ORBi Collection: Physique
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Competition of phonon and magnon effects in the temperature dependence of spinwave stiffness
http://hdl.handle.net/2268/202099
Titre: Competition of phonon and magnon effects in the temperature dependence of spinwave stiffness
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<br/>Auteur, co-auteur: Ostler, Thomas; Verstraete, Matthieu; Di Gennaro, Marco; Miranda, Alonso; Romero, Aldo
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<br/>Résumé: Magnons are the elementary magnetic excitations in ordered solids. Understanding such collective excitations is important for a number of technologically relevant fields, such as, magnonics [1] or spin caloritronics [2]. The central interactions in spin caloritronics are the couplings of phonons with electrons and spin degrees of freedom. Furthermore, understanding the effects of temperature on the phonon and spin degrees of freedom adds a further complexity. In the present work we have developed a multiscale model of ferromagnetic materials and demonstrate the effect of temperature dependent phonon displacements on the magnons spectra. Our results show that the for Fe and Ni the effect of phonon displacements acts to reduce the Curie temperature, whereas for Permalloy the opposite is true due to an increased long-ranged exchange interaction. This increased exchange interaction results in an increasing spin-wave stiffness with increasing temperature, overcoming the usual decrease due to magnon softening. To determine the effects of increasing the phonon temperature we have developed a multiscale model whereby we begin by calculating the thermal displacement of phonons, $\sqrt{\langle u^2(T) \rangle}$, calculated using the phonopy software package [3] using electronic ground state and phonon properties determined using the ABINIT software package [4]. Then the exchange constants are determined using the SPRKKR package [5]. Finally, we use linear spin wave theory to determine the effect of the phonon temperature on the exchange alone, demonstrating an increasing frequency of the acoustic magnon branch. We take into account the thermal effects of the magnetic system through the use of the atomistic spin dynamics approach. Magnon softening due to thermal effects demonstrates a more modest increase in the exchange stiffness (over the purely phononic effect), however, an overall increase is still observed. \newline \newline [1] A. V. Chumak, V. I. Vasyuchka, A. A. Serga, and B. Hillebrands, Nature Physics, {\bf 11}, 453–461 (2015). \newline [2] G. E. W. Bauer, E. Saitoh, and B. J. van Wees, Nature Materials {\bf 11}, 391 (2012). \newline [3] Atsushi Togo and Isao Tanaka, Scr. Mater., {\bf 108}, 1-5 (2015) \newline [4] X. Gonze \textit{et al.} Computer Physics Communications {\bf 180}, 2582-2615 (2009). \newline [5] T. Huhne \textit{at al.} Physical Review B, {\bf 58}, 10236 (1998).Droplet manipulation on fiber networks
http://hdl.handle.net/2268/202077
Titre: Droplet manipulation on fiber networks
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<br/>Auteur, co-auteur: Weyer, Floriane; Pan, Zhao; Truscott, Tadd; Vandewalle, NicolasAnomalous diffusion and non-monotonic relaxation processes in Ge-Se liquids
http://hdl.handle.net/2268/202062
Titre: Anomalous diffusion and non-monotonic relaxation processes in Ge-Se liquids
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<br/>Auteur, co-auteur: Yildirim, Can; Raty, Jean-Yves; Micoulaut, Matthieu
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<br/>Résumé: We investigate the dynamical properties of liquid GexSe100−x as a function of Ge content by
first-principles molecular dynamic simulations for a certain number of temperatures in the liquid
state. The focus is set on ten compositions (where x ≤ 33%) encompassing the reported flexible to
rigid and rigid to stressed-rigid transitions. We examine diffusion coefficients, diffusion activation
energies, glassy relaxation behavior, and viscosity of these liquids from Van Hove correlation and
intermediate scattering functions. At fixed temperature, all properties/functions exhibit an anomalous
behavior with Ge content in the region 18%-22%, and provide a direct and quantitative link to the
network rigidity.Performances comparison of a laser ultrasonic system using 10.6 µm infrared or 532 nm visible generation beam for the investigation of CFRP
http://hdl.handle.net/2268/202055
Titre: Performances comparison of a laser ultrasonic system using 10.6 µm infrared or 532 nm visible generation beam for the investigation of CFRP
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<br/>Auteur, co-auteur: Vandenrijt, Jean-François; Walter, Julien; Brouillette, Tomy; Georges, Marc
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<br/>Résumé: The investigation of complex shaped carbon fiber parts is a common need of the industry. Classical ultrasonic systems are commonly used, wide-spread and very efficient. However, these techniques are often limited to simple shape objects. Major problems arise when the shape of the element to be investigated is complex (peak, valley, small radius of curvature…). To overcome these problems laser ultrasonic systems can be used and the recent developments show promising results.
