A. Kasumov

Induced and intrinsic superconductivity in carbon nanotubes

Metallic single wall carbon nanotubes have attracted considerable interest as 1D quantum wires combining a low carrier density and a high mobility. It was believed for a long time that low temperature transport was exclusively dominated by the existence of unscreened Coulomb interactions leading to insulating behaviour at low temperature. However, experiments have also shown evidence of superconductivity in carbon nanotubes. We distinguish two fundamentally different physical situations. When carbon nanotubes are connected to superconducting electrodes, they exhibit proximity-induced superconductivity strongly dependent on the transmission of the electrodes. On the other hand, intrinsic superconductivity was also observed in suspended ropes of carbon nanotubes, in doped or very small diameter individual tubes. These experiments indicate the presence of attractive interactions in carbon nanotubes which overcome Coulomb repulsion at low temperature and enable investigation of superconductivity in a 1D limit never explored before.

Ballistic edge states in Bismuth nanowires revealed by SQUID interferometry

The protection against backscattering provided by topology is a striking property. In two-dimensional insulators, a consequence of this topological protection is the ballistic nature of the one-dimensional helical edge states. One demonstration of ballisticity is the quantized Hall conductance. Here we provide another demonstration of ballistic transport, in the way the edge states carry a supercurrent. The system we have investigated is a micrometre-long monocrystalline bismuth nanowire with topological surfaces, that we connect to two superconducting electrodes. We have measured the relation between the Josephson current flowing through the nanowire and the superconducting phase difference at its ends, the current–phase relation. The sharp sawtooth-shaped phase-modulated current–phase relation we find demonstrates that transport occurs selectively along two ballistic edges of the nanowire. In addition, we show that a magnetic field induces 0–π transitions and φ0-junction behaviour, providing a way to manipulate the phase of the supercurrent-carrying edge states and generate spin supercurrents.

Geometry-related magnetic interference patterns in long SNSJosephson junctions

We have measured the critical current dependence on the magnetic flux of two long SNS junctions differing by the normal wire geometry. The samples are made by a Au wire connected to W contacts, via Focused Ion Beam assisted deposition. We could tune the magnetic pattern from the monotonic gaussian-like decay of a quasi 1D normal wire to the Fraunhofer-like pattern of a square normal wire. We explain the monotonic limit with a semiclassical 1D model, and we fit both field dependences with numerical simulations of the 2D Usadel equation. Furthermore, we observe both integer and fractional Shapiro steps. The magnetic flux dependence of the integer steps reproduces as expected that of the critical current Ic, while fractional steps decay slower with the flux than Ic.

Manipulating the magnetic state of a carbon nanotube Josephson junction using the superconducting phase

The magnetic state of a quantum dot attached to superconducting leads is experimentally shown to be controlled by the superconducting phase difference across the dot. This is done by probing the relation between the Josephson current and the superconducting phase difference of a carbon nanotube junction whose Kondo energy and superconducting gap are of comparable size. It exhibits distinctively anharmonic behavior, revealing a phase mediated singlet to doublet transition. We obtain an excellent quantitative agreement with numerically exact quantum Monte Carlo calculations. This provides strong support that we indeed observed the finite temperature signatures of the phase controlled zero temperature level-crossing transition originating from strong local electronic correlations.

Higher-order topology in bismuth

The mathematical field of topology has become a framework in which to describe the low-energy electronic structure of crystalline solids. Typical of a bulk insulating three-dimensional topological crystal are conducting two-dimensional surface states. This constitutes the topological bulk–boundary correspondence. Here, we establish that the electronic structure of bismuth, an element consistently described as bulk topologically trivial, is in fact topological and follows a generalized bulk–boundary correspondence of higher-order: not the surfaces of the crystal, but its hinges host topologically protected conducting modes. These hinge modes are protected against localization by time-reversal symmetry locally, and globally by the three-fold rotational symmetry and inversion symmetry of the bismuth crystal. We support our claim theoretically and experimentally. Our theoretical analysis is based on symmetry arguments, topological indices, first-principles calculations, and the recently introduced framework of topological quantum chemistry. We provide supporting evidence from two complementary experimental techniques. With scanning-tunnelling spectroscopy, we probe the signatures of the rotational symmetry of the one-dimensional states located at the step edges of the crystal surface. With Josephson interferometry, we demonstrate their universal topological contribution to the electronic transport. Our work establishes bismuth as a higher-order topological insulator.The study of the band structure and crystal symmetry of the semimetal bismuth indicates that this material is a higher-order topological insulator hosting robust one-dimensional metallic states on the hinges of the crystal.

Long range electronic transport in DNA molecules deposited across a disconnected array of metallic nanoparticles

Abstract We report in detail our experiments on the conduction of DNA molecules over a wide range of temperature deposited across slits in a few nanometer thick platinum film. These insulating slits were fabricated using focused ion beam etching and characterized extensively using near field and electron microscopy. This characterization revealed the presence of metallic Ga nanoparticles inside the slits, as a result of the ion etching. After deposition of DNA molecules, using a protocol that we describe in detail, some of the slits became conducting and exhibited superconducting fluctuations at low temperatures. We argue that the observed conduction was due to transport along DNA molecules, that interacted with the Ga nanoparticles present in the slit. At low temperatures when Ga becomes superconducting, induced superconductivity could therefore be observed. These results indicate that minute metallic particles can easily transfer charge carriers to attached DNA molecules and provide a possible reconciliation between apparently contradictory previous experimental results concerning the length over which DNA molecules can conduct electricity.

Shot noise in carbon nanotubes

Carbon nanotubes may constitute the ultimate conducting wires for nano-electronics, with their diameters as small as a few tens of atoms and their length of order one micrometer. Because of the particular band structure of graphite, nanotubes have at most two conducting channels, which makes them a one dimensional conductor with very exotic properties. Experimental investigations have indeed shown non conventional features, such as non-ohmic behavior, superconductivity and an ability to carry a huge current density. We have carried out shot noise measurements on nanotubes which are suspended between metallic electrodes. One consequence of the suspended geometry is a very low 1/f noise, thereby enabling the extraction of shot noise. In bundles of nanotubes, we find a reduction of shot noise by more than a factor 100 compared to the full noise 2.e.I expected for uncorrelated electrons. A low noise is also found in an isolated single wall nanotube. In a simple non-interacting-electron picture, such a low shot noise implies that the electrical conduction through a bundle of nanotubes is concentrated in a few ballistic tubes. Another interpretation however would be that a substantial fraction of the tubes conduct with a strong reduction of the effective charge (more than a factor 50) due to electron-electron interaction.

Superconducting nanowires by electron-beam-induced deposition

Superconducting nanowires can be fabricated by decomposition of an organometallic gas using a focused beam of Ga ions. However, physical damage and unintentional doping often results from the exposure to the ion beam, motivating the search for a means to achieve similar structures with a beam of electrons instead of ions. This has so far remained an experimental challenge. We report the fabrication of superconducting tungsten nanowires by electron-beam-induced-deposition, with critical temperature of 2.0 K and critical magnetic field of 3.7 T, and compare them with superconducting wires made with ions. This work opens up new possibilities for the realization of nanoscale superconducting devices, without the requirement of an ion beam column.