arXiv subjects

M. Marsi

Publications and source records attributed to M. Marsi.

At least 19 recordsLinked to original sources

arXiv2025

About carrier's self-trapping and dynamical Rashba splitting in the two-dimensional hybrid perovskite (BA)$_2$(MA)$_2$Pb$_3$I$_{10}$

Time- and Angle-Resolved Photoelectron Spectroscopy (tr-ARPES) is employed to monitor photoexcited electrons in the two-dimensional hybrid perovskite (BA)$_2$(MA)$_2$Pb$_3$I$_{10}$. Photoelectron intensity maps are in good agreement with ab-initio calculations of the band structure. The effective mass is $-0.18 \pm 0.02 m_e$ and $0.12 \pm 0.02 m_e$ for holes and electrons, respectively. In the photoexcited state, spin-orbit splitting of the conduction band cannot be resolved. This sets the upper bound of photoinduced Rashba coupling to $\alpha_C<2.5$ eV\AA. The correlated electron-hole plasma evolves in Wannier excitons with Bohr radius of 2.8 nm, while no sign of self-trapping in small polarons is found within the investigated time window of up to 120 ps following photoexcitation.

cond-mat.other↗

arXiv2022

Dichroism in time-resolved ARPES and valence band orbital nature in BaNiS2

Time-resolved ARPES gives access to the band structure and ultrafast dynamics of excited electronic states in solids. The orbital character of the bands close to the Fermi level is essential to understand the origin of several exotic phenomena in quantum materials. By performing polarization dependent time- and angle-resolved photoemission spectroscopy and by analyzing the chirality of the photoelectron yield for two different crystal orientations, we identify the orbital character of bands below and above the chemical potential for the Dirac semimetal BaNiS2. Our results illustrate how the control and understanding of matrix elements effects in time-resolved photoemission spectroscopy can be a powerful tool for the study of quantum materials.

cond-mat.str-el↗

arXiv2021

Low-Temperature Insulating Phase of the Si(111)--7$\times$7 Surface

We investigated the electronic structure of the Si(111)--7$\times$7 surface below 20 K by scanning tunneling and photoemission spectroscopies and by density functional theory calculations. Previous experimental studies have questioned the ground state of this surface, which is expected to be metallic in a band picture because of the odd number of electrons per unit cell. Our differential conductance spectra instead show the opening of an energy gap at the Fermi level and a significant temperature dependence of the electronic properties, especially for the adatoms at the center of the unfaulted half of the unit cell. Complementary photoemission spectra with improved correction of the surface photovoltage shift corroborate the differential conductance data and demonstrate the absence of surface bands crossing the Fermi level at 17 K. These consistent experimental observations point to an insulating ground state and contradict the prediction of a metallic surface obtained by density functional theory in the generalized gradient approximation. The calculations indicate that this surface has or is near a magnetic instability, but remains metallic in the magnetic phases even including correlation effects at mean-field level. We discuss possible origins of the observed discrepancies between experiments and calculations.

cond-mat.str-el↗

arXiv2019

Tuning Dirac nodes with correlated d-electrons in BaCo_{1-x}Ni_{x}S_{2}

Dirac fermions play a central role in the study of topological phases, for they can generate a variety of exotic states, such as Weyl semimetals and topological insulators. The control and manipulation of Dirac fermions constitute a fundamental step towards the realization of novel concepts of electronic devices and quantum computation. By means of ARPES experiments and ab initio simulations, here we show that Dirac states can be effectively tuned by doping a transition metal sulfide, BaNiS2, through Co/Ni substitution. The symmetry and chemical characteristics of this material, combined with the modification of the charge transfer gap of BaCo_{1-x}Ni_{x}S_{2} across its phase diagram, lead to the formation of Dirac lines whose position in k-space can be displaced along the Gamma M symmetry direction, and their form reshaped. Not only does the doping x tailor the location and shape of the Dirac bands, but it also controls the metal-insulator transition in the same compound, making BaCo_{1-x}Ni_{x}S_{2} a model system to functionalize Dirac materials by varying the strength of electron correlations.

