ORBi Collection: Physics
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Ab-initio Study of Ferecrystals
http://hdl.handle.net/2268/173363
Title: Ab-initio Study of Ferecrystals
<br/>
<br/>Author, co-author: Dewandre, Antoine; Verstraete, Matthieu
<br/>
<br/>Abstract: Ferecrystals are a new family of compounds first synthesized in 2007 by the
group of D. Johnson at the University of Oregon. These materials consist
of inter-growths of dichalcogenide and chalcogenide layers, and can be written
as [(M X)_{1+δ} ]_m [T X_2 ]_n where M = Sn, Pb, Sb, Ni and some rare earths; T = Ti,
V, Cr, Nb and Ta; X= S and Se. The integers m and n denote the numbers of
consecutive formula unit layers in the different components of the inter-growth.
The δ parameter reflects the difference of the in-plane cell constants between
components of the inter-growth. This family of nanostructured materials shows
promising properties for thermoelectric devices. The compounds studied here
are [(SnSe)_1.29 ]_{234} [M oSe_2 ]_1 . We performed structural characterisation and
examined the transfer of charge at the interface between the two materials. We
show that there is a depletion of charge at the interface between the two compo-
nents of the heterostructures and that structural distortions of the SnSe layers
in the supercell are similar to those observed in slab calculations.Theoretical studies of PbTiO3 and SrTiO3 under uniaxial mechanical constraints combining first-principles calculations and phenomenological Landau theory
http://hdl.handle.net/2268/173315
Title: Theoretical studies of PbTiO3 and SrTiO3 under uniaxial mechanical constraints combining first-principles calculations and phenomenological Landau theory
<br/>
<br/>Author, co-author: Sharma, Henu
<br/>
<br/>Abstract: In the present thesis we present theoretical studies of perovskite compounds under uniaxial mechanical constraints combining first-principles DFT calculations approach and phenomenological Landau theory. ABO$_3$ perovskites form a very important class of functional materials that can exhibit a broad range of properties (e.g., superconductivity, magnetism, ferroelectricity, multiferroism, metal-insulator transitions\ldots) within small distortions of the same simple prototype cubic structure. Though these compounds have been extensively studied both experimentally and computationally, there are still unresolved issues regarding the effect of pressure. In recent years, strain engineering has reported to be an original approach to tune the ferroelectric properties of perovskite ABO$_3$ compounds. While the effect of epitaxial biaxial strain and hydrostatic strain is rather well understood in this class of materials, very little is yet known regarding the effect of uniaxial mechanical constraints. Our study is motivated by the little existing understanding of the effect of uniaxial strain and stress, that has been up to now almost totally neglected. Two prototype compounds are studied in detail: PbTiO$_3$ and SrTiO$_3$. After a general introduction on ABO$_3$ compounds and calculations techniques (ab initio and phenomenological Landau model), we studied the effect of mechanical constraints in these compounds in our thesis.
PbTiO$_3$ is a prototypical ferroelectric compound and also one of the parent components of the Pb(Zr,Ti)O$_3$ solid solution (PZT), which is the most widely used piezoelectrics. For PbTiO$_3$, we have shown that irrespectively of the uniaxial mechanical constraint applied, the system keeps a purely ferroelectric ground-state, with the polarization aligned either along the constraint direction ($FE_z$ phase) or along one of the pseudo-cubic axis perpendicular to it ($FE_x$ phase). This contrasts with the case of isotropic or biaxial mechanical constraints for which novel phases combining ferroelectric and antiferrodistortive motions have been previously reported. Under uniaxial strain, PbTiO$_3$ switches from a $FE_x$ ground state under compressive strain to $FE_z$ ground-state under tensile strain, beyond a critical strain $\eta_{zz}^c \approx +1$\%. Under uniaxial stress, PbTiO$_3$ exhibits either a $FE_x$ ground state under compression ($\sigma_{zz} < 0$) or a $FE_z$ ground state under tension ($\sigma_{zz} > 0$). Here, however, an abrupt jump of the structural parameters is also predicted under both compressive and tensile stresses at critical values $\sigma_{zz} \approx$ $+2$ GPa and $- 8$ GPa. This behavior appears similar to that predicted under negative isotropic pressure and might reveal practically useful to enhance the piezoelectric response in nanodevices.
