Publications

2019

• Electronic coupling in the F4-TCNQ/single-layer GaSe heterostructure
  
  • Khalil Lama
  • Pierucci Debora
  • Papalazarou Evangelos
  • Chaste Julien
  • Silly Mathieu G
  • Sirotti Fausto
  • Eddrief Mahmoud
  • Perfetti Luca
  • Lhuillier Emmanuel
  • Ouerghi Abdelkarim
  Physical Review Materials, American Physical Society, 2019, 3 (8). Hybrid heterostructures, made of organic molecules adsorbed on two-dimensional metal monochalcogenide, generally unveil interfacial effects that improve the electronic properties of the single constitutive layers. Here, we investigate the interfacial electronic characteristics of the F4-TCNQ/single-layer GaSe heterostructure. A sharp F4-TCNQ/GaSe interface has been obtained and characterized by x-ray photoemission spectroscopy. We demonstrate that a high electron transfer from 1TL GaSe into the adsorbed F4-TCNQ molecules takes place, thereby yielding a reduction in the excess negative charge density of GaSe. Additionally, the direct band-structure determination of the heterostructure has been carried out using angle-resolved photoemission spectroscopy, shedding light on essential features such as doping and band offset at the interface. Our results indicate that the buried 1TL GaSe below the F4-TCNQ layer exhibits a robust inversion of the valence dispersion at the Γ point, forming a Mexican-hat-shaped dispersion with 120 ± 10 meV of depth. Our experiments also reveal that F4-TCNQ can significantly tune the electronic properties of 1TL GaSe by shifting the band offset of about 0.16 eV toward lower binding energies with respect to the Fermi level, which is a key feature for envisioning its applications in nanoelectronics. (10.1103/physrevmaterials.3.084002)
  DOI : 10.1103/physrevmaterials.3.084002
• What is the reason for the asymmetry between the twins in the twin paradox?
  
  • Coddens Gerrit
  , 2019. The true difficulty of the twin paradox does not reside in the algebra that shows that the traveling twin ages less than the twin who stays at home. The truly startling part of the paradox resides in the much more difficult question why the argument cannot be reversed by symmetry, because there is no such thing as a preferred reference frame, and motion ought to be relative. Can the traveling twin not claim with equal rights to have stayed at home while the other twin has made the journey? The answer to this question is not what anyone has thought. Most of the time text books invoke the accelerations intervening in the trip to explain the asymmetry. We will show that one can formulate and solve the paradox without making any reference to accelerations. There is actually something very simple that has been overlooked. In drafting the protocol which defines the journey, we unwittingly pick a preferred reference frame, because we define the protocol with respect to a given frame, which thereby becomes special. It is this selection of a special reference frame which introduces the asymmetry. Hence, the reference frame wherein we define the protocol for the journey will act like an absolute frame and it is its unavoidable introduction which breaks the symmetry between the twins. There is an infinity of protocols that can be selected to define a trip and each of these trips leads to its own corresponding twin paradox, with its own outcome as to which twin will age less. Whereas the individual trips of the two twins within a given protocol are asymmetrical, the set of all possible trips is symmetrical, such that the symmetry of the Lorentz group is indeed respected.
• Injection Locking and Parametric Locking in a Superconducting Circuit
  
  • Marković Danijela
  • Pillet Jean-Damien
  • Flurin Emmanuel
  • Roch Nicolas
  • Huard Benjamin
  Physical Review Applied, American Physical Society, 2019, 12 (2), pp.024034. When a signal is injected in a parametric oscillator close enough to its resonance, the oscillator frequency and phase are locked to those of the injected signal. Here we demonstrate two frequency-locking schemes using a Josephson mixer in the parametric down-conversion regime, pumped beyond the parametric oscillation threshold. The circuit then emits radiation out of two spectrally and spatially separated resonators at frequencies determined by the locking schemes that we choose. When we inject the signal close to a resonance, it locks the oscillator emission to the signal frequency by injection locking. When we inject the signal close to the difference of resonances, it locks the oscillator emission by parametric locking. We compare both schemes and investigate the dependence of the parametric locking range on the pump and the injected-signal power. Our results can be interpreted using Adler’s theory for lasers, which provides a link between laser physics and superconducting circuits that could enable better understanding of pumped circuits for quantum-information applications such as error correction, circulators, and photon-number detectors. (10.1103/PhysRevApplied.12.024034)
  DOI : 10.1103/PhysRevApplied.12.024034
• Enhancement of penetration field in vortex matter in mesoscopic superconductors due to Andreev bound states
  
