Collaborative Research: Collective Mode Spectroscopy in Unconventional Superconductors

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Non - Technical Since the discovery of quantum mechanics, understanding the properties of solids has progressed steadily, with many well-known examples of important applications. One class of materials that has proved especially difficult to treat is the so-called strongly correlated metals, which are typically intermetallic compounds involving elements from the rare earth and actinide (uranium) groups of the periodic table. In these elements some of the electrons (designated d and f) are closely associated with their respective nuclei but not so closely that they do not participate in binding the atoms together and electrical transport. During conduction, in which such electrons move from atom to atom, they spend a considerable time orbiting the d and f atoms and when another electron attempts to move in the immediate vicinity of one that is already present the two must adjust (correlate) their motion so that the electrostatic repulsion is minimized; this correlation turns out to be difficult to treat mathematically and hence presents a challenge. These materials are often superconducting and among those that are it is thought that some may exhibit a new kind of superconductivity, which has been termed unconventional superconductivity. All superconductors are characterized by what is called an order parameter and, if suitably excited, this order parameter can oscillate (vibrate). The nature of the vibrations for ordinary and unconventional superconductors differs radically, and characterizing this difference should permit an unambiguous characterization of the key characteristic of the new superconductors. Electromagnetic waves (microwaves) with appropriate frequencies will excite these vibrations and allow such a characterization; establishing the nature of the superconductivity using the microwave probe is the goal of this research. The educational component of this research lies in the training of a future high-tech work force; the program will equip post docs and graduate students with important laboratory skills involving materials preparation, advanced microwave techniques, and cryogenics.TechnicalThis proposal is directed at the detection and characterization of order-parameter collective modes in strongly correlated inter-metallic compounds that are simultaneously superconducting. The strongly correlated materials are of interest because the motion of the individual electrons involves a highly-coordinated response of the remaining electrons, and in those materials that are superconducting the pairing is thought to arise from an unconventional mechanism. Conventional superconductors (most superconductors) pair in a state with zero angular momentum and overwhelmingly via an attraction originating from electron-phonon interactions. It is those superconductors that pair with non-zero angular momentum (or an antisymmetric behavior under time inversion) that are termed unconventional, and the attraction leading to the pairing is suspected to be electronic in character. Collective modes can be visualized as finite frequency "vibrations" of the associated order parameter (the gap function); for unconventional order parameters the modes are generally anisotropic and multiple modes exist. Establishing the presence of unconventional pairing presently rests on indirect thermodynamic or transport evidence and most reports are greeted with some skepticism, a situation that persists (in some materials for more than a decade), and is in some sense a crisis; however microwave collective mode studies are expected to have the required selectivity to remove ambiguity. The goal of the present proposal is to use microwave absorption to probe for collective modes in some strongly correlated materials where the indirect evidence for unconventional pairing appears to be compelling, but for which the precise form of the order parameter remains controversial.