Stuart Tessmer - Academia.edu (original) (raw)

Papers by Stuart Tessmer

Bulletin of the American Physical Society, 2019

Solid State Communications

Using scanning tunneling spectroscopy we have studied the effects of nitrogen gas exposure on the... more 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, Bi2Se3 crystals were initially cleaved in a helium gas environment and then exposed to a 22 SCFH flow of ultra-high purity N2 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 N2 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.

PLOS ONE, 2020

The importance of improving STEM education is of perennial interest, and to this end, the educati... more 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

Submitted for the MAR10 Meeting of The American Physical Society Scanning Tunneling Microscopy an... more Submitted for the MAR10 Meeting of The American Physical Society Scanning Tunneling Microscopy and Spectroscopy of Topological Insulators SERGEI URAZHDIN, West Virginia University, STUART TESSMER, Michigan State University, THEODORA KYRATSI, MERCOURI KANATZIDIS, Northwestern University — We performed cryogenic scanning tunneling microscopy and spectroscopy measurements of the surface and defect states in topological insulators Bi2Se3 and Bi2Te3. Our measurements demonstrate gapless surface states in both materials. While Bi2Te3 is naturally p-doped, we observed both pand n-doped regions on the cleaved surface. These observations can be correlated with local cleaving within quintuple layers seen from topography measurements, suggesting a new route for surface doping of topological insulators. We also describe unique clover-shaped defects states appearing inside the bulk valence band. Our band structure calculations show that these states originate from a combination of bonding anisot...

Journal of Vacuum Science & Technology B, 1991

A variable‐temperature scanning tunneling microscope system, which operates in the temperature ra... more A variable‐temperature scanning tunneling microscope system, which operates in the temperature range of 4<T<400 K, was employed to perform topographic and spectroscopic studies of conventional superconductors at low temperatures. Images of Pb thin films were obtained and local tunneling spectroscopy was performed. The N–I–S tunneling characteristic revealing superconducting gap structure was observed. In mapping the electronic density of states over the Pb surface, abrupt changes in the measured N–I–S tunneling characteristics were found, showing the existence of apparent normal surface regions. The implications of the tunneling spectroscopy results are discussed.

arXiv: Superconductivity, 2018

In this paper we present scanning tunneling microscopy of Bi$_2$Se$_3$ with superconducting Nb de... more In this paper we present scanning tunneling microscopy of Bi$_2$Se$_3$ with superconducting Nb deposited on the surface. We find that the topologically protected surface states of the Bi$_2$Se$_3$ leak into the superconducting over-layer, suggesting a dual topological proximity effect. Coupling between theses states and the Nb states leads to an effective pairing mechanism for the surface states, leading to a modified model for a topological superconductor in these systems. This model is confirmed by fits between the experimental data and the theory.

Bulletin of the American Physical Society, 2019

Bulletin of the American Physical Society, 2017

Bulletin of the American Physical Society, 2018

Journal of Material Science & Engineering, 2016

We studied the electronic properties of some new kinds of hydrogenated graphenes by first-princip... more We studied the electronic properties of some new kinds of hydrogenated graphenes by first-principles calculations. The designed systems, depending on the position and concentration of the hydrogen atoms, may show interesting band structures that are different from that of the pure graphene. For example, we can obtain semiconductor of a gap about 3eV with flat valance and conduction bands, or semimetal with anisotropic Dirac cones in which the position of Dirac points are shifted from K points, or semimetal with flat band crossing the Dirac cone at the Dirac point. The consequences of these features will be presented. We are able to get good metal with considerable density of states at Fermi level. The phonon dispersions and spectra as well as the electron-phonon couplings of such metallic systems are currently being investigated by the first-principles calculation, their superconductivity transition temperatures Tc should thus be predictable by assuming the electron-phonon coupling as the pairing interaction. We expect that the phonon frequencies are quite large, and the Tc could be high for some of the metallic systems.

Physical Review B, 2016

In this paper we present scanning tunneling microscopy of a large Bi2Se3 crystal with superconduc... more In this paper we present scanning tunneling microscopy of a large Bi2Se3 crystal with superconducting PbBi islands deposited on the surface. Local density of states measurements are consistent with induced superconductivity in the topological surface state with a coherence length of order 540 nm. At energies above the gap the density of states exhibits oscillations due to scattering caused by a nonuniform order parameter. Strikingly, the spectra taken on islands also display similar oscillations along with traces of the Dirac cone, suggesting an inverse topological proximity effect.

