Condensed Matter

We investigate theoretically the existence at low temperature of a commensurate (4/7) crystalline phase of a layer of either He isotope on top of a He-4 layer adsorbed on graphite. We make use of a recently developed, systematically improvable variational approach which allows us to treat both isotopes on an equal footing. We confirm that no commensurate crystalline second layer of He-4 forms, in agreement with all recent calculations. Interestingly and more significantly, we find that even for He-3 there is no evidence of such a phase, as the system freezes into an {\it incommensurate} crystal at a coverage lower than that (4/7) at which a commensurate one has been predicted, and for which experimental claims have been made. Implications on the interpretation of recent experiments with helium on graphite are discussed.

Upon excitation by a laser pulse, broken-symmetry phases of a wide variety of solids demonstrate similar order parameter dynamics characterized by a dramatic slowing down of relaxation for stronger pump fluences. Motivated by this recurrent phenomenology, we develop a simple non-perturbative effective model of dynamics of collective bosonic excitations in pump-probe experiments. We find that as the system recovers after photoexcitation, it shows universal prethermalized dynamics manifesting a power-law, as opposed to exponential, relaxation, explaining the slowing down of the recovery process. For strong quenches, long-wavelength over-populated transverse modes dominate the long-time dynamics; their distribution function exhibits universal scaling in time and space, whose universal exponents can be computed analytically. Our model offers a unifying description of order parameter fluctuations in a regime far from equilibrium, and our predictions can be tested with available time-resolved techniques.

Inversive geometry can be used to generate exactly self-similar space-filling sphere packings. We present a construction method in two dimensions and generalize it to search for packings in higher dimensions. We newly discover 29 three-dimensional and 13 four-dimensional topologies of which 10 and 5, respectively, are bearings. To distinguish and characterize the packing topologies, we numerically estimate their fractal dimensions and we analyze their contact networks.

Long-range spin transport in magnetic systems can be achieved by means of exchange-mediated spin textures with robust topological winding -- a phenomenon referred to as spin superfluidity. Its experimental signatures have been discussed in antiferromagnets which are nearly free of dipolar interaction. However, in ferromagnets, which possess non-negligible dipole fields, realization of such spin transport has remained a challenge. Using micromagnetic simulations, we investigate coherent exchange-mediated spin transport in extended thin ferromagnetic films. We uncover a two-fluid state, in which the long-range spin transport by spin textures co-exists with spin waves, as well as a soliton-screened spin transport regime at high spin injection biases. Both states are associated with distinct spin texture reconstructions near the spin injection region and sustain spin transport over large distances.

Previous single-pulse extreme ultraviolet and X-ray coherent diffraction studies revealed that superfluid 4He droplets obtained in free jet expansion acquire sizable angular momentum, resulting in significant centrifugal distortion. Similar experiments with normal fluid 3He droplets may help elucidating the origin of the of the large degree of rotational excitation and highlight similarities and differences of dynamics in normal and superfluid droplets. Here, we present the first comparison of the shapes of isolated 3He and 4He droplets following expansion of the corresponding fluids in vacuum at temperatures as low as ~ 2 K. Large 3He and 4He droplets with average radii of ~160 nm and ~350 nm, respectively, were produced. We find that the majority of the 3He droplets in the beam correspond to rotating oblate spheroids with reduced average angular momentum ( Λ ) and reduced angular velocities ( Ω ) similar to that of 4He droplets. Given the different physical nature of 3He and 4He, this similarity in Λ and Ω may be surprising and suggest that similar mechanisms induce rotation regardless of the isotope. We hypothesized that the observed distribution of droplet sizes and angular momenta stem from processes in the dense region close to the nozzle. In this region, the significant velocity spread and collisions between the droplets induce excessive rotation followed by droplet fission. The process may repeat itself several times before the droplets enter the collision-fee high vacuum region further downstream.

We derive exact analytical solution of the Shockley-Queisser (SQ) model and present all photovoltaic (PV) characteristics in compact and convenient form via the Lambert W function. We show that the SQ condition of chemical of equilibrium between photocarriers and emitted phonons leads to a new thermodynamic relation between the maximal conversion efficiency and the photo-induced chemical potential. Also, we consider kinetics of PV conversion and establish relation between the photocarrier collection time and photocarrier lifetime. We consider photonic and electronic requirements for approaching of SQ limit in semiconductor PV devices and highlight a role of photon reabsorption.

The existence of the phonon-roton minimum has been widely observed for both the solid and liquid phases but so far there is no sufficient theoretical explanation of its origin. In this paper we use a range of techniques to study the dynamics and short-range order for a range of simple materials in their crystalline, amorphous and liquid phases. We perform inelastic neutron scattering (INS) experiments of polycrystalline and liquid barium to study the high-frequency dynamics and understand the mechanisms underlying the atomic motion. Moreover we perform INS simulations for crystals and supercooled liquids, compare the collective excitation spectra and identify similarities. We perform molecular dynamics (MD) simulations for the same materials and present results of population and bond angle distribution showing a short-range order dependence of the different phases, expanding the current knowledge in literature. Finally, we support our findings with a theoretical explanation of the origin of the phonon-roton minima which is observed in both solids and liquids. We study this as a classical phenomenon and we base our explanation on short range interatomic interactions.

The fast evaporative cooling of micrometer-sized water droplets in vacuum offers the appealing possibility to investigate supercooled water - below the melting point but still a liquid - at temperatures far beyond the state-of-the-art. However, it is challenging to obtain a reliable value of the droplet temperature under such extreme experimental conditions. Here, the observation of morphology-dependent resonances in the Raman scattering from a stream of perfectly uniform water droplets has allowed us to measure with an absolute precision of better than 0.2% the variation in droplet size resulting from evaporative mass losses. This finding proved crucial to an unambiguous determination of the droplet temperature. In particular, a fraction of water droplets with initial diameter of 6379 ± 12 nm were found to remain liquid down to 230.6 ± 0.6 K. Our results question temperature estimates reported recently for larger supercooled water droplets, and provide valuable information on the hydrogen-bond network in liquid water in the hard-to-access deeply supercooled regime.

We recently proposed polariton graphs as a novel platform for solving hard optimization problems that can be mapped into the XY model. Here, we elucidate a relationship between the energy spectrum of the XY Hamiltonian and the total number of condensed polariton particles. Using as a test-bed the hexagonal unit lattice we show that the lower energy states of the XY Hamiltonian are faithfully reproduced by mean-field numerical simulations utilising the Ginzburg--Landau equation coupled to an exciton reservoir. Our study paves the way to simulating the spectral gap of the XY model using polariton graphs.

The friction of a stationary moving skate on smooth ice is investigated, in particular in relation to the formation of a thin layer of water between skate and ice. It is found that the combination of ploughing and sliding gives a friction force that is rather insensitive for parameters such as velocity and temperature. The weak dependence originates from the pressure adjustment inside the water layer. For instance, high velocities, which would give rise to high friction, also lead to large pressures, which, in turn, decrease the contact zone and so lower the friction. The theory is a combination and completion of two existing but conflicting theories on the formation of the water layer.