Publications

2020

• Electronic properties of silver chloride : influence of excitons on the charge dynamics
  
  • Lorin Arnaud
  , 2020. In 1848, Edmond Becquerel proposed one of the first techniques of color photography. This technique raised a long-standing debate in the scientific community concerning the origin of the observed colors. In 2019, Victor de Seauve, during his thesis at the Muséum National d'Histoire Naturelle in Paris, could clarify some important issues, proposing a new explanation. The aim of the present thesis was to participate in this effort of comprehension, by adding theoretical insight. The first question is which level of theory one may adopt in order to correctly describe the electronic and optical properties of AgCl. Our calculations show that only the solution of the Bethe-Salpeter equation is able to describe the optical spectrum of AgCl, since the spectrum is dominated by a strong peak at the absorption onset, due to a bound exciton. Moreover, the calculation has to be based on a band structure that results from a self-consistent GW calculation. In order to make the Bethe-Salpeter calculations possible, a previously proposed model dielectric function was used in an improved way. Our results well describe the measured absorption spectra, and enable us to analyse and interpret the observations. In the hypothesis brought forward by the team at the Museum, the light excites a silver nanoparticle, followed by a transfer of charge at the interface between the nanoparticle and AgCl. In order to be able to study this phenomenon, we have developed a numerical approach describing the charge dynamics due to an external perturbation. In particular, this approach has allowed us to show that a bound exciton influences the charge dynamics significantly. Finally, we have set up a first model meant to simulate the charge transfer between a silver nanoparticle and AgCl, based on a simplified heterostructure. We have shown how the absorption spectrum of AgCl changes due to its neighbourhood to the silver metal, and how these changes can be described by the effective medium theory. Calculations of the density induced by a periodic perturbation have been carried out. These calculations remain cumbersome, and the study will continue with the idea to combine ab initio calculations with effective medium theory.
• Spin-VCSELs with local optical anisotropies: toward terahertz polarization modulation
  
  • Drong M.
  • Fördös T.
  • Jaffrès H.Y. y
  • Ciompa P.
  • Peřina J.
  • Postava K.
  • Pištora J.
  • Drouhin H.-J.
  Physical Review Applied, American Physical Society, 2020, 15 (1). We present a semi-classical model for spin-injected vertical-cavity surface-emitting lasers (spin-VCSELs) with local optical anisotropies. Particular focus is put on highly-anisotropic spin-lasers with broad application potential. A generalized matrix formalism for extraction of the laser modes is introduced, which enables to calculate spatial distribution of vectorial modes in arbitrary spin-VCSELs. Time-dependence of such laser modes is further studied using the generalized coupled mode theory (CMT). It is the natural anisotropic generalization of the conventional modedecomposition approach. We use the circularly-polarized optical modes as the basis for CMT, which leads to extension of the well-known spin-flip model (SFM). In contrary to conventional SFM, the only input parameters are the geometric and local optical properties of the multilayer structure and properties of the gain media. The advantages of the theory are demonstrated on design and optimization of spin-VCSEL structure with high-contrast grating. We show that the proposed structures can be used for i) polarization modulation in THz range with tremendous applications for future ultrafast optical communication and ii) as perspective compact THz sources. (10.1103/PhysRevApplied.15.014041)
  DOI : 10.1103/PhysRevApplied.15.014041
• The color photography of Edmond Becquerel and quantum mechanics
  
  • Sottile Francesco
  • Lorin Arnaud
  • Gatti Matteo
  • Reining Lucia
  , 2020.
• Polymerization Reactions and Modifications of Polymers by Ionizing Radiation
  