Scientific Reports, 2016

The bacterium Geobacter sulfurreducens requires the expression of conductive protein filaments or... more The bacterium Geobacter sulfurreducens requires the expression of conductive protein filaments or pili to respire extracellular electron acceptors such as iron oxides and uranium and to wire electroactive biofilms, but the contribution of the protein fiber to charge transport has remained elusive. Here we demonstrate efficient long-range charge transport along individual pili purified free of metal and redox organic cofactors at rates high enough to satisfy the respiratory rates of the cell. Carrier characteristics were within the orders reported for organic semiconductors (mobility) and inorganic nanowires (concentration), and resistivity was within the lower ranges reported for moderately doped silicon nanowires. However, the pilus conductance and the carrier mobility decreased when one of the tyrosines of the predicted axial multistep hopping path was replaced with an alanine. Furthermore, low temperature scanning tunneling microscopy demonstrated the thermal dependence of the di...

Physical Review Letters, 1993

Cryogenic scanning tunneling microscopy has been used to measure the structure and the local elec... more Cryogenic scanning tunneling microscopy has been used to measure the structure and the local electronic density of states of thin gold islands in metallic contact with a bulk superconducting NbSe2 crystal. Peaks are observed in the tunneling conductance below the energy gap corresponding to discrete bound quasiparticle states in the gold. The energies and spatial dependence of the bound states imply that the superconducting pair potential falls OA abruptly at the Au-NbSe2 interface.

Submitted for the MAR06 Meeting of The American Physical Society Scanning tunneling microscopy st... more Submitted for the MAR06 Meeting of The American Physical Society Scanning tunneling microscopy study of the charge density wave in rare-earth tritellurides 1 ALEKSANDRA TOMIC, CHRISTOS MALLIAKAS, HYUN-JEONG KIM, MERCOURI KANATZIDIS, SIMON BILLINGE, STUART TESSMER, Michigan State University-A number of correlated electron oxides exhibit surprisingly intricate ordered density modulations which underlie their novel properties. These modulations arise from incommensurate charge density waves (IC-CDWs). We have applied scanning tunneling microscopy (STM) to study the nature of IC-CDWs in the rare-earth tritellurides-simple, cleavable, layered materials. In particular, for CeTe3 we have observed both the atomic lattice of surface Te atoms and the CDW modulations oriented at 45 degrees with respect to the Te net. Two-dimensional Fourier transforms of the STM images were obtained to search for discommensurations. These are essentially domain walls whose presence has been strongly suggested by atomic pair distribution function studies. We observe satellite peaks in addition to the principal CDW peaks. These represent strong evidence for the presence of discommensurations with a characteristic length scale of about 38 angstroms.

Physical Review B, 2003

We have applied a novel scanned probe method to directly resolve the interior structure of a GaAs... more We have applied a novel scanned probe method to directly resolve the interior structure of a GaAs/AlGaAs two-dimensional electron system in a tunneling geometry. We find that the application of a perpendicular magnetic field can induce surprising density modulations that are not static as a function of the field. Near six and four filled Landau levels, stripe-like structures emerge with a characteristic length scale ~2 µm. Present theories do not account for ordered density modulations on this scale.

Ultramicroscopy, 2004

We present a numerical method to model electric field sensitive scanning probe microscopy measure... more We present a numerical method to model electric field sensitive scanning probe microscopy measurements which allows for a tip of arbtrary shape and invokes image charges to exactly account for a sample dielectric overlayer. The method is applied to calculate the spatial resolution of a subsurface charge accumulation imaging system, achieving reasonable agreement with experiment.

Review of Scientific Instruments, 1994

A scanning tunneling microscope (STM) operable from room temperature to 1.5 K with an integrated ... more A scanning tunneling microscope (STM) operable from room temperature to 1.5 K with an integrated sample preparation chamber has been constructed and successfully tested. Both the sample and the tunneling tip can be cleaned and/or modified in a UHV processing chamber which is connected at the top of a liquid-helium flow cryostat. The STM is mounted to a threaded rod inside the cryostat so that it can be translated vertically. This allows a newly prepared sample/tip to be transferred onto the STM and lowered into a cryogenic environment without breaking vacuum, minimizing surface contamination which causes significant tunneling problems at cryogenic temperatures.

