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Contains fulltext : 159311.pdf (Publisher’s version ) (Open Access) RU Radboud Universiteit, 20 oktober 2016 Promotores : Christianen, P.C.M., Hest, J.C.M. van Co-promotores : Wilson, D.A., Engelkamp, H. VIII, 213 p.

The thermal motion of polymer chains in a crowded environment is anisotropic and highly confined. Whereas theoretical and experimental progress has been made, typically only indirect evidence of polymer dynamics is obtained either from scattering or mechanical response. Toward a complete understanding of the complicated polymer dynamics in crowded media such as biological cells, it is of great importance to unravel the role of heterogeneity and molecular individualism. In the present work, we investigate the dynamics of synthetic polymers and the tube-like motion of individual chains using time-resolved fluorescence microscopy. A single fluorescently labeled polymer molecule is observed in a sea of unlabeled polymers, giving access to not only the dynamics of the probe chain itself but also to that of the surrounding network. We demonstrate that it is possible to extract the characteristic time constants and length scales in one experiment, providing a detailed understanding of polymer dynamics at the single chain level. The quantitative agreement with bulk rheology measurements is promising for using local probes to study heterogeneity in complex, crowded systems.

Polymersomes are bilayer vesicles, self-assembled from amphiphilic block copolymers. They are versatile nanocapsules with adjustable properties, such as flexibility, permeability, size and functionality. However, so far no methodological approach to control their shape exists. Here we demonstrate a mechanistically fully understood procedure to precisely control polymersome shape via an out-of-equilibrium process. Carefully selecting osmotic pressure and permeability initiates controlled deflation, resulting in transient capsule shapes, followed by reinflation of the polymersomes. The shape transformation towards stomatocytes, bowl-shaped vesicles, was probed with magnetic birefringence, permitting us to stop the process at any intermediate shape in the phase diagram. Quantitative electron microscopy analysis of the different morphologies reveals that this shape transformation proceeds via a long-predicted hysteretic deflation–inflation trajectory, which can be understood in terms of bending energy. Because of the high degree of controllability and predictability, this study provides the design rules for accessing polymersomes with all possible different shapes., Polymersomes are used in many applications, ranging from drug delivery to microfluidics, but controlling their shape is difficult. Here, Christianen and co-workers have precisely controlled polymersome shape via an out-of-equilibrium process and fully understood the mechanism of this procedure.

Magnetic fields have only recently been included in theoretical simulations of high-mass star formation. The simulations show that magnetic fields can play a crucial role not only in the formation and dynamics of molecular outflows, but also in the evolution of circumstellar disks. Therefore, new measurements of magnetic fields at milliarcsecond resolution close to massive young stellar objects (YSOs) are fundamental for providing new input for numerical simulations and for understanding the formation process of massive stars. The polarized emission of 6.7 GHz CH3OH masers allows us to investigate the magnetic field close to the massive YSO where the outflows and disks are formed. Recently, we have detected with the EVN CH3OH maser polarized emission towards 10 massive YSOs. From a first statistical analysis we have found evidence that magnetic fields are primarily oriented along the molecular outflows. To improve our statistics we are carrying on a large observational EVN campaign for a total of 19 sources, the preliminary results of the first seven sources are presented in this contribution. Furthermore, we also describe our efforts to estimate the Lande' g-factors of the CH3OH maser transition to determine the magnetic field strength from our Zeeman-splitting measurements., Accepted for publication in the proceeding of the "12th European VLBI Network Symposium and Users Meeting", eds Tarchi et al. PoS(EVN 2014)041

International audience; We report on high-field electron spin resonance (ESR) studies of magnetic excitations in the spin-1/2 triangular-lattice antiferromagnet Cs2CuBr4. Frequency-field diagrams of ESR excitations are measured for different orientations of magnetic fields up to 25 T. We show that the substantial zero-field energy gap, Delta approximate to 9.5 K, observed in the low-temperature excitation spectrum of Cs2CuBr4, (Zvyagin et al. 2014 Phys. Rev. Lett. 112 077206) is present well above TN. Noticeably, the transition into the long-range magnetically ordered phase does not significantly affect the size of the gap, suggesting that even below TN the high-energy spin dynamics in Cs2CuBr4 is determined by short-range-order spin correlations. The experimental data are compared with results of model spin-wave-theory calculations for spin-1/2 triangular-lattice antiferromagnet.

Shallow donor impurities in silicon, once frozen out at low temperature, share many properties in common with free hydrogen atoms [1]. They have long been the subject of spectroscopic investigation, but it is only very recently [2,3] that it has been possible to investigate the time-domain dynamics of orbital excitations such as the 1 s to 2p, due to the difficulty of obtaining short, intense pulses in the relevant wavelength range. These new techniques make shallow donors (and also acceptors [4]) attractive for studying atomic physics effects, and for applications in quantum information. We have measured the population dynamics of electrons orbiting around phosphorus impurities in commercially-available silicon, and shown that the lattice relaxation lifetime is about 200 ps, only 1 order of magnitude shorter than the radiative lifetime of free hydrogen.

