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1.
Uncovering the nature of transient and metastable nonequilibrium phases in 1T − ▫$TaS_2$ ▫
Tanusree Saha, Arindam Pramanik, Barbara Ressel, Alessandra Ciavardini, Fabio Frassetto, Federico Galdenzi, Luca Poletto, Arun Ravindran, Primož Rebernik Ribič, Giovanni De Ninno, 2023, izvirni znanstveni članek

Opis: Complex systems are characterized by strong coupling between different microscopic degrees of freedom. Photoexcitation of such materials can drive them into new transient and metastable hidden phases that may not have any counterparts in equilibrium. By exploiting femtosecond time- and angle-resolved photoemission spectroscopy, we probe the photoinduced transient phase and the recovery dynamics of the ground state in a complex material: the charge density wave (CDW)–Mott insulator 1T-TaS2. We reveal striking similarities between the band structures of the transient phase and the (equilibrium) structurally undistorted metallic phase, with evidence for the coexistence of the low-temperature Mott insulating phase and high-temperature metallic phase. Following the transient phase, we find that the restorations of the Mott and CDW orders begin around the same time. This highlights that the Mott transition is tied to the CDW structural distortion, although earlier studies have shown that the collapses of Mott and CDW phases are decoupled from each other. Interestingly, as the suppressed order starts to recover, a metastable phase emerges before the material recovers to the ground state. Our results demonstrate that it is the CDW lattice order that drives the material into this metastable phase, which is indeed a commensurate CDW–Mott insulating phase but with a smaller CDW amplitude. Moreover, we find that the metastable phase emerges only under strong photoexcitation (∼3.6 mJ/cm2) and has no evidence when the photoexcitation strength is weak (∼1.2 mJ/cm2).
Ključne besede: angle resolved photoemission, time resolved photoemission, 2D materials, charge density wave, Mott insulator
Objavljeno v RUNG: 15.01.2024; Ogledov: 390; Prenosov: 4
.pdf Celotno besedilo (2,30 MB)
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2.
Modulation of charge transfer exciton dynamics in organic semiconductors using different structural arrangements
Cristian Soncini, Abhishek Kumar, Federica Bondino, Elena Magnano, Matija Stupar, Barbara Ressel, Giovanni De Ninno, Antonis Papadopoulos, Efthymis Serpetzoglou, Emmanuel Stratakis, Maddalena Pedio, 2023, izvirni znanstveni članek

Opis: 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.
Ključne besede: charge transfer, organic molecules, time resolved spectroscopies
Objavljeno v RUNG: 30.06.2023; Ogledov: 936; Prenosov: 4
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3.
4.
ULTRAFAST ELECTRON DYNAMICS IN CORRELATED SYSTEMS PROBED BY TIME-RESOLVED PHOTOEMISSION SPECTROSCOPY
Tanusree Saha, 2023, doktorska disertacija

Opis: 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.
Ključne besede: complex systems, charge density wave, excitonic insulator, metastable phase, Mott insulator, non-equilibrium, spin density wave, timescales, time- and angle-resolved photoemission, ultrafast dynamics
Objavljeno v RUNG: 01.06.2023; Ogledov: 1238; Prenosov: 26
.pdf Celotno besedilo (13,34 MB)

5.
Hot-carrier and optical-phonon ultrafast dynamics in the topological insulator Bi2Te3 upon iron deposition on its surface
M Weis, K Balin, T Sobol, A Ciavardini, G Vaudel, V Juvè, B Arnaud, Barbara Ressel, M Stupar, K.C. Prince, Giovanni De Ninno, P Ruello, J Szade, 2021, izvirni znanstveni članek

Opis: This paper presents a complete study of electronic structures and photoexcited carrier dynamics in topological insulators capped with iron and iron oxide. We combine static and time-resolved angle-resolved photoemission spectroscopies (ARPES, TR-ARPES) with time-resolved optical methods (transient optical reflectivity and transmission). Both single crystal and thin films of Bi2Te3 are studied. We show that monolayers of iron and iron oxide significantly affect the electronic band structure at the interface by shifting the Fermi level into the conduction band, which we explain by a band bending effect, and is confirmed by in situ XPS measurements
Ključne besede: time resolved spectroscopies, topological insulators, interfaces
Objavljeno v RUNG: 13.12.2021; Ogledov: 1853; Prenosov: 26
.pdf Celotno besedilo (2,90 MB)

6.
Orbital selective dynamics in Fe-pnictides triggered by polarized pump pulse excitations
Ganesh Adhikary, Tanusree Saha, Primož Rebernik Ribič, Matija Stupar, Barbara Ressel, Jurij Urbančič, Giovanni De Ninno, A. Thamizhavel, Kalobaran Maiti, 2021, izvirni znanstveni članek

Opis: 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.
Ključne besede: Electronic structure, Pnictides and chalcogenides, Time-resolved spectroscopy
Objavljeno v RUNG: 13.10.2021; Ogledov: 1784; Prenosov: 6
.pdf Celotno besedilo (9,56 MB)