Bulletin of the American Physical Society, 2019

Solid State Communications

Using scanning tunneling spectroscopy we have studied the effects of nitrogen gas exposure on the... more 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, Bi2Se3 crystals were initially cleaved in a helium gas environment and then exposed to a 22 SCFH flow of ultra-high purity N2 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 N2 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.

PLOS ONE, 2020

The importance of improving STEM education is of perennial interest, and to this end, the educati... more 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

Submitted for the MAR10 Meeting of The American Physical Society Scanning Tunneling Microscopy an... more Submitted for the MAR10 Meeting of The American Physical Society Scanning Tunneling Microscopy and Spectroscopy of Topological Insulators SERGEI URAZHDIN, West Virginia University, STUART TESSMER, Michigan State University, THEODORA KYRATSI, MERCOURI KANATZIDIS, Northwestern University — We performed cryogenic scanning tunneling microscopy and spectroscopy measurements of the surface and defect states in topological insulators Bi2Se3 and Bi2Te3. Our measurements demonstrate gapless surface states in both materials. While Bi2Te3 is naturally p-doped, we observed both pand n-doped regions on the cleaved surface. These observations can be correlated with local cleaving within quintuple layers seen from topography measurements, suggesting a new route for surface doping of topological insulators. We also describe unique clover-shaped defects states appearing inside the bulk valence band. Our band structure calculations show that these states originate from a combination of bonding anisot...

Journal of Vacuum Science & Technology B, 1991

A variable‐temperature scanning tunneling microscope system, which operates in the temperature ra... more A variable‐temperature scanning tunneling microscope system, which operates in the temperature range of 4<T<400 K, was employed to perform topographic and spectroscopic studies of conventional superconductors at low temperatures. Images of Pb thin films were obtained and local tunneling spectroscopy was performed. The N–I–S tunneling characteristic revealing superconducting gap structure was observed. In mapping the electronic density of states over the Pb surface, abrupt changes in the measured N–I–S tunneling characteristics were found, showing the existence of apparent normal surface regions. The implications of the tunneling spectroscopy results are discussed.

arXiv: Superconductivity, 2018

In this paper we present scanning tunneling microscopy of Bi$_2$Se$_3$ with superconducting Nb de... more In this paper we present scanning tunneling microscopy of Bi$_2$Se$_3$ with superconducting Nb deposited on the surface. We find that the topologically protected surface states of the Bi$_2$Se$_3$ leak into the superconducting over-layer, suggesting a dual topological proximity effect. Coupling between theses states and the Nb states leads to an effective pairing mechanism for the surface states, leading to a modified model for a topological superconductor in these systems. This model is confirmed by fits between the experimental data and the theory.

Bulletin of the American Physical Society, 2019

Bulletin of the American Physical Society, 2017

Bulletin of the American Physical Society, 2018

Journal of Material Science & Engineering, 2016

We studied the electronic properties of some new kinds of hydrogenated graphenes by first-princip... more We studied the electronic properties of some new kinds of hydrogenated graphenes by first-principles calculations. The designed systems, depending on the position and concentration of the hydrogen atoms, may show interesting band structures that are different from that of the pure graphene. For example, we can obtain semiconductor of a gap about 3eV with flat valance and conduction bands, or semimetal with anisotropic Dirac cones in which the position of Dirac points are shifted from K points, or semimetal with flat band crossing the Dirac cone at the Dirac point. The consequences of these features will be presented. We are able to get good metal with considerable density of states at Fermi level. The phonon dispersions and spectra as well as the electron-phonon couplings of such metallic systems are currently being investigated by the first-principles calculation, their superconductivity transition temperatures Tc should thus be predictable by assuming the electron-phonon coupling as the pairing interaction. We expect that the phonon frequencies are quite large, and the Tc could be high for some of the metallic systems.

Physical Review B, 2016

In this paper we present scanning tunneling microscopy of a large Bi2Se3 crystal with superconduc... more In this paper we present scanning tunneling microscopy of a large Bi2Se3 crystal with superconducting PbBi islands deposited on the surface. Local density of states measurements are consistent with induced superconductivity in the topological surface state with a coherence length of order 540 nm. At energies above the gap the density of states exhibits oscillations due to scattering caused by a nonuniform order parameter. Strikingly, the spectra taken on islands also display similar oscillations along with traces of the Dirac cone, suggesting an inverse topological proximity effect.

