Helium focused ion beam irradiation with subsequent chemical etching for the fabrication of nanostructures.

In this paper we demonstrate a nanofabrication technique based on local ion irradiation of silicon dioxide with a focused helium ion beam. The wet etching of silicon dioxide irradiated with a focused helium ion beam is described in a two-dimensional case both numerically and experimentally. We suggest a model for the etching process based on the distribution of ion induced defects in the irradiated material. The profile of the surface of the etched silicon dioxide is simulated and compared with the results from scanning electron microscopy. Fabrication of a suspended nanostring with a diameter of less than 20 nm by means of etching ion-irradiated material is demonstrated.

Modeling and optimization of the excitonic diffraction grating.

Periodical spatial modulation of the excitonic resonance in a quantum well could lead to the formation of a new highly directional and resonant coherent optical response-resonant diffraction. Such excitonic diffraction gratings were demonstrated in epitaxially grown quantum wells patterned by low-dose ion beam irradiation before or after the growth. In this paper we present a theoretical model of the resonant diffraction formation based on the step-by-step approximation of the Maxwell equation solution. The resulting theory allows us to reliably describe experimental data as well as to predict a way to increase the diffraction efficiency.

Tomography diagnostic of plasma radiated power on the spherical tokamak Globus-M.

A diagnostic system for plasma radiated power tomography on Globus-M was upgraded with a new linear array based on silicon photodiodes. The hardware of the diagnostic system based on a 256-channel tangential matrix array and a 24-channel linear array is described. The algorithm for 2D tomographic reconstruction of plasma emissivity is presented. It uses the Tikhonov regularization method and anisotropic diffusion functional as a side constraint. Simulated plasma emissivity profiles were used to test and assess the capabilities of the developed algorithm. Data analysis was carried out in ohmic and neutral beam heated discharges to demonstrate the performance of the tomography diagnostic in plasmas with different parameters. The presented results illustrate the proper functioning of the tomography system that allowed to reveal and study various plasma phenomena, including magnetohydrodynamic mode.

Magnetic Force Microscopy of Nanostructured Co/Pt Multilayer Films with Perpendicular Magnetization.

We present the results of magnetic force microscopy investigations of domain structures in multilayer [Co (0.5 nm)/Pt (1 nm)]5 thin film structures (denoted hereafter as Co/Pt) modified by additional Co capping layers and by ion irradiation. It is demonstrated that a Co capping layer essentially changes the domain structure and decreases the threshold of magnetization reversal, due to the formation of noncollinear magnetization in Co/Pt. It is shown that local irradiation with a focused He⁺ ion beam enables the formation of regions with decreased easy-axis anisotropy (magnetic bubbles) that have the inverse magnetization direction in the demagnetized state of Co/Pt. The experimental results demonstrate that the magnetic bubbles can be switched using a probe of a magnetic force microscope. The possible application of these effects for the development of magnetic logic and data storage systems is discussed.

Ion-beam-assisted spatial modulation of inhomogeneous broadening of a quantum well resonance: excitonic diffraction grating.

We propose a method of spatial modulation of inhomogeneous broadening of a quantum-well excitonic resonance based on local generation of defects produced by a focused ion beam. The method is applied to fabrication of excitonic diffraction grating in a single quantum-well InGaAs/GaAs structure by irradiating the sample with a beam of 35-keV He+ ions of exposure doses <1012 cm-2. The spectrum of resonant diffraction on such a structure is narrower than that of reflectivity and decreases much faster with increasing temperature. A proposed model of formation of the diffractive response based on the single scattering approximation well describes the results of the spectral and temperature measurements.

Transport of massless Dirac fermions in non-topological type edge states.

There are two types of intrinsic surface states in solids. The first type is formed on the surface of topological insulators. Recently, transport of massless Dirac fermions in the band of "topological" states has been demonstrated. States of the second type were predicted by Tamm and Shockley long ago. They do not have a topological background and are therefore strongly dependent on the properties of the surface. We study the problem of the conductivity of Tamm-Shockley edge states through direct transport experiments. Aharonov-Bohm magneto-oscillations of resistance are found on graphene samples that contain a single nanohole. The effect is explained by the conductivity of the massless Dirac fermions in the edge states cycling around the nanohole. The results demonstrate the deep connection between topological and non-topological edge states in 2D systems of massless Dirac fermions.