J.C. Maan

Magnetic field induced alignment of cyanine dye J-aggregates

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.

Exciton spectroscopy of single CdTe and CdMnTe quantum dots

Single dot optical spectroscopy results obtained for CdTe and CdMnTe self-assembled quantum dots in ZnTe matrix are reported. In order to resolve the emission from individual dots apertures with diameter below 200 nm were opened in a metal masks. In the case of CdTe dots, sharp emission lines with a linewidth below 200 μeV were observed. From a magnetic field dependence of the emission energy the exciton effective g-factor is deduced to be equal to -3 and it is found to be independent of the energy of the quantum dot emission at B = 0 T. On the other hand, for CdMnTe quantum dots, broad (∼3-5 meV) emission lines related to a single dot emission were observed. Moreover, the Zeeman splitting of these lines is found to be comparable with their linewidth. The observed behavior is attributed to either high manganese concentration in the dots or to confinement induced decrease of exchange constants in magnetic quantum structures.

A multipurpose torsional magnetometer with optical detection

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 10−12u2009J/T in a Bitter-magnet and 2×10−13u2009J/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.

Scaling behavior of metal–insulator transitions in a Si/SiGe two dimensional hole gas

We report the temperature dependence of magnetotransport measurements in a Si/SiGe two-dimensional hole gas (2DHG) and we analyze the curves in terms of scaling. A reentrant insulating transition is observed at filling factor ν=1.5, followed by a second high field insulating phase at ν<1. A scaling behavior in temperature of the width of the longitudinal conductivity, its second derivative and the slope of the Hall conductivity has been observed, for both the transitions to the insulating state.

Diffusion of carriers induced by exchange interaction with magnetic-ion system in (Zn,Mn)Se/(Zn, BE)Se quantum wells

We studied the diffusion of carriers in dilute magnetic semiconductors for conditions when an additional force caused by a spatially inhomogeneous exchange potential is active. The spatially inhomogeneous exchange potential arises from a creation of photocarriers in the Zn 0.988 Mn 0.012 Se/ Zn 0.94 Be 0.06 Se quantum well. The photocarriers enhance the spin temperature of the Mn ions by a spin-flip scattering with the magnetic ions, which results in a reduction of the giant Zeeman splitting. The gaussian like intensity profile of the exciting laser beam causes a locally varying exchange potential for the carriers. A strong influence of the gradient in the exchange potential on the diffusion of the carriers was found. The diffusion of the carriers was examined by a spatially resolved photoluminescence technique.

Quenching of the Hall effect in localised high magnetic field regions

Abstract We report the suppression of the Hall effect in a mesoscopic Hall cross with a strong magnetic field only in the centre and vanishingly small outside. The local magnetic field is produced by placing Dy pillar on top of a structure with a high-mobility two-dimensional electron gas. The effect is found to be due to a sharp increase of the number of back-scattered and quasi-localised electron orbits. The possibility of localising electrons inside the magnetic inhomogeneity region is discussed.

Side chain polymer liquid crystals in high magnetic fields

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.

Dynamical equilibrium between excitons and trions in CdTe quantum well structures

The trion formation out of excitons and electrons is studied using cw photoluminescence experiments on a 8 nm CdTe quantum well. Measurements as a function of excitation intensity show that for intense non-resonant excitation free electrons and holes should be included in the trion formation process. Temperature dependent measurements suggest that hot excitons contribute to the trion formation. The data are described successfully using a rate equation model which includes free electrons and holes, and the hot excitons.

Anomalous In-Plane Motion of Excitons in Single GaAs Quantum Wells

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.

Feasibility studies for the implementation of nuclear magnetic resonance in a 25T hybrid magnet

As an exploratory study for NMR experiments in the future 20 MW Nijmegen high field magnet laboratory, the possibilities of field stabilization and field gradient compensation in a 25 T hybrid magnet in the present installation were evaluated. High frequency field fluctuations from the power supply can be compensated to better than 10-3 ppm in the 10 Hz-10 kHz range using a computer controlled feedback system. Field mapping by 2H magnetic resonance using a homebuilt device not only showed that there are substantial axial but also strong radial field gradients. It can be shown that for any cylindrical multicoil Bitter magnet the main components of these gradients can be compensated with simple ferromagnetic inserts. In this way we achieved a linewidth under 5 ppm in 1 mm3 without further shimming or optimization. The low frequency drift of the field due to instabilities of the present power supply and the effects of temperature fluctuations of the coil are determined by simultaneous acquisition of an in-situ deuterium reference signal together with the signal of interest. This allows for a full compensation of the field fluctuations by deconvolution techniques. We will report preliminary NMR results on solid27 Al samples in fields up to 25 T.