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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, original scientific article

Abstract: 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).
Keywords: angle resolved photoemission, time resolved photoemission, 2D materials, charge density wave, Mott insulator
Published in RUNG: 15.01.2024; Views: 667; Downloads: 4
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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, 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: 2017; Downloads: 6
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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, 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: 2615; Downloads: 0
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Photoelectric effect with a twist
Giovanni De Ninno, Jonas Wätzel, Primož Rebernik Ribič, Enrico Allaria, Marcello Coreno, Miltcho B. Danailov, Christian David, Alexander Demidovich, Michele Di Fraia, Luca Giannessi, Klaus Hansen, Špela Krušič, Michele Manfredda, Micheal Meyer, Andrej Mihelič, Najmeh Mirian, Oksana Plekan, Barbara Ressel, Benedikt Rösner, Alberto Simoncig, Simone Spampinati, Janez Štupar, Matjaž Žitnik, Marco Zangrando, Carlo Callegari, Jamal Berakdar, 2020, original scientific article

Abstract: Photons have fixed spin and unbounded orbital angular momentum (OAM). While the former is manifested in the polarization of light, the latter corresponds to the spatial phase distribution of its wavefront1. The distinctive way in which the photon spin dictates the electron motion upon light– matter interaction is the basis for numerous well-established spectroscopies. By contrast, imprinting OAM on a mat- ter wave, specifically on a propagating electron, is gener- ally considered very challenging and the anticipated effect undetectable2. In refs. 3,4, the authors provided evidence of OAM-dependent absorption of light by a bound electron. Here, we seek to observe an OAM-dependent dichroic photo- electric effect, using a sample of He atoms. Surprisingly, we find that the OAM of an optical field can be imprinted coher- ently onto a propagating electron wave. Our results reveal new aspects of light–matter interaction and point to a new kind of single-photon electron spectroscopy.
Keywords: FEL, OAM, Photoelectric effect
Published in RUNG: 09.09.2020; Views: 3063; Downloads: 0
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Coupling of autoionizing states by a chirped laser pulse
Matjaž Žitnik, Andrej Mihelič, Klemen Bučar, Mateja Hrast, Žiga Barba, Špela Krušič, Primož Rebernik Ribič, Jurij Urbančič, Barbara Ressel, Matija Stupar, David Gauthier, Giovanni De Ninno, 2020, published scientific conference contribution abstract

Abstract: We have observed the autoionization of the laser-coupled 2s2p 1Po and 2p2 1Se resonances in helium. The ions were collected while varying the frequency and delay of the extreme-ultraviolet (EUV) excitation pulse with respect to the linearly chirped visible (VIS) laser pulse. From the measured frequency-delay map the Autler- Townes splitting, the EUV-VIS cross-correlation and the linear chirp parameter were extracted.
Keywords: High Harmonic Generation
Published in RUNG: 29.06.2020; Views: 3312; Downloads: 0
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