arXiv subjects

Azzedine Bendounan

Publications and source records attributed to Azzedine Bendounan.

16 recordsLinked to original sources

arXiv2026

Surface reconstruction-driven band folding and spin-orbit enhancement at the $α$-antimonene/Au(111) interface

The electronic properties of the two-dimensional (2D) $α$ phase of antimonene are unique, featuring unpinned Dirac cones that can be moved with strain. Here we investigate the structural and electronic properties of an epitaxial 2D $α$-antimonene, grown on Au(111). Using angle-resolved photoemission spectroscopy and density-functional theory, we reveal a strong hybridization at the Sb/Au interface, which imprints a rectangular reconstruction in the Au states, producing a band folding and hybrid bands exhibiting trigonal pockets. Additionally, hybridization displaces part of the Au wavefunction in regions of large electrostatic potential gradient, thereby enhancing spin-orbit splitting. Our work underscores that the pristine electronic properties of $α$-antimonene may be deeply modified by its substrate, and even overwhelmed by the bands of the latter, and also shows that spin-orbit interaction in a heavy metal (Au) can be substantially enhanced by a lighter element (Sb).

cond-mat.mtrl-sci↗

arXiv2024

Direct observation of electronic band gap and hot carrier dynamics in GeAs semiconductor

Germanium arsenide (GeAs) is a layered semiconductor with remarkably anisotropic physical, thermoelectric and optical properties, and a promising candidate for multifunctional devices based on in-plane polarization dependent response. Understanding the underlying mechanism of such devices requires the knowledge of GeAs electronic band structure and of the hot carrier dynamics in its conduction band, whose details are still unclear. In this work, we investigated the properties of occupied and photoexcited states of GeAs in energy-momentum space, by combining scanning tunneling spectroscopy (STS), angle-resolved photoemission spectroscopy (ARPES) and time-resolved ARPES. We found that, GeAs is an indirect gap semiconductor having an electronic gap of 0.8 eV, for which the conduction band minimum (CBM) is located at the Gamma point while the valence band maximum (VBM) is out of Gamma. A Stark broadening of the valence band is observed immediately after photoexcitation, which can be attributed to the effects of the electrical field at the surface induced by inhomogeneous screening. Moreover, the hot electrons relaxation time of 1.56 ps down to the CBM which is dominated from both inter-valley and intra-valley coupling. Besides their relevance for our understanding of GeAs, these findings present general interest for the design on high performance thermoelectric and optoelectronic devices based on 2D semiconductors.

cond-mat.mtrl-sci↗

arXiv2020

Silicene nanoribbons on an insulating thin film

Silicene, a new two-dimensional (2D) material has attracted intense research because of the ubiquitous use of silicon in modern technology. However, producing free-standing silicene has proved to be a huge challenge. Until now, silicene could be synthesized only on metal surfaces where it naturally forms strong interactions with the metal substrate that modify its electronic properties. Here, we report the first experimental evidence of silicene sheet on an insulating NaCl thin film. This work represents a major breakthrough; for the study of the intrinsic properties of silicene, and by extension to other 2D materials that have so far only been grown on metal surfaces.

cond-mat.mtrl-sci↗

arXiv2020

Phosphorus Pentamers: Floating Nanoflowers form a 2D Network

We present an experimental investigation of a new polymorphic 2D single layer of phosphorus on Ag(111). The atomically-resolved scanning tunneling microscopy (STM) images show a new 2D material composed of freely-floating phosphorus pentamers organized into a 2D layer, where the pentamers are aligned in close-packed rows. The scanning tunneling spectroscopy (STS) measurements reveal a semiconducting character with a band gap of 1.20 eV. This work presents the formation at low temperature (LT) of a new polymorphic 2D phosphorus layer composed of a floating 2D pentamer structure. The smooth curved terrace edges and a lack of any clear crystallographic orientation with respect to the Ag(111) substrate at room temperature indicates a smooth potential energy surface that is reminiscent of a liquid-like growth phase. This is confirmed by density functional theory (DFT) calculations that find a small energy barrier of only 0.17 eV to surface diffusion of the pentamers (see Supplemental Material). The formation of extended, homogeneous domains is a key ingredient to opening a new avenue to integrate this new 2D material into electronic devices.

cond-mat.mtrl-sci↗

arXiv2020

Tip-induced oxidation of silicene nano-ribbons

We report on the oxidation of self-assembled silicene nanoribbons grown on the Ag(110) surface using Scanning Tunneling Microscopy and High-Resolution Photoemission Spectroscopy. The results show that silicene nanoribbons present a strong resistance towards oxidation using molecular oxygen. This can be overcome by increasing the electric field in the STM tunnel junction above a threshold of +2.6 V to induce oxygen dissociation and reaction. The higher reactivity of the silicene nanoribbons towards atomic oxygen is observed as expected. The HR-PES confirm these observations: Even at high exposures of molecular oxygen, the Si 2p core-level peaks corresponding to pristine silicene remain dominant, reflecting a very low reactivity to molecular oxygen. Complete oxidation is obtained following exposure to high doses of atomic oxygen; the Si 2p core level peak corresponding to pristine silicene disappears.

