Narrow Momentum Distribution Research Articles

A unified description of the halo nucleus 37Mg from microscopic structure to reaction observables

Based on the structure input directly from the deformed relativistic Hartree-Bogoliubov theory in continuum (DRHBc), we fully study reaction observables to search for halo evidence of 37Mg with the Glauber model. In this scheme, we evaluate the reaction cross sections of 20–37Mg bombarding a C target at 240 MeV/nucleon and find that most of the results agree well with the experimental data within 1σ uncertainty, especially good for the case of 37Mg. Our predicted doubling of the cross section change from 36Mg to 37Mg over that from 34Mg to 35Mg is consistent with a halo structure in 37Mg. Additionally, our calculated narrow longitudinal momentum distribution of 36Mg residues from 37Mg + 12C breakup agrees well with data and imply a nuclear size much larger than 35Mg. Furthermore, our study shows a dominant p-wave contribution to the longitudinal momentum distribution, supporting the conclusion that 37Mg is a p-wave halo nucleus. Our study provides the first unified description of the halo characteristics of 37Mg from nuclear structure to reaction dynamics.

Engineering the radiative dynamics of thermalized excitons with metal interfaces

As a platform for optoelectronic devices based on exciton dynamics, monolayer transition metal dichalcogenides (TMDCs) are often placed near metal interfaces or inside planar cavities. While the radiative properties of point dipoles at metal interfaces has been studied extensively, those of excitons, which are delocalized and exhibit a temperature-dependent momentum distribution, lack a thorough treatment. Here, we analyze the emission properties of excitons in TMDCs near planar metal interfaces and explore their dependence on exciton center-of-mass momentum, transition dipole orientation, and temperature. Defining a characteristic energy scale k B T c = (ℏk)2/2m (k being the radiative wavevector and m the exciton mass), we find that at temperatures T ≫ T c and low densities where the momentum distribution can be characterized by Maxwell–Boltzmann statistics, the modified emission rates (normalized to free space) behave similarly to point dipoles. This similarity in behavior arises due to the broad nature of wavevector components making up the exciton and point dipole emission. On the other hand, the narrow momentum distribution of excitons for T < T c can result in significantly different emission behavior as compared to point dipoles. These differences can be further amplified by considering excitons with a Bose Einstein distribution at high phase space densities, such as in a condensate phase. We find suppression or enhancement of emission relative to the point dipole case by several orders of magnitude. These insights can help optimize the performance of optoelectronic devices that incorporate 2D semiconductors near metal electrodes and can inform future studies of exciton radiative dynamics at low temperatures. Additionally, these studies show that nanoscale optical cavities are a viable pathway to generating long-lifetime exciton states in TMDCs.

Single-cycle versus multicycle nonsequential double ionization of argon

Using an improved quantitative rescattering model, we calculate the correlated two-electron momentum distributions (CMDs) for nonsequential double ionization of Ar exposed to intense laser pulses with a wavelength of 790 nm at a peak intensity of $1.0\ifmmode\times\else\texttimes\fi{}{10}^{14}\phantom{\rule{4pt}{0ex}}\mathrm{W}/{\mathrm{cm}}^{2}$. We analyze the drastic variations in the CMDs that were observed by K\"ubel et al. [New J. Phys. 16, 033008 (2014)] in the transition from near-single-cycle to multicycle driving laser pulses. Our model reproduces their experimental data well. We also find that the transition from near-single-cycle to multicycle driving laser pulses depends strongly on the details of the pulse envelope. Special attention is paid to the mechanisms responsible for the cross-shaped structure observed experimentally with 4 fs pulses. Our analysis reveals that the cross-shaped structure in the carrier-envelope phase-averaged CMD for near-single-cycle pulses can be attributed to strong backward scattering of the recolliding electron as well as the narrow momentum distributions of the tunnel-ionized electrons compared to those for long pulses. This also explains why the cross-shaped distributions collapse to a rather structureless distribution when the pulse duration is increased to 8 fs.