  • Ashfaq Aiysha
  • Clochard Marie-Claude
  • Coqueret Xavier
  • Dispenza Clelia
  • Driscoll Mark
  • Ulański Piotr
  • Al-Sheikhly Mohamad
  Polymers, MDPI, 2020, 12 (12), pp.2877. Ionizing radiation has become the most effective way to modify natural and synthetic polymers through crosslinking, degradation, and graft polymerization. This review will include an in-depth analysis of radiation chemistry mechanisms and the kinetics of the radiation-induced C-centered free radical, anion, and cation polymerization, and grafting. It also presents sections on radiation modifications of synthetic and natural polymers. For decades, low linear energy transfer (LLET) ionizing radiation, such as gamma rays, X-rays, and up to 10 MeV electron beams, has been the primary tool to produce many products through polymerization reactions. Photons and electrons interaction with polymers display various mechanisms. While the interactions of gamma ray and X-ray photons are mainly through the photoelectric effect, Compton scattering, and pair-production, the interactions of the high-energy electrons take place through coulombic interactions. Despite the type of radiation used on materials, photons or high energy electrons, in both cases ions and electrons are produced. The interactions between electrons and monomers takes place within less than a nanosecond. Depending on the dose rate (dose is defined as the absorbed radiation energy per unit mass), the kinetic chain length of the propagation can be controlled, hence allowing for some control over the degree of polymerization. When polymers are submitted to high-energy radiation in the bulk, contrasting behaviors are observed with a dominant effect of cross-linking or chain scission, depending on the chemical nature and physical characteristics of the material. Polymers in solution are subject to indirect effects resulting from the radiolysis of the medium. Likewise, for radiation-induced polymerization, depending on the dose rate, the free radicals generated on polymer chains can undergo various reactions, such as inter/intramolecular combination or inter/intramolecular disproportionation, b-scission. These reactions lead to structural or functional polymer modifications. In the presence of oxygen, playing on irradiation dose-rates, one can favor crosslinking reactions or promotes degradations through oxidations. The competition between the crosslinking reactions of C-centered free radicals and their reactions with oxygen is described through fundamental mechanism formalisms. The fundamentals of polymerization reactions are herein presented to meet industrial needs for various polymer materials produced or degraded by irradiation. Notably, the medical and industrial applications of polymers are endless and thus it is vital to investigate the effects of sterilization dose and dose rate on various polymers and copolymers with different molecular structures and morphologies. The presence or absence of various functional groups, degree of crystallinity, irradiation temperature, etc. all greatly affect the radiation chemistry of the irradiated polymers. Over the past decade, grafting new chemical functionalities on solid polymers by radiation-induced polymerization (also called RIG for Radiation-Induced Grafting) has been widely exploited to develop innovative materials in coherence with actual societal expectations. These novel materials respond not only to health emergencies but also to carbon-free energy needs (e.g., hydrogen fuel cells, piezoelectricity, etc.) and environmental concerns with the development of numerous specific adsorbents of chemical hazards and pollutants. The modification of polymers through RIG is durable as it covalently bonds the functional monomers. As radiation penetration depths can be varied, this technique can be used to modify polymer surface or bulk. The many parameters influencing RIG that control the yield of the grafting process are discussed in this review. These include monomer reactivity, irradiation dose, solvent, presence of inhibitor of homopolymerization, grafting temperature, etc. Today, the general knowledge of RIG can be applied to any solid polymer and may predict, to some extent, the grafting location. A special focus is on how ionizing radiation sources (ion and electron beams, UVs) may be chosen or mixed to combine both solid polymer nanostructuration and RIG. LLET ionizing radiation has also been extensively used to synthesize hydrogel and nanogel for drug delivery systems and other advanced applications. In particular, nanogels can either be produced by radiation-induced polymerization and simultaneous crosslinking of hydrophilic monomers in “nanocompartments”, i.e., within the aqueous phase of inverse micelles, or by intramolecular crosslinking of suitable water-soluble polymers. The radiolytically produced oxidizing species from water, •OH radicals, can easily abstract H-atoms from the backbone of the dissolved polymers (or can add to the unsaturated bonds) leading to the formation of C-centered radicals. These C-centered free radicals can undergo two main competitive reactions; intramolecular and intermolecular crosslinking. When produced by electron beam irradiation, higher temperatures, dose rates within the pulse, and pulse repetition rates favour intramolecular crosslinking over intermolecular crosslinking, thus enabling a better control of particle size and size distribution. For other water-soluble biopolymers such as polysaccharides, proteins, DNA and RNA, the abstraction of H atoms or the addition to the unsaturation by •OH can lead to the direct scission of the backbone, double, or single strand breaks of these polymers. (10.3390/polym12122877)
  DOI : 10.3390/polym12122877
• Non-Gaussian tail in the force distribution: a hallmark of correlated disorder in the host media of elastic objects
  
