Close Approach of a Spherical Particle and a Quantised Vortex in Helium II

To interpret the results of recent experiments which used the Particle Image Velocimetry (PIV) method it is necessary to understand the interaction of the particles with the quantised vortices. We present numerical calculations of the close approach of a small spherical particle to a vortex line. The collision time scale compares well with approximate analytical results.     

Dissipation of Gross-Pitaevskii Turbulence Coupled with Thermal Excitations

Microscopic dissipation mechanism of quantized vortices in quantum fluid is studied numerically by solving the Gross-Pitaevskii equation coupled with the Bogoliubov-de-Gennes equation for thermal excitations. At low temperatures, dissipation works at smaller scales than the vortex core size, which supports the self-similar cascade process of quantized vortices at large scales and the Kolmogorov energy spectrum of quantum turbulence. On the other hand, this dissipation spreads to larger scales at high temperatures, and directly affects the vortex dynamics. This effect of dissipation at high temperatures is qualitatively similar to the mutual friction in superfluid ⁴He.     

Dynamics of Vortex Lattice Formation in a Rotating Bose-Einstein Condensate with an Optical Lattice Potential

We study dynamics of quantized vortex lattice formation in a rotating Bose-Einstein condensate with a square blue-detuned optical lattice by solving the Gross-Pitaevskii equation. This dynamics depends on the depth of the optical lattice. Vortices tend to form a triangular lattice under the rotation, while an optical lattice likes to pin vortices at their peaks. Such a competition of two effects makes this system more interesting and complicated.     

Turbulence of Capillary Waves on the Surface of Quantum Liquids

The techniques developed earlier for study of turbulence on the surface of liquid hydrogen was used for generation of nonlinear waves on the charged surface of liquid ⁴He. This permitted us to observe for the first time capillary turbulence on the surface of liquid ⁴He in normal and superfluid state, and to expand in a few times the relative width of the inertial interval, where the Kolmogorov-Zakharov spectrum of turbulence was observed.     

Transition to Superfluid Turbulence

Turbulence in superfluids depends crucially on the dissipative damping in vortex motion. This is observed in the B phase of superfluid ³He where the dynamics of quantized vortices changes radically in character as a function of temperature. An abrupt transition to turbulence is the most peculiar consequence. As distinct from viscous hydrodynamics, this transition to turbulence is not governed by the velocity-dependent Reynolds number, but by a velocity-independent dimensionless parameter 1/q which depends only on the temperature-dependent mutual friction—the dissipation which sets in when vortices move with respect to the normal excitations of the liquid. At large friction and small values of the dynamics is vortex number conserving, while at low friction and large vortices are easily destabilized and proliferate in number. A new measuring technique was employed to identify this hydrodynamic transition: the injection of a tight bundle of many small vortex loops in applied vortex-free flow at relatively high velocities. These vortices are ejected from a vortex sheet covering the AB interface when a two-phase sample of ³He-A and ³He-B is set in rotation and the interface becomes unstable at a critical rotation velocity, triggered by the superfluid Kelvin–Helmholtz instability.      

Switching Phenomena between Laminar and Turbulent Flows of Superfluid 4He Generated by a Vibrating Wire

We have investigated the turbulence transition of the superfluid ⁴He flow generated by a vibrating wire. For a 1.2-kHz vibrating wire, we observed intermittent switchings between laminar and turbulent flows. The switching rate decreases with increasing temperature above 100 mK, until no occurrence of the switchings at 350 mK. For a 2.4-kHz vibrating wire, we find that the switching rate is much lower than that of the 1.2-kHz vibrating wire even at low temperatures. This result indicates that a mechanism causing the switchings is influenced by the temperature and the oscillation frequency of the superfluid flow.     

