Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Direct observation of lattice symmetry breaking at the hidden-order
  transition in URu2Si2   Since the 1985 discovery of the phase transition at $T_{\rm HO}=17.5$ K in
the heavy-fermion metal URu$_2$Si$_2$, neither symmetry change in the crystal
structure nor magnetic ordering have been observed, which makes this "hidden
order" enigmatic. Some high-field experiments have suggested electronic
nematicity which breaks fourfold rotational symmetry, but direct evidence has
been lacking for its ground state at zero magnetic field. Here we report on the
observation of lattice symmetry breaking from the fourfold tetragonal to
twofold orthorhombic structure by high-resolution synchrotron X-ray diffraction
measurements at zero field, which pins down the space symmetry of the order.
Small orthorhombic symmetry-breaking distortion sets in at $T_{\rm HO}$ with a
jump, uncovering the weakly first-order nature of the hidden-order transition.
This distortion is observed only in ultrapure sample, implying a highly unusual
coupling nature between the electronic nematicity and underlying lattice.

26
The categories of this paper are: bayes-an
16
The categories of this paper are: stat
36
The categories of this paper are: cmp-lg
12
The categories of this paper are: nucl-th
34
The categories of this paper are: chem-ph
21
The categories of this paper are: adap-org
33
The categories of this paper are: plasm-ph
27
The categories of this paper are: comp-gas
31
The categories of this paper are: ao-sci
25
The categories of this paper are: patt-sol
1
The categories of this paper are: cs
18
The categories of this paper are: econ
9
The categories of this paper are: nlin
20
The categories of this paper are: acc-phys
30
The categories of this paper are: q-alg
6
The categories of this paper are: astro-ph