Laser ultrasonic systems can use different wavelengths for ultrasound generation. Usually CO2 lasers emitting at 10.6 µm wavelength are used. When a laser ultrasonic system is mounted on a robotic arm, very complex shaped objects can be considered. However, the optical fibers for 10.6 µm wavelength are not capable to cope with laser ultrasonic system requirements. Therefore, infrared systems use jointed articulated beam delivery systems which reduce the flexibility of the robot arm and significantly limit the feasible scan paths.
To circumvent this limitation, an all-fibered laser ultrasonic system can be used. In our case the ultrasound is generated with a pulsed laser operating at 532 nm. This system is placed on a robotic
arm, the beam delivery is performed through an optical fiber only. Therefore, this system is capable of analyzing very complex shaped objects due to the use of optical fiber only for laser beam transport.
But visible generation is known to be less efficient and produces lower quality signals. In order to balance the advantages and limitations of both of these systems a CFRP plate including artificial defects has been investigated with different ultrasonic systems. First we used classical phased-array ultrasounds as a reference to compare the performances of visible and infrared generation systems. The plate has then been investigated with a 10.6 µm laser ultrasonic system. The results are compared with an all-fibered laser ultrasonic system working at 532 mn wavelength.
Data acquired by each system allow comparing the visibility of the ultrasonic echoes and the amplitude of background noise. We observe the impact of frequency filtering. We show the main differences on the A-scans and C-scan generated by each system. From these elements, we show the advantages and limitations of each system for the investigation of CFRP with a focus on complex shaped object.Coupled Boltzmann Equation Solver: Effects of the Electron-Phonon Interaction on the Transport Coefficients
http://hdl.handle.net/2268/202014
Titre: Coupled Boltzmann Equation Solver: Effects of the Electron-Phonon Interaction on the Transport Coefficients
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<br/>Auteur, co-auteur: Pike, Nicholas; Dewandre, Antoine; Verstraete, Matthieu
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<br/>Résumé: Recent experimental and theoretical calculations point to a complex interplay between the electron and phonon baths in a wide variety of materials [1,2]. We propose a method of coupling the Boltzmann equations for the electron and phonon baths within the relaxation time approximation which we use to calculate the thermoelectric transport coefficients. Our model for the coupled Boltzmann Equation solver includes analytic models, including ${\bf k \cdot p}$ Hamiltonians and tight-binding Hamiltonians, for both the electron and phonon energies and analytic models for the electron and phonon relaxation mechanisms. From these calculations we hope to better understand the role and interplay of electron-phonon and phonon-phonon interactions on the thermoelectric transport coefficients.