  • Dolz M.
  • Bolecek N.
  • Puig J.
  • Pastoriza H.
  • Nieva G.
  • Guimpel J.
  • van Der Beek C.
  • Konczykowski M.
  • Fasano Y.
  Physical Review B: Condensed Matter and Materials Physics (1998-2015), American Physical Society, 2019, 100 (6). (10.1103/PhysRevB.100.064508)
  DOI : 10.1103/PhysRevB.100.064508
• Ion Irradiation Shaping of Dense Two-dimensional Arrays of Au Nanoparticles Embedded in Silica Studied via TEM
  
  • Mota-Santiago P
  • Kremer F
  • Rizza G.
  • Dufour C.
  • Notthoff C
  • Hadley A
  • Hussain Uh
  • Kluth P.
  Microscopy and Microanalysis, Cambridge University Press, 2019, 25 (S2), pp.1610-1611. (10.1017/S143192761900878X)
  DOI : 10.1017/S143192761900878X
• Early warning sensors for monitoring mercury in water
  
  • Pinaeva U.
  • Lairez D.
  • Oral O.
  • Faber A.
  • Clochard M-C.
  • Wade T.L.
  • Moreau Pauline
  • Ghestem Jean Philippe
  • Vivier M.
  • Ammor S.
  • Nocua R.
  • Soulé A.
  Journal of Hazardous Materials, Elsevier, 2019, 376, pp.37-47. Poly-4-vinylpyridine grafted poly(vinylidene difluoride) (P4VP-g-PVDF) nanoporous polymer electrodes were found to be sensitive for Hg(II) analysis. The fabrication and characterization of functionalized nanoporous membrane-electrodes by FESEM and FTIR are presented. Functionalized nanopore charge state versus a large range of pH (1–10) was investigated by registering the streaming potential. This isoelectric point is achieved at the pKa of P4VP (pH = 5). Mercury adsorption at solid-liquid interface obeys a Langmuir law. A protocol for accurate Hg(II) analysis at ppb level was established. Calibration curves were performed and different real water samples (mineral water, ground water, surface water) were spiked and analyzed. The resulting sensor is intended to be integrated into existing systems or used standalone as portable devices. A first generation prototype exhibiting its own integrated potentiostat, its software and set of membrane-electrode pads is presented. (10.1016/j.jhazmat.2019.05.023)
  DOI : 10.1016/j.jhazmat.2019.05.023
• Holographic vector field electron tomography of three-dimensional nanomagnets
  
  • Wolf Daniel
  • Biziere Nicolas
  • Sturm Sebastian
  • Reyes Vasquez David Fernando
  • Wade Travis
  • Niermann Tore
  • Krehl Jonas
  • Warot-Fonrose Bénédicte
  • Büchner Bernd
  • Snoeck Etienne
  • Gatel Christophe
  • Lubk Axel
  Communications Physics, Nature Research, 2019, 2 (1). Complex 3D magnetic textures in nanomagnets exhibit rich physical properties, e.g., in their dynamic interaction with external fields and currents, and play an increasing role for current technological challenges such as energy-efficient memory devices. To study these magnetic nanostructures including their dependency on geometry, composition, and crystallinity, a 3D characterization of the magnetic field with nanometer spatial resolution is indispensable. Here we show how holographic vector field electron tomography can reconstruct all three components of magnetic induction as well as the electrostatic potential of a Co/Cu nanowire with sub 10 nm spatial resolution. We address the workflow from acquisition, via image alignment to holographic and tomographic reconstruction. Combining the obtained tomographic data with micromagnetic considerations, we derive local key magnetic characteristics, such as magnetization current or exchange stiffness, and demonstrate how magnetization configurations, such as vortex states in the Co-disks, depend on small structural variations of the as-grown nanowire. (10.1038/s42005-019-0187-8)
  DOI : 10.1038/s42005-019-0187-8
• Growth of vertically aligned carbon nanotubes on aluminium substrate through a one-step thermal CVD process
  
