Subsurface Charge Accumulation Imaging

Contains an introduction and a list of publications.Joint Services Electronics Program Grant DAAH04-95-1-0038National Science Foundation Young Investigator AwardU.S. Navy - Office of Naval Research Grant N00014-93-1-063

Scanning-probe Single-electron Capacitance Spectroscopy

The integration of low-temperature scanning-probe techniques and single-electron capacitance spectroscopy represents a powerful tool to study the electronic quantum structure of small systems - including individual atomic dopants in semiconductors. Here we present a capacitance-based method, known as Subsurface Charge Accumulation (SCA) imaging, which is capable of resolving single-electron charging while achieving sufficient spatial resolution to image individual atomic dopants. The use of a capacitance technique enables observation of subsurface features, such as dopants buried many nanometers beneath the surface of a semiconductor material. In principle, this technique can be applied to any system to resolve electron motion below an insulating surface. As in other electric-field-sensitive scanned-probe techniques, the lateral spatial resolution of the measurement depends in part on the radius of curvature of the probe tip. Using tips with a small radius of curvature can enable spatial resolution of a few tens of nanometers. This fine spatial resolution allows investigations of small numbers (down to one) of subsurface dopants. The charge resolution depends greatly on the sensitivity of the charge detection circuitry; using high electron mobility transistors (HEMT) in such circuits at cryogenic temperatures enables a sensitivity of approximately 0.01 electrons/Hz[superscript ½] at 0.3 K[superscript 5].National Science Foundation (U.S.) (DMR-0305461)National Science Foundation (U.S.) (DMR-0906939)National Science Foundation (U.S.) (DMR-0605801)Michigan State University. Institute for Quantum Science

Single-Electron Spectroscopy

Contains an introduction, reports on four research projects and a list of publications,Joint Services Electronics Program Grant DAAH04-95-1-0038National Science Foundation Young Investigator AwardU.S. Navy - Office of Naval Research Grant N00014-93-1-063

Evidence for nitrogen gas surface doping of the Bi2_2Se3_3 topological insulator

Using scanning tunneling spectroscopy we have studied the effects of nitrogen gas exposure on the bismuth selenide density of states. We observe a shift in the Dirac point which is qualitatively consistent with theoretical modeling of nitrogen binding to selenium vacancies. In carefully controlled measurements, Bi$_2$Se$_3$ crystals were initially cleaved in a helium gas environment and then exposed to a 22 SCFH flow of ultra-high purity N$_2$ gas. We observe a resulting change in the spectral curves, with the exposure effect saturating after approximately 50 minutes, ultimately bringing the Dirac point about 50 meV closer to the Fermi level. These results are compared to density functional theoretical calculations, which support a picture of $N_2$ molecules physisorbing near Se vacancies and dissociating into individual N atoms which then bind strongly to Se vacancies. In this interpretation, the binding of the N atom to a Se vacancy site removes the surface defect state created by the vacancy and changes the position of the Fermi energy with respect to the Dirac point.Comment: 9 pages, 3 figure

Single-Electron Spectroscopy

Contains research goals and objectives, reports on four research projects and a list of publications.David and Lucille Packard FoundationJoint Services Electronics Program Grant DAAH04-95-1-0038U.S. Navy - Office of Naval Research Grant N00014-93-1-0633National Science Foundation Young Investigator Awar

Characterizing college science instruction: The Three-Dimensional Learning Observation Protocol

The importance of improving STEM education is of perennial interest, and to this end, the education community needs ways to characterize transformation efforts. Three-dimensional learning (3DL) is one such approach to transformation, in which core ideas of the discipline, scientific practices, and crosscutting concepts are combined to support student development of disciplinary expertise. We have previously reported on an approach to the characterization of assessments, the Three-Dimensional Learning Assessment Protocol (3D-LAP), that can be used to identify whether assessments have the potential to engage students in 3DL. Here we present the development of a companion, the Three-Dimensional Learning Observation Protocol (3D-LOP), an observation protocol that can reliably distinguish between instruction that has potential for engagement with 3DL and instruction that does not. The 3D-LOP goes beyond other observation protocols, because it is intended not only to characterize the pedagogical approaches being used in the instructional environment, but also to identify whether students are being asked to engage with scientific practices, core ideas, and crosscutting concepts. We demonstrate herein that the 3D-LOP can be used reliably to code for the presence of 3DL; further, we present data that show the utility of the 3D-LOP in differentiating between instruction that has the potential to promote 3DL from instruction that does not. Our team plans to continue using this protocol to evaluate outcomes of instructional transformation projects. We also propose that the 3D-LOP can be used to support practitioners in developing curricular materials and selecting instructional strategies to promote engagement in three-dimensional instruction

Quasiparticle bound states in normal metal/superconductor structures probed by scanning tunneling microscopy

In this work we consider systems in which small normal-metal structures (N) are put into contact with a large superconductor (S), with the goal of spatially characterizing the superconductivity. Confined regions with suppressed superconductivity will support quasiparticle bound states, which can be measured spectroscopically. Using a custom-built cryogenic scanning tunneling microscopy system, we have probed the bound states of an NS system consisting of Au (N) droplets of nanometer dimensions in electrical contact with bulk NbSe\sb 2 (S). A quasiparticle bound state was observed even when tunneling directly into the NbSe\sb 2, clear evidence for a significant reduction of the superconductivity inside the NbSe\sb 2 induced by the proximity of the Au over-layer. By invoking a proximity effect model, we are able to characterize the vertical and lateral variation of the pair potential $\Delta$ inside the superconductor. We find that a severe suppression occurs which is beyond the conventional theory. We believe that this effect arises from the short coherence length of the superconductor, so that the spatial variation of the interaction parameter g becomes important. The profile of $\Delta$ that we extract from our data then combines both the conventional proximity suppression of $\Delta$ and its modulation by the profile of g, representing the first observation of spatial structure of the interaction parameter.U of I OnlyETDs are only available to UIUC Users without author permissio