We report the direct measurement of the persistent current carried by a single electron by means of magnetization experiments on self-assembled InAs=GaAs quantum rings. We measured the first Aharonov-Bohm oscillation at a field of 14 T, in perfect agreement with our model based on the structural properties determined by cross-sectional scanning tunneling microscopy measurements. The observed oscillation magnitude of the magnetic moment per electron is remarkably large for the topology of our nanostructures, which are singly connected and exhibit a pronounced shape asymmetry. DOI: 10.1103/PhysRevLett.99.146808 PACS numbers: 73.21.La, 73.23.Ra, 78.67.Hc In quantum mechanics, particular attention is paid to phenomena occurring due to the phase coherence of charge carriers in doubly connected (ring) topologies. Electrons confined to a submicron ring manifest a topologically determined quantum-interference phenomenon, known as the Aharonov-Bohm (AB) effect [1], as a result of the oscillatory behavior of their energy levels as a function of an applied magnetic field. This behavior is usually associated with the occurrence of oscillatory persistent currents in the ring [2 -4]. Experimental evidence for AB oscillations has been detected in the mesoscopic regime in metallic [5,6] and semiconducting [7,8] rings, containing many electrons. We address the occurrence of the AB effect in defect-free self-assembled semiconductor nanostructures [9][10][11][12][13]. The ability to fill nanostructures with only a few (1-2) electrons offers the unique possibility to detect magnetic field induced oscillations in the persistent current carried by single electron states. We report the first direct measurement by means of ultrasensitive magnetization experiments of the oscillatory persistent current carried by a single electron in self-assembled InAs/GaAs ''volcanolike'' nanostructures. Remarkably, this single electron current occurs even in the absence of an opening [14] in our nanostructures, which is required for the AB effect in the standard treatment [1]. The magnetic field at which the first oscillation in the magnetic moment arises is much higher than expected from the diameter of the quantum rings as determined by atomic force microscopy [13]. However, the experiments are in good agreement with a model based on the structural parameters as determined with cross-sectional scanning tunneling microscopy (XSTM) measurements.The persistent current was determined via the magnetic moment of electrons in a highly homogeneous ensemble of InAs self-assembled nanostructures. The sample was grown by molecular beam epitaxy and contains 29 mutually decoupled periods [ Fig. 1(a)] [15]. Each period consists of a nanostructured InAs layer, between two 24 nm GaAs layers, and a 2 nm doped (7 10 16 cm ÿ3 Si) GaAs layer that provides electrons to the InAs nanostructures. We used a one-dimensional Poisson solver [16] to estimate the average number of electrons per nanostructure to be about 1.5. Considering the two possible spin orientations we ...

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.

Materials that are built up out of molecular building blocks via a spontaneous hierarchical self-assembly process have intrigued scientists for many years. Nature provides, in this respect, beautiful examples, where, through extreme control over recognition and consequent directed self-assembly on a molecular level, discrete objects are constructed, such as the triple helical rods found in collagen, [1] virus capsids, [2] or the ribosome.[3]Our present synthetic capabilities allow us to introduce many noncovalent interactions into molecules to construct supramolecular assemblies. Besides hydrogen bonding and ionic interactions, one of the main driving forces for self-assembly in an aqueous environment is hydrophobic forces. It is the subtle interplay and balance between all forces that enable a system to smoothly generate well-defined assemblies. In this respect, peptide amphiphiles have gained much interest as molecular building blocks for the development of new surfactants with unprecedented properties, leading to novel applications in the fields of materials and biomedical research [4] . Peptide amphiphiles have been shown to provide a starting point for the generation of materials with a high degree of order on a nanoscopic level that imparts a functional scaffold that can be applied for, for example, biomineralization [5] or cell proliferation [6] . However, the designer process normally stops at the stage of spontaneous assembly, and macroscopic control over the organization of the assemblies is often not obtained. One of the methods to introduce this next level of organization is a further processing step of the supramolecular materials using external forces. There are many different external forces that can be applied. Traditionally, mechanical alignment and electric forces for orientation of materials have often been used. [7,8] Another interesting, although less often used directional force, is a strong magnetic field. [9,10] Magnetic alignment is an extremely versatile technique that exploits the anisotropy in diamagnetic susceptibility of assemblies of molecules.[11] Such a procedure is very attractive as it is contact free, homogeneously effective over the whole sample, and can be used to produce structured thin films as well as bulk material. One of the drawbacks of the use of magnetic fields is that the aligning force is very small. The ability to align structures depends on the anisotropy of the diamagnetic susceptibility of the molecules and the strength of the applied magnetic field, which results in an energy difference DE between parallel and perpendicularly aligned molecules determined by using Equation 1where Dv m is the maximal diamagnetic susceptibility difference of the molecule between two orthogonal axes. Therefore, for single molecules, this energy, determined by Dv m , is very small compared to its inherent thermal energy and only a collective behavior of molecules will allow for alignment to occur. Normal Brownian motion can already disrupt any alignment of single molecules, and this...

We investigate the electronic properties of GaAs1−xBix by photoluminescence at variable temperature (T=10–430K) and high magnetic field (B=0–30T). In GaAs0.981Bi0.019, localized state contribution to PL is dominant up to 150K. At T=180K the diamagnetic shift of the free-exciton states reveals a sizable increase in the carrier effective mass with respect to GaAs. Such an increase cannot be accounted for by an enhanced localized character of the valence band states, solely. Instead, it suggests that also the Bloch states of the conduction band are heavily affected by the presence of bismuth atoms.

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