  • Nassoy F.
  • Pinault M.
  • Descarpentries J.
  • Hauf H.
  • Coulon P-E.
  • Reynaud C.
  • Mayne-L'Hermite M.
  , 2019. The aim is to grow vertically aligned carbon nanotubes (VACNT) on aluminium by a single-step process, namely the thermal aerosol assisted CCVD, in order to get a scalable process to fabricate ultracapacitor electrodes. The one-step synthesis of VACNT on such substrates requires a significant reduction in the growth temperature as compared to conventional substrates . According to previous work, our process is based on the adjunction of hydrogen in the gas phase to promote the decomposition of the catalyst precursor at low temperature, and of acetylene, easy to decompose at low temperature . Our approach is first to identify the most relevant synthesis parameters to reach VACNT growth on aluminium substrates by subsequently analysing the product features. This optimization study enabled to obtain clean, long and dense VACNTs at 580°C or 620°C with a growth rate (ca. 5$\mu$m/min) at the best level of the state of the art. Attention is paid on the study of VACNT thickness variation versus synthesis duration showing a limitation of CNT length from a certain time of synthesis. In order to understand this phenomenon, the trend in CNT length is examined according to different models reported in the literature, which gives evidence of a catalyst deactivation phenomenon occurring during the one step-CVD process performed at low temperature due to the additional formation of disordered carbon. In addition, the CNT/Al interface was analysed, enabling to clearly identify catalytic particles located at the CNT base and on the surface of a well-defined oxide interface layer suggesting the availability of catalyst nanoparticles for VACNT growth.
• Elongation mechanism of the ion shaping of embedded gold nanoparticles under swift heavy ion irradiation
  
  • Vu T.H.Y.
  • Dufour C.
  • Khomenkov V.
  • Leino A.A.
  • Djurabekova F.
  • Nordlund K.
  • Coulon P.-E.
  • Rizza G.
  • Hayoun M.
  Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, Elsevier, 2019, 451, pp.42-48. The elongation process under swift heavy ion irradiation (74 MeV Kr ions) of gold NPs, with a diameter in the range 10–30 nm, and embedded in a silica matrix has been investigated by combining experiment and simulation techniques: three-dimensional thermal spike (3DTS), molecular dynamics (MD) and a phenomenological simulation code specially developed for this study. 3DTS simulations evidence the formation of a track in the host matrix and the melting of the NP after the passage of the impinging ion. MD simulations demonstrate that melted NPs have enough time to expand after each ion impact. Our phenomenological simulation relies on the expansion of the melted NP, which flows in the track in silica with modified (lower) density, followed by its recrystallization upon cooling. Finally, the elongation of the spherical NP into a cylindrical one, with a length proportional to its initial size and a width close to the diameter of the track, is the result of the superposition of the independent effects of each expansion/recrystallization process occurring for each ion impact. In agreement with experiment, the simulation shows the gradual elongation of spherical NPs in the ion-beam direction until their widths saturate in the steady state and reach a value close to the track diameter. Moreover, the simulations indicate that the expansion of the gold NP is incomplete at each ion impact. (10.1016/j.nimb.2019.04.067)
  DOI : 10.1016/j.nimb.2019.04.067
• Plasmon dispersion in graphite: A comparison of current ab initio methods
  
  • Anderson Sean
  • Mendoza Bernardo
  • Fugallo Giorgia
  • Sottile Francesco
  Physical Review B: Condensed Matter and Materials Physics (1998-2015), American Physical Society, 2019, 100 (4). We perform a systematic study of the macroscopic dielectric function and electron energy loss (EEL) spectra for graphite. We obtain the dispersion behavior for the π plasmon, as a function of the momentum transfer q for two nonequivalent paths that traverse the first four Brillouin zones. We carry out these calculations within both time-dependent density functional theory (with two exchange-correlation functionals) and the Bethe-Salpeter equation. Additionally, we explore the effects of using the complete excitonic Hamiltonian (with all electron-hole pairs and antipairs), and within the Tamm-Dancoff approximation (neglecting antipairs). By analyzing the behavior of the macroscopic dielectric function, we are able to determine which peaks are predominantly from plasmonic behavior or only interband transitions. We compare the calculated spectra against several experiments that span almost five decades; our results present clear trends that follow the physical origins of the observed peaks. We carry out this study over a large range of momentum transfer in order to better evaluate the different theoretical methods compared to experiment, and predict the plasmonic behavior beyond available experimental data. Our results indicate that including the complete Hamiltonian with the exciton coupling included is essential for accurately describing the observed EEL spectra and plasmon dispersion of graphite, particularly for low values of momentum transfer. However, the solution of the Bethe-Salpeter equation is computationally intensive, so time-dependent density functional theory methods used in conjunction with the complete Hamiltonian may be an attractive alternative. (10.1103/PhysRevB.100.045205)
  DOI : 10.1103/PhysRevB.100.045205
• Spin-Orbit Currents, Spin-Transfer Torque and Anomalous Tunneling in III–V Heterostructures Probed by Advanced 30- and 40-Bands ${k}\cdot{p}$ Tunneling Methods
  