  • Sánchez Jazmín Aragón
  • Rumi Gonzalo
  • Maldonado Raúl Cortés
  • Bolecek Néstor René Cejas
  • Puig Joaquín
  • Pedrazzini Pablo
  • Nieva Gladys
  • Dolz Moira I.
  • Konczykowski Marcin
  • Beek Cornelis J. van Der
  • Kolton Alejandro B.
  • Fasano Yanina
  Scientific Reports, Nature Publishing Group, 2020, 10, pp.19452. Inferring the nature of disorder in the media where elastic objects are nucleated is of crucial importance for many applications but remains a challenging basic-science problem. Here we propose a method to discern whether weak-point or strong-correlated disorder dominates based on characterizing the distribution of the interaction forces between objects mapped in large fields-of-view. We illustrate our proposal with the case-study system of vortex structures nucleated in type-II superconductors with different pinning landscapes. Interaction force distributions are computed from individual vortex positions imaged in thousands-vortices fields-of-view in a two-orders-of-magnitude-wide vortex-density range. Vortex structures nucleated in point-disordered media present Gaussian distributions of the interaction force components. In contrast, if the media have dilute and randomly-distributed correlated disorder, these distributions present non-Gaussian algebraically-decaying tails for large force magnitudes. We propose that detecting this deviation from the Gaussian behavior is a fingerprint of strong disorder, in our case originated from a dilute distribution of correlated pinning centers. (10.1038/s41598-020-76529-w)
  DOI : 10.1038/s41598-020-76529-w
• Suppression of anharmonic phonons and s -wave superconductivity by defects in the filled skutterudite LaRu$_4$As$_{12}$
  
  • Mizukami Y.
  • Kończykowski M.
  • Tanaka O.
  • Juraszek J.
  • Henkie Z.
  • Cichorek T.
  • Shibauchi T.
  Physical Review Research, American Physical Society, 2020, 2 (4), pp.043428. In filled-skutterudite compounds, guest atoms are centered at highly symmetric cages essentially formed by pnictogens. The weak bonding of the guest atoms to the cage-forming elements is expected to give rise to local atomic vibrations in anharmonic potential which can influence electronic properties and may even promote superconductivity. However, the relation between the unusual low-energy phonon excitations and the electronic properties is still elusive, and evidence establishing a direct link between them is a long-standing issue. Here, we investigate the impact of artificial atomic defects introduced by electron irradiation in the filled-skutterudite superconductor LaRu 4 As 12. Our high-resolution heat capacity measurements reveal that the electronic specific heat is substantially reduced with increasing concentration of atomic defects. Moreover, the irradiation suppresses the fully gapped s-wave superconductivity as well as the contribution of anharmonic phonons in the specific heat in a correlated fashion. Our findings imply that the anharmonic phonons play an important role in enhancing electronic specific heat and superconductivity. (10.1103/PhysRevResearch.2.043428)
  DOI : 10.1103/PhysRevResearch.2.043428
• An uranyl sorption study inside functionalised nanopores
  
  • Pinaeva U.
  • Ollier N.
  • Cavani O.
  • Balanzat E.
  • Al-Sheikhly M.
  • Wade T.
  • Clochard M.-C.
  Scientific Reports, Nature Publishing Group, 2020, 10 (1), pp.5776. Abstract Sorption mechanism of uranyl by poly(bis[2-(methacryloyloxy)ethyl] phosphate) (PB2MP) functionalised polyvinylidene fluoride (PVDF) track-etched membranes, PB2MP-g-PVDF, was investigated. It was found that uranyl sorption obeyed Langmuir isotherm model giving a maximum U(VI) membrane uptake of 6.73 μ mol g −1 and an affinity constant of 9.85 ⋅ 10 6 L mol −1 . XPS and TRPL measurements were performed to identify sorbed uranyl oxidation state and its environment. Uranyl was found to be mainly in its hexavalent state, i.e . U(VI), showing that the trapping inside the PB2MP-g-PVDF nanoporous membranes did not change the ion speciation. Two sorbed uranyl life-times ( τ 1 = 8.8 μ s and τ 2 = 102.8 μ s) were measured by TRPL which pointed out different complexations taking place inside the nanopores. Uranyl sorption by PB2MP-g-PVDF membranes was also found to be pH dependent demonstrating the highest performance at circumneutral pH. In addition, TRPL was demonstrated to be not only a remarkable technique for U(VI) characterization, but also an alternative to voltammetry detection for trace on-site uranyl monitoring using PB2MP-g-PVDF nanoporous membranes. (10.1038/s41598-020-62792-4)
  DOI : 10.1038/s41598-020-62792-4
• An uranyl sorption study inside functionalised nanopores
  
