Advances in Casimir-Polder Interactions Between Atoms and Substrates

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Subtle quantum mechanical effects can produce unusual forces between objects. The quantum uncertainty associated with electric and magnetic field fluctuations, for example, can produce unusual forces between molecules, or between molecules and surfaces. These effects, generally known as Casimir interactions, were discovered long ago and thought to generate very tiny forces between macroscopic objects. They are now understood to be especially important for nanometer-sized objects, such as nano-machines and macro-molecules, and so are becoming the focus of much research on sensor technology and precision measurement physics. Accurate calculation of Casimir forces from first principles is a difficult challenge, as it involves delicate averaging of fluctuating fields, with strange consequences; the forces can be either attractive or repulsive, and can either contribute to or detract from the system's entropy. This project will enhance our understanding of Casimir forces through explicit calculations focused on systems of current experimental interest. Results of this research have applications to chemistry, biology, nanoscience, and atomic and nuclear physics. This work is being carried out in collaboration with researchers world-wide, including senior scientists, postdocs, graduate students, undergraduate students, and high-school students.This project seeks to advance our understanding of Casimir (quantum-fluctuation) energies and forces at zero and finite temperatures, for example, by using Green's function and other analytic techniques to compute Casimir self-entropies for a spherical model of an atom. This requires isolating, regulating, and renormalizing the free energy, processes that also have to be done for the energy associated with quantum fields external and internal to surfaces, either flat or curved. It has recently been discovered that the negative entropy of interaction between a perfectly conducting sphere and a perfectly conducting plate is exactly balanced by the positive self-entropy of the sphere. Negative entropies are not unrelated to repulsive forces, which can be realized between atoms and other bodies in vacuum. The nonadditivity of Casimir forces plays an important role in these phenomena. Although most work in this field has concentrated on the normal force between bodies, lateral forces are also being explored, along with Casimir torque and Casimir friction, with the hope that these latter two phenomena may be soon seen in laboratory experiments. Applications, such as to the freezing of ice on surfaces, and properties of new materials such as phosphorene, are being studied in collaboration with a diverse international group of students and advanced researchers.