cond-mat.str-el↗

arXiv2018

Direct observation of electron thermalization and electron-phonon coupling in photoexcited bismuth

We investigate the ultrafast response of the bismuth (111) surface by means of time resolved photoemission spectroscopy. The direct visualization of the electronic structure allows us to gain insights on electron-electron and electron-phonon interaction. Concerning electron-electron interaction, it is found that electron thermalization is fluence dependent and can take as much as several hundreds of femtoseconds at low fluences. This behavior is in qualitative agreement with Landau's theory of Fermi liquids but the data show deviations from the behavior of a common 3D degenerate electron gas. Concerning electron-phonon interaction, our data allows us to directly observe the coupling of individual Bloch state to the coherent $A_{1g}$ mode. It is found that surface states are much less coupled to this mode when compared to bulk states. This is confirmed by \textit{ab initio} calculations of surface and bulk bismuth.

cond-mat.mtrl-sci↗

arXiv2017

Electron-hole balanced dynamics in the type-II Weyl semimetal candidate WTe2

We present a time- and angular-resolved photoemission (TR-ARPES) study of the transition- metal dichalcogenide WTe2, a candidate type II Weyl semimetal exhibiting extremely large magne- toresistence. Using femtosecond light pulses, we characterize the unoccupied states of the electron pockets above the Fermi level. We track the relaxation dynamics of photoexcited electrons along the unoccupied band structure and into a bulk hole pocket. Following the ultrafast carrier relaxation, we report remarkably similar decay dynamics for electrons and holes. Our results corroborate the hypothesis that carrier compensation is a key factor in the exceptional magnetotransport properties of WTe2.

cond-mat.mes-hall↗

arXiv2017

Photoinduced filling of near nodal gap in Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$

We report time and angle resolved spectroscopic measurements in optimally doped Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$. The spectral function is monitored as a function of temperature, photoexcitation density and delay time from the pump pulse. According to our data, the superconducting gap becomes slightly stiffer when moving off the nodal direction. The nodal quasiparticles develop a faster dynamics when pumping the superconductor with a fluence that is large enough to induce the total collapse of the gap. We discuss the observed relaxation in terms of a dynamical reformation of Cooper pairs.

cond-mat.supr-con↗

arXiv2016

Ultrafast evolution and transient phases of the prototype out-of-equilibrium Mott-Hubbard material V2O3

The study of photoexcited strongly correlated materials is attracting growing interest since their rich phase diagram often translates into an equally rich out-of-equilibrium behavior, including non-thermal phases and photoinduced phase transitions. With femtosecond optical pulses, electronic and lattice degrees of freedom can be transiently decoupled, giving the opportunity of stabilizing new states of matter inaccessible by quasi-adiabatic pathways. Here we present a study of the ultrafast non-equilibrium evolution of the prototype Mott-Hubbard material V2O3, which presents a transient non-thermal phase developing immediately after photoexcitation and lasting few picoseconds. For both the insulating and the metallic phase, the formation of the transient configuration is triggered by the excitation of electrons into the bonding a1g orbital, and is then stabilized by a lattice distortion characterized by a marked hardening of the A1g coherent phonon. This configuration is in stark contrast with the thermally accessible ones - the A1g phonon frequency actually softens when heating the material. Our results show the importance of selective electron-lattice interplay for the ultrafast control of material parameters, and are of particular relevance for the optical manipulation of strongly correlated systems, whose electronic and structural properties are often strongly intertwinned.

