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Yu-Tsun Shao and David Muller/Provided Transmission electron microscopy image showing moir
Yu-Tsun Shao and David Muller/Provided Transmission electron microscopy image showing molybdenum ditelluride and tungsten selenide lattices.

Abstract:
A model system created by stacking pairs of monolayer semiconductors provides physicists with a simpler way to study the behavior of quantum disruptors, from heavy fermions to exotic quantum phase transitions.

Semiconductor lattice couples electrons and magnetic moments

Ithaca, New York | Posted on March 24, 2023

The group’s paper, Gate-Tunable Heavy Fermions in Moir Kondo Lattice, was published March 15 in Nature. The lead author is postdoctoral fellow Wenjin Zhao at the Kavli Institute at Cornell.

The project was led by Kin Fai Mak, professor of physics at the College of Arts and Sciences, and Jie Shan, professor of applied physics and engineering at Cornell Engineering and at A&S, the paper’s senior co-author. The two researchers are members of the Kavli Institute; they came to Cornell through the provost’s Nanoscale Science and Microsystems Engineering (NEXT Nano) initiative.

The team set out to overcome what is known as the Kondo effect, named after Japanese theoretical physicist Jun Kondo. About six decades ago, experimental physicists discovered that by taking a metal and replacing even a small number of atoms with magnetic impurities, they could scatter the material’s conduction electrons and radically change its resistivity.

The phenomenon baffled physicists, but Kondo explained it with a model showing how conduction electrons can filter out magnetic impurities, so that the electron spins pair with the magnetic impurity spins in the opposite direction, forming a singlet.

While the Kondo impurity problem is now well understood, the Kondo lattice problem with regular lattice magnetic moments instead of random magnetic impurities is far more complicated and continues to baffle physicists. Experimental studies of the Kondo lattice problem usually involve intermetallic compounds of rare earth elements, but these materials have their own limitations.

As you move down the Periodic Table, you end up with about 70 electrons in an atom, Mak said. The electronic structure of the material becomes very complicated. It’s really hard to describe what happened even without Kondo’s interaction.

The researchers simulated the Kondo lattice by stacking ultrathin monolayers of two semiconductors: molybdenum ditelluride, tuned to the Mott insulating state, and tungsten selenide, doped with roving conduction electrons. These materials are much simpler than the bulky intermetallic compounds, and stacked with a clever twist. By rotating the layers at an angle of 180 degrees, their overlap produces a moir lattice pattern that traps individual electrons in tiny slots, similar to an egg in an egg carton.

This configuration avoids the hassle of dozens of electrons jumbling together in the rare earth elements. And instead of requiring chemistry to prepare the usual magnetic moment arrays in intermetallic compounds, the simplified Kondo lattice requires only a battery. When the voltage is applied just right, the material is ordered to form a lattice of spins, and when one cranks to a different voltage, the spins are turned off, resulting in a continuously tunable system.

Everything became simpler and more controllable, says Mak.

The researchers were able to continuously tune the electron mass and spin density, which is not possible in conventional materials, and in the process they observed that electrons coated with a spin lattice could be 10 to 20 times heavier than naked electrons. , depending on the applied voltage.

Tunability can also induce quantum phase transitions in which heavy electrons change to light electrons with, among other things, the possibility of appearance of odd metallic phases, in which the electrical resistance increases linearly with temperature. Realization of this type of transition could be very useful for understanding the phenomenology of high-temperature superconductors in copper oxide.

Our results can provide laboratory benchmarks for theorists, said Mak. In condensed matter physics, theorists try to deal with the complex problem of one trillion interacting electrons. It would be nice if they didn’t have to worry about other complications, like chemistry and materials science, in real materials. So they often study these materials with the spherical shape of the Kondo cow lattice model. In the real world you can’t make a round cow, but in our materials we have now made one for the Kondo grid.

Co-authors include doctoral students Bowen Shen and Zui Tao; postdoctoral researchers Kaifei Kang and Zhongdong Han; and researchers from the National Institute for Materials Science in Tsukuba, Japan.

This research was primarily supported by the Air Force Office of Scientific Research, the National Science Foundation, the US Department of Energy, and the Gordon and Betty Moore Foundation.

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Contact:
Beck Bowyer
Cornell University

Office: 607-220-4185

Copyright © Cornell University

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