  • To Duy Quang
  • Dang Thi-Huong
  • Nguyen Hoai Viet
  • Safarov Viatcheslav I.
  • George Jean-Marie
  • Drouhin Henri-Jean
  • Jaffrès Henri
  IEEE Transactions on Magnetics, Institute of Electrical and Electronics Engineers, 2019, 55 (7), pp.1-7. On the basis of tunneling magnetoresistance (TMR) spin-transfer torque (STT) experiments in (Ga,Mn)As/GaAs/(Ga,Mn)As submicronic magnetic tunnel junctions, we have analyzed the anatomy of the spin-current profiles within the heterostructures, location of strong spin-orbit interactions. Beyond the TMR, our robust 30- and 40-band k · p numerical methods reveal the strong peculiarity of the spin-currents in the antiparallel state with the evidence of a tunneling anomalous Hall effect. Using the boundary conditions corresponding to heavy-hole (HH)-to-light-hole (LH) mixing of the relevant C 2v symmetry at III-V interfaces of the junctions, we demonstrate that the efficiency of the transverse spin-current necessary to STT is surprisingly enhanced by the HH-to-LH mixing taking benefit of the lighter mass of the LH state. (10.1109/TMAG.2019.2894571)
  DOI : 10.1109/TMAG.2019.2894571
• Ultrafast relaxation dynamics of highly excited hot electrons in silicon
  
  • Tanimura Hiroshi
  • Kanasaki Jun'Ichi
  • Tanimura Katsumi
  • Sjakste Jelena
  • Vast Nathalie
  Physical Review B, American Physical Society, 2019, 100 (3). Ultrafast relaxation dynamics of hot electrons with excess energies exceeding 1 eV in Si is studied using time-resolved photoemission spectroscopy and ab initio calculations. Experimentally, the photoemission peaks from hot electrons excited in bulk electronic states along the Γ−L and Γ−X directions with excess energy (Eex) 1.1–3.2 eV with respect to the conduction band minimum are identified, and the time constants that characterize the decay of transient populations are determined. The decay time, which is 30±3fs at Eex=3.0eV and increases to 115±5fs at Eex=1.1eV, has the same scaling with Eex irrespective of the location of hot electrons in the Brillouin zone. The calculations show that the momentum scattering time due to electron-phonon coupling is shorter than 10 fs for Eex larger than 1.5 eV, being too short to be measured. The combination of theoretical and experimental results reveals that hot electrons with high excess energy in Si are transformed into hot-electron ensembles quasiequilibrated only in momentum space by the ultrafast momentum scattering, and that the experimentally determined time constant of population decay corresponds to the energy relaxation taking place as a whole on a time scale ten times longer than that of the momentum relaxation. The detailed methodology of the analysis of experimental data which we provide in this work, as well as our conclusions which concern the relaxation dynamics of electrons with Eex exceeding 1 eV in Si, can be applied to interpret hot-carrier relaxation phenomena in a wide range of semiconducting materials. (10.1103/PhysRevB.100.035201)
  DOI : 10.1103/PhysRevB.100.035201
• Dynamics of Highly Excited Electrons in 3D and 2D Semiconductors: Theory and Experiments
  
  • Sjakste Jelena
  • Perfetti Luca
  • Vast Nathalie
  • Tanimura Katsumi
  , 2019. The rapid development of the computational methods based on density functional theory, on the one hand, and of the time-energy-and momentum-resolved spectroscopy, on the other hand, allows today an unprecedently detailed insight into the processes governing hot electron relaxation dynamics, and, in particular, into the role of the electron-phonon coupling [1]. Recently, we have developed a computational method, based on density functional theory and on interpolation of the electron-phonon matrix elements in Wannier space, for the calculation of the electron-phonon coupling in polar materials [2]. This method allowed us to successfully interpret the dynamics of hot electron relaxation in bulk GaAs, in excellent agreement with time-and angle-resolved photoemission experiments. We have demonstrated, for the relaxation of hot carriers in GaAs, the existence of two distinct relaxation regimes, one related with the momentum, and the other with energy relaxation [3]. Interestingly, the energy relaxation times become faster at lower energies [4]. In this work, we will present our new results, both experimental and theoretical, on hot electron relaxation in silicon. Numerous additional experiments were performed with respect to the work of [5], and a new interpretation of the measured relaxation times is provided, based on our ab initio calculations and on the concept of hot electron ensembles proposed recently in [3]. Moreover, we will present our recent results, both experimental and theoretical, on the hot electron relaxation and cooling in InSe. InSe is a quasi-2D material which was shown recently to have potential interest for optoelectronics [6]. In this work, we will discuss our new results on the relaxation and cooling dynamics in doped InSe. (10.1109/CLEOE-EQEC.2019.8873208)
  DOI : 10.1109/CLEOE-EQEC.2019.8873208
• Theoretical phase diagram of boron carbide from ambient to high pressure and temperature
  