cond-mat.str-el↗

arXiv2016

Time resolved photoemission of Sr$_2$IrO$_4$

We investigate the temporal evolution of electronic states in strontium iridate Sr$_2$IrO$_4$. The time resolved photoemission spectra of intrinsic, electron doped and the hole doped samples are monitored in identical experimental conditions. Our data on intrinsic and electron doped samples, show that primary doublon-holon pairs relax near to the chemical potential on a timescale shorter than $70$ fs. The subsequent cooling of low energy excitations takes place in two step: a rapid dynamics of $\cong120$ fs is followed by a slower decay of $\cong 1$ ps. The reported timescales endorse the analogies between Sr$_2$IrO$_4$ and copper oxides.

cond-mat.str-el↗

arXiv2015

Surface effects on the Mott-Hubbard transition in archetypal V$_2$O$_3$

We present an experimental and theoretical study exploring surface effects on the evolution of the metal-insulator transition in the model Mott-Hubbard compound Cr-doped V$_2$O$_3$. We find a microscopic domain formation that is clearly affected by the surface crystallographic orientation. Using scanning photoelectron microscopy and X-ray diffraction, we find that surface defects act as nucleation centers for the formation of domains at the temperature-induced isostructural transition and favor the formation of microscopic metallic regions. A density functional theory plus dynamical mean field theory study of different surface terminations shows that the surface reconstruction with excess vanadyl cations leads to doped, and hence more metallic surface states, explaining our experimental observations.

cond-mat.str-el↗

arXiv2015

Quasiparticles dynamics in high-temperature superconductors far from equilibrium: an indication of pairing amplitude without phase coherence

We perform time resolved photoelectron spectroscopy measurements of optimally doped $\tn{Bi}_2\tn{Sr}_2\tn{CaCu}_2\tn{O}_{8+\delta}$ (Bi-2212) and $\tn{Bi}_2\tn{Sr}_{2-x}\tn{La}_{x}\tn{Cu}\tn{O}_{6+\delta}$ (Bi-2201). The electrons dynamics show that inelastic scattering by nodal quasiparticles decreases when the temperature is lowered below the critical value of the superconducting phase transition. This drop of electronic dissipation is astonishingly robust and survives to photoexcitation densities much larger than the value sustained by long-range superconductivity. The unconventional behaviour of quasiparticle scattering is ascribed to superconducting correlations extending on a length scale comparable to the inelastic path. Our measurements indicate that strongly driven superconductors enter in a regime without phase coherence but finite pairing amplitude. The latter vanishes near to the critical temperature and has no evident link with the pseudogap observed by Angle Resolved Photoelectron Spectroscopy (ARPES).

cond-mat.supr-con↗

arXiv2015

Optical properties of V2O3 in its whole phase diagram

Vanadium sesquioxide V2O3 is considered a textbook example of Mott-Hubbard physics. In this paper we present an extended optical study of its whole temperature/doping phase diagram as obtained by doping the pure material with M=Cr or Ti atoms (V1-xMx)2O3. We reveal that its thermodynamically stable metallic and insulating phases, although macroscopically equivalent, show very different low-energy electrodynamics. The Cr and Ti doping drastically change both the antiferromagnetic gap and the paramagnetic metallic properties. A slight chromium content induces a mesoscopic electronic phase separation, while the pure compound is characterized by short-lived quasiparticles at high temperature. This study thus provides a new comprehensive scenario of the Mott-Hubbard physics in the prototype compound V2O3.

cond-mat.str-el↗

arXiv2013

Tuning a Schottky barrier in a photoexcited topological insulator with transient Dirac cone electron-hole asymmetry

The advent of Dirac materials has made it possible to realize two dimensional gases of relativistic fermions with unprecedented transport properties in condensed matter. Their photoconductive control with ultrafast light pulses is opening new perspectives for the transmission of current and information. Here we show that the interplay of surface and bulk transient carrier dynamics in a photoexcited topological insulator can control an essential parameter for photoconductivity - the balance between excess electrons and holes in the Dirac cone. This can result in a strongly out of equilibrium gas of hot relativistic fermions, characterized by a surprisingly long lifetime of more than 50 ps, and a simultaneous transient shift of chemical potential by as much as 100 meV. The unique properties of this transient Dirac cone make it possible to tune with ultrafast light pulses a relativistic nanoscale Schottky barrier, in a way that is impossible with conventional optoelectronic materials.