  • Pinaeva U.
  • Ollier N.
  • Cavani O.
  • Balanzat E.
  • Al-Sheikhly M.
  • Wade Travis L.
  • Clochard M.-C.
  Scientific Reports, Nature Publishing Group, 2020, 10 (1), pp.5776. Abstract Sorption mechanism of uranyl by poly(bis[2-(methacryloyloxy)ethyl] phosphate) (PB2MP) functionalised polyvinylidene fluoride (PVDF) track-etched membranes, PB2MP-g-PVDF, was investigated. It was found that uranyl sorption obeyed Langmuir isotherm model giving a maximum U(VI) membrane uptake of 6.73 μ mol g −1 and an affinity constant of 9.85 ⋅ 10 6 L mol −1 . XPS and TRPL measurements were performed to identify sorbed uranyl oxidation state and its environment. Uranyl was found to be mainly in its hexavalent state, i.e . U(VI), showing that the trapping inside the PB2MP-g-PVDF nanoporous membranes did not change the ion speciation. Two sorbed uranyl life-times ( τ 1 = 8.8 μ s and τ 2 = 102.8 μ s) were measured by TRPL which pointed out different complexations taking place inside the nanopores. Uranyl sorption by PB2MP-g-PVDF membranes was also found to be pH dependent demonstrating the highest performance at circumneutral pH. In addition, TRPL was demonstrated to be not only a remarkable technique for U(VI) characterization, but also an alternative to voltammetry detection for trace on-site uranyl monitoring using PB2MP-g-PVDF nanoporous membranes. (10.1038/s41598-020-62792-4)
  DOI : 10.1038/s41598-020-62792-4
• Spin memory of the topological material under strong disorder
  
  • Korzhovska Inna
  • Deng Haiming
  • Zhao Lukas
  • Deshko Yury
  • Chen Zhiyi
  • Konczykowski Marcin
  • Zhao Shihua
  • Raoux Simone
  • Krusin-Elbaum Lia
  Npj Quantum Materials, Nature publishing, 2020, 5, pp.39. Robustness to disorder is the defining property of any topological state. The ultimate disorder limits to topological protection are still unknown, although a number of theories predict that even in the amorphous state a quantized conductance might yet reemerge. Here we report that in strongly disordered thin films of the topological material Sb$_2$Te$_3$ disorder-induced spin correlations dominate transport of charge—they engender a spin memory phenomenon, generated by the nonequilibrium charge currents controlled by localized spins. We directly detect a glassy yet robust disorder-induced magnetic signal in films free of extrinsic magnetic dopants, which becomes null in a lower-disorder crystalline state. This is where large isotropic negative magnetoresistance (MR)—a hallmark of spin memory—crosses over to positive MR, first with only one e$^2$/h quantum conduction channel, in a weakly antilocalized diffusive transport regime with a 2D scaling characteristic of the topological state. A fresh perspective revealed by our findings is that spin memory effect sets a disorder threshold to the protected topological state. It also points to new possibilities of tuning spin-dependent charge transport by disorder engineering of topological materials (10.1038/s41535-020-0241-5)
  DOI : 10.1038/s41535-020-0241-5
• Spin memory of the topological material under strong disorder
  
  • Korzhovska Inna
  • Deng Haiming
  • Zhao Lukas
  • Deshko Yury
  • Chen Zhiyi
  • Konczykowski Marcin
  • Zhao Shihua
  • Raoux Simone
  • Krusin-Elbaum Lia
  Npj Quantum Materials, Nature publishing, 2020, 5 (1). Robustness to disorder is the defining property of any topological state. The ultimate disorder limits to topological protection are still unknown, although a number of theories predict that even in the amorphous state a quantized conductance might yet reemerge. Here we report that in strongly disordered thin films of the topological material Sb2Te3 disorder-induced spin correlations dominate transport of charge—they engender a spin memory phenomenon, generated by the nonequilibrium charge currents controlled by localized spins. We directly detect a glassy yet robust disorder-induced magnetic signal in films free of extrinsic magnetic dopants, which becomes null in a lower-disorder crystalline state. This is where large isotropic negative magnetoresistance (MR)—a hallmark of spin memory—crosses over to positive MR, first with only one e2/h quantum conduction channel, in a weakly antilocalized diffusive transport regime with a 2D scaling characteristic of the topological state. A fresh perspective revealed by our findings is that spin memory effect sets a disorder threshold to the protected topological state. It also points to new possibilities of tuning spin-dependent charge transport by disorder engineering of topological materials. (10.1038/s41535-020-0241-5)
  DOI : 10.1038/s41535-020-0241-5
• Exploratory routes to reinforce boron carbides using high pressure syntheses
  
