Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Polarized phonons carry the missing angular momentum in femtosecond
  demagnetization   Magnetic phenomena are ubiquitous in our surroundings and indispensable for
modern science and technology, but it is notoriously difficult to change the
magnetic order of a material in a rapid way. However, if a thin nickel film is
subjected to ultrashort laser pulses, it can lose its magnetic order almost
completely within merely femtosecond times. This phenomenon, in the meantime
also observed in many other materials, has connected magnetism with femtosecond
optics in an efficient, ultrafast and complex way, offering opportunities for
rapid information processing or ultrafast spintronics at frequencies
approaching those of light. Consequently, the physics of ultrafast
demagnetization is central to modern material research, but a crucial question
has remained elusive: If a material loses its magnetization within only
femtoseconds, where is the missing angular momentum in such short time? Here we
use ultrafast electron diffraction to reveal in nickel an almost instantaneous,
long-lasting, non-equilibrium population of anisotropic high-frequency phonons
that appear as quickly as the magnetic order is lost. The anisotropy plane is
perpendicular to the direction of the initial magnetization and the atomic
oscillation amplitude is 2 pm. We explain these observations by means of
circularly polarized phonons that quickly absorb the missing angular momentum
of the spin system before the slower onset of a macroscopic sample rotation.
The time that is needed for demagnetization is related to the time it takes to
accelerate the atoms. These results provide an atomistic picture of ultrafast
demagnetization under adherence to all conservation laws but also demonstrate
the general importance of polarized phonons for non-equilibrium dynamics and
provide innovative ways for controlling materials on atomic dimensions.

11
The categories of this paper are: hep-lat
15
The categories of this paper are: nucl-ex
7
The categories of this paper are: hep-th
24
The categories of this paper are: dg-ga
10
The categories of this paper are: q-bio
21
The categories of this paper are: adap-org
9
The categories of this paper are: nlin
36
The categories of this paper are: cmp-lg
3
The categories of this paper are: physics
23
The categories of this paper are: solv-int
0
The categories of this paper are: hep-ph
13
The categories of this paper are: quant-ph
20
The categories of this paper are: acc-phys
34
The categories of this paper are: chem-ph
1
The categories of this paper are: cs
8
The categories of this paper are: hep-ex