Search Results (60 total)

We investigate experimentally a two-dimensional rocking ratchet for cold atoms, realized by using a driven three-beam dissipative optical lattice. ac forces are applied in perpendicular directions by phase modulating two of the lattice beams. As predicted by the general theory [S. Denisov, Y. Zolotaryuk, S. Flach, and O. Yevtushenko, Phys. Rev. Lett. 100, 224102 (2008)], we observe a rectification phenomenon unique to high-dimensional rocking ratchets, as determined by two single-harmonic drivings applied in orthogonal directions. Also, by applying two biharmonic forces in perpendicular directions, we demonstrate the possibility of generating a current in an arbitrary direction within the optical lattice plane.

A phenomenological description of transition from an unstable to a (meta)stable phase state, including microscopic and mesoscopic scales, is presented. It is based on the introduction of specific memory functions which take into account contributions to the driving force of transformation from the past. A region of applicability for phase-field crystals and Swift-Hohenberg-type models is extended by inclusion of inertia effects into the equations of motion through a memory function of an exponential form. The inertia allows us to predict fast degrees of freedom in the form of damping perturbations with finite relaxation time in the instability of homogeneous and periodic model solutions.

We present photoluminescence measurements under intense magnetic fields (B up to 30 T) in n-doped indium nitride samples with carrier concentration ranging from about 7.5×10¹⁷ cm⁻³ to 5×10¹⁸ cm⁻³. The observation of transitions involving several Landau levels permits to determine the carrier-reduced mass μ around the Γ point. Depending on the carrier concentration, we find μ ranging between 0.093m₀ and 0.107m₀ (m₀ is the electron mass in vacuum). This finding poses a lower limit to the electron effective mass, whose unexpectedly large value (m_e≥0.093m₀) indicates that the sources of n doping in InN perturb strongly the crystal conduction band near its minimum.

We have observed bulk solidification of ⁴He induced by nucleation on positive alkali ions in pressurized superfluid helium. The ions are extracted into the liquid from alkali-doped solid He by a static electric field. The experiments prove the existence of charged particles in a solid structure composed of doped He that was recently shown to coexist with superfluid helium below the He solidification pressure. This supports our earlier suggestion that the Coulomb interaction of positive ions surrounded by a solid He shell (snowballs) and electrons trapped in spherical cavities (electron bubbles), together with surface tension, is responsible for the stability of that structure against melting. We have determined the density of charges in the sample by two independent methods.

Transverse beam stability is strongly affected by the beam space charge. Usually it is analyzed with the rigid-beam model. However, this model is only valid when a bare (not affected by the space charge) tune spread is small compared to the space charge tune shift. This condition specifies a relatively small area of parameters which, however, is the most interesting for practical applications. The Landau damping rate and the beam Schottky spectra are computed assuming that validity condition is satisfied. The results are applied to a round Gaussian beam. The stability thresholds are described by simple fits for the cases of chromatic and octupole tune spreads.

A setup of the experiment on the production of the isotopes with Z=108 in the fusion reaction ¹³⁶Xe+¹³⁶Xe and the obtained results are presented. At the excitation energy 0≤E_x≤30 MeV of the ²⁷²Hs* compound nucleus the upper limit of the cross section for evaporation residues σ_(1-3)n≤4 pb has been measured. The experimental results together with the data from asymmetric reactions point to a strong limitation of the Hs compound nucleus formation with increasing Coulomb forces in the entrance channel of the reaction.

We present time-resolved measurements of the laser-induced fluorescence from Cs atoms and Cs^*He_N exciplexes in a solid ⁴He matrix. The linear Cs^*He_N=2 exciplex is strongly quenched by a radiationless transformation into a ring-shaped Cs^*He_N=6,7 exciplex, which decays radiatively with a lifetime of 88±2  ns. The analysis of this lifetime provides information on the formation rate of Cs^*He_N=6,7 and on perturbations of the electronic states of the Cs atom by the bound helium atoms.

The results of the theoretical studies on the leakage of the optical radiation through a truncated waveguide with a subwavelength-sized exit hole are reported. We develop a self-consistent approach for the description of the fields behavior by taking into account the transformation of the initial wave into all possible modes, which appear due to the influence of the exit aperture. Our approach allows us to evaluate the amplitude reflection coefficient for the initial evanescent wave and to establish its relationship with the impedance of an infinite waveguide and the impedance of the exit aperture coupling a truncated waveguide with a free space. We determine the complex flow through the exit hole and the transmission coefficient to the far-field zone and express them through the reflection coefficient. Special attention in the work is paid to the evaluation of specific dependencies of the reflection and transmission coefficients on the ratio of the aperture radius to the light wavelength, on the dielectric constants of the waveguide core and the surrounding matter, and on the type of the waveguide mode. It is demonstrated that in the case of a metallic matter at the exit of a waveguide there is a strong resonance enhancement in the amplitude of the reflected wave and in the resulting amplitude of the tangential component of the electric field.

