Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: A15 Nb$_3$Si -- A "high" Tc superconductor synthesized at a pressure of
  one megabar and metastable at ambient conditions   A15 Nb$_3$Si is, until now, the only high temperature superconductor produced
at high pressure (~110 GPa) that has been successfully brought back to room
pressure conditions in a metastable condition. Based on the current great
interest in trying to create metastable-at-room-pressure high temperature
superconductors produced at high pressure, we have restudied explosively
compressed A15 Nb$_3$Si and its production from tetragonal Nb$_3$Si. First,
diamond anvil cell pressure measurements up to 88 GPa were performed on
explosively compressed A15 Nb$_3$Si material to trace Tc as a function of
pressure. Tc is suppressed to ~ 5.2 K at 88 GPa. Then, using these Tc (P) data
for A15 Nb$_3$Si, pressures up to 92 GPa were applied at room temperature
(which increased to 120 GPa at 5 K) on tetragonal Nb$_3$Si. Measurements of the
resistivity gave no indication of any A15 structure production, i.e., no
indications of the superconductivity characteristic of A15 Nb$_3$Si. This is in
contrast to the explosive compression (up to P~110 GPa) of tetragonal Nb$_3$Si,
which produced 50-70% A15 material, Tc = 18 K at ambient pressure, in a 1981
Los Alamos National Laboratory experiment. Our theoretical calculations show
that A15 Nb$_3$Si has an enthalpy vs the tetragonal structure that is 0.07
eV/atom smaller at 100 GPa, implying that the accompanying high temperature
(1000 deg C) caused by explosive compression is necessary to successfully drive
the reaction kinetics of the tetragonal -> A15 Nb$_3$Si structural
transformation. Annealing experiments on the A15 explosively compressed
material reaching time scales of 39 years are consistent with this viewpoint.

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The categories of this paper are: hep-ex
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The categories of this paper are: bayes-an
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The categories of this paper are: cs
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The categories of this paper are: astro-ph
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The categories of this paper are: quant-ph
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The categories of this paper are: ao-sci
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The categories of this paper are: hep-lat
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The categories of this paper are: nucl-th
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The categories of this paper are: nlin
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The categories of this paper are: patt-sol
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The categories of this paper are: gr-qc
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The categories of this paper are: comp-gas
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The categories of this paper are: dg-ga
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The categories of this paper are: q-alg
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The categories of this paper are: cmp-lg
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The categories of this paper are: solv-int