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Published: Ultrafast dynamics in (TaSe4)2I triggered by valence and core-level excitation

We make use of both optical laser and free-electron laser (FEL) based time-resolved spectroscopies in order to study the effect of a selective excitation on the normal-state and on the CDW phases by probing the near-infrared/visible optical properties both along and perpendicularly to the direction of the CDW, where the system is metallic and insulating, respectively.
  W. Bronsch et al., Faraday Discussions, Advanced Article (2022)

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Published: Photoinduced long-lived state in FeSe0.4Te0.6

We exploit tr-ARPES to study ultrafast electron dynamics in FeSe_0.4Te_0.6 following photoexcitation by near-infrared pump pulses. By exploiting probe-polarization-dependent matrix element effects, we reveal a photoinduced long-lived state, lasting for a few tens of picoseconds, showing features compatible with a nematic state.

L. Fanfarillo et al., Journal of Electron Spectroscopy and Related Phenomena 250, 147090 (2021).
 

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Published: Ultrafast broadband optical spectroscopy for quantifying subpicometric coherent atomic displacements in WTe2

Here we show how time-resolved broadband optical spectroscopy can be used to quantify, with femtometer resolution, the oscillation amplitudes of coherent phonons through a displacive model without free tuning parameters, except an overall scaling factor determined by comparison between experimental data and DFT calculations.
D. Soranzio et al., Physical Review Research 1, 032033(R) (2019).

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Revealing cuprates high energy electronic excitations

In strongly correlated systems the electronic properties at the Fermi energy are intertwined with those at high-energy scales. One of the pivotal challenges in the field of high-temperature superconductivity is to understand how the high-energy scale physics is correlated to Mott-like excitations. By using a novel time-resolved optical spectrosocpy we faced the problem and give clear answers in this very hot topic.

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Effective Fermi-Dirac distribution of the Dirac particles in Bi2Se3

By time- and angle-resolved photoemission spectroscopy we determined the evolution of the out-of-equilibrium electronic structure of the topological insulator Bi2Se3.
We found that the energy dependence of the nonequilibrium charge population is solely determined by the analytical form of the effective Fermi-Dirac distribution.







Figure Caption:
ARPES band dispersion of Bi2Se3 acquired with the 4th harmonic of our laser system, at 6.3 eV. In the figure the topological surface state (SS) and conduction band (CB) are clearly visible. The chemical potential energy is marked with μ and a green line.
The graph shows the snapshot for one particular delay time of the the pump-probe tr-ARPES. The signal is obtained as the difference between the ARPES image at +600 fs and an ARPES image at a negative delay. Red (blue) represents an increase (decrease) of the spectral weight.



The response of the Fermi-Dirac distribution to ultrashort IR laser pulses has been studied by modeling the dynamics of hot electrons after optical excitation. We disentangled a large increase in the effective temperature (T*) from a shift of the chemical potential (μ*), which is consequence of the ultrafast photodoping of the conduction band. We demonstrated that the relaxation dynamics of T* and μ* are k independent and these two quantities uniquely define the evolution of the excited charge population. 

Physical Review B 86, 205133 (2012)

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Ultrafast optical control of the ZrTe5 electronic properties

Ultrafast optoelectronics consists in the capability to manipulate electronic transport properties via light at the sub picosecond (10-12 s) time scale. In this letter, we have addressed the origin of the resistivity anomaly in ZrTe5 and we have proven the possibility to manipulate its electronic properties at the ultra-short time scale via optical excitation with laser light.



Nowadays, optical switches are realized in oxides by exploiting phase transitions between metallic and insulating states. However, to meet the full integration with the current technology, optical control of semiconductors electronic properties is of pivotal importance.
In this respect, ZrTe5 represents an ideal system, which is fascinating the condensed matter community with its amazing set of transport properties. A resistivity peak is accompanied by the switch of the charge carriers, from holes to electrons. Magneto-resistivity is observed with both positive and negative sign, as a result of either the presence of three-dimensional Dirac particles or spin polarized two-dimensional Dirac particles.
Angle resolved photoemission spectroscopy (ARPES) and Time resolved ARPES measurements have been carried out at the T-Rex laboratory giving a thorough insight in the origin of the unique behaviour of ZrTe5 band structure at the Fermi level.


  We report an energy shift of the band structure across the Fermi level by varying the temperatures.
We prove the capability to control it at the ultrafast scale by changing the material (electronic and lattice) temperature with a pulsed laser pulse. Therefore, by optically controlling the band structure binding energy and the charge carriers' lifetime, we unlock the route for a unique platform for magneto, optical and thermoelectric transport applications.
 

This research was conducted by the following research team:

Giulia Manzoni, Università degli studi di Trieste, Trieste, Italy
Andrea Sterzi, Università degli studi di Trieste, Trieste, Italy
Alberto Crepaldi, Sincrotrone Trieste S.C.p.A., Trieste, Italy
Michele Diego, Università degli studi di Trieste, Trieste, Italy
Federico Cilento, Sincrotrone Trieste S.C.p.A., Trieste, Italy
Michele Zacchigna, CNR-IOM Trieste, Trieste, Italy
Philippe Bougnon, EPFL Lausanne, Switzerland
Helmuth Berger, EPFL Lausanne, Switzerland
Arnaud Magrez, EPFL Lausanne, Switzerland
Marco Grioni, EPFL Lausanne, Switzerland
Fulvio Parmigiani, Università degli studi di Trieste, Trieste, Italy; Sincrotrone Trieste S.C.p.A., Trieste, Italy; International faculty, University of Köln, Germany.

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