cond-mat.mtrl-sci↗

arXiv2020

Single-layer graphene on epitaxial FeRh thin films

Graphene is a 2D material that displays excellent electronic transport properties with prospective applications in many fields. Inducing and controlling magnetism in the graphene layer, for instance by proximity of magnetic materials, may enable its utilization in spintronic devices. This paper presents fabrication and detailed characterization of single-layer graphene formed on the surface of epitaxial FeRh thin films. The magnetic state of the FeRh surface can be controlled by temperature, magnetic field or strain due to interconnected order parameters. Characterization of graphene layers by X-ray Photoemission and X-ray Absorption Spectroscopy, Low-Energy Ion Scattering, Scanning Tunneling Microscopy, and Low-Energy Electron Microscopy shows that graphene is single-layer, polycrystalline and covers more than 97% of the substrate. Graphene displays several preferential orientations on the FeRh(001) surface with unit vectors of graphene rotated by 30°, 15°, 11°, and 19° with respect to FeRh substrate unit vectors. In addition, the graphene layer is capable to protect the films from oxidation when exposed to air for several months. Therefore, it can be also used as a protective layer during fabrication of magnetic elements or as an atomically thin spacer, which enables incorporation of switchable magnetic layers within stacks of 2D materials in advanced devices.

cond-mat.mtrl-sci↗

arXiv2018

Dispersing and non-dispersing satellites in the photoemission spectra of aluminum

Satellites in electronic spectra are pure many-body effects, and their study has been of increasing interest in both experiment and theory. The presence of satellites due to plasmon excitations can be understood with simple models of electron-boson coupling. It is far from obvious how to match such a model to real spectra, where more than one kind of quasi-particle and of satellite excitation coexist. Our joint experimental and theoretical study shows that satellites in the angle-resolved photoemission spectra of the prototype simple metal aluminum consist of a superposition of dispersing and non-dispersing features. Both are due to electron-electron interaction, but the non-dispersing satellites also reflect the thermal motion of the atoms. Moreover, besides their energy dispersion, we also show and explain a strong shape dispersion of the satellites. By taking into account these effects, our first principles calculations using the GW+C approach of many-body perturbation theory reproduce and explain the experimental spectra to an unprecedented extent.

cond-mat.str-el↗

arXiv2018

Epitaxial Synthesis of Blue Phosphorene

Phosphorene is a new two-dimensional material composed of a single or few atomic layers of black phosphorus. Phosphorene has both an intrinsic tunable direct band gap and high carrier mobility values, which make it suitable for a large variety of optical and electronic devices. However, the synthesis of single-layer phosphorene is a major challenge. The standard procedure to obtain phosphorene is by exfoliation. More recently, the epitaxial growth of single-layer phosphorene on Au(111) has been investigated by molecular beam epitaxy and the obtained structure has been described as a blue-phosphorene sheet. In the present study, large areas of high-quality monolayer phosphorene, with a band gap value at least equal to 0.8 eV, have been synthesized on Au(111). Our experimental investigations, coupled with DFT calculations, give evidence of two distinct phases of blue phosphorene on Au(111), instead of one as previously reported, and their atomic structures have been determined.

cond-mat.mtrl-sci↗

arXiv2018

Laser induced phase transition in epitaxial FeRh layers studied by pump-probe valence band photoemission

We use time-resolved X-ray photoelectron spectroscopy to probe the electronic and magnetization dynamics in FeRh films after ultrafast laser excitations. We present experimental and theoretical results which investigate the electronic structure of the FeRh during the first-order phase transition identifying a clear signature of the magnetic phase. We find that a spin polarized feature at the Fermi edge is a fingerprint of the magnetic status of the system that is independent of the long-range ferromagnetic alignment of the magnetic domains. We use this feature to follow the phase transition induced by a laser pulse in a pump-probe experiment and find that the magnetic transition occurs in less than 50 ps, and reaches its maximum in 100 ps.

cond-mat.str-el↗

arXiv2016

Giant Rashba effect at the topological surface of PrGe revealing antiferromagnetic spintronics

Rashba spin-orbit splitting in the magnetic materials opens up a new perspective in the field of spintronics. Here, we report a giant Rashba-type spin-orbit effect on PrGe [010] surface in the paramagnetic phase with Rashba coefficient α_R=5 eVÅ. Significant changes in the electronic band structure has been observed across the phase transitions from paramagnetic to antiferromagnetic (44 K) and from antiferromagnetic to the ferromagnetic ground state (41.5 K). We find that Pr 4f states in PrGe is strongly hybridized with the Pr 5d and Ge 4s-4p states near the Fermi level. The behavior of Rashba effect is found to be different in the k_x and the k_y directions showing electron-like and the hole-like bands, respectively. The possible origin of Rashba effect in the paramagnetic phase is related to the anti-parallel spin polarization present in this system. First-principles density functional calculations of Pr terminated surface with the anti-parallel spins shows a fair agreement with the experimental results. We find that the anti-parallel spins are strongly coupled to the lattice such that the PrGe system behaves like weak ferromagnetic system. Analysis of the energy dispersion curves at different magnetic phases showed that there is a competition between the Dzyaloshinsky-Moriya interaction and the exchange interaction which gives rise to the magnetic ordering in PrGe. Supporting evidences of the presence of Dzyaloshinsky-Moriya interaction are observed as anisotropic magnetoresistance with respect to field direction and first-order type hysteresis in the X-ray diffraction measurements. A giant negative magnetoresistance of 43% in the antiferromagnetic phase and tunable Rashba parameter with temperature across the magnetic transitions makes this material a suitable candidate for technological application in the antiferromagnetic spintronic devices.

