Leonid P. Rokhinson

The fractional a.c. Josephson effect in a semiconductor-superconductor nanowire as a signature of Majorana particles

Topological superconductors which support Majorana fermions are thought to be realized in one-dimensional semiconducting wires coupled to a superconductor [1–3]. Such excitations are expected to exhibit non-Abelian statistics and can be used to realize quantum gates that are topologically protected from local sources of decoherence [4, 5]. Here we report the observation of the fractional a.c. Josephson effect in a hybrid semiconductor/superconductor InSb/Nb nanowire junction, a hallmark of topological matter. When the junction is irradiated with a radio-frequency f0 in the absence of an external magnetic field, quantized voltage steps (Shapiro steps) with a height ∆V = hf0/2e are observed, as is expected for conventional superconductor junctions, where the supercurrent is carried by charge-2e Cooper pairs. At high magnetic fields the height of the first Shapiro step is doubled to hf0/e, suggesting that the supercurrent is carried by charge-e quasiparticles. This is a unique signature of Majorana fermions, elusive particles predicted ca. 80 years ago [6].

Atomic force microscope local oxidation nanolithography of graphene

We demonstrate the local oxidation nanopatterning of graphene films by an atomic force microscope. The technique provides a method to form insulating trenches in graphene flakes and to fabricate nanodevices with sub-nanometer precision. We demonstrate fabrication of a 25-nm-wide nanoribbon and submicron size nanoring from a graphene flake. We also found that we can write either trenches or bumps on the graphene surface depending on the lithography conditions. We attribute the bumps to partial oxidation of the surface and incorporation of oxygen into the graphene lattice.

Magnetoconductance oscillations in graphene antidot arrays

Epitaxial graphene films have been formed on the C-face of semi-insulating 4H-SiC substrates by a high temperature sublimation process. Nanoscale square antidot arrays have been fabricated on these graphene films. At low temperatures, magnetoconductance in these films exhibits pronounced Aharonov–Bohm oscillations with the period corresponding to magnetic flux quanta added to the area of a single antidot. At low fields, weak localization is observed and its visibility is enhanced by intervalley scattering on antidot edges. At high fields, we observe two distinctive minima in magnetoconductance, which can be attributed to commensurability oscillations between classical cyclotron orbits and antidot array. All mesoscopic features, surviving up to 70K, reveal the unique electronic properties of graphene.

Double-dot charge transport in Si single-electron/hole transistors

We studied transport through ultrasmall Si quantum-dot transistors fabricated from siliconon-insulator wafers. At high temperatures, 4<T<100 K, the devices show single-electron or single-hole transport through the lithographically defined dot. At T<4 K, current through the devices is characterized by multidot transport. From the analysis of the transport in samples with double-dot characteristics, we conclude that extra dots are formed inside the thermally grown gate oxide which surrounds the lithographically defined dot.

Weak localization in Ga1-xMnxAs: Evidence of impurity band transport

We report the observation of negative magnetoresistance in the ferromagnetic semiconductor GaMnAs at low temperatures (

Spin transitions in a small Si quantum dot

T<3

Corrugated quantum well infrared photodetectors for polarization detection

K) and low magnetic fields (

Two-color quantum-well infrared photodetector with voltage tunable peaks

0< B <20

Nanopatterning of Si/SiGe electrical devices by atomic force microscopy oxidation

mT). We attribute this effect to weak localization. Observation of weak localization provides a strong evidence of impurity band transport in these materials, since for valence band transport one expects either weak anti-localization due to strong spin-orbit interactions or total suppression of interference by intrinsic magnetization. In addition to the weak localization, we observe Altshuler-Aronov electron-electron interactions effect in this material.

Magnetic field-induced helical mode and topological transitions in a topological insulator nanoribbon

We have studied the magnetic field dependence of the ground state energies in a small Si quantum dot. At low fields the first five electrons are added in a spin-up -- spin-down sequence minimizing the total spin. This sequence does not hold for larger number of electrons in the dot. At high fields the dot undergoes transitions between states with different spins driven entirely by Zeeman energy. We identify some features that can be attributed to transitions between different spin configurations preserving the total spin of the dot. For a few peaks we observed large linear shifts that correspond to the change of the spin of the dot by 3/2. Such a change requires that an electron in the dot flips its spin during every tunneling event.