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Spin Transport in a Mott Insulator of Ultracold Fermions
Superconductivity is a phenomenon in materials whereby electron pairs can flow freely without resistance. As a consequence, no energy is lost while electrical current passes through the superconductor. The benefits, therefore, of superconducting materials which operate at room temperature are countless, and range from revolutionizing the electrical power transmission industry, to providing sweeping improvements in the world of transportation. While there are no known materials which superconduct at room temperature, there are a class of materials, known as the cuprates, which superconduct at some of the highest temperatures known to man.
By understanding the mechanisms underlying the phenomenon of superconductivity in the cuprates, researchers hope to be able to design new materials with even higher superconducting transition temperatures. However, such an understanding has been impeded by the complexities of these materials, especially the strong interactions between the electrons. It is not known, for instance, what separate roles the spin or the charge of the electron play in determining the electrical conduction properties of the cuprates, nor how these degrees of freedom are intertwined.
A research group led by Martin Zwierlein at MIT has recently developed a new method to study transport in a model system for the cuprates, where ultracold atoms stand in for the electrons, a technique referred to as quantum simulation. In this quantum simulator, interfering lasers, which form standing waves in space, generate an artificial crystal, known as an optical lattice, in which atoms are able to hop around between neighboring lattice sites, and can interact with each other on the same site, a nearly ideal realization of a simple model for the behavior of the electrons in the cuprates, the Fermi-Hubbard model.
The researchers then confined roughly 400 atoms in a square box in this optical lattice, and tilted it in one direction to observe how the atoms flow. With the ability to detect each individual atom in the sample, something not possible in real materials, they were able to measure the spin current in this system, in order to understand how it moves differently than the charge. They found that increasing the strength of the interactions in the system allowed them to suppress the overall spin flow. By measuring the spin current as a function of the inter-particle interactions, the researchers were able to obtain the spin conductivity, a fundamental property of the system that is difficult to calculate on classical computers, and which is challenging to measure in the cuprates, where impurities and other complications limit one’s ability to control spin currents. The results of this experiment therefore represent a major step towards understanding the role that spin plays in electrical conduction in the cuprates.
Read the Publication in Science.
Read the “Science Perspective” in Science Mag.
Read the MIT News Article.