DOI: 10.1103/PhysRevB.102.165305
WoS: WOS:000576600700006
Résumé:

We report on the observation of an unconventional structure of the quantum Hall effect (QHE) in a $p$-type HgTe/Cd$_x$Hg$_{1-x}$Te double quantum well (DQW) consisting of two HgTe layers of critical thickness. The observed QHE is a reentrant function of magnetic field between two $i=2$ states (plateaus at $\rho_{xy}=h/ie^2$) separated by an intermediate $i=1$ state in the shape of a flat-top peak placed on an remarkably long $i=2$ plateau. This anomalous $i=1$ peak separates two different regimes: i) a QHE at low fields corresponding to a small density of mobile holes $p_s$ and ii) a high-field QHE with a $2-1$ plateau--plateau transition corresponding to a much larger $p_s$. The high-field part is strongly sensitive to external influences, like gate voltages, by contrast to the low field part which is much less responsive. We explain the observed behavior by analyzing the trajectories of the Fermi level $E_F(B)$ between hole-like and electron-like LLs. At low fields, below the QHE reentrance, only the 2D holes at the $\Gamma$ point of the first Brillouin zone contribute into the QHE, while the holes of the lateral maximum (LM) of the valence subband are inactive. At the contrary, at fields above the reentrance, $E_F$ comes close to the zero-mode hole LLs and all the holes come into play in the QHE. At intermediate fields, the reentrance itself is caused by a combination of two factors in the specific energy spectrum of this DQW: i) a superposition of electron-like LL and hole-like LLs and ii) a stabilizing influence of the LM reservoir on $E_F(B)$.

DOI: 10.1016/j.ssc.2020.114019
WoS: WOS:000576648100003
Résumé:

The electrically excited impurity emission from AlGaN/GaN high electron mobility transistor (HEMT) structures in external magnetic field varying from 1.2 T to 1.5 T was investigated in this work at temperature of 4.2 K. We have observed the sharp emission line at (263 ± 1) cm−1 with FWHM of (5.8 ± 0.8) cm−1, which we have attributed to radiative electronic transitions in residual oxygen impurity atoms. The blueshift of resonant transition frequency due to presence of external magnetic field was observed. The width of the peak was found to be defined mainly by the thermal broadening at 4.2 K.

DOI: 10.1021/acs.macromol.0c01506
WoS: WOS:000575432700049
Résumé:

Industrial and model polymer nanocomposites are often formulated with coating agents to improve polymer-nanoparticle (NP) compatibility. Here the localization of silane coating agents in styrene-butadiene nanocomposite is investigated through the segmental dynamics of the polymer matrix by broadband dielectric spectroscopy (BDS), allowing the detection of silanes in the matrix through their plasticization effect. This acceleration of dynamics was followed via the shift of τmax of the α-relaxation induced by the presence of coating agents of different molecular weight and quantity, for different amounts of incorporated colloidal silica NPs (R ≈ 12.5 nm, polydispersity 12%). Any noteworthy contribution of interfacial polymer layers on τmax has been excluded by reference measurements with bare NPs. Our approach allowed quantifying the partition between the matrix and the NP interfaces, and was confirmed independently by calorimetry. As a control parameter, the silane grafting reaction could be activated or not, which was confirmed by the absence (resp. presence) of partitioning with the matrix. Our main result is that in the first steps of material formulation, before any grafting reaction, coating agents both cover the silica surface by adsorption and mix with the polymer matrix – in particular if the latter has chemical compatibility via its functional groups. Silane adsorption was found to be comparable to the grafted amount (1.1 nm-2), and does not increase further, confirming that the plateau of the adsorption isotherm is reached in industrial formulations. These results are hoped to contribute to a better understanding of the surface reactions taking place during complex formulation processes of nanocomposites, namely the exact amounts at stake, e.g., in industrial mixers. Final material properties are affected both through NP-matrix compatibility and plasticization of the latter by unreacted molecules.

DOI: 10.1063/5.0019514

Ref HAL: hal-02973233_v1
Ref Arxiv: 2009.14769
Ref INSPIRE: 1820612
Ref. & Cit.: NASA ADS
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Résumé:

We derive the Casimir-Polder force and Casimir torque expressions for an anisotropic molecule close to a conducting plane with a tensorial conductivity. We apply our general expressions to the case of a carbon dioxide CO$_2$ molecule close to a plane with pure Hall conductivity and to graphene. We show that the equilibrium position of this linear molecule is with its main axis perpendicular to the surface. We hence conjecture a possible way to exploit the Casimir torque to mechanically improve the performance of CO$_2$ separation membranes useful for an efficient atmospheric CO$_2$ reduction.

Ref HAL: hal-02965425_v1
DOI: 10.1039/d0py01147a
Exporter : BibTex | endNote
Résumé:

The synthesis of a copolymer derived from poly(3-hexylthiophene), one of the monomers bearing an aniline group, is described. Like the homopolymer, this copolymer is able to functionalize carbon nanotubes by π-stacking of the thiophene units onto the hexagonal network of the nanotubes. It can as well be covalently attached onto carbon nanotube walls without the need for an intermediate chemical step. While the dispersion of non-covalent nanohybrids in THF is stable for only a few hours, one of the covalent nanohybrids is stable for weeks, showing the efficiency of the functionalization. The comparison between the spectroscopic characterization of both covalent and non-covalent systems shows that the copolymer organizes differently at the interface with the nanotubes in each case, in agreement with the effectiveness of the addition reaction.

Ref HAL: hal-02965925_v1
Exporter : BibTex | endNote