Search

This study conducts experiments on a planetary three-roll experimental apparatus and uses plasticine as the working material. Certain rolling characteristics were observed and compared with the data from finite element simulation, which include diameter of workpieces after deformation, the pitch length and threaded angle of spiral marks, and the cavity at the leading end of rolled plasticine. The comparison confirms the correctness of the simulation model for PSW.

We performed extensive studies on quantum decoherence processes of excitons trapped in the various excited states of SAQDs. Energy level structure and dephasing times of excited states were first determined by conducting photoluminescence excitation spectroscopy and wave-packet interferometry on a large number of individual SAQDs. This large statistical basis allows us to extract the correlation between the energy level structure and dephasing times. The major decoherence mechanisms and their active regime were identified from this correlation. A significant suppression of decoherence was also observed in some of the energetically isolated excited states, providing an experimental evidence for the theoretical prediction, known as “phonon bottleneck effect”. Furthermore, we observed the direct experimental evidence of Rabi oscillation in these excited states with long decoherence times. In addition, a new type of quantum interference (QI) phenomenon was discovered in the wave-packet interferometry experiments performed in the strong excitation regime where the non-linear effects of Rabi oscillation become important. Detailed theoretical investigations attribute this phenomenon to the coherent dynamics resulting from the interplay of Rabi oscillation and QI.

We investigated and compared the growth morphology of Ag films deposited on different Si surfaces at low temperatures and annealed at room temperature with scanning tunneling microscopy. Ag films on clean Si(1 1 1) 7×7 and Si(0 0 1) 2×1 surfaces exhibit with increasing film thickness: clusters (interconnected), islands with flat top terraces, flat films with voids extending down to the wetting layer, and flat films with no voids. The Ag films on Au-reconstructed Si(1 1 1) surfaces exhibit the same trend, with the exception that the initial islands exhibit no well defined heights. The growth morphology observed does not allow the identification of a single critical or minimum thickness of the Ag film as expected in the electronic growth model. Two different critical thicknesses are defined, which are higher for Si(0 0 1) 2×1 than for Si(1 1 1) 7×7 surfaces. The importance of surface states and pinning levels within the electronic growth model is discussed.

In this paper, the finite element method and the dual-stream functions upper bound method are selected to analyze the tube rolling process with a planetary rolling mill. Three-dimensional elastic–plastic finite element simulations with special emphasis on the determination of the roller profile and realistic contact conditions between the rollers, workpiece and mandrel are used to reveal the deformation characteristics of seamless tube during the rolling process. In addition, the kinematically admissible velocity field of the deforming tube is described in terms of two stream functions so that the velocity components of the deforming tube can be obtained through the upper bound analysis. In both analysis methods, the equation of meshing is used to determine the contact condition between the rollers and the workpiece. The results from both methods with different variables are observed and compared.

Up to now, significantly reducing the size of semiconductor lasers in all three dimensions is the ultimate challenge for the development of nanolasers, which is a key component for long-waited on-chip optical communications and computing systems. However, the minimum size of conventional semiconductor lasers utilizing dielectric resonators is governed by the optical diffraction limit (λ/2n)3. Recently, we have published the world’s smallest semiconductor laser [1] based on a new concept in laser feedback mechanism. We report on the low-threshold, continuous-wave operation of a sub-diffraction nanolaser based on surface plasmon amplification by stimulated emission of radiation (spaser ) [2]. The plasmonic nanocavity is formed between an atomically smooth epitaxial silver film and a single optically pumped nanorod consisting of an epitaxial gallium nitride shell and an indium gallium nitride core acting as gain medium.

A three-dimensional (3D) topological insulator (TI) is a quantum state of matter with a gapped insulating bulk yet a conducting surface hosting topologically-protected gapless surface states. One of the most distinct electronic transport signatures predicted for such topological surface states (TSS) is a well-defined half-integer quantum Hall effect (QHE) in a magnetic field, where the surface Hall conductivities become quantized in units of (1/2)e2/h (e being the electron charge, h the Planck constant) concomitant with vanishing resistance. Here, we observe well-developed QHE arising from TSS in an intrinsic TI of BiSbTeSe2. Our samples exhibit surface dominated conduction even close to room temperature, while the bulk conduction is negligible. At low temperatures and high magnetic fields perpendicular to the top and bottom surfaces, we observe well-developed integer quantized Hall plateaus, where the two parallel surfaces each contributing a half integer e2/h quantized Hall (QH) conductance, accompanied by vanishing longitudinal resistance. When the bottom surface is gated to match the top surface in carrier density, only odd integer QH plateaus are observed, representing a half-integer QHE of two degenerate Dirac gases. This system provides an excellent platform to pursue a plethora of exotic physics and novel device applications predicted for TIs, ranging from magnetic monopoles and Majorana particles to dissipationless electronics and fault-tolerant quantum computers., 23 pages, 14 figures, accepted by Nature Physics