The second compound of interest is SrTiO$_3$. It has been widely studied in the past decades due to its unusual properties at low temperature. In this work, we have extended our previous investigations on PbTiO$_3$ by exploring theoretically the pressure effects on perovskite SrTiO$_3$ combining the first-principles calculations and a phenomenological Landau model. We have discussed the evolution of phonon frequencies of SrTiO$_3$ with the three isotropic, uniaxial and biaxial strains using first-principles calculations. We also reproduce the previous work done in SrTiO$_3$ with epitaxial strain and hydrostatic strain. Finally, we have calculated the phase diagram of SrTiO$_3$ under uniaxial strain, as obtained from Landau theory and discussed how it compares with the first-principles calculations.Combined holography and thermography in a single sensor through image-plane holography at thermal infrared wavelengths
http://hdl.handle.net/2268/173002
Title: Combined holography and thermography in a single sensor through image-plane holography at thermal infrared wavelengths
<br/>
<br/>Author, co-author: Georges, Marc; Vandenrijt, Jean-François; Thizy, Cédric; Alexeenko, Igor; Pedrini, Giancarlo; Vollheim, Birgit; Lopez, Ion; Jorge, Iagoba; Rochet, Jonathan; Osten, Wolfgang
<br/>
<br/>Abstract: Holographic interferometry in the thermal wavelengths range, combining a CO2 laser and digital hologram recording with a microbolometer array based camera, allows simultaneously capturing temperature and surface shape information about objects. This is due to the fact that the holograms are affected by the thermal background emitted by objects at room temperature. We explain the setup and the processing of data which allows decoupling the two types of information. This natural data fusion can be advantageously used in a variety of nondestructive testing applications.The Heterogeneous Mineral Content of Bone-Using Stochastic Arguments and Simulations to Overcome Experimental Limitations
http://hdl.handle.net/2268/172856
Title: The Heterogeneous Mineral Content of Bone-Using Stochastic Arguments and Simulations to Overcome Experimental Limitations
<br/>
<br/>Author, co-author: Lukas, C.; Kollmannsberger, P.; Ruffoni, Davide; Roschger, P.; Fratzl, P.; Weinkamer, R.
<br/>
<br/>Abstract: On a sub-millimeter length scale, bone is a very heterogeneous material with varying mineral content. This heterogeneity can be measured by quantitative backscattered electron imaging (qBEI) and quantified by a probability distribution called the bone mineralization density distribution (BMDD). The stochastic nature of the backscattering of electrons during the measurement makes the results dependent on the acquisition time. In this work the influence of the measurement conditions was quantified and was corrected for using Tikhonov regularization. Deconvolution reduces the width of the BMDD and allows a more precise definition of a reference BMDD for healthy adults. The corrected information was used as input for a mathematical model that predicts the time evolution of the BMDD. Simulations of osteoporosis treatment reveal a double peak in the BMDD that is not observed in experiments due to limited acquisition time. Our method allows determining the necessary acquisition time to resolve such double peaks.Effect of concentration in Ge-Te liquids: A combined density functional and neutron scattering study
http://hdl.handle.net/2268/172785
Title: Effect of concentration in Ge-Te liquids: A combined density functional and neutron scattering study
<br/>
<br/>Author, co-author: Micoulaut, Matthieu; Coulet, M.V.; Piarristeguy, A.; Johnson, M.; Cuello, G.; Bichara, C.; Raty, Jean-Yves; Flores-Ruiz, H.; Pradel, A.
