Dynamics in stretched nanocomposites

Cooling of nuclear spin system (NSS) in doped semiconductors via dynamic polarization by optical pumping is a powerful method for harnessing ubiquitous fluctuations of nuclear spin. The idea of NSS cooling is based on the hypothesis of spin temperature, which states that NSS reaches an internal thermal equilibrium long before it comes to equilibrium with the external bath (crystal lattice). Although thermodynamic framework has been successfully employed for the description of a variety of the experimental data, a rigorous check of this concept in semiconductors was impossible until recently, in particular at low magnetic field. The reason for that is the lack of experimental techniques allowing nonperturbative optical control over adiabatic transformation of the NSS. We have recently developed such methods, based on off-resonant Faraday rotation and spin noise spectroscopy [1,2]. Using these techniques, combined with photoluminescence spectroscopy, we established a comprehensive picture of the nuclear spin relaxation efficiency, its magnetic field, temperature, and carrier concentration dependence in both n- and p-doped GaAs, a model system in the field of nuclear spin physics in semiconductors [3-5]. We also analyzed the interplay between four relevant relaxation mechanisms: hyperfine interaction, quadrupole interaction, spin diffusion towards paramagnetic impurities, and Korringa mechanisms. Figure 1 illustrates these processes in a nuclei-electron coupled system. Understanding of field dependence of NSS dynamics allowed us to obtain a new insight into the NSS thermodynamics, and verify the spin temperature concept in GaAs bulk material and microcavities [6]. We have demonstrated that NSS exactly follows the predictions of the spin temperature theory, despite the quadrupole interaction that was earlier reported to disrupt nuclear spin thermalization in quantum dots [7]. These results open a way for the deep cooling of nuclear spins in semiconductor structures, with the prospect of realizing nuclear spin-ordered states for high-fidelity spin-photon interfaces. References[1] R. Giri et al, Physical Review Letters, 111, 087603 (2013)[2] I. I. Ryzhov et al, Applied Physics Letters, 106, 242405 (2015)[3] M. Kotur et al, Physical Review B, 94, 081201(R) (2016)[4] M. Vladimirova et al, Physical Review B, 95, 125312 (2017)[5] M. Kotur et al, Physical Review B, 97, 165206 (2018)[6] M. Vladimirova et al, Physical Review B, 97, 041301 (2018)[7] P. Maletinsky et al, Nature Physics, 5, 407 (2009)

Different kinds of structure in alcogels and aerogels (fractal or not fractal) can be synthesized by a control of the chemical parameters and also by different steps in the preparation such as sintering and plastic compaction. The porosity of the gelsis affected either by the adjustment of the gelifying concentration, by a precise control of the viscous flow sintering process, or by an isostatic pressure deformation. The different kinds of gels cover the whole range of porosity between 99% and 0%, and their mechanical properties (elastic modulus, strength, toughness) are strongly dependent on the porosity but also on their structure. We follow the mechanical properties of the over the whole process alcogel–aerogel–glass. They vary by five orders of magnitude as a function of the density, and for the same relative density, the elastic modulus and strength can increase by one order of magnitude due to a change in connectivity. The influence of the sintering process compared to isostatic pressure on the mechanical properties is explained by the associated structural changes.

Efficient bacterial chromosome segregation typically requires the coordinated action of a three-component, fueled by adenosine triphosphate machinery called the partition complex. We present a phenomenological model accounting for the dynamic activity of this system that is also relevant for the physics of catalytic particles in active environments. The model is obtained by coupling simple linear reaction-diffusion equations with a proteophoresis, or “volumetric” chemophoresis, force field that arises from protein-protein interactions and provides a physically viable mechanism for complex translocation. This minimal description captures most known experimental observations: dynamic oscillations of complex components, complex separation and subsequent symmetrical positioning. The predictions of our model are in phenomenological agreement with and provide substantial insight into recent experiments. From a non-linear physics view point, this system explores the active separation of matter at micrometric scales with a dynamical instability between static positioning and travelling wave regimes triggered by the dynamical spontaneous breaking of rotational symmetry.

As an opening lecture to the French-Swedish neutron scattering school held in Uppsala (6th to 9th of December 2016), the basic concepts ofboth soft matter science and neutron scattering are introduced. Typical soft matter systems like self-assembled surfactants in water, microemulsions, (co-)polymers, and colloids are presented. It will be shown that widely different systems have a common underlying physics dominated by the thermal energy, with astonishing consequences on their statistical thermodynamics, and ultimately rheological properties – namely softness. In the second part, the fundamentals of neutron scattering techniques and in particular small-angle neutron scattering as a powerful method to characterize soft matter systems will be outlined.

Dye encapsulation into host single-walled carbon nanotubes is an easy way to create hybrid nano-systems with tunable opto-electronic properties. In this work, we will discuss the supramolecular organization inside the nanotube, the optical properties and the charge transfer as the function of the type of molecules, diameter and metallic or semiconducting character of the host nanotube. A significant electron transfer from encapsulated dyes to the carbon material is reported, whose magnitude strongly depends on the nanotube diameter, in good agreement with DFT calculations, and on the metallic or semiconducting character. Experiments also suggest a photo-activated electron transfer for small diameter (~9 Å) semiconducting and metallic tubes. Dye encapsulation is shown to significantly alter electron-phonon coupling into metallic nanotubes as revealed by the value of the coupling factor extracted from the Fano equation. The charge transfer is consistent with an important enhancement of the photoluminescence intensity up to a factor of nearly six for optimal confinement configuration. This charge transfer shifts the Fermi level, acting on the photoluminescence efficiency and the electron-phonon-coupling.

In the implementation of field effect transistor (FET) terahertz (THz) detectors, the integration of properly designed planar antennas could effectively enhance the coupling efficiencies between the transistors and THz radiation, thus improving the responsivities of THz detectors. A method to design the planar antenna which is based on the simulation of channel electric field at the gate edge of FET is reported here. This method is suitable for the situation where the input impedances of FETs may not be conveniently obtained in the THz regime. The validity of this method in the antenna design is confirmed by the measurements of the fabricated GaN/AlGaN FET THz detectors. The maximum responsivities of the bowtie detector and the dual-dipole detector are obtained at 170.7 GHz (1568.4 V/W) and 124.3 GHz (1047.2 V/W) respectively, which are close to the simulation results of channel electric field at the gate edge of the bowtie detector and the dual-dipole detector.