Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Broadly Tunable Sub-terahertz Emission from Internal Branches of the
  Current-voltage Characteristics of Superconducting Bi2Sr2CaCu2O8+d Single
  Crystals   Continuous, coherent sub-terahertz radiation arises when a dc voltage is
applied across a stack of the many intrinsic Josephson junctions in a
Bi2Sr2CaCu2O8+d single crystal. The active junctions produce an equal number of
I-V characteristic branches. Each branch radiates at a slightly tunable
frequency obeying the ac Josephson relation. The overall output is broadly
tunable and nearly independent of heating effects and internal cavity
frequencies. Amplification by a surrounding external cavity to allow for the
development of a useful high-power source is proposed.

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The categories of this paper are: physics
37
The categories of this paper are: supr-con
20
The categories of this paper are: acc-phys
13
The categories of this paper are: quant-ph
11
The categories of this paper are: hep-lat
30
The categories of this paper are: q-alg
25
The categories of this paper are: patt-sol
9
The categories of this paper are: nlin
14
The categories of this paper are: math-ph
16
The categories of this paper are: stat
27
The categories of this paper are: comp-gas
7
The categories of this paper are: hep-th
17
The categories of this paper are: q-fin
23
The categories of this paper are: solv-int
0
The categories of this paper are: hep-ph
6
The categories of this paper are: astro-ph