<br/>
<br/>Abstract: The structural properties of three compositions of Ge-Te liquids (Ge10Te90, Ge15Te85, Ge20Te80) are studied from a combination of density functional based molecular dynamics simulations and neutron scattering experiments. We investigate structural properties including structure factors, pair distribution functions, angular distributions, coordination numbers, neighbor distributions and compare our results with experimental findings. Most noticeable is the good agreement found in the reproduction of the structure in real and reciprocal space, resulting from the incorporation of dispersion forces in the simulation. This leads to Ge and Te coordination numbers which are lower than in previous studies and which can now be followed with temperature, while also strongly depending on the chosen cutoff distance. Results show a gradual conversion of higher coordinated species (TeIV, GeV) into lower coordinated ones at lower temperature, while leaving anticipated coordinations from the octet rule (TeII and GeIV) nearly unchanged. Structural correlations are characterized as a function of temperature and composition. The vibrational density of states is also measured from inelastic neutron scattering for different compositions and temperatures, and compared to the simulated counterpart which exhibits a reasonable agreement at low frequency.Atomically precise interfaces from non-stoichiometric deposition
http://hdl.handle.net/2268/172773
Title: Atomically precise interfaces from non-stoichiometric deposition
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<br/>Author, co-author: Nie, Y. F.; Zhu, Y.; Lee, C.-H.; Kourkoutis, L. F.; Mundy, J. A.; Junquera, Javier; Ghosez, Philippe; Baek, D. J.; Sung, S.; Xi, X. X.; Shen, K. M.; Muller, D. A.; Schlom, D. G.
<br/>
<br/>Abstract: Complex oxide heterostructures display some of the most chemically abrupt, atomically
precise interfaces, which is advantageous when constructing new interface phases with
emergent properties by juxtaposing incompatible ground states. One might assume that
atomically precise interfaces result from stoichiometric growth. Here we show that the most
precise control is, however, obtained by using deliberate and specific non-stoichiometric
growth conditions. For the precise growth of Srnþ1TinOnþ1 Ruddlesden–Popper (RP) phases,
stoichiometric deposition leads to the loss of the first RP rock-salt double layer, but growing
with a strontium-rich surface layer restores the bulk stoichiometry and ordering of the
subsurface RP structure. Our results dramatically expand the materials that can be prepared
in epitaxial heterostructures with precise interface control—from just the n¼N end
members (perovskites) to the entire RP homologous series—enabling the exploration of novel
quantum phenomena at a richer variety of oxide interfaces.Phase diagram of BiFeO3/LaFeO3 superlattices studied by x-ray diffraction experiments and first-principles calculations
http://hdl.handle.net/2268/172771
Title: Phase diagram of BiFeO3/LaFeO3 superlattices studied by x-ray diffraction experiments and first-principles calculations
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<br/>Author, co-author: Rispens, Gijsbert; Ziegler, Benedikt; Paruch, Patrycja; Zanolli, Zeila; Iniguez, Jorge; Ghosez, Philippe
<br/>
<br/>Abstract: Combining structural and functional measurements, we have mapped the phase diagram of BiFeO3/LaFeO3 superlattices grown by off-axis sputtering on (110)o DyScO3 substrates. The phase diagram displays three distinct regions as a function of BiFeO3 fraction, with a BiFeO3-like ferroelectric phase and a LaFeO3-like paraelectric phase at its extremities, and a complex intermediate region, as supported by first-principles calculations. This intermediate region shows unusual, mixed functional behavior, most likely due to competing phases driven by substitution with a same-size central ion and the specific boundary conditions imposed by the superlattice
structure. In the BiFeO3 rich superlattices, scaling of the ferroelectric-to-paraelectric transition temperature with the BiFeO3 thickness could provide an alternate route for studying ferroelectric size effects in BiFeO3.First-principles characterization of the P21ab ferroelectric phase of Aurivillius Bi2WO6
http://hdl.handle.net/2268/172770
Title: First-principles characterization of the P21ab ferroelectric phase of Aurivillius Bi2WO6
<br/>
<br/>Author, co-author: Djani-Ait, Hania; Hermet, Patrick; Ghosez, Philippe
<br/>
<br/>Abstract: The structural, dielectric, dynamical, elastic, piezoelectric and nonlinear
optical (second-order susceptibility and Pockels tensors) properties of Bi2WO6 in
its P21ab ferroelectric ground state are determined using density functional theory.
The calculation of infrared and Raman spectra on single crystal allowed us to
clarify the assignment of experimental phonon modes, considering the good agreement
between the calculated and the experimental Raman spectra obtained on polycrystal.
