This illustration depicts the magnetic excitations of cobalt phthalocyanine molecules, where entangled electrons propagate into triplons. Image: Jose Lado/Aalto University.
This illustration depicts the magnetic excitations of cobalt phthalocyanine molecules, where entangled electrons propagate into triplons. Image: Jose Lado/Aalto University.

A team from Aalto University and the University of Jyväskylä, both in Finland, have created an artificial quantum magnet that features a quasiparticle made of entangled electrons known as a triplon.

Triplons are tricky little things. Experimentally, they’re exceedingly difficult to observe. Even then, researchers usually conduct the tests on macroscopic materials, in which measurements are expressed as an average across the whole sample.

That’s where designer quantum materials offer a unique advantage, says Robert Drost, a research fellow at Aalto University and first author of a paper on this work in Physical Review Letters. Such designer quantum materials let researchers create phenomena not found in natural compounds, ultimately leading to the realization of exotic quantum excitations.

“These materials are very complex,” says Peter Liljeroth, head of the Atomic Scale physics research group at Aalto University. “They give you very exciting physics, but the most exotic ones are also challenging to find and study. So, we are trying a different approach here by building an artificial material using individual components.”

Quantum materials are governed by the interactions between electrons at the microscopic level. These electronic correlations can lead to unusual phenomena like high-temperature superconductivity or complex magnetic states, and quantum correlations can give rise to new electronic states.

In the case of two electrons, there are two entangled states known as singlet and triplet states. Supplying energy to the electron system can excite it from the singlet to the triplet state. In some cases, this excitation can propagate through a material via an entanglement wave known as a triplon. These excitations are not present in conventional magnetic materials and measuring them has remained an open challenge in quantum materials.

In this study, the team used a small organic molecule known as cobalt phthalocyanine to create an artificial quantum material with unusual magnetic properties. Each cobalt phthalocyanine molecule contains two frontier electrons.

“Using very simple molecular building blocks, we are able to engineer and probe this complex quantum magnet in a way that has never been done before, revealing phenomena not found in its independent parts,” Drost says. “While magnetic excitations in isolated atoms have long been observed using scanning tunnelling spectroscopy, it has never been accomplished with propagating triplons.

“We use these molecules to bundle electrons together, we pack them into a tight space and force them to interact. Looking into such a molecule from the outside, we will see the joint physics of both electrons. Because our fundamental building block now contains two electrons, rather than one, we see a very different kind of physics.”

The team monitored magnetic excitations first in individual cobalt phthalocyanine molecules and then in larger structures like molecular chains and islands. By starting with the very simple and working towards increasing complexity, the researchers hope to understand emergent behaviour in quantum materials. In the present study, they were able to demonstrate that the singlet-triplet excitations of their building blocks can traverse molecular networks as triplons.

“We show that we can create an exotic quantum magnetic excitation in an artificial material,” says Jose Lado, who heads the Correlated Quantum Materials research group at Aalto University and is a co-author of the paper. “This strategy shows that we can rationally design material platforms that open up new possibilities in quantum technologies.”

By extending their approach towards more complex building blocks, the researchers plan to design other exotic magnetic excitations and ordering in quantum materials. Rational design from simple ingredients will not only enhance understanding of the complex physics of correlated electron systems but also establish new platforms for designer quantum materials.

This story is adapted from material from Aalto University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.