90 years later, a staff of physicists co-led by Hongkun Park and Eugene Demler within the School of Arts and Sciences has lastly experimentally documented this transition.

The work is described in a new examine revealed within the journal Nature and marks a large step towards creating a system for learning these sorts of transitions between states of matter on the quantum stage, a long-sought-after purpose within the area.

“That is proper on the border of matter of altering from partially quantum materials to partially classical materials and has many uncommon and attention-grabbing phenomena and properties,” stated Eugene Demler, a senior writer on the paper. “The crystal themselves have been seen, however this, type of, pristine transition — when quantum mechanics and classical interactions are competing with one another — has not been seen. It has taken 86 years.”

Led by Park and Demler, the analysis staff targeted on observing Wigner crystals and their part transitions within the examine. In chemistry, physics, and thermodynamics, part transitions occur when a substance modifications from a stable, liquid, or fuel to a completely different state. When quantum fluctuations close to absolute zero temperature drive these transitions, they’re referred to as quantum part transitions. These quantum transitions are thought to play an necessary position in lots of quantum programs.

Within the case of a Wigner crystal, the crystal-to-liquid transition occurs from a competitors between the classical and quantum elements of the electrons – the previous dominating within the stable part, during which electrons are “particle-like,” and the latter dominating within the liquid, during which electrons are “wave-like.” For a single electron, quantum mechanics tells us that the particle and wave nature are complementary.

“It’s hanging that, in a system of many interacting electrons, these completely different behaviors manifest in distinct phases of matter,” stated Park. “For these causes, the character of the electron solid-liquid transition has drawn large theoretical and experimental curiosity.”

The Harvard scientists report utilizing a novel experimental method developed by You Zhou, Jiho Sung, and Elise Brutschea — researchers from the Park Analysis Group and lead authors on the paper — to watch this stable to liquid transition in atomically skinny semiconductor bilayers. Usually, Wigner crystallization requires very low electron density, making its experimental realization a main experimental problem. By setting up two interacting electron layers from two atomically skinny semiconductors, experimentalists created a scenario during which the crystallization is stabilized at larger densities.

To see the transition, the researchers used a methodology referred to as exciton spectroscopy. They use mild to excite an electron within the system and bind it to the electron emptiness, or gap, it leaves behind, forming hydrogen-like electron-hole pair referred to as an exciton. This pair interacts with the opposite electrons within the materials and modifies its properties to allow them to be optically seen.

The findings from the paper had been largely unintended and got here as a shock, in response to the researchers. The Park group initially set out in a completely different path and had been puzzled after they seen the electrons of their materials displayed insulating conduct. They consulted with theorists from Demler’s lab and shortly realized what they’d.

The researchers plan on utilizing their new methodology to proceed to research different quantum part transitions.

“We now have an experimental platform the place all these [different quantum phase transition] predictions can now be examined,” Demler stated.

Reference: “Bilayer Wigner crystals in a transition metallic dichalcogenide heterostructure” by You Zhou, Jiho Sung, Elise Brutschea, Ilya Esterlis, Yao Wang, Giovanni Scuri, Ryan J. Gelly, Hoseok Heo, Takashi Taniguchi, Kenji Watanabe, Gergely Zaránd, Mikhail D. Lukin, Philip Kim, Eugene Demler and Hongkun Park, 30 June 2021, Nature.
DOI: 10.1038/s41586-021-03560-w

Funding: US Division of Protection, US Air Pressure, Air Pressure Workplace of Scientific Analysis, US Division of EnergyUMD | A. James Clark Faculty of Engineering, Nationwide Science Basis
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