17 research outputs found
Theory of Non-equilibrium Single Electron Dynamics in STM Imaging of Dangling Bonds on a Hydrogenated Silicon Surface
During fabrication and scanning-tunneling-microscope (STM) imaging of
dangling bonds (DBs) on a hydrogenated silicon surface, we consistently
observed halo-like features around isolated DBs for specific imaging
conditions. These surround individual or small groups of DBs, have abnormally
sharp edges, and cannot be explained by conventional STM theory. Here we
investigate the nature of these features by a comprehensive 3-dimensional model
of elastic and inelastic charge transfer in the vicinity of a DB. Our essential
finding is that non-equilibrium current through the localized electronic state
of a DB determines the charging state of the DB. This localized charge distorts
the electronic bands of the silicon sample, which in turn affects the STM
current in that vicinity causing the halo effect. The influence of various
imaging conditions and characteristics of the sample on STM images of DBs is
also investigated.Comment: 33 pages, 9 figure
New fabrication technique for highly sensitive qPlus sensor with well-defined spring constant
A new technique for the fabrication of highly sensitive qPlus sensor for
atomic force microscopy (AFM) is described. Focused ion beam was used to cut
then weld onto a bare quartz tuning fork a sharp micro-tip from an
electrochemically etched tungsten wire. The resulting qPlus sensor exhibits
high resonance frequency and quality factor allowing increased force gradient
sensitivity. Its spring constant can be determined precisely which allows
accurate quantitative AFM measurements. The sensor is shown to be very stable
and could undergo usual UHV tip cleaning including e-beam and field evaporation
as well as in-situ STM tip treatment. Preliminary results with STM and AFM
atomic resolution imaging at of the silicon
surface are presented.Comment: 5 pages, 3 figure
Single Electron Dynamics of an Atomic Silicon Quantum Dot on the H-Si(100) 2x1 Surface
Here we report the direct observation of single electron charging of a single
atomic Dangling Bond (DB) on the H-Si(100) 2x1 surface. The tip of a scanning
tunneling microscope is placed adjacent to the DB to serve as a single electron
sensitive charge-detector. Three distinct charge states of the dangling bond,
positive, neutral, and negative, are discerned. Charge state probabilities are
extracted from the data, and analysis of current traces reveals the
characteristic single electron charging dynamics. Filling rates are found to
decay exponentially with increasing tip-DB separation, but are not a function
of sample bias, while emptying rates show a very weak dependence on tip
position, but a strong dependence on sample bias, consistent with the notion of
an atomic quantum dot tunnel coupled to the tip on one side and the bulk
silicon on the other.Comment: 7 pages, 6 figure
Characterizing the rate and coherence of single-electron tunneling between two dangling bonds on the surface of silicon
We devise a scheme to characterize tunneling of an excess electron shared by
a pair of tunnel-coupled dangling bonds on a silicon surface -- effectively a
two-level system. Theoretical estimates show that the tunneling should be
highly coherent but too fast to be measured by any conventional techniques. Our
approach is instead to measure the time-averaged charge distribution of our
dangling-bond pair by a capacitively coupled atomic-force-microscope tip in the
presence of both a surface-parallel electrostatic potential bias between the
two dangling bonds and a tunable midinfrared laser capable of inducing Rabi
oscillations in the system. With a nonresonant laser, the time-averaged charge
distribution in the dangling-bond pair is asymmetric as imposed by the bias.
However, as the laser becomes resonant with the coherent electron tunneling in
the biased pair the theory predicts that the time-averaged charge distribution
becomes symmetric. This resonant symmetry effect should not only reveal the
tunneling rate, but also the nature and rate of decoherence of single-electron
dynamics in our system
Nonperturbative harmonic generation in graphene from intense midinfrared pulsed light
In solids, high harmonic radiation arises from the subcycle dynamics of electrons and holes under the action of an intense laser field. The strong-field regime opens new opportunities to understand and control carrier dynamics on ultrafast time scales, including the coherent dynamics of quasiparticles such as massless Dirac fermions. Here, we irradiate monolayer and few-layer graphene with intense infrared light to produce nonperturbative harmonics of the fundamental up to the seventh order. We find that the polarization dependence shows surprising agreement with gas-phase harmonics. Using a two-band model, we explore the nonlinear current due to electrons near the Dirac points, and we discuss the interplay between intraband and interband contributions to the harmonic spectrum. This interplay opens new opportunities to access ultrafast and strong-field physics of graphene.Peer reviewed: YesNRC publication: Ye
Chiral high-harmonic generation and spectroscopy on solid surfaces using polarization-tailored strong fields
Strong nonlinearities in solid state materials can lead to interesting applications in photonics. Here the authors study chiral high-harmonic generation at SiO2 and MgO surfaces using bi-circular two-color driving fields and extract information on crystal properties