  • Jay Antoine
  • Hardouin Duparc Olivier
  • Sjakste Jelena
  • Vast Nathalie
  Journal of Applied Physics, American Institute of Physics, 2019, 125 (18), pp.185902. The phase diagram of boron carbide is calculated within the density functional theory as a function of temperature and pressure up to 80 GPa, accounting for icosahedral, graphitelike, and diamondlike atomic structures. Only some icosahedral phases turn out to be thermodynamically stable with atomic carbon concentrations (c) of 8.7% (B10.5C), 13.0% (B6.7C), 20% (B4C), and 28.6% (B2.5C), respectively. Their respective ranges of stability under pressure and temperature are calculated, and the theoretical T-P-c phase diagram boundaries are discussed. At ambient conditions, the introduction in the phase diagram of the new phase B10.5C with an ordered crystalline motif of 414 atoms is shown to bring the theoretical solubility range of carbon in boron close to the experimental one. The link with the experimental phase diagram consisting of one single phase having the R3⎯⎯⎯m space group is discussed, and the concept of partial occupation of Wyckoff’s site is introduced. At high pressure, the phase diagram is defined by a new carbon-rich phase B2.5C, which is stabilized by both pressure and temperature in our calculations. All of the other diamond and graphite phases reported previously turn out to be thermodynamically unstable in our calculations, although some of them are observed in high pressure experiments. (10.1063/1.5091000)
  DOI : 10.1063/1.5091000
• Light-Heat Conversion: Tracking Photothermal Energy Flow in Highly Diversified Water-Dispersed Hydrophobic Nanocrystal Assemblies
  
  • Mazzanti Andrea
  • Yang Zhijie
  • Silva Mychel G
  • Yang Nailiang
  • Rizza Giancarlo
  • Coulon Pierre-Eugène
  • Manzoni Cristian
  • de Paula Ana Maria
  • Cerullo Giulio
  • Della Valle Giuseppe
  • Pileni Marie-Paule
  Proceedings of the National Academy of Sciences of the United States of America, National Academy of Sciences, 2019, 116 (17), pp.8161-8166. We investigate, with a combination of ultrafast optical spectroscopy and semiclassical modeling, the photothermal properties of various water-soluble nanocrystal assemblies. Broadband pump–probe experiments with ∼100-fs time resolution in the visible and near infrared reveal a complex scenario for their transient optical response that is dictated by their hybrid composition at the nanoscale, comprising metallic (Au) or semiconducting (Fe3O4) nanostructures and a matrix of organic ligands. We track the whole chain of energy flow that starts from light absorption by the individual nanocrystals and subsequent excitation of out-of-equilibrium carriers followed by the electron–phonon equilibration, occurring in a few picoseconds, and then by the heat release to the matrix on the 100-ps timescale. Two-dimensional finite-element method electromagnetic simulations of the composite nanostructure and multitemperature modeling of the energy flow dynamics enable us to identify the key mechanism presiding over the light–heat conversion in these kinds of nanomaterials. We demonstrate that hybrid (organic–inorganic) nanocrystal assemblies can operate as efficient nanoheaters by exploiting the high absorption from the individual nanocrystals, enabled by the dilution of the inorganic phase that is followed by a relatively fast heating of the embedding organic matrix, occurring on the 100-ps timescale. (10.1073/pnas.1817850116)
  DOI : 10.1073/pnas.1817850116
• Evidence of direct electronic band gap in two-dimensional van der Waals indium selenide crystals
  