  • Chakraborti Amrita
  , 2020. The goal of this work is to explore different ways to strengthen boron carbide using high pressure synthesis as a tool. To this end, different routes of reinforcement have been followed over the last three years.I have used torsion under high pressure using the Rotational Tomography Paris-Edinburgh Cell (RoToPEC) to study the vacancies developed in boron carbide, which is a possible reason for its diminishing strength beyond its Hugoniot Elastic Limit (HEL).High pressure devices like Paris-Edinburgh press, multi-anvil press and diamond anvil cell were used to synthesise new phases of boron carbide. It had already been predicted that these phases would have improved mechanical strength through Density Functional Theory calculations. As part of these experiments, synthesis parameters of boron carbide at high pressure high temperature conditions have been optimised and new synthesis techniques for other industrially relevant materials like alpha boron have also been established.An idea to use silicon as a reinforcing material for boron carbide has also been pursued. High pressure tools were used to advance towards synthesising a new ternary compound of boron, carbon and silicon with improved mechanical properties.
• Unraveling intrinsic correlation effects with angle-resolved photoemission spectroscopy
  
  • Zhou Jianqiang Sky
  • Reining Lucia
  • Nicolaou Alessandro
  • Bendounan Azzedine
  • Ruotsalainen Kari
  • Vanzini Marco
  • Kas J.
  • Rehr J.
  • Muntwiler Matthias
  • Strocov Vladimir
  • Sirotti Fausto G
  • Gatti Matteo
  Proceedings of the National Academy of Sciences of the United States of America, National Academy of Sciences, 2020, 117 (46), pp.28596-28602. (10.1073/pnas.2012625117)
  DOI : 10.1073/pnas.2012625117
• Ultrafast dynamics of photoexcited carriers in bulk semiconductors and in accumulation layer: energy relaxation and screening effects
  
  • Sjakste Jelena
  , 2020.
• Radiolysis and corrosion of stainless steel in high temperature water environment
  
  • Feron Damien
  • Wang Mi
  • Perrin Sébastien
  • Corbel Catherine
  , 2020. Radiolysis influences not only the electrochemical behaviour but also the oxide formation of stainless steel in high temperature water environments, like primary water of Pressurised Water Reactors (PWR). This paper focus on the corrosion behaviour of austenitic 316L stainless steel in PWR primary water under the influence of radiolysis, including experimental devices and main obtained results. After a short overview of the literature in the field, the approach is described with the use of a high energy proton beam to control the production of radiolytic species at the interface between stainless steel (316L type) and PWR water solution in a high temperature and high pressure electrochemical cell working up to 300°C and 100 bar. The corrosion potential of stainless steel was enhanced by the radiolysis causing by high energy proton beam. A high similarity of electrochemical behaviour was also observed under electron beam. The electrochemical oxidative response of the 316L/PWR solution interface under radiolysis is related to the surface characterization analysis (SEM, XPS, Raman spectroscopy, NRA…) on the oxide layers of 316L which are formed under or without irradiation. The radiolysis effect on the oxide film includes micron scale cavities which were observed in a highly irradiated oxide film. The observation of -Fe2O3 hematite on the outer oxide film where cavities were formed is in accordance with the electrochemical oxidative response.
• Effect of controlled pointlike disorder induced by 2.5-MeV electron irradiation on the nematic resistivity anisotropy of hole-doped (Ba, K)Fe$_2$As$_ 2$
  
  • Tanatar M A
  • Timmons Erik I
  • Kończykowski M.
  • Cavani O.
  • Cho Kyuil
  • Liu Yong
  • Lograsso T A
  • Prozorov R.
  Physical Review B, American Physical Society, 2020, 102 (14), pp.144511. In-plane anisotropy of electrical resistivity was studied in samples of the hole-doped Ba$_{1−x}$K$_x$Fe$_2$As$_2$ in the composition range 0.21 $\le$ $x$ $\le$ 0,26 where anisotropy changes sign. Low-temperature (∼20 K) irradiation with relativistic 2.5 MeV electrons was used to control the level of disorder and residual resistivity of the samples. Modification of the stress-detwinning technique enabled measurements of the same samples before and after irradiation, leading to the conclusion of anisotropic character of predominantly inelastic scattering processes. Our main finding is that the resistivity anisotropy is of the same sign irrespective of residual resistivity, and remains the same in the orthorhombic $C_2$ phase above the reentrant tetragonal transition. Unusual $T$-linear dependence of the anisotropy $\Delta$ρ ≡ ρ$_a$($T$) − ρ$_b$($T$) is found in pristine samples with x = 0.213 and x = 0.219, without similar signatures in either ρ$_a$ ($T$) or ρ$_b$ ($T$). We show that this feature can be reproduced by a phenomenological model of R. M. Fernandes et al. [Phys. Rev. Lett. 107, 217002 (2011)]. We speculate that onset of fluctuations of nematic order on approaching the instability towards the reentrant tetragonal phase contributes to this unusual dependence. (10.1103/PhysRevB.102.144511)
  DOI : 10.1103/PhysRevB.102.144511
• Excitation pathways in resonant inelastic x-ray scattering of solids
  
