49 research outputs found
Mapping the wavefunction of transition metal acceptor states in the GaAs surface
We utilize a single atom substitution technique with spectroscopic imaging in
a scanning tunneling microscope (STM) to visualize the anisotropic spatial
structure of magnetic and non-magnetic transition metal acceptor states in the
GaAs (110) surface. The character of the defect states play a critical role in
the properties of the semiconductor, the localization of the states influencing
such things as the onset of the metal-insulator transition, and in dilute
magnetic semiconductors the mechanism and strength of magnetic interactions
that lead to the emergence of ferromagnetism. We study these states in the GaAs
surface finding remarkable similarities between the shape of the acceptor state
wavefunction for Mn, Fe, Co and Zn dopants, which is determined by the GaAs
host and is generally reproduced by tight binding calculations of Mn in bulk
GaAs [Tang, J.M. & Flatte, M.E., Phys. Rev. Lett. 92, 047201 (2004)]. The
similarities originate from the antibonding nature of the acceptor states that
arise from the hybridization of the impurity d-levels with the host. A second
deeper in-gap state is also observed for Fe and Co that can be explained by the
symmetry breaking of the surface.Comment: 19 pages, 6 figure
Characterizing the Structure of Topological Insulator Thin Films
We describe the characterization of structural defects that occur during
molecular beam epitaxy of topological insulator thin films on commonly used
substrates. Twinned domains are ubiquitous but can be reduced by growth on
smooth InP (111)A substrates, depending on details of the oxide desorption.
Even with a low density of twins, the lattice mismatch between (Bi,Sb)2Te3 and
InP can cause tilts in the film with respect to the substrate. We also briefly
discuss transport in simultaneously top and back electrically gated devices
using SrTiO3 and the use of capping layers to protect topological insulator
films from oxidation and exposure
Low temperature saturation of phase coherence length in topological insulators
Implementing topological insulators as elementary units in quantum
technologies requires a comprehensive understanding of the dephasing mechanisms
governing the surface carriers in these materials, which impose a practical
limit to the applicability of these materials in such technologies requiring
phase coherent transport. To investigate this, we have performed
magneto-resistance (MR) and conductance fluctuations\ (CF) measurements in both
exfoliated and molecular beam epitaxy grown samples. The phase breaking length
() obtained from MR shows a saturation below sample dependent
characteristic temperatures, consistent with that obtained from CF
measurements. We have systematically eliminated several factors that may lead
to such behavior of in the context of TIs, such as finite size
effect, thermalization, spin-orbit coupling length, spin-flip scattering, and
surface-bulk coupling. Our work indicates the need to identify an alternative
source of dephasing that dominates at low in topological insulators,
causing saturation in the phase breaking length and time
Persistent Optical Gating of a Topological Insulator
Topological insulators (TIs) have attracted much attention due to their
spin-polarized surface and edge states, whose origin in symmetry gives them
intriguing quantum-mechanical properties. Robust control over the chemical
potential of TI materials is important if these states are to become useful in
new technologies, or as a venue for exotic physics. Unfortunately, chemical
potential tuning is challenging in TIs in part because the fabrication of
electrostatic top-gates tends to degrade material properties and the addition
of chemical dopants or adsorbates can cause unwanted disorder. Here, we present
an all-optical technique which allows persistent, bidirectional gating of a
(Bi,Sb)2Te3 channel by optically manipulating the distribution of electric
charge below its interface with an insulating SrTiO3 substrate. In this fashion
we optically pattern p-n junctions in a TI material, which we subsequently
image using scanning photocurrent microscopy. The ability to dynamically write
and re-write mesoscopic electronic structures in a TI may aid in the
investigation of the unique properties of the topological insulating phase. The
optical gating effect may be adaptable to other material systems, providing a
more general mechanism for reconfigurable electronics.Comment: 5 pages, 5 figure