  • Henck Hugo
  • Pierucci Debora
  • Zribi Jihene
  • Bisti Federico
  • Papalazarou Evangelos
  • Girard Jean-Christophe
  • Chaste Julien
  • Bertran François
  • Le Fèvre Patrick
  • Sirotti Fausto
  • Perfetti Luca
  • Giorgetti Christine
  • Shukla Abhay
  • Rault Julien
  • Ouerghi Abdelkarim
  Physical Review Materials, American Physical Society, 2019, 3 (3). Metal monochalcogenide compounds offer a large variety of electronic properties depending on chemical composition, number of layers, and stacking order. Among them, the InSe has attracted much attention due to the promise of outstanding electronic properties, attractive quantum physics, and high photoresponse. Precise experimental determination of the electronic structure of InSe is sorely needed for better understanding of potential properties and device applications. Here, combining scanning tunneling spectroscopy (STS) and two-photon photoemission spectroscopy, we demonstrate that InSe exhibits a direct band gap of about 1.25 eV located at the Γ point of the Brillouin zone. STS measurements underline the presence of a finite and almost constant density of states (DOS) near the conduction-band minimum. This particular DOS is generated by a poorly dispersive nature of the top valence band, as shown by angle-resolved photoemission spectroscopy (ARPES) investigation. In fact, a hole effective mass of about m∗/m0=−0.95 (¯¯¯¯¯¯ΓK direction) was measured. Moreover, using ARPES measurements a spin-orbit splitting of the deeper-lying bands of about 0.35 eV was evidenced. These findings allow a deeper understanding of the InSe electronic properties underlying the potential of III-VI semiconductors for electronic and photonic technologies. (10.1103/PhysRevMaterials.3.034004)
  DOI : 10.1103/PhysRevMaterials.3.034004
• Micromagnetic study of magnetic states in Co/Cu multilayered cylinders observed by electron holography
  
  • Biziere Nicolas
  • Reyes Vasquez David Fernando
  • Wade T
  • Warot-Fonrose Bénédicte
  • Gatel Christophe
  , 2019.
• Isotope effect on hydrogen bond symmetrization in hydrogen and deuterium fluoride crystals by molecular dynamics simulation
  
  • Dammak Hichem
  • Brieuc Fabien
  • Geneste Grégory
  • Torrent Marc
  • Hayoun Marc
  Physical Chemistry Chemical Physics, Royal Society of Chemistry, 2019, 21 (6), pp.3211-3217. The isotope effect on the collective proton/deuteron transfer in hydrogen and deuterium fluoride crystals has been investigated at 100 K by ab initio quantum-thermal-bath path-integral molecular dynamics (QTB-PIMD) simulation. The deuterons within a planar zigzag chain of the orthorhombic structure simultaneously flip between covalent and hydrogen bonds due to the barrier crossing through tunnelling. The height of the corresponding static barrier normalized for one deuteron is 29.2 meV. In the HF crystal, all the protons are located at the center of the heavy-atom distance. This evidences the symmetrization of the H-bonds, and indicates that the proton zero-point energy is above the barrier top. The decrease of the heavy-atom distance due to quantum fluctuations in both HF and DF crystals corresponds to a large decrease and an increase of the hydrogen and covalent bond lengths, respectively. Upon deuteration, the increase of the heavy-atom distance (Ubbelohde effect) is in agreement with experimental data. (10.1039/c8cp06949b)
  DOI : 10.1039/c8cp06949b
• Emersion Project (AO2015) : Nanoscale assembly of non-miscible metals by numerical engineering of their interfaces.
  
  • Pontikis Vassilis
  • Baldinozzi Gianguido
  , 2019. The modeling of the effects related to the existing interactions between defects and interfaces of ultra-thin heterophase layers composed of two metals with positive enthalpy of mixing is the key to improve the understanding of the fundamental mechanisms that modify the physics of these systems in the region surrounding the interface. The objective of the project is to capitalize this knowledge to eventually develop systems with well defined interface characteristics, exceeding the performance of current materials (thermal barriers, coatings, ...). We team up theoretical models, numerical and experimental approaches to solve this problem, to accelerate the discovery of the most promising metal couples, to predict their behavior in deployment conditions (these conditions are intrinsically out-of-equilibrium), and to develop and validate predictive models that ultimately help us designing advanced materials that exploit the metastability of particular architectures and microstructures.
• Structuration of the surface layer during drying of colloidal dispersions
  