  • Vorwerk Christian
  • Sottile Francesco
  • Draxl Claudia
  Physical Review Research, American Physical Society, 2020, 2 (4), pp.042003. (10.1103/PhysRevResearch.2.042003)
  DOI : 10.1103/PhysRevResearch.2.042003
• Ultrafast dynamics of hot carriers in a quasi–two-dimensional electron gas on InSe
  
  • Chen Zhesheng
  • Sjakste Jelena
  • Dong Jingwei
  • Taleb-Ibrahimi Amina
  • Rueff Jean-Pascal
  • Shukla Abhay
  • Peretti Jacques
  • Papalazarou Evangelos
  • Marsi Marino
  • Perfetti Luca
  Proceedings of the National Academy of Sciences of the United States of America, National Academy of Sciences, 2020, 117 (36), pp.21962-21967. Two-dimensional electron gases (2DEGs) are at the base of current nanoelectronics because of their exceptional mobilities. Often the accumulation layer forms at polar interfaces with longitudinal optical (LO) modes. In most cases, the many-body screening of the quasi-2DEGs dramatically reduces the Fröhlich scattering strength. Despite the effectiveness of such a process, it has been recurrently proposed that a remote coupling with LO phonons persists even at high carrier concentration. We address this issue by perturbing electrons in an accumulation layer via an ultrafast laser pulse and monitoring their relaxation via time- and momentum-resolved spectroscopy. The cooling rate of excited carriers is monitored at doping level spanning from the semiconducting to the metallic limit. We observe that screening of LO phonons is not as efficient as it would be in a strictly 2D system. The large discrepancy is due to the remote coupling of confined states with the bulk. Our data indicate that the effect of such a remote coupling can be mimicked by a 3D Fröhlich interaction with Thomas–Fermi screening. These conclusions are very general and should apply to field effect transistors (FET) with high- dielectric gates, van der Waals heterostructures, and metallic interfaces between insulating oxides. (10.1073/pnas.2008282117)
  DOI : 10.1073/pnas.2008282117
• Structural and electronic transitions in few layers of isotopically pure hexagonal boron nitride
  
  • Zribi Jihene
  • Khalil Lama
  • Avila José
  • Chaste Julien
  • Henck Hugo
  • Oehler Fabrice
  • Gil Bernard
  • Liu Song
  • Edgar James
  • Giorgetti Christine
  • Dappe Yannick J
  • Lhuillier Emmanuel
  • Cassabois Guillaume
  • Ouerghi Abdelkarim
  • Pierucci Debora
  Physical Review B, American Physical Society, 2020, 102 (11). Hexagonal boron nitride (hBN) is attracting tremendous interest as an essential component in van der Waals heterostructures due to its ability to provide weakly interacting interfaces and because of its large bandgap. Although most of theoretical calculations yield the standard AA′ stacking for few-layer hBN, the exact determination of its structural and electronic properties remains unrevealed to date. Here, we provide the direct observation of structural and electronic transitions in few layers of isotopically pure exfoliated h11BN flakes. Our nanoscopic angle-resolved photoemission spectroscopy measurements combined with density-functional theory calculations indicate that the stacking and the band structure can be strongly affected by the thickness of h11BN. Hence, we show that hBN presents an AA′ stacking in its bulk form and another more exotic stacking for three and four layers. Our findings open perspectives in understanding and controlling the stackings in hBN, which could be of great interest for optoelectronic applications. (10.1103/PhysRevB.102.115141)
  DOI : 10.1103/PhysRevB.102.115141
• Ion shaping of single-layer Au nanoparticles in amorphous silicon dioxide, in silicon nitride, and at their interface
  