cond-mat.mtrl-sci↗

arXiv2015

Stable room-temperature ferromagnetic phase at the FeRh(100) surface

Interfaces and low dimensionality are sources of strong modifications of electronic, structural, and magnetic properties of materials. FeRh alloys are an excellent example because of the first-order phase transition taking place at $\sim$400 K from an antiferromagnetic phase at room temperature to a high temperature ferromagnetic one. It is accompanied by a resistance change and volume expansion of about 1\%. We have investigated the electronic and magnetic properties of FeRh(100) epitaxially grown on MgO by combining spectroscopies characterized by different probing depths, namely X-ray magnetic circular dichroism and photoelectron spectroscopy. We thus reveal that the symmetry breaking induced at the Rh-terminated surface stabilizes a surface ferromagnetic layer involving five planes of Fe and Rh atoms in the nominally antiferromagnetic phase at room temperature. First-principles calculations provide a microscopic description of the structural relaxation and the electron spin-density distribution that fully support the experimental findings.

cond-mat.mtrl-sci↗

arXiv2014

Estimate of the Coulomb Correlation Energy in CeAg$_2$Ge$_2$ from Inverse Photoemission and High Resolution Photoemission Spectroscopy

The occupied and the unoccupied electronic structure of CeAg$_2$Ge$_2$ single crystal has been studied using high resolution photoemission and inverse photoemission spectroscopy respectively. High resolution photoemission reveals the clear signature of Ce $4f$ states in the occupied electronic structure which was not observed earlier due to the poor resolution. The coulomb correlation energy in this system has been determined experimentally from the position of the $4f$ states above and below the Fermi level. Theoretically the correlation energy has been determined by using the first principles density functional calculations within the generalized gradient approximations taking into account the strong intra-atomic (on-site) interaction Hubbard $U_{eff}$ term. Although the valence band calculated with different $U_{eff}$ does not show significant difference, but the substantial changes are observed in the conduction band. The estimated value of correlation energy from both the theory and the experiment is $\approx$4.2~eV for CeAg$_2$Ge$_2$.

cond-mat.mtrl-sci↗

arXiv2013

Silicon Sheets By Redox Assisted Chemical Exfoliation

In this paper, we report the direct chemical synthesis of silicon sheets in gram-scale quantities by chemical exfoliation of pre-processed calcium di-silicide (CaSi2). We have used a combination of X-ray photoelectron spectroscopy, transmission electron microscopy and Energy-dispersive X-ray spectroscopy to characterize the obtained silicon sheets. We found that the clean and crystalline silicon sheets show a 2-dimensional hexagonal graphitic structure.

physics.chem-ph↗

arXiv2013

Formation of one-dimensional self-assembled silicon nanoribbons on Au(110)-(2x1)

We report results on the self-assembly of silicon nanoribbons on the (2x1) reconstructed Au(110) surface under ultra-high vacuum conditions. Upon adsorption of 0.2 monolayer (ML) of silicon the (2x1) reconstruction of Au(110) is replaced by an ordered surface alloy. Above this coverage a new superstructure is revealed by low electron energy diffraction (LEED) which becomes sharper at 0.3 Si ML. This superstructure corresponds to Si nanoribbons all oriented along the [-110] direction as revealed by LEED and scanning tunneling microscopy (STM). STM and high-resolution photoemission spectroscopy indicate that the nanoribbons are flat and predominantly 1.6 nm wide. In addition the silicon atoms show signatures of two chemical environments corresponding to the edge and center of the ribbons.

cond-mat.mtrl-sci↗

arXiv2010

Electronic Properties of low dimensional structures

Exotic phenomena about the behavior of electrons inside the solid were a long time ago predicted by the quantum mechanic physics and are only recently experimentally observed, in particular for systems of extremely reduced dimensions. Here, I report on recent experimental observation of fundamental effect concerning the dispersion properties of the surface state influenced by the presence of surface reconstruction.

physics.chem-ph↗

arXiv2009

Asymmetric hysteresis of Néel caps in flux-closure magnetic dots

We investigated with XMCD-PEEM magnetic imaging the magnetization reversal processes of Néel caps inside Bloch walls in self-assembled, micron-sized Fe(110) dots with flux-closure magnetic state. In most cases the magnetic-dependent processes are symmetric in field, as expected. However, some dots show pronounced asymmetric behaviors. Micromagnetic simulations suggest that the geometrical features (and their asymmetry) of the dots strongly affect the switching mechanism of the Néel caps.

cond-mat.mtrl-sci↗