Physicists have created a new ultra-thin, two-layer material with quantum
properties that normally require rare earth compounds. This material, which
is relatively easy to make and does not contain rare earth metals, could
provide a new platform for quantum computing and advance research into
unconventional superconductivity and quantum criticality.
The researchers showed that by starting from seemingly common materials, a
radically new quantum state of matter can appear. The discovery emerged from
their efforts to create a quantum spin liquid which they could use to
investigate emergent quantum phenomena such as gauge theory. This involves
fabricating a single layer of atomically thin tantalum disulphide, but the
process also creates islands that consist of two layers.
When the team examined these islands, they found that interactions between
the two layers induced a phenomenon known as the Kondo effect, leading to a
macroscopically entangled state of matter producing a heavy-fermion system.
The Kondo effect is an interaction between magnetic impurities and electrons
that causes a material's electrical resistance to change with temperature.
This results in the electrons behaving as though they have more mass,
leading these compounds to be called heavy fermion materials. This
phenomenon is a hallmark of materials containing rare earth elements.
Heavy fermion materials are important in several domains of cutting-edge
physics, including research into quantum materials. "Studying complex
quantum materials is hindered by the properties of naturally occurring
compounds. Our goal is to produce artificial designer materials that can be
readily tuned and controlled externally to expand the range of exotic
phenomena that can be realized in the lab," says Professor Peter Liljeroth.
For example, heavy fermion materials could act as topological
superconductors, which could be useful for building qubits that are more
robust to noise and perturbation from the environment, reducing error rates
in quantum computers. "Creating this in real life would benefit enormously
from having a heavy fermion material system that can be readily incorporated
into electrical devices and tuned externally," explains Viliam Vaňo, a
doctoral student in Liljeroth's group and the paper's lead author.
Although both layers in the new material are tantalum sulfide, there are
subtle but important differences in their properties. One layer behaves like
a metal, conducting electrons, while the other layer has a structural change
that causes electrons to be localized into a regular lattice. The
combination of the two results in the appearance of heavy fermion physics,
which neither layer exhibits alone.
This new heavy fermion material also offers a powerful tool for probing
quantum criticality. "The material can reach a quantum-critical point when
it begins to move from one collective quantum state to another, for example,
from a regular magnet towards an entangled heavy fermion material," explains
Professor Jose Lado. "Between these states, the entire system is critical,
reacting strongly to the slightest change, and providing an ideal platform
to engineer even more exotic quantum matter."
"In the future, we will explore how the system reacts to the rotation of
each sheet relative to the other and try to modify the coupling between the
layers to tune the material towards quantum critical behavior," says
Liljeroth.
Reference:
Peter Liljeroth, Artificial heavy fermions in a van der Waals
heterostructure, Nature (2021).
DOI: 10.1038/s41586-021-04021-0.
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Physics