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Optimal measurement counting time and statistics in gamma spectrometry analysis: The time balance
http://hdl.handle.net/2268/209950
Title: Optimal measurement counting time and statistics in gamma spectrometry analysis: The time balance
<br/>
<br/>Author, coauthor: Guembou Shouop, Cébastien Joel; Penabei, Samafou; Ndontchueng Moyo, Maurice; Chene, Grégoire; Nguelem Mekontso, Eric Jilbert; Takoukam, Serge Didier; Werner, Volker; Strivay, David
<br/>
<br/>Abstract: The optimal measurement counting time for gammaray spectrometry analysis using HPGe detectors was determined in our laboratory by comparing twelve hours measurement counting time at day and twelve hours measurement counting time at night. The day spectrum does not fully cover the night spectrum for the same sample. It is observed that the perturbation come to the sunlight. After several investigations became clearer: to remove all effects of radiation from outside (earth, the sun, and universe) our system, it is necessary to measure the background for 24, 48 or 72 hours. In the same way, the samples have to be measured for 24, 48 or 72 hours to be safe to be purified the measurement (equality of day and night measurement). It is also possible to not use the background of the winter in summer. Depend on to the energy of radionuclide we seek, it is clear that the most important steps of a gamma spectrometry measurement are the preparation of the sample and the calibration of the detector.

Ribbons of superparamagnetic colloids in magnetic field
http://hdl.handle.net/2268/209917
Title: Ribbons of superparamagnetic colloids in magnetic field
<br/>
<br/>Author, coauthor: Darras, Alexis; Fiscina, Jorge; Pakpour, Maryam; Vandewalle, Nicolas; Lumay, Geoffroy
<br/>
<br/>Abstract: While the aggregation process of superparamagnetic colloids in strong magnetic eld is well
known on short time since a few decades, recent theoretical works predicted an equilibrium state reached after a long time. In this talk, we present experimental observations of this equilibrium state with a twodimensional system and we compare our data with the predictions of a preexisting model. Above a critical aggregation size, a deviation between the model and the experimental data is observed. This deviation is explained by the formation of ribbonshaped aggregates. The ribbons are formed due to lateral aggregation of chains. An estimation of the magnetic energy for chains and ribbons shows that ribbons are stable structures when the number of magnetic grains is higher than N=30.

Ribbons of superparamagnetic colloids
http://hdl.handle.net/2268/209916
Title: Ribbons of superparamagnetic colloids
<br/>
<br/>Author, coauthor: Darras, Alexis; Fiscina, Jorge; Pakpour, Maryam; Vandewalle, Nicolas; Lumay, Geoffroy

Phasechange materials and rigidity
http://hdl.handle.net/2268/209908
Title: Phasechange materials and rigidity
<br/>
<br/>Author, coauthor: Piarristeguy, A.; Pradel, A.; Raty, JeanYves
<br/>
<br/>Abstract: Rigidity theory is an extraordinary tool to understand glasses. This article demonstrates how this model can help in understanding the link between structure, dynamics, and subtler properties such as drift and aging, in particular, in phasechange materials (PCMs). First, a map of flexible/rigid regions in the Ge(Sb)Te system is drawn on the basis of atomistic structures modeled either by ab initio or reverse Monte Carlo techniques. A clear link between the flexible/rigid nature of the glass and its aging behavior is shown through resistivity drift as a function of composition measurements in amorphous GexTe100x. In the particular case of amorphous GeTe, application of rigidity theory indicates that the average number of mechanical constraints decreases during aging, making the glass less stressedrigid. Finally, the stability of PCMs also depends on the topology of the materials. The increasing number of constraints in GeTe when doped with C or N results in increased stability of the PCM. © 2017 Materials Research Society.

