Condensed Matter

The nuclear spin of a He 3 quasiparticle dissolved in superfluid He 4 sees an apparent magnetic field proportional to the Fermi coupling constant, the superfluid condensate density, and the electron current at the He 3 nucleus. Whereas the direction of the current must be parallel to the quasiparticle momentum, calculating its magnitude presents an interesting theoretical challenge because it vanishes in the Born-Oppenheimer approximation. We find the effect is too small to be observed and present our results in the hope others will be inspired to look for similar effects in other systems.

This paper presents a finite particle approximation of the two-fluid model for liquid 4 He using smoothed particle hydrodynamics (SPH). In recent years, several studies have combined the vortex filament model (VFM), which describes quantized vortices in superfluid components, with the Navier-Stokes equations, which describe the motion of normal fluids. These studies led us to assume that coupling both components of the two-fluid model instead of using the VFM to describe the superfluid component enables us to approximate the system. In this study, we formulated a new SPH model that simultaneously solves both equations of motion of the two-fluid model. We then performed a numerical simulation of the rotating liquid 4 He using our SPH. The results showed that the two major phenomena, the emergence of multiple independent vortices parallel to the circular axis and that of the so-called rigid-body rotation, can be reproduced by solving the two-fluid model using SPH. This finding is interesting because it was previously assumed that only a single vortex emerges when addressing similar problems without considering quantum mechanics. Our further analysis found that the emergence of multiple independent vortices can be realized by reformulating the viscosity term of the two-fluid model to conserve the angular momentum of the particles around their axes. Consequently, our model succeeded in reproducing the phenomena observed in quantum cases, even though we solve the phenomenological governing equations of liquid 4 He.

Although seawater is abundant, desalination is energy-intensive and expensive. Using the sun as an energy source is attractive for desalinating seawater; however, the performance of state-of-the-art passive devices is unsatisfactory when operated at less than one sun ( < 1 kW m ?? ). Here, we present a completely passive, modular, and low-cost solar thermal distiller for seawater desalination. Each distillation stage is made of two opposed hydrophilic layers separated by a hydrophobic microporous membrane, and it does not require further mechanical ancillaries. Under realistic laboratory and outdoor conditions, we obtained a distillate flow rate of almost 3 L m ?? h ?? from seawater at less than one sun - twice the yield of recent passive device reported in the literature. In perspective, theoretical modelling suggests that the distiller has the potential to further doubling the peak flow rate observed in the current experiments. This layout can satisfy freshwater needs in isolated and impoverished communities, as well as realize self-sufficient floating installations or provide freshwater in emergency conditions.

We studied a novel cooling method, in which 3 He and 4 He are mixed at the 4 He crystallization pressure at temperatures below 0.5mK . We describe the experimental setup in detail, and present an analysis of its performance under varying isotope contents, temperatures, and operational modes. Further, we developed a computational model of the system, which was required to determine the lowest temperatures obtained, since our mechanical oscillator thermometers already became insensitive at the low end of the temperature range, extending down to (90±20)μK≈ T c (29±5) ( T c of pure 3 He). We did not observe any indication of superfluidity of the 3 He component in the isotope mixture. The performance of the setup was limited by the background heat leak of the order of 30pW at low melting rates, and by the heat leak caused by the flow of 4 He in the superleak line at high melting rates up to 500μmol/s . The optimal mixing rate between 3 He and 4 He, with the heat leak taken into account, was found to be about 100..150μmol/s . We suggest improvements to the experimental design to reduce the ultimate achievable temperature further.

We explore theoretically the nonequilibrium photonic phases of an array of coupled cavities in presence of incoherent driving and dissipation. In particular, we consider a Hubbard model system where each site is a Kerr nonlinear resonator coupled to a two-level emitter, which is pumped incoherently. Within a Gutzwiller mean-field approach, we determine the steady-state phase diagram of such a system. We find that, at a critical value of the inter-cavity photon hopping rate, a second-order nonequilibrium phase transition associated with the spontaneous breaking of the U(1) symmetry occurs. The transition from an incompressible Mott-like photon fluid to a coherent delocalized phase is driven by commensurability effects and not by the competition between photon hopping and optical nonlinearity. The essence of the mean-field predictions is corroborated by finite-size simulations obtained with matrix product operators and corner-space renormalization methods.

