Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Ultracold electrons via Near-Threshold Photoemission from Single-Crystal
  Cu(100)   Achieving a low mean transverse energy or temperature of electrons emitted
from the photocathode-based electron sources is critical to the development of
next-generation and compact X-ray Free Electron Lasers and Ultrafast Electron
Diffraction, Spectroscopy and Microscopy experiments. In this paper, we
demonstrate a record low mean transverse energy of 5 meV from the cryo-cooled
(100) surface of copper using near-threshold photoemission. Further, we also
show that the electron energy spread obtained from such a surface is less than
11.5 meV, making it the smallest energy spread electron source known to date:
more than an order of magnitude smaller than any existing photoemission, field
emission or thermionic emission based electron source. Our measurements also
shed light on the physics of electron emission and show how the energy spread
at few meV scale energies is limited by both the temperature and the vacuum
density of states.

33
The categories of this paper are: plasm-ph
28
The categories of this paper are: alg-geom
30
The categories of this paper are: q-alg
23
The categories of this paper are: solv-int
27
The categories of this paper are: comp-gas
8
The categories of this paper are: hep-ex
2
The categories of this paper are: math
12
The categories of this paper are: nucl-th
15
The categories of this paper are: nucl-ex
16
The categories of this paper are: stat
32
The categories of this paper are: atom-ph
1
The categories of this paper are: cs
22
The categories of this paper are: chao-dyn
0
The categories of this paper are: hep-ph
18
The categories of this paper are: econ
13
The categories of this paper are: quant-ph