  • Mota-Santiago P.
  • Kremer F.
  • Rizza G.
  • Dufour C.
  • Khomenkov V.
  • Notthoff C.
  • Hadley A.
  • Kluth P.
  Physical Review Materials, American Physical Society, 2020, 4 (9), pp.096002. We present the shape transformation of a single layer of Au nanoparticles (NPs) when embedded in, and at the interface of, amorphous SiNx and SiOx (a−SiNx and a−SiOx) thin films upon irradiation with 185-MeV Au ions to fluences ranging from 0.3 to 30×1013cm−2. Transmission electron microscopy (TEM) and high angular annular dark field microscopy were used to study the ion-shaping process. The former allows us to follow the overall change in geometry, size, and structure, while the latter reveals information about the relative position with respect to the interface. For Au NPs embedded in a single material, a lower elongation rate for a−SiNx was found in comparison to a−SiOx. When at the interface of the two materials, TEM reveals a preferential elongation towards a−SiOx. The latter demonstrates the use of a−SiNx for confining the ion-shaping process within an intermediate a−SiOx layer. The simulation of the temperature evolution during a single-ion impact was used to understand the difference in elongation rates between a−SiNx and a−SiOx, as well as the asymmetric behavior when located at the interface using the three-dimensional inelastic thermal spike model with bulk thermophysical properties. The calculations show good agreement with the experimental observations and reveal a correlation between the thermal profile and the resulting NP geometry. (10.1103/PhysRevMaterials.4.096002)
  DOI : 10.1103/PhysRevMaterials.4.096002
• The exact theory of the Stern-Gerlach experiment and why it does not imply that a fermion can only have its spin up or down
  
  • Coddens Gerrit
  , 2020. The Stern-Gerlach experiment is notoriously counter-intuitive. The official theory is that the spin of a fermion remains always aligned with the magnetic field. Its directions are thus quantized: It can only be spin up or down. But that theory is based on mathematical errors in the way it (mis)treats spinors and group theory. We present here a mathematically rigorous theory for a fermion in a magnetic field, which is no longer counter-intuitive. It is based on an understanding of spinors in SU(2) which is only Euclidean geometry. Contrary to what Pauli has been reading into the Stern-Gerlach experiment, the spin directions are not quantized. The new corrected paradigm, which solves all conceptual problems, is that the fermions precess around the magnetic-field just like Einstein and Ehrenfest had conjectured. Surprizingly this leads to only two energy states, which should be qualified as precession-up and precession-down rather than spin-up and spin down. Indeed, despite the presence of the many different possible angles $\theta$ between the spin axis ${\mathbf{s}}$ and the magnetic field ${\mathbf{B}}$, the fermions can only have two possible energies $m_{0}c^{2}\pm\mu B$. The values $\pm\mu B$ do thus not correspond to the continuum of values $-{\boldsymbol{\mu\cdot}}{\mathbf{B}}$ Einstein and Ehrenfest had conjectured. The energy term $V= -{\boldsymbol{\mu\cdot}}{\mathbf{B}}$ is a macroscopic quantity. It is a statistical average over a large ensemble of fermions distributed over the two microscopic energy states $\pm\mu B$, and as such not valid for individual fermions. The two fermion states $\pm\mu B$ are not potential-energy states. We also explain the mathematically rigorous meaning of the up and down spinors. They represent left-handed and right-handed reference frames, such that now everything is intuitively clear and understandable in simple geometrical terms. The paradigm shift does not affect the Pauli principle.
• Surface currents in Hall devices
  
  • Creff M
  • Faisant F
  • Rubì J M
  • Wegrowe Jean-Eric
  Journal of Applied Physics, American Institute of Physics, 2020, 128. One hundred and forty years after his discovery, the Hall effect still deserves attention. If it is well-known that the Hall voltage measured in Hall bar devices is due to the electric charges accumulated at the edges in response to the magnetic field, the nature of the corresponding boundary conditions is still problematic. In order to study this out-of-equilibrium stationary state, the Onsager's least-dissipation principle is applied. It is shown that, beside the well-known expression of the charge accumulation and the corresponding Hall voltage, a longitudinal surface current proportional to the charge accumulation is generated. An expression of the surface current is given. The surface currents allow the Hall voltage to be stabilized at a stationary state, despite, e.g., the presence of leakage of charges at the edges. (10.1063/5.0013182)
  DOI : 10.1063/5.0013182
• Red luminescence and UV light generation of europium doped zinc oxide thin films for optoelectronic applications
  