  • Leang Marguerite
  • Lairez Didier
  • Cousin Fabrice
  • Giorgiutti-Dauphiné Frédérique
  • Pauchard Ludovic
  • Lee Lay-Theng
  Langmuir, American Chemical Society, 2019, 35 (7), pp.2692-2701. During evaporative drying of a colloidal dispersion, the structural behavior at the air–dispersion interface is of particular relevance to the understanding of the consolidation mechanism and the final structural and mechanical properties of the porous media. The drying interface constitutes the region of initial drying stress that, when accumulated over a critical thickness, leads to crack formation. This work presents an experimental study of top-down drying of colloidal silica dispersions with three different sizes (radius 5, 8, and 13 nm). Using specular neutron reflectivity, we focus on the structural evolution at the free drying front of the dispersion with a macroscopic drying surface and demonstrate the existence of a thick concentrated surface layer induced by heterogeneous evaporation. The reflectivity profile contains a strong structure peak due to scattering from particles in the interfacial region, from which the interparticle distance is deduced. A notable advantage of these measurements is the direct extraction of the corresponding dispersion concentration from the critical total reflection edge, providing a straightforward access to a structure-concentration relation during the drying process. The bulk reservoir of this experimental configuration renders it possible to verify the evaporation–diffusion balance to construct the surface layer and also to check reversibility of particle ordering. We follow the structural evolution of this surface layer from a sol to a soft wet-gel that is the precursor of a fragile skin and the onset of significant particle aggregation that precedes formation of the wet-crust. Separate complementary measurements on the structural evolution in the bulk dispersion are also carried out by small-angle neutron scattering, where the particle concentration is also extracted directly from the experimental curves. The two sets of data reveal similar structural evolution with concentration at the interface and in the bulk and an increase in the degree of ordering with the particle size. (10.1021/acs.langmuir.8b03772)
  DOI : 10.1021/acs.langmuir.8b03772
• Competition between orthorhombic and re-entrant tetragonal phases in underdoped Ba$_{1 − x}$K$_x$Fe$_2$As$_2$ probed by the response to controlled disorder
  
  • Timmons E. I.
  • Tanatar M. A.
  • Willa K.
  • Teknowijoyo S.
  • Cho Kyuil
  • Kończykowski M.
  • Cavani O.
  • Liu Yong
  • Lograsso T A
  • Welp U.
  • Prozorov R.
  Physical Review B, American Physical Society, 2019, 99, pp.054518. Low-temperature (22 K) irradiation with 2.5-MeV electrons, creating point defects affecting elastic scattering, was used to study the competition between stripe C$_2$ and tetragonal C$_4$ antiferromagnetic phases which exist in a narrow doping range around x = 0.25 in hole-doped Ba$_{1− x}$K$_x$Fe$_2$As$_2$. In nearby compositions outside of this range, at x = 0.22 and x = 0.19, the temperatures of both the concomitant orthorhombic/stripe antiferromagnetic transition $T_{C2}$ and the superconducting transition $T_c$ are monotonically suppressed by added disorder at similar rates of about 0.1 K/μ$\Omega$ cm, as revealed through using resistivity variation as an intrinsic measure of scattering rate. In a stark contrast, a rapid suppression of the C$_4$ phase at the rate of 0.24 K/μ$\Omega$cm is found at x = 0.25. Moreover, this suppression of the C$_4$ phase is accompanied by unusual disorder-induced stabilization of the C$_2$ phase, determined by resistivity and specific heat measurements. The rate of the C$_4$ phase suppression is notably higher than the suppression rate of the spin-vortex phase in the Ni-doped CaKFe$_4$As$_4$ (0.16 K/μ$\Omega$ cm). (10.1103/PhysRevB.99.054518)
  DOI : 10.1103/PhysRevB.99.054518
• Creation of glass-characteristic point defects in crystalline SiO$_2$ by 2.5 MeV electrons and by fast neutrons
  
  • Skuja Linards
  • Ollier N.
  • Kajiharac Koichi
  • Smits Krisjanis
  Journal of Non-Crystalline Solids, Elsevier, 2019, 505, pp.252 - 259. Point defects in crystalline SiO$_2$ , created by 2.5 MeV electron irradiation at dose below the amorphization threshold or by fast neutrons, were compared by luminescence spectroscopy. Oxygen dangling bonds ("nonbridging oxygen hole centers", NBOHCs), peculiar to amorphous state of SiO$_2$ , were detected for the first time in electron-irradiated non-amorphized α-quartz crystal. Their presence may signal the formation of nucleation centers in crystal structure as the first step to radiation-induced amorphization. Compared to crystal, irradiated by 10$^{19}$cm$^{-2}$ fast neutrons, their concentration was over 100 times lower, and their inhomogeneous broadening was at least 2.5 times smaller. Divalent silicons ("silicon oxygen deficiency centers", SiODC(II)), inherent to oxygen-deficient or irradiated SiO$_2$ glass, were detected in neutron-irradiated (10$^{19}$ n/cm$^2$) $\alpha$-quartz but were not found after the electron irradiation. Radiation-induced interstitial O$_2$ molecules, characteristic to irradiated glassy SiO$_2$ and other oxide glasses, are found in $\alpha$-quartz only after neutron irradiation. The oxygen atoms, displaced by the 2.5 MeV e$^−$ irradiation of $\alpha$-quartz for fluences up to 10$^{19}$ e$^−$ /cm$^2$ evidently stays entirely in the peroxy linkage (Si-O-O-Si bond) form. (10.1016/j.jnoncrysol.2018.11.014)
  DOI : 10.1016/j.jnoncrysol.2018.11.014
• Bulk defects and surface state dynamics in topological insulators: The effects of electron beam irradiation on the ultrafast relaxation of Dirac fermions in Bi 2 Te 3
  