[1] - Phys. Rev. Lett. ${\bf 114}$, 115901 (2015). [2] - PNAS ${\bf 112}$, 14777-14782 (2015).Cooperative spontaneous emission from indistinguishable atoms in arbitrary motional quantum states
http://hdl.handle.net/2268/201976
Titre: Cooperative spontaneous emission from indistinguishable atoms in arbitrary motional quantum states
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<br/>Auteur, co-auteur: Damanet, François; Braun, Daniel; Martin, John
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<br/>Résumé: We investigate superradiance and subradiance of indistinguishable atoms with quantized motional states, starting with an initial total state that factorizes over the internal and external degrees of freedom of the atoms. Due to the permutational symmetry of the motional state, the cooperative spontaneous emission, governed by a recently derived master equation [F. Damanet et al., Phys. Rev. A 93, 022124 (2016)], depends only on two decay rates γ and γ0 and a single parameter dd describing the dipole-dipole shifts. We solve the dynamics exactly for N = 2 atoms, numerically for up to 30 atoms, and obtain the large-N limit by a mean-field approach. We find that there is a critical difference γ0 − γ that depends on N beyond which superradiance is lost. We show that exact nontrivial dark states (i.e., states other than the ground state with vanishing spontaneous emission) only exist for γ = γ0 and that those states (dark when γ = γ0) are subradiant when γ < γ0.Excitation functions of the natCr(p,x)44Ti, 44Fe(p,x)44Ti, natNi(p,x)44Ti and 93Nb(p,x)44Ti reactions at energies up to 2.6 GeV
http://hdl.handle.net/2268/201958
Titre: Excitation functions of the natCr(p,x)44Ti, 44Fe(p,x)44Ti, natNi(p,x)44Ti and 93Nb(p,x)44Ti reactions at energies up to 2.6 GeV
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<br/>Auteur, co-auteur: Titarenko, Yu. E.; Batyaev, V. F.; Pavlov, K. V.; Titarenko, A. Yu.; Zhivun, V. M.; Chauzova, M. V.; Balyuk, S. A.; Bebenin, P. V.; Ignatyuk, A. V.; Mashnik, S. G.; Leray, Sylvie; Boudard, Alain; David, Jean-Christophe; Mancusi, Davide; Cugnon, Joseph; Yariv, Yair; Nishihara, K.; Matsuda, N.; Kumawat, H.; Stankovskiy, A. Yu.Processes involving few degrees of freedom in the frame of the Intranuclear Cascade approaches
http://hdl.handle.net/2268/201956
Titre: Processes involving few degrees of freedom in the frame of the Intranuclear Cascade approaches
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<br/>Auteur, co-auteur: Cugnon, Joseph; Boudard, Alain; David, Jean-Christophe; Leray, Sylvie; Mancusi, DavideUltra-narrow superconducting junctions: electromigration to shed light on quantum point contacts
http://hdl.handle.net/2268/201933
Titre: Ultra-narrow superconducting junctions: electromigration to shed light on quantum point contacts
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<br/>Auteur, co-auteur: Baumans, Xavier; Cerbu, Dorin; Adami, Obaïd-Allah; Zharinov, Vyacheslav; Verellen, Niels; Papari, Gianpaolo; Scheerder, Jeroen; Zhang, Gufei; Moshchalkov, Victor; Silhanek, Alejandro; Van de Vondel, Joris
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<br/>Résumé: Superconducting nanowires have been, for years now, a topic of great interest due to
their potential application in single photon detectors and in quantum computing
circuits. In this context, it is of fundamental importance to better understand the
undesired and harmful appearance of thermal and quantum fluctuations of the
superconducting order parameter [1]-[3] as a function of the wire width.
Although superconductors in the mesoscopic regime (i.e. size comparable to ξ and/or
λ) have been explored both experimentally and theoretically in depth, the
superconducting nanoworld (i.e. at scales of the fermi wavelength) has received much
less attention. The lack of experimental results is in part due to the difficulty of sample
fabrication, at dimensions beyond the limit reached by conventional lithographic
techniques. A promising direction consists of controlling the local displacement of
atom by an electron wind, a process known as electromigration (EM) [4] . This effect
relies on the combination of local temperature rise and substantial current crowding at
nanoconstrictions. While uncontrolled, EM is responsible for the breakdown of small
electronic devices, it can be used in a controllable way to further decrease locally the
cross section of the nanowire towards single atomic contacts.