7.
Dissecting Mott and charge-density wave dynamics in the photoinduced phase of 1T-TaS[sub]2
Alberto Simoncig, Matija Stupar, Barbara Ressel, Tanusree Saha, Primož Rebernik Ribič, Giovanni De Ninno, 2021, izvirni znanstveni članek

Opis: 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.
Ključne besede: ultrafast phenomena, time resolved photoemission, strongly correlated systems, transition metal dichalcogenide
Objavljeno v RUNG: 13.04.2021; Ogledov: 2319; Prenosov: 0
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8.
The role of charge transfer at reduced graphene oxide/organic semiconductor interface on the charge transport properties
Gvido Bratina, Egon Pavlica, 2019, izvirni znanstveni članek

Opis: The effect of 1-pyrenesulfonicacid sodium salt (1-PSA), tetracyanoethylene (TCNE) and tetrafluoro- tetracyanoquinodimethane (F4-TCNQ) on charge transport properties of reduced graphene oxide (RGO) is examined by measuring the transfer characteristics of field-effect transistors and co-planar time-of-flight photocurrent technique. Evidence of p-type doping and a reduction of mobility of electrons in RGO upon deposition of these materials is observed. Time-resolved photocurrent measurements show a reduction in elec- tron mobility even at submonolayer coverage of these materials. The variation of transit time with different coverages reveals that electron mobility decreases with increasing the surface coverage of 1-PSA, TCNE and F4- TCNQ to a certain extent, while at higher coverage the electron mobility is slightly recovered. All three molecules show the same trend in charge carrier mobility variation with coverage, but with different magnitude. Among all three molecules, 1-PSA acts as weak electron acceptor compared to TCNE and F4-TCNQ. The additional fluorine moieties in F4-TCNQ provides excellent electron withdrawing capability compared to TCNE. The experimental results are consistent with the density functional theory calculations.
Ključne besede: organic semiconductors, reduced graphene oxide, time-resolved photocurrent measurements, organic thin film transistors
Objavljeno v RUNG: 28.10.2019; Ogledov: 3467; Prenosov: 1
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9.
Introduction to Electronic Properties and Dynamics of Organic Complexes as Self‐Assembled Monolayers
Maddalena Pedio, 2017, samostojni znanstveni sestavek ali poglavje v monografski publikaciji

Opis: Self‐assembled monolayers (SAMs) of organic‐conjugated transition metal complexes on surfaces is a focus of both device engineering and basic science, since it is a key factor in nearly all important aspects of device performances, including operation voltages, degradation, and efficiency. The huge amount of literature results related to the first monolayer, and reorganization and self‐assembling processes are due to the general accepted result that structural and chemical properties of the first monolayer are the key parameters for controlled thin film growth. Optical and magneto‐electronic properties are intimately connected, and the accurate determination of electronic levels, excitation, and relaxation dynamics is mandatory for the optimization of electronic, photovoltaic, and opto‐electronic devices. Quite a number of electronic states is generated by the interaction of light with complex organic molecules. Time‐resolved spectroscopies are a new investigation tool that gives the possibility of correctly addressing their origin and life time. Examples of prototypical systems are presented and discussed. We review on complementary techniques, trying to single out how different approaches are fundamental to fully characterize these complex systems.
Ključne besede: self‐assembled monolayer (SAM), surface structures molecular layers, nanotechnology, electronic properties, spectroscopies, time resolved
Objavljeno v RUNG: 12.06.2017; Ogledov: 4503; Prenosov: 208
.pdf Celotno besedilo (5,78 MB)

10.
Chemical and structural investigation of the cobalt phthalocyanine
Matija Stupar, 2015, magistrsko delo

Opis: In the last two decades, studies on organic molecules mimicking substances of fundamental importance in nature, like chlorophyll or hemoglobin, have attracted researchers’ attention. These molecules are building blocks for a family of materials also referred to as “organic semiconductors”. Such compounds can be implemented in numerous applications, ranging from data-storage to light harvesting. Some of their fundamental advantages include low cost, light weight, relatively easy engineering and mechanical flexibility, compatible with bending plastic substrates. In this thesis work we investigated the chemical, structural and electronic properties of cobalt phthalocyanines (CoPc). These molecules have promising applications in the field of magnetic data storage and spintronics in general, due to the ferromagnetic properties of the cobalt atom. Several techniques like photoemission core-level spectroscopy and valence band spectroscopy, together with X-ray absorption, have been used in order to determine the CoPc properties in gaseous phase, i.e. in the absence of interaction with the surrounding environment. Another set of experiments was devoted to the commissioning of the CITIUS time-resolved photoemission setup, that will be used in future studies of CoPc molecules on surfaces.
Ključne besede: Cobalt phthalocyanine (CoPc), photoemission spectroscopy (PES), X-ray absorption spectroscopy (XAS), synchrotron radiation, laser, high order harmonic generation (HHG), time resolved spectroscopy
Objavljeno v RUNG: 29.09.2015; Ogledov: 8714; Prenosov: 280
.pdf Celotno besedilo (2,96 MB)

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