In the collider run II, the Tevatron operates with 36 high intensity bunches of 980 GeV protons and antiprotons. Particles not captured by the Tevatron rf system pose a threat since they can quench the superconducting magnets during acceleration or at beam abort. We describe the main mechanisms for the origination of this uncaptured beam, and present measurements of its main parameters by means of a newly developed diagnostics system. The Tevatron electron lens is effectively used in the collider run II operation to remove uncaptured beam and keep its intensity in the abort gaps at a safe level.

The multiform donor nature of hydrogen in n-type indium nitride is experimentally observed in samples exposed to atomic hydrogen. Photoluminescence measurements reveal a tenfold increase in the electron concentration and the formation of a shallow donor band upon hydrogen incorporation. Annealing studies show that hydrogen occupies at least two equilibrium sites having almost equivalent thermal stability.

We demonstrate experimentally a gating ratchet with cold rubidium atoms in a driven near-resonant optical lattice. A single-harmonic periodic modulation of the optical potential depth is applied, together with a single-harmonic rocking force. Directed motion is observed as a result of the breaking of the symmetries of the system.

Tank-treading, tumbling, and trembling are different types of the vesicle behavior in an external flow. We derive a dynamical equation enabling us to establish a state of nearly spherical vesicles. For a 2D external flow, the character of the vesicle dynamics is determined by two dimensionless parameters, depending on the vesicle excess area, fluid viscosities, membrane viscosity and bending modulus, strength of the flow, and ratio of the elongational and rotational components of the flow. The tank-treading to tumbling transition occurs via a saddle-node bifurcation, whereas the tank-treading to trembling transition occurs via a Hopf bifurcation. A slowdown of vesicle dynamics should be observed in a vicinity of a point separating the transition lines. We show that the slowdown can be described by a power law with two different critical exponents 1/4 and 1/2 corresponding to the slowdown of tumbling and trembling cycles.

Eukaryotic cells possess a sensible chemical compass allowing them to orient toward sources of soluble chemicals. The extracellular chemical signal triggers separation of the cell membrane into two domains populated by different phospholipid molecules and oriented along the signal anisotropy. We propose a theory of this polarization process, which is articulated into subsequent stages of germ nucleation, patch coarsening, and merging into a single domain. We find that the polarization time, t_ϵ, depends on the anisotropy degree ϵ through the power law t_ϵ∝ϵ^-2, and that in a cell of radius R there should exist a threshold value ϵ_th∝R^-1 for the smallest detectable anisotropy.

We report a numerical study, supplemented by phenomenological explanations, of “energy condensation” in forced 2D turbulence in a biperiodic box. Condensation is a finite size effect which occurs after the standard inverse cascade reaches the size of the system. It leads to the emergence of a coherent vortex dipole. We show that the time growth of the dipole is self-similar, and it contains most of the injected energy, thus resulting in an energy spectrum which is markedly steeper than the standard k^-5/3 one. Once the coherent component is subtracted, however, the remaining fluctuations have a spectrum close to k^-1. The fluctuations decay slowly as the coherent part grows.

Coherent synchrobetatron resonances can present a serious limit for low-energy synchrotrons. An excitation of a dipole transverse mode is considered at resonance condition. As an example, the growth rate for the Fermilab Booster is estimated.

Coupling of the transverse degrees of freedom modifies analysis of beam coherent motion. A general, simple, and effective rule to do that is derived.

Using two different high-pressure techniques, we have prepared boron-doped diamonds with atomic concentration of the dopant ranging from 0.04% to 4% (from 7×10¹⁹ to 7×10²¹ atom∕cm³) and studied the lattice constants and thermal expansion of the diamonds in the temperature range from 90 to 300 K. Both sets of samples demonstrate the same increasing concentration dependence of the lattice parameter with maximum shift of the lattice constant about 0.011 Å. We have established an abnormally high thermal expansion of the heavily boron-doped superconducting diamonds with respect to the undoped ones and a nearly linear correlation between lattice constant and critical temperature of the superconducting transition.

The Tevatron in Collider Run II (2001–present) is operating with 6 times more bunches, many times higher beam intensities and luminosities than in Run I (1992–1995). Electromagnetic long-range and head-on interactions of high intensity proton and antiproton beams have been significant sources of beam loss and lifetime limitations. We present observations of the beam-beam phenomena in the Tevatron and results of relevant beam studies. We analyze the data and various methods employed in operations, predict the performance for planned luminosity upgrades, and discuss ways to improve it.