Pulsed Third Sound in the Presence of a Superfluid Flow

We report measurements of pulsed third sound in the presence of a uniform superfluid flow at T=0.89 K. The superfluid flow is created using a heater technique. For increasingly large values of power dissipated in the heater used to create the flow, the shapes of the received third sound pulses evolve. The pulses propagating parallel to the flow grow while the pulses propagating against the flow decrease. When the heater power is increased further such that film thinning effects occur, the pulses propagating antiparallel to the flow qualitatively resemble the derivative of the drive pulse. Similar effects were also observed in thin films at T=1.35 K. A possible explanation for the growth and decrease of the pulses will be discussed. The cause of the pulse shapes that resemble the derivative of the drive pulse is not understood.     

QCM Measurements of Superfluid Response in 4He Films up to 180 MHz

At finite frequencies, a dynamic Kosterlitz–Thouless (KT) theory predicts a frequency dependence of the superfluid transition in ⁴He films on planar surfaces. We report results of QCM measurements to study the superfluid response on planar gold surfaces for very high frequencies up to 180 MHz in the temperature range of 0.6–1.0 K. As the frequency is increased, we observed the expected KT behavior that the superfluid transition shifts to a higher temperature from the static transition temperature T _KT and the transition temperature region broadens. The frequency dependence of the dissipation peak temperature at the transition agrees with a simple equation of the frequency dependence based on the dynamic KT theory. The microscopic parameter for the dynamic transition, the ratio of the diffusion constant to the square of the vortex core radius D/r ₀ ², is estimated to be on the order of 10¹⁰ s⁻¹.     

Nonlinear Second Sound Waves and Acoustic Turbulence in Superfluid 4He

The preliminary results of an investigation of nonlinear second sound waves in a high quality resonator filled with superfluid ⁴He are presented and discussed. It is found that, for a sufficiently strong periodic driving force, a cascade of second sound waves is formed at multiple harmonics of the driving frequency over the extremely wide frequency range 1–100 kHz. It can be described by a power law A _ω =const.×ω ^−m , where the scaling index m≈1. These observation can be attributed to the formation of a Kolmogorov-like turbulent cascade in the system of second sound waves, accompanied by directed energy flux through the frequency scales. It manifests itself as a limiting of the amplitude of a standing wave, a distortion of the shape of the initially harmonic waves, and a reduction of the effective quality factor Q of the resonator.     

Fourth Sound Measurement of Superfluid 3He in Aerogel

The fourth sound resonance experiment has been done on liquid ³He in 98.5% porosity aerogel. Aerogel was grown inside the pores among the sintered silver powder to avoid the vibration of the aerogel strands by the sound experiment. The measurement was performed at zero magnetic field and 27 bar. We observed the phase transition between the A-like and B-like phases and also their coexistent state. The A-like to B-like phase transition occurs not at a temperature but within a temperature band. In this band, the A-like phase gradually converts to the B-like phase. Possible picture of the coexistent state is discussed.     

Spin Pumping Effect in Superfluid 3He A1

A fountain effect is a common phenomenon in both ³He and ⁴He superfluids. Unique to superfluid ³He is the magnetic fountain effect, which has been used to determine the spin direction of the condensate in ³He A₁ phase. Here we present a pressure driven fountain effect in A₁ phase. The experimental cell is composed of a large reservoir connected to a small detector chamber through superleak channels of width of 20 μm. One wall of the detector chamber houses a movable circular 6 μm thick membrane which serves as a sensitive capacitive pressure sensor and also acts as a spin pump. In A₁ phase, a DC voltage applied on the capacitor induces a simultaneous mass and spin superfluid current into the small chamber. After equilibration, removal of the DC voltage causes a sudden pressure drop followed by a slow relaxation. The sudden drop is a consequence of reversed superfluid flow through the superleak. The observed decay times during the slow relaxation agree with those obtained in magnetically induced spin flow experiment. These observations show that the slow relaxation stems from spin relaxation in the absence of applied field gradient.     