Imprinting superconducting vortex footsteps in a magnetic layer
http://hdl.handle.net/2268/209621
Title: Imprinting superconducting vortex footsteps in a magnetic layer
<br/>
<br/>Author, coauthor: Brisbois, Jérémy
<br/>
<br/>Abstract: We experimentally show that the principle of local polarization of a magnetic layer, a wellknown method to store information, namely in hard drives and credit cards, can be applied for imprinting into a soft magnetic layer of permalloy (Py) the trajectory of vortices moving in a superconducting film (Nb). In full analogy with a magnetic drawing board, vortices act as tiny magnetic scribers leaving a wake of polarized magnetic media in the Py layer. We have used the magnetooptical imaging technique to investigate the mutual interaction between superconducting vortices and ferromagnetic domains. In general, we observe that the flux propagation is delayed at the border of the magnetic layer. Interestingly, in thin Py layers without stripe domains, vortices leave clear imprints of locally polarized magnetic moments along their trajectories. Furthermore, the printings were found to be stable and could still be observed at room temperature, allowing for ex situ observation of the flux penetration in superconductors. We expect our findings to pave the way for further studies for optimizing magnetic recording of superconducting vortex trajectories.

Realization of the NajafiGolestanian microswimmer
http://hdl.handle.net/2268/209570
Title: Realization of the NajafiGolestanian microswimmer
<br/>
<br/>Author, coauthor: Hubert, Maxime; Grosjean, Galien; Lagubeau, Guillaume; Vandewalle, Nicolas
<br/>
<br/>Abstract: The development of artificial microswimmers, microscopic robots that swim in a fluid like sperm cells and
motile bacteria, could cause a leap forward in various fields such as microfluidics, microsystems, or minimally
invasive medicine. Nature provides plenty of examples of efficient microswimmers. However, a bottomup
approach, looking at the simplest ingredients needed to generate a microswimmer, can lead to a deeper under
standing of the swimming problem. First described by Najafi and Golestanian 1 , a paradigmatic microswimmer
is the threelinkedspheres model, which follows a minimalist approach for propulsion by shape shifting. In this
presentation, we describe the experimental realisation of this microswimmer using selfassembled ferromagnetic
particle at an airwater interface, powered by an uniform oscillating magnetic field 2 . A model, using two har
monic oscillators, reproduces the experimental findings. Because the model remains general, the same approach
could be used to design a variety of efficient microswimmers.

Chemical Strain Engineering of Magnetism in Oxide Thin Films
http://hdl.handle.net/2268/209557
Title: Chemical Strain Engineering of Magnetism in Oxide Thin Films
<br/>
<br/>Author, coauthor: Copie, Olivier; Varignon, Julien; Rotella, Hélène; Steciuk, Gwladys; Boullay, Philippe; Pautrat, Alain; David, Adrian; Mercey, Bernard; Ghosez, Philippe; Prellier, Wilfrid
<br/>
<br/>Abstract: Transition metal oxides having a perovskite structure form a wide and technologically
important class of compounds. In these systems, ferroelectric, ferromagnetic,
ferroelastic, or even orbital and charge orderings can develop and
eventually coexist. These orderings can be tuned by external electric, magnetic,
or stress field, and the crosscouplings between them enable important
multifunctional properties, such as piezoelectricity, magnetoelectricity, or
magnetoelasticity. Recently, it has been proposed that additional to typical
fields, the chemical potential that controls the concentration of ion vacancies
in these systems may reveal an efficient alternative parameter to further tune
their properties and achieve new functionalities. In this study, concretizing
this proposal, the authors show that the control of the content of oxygen
vacancies in perovskite thin films can indeed be used to tune their magnetic
properties. Growing PrVO3 thin films epitaxially on an SrTiO3 substrate, the
authors reveal a concrete pathway to achieve this effect. The authors demonstrate
that monitoring the concentration of oxygen vacancies through the
oxygen partial pressure or the growth temperature can produce a substantial
macroscopic tensile strain of a few percent. In turn, this strain affects the
exchange interactions, producing a nontrivial evolution of Néel temperature
in a range of 30 K.

