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1. Spatial Diffusion of Hydrogen Atoms in Normal and Para-Hydrogen Molecular Films at Temperature 0.7 K

   https://doi.org/10.1007/s10909-024-03053-w  

   Sheludiakov, S; Wetzel, C K; Lee, D M; Khmelenko, V V; Järvinen, J; Ahokas, J; Vasiliev, S (February 2024, Journal of Low Temperature Physics)

   We report on electron spin resonance studies of H atoms stabilized in solid H2 films at temperature 0.7 K and in a magnetic field of 4.6 T. The H atoms were produced by bombarding H2 films with 100 eV electrons from a radiofrequency discharge run in the sample cell. We observed a one order of magnitude faster H atom accumulation in the films made of para-H2 gas with a small ortho-H2 concentration (0.2% ortho-H2 ) as compared with those made from normal H2 gas content (75% ortho-H2 ). We also studied the influence of ortho-H2 molecules on spatial diffusion of H atoms in solid H2 films. The spatial diffusion of H atoms in both normal and para-H2 films is faster than the diffusion obtained from the measurement of H atom recombination. The rate of spatial diffusion of H atoms in para-H2 films was slower in comparison with that in the normal H2 films. We discuss possible explanations of these observations. 

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2. Studies of the Structures of Nitrogen-Neon Nanoclusters Immersed into Superfluid Helium-4

   https://doi.org/10.1007/s10909-023-03026-5  

   Wetzel, C K; Lee, D M; Khmelenko, V V (January 2024, Journal of Low Temperature Physics)

   We studied the electron spin resonance (ESR) spectra of nitrogen atoms stabilized in nitrogen-neon nanoclusters immersed in superfluid 4He. The nanoclusters were formed during the condensation of the products of the discharge in N2–Ne–He gas mixtures into bulk superfluid 4He at temperature 1.5 K. We studied nanoclusters formed by injection of gas mixtures with different ratios of heavy impurities in the helium N2/(Ne + N2 ) ranging from 2% to 90%. Analysis of the ESR spectra of nitrogen atoms stabilized in nitrogen-neon nanoclusters provides important information about the environment of the stabilized atoms and a shell structure of the nanoclusters was revealed. For all samples studied, preferential stabilization of N atoms on the surfaces of the nanoclusters was observed. Annealing of the collection of the nanoclusters in the temperature range 1.1–10 K resulted in substantial changes in the structure of the nanoclusters. 

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3. Coulomb Interaction-Driven Entanglement of Electrons on Helium

   https://doi.org/10.1103/PRXQuantum.5.030324  

   Beysengulov, Niyaz R; Schøyen, Øyvind S; Bilek, Stian D; Flaten, Jonas B; Leinonen, Oskar; Hjorth-Jensen, Morten; Pollanen, Johannes; Kristiansen, Håkon Emil; Stewart, Zachary J; Weidman, Jared D; et al (August 2024, PRX Quantum)

   The generation and evolution of entanglement in many-body systems is an active area of research that spans multiple fields, from quantum information science to the simulation of quantum many-body systems encountered in condensed matter, subatomic physics, and quantum chemistry. Motivated by recent experiments exploring quantum information processing systems with electrons trapped above the surface of cryogenic noble gas substrates, we theoretically investigate the generation of entanglement between two electrons via their unscreened Coulomb interaction. The model system consists of two electrons confined in separate electrostatic traps that establish microwave-frequency quantized states of their motion. We compute the motional energy spectra of the electrons, as well as their entanglement, by diagonalizing the model Hamiltonian with respect to a single-particle Hartree product basis. We also compare our results with the predictions of an effective Hamiltonian. The computational procedure outlined here can be employed for device design and guidance of experimental implementations. In particular, the theoretical tools developed here can be used for fine-tuning and optimization of control parameters in future experiments with electrons trapped above the surface of superfluid helium or solid neon. 

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4. State-Selective Double Photoionization of Atomic Carbon and Neon

   https://doi.org/10.3390/atoms12120070  

   Yip, Frank L (December 2024, Atoms)

   Double photoionization (DPI) allows for a sensitive and direct probe of electron correlation, which governs the structure of all matter. For atoms, much of the work in theory and experiment that informs our fullest understanding of this process has been conducted on helium, and efforts continue to explore many-electron targets with the same level of detail to understand the angular distributions of the ejected electrons in full dimensionality. Expanding on previous results, we consider here the double photoionization of two 2p valence electrons of atomic carbon and neon and explore the possible continuum states that are connected by dipole selection rules to the coupling of the outgoing electrons in 3P, 1D, and 1S initial states of the target atoms. Carbon and neon share these possible symmetries for the coupling of their valence electrons. Results are presented for the energy-sharing single differential cross section (SDCS) and triple differential cross section (TDCS), further elucidating the impact of the initial state symmetry in determining the angular distributions that are impacted by the correlation that drives the DPI process. 

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5. Piezoacoustics for precision control of electrons floating on helium

   https://doi.org/10.1038/s41467-021-24452-7  

   Byeon, H.; Nasyedkin, K.; Lane, J. R.; Beysengulov, N. R.; Zhang, L.; Loloee, R.; Pollanen, J. (December 2021, Nature Communications)

   null (Ed.)

   Abstract Piezoelectric surface acoustic waves (SAWs) are powerful for investigating and controlling elementary and collective excitations in condensed matter. In semiconductor two-dimensional electron systems SAWs have been used to reveal the spatial and temporal structure of electronic states, produce quantized charge pumping, and transfer quantum information. In contrast to semiconductors, electrons trapped above the surface of superfluid helium form an ultra-high mobility, two-dimensional electron system home to strongly-interacting Coulomb liquid and solid states, which exhibit non-trivial spatial structure and temporal dynamics prime for SAW-based experiments. Here we report on the coupling of electrons on helium to an evanescent piezoelectric SAW. We demonstrate precision acoustoelectric transport of as little as ~0.01% of the electrons, opening the door to future quantized charge pumping experiments. We also show SAWs are a route to investigating the high-frequency dynamical response, and relaxational processes, of collective excitations of the electronic liquid and solid phases of electrons on helium. 

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