Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Tunable sub-luminal propagation of narrowband x-ray pulses   Group velocity control is demonstrated for x-ray photons of 14.4 keV energy
via a direct measurement of the temporal delay imposed on spectrally narrow
x-ray pulses. Sub-luminal light propagation is achieved by inducing a steep
positive linear dispersion in the optical response of ${}^{57}$Fe M\"ossbauer
nuclei embedded in a thin film planar x-ray cavity. The direct detection of the
temporal pulse delay is enabled by generating frequency-tunable spectrally
narrow x-ray pulses from broadband pulsed synchrotron radiation. Our
theoretical model is in good agreement with the experimental data.

20
The categories of this paper are: acc-phys
18
The categories of this paper are: econ
1
The categories of this paper are: cs
4
The categories of this paper are: cond-mat
25
The categories of this paper are: patt-sol
34
The categories of this paper are: chem-ph
33
The categories of this paper are: plasm-ph
36
The categories of this paper are: cmp-lg
37
The categories of this paper are: supr-con
35
The categories of this paper are: mtrl-th
2
The categories of this paper are: math
15
The categories of this paper are: nucl-ex
11
The categories of this paper are: hep-lat
6
The categories of this paper are: astro-ph
30
The categories of this paper are: q-alg
23
The categories of this paper are: solv-int