  • El Jouad Mohamed
  • Bouabdalli El Mehdi
  • Touhtouh Samira
  • Addou Mohammed
  • Ollier Nadège
  • Sahraoui Bouchta
  European Physical Journal: Applied Physics, EDP Sciences, 2020, 91 (1), pp.10501. In the present work, the Europium doped Zinc Oxide (ZnO: Eu) thin films were elaborated using spray pyrolysis technique. We are interested in investigating the structural properties, photoluminescence (PL) and third harmonic generation (THG) of the elaborated films. The structural properties of as-prepared thin films were characterized by X-ray diffraction (XRD). It confirms that all deposited thin films of Europium doped Zinc Oxide are crystallized in the hexagonal wurtzite structure. Both undoped and doped europium thin films show strong preferred c-axis orientation. Photoluminescence (PL) emission from Europium doped Zinc Oxide thin films, under excitation by 266 nm, shows characteristic transitions of Europium (5 D0 ! 7 F0, 5 D0 ! 7 F1, 5 D0 ! 7 F2, etc.). It reveals the good incorporation of Eu3+ ions in the ZnO host. Additionally, the 5 D0 ! 7 F2 is the most intense transition usually observed for Eu3+ embedded in materials of Zinc Oxide lattice. The dependence of third-order nonlinear susceptibility on doping rate was evaluated. The highest nonlinear susceptibility x3 is obtained for the 5% Europium doped ZnO sample. (10.1051/epjap/2020200133)
  DOI : 10.1051/epjap/2020200133
• Excitation of surface plasma waves and fast electron generation in relativistic laser–plasma interaction
  
  • Raynaud Michèle
  • Héron Anne
  • Adam Jean-Claude
  Scientific Reports, Nature Publishing Group, 2020, 10. The excitation of surface plasma waves (SPW) by an intense short laser pulse is a useful tool to enhance the laser absorption and the electron heating in the target. In this work, the influence of the transverse laser profile and the pulse duration used to excited SPW is investigated from Fluid and 2D Particle-in-Cell simulations. We show the existence of a lobe of surface plasma wave modes. Our results highlight surface plasma waves excitation mechanism and define the laser parameters to optimise the SPW excitation and the kinetic energy of the associated electron trapped in the wave. It opens the door to monitor the spectral mode distribution and temporal shape of the excited surface waves in the high relativistic regime. The most important result of the study is that—at least in 2D—the charge and the energy of the electron bunches depend essentially on the laser energy rather than on temporal or spatial shape of the laser pulse. (10.1038/s41598-020-70221-9)
  DOI : 10.1038/s41598-020-70221-9
• Nonlinear response in the cumulant expansion for core-level photoemission
  
  • Tzavala Marilena
  • Kas J.
  • Reining Lucia
  • Rehr J.
  Physical Review Research, American Physical Society, 2020, 2 (3). Most currently used approximations for the one-particle Green's function G in the framework of many-body perturbation theory, such as Hedin's GW approximation or the cumulant GW+C approach, are based on a linear-response approximation for the screened interaction W. The extent to which such a hypothesis is valid and ways to go beyond have been explored only very little. Here we show how to derive a cumulant Green's function beyond linear response from the equation of motion of the Green's function in a functional derivative formulation. The results can be written in a compact form, which opens the possibility to calculate the corrections in a first-principles framework using time-dependent density functional theory. In order to illustrate the potential importance of the corrections, numerical results are presented for a model system with a core level and two valence orbitals. (10.1103/PhysRevResearch.2.033147)
  DOI : 10.1103/PhysRevResearch.2.033147
• Mechanical detwinning device for anisotropic resistivity measurements in samples requiring dismounting for particle irradiation
  
  • Timmons E. I.
  • Tanatar M A
  • Liu Yong
  • Cho Kyuil
  • Lograsso T A
  • Kończykowski M.
  • Prozorov R.
  Review of Scientific Instruments, American Institute of Physics, 2020, 91 (7), pp.073904. Uniaxial stress is used to detwin samples of orthorhombic iron based superconductors to study their intrinsic electronic anisotropy. Here we describe the development of a new detwinning setup enabling variable-load stress-detwinning with easy sample mounting/dismounting without the need to re-solder the contacts. It enables the systematic study of the anisotropy evolution as a function of an external parameter when the sample is modified between the measurements. In our case, the external parameter is the dose of 2.5 MeV electron irradiation at low temperature. We illustrate the approach by studying resistivity anisotropy in single crystals of Ba$_{1−x}$K$_x$Fe$_2$As$_2$ at $x$ =0.25 where the much discussed unusual re-entrance of tetragonal C 4 phase, C 4 → C 2 → C 4 , is observed on cooling. With the described technique we found a significant anisotropy increase in $C_2$ phase after electron irradiation with dose of 2.35 C/cm$^2$ (10.1063/5.0012053)
  DOI : 10.1063/5.0012053