We report on the first demonstration of broadband tunable, single-mode plasmonic nanolasers (spas- ers) emitting in the full visible spectrum. These nanolasers are based on a single metal−oxide−semiconductor nanostructure platform comprising of InGaN/GaN semiconductor nanorods supported on an Al2O3-capped epitaxial Ag film. In particular, all-color lasing in subdiffraction plasmonic resonators is achieved via a novel mechanism based on a property of weak size dependence inherent in spasers. Moreover, we have successfully reduced the continuous-wave (CW) lasing thresh- olds to ultrasmall values for all three primary colors and have clearly demonstrated the possibility of "thresholdless" lasing for the blue plasmonic nanolaser.

This proposal is aimed at investigating the influence of substrate steps on molecular beam epitaxial growth of Bi2Se3 and Bi2Te3 topological insulators on Si (111). It uses a novel combinatorial approach by creating micro-lens structures on Si(111) substrate. The microlens contains a combination of numerous micro-surfaces with different miscut angles and miscut orientation. By using scanning probe microscopy, we can investigate how different miscut angles and orientations influence the growth process.

Using scanning tunneling microscopy and ab initio simulations, we have identified several configurations for Cu dopants in ${\text{Cu}}_{x}$${\text{Bi}}_{2}$${\text{Se}}_{3}$, with Cu intercalants being the most abundant. Through statistical analysis, we show strong short-range repulsive interactions between Cu intercalants. At intermediate range (g5 nm), the pair distribution function shows oscillatory structure along the $\ensuremath{\langle}10\overline{1}\ensuremath{\rangle}$ directions, which appear to be influenced by different diffusion barriers along the $\ensuremath{\langle}10\overline{1}\ensuremath{\rangle}$ and $\ensuremath{\langle}2\overline{1}\overline{1}\ensuremath{\rangle}$ directions.

Using Scanning Tunneling Microscopy and Spectroscopy, we probe the electronic structures of single layer ${\small MoS_2}$ on graphite. We show that the quasiparticle energy gap of single layer ${\small MoS_2}$ is 2.15 $\pm$ 0.07 eV at 77 K. Combining with temperature dependent photoluminescence studies, we deduce an exciton binding energy of 0.22 $\pm$ 0.1 eV, a value that is much lower than current theoretical predictions. Consistent with theoretical predictions we directly observed metallic edge states of single layer ${\small MoS_2}$. In the bulk region of ${\small MoS_2}$, the Fermi level is located at 1.8 eV above the valence band maximum, possibly due to the formation of a graphite/${\small MoS_2}$ heterojunction. At the edge, however, we observe an upward band bending of 0.6 eV within a short depletion length of about 5 nm, analogous to the phenomena of Fermi level pinning of a 3D semiconductor by metallic surface states., 17 pages, 4 figures

The emergence of transition metal dichalcogenides (TMDs) as 2D electronic materials has stimulated proposals of novel electronic and photonic devices based on TMD heterostructures. Here we report the determination of band offsets in TMD heterostructures by using microbeam X-ray photoelectron spectroscopy ({\mu}-XPS) and scanning tunneling microscopy/spectroscopy (STM/S). We determine a type-II alignment between $\textrm{MoS}_2$ and $\textrm{WSe}_2$ with a valence band offset (VBO) value of 0.83 eV and a conduction band offset (CBO) of 0.76 eV. First-principles calculations show that in this heterostructure with dissimilar chalcogen atoms, the electronic structures of $\textrm{WSe}_2$ and $\textrm{MoS}_2$ are well retained in their respective layers due to a weak interlayer coupling. Moreover, a VBO of 0.94 eV is obtained from density functional theory (DFT), consistent with the experimental determination., Comment: ^ These authors contributed equally. *Corresponding author E-mail: lanceli@gate.sinica.edu.tw, shih@physics.utexas.edu 20 pages, 4 figures in main text

Three kinds of 12-fold quasi-periodic subwavelength hole arrays have been designed using the same dodecahedral supercell arranged with different local rotational symmetries. Fano resonances associated with spoof surface plasmons in these structures have been studied by far-infrared transmission measurements. The resonant transmission channels of the lowest-order Fano resonance mode have been compared directly between these structures, benefitting from constant non-resonant transmission channel. It is found that the higher is the local rotational symmetry of the supercell array, the higher the transmission intensity and the narrower the linewidth of the Fano resonance.