The calculation of the elastic constants con rms the elastic stability of the crystal
and allow us to estimate the Young and shear moduli of polycrystalline samples.
The piezoelectric constants have signi cant intrinsic values comparable to those of
prototypical ABO3 ferroelectrics. The electro-optic response is strongly dominated by
the ionic contribution of transverse optic modes, yielding sizable Pockels coe cients
around 9 pm/V along the polar direction.Investigation of spectral impacts on the performance of a concentration device using a Fresnel lens combined with a double junction cell
http://hdl.handle.net/2268/172745
Title: Investigation of spectral impacts on the performance of a concentration device using a Fresnel lens combined with a double junction cell
<br/>
<br/>Author, co-author: Loicq, Jerôme; Galante, Nicolas; Thibert, Tanguy; Hellin, Marie-Laure; Languy, Fabian; Habraken, Serge; Lecat, Jean-Hervé
<br/>
<br/>Abstract: This experimental study was carried out within the context of high concentration photovoltaics.
The paper presents the results of an experimental investigation relating to the quantification of the impacts of the chromatic effect on the performance of a double junction GaInP/GaAs solar cell. Chromatic effects are the result of material dispersion caused by the refractive optics component. This study aims to evaluate the effect of the spectral modification of the incident beam on the whole solar concentrator system performance. Such considerations are fundamental in producing a highly accurate design, with which to achieve the best possible system performance. Efficiency is evaluated within the vicinity of the focus of a Fresnel lens designed for concentration. On the optical axis, rays with different wavelengths are not focalized at the same points. The spectral content of the beam depends, therefore, upon the position of the cell along the optical axis. It is assumed that spectral content modification may have an impact on cell performance and, as a consequence, on system efficiency as a whole. Efficiency of the optical Fresnel lens and of the cell were evaluated in relation to spectral content modification.Fiber based optofluidic biosensors
http://hdl.handle.net/2268/172563
Title: Fiber based optofluidic biosensors
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<br/>Author, co-author: Lismont, Marjorie; Vandewalle, Nicolas; Joris, Bernard; Dreesen, Laurent
<br/>
<br/>Abstract: Medicinal diagnosis requires the development of innovative devices allowing the detection of small amounts of biological species. Among the large variety of available biosensors, the ones based on fluorescence phenomenon are really promising. Here, we show a prototype of the basic unit of a multi-sensing biosensor combining optics and microfluidics benefits. This unit makes use of two crossed optical fibers: the first fiber is used to carry small probe molecules droplets and excite fluorescence, while the second one is devoted to target molecules droplets transport and fluorescence detection. Within this scheme, the interaction takes place in each fiber node. The main benefits of this detection setup are the absence of fibers functionalization, the use of microliter volumes of target and probe species, their separation before interaction, and a better detection limit compared to cuvettes setups.Multiqubit symmetric states with maximally mixed one-qubit reductions
http://hdl.handle.net/2268/172357
Title: Multiqubit symmetric states with maximally mixed one-qubit reductions
<br/>
<br/>Author, co-author: Baguette, Dorian; Bastin, Thierry; Martin, John
<br/>
<br/>Abstract: We present a comprehensive study of maximally entangled symmetric states of arbitrary numbers of qubits in the sense of the maximal mixedness of the one-qubit reduced density operator. A general criterion is provided to easily identify whether given symmetric states are maximally entangled in that respect or not. We show that these maximally entangled symmetric (MES) states are the only symmetric states for which the expectation value of the associated collective spin of the system vanishes, as well as in corollary the dipole moment of the Husimi function. We establish the link between this kind of maximal entanglement, the anticoherence properties of spin states, and the degree of polarization of light fields. We analyze the relationship between the MES states and the classes of states equivalent through stochastic local operations with classical communication (SLOCC). We provide a nonexistence criterion of MES states within SLOCC classes of qubit states and show in particular that the symmetric Dicke state SLOCC classes never contain such MES states, with the only exception of the balanced Dicke state class for even numbers of qubits. The 4-qubit system is analyzed exhaustively and all MES states of this system are identified and characterized. Finally the entanglement content of MES states is analyzed with respect to the geometric and barycentric measures of entanglement, as well as to the generalized N-tangle. We show that the geometric entanglement of MES states is ensured to be larger than or equal to 1/2, but also that MES states are not in general the symmetric states that maximize the investigated entanglement measures.A closer look at the low frequency dynamics of vortex matter
