Ultralow Velocities Research Articles

Cooling positronium to ultralow velocities with a chirped laser pulse train

When laser radiation is skilfully applied, atoms and molecules can be cooled1–3, allowing the precise measurements and control of quantum systems. This is essential for the fundamental studies of physics as well as practical applications such as precision spectroscopy4–7, ultracold gases with quantum statistical properties8–10 and quantum computing. In laser cooling, atoms are slowed to otherwise unattainable velocities through repeated cycles of laser photon absorption and spontaneous emission in random directions. Simple systems can serve as rigorous testing grounds for fundamental physics—one such case is the purely leptonic positronium11,12, an exotic atom comprising an electron and its antiparticle, the positron. Laser cooling of positronium, however, has hitherto remained unrealized. Here we demonstrate the one-dimensional laser cooling of positronium. An innovative laser system emitting a train of broadband pulses with successively increasing central frequencies was used to overcome major challenges posed by the short positronium lifetime and the effects of Doppler broadening and recoil. One-dimensional chirp cooling was used to cool a portion of the dilute positronium gas to a velocity distribution of approximately 1 K in 100 ns. A major advancement in the field of low-temperature fundamental physics of antimatter, this study on a purely leptonic system complements work on antihydrogen13, a hadron-containing exotic atom. The successful application of laser cooling to positronium affords unique opportunities to rigorously test bound-state quantum electrodynamics and to potentially realize Bose–Einstein condensation14–18 in this matter–antimatter system.

Organic fluid migration in low permeability reservoirs restricted by pore structure parameters

The flow of fluids in porous media is a complex but essential study; due to the limitation of experimental methods, the research on ultra-low velocity flow of fluids has been developing slowly. This paper investigated the flow of oil in shale through laboratory tests to explore the flow characteristics of organic fluids at ultra-low velocities in ultra-low permeability porous media; the experimental results show that the flow of organic fluids in a specific porous medium at ultra-low velocity still follow Darcy's law, which is different from the previous research that believed that the ultra-low velocity flow of fluids must be non-Darcy flow. However, the flow characteristics of fluids are not static and will change with external conditions (Ambient temperature or Confining pressure). Moreover, an improved mathematical model of fluid flow based on Poiseuille's law is deduced, which can be verified well by the calculation and experimental data. The model comprehensively considers the shale stress sensitivity and the effects of the boundary layer on the fluid flow. The boundary layer is cleverly calculated through the concept of shale oil occurrence and confirmed with molecular dynamics simulation results. The correlation analysis between the critical parameters in the model and the geological parameters shows that any material composition of porous media has a specific influence on the fluid flow characteristics, especially the effect of clay minerals, that cannot be ignored. The analysis results show that when the relative content of clay minerals is about 50%, the porous medium has a better flow capacity; When the relative content of brittle minerals is between 70% and 80%, it has a better compressive ability. Simultaneously, the organic matter content index TOC has a specific inhibitory effect on fluid flow. Therefore, determining the properties of porous media and the changes in the external conditions of porous media is of great significance for engineering practice (e.g., the development of shale oil).

Investigation of ultra-low insertion speeds in an inelastic artificial cochlear model using custom-made cochlear implant electrodes.

Latest research on cochlear implantations focuses on hearing preservation during insertion of the implant's electrode array by reducing insertion trauma. One parameter which may influence trauma is insertion speed. The objective of this study was to extend the range of examined insertion speeds to include ultra-low velocities, being lower than manually feasible, and investigate whether these reduce insertion forces. 24 custom-made cochlear implant test samples were fabricated and inserted into an artificial scala tympani model using 12 different insertion speeds while measuring the resulting insertion forces. Three commercially available slim straight electrode carriers were inserted using the same setup to analyze whether the results are comparable. Insertions of the test samples using high insertion speeds (2.0/2.8mm/s) showed significantly higher insertion forces than insertions done with low insertion speeds (0.2mm/s) or ultra-low insertion speeds (< 0.1mm/s). The insertions with commercial slim straight electrode arrays showed significantly reduced insertion forces when using a low insertion speed as well. Slow insertions showed significantly reduced insertion forces. Insertion speeds which are lower than manually feasible showed even lower insertion forces.

Slow and robust plate acoustic waveguiding with valley-dependent pseudospins

In this letter, we present an elastic slow-wave waveguide, which utilizes and transports valley pseudospins in a two-dimensional plate phononic crystal (PnC). Both straight and bent waveguides are made for demonstrating the backscattering suppression of elastic edge-state transport due to valley-dependent pseudospins. By band structure engineering, elastic waves in a plate waveguide can be transported at ultralow velocities with considerable robustness, which is very promising for the generation of new solid-state phononic circuits with great versatility.

