Low-velocity antiproton stopping power

Ab Initio Prediction of a Negative Barkas Coefficient for Slow Protons and Antiprotons in LiF

We report the ab initio prediction of a negative Barkas coefficient in lithium fluoride (LiF) insulator at low velocity (v<0.25  a.u., E_{kin}∼2  keV). The electronic stopping power of protons in LiF has been extensively studied both experimentally and theoretically because of a controversial threshold effect. While our time-dependent density-functional theory simulations confirm the presence of a velocity threshold below which the proton stopping power vanishes, our calculations demonstrate that the antiprotons do not experience such a threshold. The combination of those two contrasting behaviors gives rise to an unprecedented negative Barkas effect: the stopping power of antiprotons is larger than that of protons. We identify that the slow antiproton at close encounter destabilizes a p orbital of the F^{-} anion pointing toward the antiproton. This particular orbital becomes highly polarizable and hence contributes much to the stopping power.

Interactions of Single Particle with Organic Matters: A Facile Bottom-Up Approach to Low Dimensional Nanostructures

A particle induces a pack of chemical reactions in nanospace: chemical reactions confined into extremely small space provide an ultimate technique for the nanofabrication of organic matter with a variety of functions. Since the discovery of particle accelerators, an extremely high energy density can be deposited, even by a single isolated particle with MeV-ordered kinetic energy. However, this was considered to cause severe damages to organic molecules due to its relatively small bond energies, and lack of ability to control the reactions precisely to form the structures while retaining physico-chemical molecular functionalities. Practically, the severely damaged area along a particle trajectory: a core of a particle track has been simply visualized for the detection/dosimetry of an incident particle to the matters, or been removed to lead nanopores and functionalized by refilling/grafting of fresh organic/inorganic materials. The use of intra-track reactions in the so-called “penumbra” or “halo” area of functional organic materials has been realized and provided us with novel and facile protocols to provide low dimensional nano-materials with perfect size controllability in the 21st century. These protocols are now referred to as single particle nanofabrication technique (SPNT) and/or single particle triggered linear polymerization technique (STLiP), paving the way towards a new approach for nanomaterials with desired functionalities from original molecules. Herein, we report on the extremely wide applicability of SPNT/STLiP protocols for the future development of materials for opto-electronic, catalytic, and biological applications among others.

Time-dependent screening of a point charge at a metal surface

The space-time evolution of the dynamical screening charge density caused by a suddenly created point charge at the Cu(111) surface is investigated in the linear response approximation. Considering a thin slab as a model for the Cu(111) surface, we investigate the confinement effects on dynamical screening as well. The results have been obtained on the basis of self-consistent evaluation of the energy-momentum-dependent response function, taking into account the realistic surface band structure of Cu(111). At the initial stage, we observe fast long-range charge density oscillations due to excitation of the surface plasmon modes. Then we observe the propagation of the shock wave of the electron-hole excitations along the slab with velocity determined by the Fermi velocity of bulk Cu. At longer times, we have identified the propagation along the two slab surfaces of a much slower (with velocity ∼ 0.3 au, close to the Fermi velocity of the Cu(111) surface state) charge disturbance due to acoustic surface plasmon. The role of the energy band gap in the direction perpendicular to the surface in establishing the screening is also addressed.

Coupling of conduction electrons to two-level systems formed by hydrogen: a scattering approach

An effective Hamiltonian for a two-level system (TLS) which could model the interaction between a tunneling proton and the conduction electrons of a metal is investigated in a comparative way. In the conventional first-order Born approximation with plane waves, and for small-distance displacement of the tunneling particle, a simple correlation between the atomic motion and angular momentum change of the scattering electron is deduced. For such a displacement, and within a distorted wave Born approximation for initial and final states, the change in the scattering amplitude is expressed via bounded trigonometric functions of the corresponding difference of scattering phase shifts. The numerical value of this amplitude change is analyzed in the framework of a self-consistent screening description for an impurity embedding in a paramagnetic electron gas. The coupling thus obtained of the tunneling proton to a homogeneous electron gas is too weak to be in the range required for realization of the two-channel Kondo effect.

Antiproton stopping at low energies: confirmation of velocity-proportional stopping power

The stopping power for antiprotons in various solid targets has been measured in the low-energy range of 1-100 keV. In agreement with most models, in particular free-electron gas models, the stopping power is found to be proportional to the projectile velocity below the stopping-power maximum. Although a stopping power proportional to velocity has also been observed for protons, the interpretation of such measurements is difficult due to the presence of charge exchange processes. Hence, the present measurements constitute the first unambiguous support for a velocity-proportional stopping power due to target excitations by a pointlike projectile.