We report a fabrication process and characterization measurements of single crystal silicon micro-wire resonators to be used for study of quantum turbulence in superfluid $$^{4}$$ He at millikelvin temperatures. Our devices are single standing goal-post-shaped silicon structures with a width and height of the order of 7 microns. Vapour-deposited superconducting aluminium film of 120 nm thickness is used for magneto-motive drive of the resonators. In the window of each chip, two such devices of different dimensions are placed 30 µm to 1 mm apart, with the intent to study interaction due to pinned quantized vortices. With resonant frequencies below 10 kHz, the devices reach quality factors of $$\approx 2 \times 10^4$$ in cold helium vapour.

Past investigations of thermal counterflow in He II were mostly conducted in pipes/channels of constant cross-sections, which are often unduly influenced by the presence of walls. We devise and carry out an experiment using a spherically symmetric setup to study unbounded counterflow in order to gain better understanding of interactions between quantized vortices and counterflow; the preliminary analysis shows that this method is viable.

We report a large chiro-optical response from a nanostructured film of aperiodic dielectric helices decorated with ellipsoidal metal nanoparticles. The influence of the inherent fabrication variation on the chiro-optical response of the wafer-scalable nanostructured film is investigated using a computational model which closely mimics the material system. From the computational approach, we found that the chiro-optical signal is strongly dependent on the ellipticities of the metal nanoparticles and the developed computational model can account for all the variations caused by the fabrication process. We report the experimentally realized dissymmetry factor ∼1.6, which is the largest reported for wafer scalable chiro-plasmonic samples till now. The calculations incorporate strong multipolar contributions of the plasmonic interactions to the chiro-optical response from the tightly confined ellipsoidal nanoparticles, improving upon the previous studies carried in the coupled dipole approximation regime. Our analyzes confirm the large chiro-optical response in these films developed by a scalable and simple fabrication technique, indicating their applicability pertaining to manipulation of optical polarization, enantiomer selective identification and enhanced sensing and detection of chiral molecules.

Multielectron bubbles (MEBs) are cavities in liquid helium that contain electrons and possibly vapour. They provide a versatile platform for exploring properties of interacting electrons in two dimensions in a regime of densities that has not been previously available. Since the electrons are pushed against the deformable curved inner surface of the bubble, MEBs also provide an environment to study the roles of curvatures. In this review, we describe the current understanding of MEBs, with a focus on their stability issues, and highlight recent experimental studies of this interesting system.

It has been shown that a bubble in liquid helium containing two electrons is unstable against fission. In this paper, we consider the stability of electron bubbles containing 4, 6, or 12 electrons. We find that a bubble with four electrons is unstable at zero pressure and presumably breaks up into single electron bubbles. Our calculation is not accurate enough to determine whether a bubble with six electrons is stable at zero pressure. We find that in liquid He-4 a bubble with 12 electrons is stable over a pressure range from -0.32 to 0.5 bar.

We have performed calculations of the properties of bubbles in liquid helium containing small numbers of electrons. We use an iterative approach to estimate the energy of the electrons inside a bubble of given shape, and then vary the shape of the bubble to find the minimum energy. For helium-3 we show that at zero applied pressure bubbles containing 4 electrons are unstable against breakup into single electron bubbles, but are stable at pressures between -0.23 and -0.15 bars. Bubbles with 6 electrons are stable between -0.17 and -0.05 bars and bubbles with 12 electrons are stable over the pressure range -0.1 to 0.08 bars.

The normal negative ion in liquid helium consists of an electron confined in a bubble of radius approximately 19 angstrom. These bubbles have been studied in many experiments. Time-of-flight mobility measurements have revealed that there are other types of negative ion of higher mobility and unknown structure. In this note we report on a study of the fastest of these and discuss the conditions under which it can be observed.