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Abstract:
This thesis presents studies on how photoexcitation disturbs the o
rdering of the elec-
tronic degree of freedom located at the manganese 3
d
orbitals. Throughout the thesis
the model compound La
0
.
5
Sr
1
.
5
MnO
4
has been studied. This material exhibits two di-
mensional ordering of the charges and the orbitals (COO) and at a lo
wer temperature
also a three dimensional antiferromagnetic spin order.
In the first approach the sample was photoexcited at 560 nm and th
e ensuing
dynamics was probed through the optical anisotropy at 630 nm. To
this end two
independent tunable noncollinear optical parameteric amplifiers (NO
PA) were used.
The cross correlation between them was measured to be 16 fs, muc
h shorter than
what previously has been used. The majority of the COO signal at 25
K was found
to disappear with a time constant limited by the time-integral of the c
ross correlation
between pump and probe pulse. This hints towards an electrically-dr
iven melting of
the COO.
To be more sensitive to the photo-induced changes in the COO, the t
echnique of
resonant soft x-ray diffraction was transferred from the static
to the time-resolved
domain. A diffractometer has been designed, built and comissioned wh
ich allows
time-resolved experiments on the single-layered manganite La
0
.
5
Sr
1
.
5
MnO
4
. In this
way it was possible to separate the dynamics of the orbital from the
spin ordering by
measuring the intensities of the associated diffraction peaks.
The time resolution was as good as 10 ps. On this timescale the orbital
peak
responded profoundly differently compared to the magnetic order
ing peak. It was
found that it is possible to melt the magnetic peak completely within the
time resolu-
tion, while the orbital ordered peak was much less affected. Throug
h calculation and
careful comparison with static data a temperature-driven effect
could be excluded.
Therefore, the experiments suggest that the quenching of the m
agnetic order is an
electrically-driven process.
