201 research outputs found
Cryogenic Microwave Imaging of Metal-Insulator Transition in Doped Silicon
We report the instrumentation and experimental results of a cryogenic
scanning microwave impedance microscope. The microwave probe and the scanning
stage are located inside the variable temperature insert of a helium cryostat.
Microwave signals in the distance modulation mode are used for monitoring the
tip-sample distance and adjusting the phase of the two output channels. The
ability to spatially resolve the metal-insulator transition in a doped silicon
sample is demonstrated. The data agree with a semi-quantitative finite-element
simulation. Effects of the thermal energy and electric fields on local charge
carriers can be seen in the images taken at different temperatures and DC
biases.Comment: 10 pages, 5 Figures, Accepted to Review of Scientific Instrumen
Ultra-thin Topological Insulator Bi2Se3 Nanoribbons Exfoliated by Atomic Force Microscopy
Ultra-thin topological insulator nanostructures, in which coupling between
top and bottom surface states takes place, are of great intellectual and
practical importance. Due to the weak Van der Waals interaction between
adjacent quintuple layers (QLs), the layered bismuth selenide (Bi2Se3), a
single Dirac-cone topological insulator with a large bulk gap, can be
exfoliated down to a few QLs. In this paper, we report the first controlled
mechanical exfoliation of Bi2Se3 nanoribbons (> 50 QLs) by an atomic force
microscope (AFM) tip down to a single QL. Microwave impedance microscopy is
employed to map out the local conductivity of such ultra-thin nanoribbons,
showing drastic difference in sheet resistance between 1~2 QLs and 4~5 QLs.
Transport measurement carried out on an exfoliated (\leq 5 QLs) Bi2Se3 device
shows non-metallic temperature dependence of resistance, in sharp contrast to
the metallic behavior seen in thick (> 50 QLs) ribbons. These AFM-exfoliated
thin nanoribbons afford interesting candidates for studying the transition from
quantum spin Hall surface to edge states
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