The electronic reconstruction at the interface between two insulating oxides can

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Nature © Macmillan Publishers Ltd 1998 8 letters to nature 144 NATURE | VOL 396 | 12 NOVEMBER 1998 | www.nature.comThe line¯ux from the SNR can be translated into an ejected mass of 44 Ti, if we know the age and distance of the SNR. Recent measurements of the 44 Ti lifetime (see refs 18, 19, and references therein) were used to derive a weighted mean of 90:4 6 1:3 years. We note that the effective 44 Ti lifetime in SNRs could be larger, depending on the degree of ionization of the 44 Ti and its Lorentz factor. The derived value of the ejected 44 Ti mass is mainly sensitive to the actual value of the lifetime and is less critically dependent on the distance to, and the age of, the SNR.The parameters (age and distance) are not available from the gray measurements alone. Fortunately however, a possible counterpart of the newly discovered SNR was recently (independently) detected in Rosat data 20 : an extended feature of ,28 diameter centred at Galactic longitude l 266:38, latitude b 1:28 only 0.48 away from the 44 Ti excess, well within the measurement uncertainties.By combining the g-ray line¯ux and the X-ray diameter 20 with an assumed typical 44 Ti yield of ,5 3 10 2 5 M ( for supernovae of different types 6±9 , and taking as representative an expansion velocity of ,5,000 km s -1 (ref. 21) for the supernova ejecta, we derive a distance of ,200 pc, and an age of the SNR of ,680 yr. For larger 44 Ti yields and larger expansion velocities, the distance estimate becomes larger and the age estimate becomes less. We note that the SNR expansion velocity, when evaluated from the SNR X-ray spectrum 20 , has the same value of ,5,000 km s -1 .We can only speculate about the reasons why this supernova was not observed ,700 years ago: we can consider the possible existence both of optically subluminous supernovae 22 and of absorbing material in front of the supernova. In addition, the celestial position and the time of the event might have been unfavourable for an observation. Information about the existence and type of the compact stellar-like remnant of the supernova, and the elemental abundances of the SNR, will have to await future optical, radio, Xray and g-ray measurements. M

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.