Scientists discover an invisible phenomenon

The discovery is a step toward much more accessible superconductivity.

With further information about the relationship between spin fluid and superconductivity, it may be possible to develop superconductors that operate at room temperature, which will change our daily lives.

Superconductors offer enormous technical and economic promise for applications such as high-speed hovertrains, MRI machines, efficient power lines. quantum computing, and other technologies. However, their usefulness is limited because superconductivity requires extremely low temperatures. Due to this demanding and costly requirement, integrating them with modern technology is extremely challenging.

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

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

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

Scientists were perplexed when superconductors were initially exposed to high temperatures. Scientists had assumed that metals would have good superconductivity. Contrary to predictions, it was found that insulating ceramic materials are the best superconductors.

Finding properties common to these ceramic materials may help identify where their superconductivity originated and improve control over critical temperatures. One such property is that the electrons in these substances strongly oppose each other. Thus they are unable to move independently. Instead they are trapped inside a periodic lattice structure.

Electrons have two defining properties: their charge (a moving charge results in 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 is attached to each electron. In ordinary materials, charge and spin are “inherent” to electrons and cannot be separated.

However, in special quantum materials called “quantum spin fluids”, interactions between electrons enable a unique phenomenon whereby each electron breaks up into two particles, one with charge (but no spin) and one with spin (and no charge). Such quantum spin fluids can exist in high-temperature superconductors and, in fact, their existence may explain why superconductivity is so good in these materials.

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

The current study, conducted by Professor Bina Kaliski from the Department of Physics at Bar-Ilan University, and doctoral student Alon Persky and colleagues, is an important step toward the development of a method to study spin fluids. The researchers investigated the properties of a spin liquid interacting with a superconductor. They used an engineered material composed of alternating atomic layers of the superconductor and candidate spin liquid.

“Unlike spin fluids that produce no signal, superconductors have clear magnetic signatures that are easy to measure. Therefore, we were able to study the properties of the spin liquid by measuring the tiny changes produced in the superconductor,” Persky says. The researchers used a scanning SQUID – an extremely sensitive magnetic sensor capable of detecting both magnetism and superconductivity – to probe the properties of the heterostructure.

“We have seen vortices created in superconductors. These vortices are transmitting electric currents, each carrying an amount of magnetic flux. The only way to create such vortices is to apply a magnetic field, but in our case, The vortices were created automatically,” Kalinsky explains. This observation showed that the material itself generated a magnetic field. The biggest surprise came when the region did not show itself in direct measurements. “Astonishingly, we found that the magnetic field created by the material was invisible to direct magnetic measurements,” Kalisky says.

The results point to a “hidden” magnetic phase, which was uncovered in the experiment through interactions with the superconducting layer. Collaborating with groups from Bar-Ilan University of Technology, the Weisman Institute, the University of California, Berkeley and the Georgia Institute of Technology, the researchers concluded that this magnetic phase was probably a direct result of the connection between the spin liquid layer. and superconducting layer. The hidden magnetization is the result of spin-charge separation in the spin liquid. The superconductor reacts to this magnetism and it generates vortices without the need for a “real” magnetic field.

In fact, this is the first direct observation of the relationship between these two phases of matter. These results provide access to the properties of elusive spin fluids, such as interactions between electrons. The results also open the door to additional layered materials engineering, through which the relationship between superconductivity and other electronic phases can be studied. Further study of the relationship between spin fluid and superconductivity could help design superconductors that operate at room temperature, and this, in turn, will change our daily lives.

Reference: “Magnetic memory and spontaneous vortices in a van der Waals superconductor” Alon Persky, Anders v. Björlig, Irena Feldman, Avior Almoelem, Ehud Altman, Erez Berg, Itamar Kimchi, Jonathan Ruhmann, Amit Kanigel and Bina Kaliski, 27 July 2022, Nature,
DOI: 10.1038/s41586-022-04855-2

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