Semiconductor lattice marries electrons and magnetic moments

Date:
March 22, 2023
Source:
Cornell University
Summary:
A model system created by stacking a pair of monolayer
semiconductors is giving physicists a simpler way to study
confounding quantum behavior, from heavy fermions to exotic quantum
phase transitions.


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FULL STORY ==========================================================================
A model system created by stacking a pair of monolayer semiconductors is
giving physicists a simpler way to study confounding quantum behavior,
from heavy fermions to exotic quantum phase transitions.


==========================================================================
The group's paper, "Gate-Tunable Heavy Fermions in a Moire' Kondo
Lattice," published March 15 in Nature. The lead author is postdoctoral
fellow Wenjin Zhao in the Kavli Institute at Cornell.

The project was led by Kin Fai Mak, professor of physics in the College
of Arts and Sciences, and Jie Shan, professor of applied and engineering physics in Cornell Engineering and in A&S, the paper's co-senior
authors. Both 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 address 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 substituting
even a small number of atoms with magnetic impurities, they could scatter
the material's conduction electrons and radically alter its resistivity.

That phenomenon puzzled physicists, but Kondo explained it with a model
that showed how conduction electrons can "screen" the magnetic impurities,
such that the electron spin pairs with the spin of a magnetic impurity
in opposite directions, forming a singlet.

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

"When you move all the way down to the bottom of the Periodic Table,
you end up with something like 70 electrons in an atom," Mak said. "The electronic structure of the material becomes so complicated. It is very difficult to describe what's going on even without Kondo interactions."
The researchers simulated the Kondo lattice by stacking ultrathin
monolayers of two semiconductors: molybdenum ditelluride, tuned to a
Mott insulating state, and tungsten diselenide, which was doped with
itinerant conduction electrons.

These materials are much simpler than bulky intermetallic compounds,
and they are stacked with a clever twist. By rotating the layers at
a 180-degree angle, their overlap results in a moire' lattice pattern
that traps individual electrons in tiny slots, similar to eggs in an
egg carton.

This configuration avoids the complication of dozens of electrons jumbling together in the rare earth elements. And instead of requiring chemistry
to prepare the regular array of magnetic moments in the intermetallic compounds, the simplified Kondo lattice only needs a battery. When a
voltage is applied just right, the material is ordered into forming a
lattice of spins, and when one dials to a different voltage, the spins
are quenched, producing a continuously tunable system.

"Everything becomes much simpler and much more controllable," Mak said.

The researchers were able to continuously tune the electron mass and
density of the spins, which cannot be done in a conventional material,
and in the process they observed that the electrons dressed with the
spin lattice can become 10 to 20 times heavier than the "bare" electrons, depending on the voltage applied.

The tunability can also induce quantum phase transitions whereby heavy electrons turn into light electrons with, in between, the possible
emergence of a "strange" metal phase, in which electrical resistance
increases linearly with temperature. The realization of this type
of transition could be particularly useful for understanding the high-temperature superconducting phenomenology in copper oxides.

"Our results could provide a laboratory benchmark for theorists,"
Mak said. "In condensed matter physics, theorists are trying to deal
with the complicated problem of a trillion interacting electrons. It
would be great if they don't have to worry about other complications,
such as chemistry and material science, in real materials. So they often
study these materials with a 'spherical cow' Kondo lattice model. In the
real world you cannot create a spherical cow, but in our material now
we've created one for the Kondo lattice." 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.

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

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========================================================================== Story Source: Materials provided by Cornell_University. Original written
by David Nutt, courtesy of the Cornell Chronicle. Note: Content may be
edited for style and length.


========================================================================== Journal Reference:
1. Wenjin Zhao, Bowen Shen, Zui Tao, Zhongdong Han, Kaifei Kang, Kenji
Watanabe, Takashi Taniguchi, Kin Fai Mak, Jie Shan. Gate-tunable
heavy fermions in a moire' Kondo lattice. Nature, 2023; DOI:
10.1038/s41586- 023-05800-7 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2023/03/230322140331.htm

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