Influence of spring and autumn phenological transitions on forest ecosystem productivity
http://hdl.handle.net/2268/209521
Title: Influence of spring and autumn phenological transitions on forest ecosystem productivity
<br/>
<br/>Author, coauthor: Richardson, A. D.; Black, T. A.; Ciais, P.; Delbart, N.; Friedl, M. A.; Gobron, N.; Hollinger, D. Y.; Kutsch, W. L.; Longdoz, Bernard; Luyssaert, S.; Migliavacca, M.; Montagnani, L.; Munger, J. W.; Moors, E.; Piao, S.; Rebmann, C.; Reichstein, M.; Saigusa, N.; Tomelleri, E.; Vargas, R.; Varlagin, A.
<br/>
<br/>Abstract: We use eddy covariance measurements of net ecosystem productivity (NEP) from 21 FLUXNET sites (153 siteyears of data) to investigate relationships between phenology and productivity (in terms of both NEP and gross ecosystem photosynthesis, GEP) in temperate and boreal forests. Results are used to evaluate the plausibility of four different conceptual models. Phenological indicators were derived from the eddy covariance time series, and from remote sensing and models. We examine spatial patterns (across sites) and temporal patterns (across years); an important conclusion is that it is likely that neither of these accurately represents how productivity will respond to future phenological shifts resulting from ongoing climate change. In spring and autumn, increased GEP resulting from an 'extra' day tends to be offset by concurrent, but smaller, increases in ecosystem respiration, and thus the effect on NEP is still positive. Spring productivity anomalies appear to have carryover effects that translate to productivity anomalies in the following autumn, but it is not clear that these result directly from phenological anomalies. Finally, the productivity of evergreen needleleaf forests is less sensitive to phenology than is productivity of deciduous broadleaf forests. This has implications for how climate change may drive shifts in competition within mixedspecies stands. © 2010 The Royal Society.

Scaling Theory of the Anderson Transition in Random Graphs: Ergodicity and Universality
http://hdl.handle.net/2268/209496
Title: Scaling Theory of the Anderson Transition in Random Graphs: Ergodicity and Universality
<br/>
<br/>Author, coauthor: GarciaMata, Ignacio; Giraud, Olivier; Georgeot, Bertrand; Martin, John; Dubertrand, Rémy; Lemarié, Gabriel
<br/>
<br/>Abstract: We study the Anderson transition on a generic model of random graphs with a tunable branching parameter 1 < K < 2, through large scale numerical simulations and finitesize scaling analysis. We find that a single transition separates a localized phase from an unusual delocalized phase that is ergodic at large scales but strongly nonergodic at smaller scales. In the critical regime, multifractal wave functions are located on a few branches of the graph. Different scaling laws apply on both sides of the transition: a scaling with the linear size of the system on the localized side, and an unusual volumic scaling on the delocalized side. The critical scalings and exponents are independent of the branching parameter, which strongly supports the universality of our results.

