Time resolved spectroscopy of strongly correlated materials
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Abstract
Strongly correlated materials are systems for which the complex interplay among
the electrons, spin and lattice lead to the formation of different phase transitions
and new orders with dramatically different electrical, optical, mechanical and
thermal properties. Emergence of complex phases like- Charge Density Wave
(CDW), Spin Density Wave (SDW), Superconductivity just to name a few, with
remarkable properties offer great deal (such as an order of magnitude change
in electrical conductivity) from fundamental research perspective and thought
to shape our future technologies. Non-equilibrium spectroscopic techniques are
ideal tools to investigate such correlated systems as the techniques offer simulta-
neous spectroscopic and temporal information. In addition, driving the system
out of equilibrium and tracking the relaxation dynamics and their couplings,
one can disentangle the different degrees of freedom because of the different
timescales that characterize the recovery of the initial ground state.
The work described in this thesis utilizes the ultrafast spectroscopic tech-
nique as a tool to investigate the solid state materials exhibiting Charge Density
Wave (CDW) order, Kondo Insulating (KI) behavior and Mott Insulating (MI)
ground state. All of these materials fall into the category of strongly correlated
systems where multiple phases emerges due to correlation effects. In the Kondo
Insulator YbB12 we track the photo-induced reflectivity dynamics at various temperatures and excitation densities and discuss the corresponding changes in the ow energy electronic structure. In CDW system BaNi2As2 we study the collec-
tive amplitude modes of the CDW order and their temperature and excitation
density dependence. These results provide valuable information on the nature
of the CDW order and its relation to the observed structural phase transitions. In
particular, the smooth evolution of several amplitude modes through the struc-
tural phase transitions suggest that CDW may be responsible for the triclinic
phase transition. Moreover, robustness of the CDW order against perturbation
suggest an unconventional, non-Peierls nature of the CDW order. Last, but not
least, we present the time-resolved study on photo-doping the Mott insulator
La2CuO4. By tracking the time evolution of the complex dielectric function over
the broad spectral range, we study the dynamics of the charge transfer gap and
the appearance of the mid-infrared excitations. By varying the excitation densi-
ties over three order of magnitude we demonstrate the extreme resilience of the
Mott insulating ground state against perturbation.