In this work, we explore in-situ controlled EM to fabricate nano-constrictions immersed
in cryogenic environment. We demonstrate that a transition from thermally assisted
phase slips (TAPS) to quantum phase slips (QPS) takes place when the effective cross
section becomes smaller than ~ 150 nm 2 . In the regime dominated by QPS the
nanowire loses completely its capacity to carry current without dissipation, even at the
lowest possible temperature [5] . We also demonstrate that the bow-tie shaped
constrictions exhibit a negative magnetoresistance at low magnetic fields [5] which can
be attributed to the suppression of superconductivity in the contact leads [6] . Strikingly,
the detrimental effect caused by the repeated EM can be healed by simply inverting
the current direction. These findings reveal the strong potential of the proposed
fabrication method to explore various fascinating superconducting phenomena in
atomic-size constrictions.Internal structures of clusters in driven granular gas
http://hdl.handle.net/2268/201911
Titre: Internal structures of clusters in driven granular gas
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<br/>Auteur, co-auteur: Noirhomme, MartialShell structure and few-nucleon removal in intranuclear cascade
http://hdl.handle.net/2268/201896
Titre: Shell structure and few-nucleon removal in intranuclear cascade
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<br/>Auteur, co-auteur: Mancusi, Davide; Boudard, Alain; Carbonell, Jaume; Cugnon, Joseph; David, Jean-Christophe; Leray, SylvieSelf-assembly of capillary multipoles
http://hdl.handle.net/2268/201838
Titre: Self-assembly of capillary multipoles
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<br/>Auteur, co-auteur: Poty, Martin; Lagubeau, Guillaume; Lumay, Geoffroy; Vandewalle, Nicolas
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<br/>Résumé: Self-assembly of floating particles driven by capillary forces at some liquid-air interface leads to the formation of two-dimensionnal structures. Using a 3d printer, millimeter scale objets are produced. Their 3d shape is chosen in order to create capillary multipoles. The capillary interactions between these components can be either attractive or repulsive depending on the interface local deformations along the liquid-air interface. In order to understand how the shape of an object deforms the interface, we developed an original profilometry method. The measurements show that specific structures can be programmed by selecting the 3d branched shapes.Imprinting superconducting vortex trajectories in a magnetic layer
http://hdl.handle.net/2268/201749
Titre: Imprinting superconducting vortex trajectories in a magnetic layer
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<br/>Auteur, co-auteur: Shaw, Gorky
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<br/>Résumé: We experimentally show that the principle of local polarization of a magnetic layer 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 magneto-optical 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. For thick Py layers, the stripe magnetic domain pattern guides the smooth flux penetration as well as the abrupt vortex avalanches in the Nb film. More 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.Phase Slips In Tunable Width Point Contacts Created By Electromigration
http://hdl.handle.net/2268/201706
Titre: Phase Slips In Tunable Width Point Contacts Created By Electromigration
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<br/>Auteur, co-auteur: Silhanek, Alejandro
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<br/>Résumé: Superconducting nanowires have been, for years now, a topic of great interest due to their potential application in single photon detectors and as interconnects in circuits proposed for quantum computing. In this context, it is of fundamental importance to better understand the undesired and harmful appearance of thermal and quantum fluctuations of the superconducting order parameter as a function of the wire width. In this presentation, we explore in-situ controlled electromigration (EM) to fabricate nano-constrictions immersed in cryogenic environment. We demonstrate that a transition from thermally assisted phase slips (TAPS) to quantum phase slips (QPS) takes place when the effective cross section becomes smaller than ~150 nm^2. In the regime dominated by QPS the nanowire loses completely its capacity to carry current without dissipation, even at the lowest possible temperature. We also demonstrate that the bow-tie shaped constrictions exhibit a negative magnetoresistance at low magnetic fields which can be attributed to the suppression of superconductivity in the contact leads. Strikingly, the detrimental effect caused by the repeated EM can be healed by simply inverting the current direction. These findings reveal perspectives of the proposed fabrication method for exploring various fascinating superconducting phenomena in atomic size constrictions.Quantumlike statistics of deterministic wave-particle interactions in a circular cavity
http://hdl.handle.net/2268/201674
Titre: Quantumlike statistics of deterministic wave-particle interactions in a circular cavity
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<br/>Auteur, co-auteur: Gilet, Tristan
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<br/>Résumé: A deterministic low-dimensional iterated map is proposed here to describe the interaction between a bouncing droplet and Faraday waves confined to a circular cavity. Its solutions are investigated theoretically and numerically. The horizontal trajectory of the droplet can be chaotic: it then corresponds to a random walk of average step size equal to half the Faraday wavelength. An analogy is made between the diffusion coefficient of this random walk and the action per unit mass h/m of a quantum particle. The statistics of droplet position and speed are shaped by the cavity eigenmodes, in remarkable agreement with the solution of Schrödinger equation for a quantum particle in a similar potential well.Quantum calculation of the Vavilov-Cherenkov radiation by twisted electrons
