No abstract

The Global Network of Optical Magnetometers to search for Exotic physics (GNOME) is a network of geographically separated, time-synchronized, optically pumped atomic magnetometers that is being used to search for correlated transient signals heralding exotic physics. The GNOME is sensitive to nuclear-and electron-spin couplings to exotic fields from astrophysical sources such as compact dark-matter objects (for example, axion stars and domain walls). Properties of the GNOME sensors such as sensitivity, bandwidth, and noise characteristics are studied in the present work, and features of the network's operation (e.g., data acquisition, format, storage, and diagnostics) are described. Characterization of the GNOME is a key prerequisite to searches for and identification of exotic physics signatures.

-We demonstrate that laser ablation of a gold target immersed in superfluid and normal fluid helium leads to the formation of elongated gold nano-fragments. In the superfluid phase these nano-fragments aggregate into filaments with extremely large aspect ratios displaying metallic electric conductivity. We attribute this unusual structure to the coalescence of gold particles trapped on quantized vortices. Our observations suggest new ways to visualize the structure of quantized vortex bundles and a new approach for producing centimeter-long metal nanowires. Superfluid4 He (HeII) exhibits the outstanding macroscopic quantum property of superfluidity when cooled below 2.172 K. Associated with this property is the appearance of one-dimensional structures around which the fluid motion has a circulation that is quantized in units of h/m He . Such vortex lines have been studied in detail for more than 50 years as reported in the comprehensive review by Donnelly [1]. More recently the flow of superfluid and normal fluid helium has received renewed attention in the context of quantum turbulence [2,3].Because of the strong pressure gradient in the vicinity of their core, quantized vortices can bind foreign particles. This fact has been used for the visualization of vortex bundles by the electrostatic extraction and projection of vortex-attached electrons onto a phosphor screen [4]. The pressure gradients are strong enough to attract also heavier particles, such as 3 He or other guest atoms and clusters. Because of the one-dimensional structure of the vortex line one expects [5] the formation of monoatomic chains or high-aspect-ratio filaments following the coalescence of particles on the vortex core. The formation of macroscopic filament-like structures by impurity particles suspended in superfluid helium was indeed reported by several authors [6][7][8][9] as well as the lining-up of micronsized solid hydrogen clusters injected into normal fluid He, (a) E-mail: peter.moroshkin@unifr.ch followed by a transition to the superfluid phase [10,11]. No such unidimensional structures appear in normal fluid helium. All previous studies were limited to visual observations in the He bath only, since the filament materials were either hydrogen or alkali metals which oxidize in contact with air. The experimental results presented below suggest a new approach to this problem. By doping liquid helium with a chemically inert material that keeps its shape and properties after the helium evaporation, we were able to form solid filaments and analyze them by means of optical and electron microscopy. In doped HeII we observe the production of centimeter-long gold filaments, whose analysis reveals a very rich substructure at the nanometer scale and the formation of metallic binding. On the other hand, when the doping is done in normal fluid helium we obtain only chaotically oriented gold nanonetworks which (because of their small size) escaped observation in previous studies. Our observations open new ways for visualizing the pattern...

Ultralight bosons such as axion-like particles are viable candidates for dark matter. They can form stable, macroscopic field configurations in the form of topological defects that could concentrate the dark matter density into many distinct, compact spatial regions that are small compared with the Galaxy but much larger than the Earth. Here we report the results of the search for transient signals from the domain walls of axion-like particles by using the global network of optical magnetometers for exotic (GNOME) physics searches. We search the data, consisting of correlated measurements from optical atomic magnetometers located in laboratories all over the world, for patterns of signals propagating through the network consistent with domain walls. The analysis of these data from a continuous month-long operation of GNOME finds no statistically significant signals, thus placing experimental constraints on such dark matter scenarios.

The discrimination of immobilised superparamagnetic iron oxide nanoparticles (SPIONs) against SPIONs in fluid environments via their magnetic relaxation behaviour is a powerful tool for bio-medical imaging.Here we demonstrate that a gradiometer of laser-pumped atomic magnetometers can be used to record accurate time series of the relaxing magnetic field produced by pre-polarised SPIONs. We have investigated dry in vitro maghemite nanoparticle samples with different size distributions (average radii ranging from 14 to 21 nm) and analysed their relaxation using the Néel-Brown formalism. Fitting our model function to the magnetorelaxation (MRX) data allows us to extract the anisotropy constant K and the saturation magnetisation M S of each sample. While the latter was found not to depend on the particle size, we observe that K is inversely proportional to the (time-and size-) averaged volume of the magnetised particle fraction. We have identified the range of SPION sizes that are best suited for MRX detection considering our specific experimental conditions and sample preparation technique.