http://hdl.handle.net/2268/172186
Title: A closer look at the low frequency dynamics of vortex matter
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<br/>Author, co-author: Raes, Bart; de Souza Silva, Clecio C.; Silhanek, Alejandro; Cabral, Leonardo R.E.; Moshchalkov, Victor V.; Van de Vondel, Joris
<br/>
<br/>Abstract: Using scanning susceptibility microscopy, we shed new light on the dynamics of individual superconducting vortices and examine the hypotheses of the phenomenological models traditionally used to explain the macroscopic ac electromagnetic properties of superconductors. The measurements, carried out on a 2H-NbSe2 single crystal at relatively high temperature T = 6.8 K, show a linear amplitude dependence of the global ac-susceptibility for excitation amplitudes between 0.3 and 2.6 Oe. We observe that the low amplitude response, typically attributed to the oscillation of vortices in a potential well defi ned by a single, relaxing, Labusch constant, actually corresponds to strongly non-uniform vortex shaking. This is particularly pronounced in the fi eld-cooled disordered phase, which undergoes a dynamic reorganization above 0.8 Oe as evidenced by the healing of lattice defects and a more uniform oscillation of vortices. These observations are corroborated by molecular dynamics simulations when choosing the microscopic input parameters from the experiments. The theoretical simulations allow us to reconstruct the vortex trajectories providing deeper insight in the thermally induced hopping dynamics and the vortex lattice reordering.SELF BROADENING COEFFICIENTS AND IMPROVED LINE INTENSITIES FOR THE v7 BAND OF C2H4 NEAR 10.5 µm, AND IMPACT ON ETHYLENE RETRIEVALS FROM JUNGFRAUJOCH SOLAR SPECTRA
http://hdl.handle.net/2268/172155
Title: SELF BROADENING COEFFICIENTS AND IMPROVED LINE INTENSITIES FOR THE v7 BAND OF C2H4 NEAR 10.5 µm, AND IMPACT ON ETHYLENE RETRIEVALS FROM JUNGFRAUJOCH SOLAR SPECTRA
<br/>
<br/>Author, co-author: Vander Auwera, Jean; Fayt, André; Tudorie, Marcella; Rotger, Maud; Boudon, Vincent; Franco, Bruno; Mahieu, Emmanuel
<br/>
<br/>Abstract: Relying on high-resolution Fourier transform infrared (FTIR) spectra, the present work
involved extensive measurements of individual line intensities and self-broadening
coefficients for the ν7 band of 12C2H4. The measured self-broadening coefficients exhibit
a dependence on both J and Ka. Compared to the corresponding information available in
the latest edition of the HITRAN spectroscopic database, the measured line intensities
were found to be higher by about 10 % for high J lines in the P branch and lower by about
5 % for high J lines of the R branch, varying between these two limits roughly linearly with
the line positions. The impact of the presently measured line intensities on retrievals of
atmospheric ethylene in the 949.0–952.0cm 1 microwindow was evaluated using a
subset of ground-based high-resolution FTIR solar spectra recorded at the Jungfraujoch
station. The use of HITRAN 2012 with line intensities modified to match the present
measurements led to a systematic reduction of the measured total columns of ethylene by
-4.1 +/- 0.1 %.Classical analogy for the deflection of flux avalanches by a metallic layer
http://hdl.handle.net/2268/172107
Title: Classical analogy for the deflection of flux avalanches by a metallic layer
<br/>
<br/>Author, co-author: Brisbois, Jérémy; Vanderheyden, Benoît; Colauto, Fabiano; Motta, Maycon; Ortiz, Wilson A.; Fritzsche, Joachim; Nguyen, Ngoc Duy; Hackens, Benoît; Adami, Obaïd-Allah; Silhanek, Alejandro
<br/>
<br/>Abstract: Sudden avalanches of magnetic flux bursting into a superconducting sample undergo deflections of their trajectories when encountering a conductive layer deposited on top of the superconductor. Remarkably, in some cases the flux is totally excluded from the area covered by the conductive layer. We present a simple classical model that accounts for this behaviour and considers a magnetic monopole approaching a semi-infinite conductive plane. This model suggests that magnetic braking is an important mechanism responsible for avalanche deflection.Successive Droplets Encapsulation Revealed by Optofluidics
http://hdl.handle.net/2268/171982
Title: Successive Droplets Encapsulation Revealed by Optofluidics
<br/>
<br/>Author, co-author: Lismont, Marjorie; Robert, Damien; Vandewalle, Nicolas; Dreesen, Laurent
<br/>
<br/>Abstract: Digital microfluidics: an easy way to encapsulate successively water droplets by oil.
