Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Prediction of intrinsic ferroelectricity and large piezoelectricity in
  monolayer arsenic chalcogenides   Two-dimensional materials that exhibit spontaneous electric polarization are
of notable interest for functional materials. However, despite many
two-dimensional polar materials are predicted in theory, the number of
experimentally confirmed two-dimensional ferroelectrics are still far less than
bulk ferroelectrics. We provide strong evidence that the Pmn2$_1$ phase of
arsenic chalcogenides As$_2$X$_3$ (X=S, Se, and Te), which include the recently
isolated monolayer orpiment, are intrinsic ferroelectrics and demonstrate
strong in-plane piezoelectricity. We found the calculated energy barriers for
collectively reversing the electric polarization or moving a 180$^\circ$ domain
wall are reasonable compared to previously reported ferroelectrics. We propose
a high-symmetry structure (with Pmmn space group) transforms into the
ferroelectric Pmn2$_1$ phase by a soft B$_{2u}$ phonon mode. By studying other
soft modes of the high-symmetry Pmmn structure, we identify several
undiscovered metastable polymorphs, including a polar phase (with a P2$_1$
space group) with sizable piezoelectricity.

37
The categories of this paper are: supr-con
32
The categories of this paper are: atom-ph
19
The categories of this paper are: eess
33
The categories of this paper are: plasm-ph
36
The categories of this paper are: cmp-lg
30
The categories of this paper are: q-alg
8
The categories of this paper are: hep-ex
5
The categories of this paper are: gr-qc
35
The categories of this paper are: mtrl-th
17
The categories of this paper are: q-fin
0
The categories of this paper are: hep-ph
13
The categories of this paper are: quant-ph
28
The categories of this paper are: alg-geom
24
The categories of this paper are: dg-ga
12
The categories of this paper are: nucl-th
23
The categories of this paper are: solv-int