Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Evidence of Magnetically Driven Structural Phase Transition in Parent
  Compounds RFeAsO (R = La, Sm, Gd, Tb): study of low-temperature X-ray
  diffraction   We report measurements of structural phase transition of four parent
compounds $R$FeAsO ($R$ = La, Sm, Gd, and Tb) by means of low-temperature X-ray
diffraction (XRD). Magnetic transition temperatures associated with Fe ions
($T_{N1}$) are also determined from the temperature dependence of resistivity.
As $R$ is changed from La, through Sm and Gd, to Tb, both the c-axis and a-axis
lattice constants decrease significantly. Meanwhile both the structural phase
transition temperature ($T_S$) and $T_{N1}$ decrease monotonously. It is also
found that the temperature gap between $T_S$ and $T_{N1}$ becomes smaller when
the distance between FeAs layer becomes shorter. This result is consistent with
magnetically driven structural phase transition and suggests that the
dimensionality have an important effect on the AFM ordering.

27
The categories of this paper are: comp-gas
5
The categories of this paper are: gr-qc
26
The categories of this paper are: bayes-an
34
The categories of this paper are: chem-ph
31
The categories of this paper are: ao-sci
20
The categories of this paper are: acc-phys
1
The categories of this paper are: cs
25
The categories of this paper are: patt-sol
37
The categories of this paper are: supr-con
8
The categories of this paper are: hep-ex
13
The categories of this paper are: quant-ph
35
The categories of this paper are: mtrl-th
24
The categories of this paper are: dg-ga
15
The categories of this paper are: nucl-ex
6
The categories of this paper are: astro-ph
9
The categories of this paper are: nlin