What is a quantum spin fluid

A new material for quantum spin liquids

In 1973 the physicist and later Nobel Prize winner Philip W. Anderson proposed a bizarre state of matter: the quantum spin fluid (QSL). Unlike the everyday fluids we know, QSL actually has to do with magnetism - and magnetism has to do with spin.

Disordered electron spin creates QSLs

What makes a magnet? It was a long-standing mystery, but today we finally know that magnetism arises from a special property of subatomic particles, like electrons. This property is known as "spin," and the best, albeit totally inadequate, way of imagining it is like a children's spinning top toy.

What is important for magnetism is that the spin transforms each of the billions of electrons in a material into a tiny magnet with its own magnetic "direction" (think of the north and south poles of a magnet). But the electron spins are not isolated; they interact with each other in different ways until they stabilize in different magnetic states, which gives the material they belong to magnetic properties.

In a conventional magnet, the interacting spins stabilize and the magnetic directions of the individual electrons align. This leads to a stable formation.

But in a so-called “frustrated” magnet, the electron spins cannot stabilize in the same direction. Instead, they fluctuate constantly like a liquid - hence the name “quantum spin liquid”.

Quantum Spin Liquids in Future Technologies

The exciting thing about QSLs is that they can be used in a number of applications. Since there are different variants with different properties, QSLs can be used in quantum computers, telecommunications, superconductors, spintronics (a variant of electronics that uses electron spin instead of electricity), and a variety of other quantum-based technologies.

But before we can use them, we must first gain a solid understanding of the SCI states. To do this, women scientists need to find ways to produce QSLs on request - a task that has proven difficult so far, as few materials are offered as QSL candidates.

A complex material could solve a complex problem

When it was published in PNAS, scientists led by Péter Szirmai and Bálint Náfrádi in László Forró's laboratory at the EPFL Faculty of Basic Sciences successfully created and examined a QSL in a very original material called EDT-BCO. The system was designed and synthesized by Patrick Batail's group at the Université d'Angers (CNRS).

The structure of EDT-BCO makes it possible to create a QSL. The electron spins in the EDT-BCO form triangularly organized dimers, each of which has a spin 1/2 magnetic moment, which means that the electron has to completely rotate twice to return to its original configuration. The layers of the spin 1/2 dimers are separated by a sublattice of carboxylate anions that is centered by a chiral bicyclooctane. The anions are called “rotors” because they have conformational and rotational degrees of freedom.

The unique rotor component in a magnetic system makes the material something special among the QSL candidates, which represent a new family of materials. “The subtle disruption caused by the rotor components introduces a new grip on the spin system,” says Szirmai.

The scientists and their employees used an arsenal of methods to investigate the EDT-BCO as a candidate for a QSL material: calculations of density functional theory, high-frequency electron spin resonance measurements (a trademark of the Forró laboratory), nuclear magnetic resonance and muon spin spectroscopy. All of these techniques examine the magnetic properties of EDT-BCO from different angles.

All techniques confirmed the lack of long-range magnetic order and the emergence of a QSL. In short, EDT-BCO officially joins the limited line of QSL materials and takes us one step further into the next generation of technologies. As Bálint Náfrádi puts it:

"In addition to the excellent demonstration of the QSL state, our work is of great relevance as it provides an instrument for obtaining additional QSL materials through specially designed functional rotor molecules." Bálint Náfrádi