Probing the polarization of water molecules at charged interfaces by second harmonic generation spectroscopy has been heretofore limited to isotropic materials. Here, we report non-resonant nonlinear optical measurements at the interface of anisotropic z-cut {\alpha}-quartz and water under conditions of dynamically changing ionic strength and bulk solution pH. We find that the product of the third-order susceptibility and the interfacial potential, \c{hi}(3). {\Phi}(0), is given by ( \c{hi}(3)-i\c{hi}(3)). {\Phi}(0), and that the interference between this product and the second-order susceptibility of bulk quartz depends on the rotation angle of {\alpha}-quartz around the z-axis. Our experiments show that this newly identified term, i\c{hi}(3). {\Phi}(0), which is out of phase from the surface terms, is of bulk origin. The possibility of internally phase referencing the interfacial response for the interfacial orientation analysis of species or materials in contact with {\alpha}-quartz is discussed along with the implications for conditions of resonance enhancement.

The main candidate for the superfluid pathways in solid Helium-4 are dislocations with Burgers vector along the hcp symmetry axis. Here we focus on quantum behavior of a generic edge dislocation which can perform superclimb -- climb supported by the superflow along its core. The role of the long range elastic interactions between jogs is addressed by Monte Carlo simulations. It is found that such interactions do not change qualitatively the phase diagram found without accounting for such forces. Their main effect consists of renormalizing the effective scale determining compressibility of the dislocation in the Tomonaga-Luttinger Liquid phase. It is also found that the quantum rough phase of the dislocation can be well described within the gaussian approximation which features off-diagonal long range order in 1D for the superfluid order parameter along the core.

Interaction of an electron system with a strong electromagnetic wave leads to rearrangement both the electron and vibrational energy spectra of a dissipative system. For instance, the optically coupled electron levels become split in the conditions of the ac Stark effect that gives rise to appearance of the nonadiabatic coupling between the electron and vibrational motions. The nonadiabatic coupling exerts a substantial impact on the electron and phonon dynamics and must be taken into account to determine the system wave functions. In this paper, the vibronic coupling induced by the ac Stark effect is considered. It is shown that the interaction between the electron states dressed by an electromagnetic field and the forced vibrations of reservoir oscillators under the action of rapid changing of the electron density with the Rabi frequency is responsible for establishment of the photoinduced vibronic coupling. However, if the resonance conditions for the optical phonon frequency and the transition frequency of electrons in the dressed state basis are satisfied, the vibronic coupling is due to the electron-phonon interaction. Additionally, photoinduced vibronic coupling results in appearance of the doubly dressed states which are formed by both the electron-photon and electron-vibrational interactions.

During the last decade experimental evidence is building that the mass supertransport through solid Helium-4 as well as the anomalously large matter accumulation in the bulk -- the giant isochoric compressibility (aka the syringe effect) -- are both supported by a network of dislocations with superfluid core. However, a structure of this network as well as its relation to the basal (non-superfluid) dislocations which are responsible for plasticity remain unclear. Here it is shown that superclimbing and basal edge dislocations can form bound pairs. This implies that plastic deformation should produce the syringe effect and vice versa. The experimental test is proposed. While the strength of the effect depends on the average orientation of the paired dislocations, there is a feature unique for the superfluid dislocation scenario -- the supercurrents flow in the direction perpendicular to the plastic deformation.

The time reversal symmetric polar phase of the spin-triplet superfluid 3 He has two types of Dirac nodal lines. In addition to the Dirac loop in the spectrum of the fermionic Bogoliubov quasiparticles in the momentum space ( p x , p y , p z ) , the spectrum of bosons (magnons) has Dirac loop in the 3D space of parameters -- the components of magnetic field ( H x , H y , H z ) . The bosonic Dirac system lives on the border between the type-I and type-II.