Controlled Generation of Quantized Vortex-Antivortex Pairs in a Superconducting Condensate.

Quantized vortices, as topological defects, play an important role in both physics and technological applications of superconductors. Normally, the nucleation of vortices requires the presence of a high magnetic field or current density, which allow the vortices to enter from the sample boundaries. At the same time, the controllable generation of individual vortices inside a superconductor is still challenging. Here, we report the controllable creation of single quantum vortices and antivortices at any desirable position inside a superconductor. We exploit the local heating effect of a scanning tunneling microscope (STM) tip: superconductivity is locally suppressed by the tip and vortex-antivortex pairs are generated when supercurrent flows around the hot spot. The experimental results are well-explained by theoretical simulations within the Ginzburg-Landau approach.

Tunable and switchable magnetic dipole patterns in nanostructured superconductors.

Design and manipulation of magnetic moment arrays have been at the focus of studying the interesting cooperative physical phenomena in various magnetic systems. However, long-range ordered magnetic moments are rather difficult to achieve due to the excited states arising from the relatively weak exchange interactions between the localized moments. Here, using a nanostructured superconductor, we investigate a perfectly ordered magnetic dipole pattern with the magnetic poles having the same distribution as the magnetic charges in an artificial spin ice. The magnetic states can simply be switched on/off by applying a current flowing through nanopatterned area. Moreover, by coupling magnetic dipoles with the pinned vortex lattice, we are able to erase the positive/negative poles, resulting in a magnetic dipole pattern of only one polarity, analogous to the recently predicted vortex ice. These switchable and tunable magnetic dipole patterns open pathways for the study of exotic ordering phenomena in magnetic systems.

Nanoscale assembly of superconducting vortices with scanning tunnelling microscope tip.

Vortices play a crucial role in determining the properties of superconductors as well as their applications. Therefore, characterization and manipulation of vortices, especially at the single-vortex level, is of great importance. Among many techniques to study single vortices, scanning tunnelling microscopy (STM) stands out as a powerful tool, due to its ability to detect the local electronic states and high spatial resolution. However, local control of superconductivity as well as the manipulation of individual vortices with the STM tip is still lacking. Here we report a new function of the STM, namely to control the local pinning in a superconductor through the heating effect. Such effect allows us to quench the superconducting state at nanoscale, and leads to the growth of vortex clusters whose size can be controlled by the bias voltage. We also demonstrate the use of an STM tip to assemble single-quantum vortices into desired nanoscale configurations.

Bound vortex dipoles generated at pinning centres by Meissner current.

One of the phenomena that make superconductors unique materials is the Meissner-Ochsenfeld effect. This effect results in a state in which an applied magnetic field is expelled from the bulk of the material because of the circulation near its surface of resistance-free currents, also known as Meissner currents. Notwithstanding the intense research on the Meissner state, local fields due to the interaction of Meissner currents with pinning centres have not received much attention. Here we report that the Meissner currents, when flowing through an area containing a pinning centre, generate in its vicinity two opposite sense current half-loops producing a bound vortex-antivortex pair, which eventually may transform into a fully developed vortex-antivortex pair ultimately separated in space. The generation of such vortex dipoles by Meissner currents is not restricted to superconductors; similar topological excitations may be present in other systems with Meissner-like phases.

Photoluminescence of tetrahedral quantum-dot quantum wells.

Taking into account the tetrahedral shape of a quantum-dot quantum well (QDQW) when describing excitonic states, phonon modes, and the exciton-phonon interaction in the structure, we obtain within a nonadiabatic approach a quantitative interpretation of the photoluminescence spectrum of a single CdS/HgS/CdS QDQW. We find that the exciton ground state in a tetrahedral QDQW is bright, in contrast to the dark ground state for a spherical QDQW. The position of the phonon peaks in the photoluminescence spectrum is attributed to interface optical phonons. We also show that the experimental value of the Huang-Rhys parameter can be obtained only within the nonadiabatic theory of phonon-assisted transitions.