Digital microfluidics is implemented to coat successively water droplets with a thin layer of silicon oil. Small water droplets are settled at three fibers nodes while a larger oil drop slides down the vertical fiber.
Optofluidics: an interesting way to reveal the droplet sliding motion and the encapsulation.
To distinguish the different fluids, quantum dots (QDs) and rhodamine labelled polystyrene beads are diluted in oil and water, respectively.These two fluorescent probes are lighted by a 532 nm radiation.The fluorescence signal is recorded by a high speed camera, perpendicularly to the laser and to the fibers network.
This set-up reveals that the oil drop passes through the water droplets without removing them from the nodes, leaving a thin oil layer around the water droplets.
The coating, clearly seen on top pictures of Fig 2, prevents water evaporation. The key parameters for a good encapsulation are the fiber diameter and the oil viscosity that determine droplets size and velocity, respectively.Comparative study of gold and silver based nanobiosensors
http://hdl.handle.net/2268/171981
Title: Comparative study of gold and silver based nanobiosensors
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<br/>Author, co-author: Lismont, Marjorie; Dreesen, Laurent
<br/>
<br/>Abstract: Due to their particular optical properties, resulting from the surface plasmon resonance (SPR) phenomenon, silver and gold nanoparticles (NPs) can be used as robust tools for optical biosensing [1]. Optical SPR biosensors are able to measure complex formation in real time. Indeed, the SPR absorption spectrum band of the NPs is sensitive to the shape, size, inter-particle distance and composition of the NP as well as the dielectric properties of the surrounding medium [2]. Due to the sensitivity of SPR to the local dielectric environment, plasmonic NPs can act as transducers that convert small changes in the local refractive index and the inter-particle distance into spectral shifts and broadening in the absorption spectra bands [3].
In our study, the prototypical biocytin-avidin interaction was used to study gradual changes with time in the absorption spectra bands of biotinylated 10 nm silver and gold NPs as a function of added avidin. After avidin addition, a SPR red-shift and a broadening of the SPR bands were observed with both NPs. These optical changes evolved with time and reached their final values after around 45 min for each system. The maximum SPR red-shifts were equal to 25 nm and 15 nm for silver NPs and gold NPs, respectively. The detection limit, described as the lowest concentration for clear identification of wavelength shift due to biomolecular recognition, is determined to be 4 nM for both silver and gold NPs. The specificity of the biocytin-avidin biosensors was checked by replacing avidin by BSA. When BSA was added, we observed a SPR band shift which was smaller than the detection limit of 3 nm, attesting the biosensor selectivity. Our work demonstrates the superiority of Ag over Au NPs for the elaboration of biosensors based on SPR.Dynamics of a grain-filled ball on a vibrating plate
http://hdl.handle.net/2268/171939
Title: Dynamics of a grain-filled ball on a vibrating plate
<br/>
<br/>Author, co-author: Pacheco-Vazquez, felipe; Ludewig, François; Dorbolo, StéphaneFirst-principles study of vibrational and noncollinear magnetic properties of the perovskite to postperovskite pressure transition of $NaMnF_3$
http://hdl.handle.net/2268/171832
Title: First-principles study of vibrational and noncollinear magnetic properties of the perovskite to postperovskite pressure transition of $NaMnF_3$
<br/>
<br/>Author, co-author: Garcia-Castro, A. C.; Romero, A. H.; Bousquet, Eric
<br/>