Intermittent and lateral varying ULVZ structure at the northeastern margin of the Pacific LLSVP

AbstractThin patches with ultralow velocities have been proposed to exist at the core‐mantle boundary (CMB). The detection and mapping of ultralow velocity zones (ULVZs) are difficult, in part, because of limited source‐receiver geometries of seismic phases used in ULVZ modeling. Here we develop a new approach that simultaneously utilizes ScS precursor and postcursor energies to investigate the CMB region for ULVZ structure. We stacked source‐deconvolved ScS waveforms within 1.5° geographic bins to extract ScS precursor and postcursor energies, if present, with ScS effectively removed from waveforms. We investigate the CMB beneath the central Pacific Ocean, and evidence for ULVZs is clearly apparent. Geographic bin stacks possessing similar ScS precursor‐plus‐postcursor behavior are grouped by using cluster analysis to produce more robust waveforms by enhancing the signal‐to‐noise ratios. Synthetic seismograms that demonstrate the amplitude and timing of the ULVZ arrivals are sensitive to ULVZ thickness and internal velocities. To pursue local ULVZ properties we processed 13,850 1‐D synthetic models with various ULVZ thicknesses and internal properties, using the identical ScS‐stripping method as with the data. A best fitting model was found for each geographical bin cluster by using an amplitude‐sensitive cross‐correlation algorithm. While limitations exist due to 1‐D modeling, strong lateral variations are clearly apparent in ULVZ thickness and properties across the large low shear velocity province (LLSVP) margin in our study area. Inside hypothesized LLSVP edges, ULVZs appear to distribute unevenly, suggesting 3‐D variations of convection currents.

Melting phase relations in the MgO–MgSiO3 system between 16 and 26 GPa: Implications for melting in Earth's deep interior

Melting experiments in the system MgO–MgSiO3 were performed to study the eutectic melt composition and phase relations between 16 and 26GPa using the mutlianvil apparatus. By employing a multi-chamber capsule design, several different starting compositions along this binary join could be run at a single pressure and temperature and internally consistent phase relations and liquidus compositions were obtained. A single eutectic composition was identified on this join which becomes progressively more MgO-rich over the investigated pressure range. A simple thermodynamic model is developed to describe the melting phase relations based on literature models for melting curves of end-members and a symmetric liquid mixing model.It is shown that melting relations of a natural peridotite composition at high pressure can be reasonably approximated on the basis of the phase relations of this simple binary, and a very good agreement is found once the effects of FeO on phase relations and melting temperatures are considered.The thermodynamic model, extrapolated to pressures covering the entire mantle, predicts that the eutectic composition becomes richer in MgO up to approximately 80GPa, where it becomes near constant with pressure and has a Mg/Si ratio close to that of a peridotite composition. By applying further experimental results to account for the effect of FeO on the melting temperatures, the model predicts that the solidus and liquidus for a peridotitic composition at lower mantle pressures are never more than ∼250K apart.These results can be used to examine a partial melt origin for the existence of localised zones with ultra low shear wave velocities (ULVZ) at the core–mantle boundary (CMB). The solidus temperature of peridotitic mantle at the CMB is estimated to be 4400±300K, which would require temperatures at the CMB to be at the very top end of the estimated range for melting to occur. The proximity of the solidus and liquidus temperature predicted by the model, however, implies that large melt fractions could form over small depth intervals as a result of relatively small increases in either temperature or FeO content. This is consistent with a seismically sharp transition in the upper boundary of ULVZ layers. Given the high temperatures required to melt mantle peridotite at the CMB, a partial melt origin due to raised FeO contents seems more plausible.

Multiphysics modeling of building physical constructions

The paper presents an overview of Multiphysics applications using a Multiphysics modeling package for building physical constructions simulation. The overview includes three main basic transport phenomena for building physical constructions: (1) heat transfer, (2) heat and moisture transfer and (3) heat, air and moisture (HAM) transfer. It is concluded that full 3D transient coupled HAM models for building physical constructions can be build using a Multiphysics modeling package. Regarding the heat transport, neither difficulties nor limitations are expected. Concerning the combined heat and moisture transport the main difficulties are related with the material properties but this seems to be no limitation. Regarding the HAM modeling inside solid constructions, there is at least one limitation: the validation is almost impossible due to limitation of measuring ultra low air velocities of order μm/s.