Sapphire Ultraoptics for SubmeV 121Sb and 125Te Phonon Spectroscopy
http://hdl.handle.net/2268/208956
Title: Sapphire Ultraoptics for SubmeV 121Sb and 125Te Phonon Spectroscopy
<br/>
<br/>Author, coauthor: Jafari, Atefeh
<br/>
<br/>Abstract: Nuclear resonance scattering of synchrotron radiation enables probing hyperfine interactions and element specific vibrational modes of nuclei that exhibit a Mössbauer transition. A prerequisite for this method is a monochromator with narrow bandwidth. Silicon, as the most commonly used crystal in monochromators, is not suitable for experiments above 30 keV photon energy. Using a sapphire single crystal in backscattering geometry is an alternative. An xray beam with narrow
bandwidth can be obtained from a backreflection with a Bragg angle of a few arcsec to a few
arcmin smaller than =2. The development of a sapphire backscattering monochromator with high energy resolution, better than 1 meV, would permit detailed lattice dynamics characterization of novel functional materials.
Sapphire is a very rigid material with desirable optical properties, high chemical resistance, and
high heat conduction. Crystals of large size with high quality are of interest for many industrial applications, and suited for optics and optoelectronics operating under ambient condition or extreme conditions.
The purpose of the work in this dissertation is twofold. First, study the quality of sapphire single
crystals by modern highresolution characterization techniques in order to acquire microstructural information and thereby a better understanding of the origin of lattice defects. The second goal is the study of the lattice dynamics in materials based on tellurium and antimony with Mössbauer energies of 35.49 keV and 37.13 keV using nuclear resonance scattering with energy resolution given by one of the highest quality sapphires.
White beam topography of more than thirty crystals, grown at the Shubnikov Institute of Crystallography in Moscow, revealed qualitative information about the distribution of lattice defects of which linear defects, i.e. dislocations, are the majority type. The lowest dislocation density of 10^2 10^5 cm^2 was found in Cplane crystals grown by the Kyropoulos and Bridgman techniques.
We carried out rocking curve imaging in backscattering geometry to estimate the lattice parameter variation and energy resolution from backreflections. Minimum variations of the lattice parameters
on the order of 10^8 were observed from spots with an edge length of 0.20.5 mm. There are a few spots with such a quality in one crystal which makes it suitable as backscattering monochromator in nuclear resonance scattering or as analyzer in resonant inelastic xray scattering.
The use of very high energy resolution nuclear inelastic scattering with 0.7 meV at the energy
of the nuclear transition in 121Sb and 125Te enables valuable insight into the phonon scattering of thermoelectric materials, that convert heat to prvide electricity and vice versa, composed of Sb and Te. A careful study is done on heat carrying acoustic modes in the partial density of states of (PbTe)mAgSbTe2, so called LASTm alloys. An impressive mismatch in the phonon group velocities in the 25 meV range and di erence in the force constants for the Sb and Te density of phonon states is observed, a phonon mismatch predicted to be responsible for low lattice thermal conductivity in LASTm.
An indepth understanding of the elementspecific dynamic properties of cubic and orthorhombic
antimony trioxides was achieved using nuclear resonance scattering with an energy resolution of 1 meV at the nuclear transition energy of 121Sb. A softening of the Sb bonds upon transformation from cubic molecular structured in alphaSb2O3 to chain structured orthorhombic in betaSb2O3 is observed. Furthermore, results on the lattice dynamics on alphaTeO2, with quasi molecular structure, demonstrate strong interatomic Te bonds, comparable with the strong bonds in molecular structured alphaSb2O3. The nuclear resonance data is complemented with inelastic neutron scattering data that reveals the oxygen vibrational modes. In addition, the experimental results validate the calculations of the vibrational modes in these types of materials and serve as benchmark for the calculation.

Twofrequency forcing of droplet rebounds on a liquid bath
http://hdl.handle.net/2268/208896
Title: Twofrequency forcing of droplet rebounds on a liquid bath
<br/>
<br/>Author, coauthor: Sampara, Naresh; Gilet, Tristan
<br/>
<br/>Abstract: Droplets can bounce indefinitely on a liquid bath vertically vibrated in a sinusoidal fashion. We here present experimental results that extend this observation to forcing signals composed of a combination of two commensurable frequencies. The Faraday and Goodridge thresholds are characterized. Then a number of vertical bouncing modes are reported, including walkers. The vertical motion can become chaotic, in which case the horizontal motion is an alternation of walk and stop.

Deformations of soap bubbles in a uniform electric field
http://hdl.handle.net/2268/208866
Title: Deformations of soap bubbles in a uniform electric field
<br/>
<br/>Author, coauthor: Mawet, Sébastien
<br/>
<br/>Abstract: The study of the deformations undergone by a soap bubble submitted to an electrical force
began with Taylor and Wilson 1 and the observation of the socalled Taylor’s cones. Beyond this
particular structure, few studies analyzed the bubble deformations. For example, what is the link
between the deformations and the electrical force ? Or, how do charges move in the thin soap
film formed by the bubble ? To answer those questions, we characterize the shape variations of
the surface of the bubble immersed in the uniform electric field of a plan capacitor. In particular,
our study focuses on hemispherical bubbles lying on the bottom electrode of a plane capacitor.
This study allows us to observe some interesting phenomena like the appearance of a hysteresis
cycle in the deformation amplitude.
<br/>
<br/>Commentary: Abstract de la présentation proposée au March meeting 2017 de l'aps

Growth and Characterization of Sr2FeMoO6 thin films on ceramic substrates
http://hdl.handle.net/2268/208680
Title: Growth and Characterization of Sr2FeMoO6 thin films on ceramic substrates
<br/>
<br/>Author, coauthor: Myana, Santosh Kumar

