Search For The Spin- And Velocity-Dependent Exotic Interaction Using A Magnetic Force Microscope

The Standard Model of particle physics（SM）describes all the elementary particles discovered and the three fundamental interactions except gravity.However,there are still some fundamental problems that it cannot explain.For example,the SM does not include gravity,it cannot explain the nature of dark matter and dark energy,and the strong CP problem.Theories beyond the SM usually introduce new bosons,which may also be candidates for dark matter.New interactions between fermions may arise from exchanging these particles.Under the framework of quantum field theory,there are sixteen types of interaction potentials generated between fermions by exchanging any generic spin-0 or spin-1 boson assuming rotational invariance.In this thesis,one of the interactions,which is spin-and velocity-dependent,is investigated.The interaction can be generated by exchanging spin-1 bosons such as generic Z’bosons.A new method based on magnetic force microscopy is proposed to search for the interaction between spin-polarized electrons in a magnetic tip and nucleons in a nucleon source at the micrometer range.The force is measured by a cantilever with a magnetic tip.The magnetic tip is coated with hard magnetic Co Cr alloy to provide spin-polarized electrons.The nucleon source is made of alternative high density（gold）and low density（silicon dioxide）materials to create a spatially modulated signal if the new interaction exists.The magnetic tip is driven to vibrate at the resonance frequency of the cantilever,and its displacement is measured by a home-made fiber laser interferometer.Since the new interaction is proportional to the relative velocity between spin-polarized electrons and nucleons,the cantilever oscillation amplitude changes depending on the density of the material underneath the tip.We search for the exotic interaction by looking for the amplitude variation when the magnetic tip scans over different areas of nucleon density.The characterization of the mechanical and magnetic properties of the magnetic tips are described in details.The resonance frequency and the quality factor Q are measured from frequency response curve of the cantilever.The spring constant is derived from the finite element simulation.The magnetization of the Co Cr magnetic film is found to be in the plane of the film by measuring its hysteresis loop with a vibration sample magnetometer.The distribution of the magnetic moments of Co Cr film is obtained through micromagnetic simulation,and then the magnitude of the exotic interaction is calculated based on the distribution of the magnetic moments.The magnetism of the tip was verified by performing a magnetic force microscopy measurement on a periodic magnetized structures before and after the experimental runs.The density-modulated nucleon source structures were successfully prepared with microfabrication,and its surface corrugation caused by different density materials is suppressed to below 2 nm.In order to reduce the influence of roughness and defects of the nucleon source surface,the data are taking on a plane at a constant distance between the tip and nucleon source,and images of the cantilever vibraton amplitude are obtained.No exotic signal is observed related to the density modulation,thus we derive limits on the coupling constant by using maximum likelihood estimation.The experiment sets a limit on the electron-nucleon coupling constant |g_A^eg_V^N|≤9×10^-15 for 15 μm≤λ≤180 μm at the 95% confidence level.The results provide more experimental informations for the investigation in new particles,new interactions and the extension of the standard model.