<br/>Abstract: We performed a first-principles study of the structural, vibrational, electronic, and magnetic properties of NaMnF3 under applied isotropic pressure. We found that NaMnF3 undergoes a reconstructive phase transition at 8 GPa from the Pnma distorted perovskite structure toward the Cmcm postperovskite structure. This is confirmed by a sudden change of the Mn-F-Mn bondings where the crystal goes from corner-shared octahedra in the Pnma phase to edge-shared octahedra in the Cmcm phase. The magnetic ordering also changes from a G-type antiferromagnetic ordering in the Pnma phase to a C-type antiferromagnetic ordering in the Cmcm phase. Interestingly, we found that the high-spin d-orbital filling is kept at the phase transition which has never been observed in the known magnetic postperovskite structures. We also found a highly noncollinear magnetic ordering in the Cmcm postperovskite phase that drives a large ferromagnetic canting of the spins. We discuss the validity of these results with respect to the U and J parameters of the GGA+U exchange-correlation functional used in our study and conclude that large-spin canting is a promising property of the postperovskite fluoride compounds.Collapsing and reswelling kinetics of thermoresponsive polymers on surfaces: a matter of confinement and constraints
http://hdl.handle.net/2268/171711
Title: Collapsing and reswelling kinetics of thermoresponsive polymers on surfaces: a matter of confinement and constraints
<br/>
<br/>Author, co-author: Willet, Nicolas; Gabriel, Sabine; Jérôme, Christine; Du Prez, Filip; Duwez, Anne-Sophie
<br/>
<br/>Abstract: We report on the collapsing and reswelling ability of grafted poly(methyl vinyl ether) chains of different molecular architectures. In order to study the influence of constraints and confinement of the chains, the polymer was grafted onto AFM tips, as a model of a curved nano-sized surface, and onto macroscopic silicon substrates for comparison purposes. AFM-based force spectroscopy experiments were performed to characterise at the nanoscale the temperature-dependent collapsing process and the reversibility to the swollen state on both substrates. The reversible character of the thermoresponsive transition and its kinetics were shown to greatly depend on the polymer architecture and the constraints encountered by the chains.Lower critical field and SNS-Andreev spectroscopy of 122-arsenides: Evidence of nodeless superconducting gap
http://hdl.handle.net/2268/171692
Title: Lower critical field and SNS-Andreev spectroscopy of 122-arsenides: Evidence of nodeless superconducting gap
<br/>
<br/>Author, co-author: Abdel-Hafiez, M.; Pereira, P.J.; Kuzmichev, S.A.; Kuzmicheva, T.E.; Pudalov, V.M.; Harnagea, L.; Kordyuk, A.A.; Silhanek, Alejandro; Moshchalkov, V.V.; Shen, B.; Wen, Hai-Hu; Vasiliev, A.N.; Chen, Xiao-Jia
<br/>
<br/>Abstract: Using two experimental techniques, we studied single crystals of the 122-FeAs family with almost the same critical temperature, Tc. We investigated the temperature dependence of the lower critical field Hc1(T) of a Ca0.32Na0.68Fe2As2 (Tc ≈ 34 K) single crystal under static magnetic fields H parallel to the c axis. The temperature dependence of the London penetration depth can be described equally well either by a single anisotropic s-wave-like gap or by a two-gap model, while a d-wave approach cannot be used to fit the London penetration depth data. Intrinsic multiple Andreev reflection effect spectroscopy was used to detect bulk gap values in single crystals of the intimate compound Ba0.65K0.35Fe2As2, with the same Tc. We estimated the range of the large gap value L = 6–8 meV (depending on small variation of Tc) and its a k space anisotropy of about 30%, and the small gap Delta ≈ 1.7 ± 0.3 meV. This clearly indicates that the gap structure of our investigated systems more likely corresponds to a nodeless s-wave two gaps.