Evidence of a Proton Halo in 23Al: A Mean Field Analysis

We have studied the nuclear structure properties like binding energy, charge radius, quadrupole deformation parameter for various isotopes of Al from the valley of stability to drip line region using the well known relativistic mean field formalism (RMF) with NL3 parameter set. We have compared our results with experimental data and found reasonable agreement. Further, we have taken spherical and deformed RMF densities to estimate the reaction dynamics of $^{23-28}$Al isotopes as projectiles and $^{12}$C as target by conjunction in Glauber model. The estimated results are also compared with the experimental data. The analysis of angular elastic differential and one proton removal cross sections are also studied with Glauber model many body system to investigate the structural feature of $^{23}$Al. The evidence of enhanced total reaction cross section, higher value of rms radius, narrow longitudinal momentum distribution and small proton separation energy of $^{23}$Al support its proton halo structure.

Excited-state one-neutron halo nuclei within a parallel momentum distribution analysis

Using a fully quantum mechanical post-form finite-range distorted-wave Born approximation theory of Coulomb breakup, I study the parallel momentum distribution of the core in the Coulomb breakup of suggested excited-state one-neutron halo nuclei considered in their different bound excited states. Narrow momentum distributions obtained in the present calculations for some cases indicate the possibilities of the excited-state halo structure in the nuclei under consideration and therefore favor the previous predictions.

A three-dimensional time-dependent Schrödinger equation solver: an application to hydrogen atoms in an elliptical laser field

We extended the two-dimensional generalized pseudo-spectral time-propagator to a three-dimensional time-propagator. With this time-propagator, we theoretically investigated the above-threshold ionization of hydrogen atoms in an elliptical field by solving the time-dependent Schrödinger equation. We found that the total ionization probabilities are suppressed about one order, while the excitation probabilities drop about eight orders from a linearly polarized laser to a circularly polarized one with a given laser intensity. The width of the lateral (perpendicular to the laser polarization plane) momentum distribution increases as the ellipticity increases, which is against our intuition that the large peak laser field results in a broad distribution in the lateral direction. Further analysis shows that the narrow lateral momentum distribution for linearly polarized laser fields originates from Coulomb focusing, which is as less or not important for the circularly polarized field. Such a high precision simulation can be used to calibrate the infrared laser intensity to a high precision.

The parent populations of six groups identified from chemical tagging in the solar neighbourhood

We estimate the size and distribution of the parent populations for the 6 largest (at least 20 stars in the Solar neighborhood) chemical groups identified in the Chemical Tagging experiment by Mitschang et al.~2014. Stars in the abundance groups tend to lie near a boundary in angular momentum versus eccentricity space where the probability is highest for a star to be found in the Solar neighborhood and where orbits have apocenter approximately equal to the Sun's galactocentric radius. Assuming that the parent populations are uniformly distributed at all azimuthal angles in the Galaxy, we estimate that the parent populations of these abundance groups contain at least 200,000 members. The spread in angular momentum of the groups implies that the assumption of a uniform azimuthal distribution only fails for the two youngest groups and only for the highest angular momentum stars in them. The parent populations of three thin disk groups have narrow angular momentum distributions, but tails in the eccentricity and angular momentum distributions suggest that only a small fraction of stars have migrated and increased in eccentricity. In contrast, the parent populations of the thick disk groups exhibit both wide angular momentum and eccentricity distributions implying that both heating and radial migration has taken place.

Coulomb breakup of 37Mg and its ground state structure

We calculate Coulomb breakup of the neutron rich nucleus 37Mg on a Pb target at the beam energy of 244 MeV/nucleon within the framework of a finite range distorted wave Born approximation theory that is extended to include the effects of projectile deformation. In this theory, the breakup amplitude involves the full wave function of the projectile ground state. Calculations have been carried out for the total one-neutron removal cross section (σ−1n), the neutron–core relative energy spectrum, the parallel momentum distribution of the core fragment, the valence neutron angular, and energy–angular distributions. The calculated σ−1n has been compared with the recently measured data to put constraints on the spin parity, and the one-neutron separation energy (Sn) of the 37Mg ground state (37Mggs). The dependence of σ−1n on the deformation of this state has also been investigated. While a spin parity assignment of 7/2− for the 37Mggs is ruled out by our study, neither of the 3/2− and 1/2+ assignments can be clearly excluded. Using the spectroscopic factor of one for both the 3/2− and 1/2+ configurations and ignoring the projectile deformation effects, the Sn values of 0.35±0.06 MeV and 0.50±0.07 MeV, respectively, are extracted for the two configurations. However, the extracted Sn is strongly dependent on the spectroscopic factor and the deformation effects of the respective configuration. The narrow parallel momentum distribution of the core fragment and the strong forward peaking of the valence neutron angular distribution suggest a one-neutron halo configuration in either of the 2p3/2 and 2s1/2 configurations of the 37Mg ground state.

