Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Stable $\pi$-radical 2,2-diphenyl-1-picrylhydrazyl (DPPH) adsorbed at
  the elbows of $22\times\sqrt{3}$ reconstructed Au(111)   Stable organic radicals serve as model systems for investigating metal-free
magnetic phenomena at (sub)nanometer length scales. We have investigated at the
single-molecule level the stable $\pi$-radical 2,2-diphenyl-1-picrylhydrazyl
(DPPH) adsorbed at the elbow sites of the $22\times\sqrt{3}$ reconstructed
Au(111) surface. Low-temperature scanning tunneling microscopy (STM) and
-spectroscopy under ultrahigh vacuum conditions at 8~K reveal structural as
well as frontier-orbital related electronic details of DPPH/Au(111). A
Kondo-like spectroscopic signature indicates preservation of the unpaired
electron spin state.

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The categories of this paper are: hep-ph
19
The categories of this paper are: eess
29
The categories of this paper are: funct-an
12
The categories of this paper are: nucl-th
34
The categories of this paper are: chem-ph
27
The categories of this paper are: comp-gas
15
The categories of this paper are: nucl-ex
10
The categories of this paper are: q-bio
1
The categories of this paper are: cs
24
The categories of this paper are: dg-ga
30
The categories of this paper are: q-alg
13
The categories of this paper are: quant-ph
17
The categories of this paper are: q-fin
2
The categories of this paper are: math
11
The categories of this paper are: hep-lat
16
The categories of this paper are: stat