The ultra-thin copper oxide layer behaves like a quantum spin liquid

1. The ultra-thin copper oxide layer behaves like a quantum spin liquid The magnetic study of ultra-thin copper oxide material plates shows that at extremely low temperatures, the thinnest isolated layers lose their long magnetic field order and behave like "quantum spin liquid" - a material state in which the direction of electron spin fluctuates violently. This unexpected discovery by scientists from the Brookhaven National Laboratory of the Department of Energy (DOE) and collaborators from the Paul Scheler Institute in Switzerland may support the view that this novel condensed matter is the precursor of high-temperature superconductivity - the ability to carry current without resistance. This research just published in the Physical Review Letters hopes to enable people to have a deeper understanding of the physics of high temperature superconductivity and promote the exploration of new and better superconductors to meet the energy needs of the United States and the world. The idea of quantum spin liquid is attributed to Nobel Prize winner Philip W. Anderson, who also proposed that when copper oxide (or "cuprate") materials doped with mobile carriers, the emergence of high temperature superconductivity may be related to quantum spin liquid - that is, when atoms provide additional electrons or electron vacancies. However, some previous experimental results do not support this proposal: without doping, lanthanum copper oxide, one of the most studied cuprates, shows a form of long-range magnetic order called antiferromagnetism - even at room temperature, the spin directions of adjacent electrons alternately point in the opposite direction. But new experiments at the Brookhaven Laboratory/Scheller Institute show that when people look at layers that are thin enough, things are different. "The crystal structure of lanthanum oxide copper is layered; it consists of parallel copper oxide and lanthanum oxide sheets," explained Brookhaven physicist Ivan Bozovich, one of the main authors of the paper. "In a copper oxide plane, the interaction between spins is very strong, while their interaction with the nearest copper oxide plane (about 0.66 nanometers away) is 10000 times weaker. However, the weak interaction between layers may be sufficient to suppress fluctuations and stabilize the antiferromagnetic order." To find out whether there is an interaction between layers to suppress fluctuations, the key is to find the magnetic order in a thinner film with fewer layers and better insulation.

2. Bozovic used a special atomic layer by layer molecular beam epitaxy method developed by him to assemble lanthanum copper oxide samples with different layers. These layers are well separated and insulated to prevent any "crosstalk". The thickness is controlled at atomic precision and digitally varies up to a single copper oxide plane. This accuracy is critical to the success of the experiment. Elvezio Morenzoni and his team studied these unique samples at the Paul Scherrer Institute, and they developed a new type of energy called low energy μ The fine diagnostic technique of sub spin spectroscopy is used to detect and study the magnetism in this ultrathin layer. Magnetic measurements show that when the plates contain four or more copper oxide layers, they exhibit an antiferromagnetic sequence - just like thick blocky crystals of the same material, even at the same temperature. However, the thinner plate containing only one or two layers of copper oxide showed an unexpected result: "Although the magnetic moment or spin still exists and has roughly the same size, there is no long-term static antiferromagnetic order, even on the scale of a few nanometers. Conversely, the spin fluctuates violently and changes direction very quickly," Bozovic said. More telling is that the lower the sample temperature, the stronger this effect. Bozovic explained: "This means that these waves cannot be of thermal origin, but must be of quantum origin - quantum objects will fluctuate even at zero temperature." "In general, this experiment shows that once the copper oxide plane is well isolated and does not interact with other such layers, it actually seems to behave like some kind of quantum spin liquid at low temperatures." Bozovic said. Therefore, the idea that high temperature superconductivity originates from this quantum spin liquid may be correct. Bozovic said, "Of course, we need to do more experiments to test the meaning of our findings and its relationship with theoretical predictions."