Experimental realization of a quantum ratchet through phase modulation

We report on an experimental realization of unidirectional transporting island structures in an otherwise chaotic phase space of the delta-kicked rotor system. Using a Bose-Einstein Condensate as a source of ultracold atoms, we employ asymmetric phase modulation in the kicks, with the narrow momentum distribution of the atoms allowing us to address individual island structures. We observe quantum ratchet behavior in this system, with clear directed momentum current in the absence of a directional force, which we characterize and connect to \epsilon-classical theory.

One- and two-neutron removal reactions from the most neutron-rich carbon isotopes

The structure of 19,20,22C has been investigated using high-energy (around 240 MeV/nucleon) one- and two-neutron removal reactions on a carbon target. Measurements were made of the inclusive cross sections and momentum distributions for the charged residues. Narrow momentum distributions were observed for one-neutron removal from 19C and 20C and two-neutron removal from 22C. Two-neutron removal from 20C resulted in a relatively broad momentum distribution. The results are compared with eikonal-model calculations combined with shell-model structure information. The neutron removal cross sections and associated momentum distributions are calculated for transitions to both the particle-bound and particle-unbound final states. The calculations take into account the population of the mass A−1 reaction residues A−1C and, following one-neutron emission after one-neutron removal, the mass A−2 two-neutron removal residues A−2C. The smaller contributions of direct two-neutron removal, that populate the A−2C residues in a single step, are also computed. The data and calculations are shown to be in good overall agreement and consistent with the predicted shell-model ground-state configurations and one-neutron overlaps with low-lying states in 18−21C. These suggest significant νs1/22 valence neutron configurations in both 20C and 22C. The results for 22C strongly support the picture of 22C as a two-neutron halo nucleus with a dominant νs1/22 ground-state configuration.Received 30 November 2011DOI:https://doi.org/10.1103/PhysRevC.86.054604©2012 American Physical Society

SCATTERING TIMES IN THE MODIFIED PÖSCHL–TELLER POTENTIAL WELL

We study the scattering of a Gaussian wave packet in the modified Pöschl–Teller potential well in time respect. Analytical expressions of average dwell time, phase time, and average traversal time due to Larmor-clock times are obtained, from which the average traversal time can be identified with the average dwell time. We observe that the average dwell time is appreciably longer than the phase time at low energies, but they tend to coincide in the limit of narrow momentum distribution, high energy, and large interaction region. We also find that both of them are longer than the classical scattering time in the potential. All of these times are, however, shorter than the free-particle time, demonstrating the acceleration effect of the potential in temporal aspect of the scattering.

Structure ofN22and theN=14subshell

One-neutron knockout data for N18−22 are analyzed in the eikonal approximation of the Glauber model. The role of the s-d shell and the crossing of the N=14 neutron subshell are discussed. Of particular interest is the nucleus N22, where the knockout data provide a sensitive experimental test for a possible halo structure of its ground state. The observation of a narrow momentum distribution of the N21 fragments, together with an essential 1s1/2 contribution needed to describe the observed longitudinal-momentum distribution, allow the firm conclusion that the ground state of N22 is a well-developed nuclear halo. The results also show that the N=14 subshell in N22 is somewhat reduced as compared to that of O23.Received 8 February 2011DOI:https://doi.org/10.1103/PhysRevC.83.064313©2011 American Physical Society

Exploring the Phase Space of the Quantumδ-Kicked Accelerator

We experimentally explore the underlying pseudoclassical phase space structure of the quantum delta-kicked accelerator. This was achieved by exposing a Bose-Einstein condensate to the spatially corrugated potential created by pulses of an off-resonant standing light wave. For the first time quantum accelerator modes were realized in such a system. By utilizing the narrow momentum distribution of the condensate we were able to observe the discrete momentum state structure of a quantum accelerator mode and also to directly measure the size of the structures in the phase space.

High-Order Quantum Resonances Observed in a Periodically Kicked Bose-Einstein Condensate

We have observed high-order quantum resonances in a realization of the quantum delta-kicked rotor, using Bose-condensed Na atoms subjected to a pulsed standing wave of laser light. These resonances occur for pulse intervals that are rational fractions of the Talbot time, and are characterized by ballistic momentum transfer to the atoms. The condensate's narrow momentum distribution not only permits the observation of the quantum resonances at 3/4 and 1/3 of the Talbot time, but also allows us to study scaling laws for the resonance width in quasimomentum and pulse interval.

