Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: First order transition in trigonal structure
  ${\textbf{Ca}}{\textbf{Mn}}_{2}{\textbf{P}}_{2}$   We report structural and physical properties of the single crystalline
${\mathrm{Ca}}{\mathrm{Mn}}_{2}{\mathrm{P}}_{2}$. The X-ray diffraction(XRD)
results show that ${\mathrm{Ca}}{\mathrm{Mn}}_{2}{\mathrm{P}}_{2}$ adopts the
trigonal ${\mathrm{Ca}}{\mathrm{Al}}_{2}{\mathrm{Si}}_{2}$-type structure.
Temperature dependent electrical resistivity $\rho(T)$ measurements indicate an
insulating ground state for ${\mathrm{Ca}}{\mathrm{Mn}}_{2}{\mathrm{P}}_{2}$
with activation energies of 40 meV and 0.64 meV for two distinct regions,
respectively. Magnetization measurements show no apparent magnetic phase
transition under 400 K. Different from other
${\mathrm{A}}{\mathrm{Mn}}_{2}{\mathrm{Pn}}_{2}$ (A = Ca, Sr, and Ba, and Pn =
P, As, and Sb) compounds with the same structure, heat capacity
$C_{\mathrm{p}}(T)$ and $\rho(T)$ reveal that
${\mathrm{Ca}}{\mathrm{Mn}}_{2}{\mathrm{P}}_{2}$ has a first-order transition
at $T$ = 69.5 K and the transition temperature shifts to high temperature upon
increasing pressure. The emergence of plenty of new Raman modes below the
transition, clearly suggests a change in symmetry accompanying the transition.
The combination of the structural, transport, thermal and magnetic
measurements, points to an unusual origin of the transition.

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The categories of this paper are: chem-ph
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The categories of this paper are: hep-lat
2
The categories of this paper are: math
27
The categories of this paper are: comp-gas
7
The categories of this paper are: hep-th
26
The categories of this paper are: bayes-an
30
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: ao-sci
3
The categories of this paper are: physics
8
The categories of this paper are: hep-ex
28
The categories of this paper are: alg-geom
9
The categories of this paper are: nlin
16
The categories of this paper are: stat
22
The categories of this paper are: chao-dyn
6
The categories of this paper are: astro-ph