http://hdl.handle.net/2268/201645
Titre: Quantum calculation of the Vavilov-Cherenkov radiation by twisted electrons
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<br/>Auteur, co-auteur: Ivanov, Igor; Serbo, Valery; Zaytsev, Vladimir
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<br/>Résumé: We present a detailed quantum electrodynamical description of Vavilov-Cherenkov radiation emitted by a relativistic twisted electron in the transparent medium. Simple expressions for the spectral and spectral-angular distributions as well as for the polarization properties of the emitted radiation are obtained. Unlike the plane-wave case, the twisted electron produces radiation within the annular angular region, with enhancement towards its boundaries. Additionally, the emitted photons can have linear polarization not only in the scattering plane but also in the orthogonal direction. We find that the Vavilov-Cherenkov radiation emitted by an electron in a superposition of two vortex states exhibits a strong azimuthal asymmetry. Thus, the Vavilov-Cherenkov radiation offers itself as a convenient diagnostic tool of such electrons and complements the traditional microscopic imaging.CP-conserving multi-Higgs model with irremovable complex coefficients
http://hdl.handle.net/2268/201644
Titre: CP-conserving multi-Higgs model with irremovable complex coefficients
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<br/>Auteur, co-auteur: Ivanov, Igor; Silva, Joao
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<br/>Résumé: Models beyond the Standard Model (bSM) often involve elaborate Higgs sectors, which can be a source of CP violation. It brings up the question of recognizing in an efficient way whether a model is CP violating. There is a diffuse belief corroborated by all specific multi-Higgs models considered so far that the issue of explicit CP invariance can be linked to the existence of a basis in which all coefficients are real. Here, we prove that this belief is not justified. We present a CP-conserving three-Higgs-doublet model for which no real basis exists. The generalized CP symmetry of this model is of order 4, which leads to the peculiar property of the extra neutral Higgs bosons of being neither CP-even nor CP-odd but “half-odd” under this symmetry transformation.Group-theoretic restrictions on generation of CP-violation in multi-Higgs-doublet models
http://hdl.handle.net/2268/201643
Titre: Group-theoretic restrictions on generation of CP-violation in multi-Higgs-doublet models
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<br/>Auteur, co-auteur: Branco, Gustavo; Ivanov, Igor
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<br/>Résumé: It has been known since decades that imposing a symmetry group G on the scalar sector of multi-Higgs-doublet models has consequences for CP -violation. In all examples of two- and three-Higgs-doublet models equipped with symmetries, one observes the following intriguing property: if G prevents explicit CP -violation (CPV), at least in the neutral Higgs sector, then it also prevents spontaneous CPV, and if G allows explicit CPV, then it allows for spontaneous CPV. One is led to conjecture that this is a general phenomenon. In this paper, we prove this conjecture for any rephasing symmetry group G and any number of doublets.SO(10) models with flavour symmetries: Classification and examples
http://hdl.handle.net/2268/201642
Titre: SO(10) models with flavour symmetries: Classification and examples
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<br/>Auteur, co-auteur: Ivanov, Igor; Lavoura, Luis
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<br/>Résumé: Renormalizable SO(10) grand unified theory (GUT) models equipped with flavour symmetries are a popular framework for addressing the flavour puzzle. Usually, the flavour symmetry group has been an ad hoc choice, and no general arguments limiting this choice were known. In this paper, we establish the full list of flavour symmetry groups which may be enforced, without producing any further accidental symmetry, on the Yukawa-coupling matrices of an SO(10) GUT with arbitrary numbers of scalar multiplets in the ${\bf{10}}$, $\bar{{\bf{126}}}$, and ${\bf{120}}$ representations of SO(10). For each of the possible discrete non-Abelian symmetry groups, we present examples of minimal models which do not run into obvious contradiction with the phenomenological fermion masses and mixings.Tree-level metastability bounds for the most general two Higgs doublet model
http://hdl.handle.net/2268/201640
Titre: Tree-level metastability bounds for the most general two Higgs doublet model
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<br/>Auteur, co-auteur: Ivanov, Igor; Silva, João
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<br/>Résumé: Within two Higgs doublet models, it is possible that the current vacuum is not the global minimum, in which case it could possibly decay at a later stage. We discuss the tree-level conditions which must be obeyed by the most general scalar potential in order to preclude that possibility. We propose a new procedure which is not only more general but also easier to implement than the previously published one, including CP conserving as well as CP violating scalar sectors. We illustrate these conditions within the context of the Z2 model, softly broken by a complex, CP violating parameter.