This paper is focused on using the finite element method to simulate and analyze the planetary rolling process. First, a basic geometric model of the planetary rolling mill that considers roll profiles and offset angle of the rolls was constructed. Then, a three-dimensional elastic–plastic finite element simulation was used to analyze both the deformation characteristics of this process and the distributions of stress and strain in the workpieces. During simulations, an algorithm called Equation of Meshing was proposed by which the initial contact conditions between the rollers and the workpiece were successfully derived. Finally, an optimum design method was integrated into this analysis for seeking the best design variables in order to reduce the cavity conditions in the leading end of milled steel rods.

We demonstrate that the limited coherence length of a one-dimensional quasiperiodically ordered superstructure of a thin Ag film on GaAs(1 1 0) surfaces is due to structural defects. We identify dislocations, phason defects, and domain walls. The domain walls end at dislocations, whose stress field is found to induce a wide distribution of oriented phason defects.

We use cross-sectional scanning tunneling microscopy to examine the shape and composition distribution of ${\mathrm{In}}_{0.5}{\mathrm{Ga}}_{0.5}\mathrm{As}$ quantum dots (QDs) formed by capping heteroepitaxial islands. The QDs have a truncated pyramid shape. The composition appears highly nonuniform, with an In-rich core having an inverted-triangle shape. Thus the electronic properties will be drastically altered, relative to the uniform composition generally assumed in device modeling. Theoretical analysis of the QD growth suggests a simple explanation for the unexpected shape of the In-rich core.

We present a novel {open_quotes}electronic growth{close_quotes} model for metallic thin films on semiconductor substrates. Depending on the competition between the effects of quantum confinement, charge spilling, and interface-induced Friedel oscillations, different types of film stability are defined, as characterized by the existence of critical/magic thicknesses for smooth growth. In particular, smooth growth can be achieved only above a few monolayers for noble metals, and only for the first layer for alkali metals. {copyright} {ital 1998} {ital The American Physical Society}

Intra- and interlayer atom-atom correlations in molecular beam epitaxially grown dilute InGaAs alloys were studied using cross-sectional scanning tunneling microscopy. By imaging individual chemical constituents we construct a large ensemble of ``atom maps'' from which two-dimensional In-In pair correlation functions were deduced. We found a total absence of interlayer pair correlation along $[001]$ and a strong negative correlation for the nearest neighbor (nn) pair along $[110]$, corresponding to a repulsive interaction energy of 0.1 eV for the nn In pairs along $[110]$. In addition, a weak long-range oscillation in the correlation function along $[110]$ is observed.

Self-assembled InGaAs quantum dots (QDs) were fabricated inside a planar microcavity with two vertical cavity modes. This allowed us to excite the QDs coupled to one of the vertical cavity modes through two propagating cavity modes to study their down- and up-converted photoluminescence (PL). The up-converted PL increased continuously with the increasing temperature, reaching an intensity level comparable to that of the down-converted PL at ~120 K. This giant efficiency in the up-converted PL of InGaAs QDs was enhanced by about 2 orders of magnitude with respect to a similar structure without cavity. We tentatively explain the enhanced up-converted signal as a direct consequence of the modified spontaneous emission properties of the QDs in the microcavity, combined with the phonon absorption and emission effects.

Bi2Se3 initially emerged as a particularly promising host of topological physics. However, in actual materials, several issues have been uncovered including strong surface band bending and potential fluctuations. To investigate these concerns, we study nominally stoichiometric Bi2Se3 using scanning tunnelling microscopy. Here we identify two distinct distributions of Bi-Se antisites that act as nanometer-scale sensors for the surface band-bending field. To confirm this, we examine bulk Cu-doped Bi2Se3 and demonstrate a significantly reduced surface band-bending field. In addition, we find that in the case of unintentionally doped Bi2Se3, lateral fluctuations of the Dirac point can be directly correlated with specific near-surface point defects, namely Se vacancies.

Quantum oscillations of work function and film stability as a function of the film thickness in Pb thin films on Si(111) are measured directly using scanning tunneling microscopy and spectroscopy in order to determine their phase relationship. The comparison of the phase relationship in quantum oscillations (surface energy vs work function) reveals a complete surprise: In contrast to a theoretically predicted quarter wavelength phase shift in the phase relationship, we found that their quantum oscillations have identical phase for this particular system. A conjecture to resolve this contradiction is also provided.

