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    Physics: Spin Liquid Simulated for the First Time


    Electrons in honeycomb structures can switch over to a very rare state, referred to as "spin liquid" by researchers. This has been demonstrated by physicists of the Universities of Würzburg and Stuttgart in a publication in the scientific journal Nature.

    Electrons usually assume one of two states in solids: Either they are freely movable, thus allowing the solid to conduct electricity. Or they are immobile, turning the material into an insulator. In this case, the mutual repulsion between the electrons is so great that they are trapped in position.

    However, there is yet another state of the electrons, which is called "spin liquid" by physicists. This state cannot be represented in a drawing and it is very hard to describe figuratively, as it is a "state without order", which is very rare in models and in nature.

    Spin liquid: Like water unwilling to freeze

    "This state is possibly best compared with water unwilling to freeze," says Professor Fakher Assaad of the Department of Theoretical Physics at the University of Würzburg. The characteristic of a spin liquid is that its electrons remain in a disordered and dynamic state even at low temperatures all the way down to absolute zero at minus 273 degrees Celsius. Usually, materials tend to increase in order with decreasing temperatures – as in the example of water where the molecules develop a regular crystal lattice in the transition to ice.

    The theoretical physicists of Würzburg and Stuttgart have now succeeded in demonstrating for the first time the occurrence of a spin liquid in a realistic model – with a "100-percent-controlled" simulation computation, as Professor Assaad puts it: "For the first time, we have described the spin liquid in a clearly realistic model, which is more than just an abstract approximation." For this reason the prestigious journal Nature did not hesitate to publish the study.

    Spin liquid can develop in graphene

    The simulation predicts: Spin liquid can be generated, for instance, in graphene. This is a material made of carbon atoms, which are arranged in a planar sheet of regular hexagons forming a honeycomb pattern. If the electrostatic interactions between the electrons in this structure are successfully influenced from outside in a targeted way, "the highly interesting electronic state called spin liquid can be generated in this material," explains Thomas Lang, who is doctoral student under the supervision of Fakher Assaad.

    This problem is at the heart of the newly formed DFG-funded Würzburg research group "Electron Correlation-Induced Phenomena in Surfaces and Interfaces with Tunable Interactions", of which Professor Ralph Claessen is the spokesperson.

    Possible starting point for a superconductor

    One of the reasons why the scientists find the spin liquid so interesting is the fact that it might serve as starting point in the development of a so-called superconductor: If this were to be achieved, electrical current would flow through the solid with zero resistance and thus without energy dissipation. This effect could be used for many exciting technological applications, such as super-fast computer chips or power supply systems that are free from energy loss.

    This is the theory. Its practical implementation is still a long way off. With state-of-the-art technology, the interactions between the electrons in a solid cannot be regulated at will from the outside. However: With their simulation, the physicists have determined for the first time under which precise conditions a spin liquid could be generated.

    Surprising discovery

    The scientists did not specifically search for a spin liquid state. "We basically look into the possible states of many-body systems in solids," explains Professor Assaad.

    His study group performs this research on various structures and materials. In the graphene model, the researchers had been analyzing possible transitions between the "metal" and the "insulator" state. "Naturally, we always hope for some unusual findings," says Professor Assaad. "To find a spin liquid state in this model, however, was a complete surprise."

    Participating scientists

    Besides Professor Fakher Assaad and his doctoral student Thomas Lang, the scientists Zi Yang Meng, Stefan Wessel and Professor Alejandro Muramatsu of the Institute for Theoretical Physics III at the Universität of Stuttgart were equally involved in the study.

    Quantum spin-liquid emerging in two-dimensional correlated Dirac fermions, Z.Y. Meng, T.C. Lang, S. Wessel, F.F. Assaad, and A. Muramatsu, Nature 464, 847-851 (8 April 2010), doi:10.1038/nature08942


    Prof. Dr. Fakher Assaad, Institute of Theoretical Physics, University of Würzburg, phone ++ 49 931 31-83652, assaad@physik.uni-wuerzburg.de

    DFG-funded Würzburg research group „Electron Correlation-Induced Phenomena in Surfaces and Interfaces with Tunable Interactions"