Suhl Instability as a Mechanism of the Catastrophic Relaxation in the Superfluid 3He–B

In order to explain catastrophic relaxation, bulk mechanism based on Suhl instability (J. Phys. Chem. Solids 1, 209, 1957) is studied. It is shown, that at sufficiently low temperatures homogeneous precession of spin becomes unstable in the whole region of tipping angles of spin 0≤β≤π. In comparison with the previous publication of Surovtsev and Fomin (J. Exp. Theor. Phys. Lett. 83, 410, 2006) the leading zero temperature increments for the angles θ ₀≃104°≤β≤π are found. Estimation of the temperature of transition to the unstable state for the angle of 105°, that corresponds to the region of tipping angles in homogeneously precessing domain (HPD), is made.     

Characterization of iron nanoparticles synthesized by high pressure sputtering

The study of magnetic nanoparticles is interesting because of their importance and applications in spintronics, biology, and medicine. We have used a high pressure sputtering technique to deposit iron nanoparticles on a silicon substrate. The nanoparticles are then analyzed using atomic force microscopy (AFM), X-ray diffraction, and transmission electron microscopy. AFM data show that the size of the particles depends on different deposition conditions. X-ray diffraction data show that the nanoparticles adopt a body-centered cubic crystal structure. Overall it was found particle size could be tuned by adjusting the deposition conditions.     

The Theory of Local Superconductivity and Phase Separation Applied to Cuprates

In this paper we present a theory of the high critical temperature superconductors (HTSC). As has been shown directly by Scanning Tunneling Microscopy (STM) and indirectly by neutron diffraction, these materials are not homogeneous. We suppose that, at temperatures detected by some experiments, these compounds go through a phase separation which essentially creates regions with low and large doping around the average doping level. Using a Bogoliubov–deGennes local theory, we calculate the superconducting local gap as function of the density, for islands with 10–14 lattice parameters on average. We argue that this procedure is suitable to explain many HTSC properties and their universal phase diagram.     

Ion beam effects on dust grains

Our experimental set-up allows us to keep a single dust grain trapped in a high-vacuum chamber for hours or days and to affect it by electron or ion beams. It apparently makes possible to study each particular charging process separately but, as our results have shown, the influence of emissions from the surfaces surrounding the experimental space cannot be neglected in interpretation of results.The paper is devoted to experimental and theoretical investigations of ion field emission from spherical non-conducting monodisperse melamine resin grains with diameters of , 4.97, and that were exposed by the ion beam with energies ranging from 0.3 to 5 keV. We present the methodology of the determination of different charging currents as well as the determination of a grain size. These quantities are then used for estimation of equilibrium potentials. Results obtained using He ions show that ion field emission from the investigated grains can be recorded at field intensities of the order of 10⁹ V/m and strongly depends on a history of the grain charging.     

Electrical transport and magnetoresistance in La0.67Ca0.33MnO3/BaTiO3 composites

The results of the structure, electrical transport and magnetoresistance of a ferromagnet-ferroelectric-type La_0.67Ca_0.33MnO₃ (LCMO)/BaTiO₃ composites fabricated by the sol-gel method are presented. The structure and morphology characterization indicates no apparent variations in morphology and particle size in spite of the existence of BaTiO₃. The insulator-metal transition temperature (T_IM) is shifted to a higher temperature and resistivity decreases with the increase of low content BaTiO₃. Magnetoresistance (MR) of the composites is enhanced over the whole temperature range as a result of the introduction of BaTiO₃. By calculating in terms of a ferromagnetic grain coupling model, we attribute these transport properties to the enhancement of the ferromagnetic coupling between the neighboring grains, which could be explained by the increase of the carrier concentration at the grain boundary due to the introduction of BaTiO₃ and the associated magnetoelectric coupling effect.     

Field emission properties of composite nano-Au/DLC films prepared by CVD technique

Nanocrystalline gold incorporated diamond-like carbon (nano-Au/DLC) films were deposited by capacitively coupled plasma (CCP) r.f. chemical vapour deposition (CVD) technique. Gold content in the DLC matrix was controlled by the amount of argon in the argon + methane mixture in the plasma. Field emission properties of these films were studied critically. Bonding environment (sp²/sp³ ratio) in these films was obtained from Raman measurements. Modification of the surface with the incorporation of gold nanocrystallites and associated modulation of sp²/sp³ ratio in the films culminated in improved field emission properties. Fowler-Nordheim model was used to ascertain the work function (?) which varied between 19 and 64 meV. The field factor (β) varied between 172 and 1050.     