Preparation of ultralow atomic velocities by transforming bound states into tunneling resonances

A procedure is proposed to prepare the average and width of the velocity distribution of ultracold atoms. The atoms are set initially in the ground state of an optical trap formed by an inner red-detuned-laser well and an outer blue-detuned-laser barrier. Then the well and barrier parameters are changed until the ground state becomes a Breit-Wigner tunneling resonance. An optimal time dependence of the switching process, between the sudden and adiabatic limits, adjusts the final translational energies of the leaking atoms to the Lorentzian distribution of the resonance state.

Thermal flow sensor for ultra-low velocities based on printed circuitboard technology

This paper presents a new thermal flow sensor for ultra-lowvelocities. The sensor was fabricated with the standard printed circuit board(PCB) technology. The technology and the simple sensor design would reducecosts in fabrication and packaging. A one-dimensional analytical model ispresented in order to understand the working principle and to optimize thesensitivity. A two-dimensional finite element analysis (FEA) model is alsopresented. The paper describes the experimental investigations of thetemperature field around a heater under the influence of forced convection.The experimental and theoretical optimization results lead to the design ofthe PCB sensor whose characteristics are presented at the end of the paper.With the current calibration facility, the sensor is able to measure air flowvelocities less than 80 mm s-1.

Slowing down of 3-eV Sm ions in Eu compounds.

Slowing down of ions at ultralow velocities has been studied in the case of 0.02-eV/amu $^{152}\mathrm{Sm}$ recoils in ${\mathrm{EuF}}_{2}$, ${\mathrm{EuF}}_{3}$, ${\mathrm{EuCl}}_{3}$, and ${\mathrm{Eu}}_{2}$${\mathrm{O}}_{3}$. The recoiling atoms were produced in \ensuremath{\beta} decay of $^{152}\mathrm{Eu}$. The Doppler-broadened 842- and 963-keV \ensuremath{\gamma}-rays emitted by the recoiling $^{152}\mathrm{Sm}$ nuclei were detected by a crystal spectrometer. The experimental \ensuremath{\gamma}-ray line shapes were simulated by molecular-dynamics technique using empirically derived interatomic pair potentials. The lifetime of 28\ifmmode\pm\else\textpm\fi{}6 fs was deduced for the first excited state at 963 keV in $^{152}\mathrm{Sm}$.

First-principles simulations of intrinsic collision cascades of 23-eV/amu Si and 5-eV/amu Ti atoms in single-crystal Si and Ti.

The slowing-down process of Ti atoms in crystalline Ti and Si atoms in crystalline Si is studied using the molecular-dynamics method. Based on an experimental technique available for studies of the slowing-down process of atoms at ultralow velocities in bulk matter, the initial recoil energy was 261 eV for Ti and 677 eV for Si atoms. The values correspond to the energies imparted after the thermal neutron capture by primary [gamma] rays. The effect of the crystalline structure on the slowing-down process is studied by calculating the energy distribution of the Doppler-shifted secondary [gamma] rays emitted by the recoiling nuclei. The dependence of the distribution on the interatomic potential used in the simulations is also investigated.

Molecular dynamics simulations applied to the determination of nuclear lifetimes from Doppler-broadened γ-ray line shapes produced in thermal neutron capture reactions

Molecular dynamics simulation of slowing down of nuclei produced in thermal neutron capture reactions is used to deduce lifetime values of excited nuclear states in 36 Cl[τ=63±2 fs (E x=1.959 MeV) , 48±26 (2.49), 31±5 (2.68), 21±1 (2.86), 59±3 (3.60)], 49 Ti[29±2 (1.76), 79±4 (3.18), 16.2±0.7 (3.26)] , 54 Cr [112±22 (2.62), 10.3±0.5 (3.07), 24±2 (3.72)], 57 Fe [52±4 (1.63), 38±3 (1.73)] , 59 Ni [40±7 (2.89), 37±5 (3.18), 6.5±1.4 (4.14)], and 61 Ni [73±14 (2.12)] from Doppler-broadened γ-ray line shapes. It is characteristic for the line shapes that they are produced by emission of secondary γ-rays from nuclei recoiling at ultra-low velocities, below 10 5 m/s, the recoil energy being imparted to nuclei by emission of typically 5–10 MeV primary γ-rays. The novel method of analysis used in the deduction of the lifetime values is described. The relevance of interatomic potentials on the reliability of lifetime values is considered.