We present phenomenology describing the internal structure of a turbulent zone, produced as the result of the push of a heavy fluid into a light one, for the case of immiscible fluids. One finds that the Kolmogorov cascade is realized within a range that grows with time, viz., scales between the mixing zone width, L∝t², and the viscous scale, η∝t^−1∕4. Surface-tension effects lead to formation of an emulsionlike state. Density fluctuations on scales larger than the typical drop size, l, are governed by the Obukhov-Corrsin cascade. If l⪢η, a wave energy cascade, related to capillary waves propagating along the surfaces of drops, is formed at scales below l, l∝t^−2∕5.

We develop a theory of light transmission through the aperture-type near-field optical probe by taking into account the effects associated with light absorption inside its semiconducting core. Our model is based on the exact description of the transverse-magnetic (TM) eigenmodes inside a conical waveguide with perfectly conducting metallic walls. A dissipative matter of its core is described by a complex frequency-dependent dielectric function. Analytical formulas are derived for the energy density distributions of the electric and magnetic fields inside a probe. Particular attention is paid to the evaluation of the near-field transmission coefficient of a metallized silicon probe in the spectral region from 400 nm to 830 nm. We study the dependences of the optical transmittance on the light wavelength, the aperture diameter, the taper angle as well as on the length of the probe. It is shown that the behavior of the electromagnetic fields and the transmission coefficient for the Si probe differ dramatically from the case of a loss-free dielectric core. In this work we point out that the use of a short Si probe instead of a glass one allows us to achieve a strong enhancement in the transmission efficiency in the visible and near-infrared spectral regions.

We study propagation of solitons in optical fibers with randomly varying birefringence which results in polarization mode dispersion. Due to the disorder, solitons emit radiation, i.e., the energy of the solitons is partly transferred into the delocalized modes. The radiation serves as a mediator of the intersoliton interaction leading to fluctuations of the soliton separations. We establish statistics of the fluctuations which is found to be sensitive to the phase mismatches and mutual polarizations of the solitons, and independent of the soliton separation. The theoretical results are justified by direct numerical simulations.

We consider the evolution of a passive scalar (concentration of pollutants or temperature) in a chaotic (turbulent) flow. A universal asymptotic behavior of the passive scalar decay (homogenization) related to peripheral regions (near walls) is established. The passive scalar moments and their pair correlation function in the peripheral region are analyzed. A special case investigated in our paper is the passive scalar decay along a pipe.

The nonstationary pump-probe interaction between short laser pulses propagating in a resonant optically dense coherent medium is considered. Special attention is paid to the case where the density of two-level particles is high enough that a considerable part of the energy of relatively weak external laser fields can be coherently absorbed and reemitted by the medium. Thus, the field of the medium reaction plays a key role in the interaction processes, which leads to collective behavior of an atomic ensemble in the strongly coupled light-matter system. Such behavior results in fast excitation interchanges between the field and a medium in the form of optical ringing, which is analogous to polariton beating in solid-state optics. This collective oscillating response, which can be treated as successive beats between light wave packets of different group velocities, is shown to significantly affect the propagation and amplification of the probe field under its nonlinear interaction with a nearly copropagating pump pulse. Depending on the probe-pump time delay, the probe transmission spectra show the appearance of either a specific doublet or coherent dip. The widths of these features are determined by the density-dependent field-matter coupling coefficient and increase during the propagation. Besides that, the widths of the coherent features, which appear close to resonance in the broadband probe spectrum, exceed the absorption-line width, since under the strong-coupling regime, the frequency of optical ringing exceeds the rate of incoherent relaxation. Contrary to stationary strong-field effects, the density- and coordinate-dependent transmission spectra of the probe manifest the importance of collective oscillations and cannot be obtained in the framework of a single-atom model.

We consider a linear model of optical transmission through a fiber with birefringent disorder in the presence of amplifier noise. Both disorder and noise are assumed to be weak, i.e., the average bit-error rate (BER) is small. The probability distribution function (PDF) of rare violent events leading to the values of BER much larger than its typical value is estimated. We show that the PDF has a long algebraiclike tail.

We describe the status of our effort to realize a first neutrino factory and the progress made in understanding the problems associated with the collection and cooling of muons towards that end. We summarize the physics that can be done with neutrino factories as well as with intense cold beams of muons. The physics potential of muon colliders is reviewed, both as Higgs factories and compact high-energy lepton colliders. The status and time scale of our research and development effort is reviewed as well as the latest designs in cooling channels including the promise of ring coolers in achieving longitudinal and transverse cooling simultaneously. We detail the efforts being made to mount an international cooling experiment to demonstrate the ionization cooling of muons.