18 research outputs found
Imaging coherent transport in graphene (Part II): Probing weak localization
Graphene has opened new avenues of research in quantum transport, with
potential applications for coherent electronics. Coherent transport depends
sensitively on scattering from microscopic disorder present in graphene
samples: electron waves traveling along different paths interfere, changing the
total conductance. Weak localization is produced by the coherent backscattering
of waves, while universal conductance fluctuations are created by summing over
all paths. In this work, we obtain conductance images of weak localization with
a liquid-He-cooled scanning probe microscope, by using the tip to create a
movable scatterer in a graphene device. This technique allows us to investigate
coherent transport with a probe of size comparable to the electron wavelength.
Images of magnetoconductance \textit{vs.} tip position map the effects of
disorder by moving a single scatterer, revealing how electron interference is
modified by the tip perturbation. The weak localization dip in conductivity at
B=0 is obtained by averaging magnetoconductance traces at different positions
of the tip-created scatterer. The width of the dip yields an
estimate of the electron coherence length at fixed charge density.
This "scanning scatterer" method provides a new way of investigating coherent
transport in graphene by directly perturbing the disorder configuration that
creates these interferometric effects.Comment: 18 pages, 7 figure
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Coaxial atomic force microscope probes for imaging with dielectrophoresis
We demonstrate atomic force microscope(AFM) imaging using dielectrophoresis(DEP) with coaxial probes. DEP provides force contrast allowing coaxial probes to image with enhanced spatial resolution. We model a coaxial probe as an electric dipole to provide analytic formulas for DEP between a dipole, dielectric spheres, and a dielectric substrate. AFM images taken of dielectric spheres with and without an applied electric field show the disappearance of artifacts when imaging with DEP. Quantitative agreement between our model and experiment shows that we are imaging with DEP.Engineering and Applied Science
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Imaging Coherent Transport in Graphene (Part I): Mapping Universal Conductance Fluctuations
Graphene provides a fascinating testbed for new physics and exciting opportunities for future applications based on quantum phenomena. To understand the coherent flow of electrons through a graphene device, we employ a nanoscale probe that can access the relevant length scales—the tip of a liquid-He-cooled scanning probe microscope (SPM) capacitively couples to the graphene device below, creating a movable scatterer for electron waves. At sufficiently low temperatures and small size scales, the diffusive transport of electrons through graphene becomes coherent, leading to universal conductance fluctuations (UCF). By scanning the tip over a device, we map these conductance fluctuations versus scatterer position. We find that the conductance is highly sensitive to the tip position, producing fluctuations when the tip is displaced by a distance comparable to half the Fermi wavelength. These measurements are in good agreement with detailed quantum simulations of the imaging experiment and demonstrate the value of a cooled SPM for probing coherent transport in graphene.Chemistry and Chemical BiologyEngineering and Applied SciencesPhysic
Spin dynamics and level structure of quantum-dot quantum wells
We have characterized CdS/CdSe/CdS quantum-dot quantum wells using
time-resolved Faraday rotation (TRFR). The spin dynamics show that the electron
g-factor varies as a function of quantum well width and the transverse spin
lifetime of several nano-seconds is robust up to room temperature. As a
function of probe energy, the amplitude of the TRFR signal shows pronounced
resonances, which allow one to identify individual exciton transitions. While
the TRFR data are inconsistent with the conduction and valence band level
scheme of spherical quantum-dot quantum wells, a model in which broken
spherical symmetry is taken into account captures the essential features.Comment: 5 pages, 3 figure
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Imaging Universal Conductance Fluctuations in Graphene
We study conductance fluctuations (CF) and the sensitivity of the conductance to the motion of a single scatterer in two-dimensional massless Dirac systems. Our extensive numerical study finds limits to the predicted universal value of CF. We find that CF are suppressed for ballistic systems near the Dirac point and approach the universal value at sufficiently strong disorder. The conductance of massless Dirac fermions is sensitive to the motion of a single scatterer. CF of order e2/h result from the motion of a single impurity by a distance comparable to the Fermi wavelength. This result applies to graphene systems with a broad range of impurity strength and concentration while the dependence on the Fermi wavelength can be explored via gate voltages. Our prediction can be tested by comparing graphene samples with varying amounts of disorder and can be used to understand interference effects in mesoscopic graphene devices.Physic