Spin waves on chains of YIG particles: dispersion relations, Faraday rotation, and power transmission
http://hdl.handle.net/2268/208574
Title: Spin waves on chains of YIG particles: dispersion relations, Faraday rotation, and power transmission
<br/>
<br/>Author, coauthor: Pike, Nicholas; Stroud, David
<br/>
<br/>Abstract: We calculate the dispersion relations for spin waves on a periodic chain of spherical or cylindrical Yttrium Iron Garnet (YIG) particles. We use the quasistatic approximation, appropriate when kd <<1, where k is the wave number and d the interparticle spacing. In this regime, because of the magnetic dipoledipole interaction between the localized magnetic excitations on neighboring particles, dispersive spin waves can propagate along the chain. The waves are analogous to plasmonic waves generated by electric dipoledipole interactions between plasmons on neighboring metallic particles. The spin waves can be longitudinal (L), transverse (T ), or elliptically polarized. We find that a linearly polarized spin wave undergoes a Faraday rotation as it propagates along the chain. The amount of Faraday rotation can be tuned by varying the offdiagonal component of the permeability tensor. We also discuss the possibility of wireless power transmission along the chain using these coupled spin waves.

Formation and characterization of artificial lipid bilayers on optical fibers
http://hdl.handle.net/2268/208542
Title: Formation and characterization of artificial lipid bilayers on optical fibers
<br/>
<br/>Author, coauthor: Toussaint, Pauline; Dreesen, Laurent
<br/>
<br/>Abstract: Transports across cellular membranes are at the basis of a lot of biological processes such as the transmission of information in neurons. Their characterization is therefore of crucial interest. As they are equivalent to biological membranes, artificial lipid bilayers can be created to study membranes and transmembrane proteins properties or transmembrane transports. The aim of this work is to develop a new method for the fabrication of artificial membranes based on the use of optical fibers as support for the bilayer, and for their characterization by fluorescence measurements. We use microfluidics on fibers to create two phospholipid monolayers that we approach close enough to form a bilayer. The membrane formation is checked using fluorescein or a fluorescent sodium probe, Tetra (tetramethylammonium) salt (sodium green), whose optical signal depends on sodium concentration.

Origin of the Counterintuitive Dynamic Charge in the Transition Metal Dichalcogenides
http://hdl.handle.net/2268/208462
Title: Origin of the Counterintuitive Dynamic Charge in the Transition Metal Dichalcogenides
<br/>
<br/>Author, coauthor: Pike, Nicholas; Dewandre, Antoine; Van Troeye, Benoit; Gonze, Xavier; Verstraete, Matthieu
<br/>
<br/>Abstract: Our recent firstprinciples calculations of the electronic and vibrational properties of the hexagonal transitionmetal dichalcogenides reveal that their Born effective charges display a counterintuitive sign when compared to most other materials or transitionmetal dichalcogenides with trigonal symmetry. We determine the origin of this counterintuitive sign by calculating the electronic, vibrational, and optical properties of these systems. We show that the sign of the Born effective charge is directly related to the electric field response of the electronic density, and, in turn, to the bonding characteristics of the material.There is a filled antibonding molecular orbital at the Fermi level, which is localized on the transitionmetal atom and corresponds to a form of solid state $\pi$ backbonding in these material. We propose a method of determining if other materials display a similar counterintuitive sign, based on their bonding characteristics, and propose experiments which could measure the sign of the Born effective charge using different spectroscopies.