Scientific Reports, 2016

The bacterium Geobacter sulfurreducens requires the expression of conductive protein filaments or... more The bacterium Geobacter sulfurreducens requires the expression of conductive protein filaments or pili to respire extracellular electron acceptors such as iron oxides and uranium and to wire electroactive biofilms, but the contribution of the protein fiber to charge transport has remained elusive. Here we demonstrate efficient long-range charge transport along individual pili purified free of metal and redox organic cofactors at rates high enough to satisfy the respiratory rates of the cell. Carrier characteristics were within the orders reported for organic semiconductors (mobility) and inorganic nanowires (concentration), and resistivity was within the lower ranges reported for moderately doped silicon nanowires. However, the pilus conductance and the carrier mobility decreased when one of the tyrosines of the predicted axial multistep hopping path was replaced with an alanine. Furthermore, low temperature scanning tunneling microscopy demonstrated the thermal dependence of the di...

Physical Review Letters, 1993

Cryogenic scanning tunneling microscopy has been used to measure the structure and the local elec... more Cryogenic scanning tunneling microscopy has been used to measure the structure and the local electronic density of states of thin gold islands in metallic contact with a bulk superconducting NbSe2 crystal. Peaks are observed in the tunneling conductance below the energy gap corresponding to discrete bound quasiparticle states in the gold. The energies and spatial dependence of the bound states imply that the superconducting pair potential falls OA abruptly at the Au-NbSe2 interface.

Submitted for the MAR06 Meeting of The American Physical Society Scanning tunneling microscopy st... more Submitted for the MAR06 Meeting of The American Physical Society Scanning tunneling microscopy study of the charge density wave in rare-earth tritellurides 1 ALEKSANDRA TOMIC, CHRISTOS MALLIAKAS, HYUN-JEONG KIM, MERCOURI KANATZIDIS, SIMON BILLINGE, STUART TESSMER, Michigan State University-A number of correlated electron oxides exhibit surprisingly intricate ordered density modulations which underlie their novel properties. These modulations arise from incommensurate charge density waves (IC-CDWs). We have applied scanning tunneling microscopy (STM) to study the nature of IC-CDWs in the rare-earth tritellurides-simple, cleavable, layered materials. In particular, for CeTe3 we have observed both the atomic lattice of surface Te atoms and the CDW modulations oriented at 45 degrees with respect to the Te net. Two-dimensional Fourier transforms of the STM images were obtained to search for discommensurations. These are essentially domain walls whose presence has been strongly suggested by atomic pair distribution function studies. We observe satellite peaks in addition to the principal CDW peaks. These represent strong evidence for the presence of discommensurations with a characteristic length scale of about 38 angstroms.

Physical Review B, 2003

We have applied a novel scanned probe method to directly resolve the interior structure of a GaAs... more We have applied a novel scanned probe method to directly resolve the interior structure of a GaAs/AlGaAs two-dimensional electron system in a tunneling geometry. We find that the application of a perpendicular magnetic field can induce surprising density modulations that are not static as a function of the field. Near six and four filled Landau levels, stripe-like structures emerge with a characteristic length scale ~2 µm. Present theories do not account for ordered density modulations on this scale.

Ultramicroscopy, 2004

We present a numerical method to model electric field sensitive scanning probe microscopy measure... more We present a numerical method to model electric field sensitive scanning probe microscopy measurements which allows for a tip of arbtrary shape and invokes image charges to exactly account for a sample dielectric overlayer. The method is applied to calculate the spatial resolution of a subsurface charge accumulation imaging system, achieving reasonable agreement with experiment.

Review of Scientific Instruments, 1994

A scanning tunneling microscope (STM) operable from room temperature to 1.5 K with an integrated ... more A scanning tunneling microscope (STM) operable from room temperature to 1.5 K with an integrated sample preparation chamber has been constructed and successfully tested. Both the sample and the tunneling tip can be cleaned and/or modified in a UHV processing chamber which is connected at the top of a liquid-helium flow cryostat. The STM is mounted to a threaded rod inside the cryostat so that it can be translated vertically. This allows a newly prepared sample/tip to be transferred onto the STM and lowered into a cryogenic environment without breaking vacuum, minimizing surface contamination which causes significant tunneling problems at cryogenic temperatures.