1. Modulation of charge transfer exciton dynamics in organic semiconductors using different structural arrangements : noneCristian Soncini, Abhishek Kumar, Federica Bondino, Elena Magnano, Matija Stupar, Barbara Ressel, Giovanni De Ninno, Antonis Papadopoulos, Efthymis Serpetzoglou, Emmanuel Stratakis, Maddalena Pedio, 2023, original scientific article Abstract: In devices based on organic semiconductors, aggregation and inter-molecular interactions play a key role in affecting the photo-physical and dynamical carrier properties of the material, potentially becoming a limiting factor to achieving high efficiency. As a consequence, a detailed understanding of the interplay between the film molecular structure and the material properties is essential to properly
design devices with optimized performance. Here we demonstrate how different molecular structural arrangements modulate the charge transfer (CT) dynamics in cobalt phthalocyanine (CoPc) thin films. By transient absorption spectroscopy and time-resolved photoemission spectroscopy, we study the influence of different CoPc structures on the dynamical electronic properties, the CoPc intra and inter- molecular de-excitation pathways up to 7 ns. We rationalize the ultrafast formation of triplet states in the CoPc through an electron exchange process between the single-occupied Co3dz2 orbital and p orbitals of the macrocycle, which obviate for an energetically unfavourable spin-flip. We found enhanced CT exciton lifetime in the case of the herringbone structure with respect to the brickwork one, possibly explainable by a more efficient CT exciton delocalization along the stacking axis.
Keywords: Charge transfer, organic molecules, time resolved spectroscopies
Published in RUNG: 30.06.2023; Views: 363; Downloads: 2
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2. ULTRAFAST ELECTRON DYNAMICS IN CORRELATED SYSTEMS PROBED BY TIME-RESOLVED PHOTOEMISSION SPECTROSCOPYTanusree Saha, 2023, doctoral dissertation Abstract: Complex systems in condensed matter are characterized by strong coupling
between different degrees of freedom constituting a solid. In materials
described by many-body physics, these interactions may lead to
the formation of new ground states such as excitonic insulators, Mott
insulators, and charge and spin density waves. However, the inherent
complexity in such materials poses a challenge to identifying the
dominant interactions governing these phases using equilibrium studies.
Owing to the distinct timescales associated with the elementary interactions,
such complexities can be readily addressed in the non-equilibrium
regime. Additionally, these materials might also show the emergence
of new, metastable “hidden“ phases under non-equilibrium. The thesis
investigates the ultrafast timescales of fundamental interactions in candidate
systems by employing time-and angle-resolved photoemission spectroscopy
in the femtosecond time domain. In the (supposed) excitonic
insulator model system Ta2NiSe5, the timescale of band gap closure
and the dependence of rise time (of the photoemission signal) on the
photoexcitation strength point to a predominantly electronic origin of
the band gap at the Fermi level. The charge density wave (CDW) -
Mott insulator 1T-TaS2 undergoes photoinduced phase transition to two
different phases. The initial one is a transient phase which resembles
the systems’s high temperature equilibrium phase, followed by a long-lived
“hidden“ phase with a different CDW amplitude and is primarily
driven by the CDW lattice order. For the spin density wave system
CaFe2As2 where multiple bands contribute in the formation of Fermi surfaces,
selective photoexcitation was used to disentangle the role played
by different electron orbitals. By varying the polarization of photoexcitation
pulses, it is observed that dxz/dyz orbitals primarily contribute to
the magnetic ordering while the dxy orbitals have dominant role in the
structural order. The findings of the present study provide deeper perspectives
on the underlying interactions in complex ground phases of
matter, therefore, initiating further experimental and theoretical studies
on such materials.
