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We search for dark matter in the form of axionlike particles (ALPs) in the mass range $5.576741\mathrm{neV}/{\mathrm{c}}^2-5.577733\mathrm{neV}/{\mathrm{c}}^2$by probing their possible coupling to fermion spins through the ALP field gradient. This is achieved by performing proton nuclear magnetic resonance spectroscopy on a sample of methanol as a technical demonstration of the Cosmic Axion Spin Precession Experiment Gradient (CASPEr-Gradient) Low-Field apparatus. Searching for spin-coupled ALP dark matter in this mass range with associated Compton frequencies in a 240 Hz window centered at 1.348570 MHZ resulted in a sensitivity to the ALP-proton coupling constant of $g_{\mathrm{ap}}\approx 3\times {10}^{-2}{\mathrm{GeV}}^{-1}$. This narrow-bandwidth search serves as a proof-of-principle and a commissioning measurement, validating our methodology and demonstrating the experiment’s capabilities. CASPEr-Gradient Low-Field will probe the mass range from $4.1\mathrm{peV}/{\mathrm{c}}^2$to $17\mathrm{neV}/{\mathrm{c}}^2$with hyperpolarized samples to boost the sensitivity beyond the astronomical limits. 

We investigate the morphologies of the Ge(001) surface that are produced by bombardment with a normally incident, broad argon ion beam at sample temperatures above the recrystallization temperature. Two previously observed kinds of topographies are seen, i.e., patterns consisting of upright and inverted rectangular pyramids, as well as patterns composed of shallow, isotropic basins. In addition, we observe the formation of an unexpected third type of pattern for intermediate values of the temperature, ion energy, and ion flux. In this type of intermediate morphology, isolated peaks with rectangular cross-sections stand above a landscape of shallow, rounded basins. We also extend past theoretical work to include a second-order correction term that comes from the curvature dependence of the sputter yield. For a range of parameter values, the resulting continuum model of the surface dynamics produces patterns that are remarkably similar to the intermediate morphologies we observe in our experiments. The formation of the isolated peaks is the result of a term that is not ordinarily included in the equation of motion, a second-order correction to the curvature dependence of the sputter yield.