Magnetic and electrical characterization of superconductors
http://hdl.handle.net/2268/208460
Title: Magnetic and electrical characterization of superconductors
<br/>
<br/>Author, coauthor: Vanderbemden, Philippe

Statistics of localized phase slips in tunable width planar point contacts
http://hdl.handle.net/2268/208276
Title: Statistics of localized phase slips in tunable width planar point contacts
<br/>
<br/>Author, coauthor: Baumans, Xavier
<br/>
<br/>Abstract: The main dissipation mechanism in superconducting nanowires arises from phase slips. Thus far,
most of the studies focus on long nanowires where coexisting events appear randomly along the
nanowire. In the present work we investigate highly confined phase slips at the contact point of two superconducting leads. Profiting from the high current crowding at this spot, we are able to shrink insitu the nanoconstriction. This procedure allows us to investigate, in the very same sample, thermally activated phase slips and the probability density function of the switching current I sw needed to trigger an avalanche of events. Furthermore, for an applied current larger than I sw , we unveil the existence of two distinct thermal regimes. One corresponding to efficient heat removal where the constriction and bath temperatures remain close to each other, and another one in which the constriction temperature can be substantially larger than the bath temperature leading to the formation of a hot spot. Considering that the switching current distribution depends on the exact thermal properties of the sample, the identification of different thermal regimes is of utmost importance for properly interpreting the dissipation mechanisms in narrow point contacts.

Longrange Rydbergblockade entangling gate mediated by auxiliary atoms
http://hdl.handle.net/2268/208234
Title: Longrange Rydbergblockade entangling gate mediated by auxiliary atoms
<br/>
<br/>Author, coauthor: Cesa, Alexandre; Martin, John
<br/>
<br/>Abstract: Arrays of qubits encoded in the ground state manifold of trapped neutral atoms appear as a promising platform for the realisation of a scalable quantum computer. Indeed, such physical qubits have a long coherence time and allow for highfidelity singlequbit operations [1]. In such a platform, entangling twoqubit gates can be implemented by exploiting the Rydbergblockade mechanism to produce a phase shift or a flip of the state of a target atom conditioned on the state of a control atom [2]. However, because dipoledipole interactions fall off rapidly with the interatomic distance, such entangling gates based on Rydbergblockade are impractical between distant qubits. In this work, we propose a protocol to implement longrange Rydbergblockade gates (CZ or CNot) using auxillary noncoding atoms to transfer the Rydberg excitation from the control to the target qubit. The dependence of the fidelity on the number of auxillary atoms, the blockade strength and the decay rates of the Rydberg states are determined. When compared to a sequential application of nearest neighbours entangling gates, our protocol leads to a larger fidelity and a reduction of the overall gate duration (which scales linearly with the number of auxillary atoms).
[1] M. Saffman, J. Phys. B: At. Mol. Opt. Phys. 49, 202001 (2016). [2] D. Jaksch, J. I. Cirac, P. Zoller, S. L. Rolston, R. Côté, and M. D. Lukin, Phys. Rev. Lett. 85, 2208 (2000).

Longrange Rydbergblockade entangling gate mediated by auxiliary atoms
http://hdl.handle.net/2268/208234
Title: Longrange Rydbergblockade entangling gate mediated by auxiliary atoms
<br/>
<br/>Author, coauthor: Cesa, Alexandre; Martin, John
<br/>
<br/>Abstract: Arrays of qubits encoded in the ground state manifold of trapped neutral atoms appear as a promising platform for the realisation of a scalable quantum computer. Indeed, such physical qubits have a long coherence time and allow for highfidelity singlequbit operations [1]. In such a platform, entangling twoqubit gates can be implemented by exploiting the Rydbergblockade mechanism to produce a phase shift or a flip of the state of a target atom conditioned on the state of a control atom [2]. However, because dipoledipole interactions fall off rapidly with the interatomic distance, such entangling gates based on Rydbergblockade are impractical between distant qubits. In this work, we propose a protocol to implement longrange Rydbergblockade gates (CZ or CNot) using auxillary noncoding atoms to transfer the Rydberg excitation from the control to the target qubit. The dependence of the fidelity on the number of auxillary atoms, the blockade strength and the decay rates of the Rydberg states are determined. When compared to a sequential application of nearest neighbours entangling gates, our protocol leads to a larger fidelity and a reduction of the overall gate duration (which scales linearly with the number of auxillary atoms).
[1] M. Saffman, J. Phys. B: At. Mol. Opt. Phys. 49, 202001 (2016). [2] D. Jaksch, J. I. Cirac, P. Zoller, S. L. Rolston, R. Côté, and M. D. Lukin, Phys. Rev. Lett. 85, 2208 (2000).