  • Khalil L.
  • Papalazarou E.
  • Caputo M.
  • Nilforoushan N.
  • Perfetti L.
  • Taleb-Ibrahimi A.
  • Konczykowski M.
  • Hruban A.
  • Wołoś A.
  • Krusin-Elbaum L.
  • Marsi M.
  Journal of Applied Physics, American Institute of Physics, 2019, 125 (2), pp.025103. One of the most important challenges in the study of topological insulators is the realization of materials that are really insulating in the bulk, in order to emphasize quantum transport in the protected surface states. Irradiation with electron beams is a very promising approach toward this goal. By studying a series of samples of the prototype 3D topological insulator Bi 2 Te 3 , we show that while the topological properties of Dirac surface states are preserved after electron irradiation, their relaxation dynamics are very sensitive to the related modifications of the bulk properties. Using time-and angle-resolved photoelectron spectroscopy, we can reveal two distinct relaxation regimes after optical excitation for non-irradiated and irradiated samples. While the faster regime, corresponding to the first few picoseconds, presents a similar temporal evolution of the photoexcited population for all studied samples, the slower regime is strongly influenced by the controlled generation of defects in the bulk lattice. By adjusting the irradiation parameters in this class of materials, one can thus not only change the bulk transport properties but also tune the ultrafast response of the topological surface states. (10.1063/1.5057754)
  DOI : 10.1063/1.5057754
• Evidence of Pure Spin-Current Generated by Spin Pumping in Interface-Localized States in Hybrid Metal–Silicon–Metal Vertical Structures
  
  • Cerqueira Carolina
  • Qin Jian Yin
  • Dang Huong
  • Djeffal Abdelhak
  • Le Breton Jc
  • Hehn Michel
  • Rojas-Sánchez J.-C.
  • Devaux Xavier
  • Suire Stéphane
  • Migot Sylvie
  • Schieffer Philippe
  • Mussot Jean-Georges
  • Łaczkowski Piotr
  • Anane Abdelmadjid
  • Petit-Watelot S.
  • Stoffel Mathieu
  • Mangin Stéphane
  • Liu Zhi
  • Cheng Bu Wen
  • Han Xiu Feng
  • Jaffrès Henri
  • George Jean-Marie
  • Lu Yuan
  Nano Letters, American Chemical Society, 2019, 19 (1), pp.90-99. Due to the difficulty of growing high-quality semiconductors on ferromagnetic metals, the study of spin diffusion transport in Si was limited to lateral geometry devices. In this work, by using an ultrahigh-vacuum wafer-bonding technique, we have successfully fabricated metal–semiconductor–metal CoFeB/MgO/Si/Pt vertical structures. We hereby demonstrate pure spin-current injection and transport in the perpendicular current flow geometry over a distance larger than 2 μm in n-type Si at room temperature. In those experiments, a pure propagating spin current is generated via ferromagnetic resonance spin pumping and converted into a measurable voltage by using the inverse spin Hall effect occurring in the top Pt layer. A systematic study varying both Si and MgO thicknesses reveals the important role played by the localized states at the MgO–Si interface for the spin-current generation. Proximity effects involving indirect exchange interactions between the ferromagnet and the MgO–Si interface states appears to be a prerequisite to establishing the necessary out-of-equilibrium spin population in Si under the spin-pumping action. (10.1021/acs.nanolett.8b03386)
  DOI : 10.1021/acs.nanolett.8b03386
• Entropy jump at the first-order vortex phase transition in Bi2Sr2CaCu2O8+δ with columnar defects
  
  • Rumi G.
  • Albornoz L.J.
  • Pedrazzini P.
  • Dolz M.I.
  • Pastoriza H.
  • van Der Beek C.J.
  • Konczykowski M.
  • Fasano Y.
  Materials Today: Proceedings, Elsevier, 2019, 14, pp.30-33. (10.1016/j.matpr.2019.05.046)
  DOI : 10.1016/j.matpr.2019.05.046