Both the collective (plasmon) and the single particle (Drude) excitations of an electron gas can be controlled and modified by an external magnetic field B. At finite B, plasmon gives rise to a magnetoplasmon mode and the Drude term to a cyclotron resonance. These magnetic effects are expected to be extremely strong for Dirac electrons with a linear energy-momentum dispersion, like those present in graphene and topological insulators (TIs). Here, we investigate both the plasmon and the Drude response versus B in Bi2Se3 topological insulator. At low B, the cyclotron resonance is still well separated in energy from the magnetoplasmon mode; meanwhile, both excitations asymptotically converge at the same energy for increasing B, consistently with a dynamical mass for Dirac carriers of mD* = 0.18 ± 0.01 me. In TIs, one then achieves an excellent magnetic control of plasmonic excitations and this could open the way toward plasmon controlled terahertz magneto-optics.

Optical spectroscopy in high magnetic fields $B\leq65$ T is used to reveal the very different nature of carriers in monolayer and bulk transition metal dichalcogenides. In monolayer WSe$_{2}$, the exciton emission shifts linearly with the magnetic field and exhibits a splitting which originates from the magnetic field induced valley splitting. The monolayer data can be described using a single particle picture with a Dirac-like Hamiltonian for massive Dirac fermions, with an additional term to phenomenologically include the valley splitting. In contrast, in bulk WSe$_{2}$ where the inversion symmetry is restored, transmission measurements show a distinctly excitonic behavior with absorption to the 1s and 2s states. Magnetic field induces a spin splitting together with a small diamagnetic shift and cyclotron like behavior at high fields, which is best described within the hydrogen model., just accepted in Nano Letters http://pubs.acs.org/doi/abs/10.1021/acs.nanolett.5b00626 (equations corrected)

Polymer-stabilized liquid crystals (PSLCs) are materials composed of a polymer mesh in a continuous phase of liquid crystal. The polymer mesh provides an anchor point for alignment of the liquid crystalline bulk. The macroscopic extent of order in such systems depends on the order parameter of the liquid crystal (given by the temperature) and the domain order parameter, induced by external stimuli, such as (rubbed) substrates or magnetic or electric fields. We studied thick PSLCs where substrate interactions cannot be employed and used magnetic fields instead. We show how the polymerization conditions, i.e., the temperature and the magnetic field, influence the overall order parameter in 4-octyl-4′-cyanobiphenyl (8CB)-based PSLCs. Optimal macroscopic alignment was obtained in samples polymerized at room temperature and at magnetic fields in excess of 5 T. The effect of mesh network can be quantified by introducing a phenomenological constant, which is correlated to the order parameter at the polymerizatio...

The fanned out, noncollinear spin structure of the Fe(Co) sublattice in amorphous NdFeCo and PrFeCo alloys is shown to strongly affect its ultrafast laser-induced magnetization dynamics. An overshooting effect is discovered at low applied magnetic fields, where the magnetization temporarily increases above its equilibrium value. We explain this phenomenon by considering the dynamics of the noncollinear spin structure. After femtosecond laser excitation the system first reconstructs magnetic order on a time scale of 50--150 ps arriving at a state with a smaller opening angle of the fan. Subsequently, the original opening angle is restored on a time scale of nanoseconds. Increasing the field up to 0.6 T we can fully close the fan and therewith suppress the overshooting behavior.

A micrometric photoluminescence imaging technique is used to study the in-plane motion of excitons in single GaAs quantum wells, as a function of well width, temperature and magnetic field. Contrary to previous investigations, it is found that the motion of resonantly excited excitons can be resolved by far-field optics. For relatively wide wells anomalies in the exciton motion are observed, which suggests a contribution of exciton polaritons to the in-plane motion.

We report an optical study of Side Chain Polymer Liquid Crystals (SCPLCs) in magnetic fields up to 20T. A novel sensitive set-up, which allows simultaneous birefringence and static light scattering measurements, was used to study polynorborene and polyacrylate based SCPLCs. We find that only magnetic fields bigger than a threshold value B min are able to induce orientation effects and this only at the crossing of the Isotropic-Nematic (IN) transition when cooling from the melt. The existence of a minimum field B min proves to be general, while its value is strongly dependent on the polymeric backbone elasticity, polymerization degree and spacer length. We have produced bulk and thin film aligned samples, stable at room temperature, characterized by an optical uniaxial anisotropy, with a director order parameter S D ∼0.9999.

The free electron laser Free electron Laser for Advanced spectroscopy and high Resolution Experiments (FLARE) at the FELIX Laboratory generates powerful radiation in the frequency range of 0.3–3 THz. This light, in combination with 33 T Bitter magnets at the High Field Magnet Laboratory, provides the unique opportunity to perform THz magneto spectroscopy with light intensities many orders of magnitude higher than provided by conventional sources. The performance of the THz spectrometer is measured via high-field electron spin resonance (ESR) in the paramagnetic benchmark system 2,2-diphenyl-1-picrylhydrazyl (DPPH). The narrow ESR linewidth of DPPH allows us to resolve a fine structure with 3 GHz spacing, demonstrating a considerable coherence of the individual THz micropulses of FLARE. The spectral resolution Δ ν / ν is better than 0.1%, which is an order of magnitude higher than typical values for a rf-linac based free electron laser. The observed coherence of the high power THz micropulses is a prerequi...