Keywords: complex systems, charge density wave, excitonic insulator, metastable phase, Mott insulator, non-equilibrium, spin density wave, timescales, time- and angle-resolved photoemission, ultrafast dynamics
Published in RUNG: 01.06.2023; Views: 473; Downloads: 12
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3. Light-Induced Magnetization at the NanoscaleJonas Wätzel, Primož Rebernik Ribič, Marcello Coreno, Miltcho Danailov, Christian David, Alexander Demidovich, Michele Di Fraia, Luca Giannessi, Klavs Hansen, Špela Krušič, Michele Manfredda, Michael Meyer, Andrej Mihelič, Najmeh Mirian, Oksana Plekan, Barbara Ressel, Benedikt Rösner, Alberto Simoncig, Simone Spampinati, Matija Stupar, Matjaž Žitnik, Marco Zangrando, Carlo Callegari, Jamal Berakdar, Giovanni De Ninno, 2022, original scientific article Keywords: FEL, orbital angular momentum, magnetisation
Published in RUNG: 16.01.2023; Views: 466; Downloads: 0
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4. Generation and measurement of intense few-femtosecond superradiant extreme-ultraviolet free-electron laser pulsesNajmeh S. Mirian, Michele Di Fraia, Simone Spampinati, Filippo Sottocorona, Enrico Allaria, Laura Badano, Miltcho Bojanov Danailov, Alexander Demidovich, Giovanni De Ninno, Primož Rebernik Ribič, 2021, original scientific article Keywords: free electron laser, superradiance, femtosecond
Published in RUNG: 03.01.2022; Views: 1136; Downloads: 0
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5. Orbital selective dynamics in Fe-pnictides triggered by polarized pump pulse excitationsGanesh Adhikary, Tanusree Saha, Primož Rebernik Ribič, Matija Stupar, Barbara Ressel, Jurij Urbančič, Giovanni De Ninno, A. Thamizhavel, Kalobaran Maiti, 2021, original scientific article Abstract: Quantum materials display exotic behaviours related to the interplay between temperature-driven phase transitions. Here, we study the electron dynamics in one such material, CaFe$_2$As$_2$, a parent Fe-based superconductor, employing time and angle-resolved photoemission spectroscopy. CaFe$_2$As$_2$ exhibits concomitant transition to spin density wave state and tetragonal to orthorhombic structure below 170 K. The Fermi surface of this material consists of three hole pockets ($\alpha$, $\beta$ and $\gamma$) around $\Gamma$-point and two electron pockets around $X$-point. The hole pockets have $d_{xy}$, $d_{yz}$ and $d_{zx}$ orbital symmetries. The $\beta$ band constituted by $d_{xz}$/$d_{yz}$ orbitals exhibit a gap across the magnetic phase transition. We discover that polarized pump pulses can induce excitations of electrons of a selected symmetry. More specifically, while $s$-polarized light (polarization vector perpendicular to the $xz$-plane) excites electrons corresponding to all the three hole bands, $p$-polarized light excites electrons essentially from ($\alpha$,$\beta$) bands which are responsible for magnetic order. Interestingly, within the magnetically ordered phase, the excitation due to the $p$-polarized pump pulses occur at a time scale of 50 fs, which is significantly faster than the excitation induced by $s$-polarized light ($\sim$ 200 fs). These results suggest that the relaxation of different ordered phases occurs at different time scales and this method can be used to achieve selective excitations to disentangle complexity in the study of quantum materials.
Keywords: Electronic structure, Pnictides and chalcogenides, Time-resolved spectroscopy
Published in RUNG: 13.10.2021; Views: 1366; Downloads: 5
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6. Dissecting Mott and charge-density wave dynamics in the photoinduced phase of 1T-TaS[sub]2Alberto Simoncig, Matija Stupar, Barbara Ressel, Tanusree Saha, Primož Rebernik Ribič, Giovanni De Ninno, 2021, original scientific article Abstract: The two-dimensional transition-metal dichalcogenide 1T−TaS2 is a complex material standing out for its puzzling low temperature phase marked by signatures amenable to both Mott-insulating and charge-density wave states. Electronic Mott states, coupled to a lattice, respond to coherent optical excitations via a modulation of the lower (valence) Hubbard band. Such dynamics is driven by strong electron-phonon coupling and typically lasts for tens of picoseconds, mimicking coherent structural distortions. Instead, the response occurring at the much faster timescale, mainly dominated by electronic many-body effects, is still a matter of intense research. By performing time- and angle-resolved photoemission spectroscopy, we investigated the photoinduced phase of 1T−TaS2 and found out that its lower Hubbard band promptly reacts to coherent optical excitations by shifting its binding energy towards a slightly larger value. This process lasts for a time comparable to the optical pump pulse length, mirroring a transient change of the onsite Coulomb repulsion energy (U). Such an observation suggests that the correction to the bare value of U, ascribed to the phonon-mediated screening which slightly opposes the Hubbard repulsion, is lost within an interval of a few tens of femtoseconds and can be understood as a fingerprint of electronic states largely decoupled from the lattice. Additionally, these results enforce the hypothesis, envisaged in the current literature, that the transient photoinduced states belong to a sort of crossover phase instead of an equilibrium metallic one.
Keywords: ultrafast phenomena, time resolved photoemission, strongly correlated systems, transition metal dichalcogenide
Published in RUNG: 13.04.2021; Views: 1852; Downloads: 0
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