Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Positronium formation in graphene and graphite   Positronium (Ps) formation on the surface of clean polycrystalline copper
(Cu), highly oriented pyrolytic graphite (HOPG) and multi layer graphene (MLG)
grown on a polycrystalline copper substrate has been investigated as a function
of incident positron kinetic energy (1.5eV to 1keV). Measurments on Cu indicate
that as the kinetic energy of the incident positrons increases from 1.5eV to
900eV, the fraction of positrons that form Ps ($f_{Ps}$) decreases from ~0.5 to
~0.3. However, in HOPG and MLG, instead of a monotonic decrease of $f_{Ps}$
with positron kinetic energy, a sharp peak is observed at ~ 5eV and above
~200eV,remains nearly constant in HOPG and MLG. We propose that in HOPG and
MLG, at low incident positron energies the Ps formation is dominated either by
a surface Plasmon assisted electron pick up process or by an energy dependent
back scattering process. Both these processes can explain the peak observed and
the present data can help to augment the understanding of Ps formation from
layered materials.

12
The categories of this paper are: nucl-th
37
The categories of this paper are: supr-con
3
The categories of this paper are: physics
28
The categories of this paper are: alg-geom
10
The categories of this paper are: q-bio
2
The categories of this paper are: math
20
The categories of this paper are: acc-phys
19
The categories of this paper are: eess
30
The categories of this paper are: q-alg
9
The categories of this paper are: nlin
22
The categories of this paper are: chao-dyn
29
The categories of this paper are: funct-an
24
The categories of this paper are: dg-ga
1
The categories of this paper are: cs
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The categories of this paper are: solv-int
5
The categories of this paper are: gr-qc