Feature Article:Structure and Reactions of Halo Nuclei

It is a challenge to artificially produce thousands of nuclei that do not exist in nature, to study their properties and to explore their roles in forming nature. Recent techniques with radioactive beams have made it possible to reach the limit of the neutron dripline for relatively light nuclei. At this limit for Z=9, one of the most fascinating discoveries was that of a neutron halo in 11Li. The measurements that led to the discovery showed enhanced interaction cross-sections, narrow momentum distributions of 9Li fragments, and large Coulomb dissociation cross-sections. Subsequently, the neutron halos were confirmed in several other nuclei near the dripline, for example, 6He, 11Be, 14Be, and 19C.

Observation of a two-proton halo in 17Ne

The measurement of longitudinal momentum distribution for two-proton removal from the proton-drip line nucleus 17Ne with a Be target at 64 A MeV is reported. The observed narrow momentum distribution and the large interaction cross-section suggests the formation of a two-proton halo. The data analyzed within the Few-body Glauber model suggests a significant probability of the two valence protons to abnormally occupy the 2s 1/2 orbit, indicating its lowering in proton-rich nuclei.

Quantum reflection times for attractive potential tails.

When a wave packet with a narrow momentum distribution is quantum reflected in a purely attractive potential proportional to -1/r(alpha), alpha>2, it generally experiences a time gain compared to a free particle reflected at r=0; for alpha=3 and very low energies there are large time delays. In quantum reflection of an atomic beam by a surface, such a time gain (delay) represents an apparent plane of reflection which is shifted in front of (behind) the surface. The quantum reflected wave is always delayed with respect to the classical particle accelerated in the attractive potential.

Stopping energetic heavy ions in one-bar helium: narrow incident momentum distributions

Various tests for stopping energetic, 100– 150 MeV /amu, heavy ions with narrow momentum distributions in a 50 cm gas cell filled with helium at 0.5– 2 bar pressure were performed. The fraction of ions stopped in the gas was measured as a function of absorber thickness, gas pressure and length of ion path within the gas cell. Information on the range distribution was obtained and the residual energy distribution of ions was measured under different conditions. The experimental results for various beams are compared with stopping power calculations in the literature. The results are discussed in the context of the proposed RIA facility.

Early time diffusion for the quantum kicked rotor with narrow initial momentum distributions.

We investigate analytically and numerically early-time momentum diffusion rates for the delta-kicked rotor across the quantum to classical transition, i.e., as increased total system action produces more macroscopic dynamics. For sufficiently narrow initial momentum distributions we find a rich structure of resonances in these diffusion rates as a function of the effective Planck's constant. Our study is set in the physical context of the atom optics kicked rotor, and numerical simulations confirm that the resonances persist with kicks of finite duration and with other typical experimental imperfections, such as spontaneous emission noise. Our results should be testable in experiments where narrow initial momentum distributions are prepared using, for example, velocity selective Raman transitions or Bose-Einstein condensates.

Positron annihilation in SiO 2–Si studied by a pulsed slow positron beam

Positron and positronium (Ps) behavior in SiO 2–Si have been studied by means of positron annihilation lifetime spectroscopy (PALS) and age-momentum correlation (AMOC) spectroscopy with a pulsed slow positron beam. The PALS study of SiO 2–Si samples, which were prepared by a dry-oxygen thermal process, revealed that the positrons implanted in the Si substrate and diffused back to the interface do not contribute to the ortho-Ps long-lived component, and the lifetime spectrum of the interface has at least two components. From the AMOC study, the momentum distribution of the ortho-Ps pick-off annihilation in SiO 2, which shows broader momentum distribution than that of crystalline Si, was found to be almost the same as that of free positron annihilation in SiO 2. A varied interface model was proposed to interpret the results of the metal-oxide-semiconductor (MOS) experiments. The narrow momentum distribution found in the n-type MOS with a negative gate bias voltage could be attributed to Ps formation and rapid spin exchange in the SiO 2–Si interface. We have developed a two-dimensional positron lifetime technique, which measures annihilation time and pulse height of the scintillation gamma-ray detector for each event. Using this technique, the positronium behavior in a porous SiO 2 film, grown by a sputtering method, has been studied.