Ion beam effects on dust grains: 2—Influence of charging history

Dust grains in space are charged by various processes. Impacts of energetic ions lead to deposition of positive charge on the grain, increasing the grain potential and, as a consequence, the electric field at its surface. The accumulated charge is spontaneously released as an emission current when the electric field reaches a threshold. This discharging current is usually attributed to field ionization of any gas surrounding the grain or to ion field emission and would thus be predominantly a function of the surface potential. However, preliminary studies [Velyhan A, Z?ilavý P, Pavl? J, S?afránková J, Něme?ek Z. Ion beam effects on dust grains. Vacuum 2004;76:447-55] using melamine formaldehyde spheres have shown that the discharging current depends strongly on the energy of primary ions. The present paper continues these investigations with the motivation to understand the whole charging/discharging process. The experiment is based on the capture of a single dust grain in an electrodynamic quadrupole. The trapped grain is exposed to an ion beam with different energies up to 5 keV and its charge and surface potential are estimated from the frequency of its oscillations in the quadrupole. The charging/discharging currents are determined from temporal changes of the grain charge. Our results suggest that the grain charge is accumulated in a thick surface layer of non-conducting samples. The thickness of this layer depends on the mass and energy of primary ions. On the other hand, the beam ions probably recombine on the metallic surfaces and create an adsorbed layer there. We believe that the main discharging process is field desorption complemented in this particular case with post-ionization.     

Combined effects of heat treatment and seed layer materials on magnetic properties of CoCrPt perpendicular media

Samples of glass/Ti(20 nm)/CoCrPt (20 nm)/Ti (3.2 nm) and glass/seed (Cr or Cu)/Ti(20 nm)/CoCrPt(20 nm)/Ti (3.2 nm) were deposited by magnetron sputtering at ambient temperature or 200 °C. All samples were post-annealed at 400 °C for 15 min. For either Cu or Cr seed layer, post-annealing can enhance the out-of-plane coercivity. For post-annealed samples with either Cr or Cu seed layers, the ambient temperature growth induce an enhancement in the coercivity but a reduction in the slope of hysteresis loop at the coercivity, in comparison with that of the elevated temperature growth. With identical post-annealing and growth conditions, Cr or Cu seed layers can enhance the out-of-plane coercivity, in comparison with those grown on bare substrates. The nucleation field is negative for Cr seed layer and positive for the Cu seed layer although they have close coercivity. With the same growth and post-annealing conditions, the magnetic properties of CoCrPt layers depend on the seed layer thickness. Evolution of magnetic properties can be explained in terms of changes of structural properties of constituent layers.     

New insight into the functional dependence rules in turbulence modelling

This paper is intended to revisit the Euclidean invariance of turbulence models, in particular of the algebraic Reynolds stress models (ARSM). The approach is motivated by the existing different interpretations of the Euclidean invariance of an equation. We show that these interpretations are not suitable to rule out functional dependencies in turbulence. We use the concept of Euclidean invariance with respect to a constitutive or flow rule class of a closure relation to analyze the Euclidean invariance properties of the ARSM. It turns out that the existing ARSM are Euclidean invariant with respect to two different classes, that are basically determined by the equilibrium assumption made in an inertial and a non-inertial reference frame, respectively. The first one is more restrictive and does not include the relative environment dependence of the Reynolds stress tensor in the implicit ARSM. The second class endeavors to involve this environment dependence. After this first step, the issue how to formulate a unified rationale for a consistent reduction procedure based on extended thermodynamics to rule out functional dependencies of the ARSM relations for Reynolds-averaged Navier-Stokes equations (RANS) or large eddy simulations (LES) is left for future work.