REPOSITORY > RESULTS

Abstract

This thesis covers a study of the photoexcited relaxation dynamics in superconducting ironbased
pnictides, cuprates and charge-density wave systems by means of femtosecond laser
spectroscopy. To examine the temperature and fluence dependence of photoinduced reflectivity transients ΔR/R in iron-based pnictides and cuprates, we apply a standard double
pump-probe technique, while for the charge-density wave systems, we introduce a new writepump-probe pulse technique.
Temperature dependence study of ΔR/R in a low-perturbation pump regime in undoped
iron-based pnictides has revealed a critical slowing down of the relaxation at spin-density
wave/structural transition temperature TSDW=Ts. At temperatures below TSDW=Ts, a bottleneck
appears in the single particle relaxation dynamics, associated with opening of a
BCS-like temperature dependent gap. We estimate the magnitude of the gap and find that
it decreases with increasing doping which is in accordance with a stability decrease of the
orthorombic/SDW state. From the relaxation time in the normal state, we determined the
electron phonon coupling constant λ ≈ 0:2 from the second moment of the Eliashberg function.
In all three studied iron-based pnictides, λ has a similar values referring to a moderate
electron phonon coupling.
In the superconducting pnictides and cuprates, we observe a saturation of the SC component
with increasing pump fluence. From the threshold pump fluence, we estimate the
SC state destruction energy density Ud and find that it exhibits an approximate square
power-law dependence on Tc. This systematic behavior is discussed in the framework of a
model based on phonon-mediated quasiparticle bottleneck mechanism.
The T-dependence of the transient reflectivity amplitude in the normal state is consistent
with the presence of a temperature-independent gap (pseudogap) in the QP density of
states. Estimated magnitudes of the pseudogap decrease with increasing doping and they
are consistent with the pseudogap values obtained from the T-dependence of the Knight
shift.
In Co-doped Ba-122 samples, we observe in nominally tetragonal phase a twofold rotational
anisotropy with respect to the probe polarization up to ~ 200 K. The anisotropy is
assigned to electronic nematic fluctuations or ordering of the Fe d orbitals.
We introduce a new write-pump-probe technique to study the symmetry breaking transition
in 1T-TaS2 single crystals. In these experiments, we quench the system with a single
intense 50 fs pulse into a high symmetry state and then monitor the temporal recovery with
a standard pump-probe sequence. A switching to a new hidden photoinduced p-TaS2 state
(ON state) was achieved after certain quench pulse threshold energy. It was shown that a
new state can be reached only via optical path and it is stable up to ~ 100 K. To return
back the system from the p-TaS2 state to the CCDW state (OFF state), we apply a train of
long 50 ps pulses. The switching ON/OFF is repeatable and accompanied with a change in
reflectivity and DC resistance, opening a new possibility for ultrafast non-volatile memory
devices.