We report a systematic study of low-energy lattice vibrations in the layered systems KY(MoO4)2, KDy(MoO4)2, KEr(MoO4)2, and KTm(MoO4)2. A layered crystal structure and low symmetry of the local environment of the rare-earth ion cause the appearance of vibrational and electronic excitations in Terahertz frequencies. The interaction between these excitations leads to sophisticated dynamical properties, including non-linear effects in paramagnetic resonance spectra. The THz study in magnetic field allows for the clear distinction between lattice vibrations and electronic excitations. We measured the THz transmission spectra and show that the low energy lattice vibrations in binary molybdates can be well described within the quasi-one-dimensional model. The developed model describes the measured far-infrared spectra, and results of our calculations agree with previous Raman and ultrasound studies.

The two dimensional diffusion of single excitons in GaAs/AlGaAs quantum wells has been measured, at 4.2 K and in magnetic fields B up to 12 T, using spatially-resolved photoluminescence spectroscopy. At zero field an exceptionally high diffusion constant D (600 cm 2 /s) has been found, which reduces quadratically with the application of a small B , due to the field induced increase of the excitonic mass. For higher magnetic fields the diffusion constant decreases as D ∼ B −2 , in agreement with theoretical predictions which include quantum corrections in the exciton motion.

The in-plane motion of excitons (X) and negatively charged excitons (X−) in a doped CdTe/CdMgTe quantum well has been measured at 4.2 K in magnetic fields up to 12 T, using spatially resolved photoluminescence (PL) spectroscopy. The populations of X and X− have been found to be in local dynamical equilibrium. The measured PL patterns in real space could therefore be described by considering free and mobile excitons in equilibrium with localised charged excitons.

We have measured the near-infrared photoluminescence spectrum of phosphorus doped silicon (Si: P) and extracted the donor-bound exciton (D0X) energy at magnetic fields up to 28 T. At high field the Zeeman effect is strongly nonlinear because of the diamagnetic shift, also known as the quadratic Zeeman effect (QZE). The magnitude of the QZE is determined by the spatial extent of the wave-function. High field data allows us to extract values for the radius of the neutral donor (D0) ground state, and the light and heavy hole D0X states, all with more than an order of magnitude better precision than previous work. Good agreement was found between the experimental state radius and an effective mass model for D0. The D0X results are much more surprising, and the radius of the mJ=±3/2 heavy hole is found to be larger than that of the mJ=±1/2 light hole.

We have developed a sensitive, multipurpose torsional magnetometer with optical detection of the torque. The use of a feedback system with a current coil mounted with the sample allows direct, quantitative determination of the magnetization with a sensitivity of 10−12 J/T in a Bitter-magnet and 2×10−13 J/T at 15 T in a superconducting magnet. The system can be used over a wide range of temperatures and up to high magnetic fields. To demonstrate the sensitivity and versatility of our magnetometer, we present magnetization measurements of a multisubband two-dimensional electron gas (3×1011 spins) and of a 0.13 mg crystal of the organic conductor κ-(BEDT-TTF)2Cu(NCS)2.

The formation of two-dimensional negatively charged excitons (negative trions) out of excitons and free electrons is found to be determined by a dynamical equilibrium. This dynamical equilibrium consists of a chemical equilibrium, relating the trion, exciton, and electron populations, modified by finite formation and recombination times of (charged) excitons, as is evidenced by a magnetic-field-dependent photoluminescence (PL) and far-infrared study of doped CdTe/CdMgTe quantum wells. The data show that the trion formation is entirely driven by the occupation of the spin-split trion, exciton and electron levels. Incorporation of the proposed trion formation scheme into a rate equation model gives a proper description of the experimental data, leading to values of the formation, recombination, and spin-flip times of trions and excitons that are in good agreement with results of time-resolved experiments in the literature. The model elucidates the effect of the heavy-hole splitting on the polarization degree of the trion PL in a magnetic field and the influence of the dark excitons on the trion formation.

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We show that cooling side chain polymer liquid crystals in a magnetic field, from the isotropic to nematic and subsequently to the glass phase, results in a macroscopically ordered, transparent, and strongly birefringent material. The aligned samples retain their properties after the field is removed and can be dealigned only by heating them to the isotropic phase. To induce alignment, a threshold field B t h is necessary, which strongly depends on the chemical structure details. B t h reflects the strength of mesogene-polymer backbone coupling, and we will show that this interaction is responsible for the stability of the induced alignment at zero field.

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High magnetic fields were used for the alignment of J-aggregates of cyanine dyes in solution leading to strongly polarized optical properties, yielding a maximum dichroic ratio of 13. The aligned aggregates were fixed by gelation resulting in samples that are stable at room temperature and exhibit strongly polarized absorption and emission spectra.