Scientists discover an invisible phenomenon

This discovery is a step towards a much more accessible superconductivity.

It might be possible to develop superconductors operating at room temperature with a better understanding of the relationship between spin liquids and superconductivity, which would transform our daily lives.

Superconductors hold enormous technical and economic promise for applications such as high-speed aerotrains, MRI machines, efficient power lines, quantum computing, and other technologies. However, their usefulness is limited because superconductivity requires extremely low temperatures. It is very difficult to integrate them with modern technology due to this demanding and costly requirement.

The electrical resistance of a superconductor has a specific critical temperature beyond which it drops sharply to zero, unlike an ordinary metallic conductor, whose resistance gradually decreases as the temperature drops, even down to near absolute zero.

The search for superconductors that do not require such low temperatures is the primary objective of current research in superconductivity. The mechanism by which these superconductors work is the biggest mystery in this field, to which no one has an answer. Understanding the process that creates high temperature superconductivity would allow for more practical applications.

A recent study by scientists at Israel’s Bar-Ilan University and recently published in the journal Nature makes progress in solving this lingering mystery. Using a scanning magnetic microscope SQUID (superconducting quantum interference device), the researchers photographed a phenomenon that was previously invisible to other techniques.

Scientists were surprised when high temperature superconductors were first discovered. Scientists had assumed that good superconductivity would be found in metals. Contrary to predictions, it was found that insulating ceramic materials are the best superconductors.

The discovery of properties common to these ceramic materials can help identify the origin of their superconductivity and improve critical temperature control. One of these properties is that the electrons of these materials strongly resist each other. So they can’t move freely. Rather, they are trapped inside a periodic lattice structure.

Electrons have two defining properties: their charge (a moving charge produces an electric current) and their spin. Spin is the quantum property of electrons responsible for their magnetic properties. It is as if a small bar magnet was attached to each electron. In ordinary materials, charge and spin are “embedded” in electrons and cannot be separated.

However, in special quantum materials called “quantum spin liquids”, interactions between electrons allow for a unique phenomenon whereby each electron is split into two particles, one charged (but spinless) and one with spin (and spinless) . Such quantum spin liquids can exist in high temperature superconductors and, in fact, their existence could explain why the superconductivity of these materials is so good.

The challenge is that these spin liquids are “invisible” to conventional measurements. Even when we suspect that a material may be a spin liquid, no experiment could verify it or probe its nature. This is similar to dark matter which does not interact with light and is therefore very difficult to detect.

The current study, led by Professor Beena Kalisky and doctoral student Eylon Persky from Bar-Ilan University’s Department of Physics and their collaborators, is an important step towards developing a method to study spin liquids. The researchers examined the properties of a spin liquid by having it interact with a superconductor. They used an engineered material composed of alternating atomic layers of the superconductor and the candidate spin liquid.

“Unlike spin liquids which generate no signal, superconductors have clear and easy to measure magnetic signatures. So we were able to study the properties of the spin liquid by measuring the small changes it generated in the superconductor,” says Persky. The researchers used a scanning SQUID – an extremely sensitive magnetic sensor capable of detecting both magnetism and superconductivity – to study the properties of the heterostructure.

“We observed vortices created in the superconductor. These vortices are circulating electric currents, each containing a quantum of magnetic flux. The only way to create such vortices is to apply a magnetic field, but in our case, the vortices created spontaneously,” Kalisky explains. This observation showed that the material itself generated a magnetic field. The biggest surprise came when this field did not show up in a direct measurement. “Amazingly, we found that the magnetic field created by the material was invisible for direct magnetic measurement,” Kalisky adds.

The results revealed a “hidden” magnetic phase, which was exposed in the experiment by interaction with the superconducting layer. Working with groups from Bar-Ilan University, the Technion, the Weizmann Institute, the University of California at Berkeley, and the Georgia Institute of Technology, the researchers concluded that this magnetic phase was likely the direct result of the relationship between the spin liquid layer and the superconducting layer. The hidden magnetism is the result of spin-charge separation in the spin liquid. The superconductor reacts to this magnetism and this generates vortices without the need for a “real” magnetic field.

This is in fact the first direct observation of the link between these two phases of matter. These results provide access to elusive properties of spin liquids, such as interactions between electrons. The results also open the door to the engineering of additional layered materials, through which the relationship between superconductivity and other electronic phases could be investigated. Further studies on the relationship between spin liquids and superconductivity could lead to the design of superconductors that work at room temperature, which, in turn, would change our daily lives.

Reference: “Magnetic memory and spontaneous vortices in a van der Waals superconductor” by Eylon Persky, Anders V. Bjørlig, Irena Feldman, Avior Almoalem, Ehud Altman, Erez Berg, Itamar Kimchi, Jonathan Ruhman, Amit Kanigel and Beena Kalisky, July 27 2022, Nature.
DOI: 10.1038/s41586-022-04855-2

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