cond-mat.mtrl-sci↗

arXiv2013

Mott-Hubbard transition in V2O3 revisited

The isostructural metal-insulator transition in Cr-doped V2O3 is the textbook example of a Mott-Hubbard transition between a paramagnetic metal and a paramagnetic insulator. We review recent theoretical calculations as well as experimental findings which shed new light on this famous transition. In particular, the old paradigm of a doping-pressure equivalence does not hold, and there is a microscale phase separation for Cr-doped V2O3.

cond-mat.str-el↗

arXiv2013

Ultrafast filling of an electronic pseudogap in an incommensurate crystal

We investigate the quasiperiodic crystal (LaS)1.196(VS2) by angle and time resolved photoemission spectroscopy. The dispersion of electronic states is in qualitative agreement with band structure calculated for the VS2 slab without the incommensurate distortion. Nonetheless, the spectra display a temperature dependent pseudogap instead of quasiparticles crossing. The sudden photoexcitation at 50 K induces a partial filling of the electronic pseudogap within less than 80 fs. The electronic energy flows into the lattice modes on a comparable timescale. We attribute this surprisingly short timescale to a very strong electron-phonon coupling to the incommensurate distortion. This result sheds light on the electronic localization arising in aperiodic structures and quasicrystals.

cond-mat.str-el↗

arXiv2012

Giant Anisotropy of Spin-Orbit Splitting at the Bismuth Surface

We investigate the bismuth (111) surface by means of time and angle resolved photoelectron spectroscopy. The parallel detection of the surface states below and above the Fermi level reveals a giant anisotropy of the Spin-Orbit (SO) spitting. These strong deviations from the Rashba-like coupling cannot be treated in $\textbf{k}\cdot \textbf{p}$ perturbation theory. Instead, first principle calculations could accurately reproduce the experimental dispersion of the electronic states. Our analysis shows that the giant anisotropy of the SO splitting is due to a large out-of plane buckling of the spin and orbital texture.

cond-mat.mtrl-sci↗

arXiv2012

Ultrafast surface carrier dynamics in the topological insulator Bi2Te3

We discuss the ultrafast evolution of the surface electronic structure of the topological insulator Bi$_2$Te$_3$ following a femtosecond laser excitation. Using time and angle resolved photoelectron spectroscopy, we provide a direct real-time visualisation of the transient carrier population of both the surface states and the bulk conduction band. We find that the thermalization of the surface states is initially determined by interband scattering from the bulk conduction band, lasting for about 0.5 ps; subsequently, few ps are necessary for the Dirac cone non-equilibrium electrons to recover a Fermi-Dirac distribution, while their relaxation extends over more than 10 ps. The surface sensitivity of our measurements makes it possible to estimate the range of the bulk-surface interband scattering channel, indicating that the process is effective over a distance of 5 nm or less. This establishes a correlation between the nanoscale thickness of the bulk charge reservoir and the evolution of the ultrafast carrier dynamics in the surface Dirac cone.

cond-mat.mtrl-sci↗

arXiv2012

Orbitally resolved lifetimes in Ba(Fe0.92Co0.08)2As2 measured by ARPES

Despite many ARPES investigations of iron pnictides, the structure of the electron pockets is still poorly understood. By combining ARPES measurements in different experimental configurations, we clearly resolve their elliptic shape. Comparison with band calculation identify a deep electron band with the dxy orbital and a shallow electron band along the perpendicular ellipse axis with the dxz/dyz orbitals. We find that, for both electron and hole bands, the lifetimes associated with dxy are longer than for dxz/dyz. This suggests that the two types of orbitals play different roles in the electronic properties and that their relative weight is a key parameter to determine the ground state.

cond-mat.supr-con↗