Diffraction-unlimited semiconductor nanolasers are demonstrated by using single shape-controlled InGaN/GaN core-shell nanorods as laser gain media on Ag films epitaxially grown on Si substrates. The use of atomically smooth Ag films allows us to fabricate low-loss plasmonic cavities for ultralow-threshold, continuous-wave (CW) nanolaser operation above liquid nitrogen temperature. Furthermore, the tunable band-gap energy of the InxGa1-xN alloy makes it possible to realize laser emission in the full visible spectrum.

Bi₂Se₃ initially emerged as a particularly promising host of topological physics. However, in actual materials, several issues have been uncovered including strong surface band bending and potential fluctuations. To investigate these concerns, we study nominally stoichiometric Bi₂Se₃ using scanning tunnelling microscopy. Here we identify two distinct distributions of BiSe antisites that act as nanometer-scale sensors for the surface band-bending field. To confirm this, we examine bulk Cu-doped Bi₂Se₃ and demonstrate a significantly reduced surface band-bending field. In addition, we find that in the case of unintentionally doped Bi₂Se₃, lateral fluctuations of the Dirac point can be directly correlated with specific near-surface point defects, namely Se vacancies.

Miniaturization of semiconductor lasers holds the key to the development of compact, low-threshold, and fast coherent on-chip light sources/amplifiers, which are critically important for emerging applications in nanophotonics, integrated optics, and information technology (1–3). However, on-chip integration of nanoscale electronic components with conventional semiconductor lasers utilizing dielectric optical cavities is impeded by the diffraction limit-i.e., ∼(λ/2n)3 for three-dimensional (3D) cavities, where λ is the free-space wavelength and n is the refractive index of the dielectric (4–7). The recent advent of nanoplasmonics based on metallodielectric structures has led to the design of optical components and optoelectronic devices in the deep subwavelength regime (8–13). In particular, a new class of lasers based on surface plasmon amplification by stimulated emission of radiation (SPASER) has recently been proposed (14, 15) and experimentally demonstrated (16–19). In the SPASER operation, surface plasmons excited in noble-metal structures adjacent to gain media dramatically increase the optical mode density, shrink the optical mode volume, and provide the necessary feedback mechanism.

We report the measurements of one-photon and two-photon interference of light scattered by two quantum dots, separated by 40 μm. We show how fringe contrasts are strongly affected by spectral diffusion.

Scanning tunneling microscopy is used to study the compensation of Zn‐dopant atoms by surface vacancies on InP and GaAs(110) surfaces as a function of the annealing time and temperature. An attractive interaction is observed between negatively charged Zn atoms and positively charged anion vacancies. Uncharged Zn‐vacancy defect complexes are formed and their structure is analyzed. The concentration of the complexes increases with the vacancy concentration. Simultaneously the concentration of charged Zn decreases. The total observable Zn concentration is found to be the sum of the concentration of isolated charged Zn atoms and of the concentration of the defect complexes. This sum is constant at low temperatures, but decreases at 480 K. The observations are explained by the formation of subsurface defect complexes at higher temperatures and surface complexes at low temperatures.

The thermal formation of Zn-dopant-vacancy defect complexes is studied as a function of annealing time and temperature between 293 and 480 K on InP(110) surfaces using scanning tunneling microscopy. The geometric and electronic structure as well as the concentrations of isolated dopant atoms, phosphorus monovacancies, and vacancy-Zn defect complexes are found to be related to each other. An attractive interaction between the vacancies and Zn atoms is found. The vacancies and Zn-dopant atoms can compensate each other's charge and form uncharged complexes. The formation of these compensated defect complexes strongly decreases the concentration of electrically active Zn atoms. The total observable Zn concentration in the surface layers remains constant with time at temperatures up to 415 K. Only defect complexes consisting of a surface vacancy and a subsurface dopant atom are formed. At 480 K the observable Zn concentration decreases, however, because defect complexes consisting of a subsurface vacancy and a Zn atom are formed. \textcopyright{} 1996 The American Physical Society.

We report evidence for the existence of anticorrelation in InGaAs∕GaAs self-assembled quantum dots (QDs). We found that, as a function of the spacer layer thickness, the QDs between the neighboring layers are either vertically correlated (at small spacer thickness) or anticorrelated (at larger spacer thickness). Moreover, in the case when the QDs are antialigned, the size distribution of individual quantum dots becomes more uniform. The implications of this work to the fundamental understanding of the self-assembly process, and the technological applications are discussed.

We report electrical transport measurements of individual carbon nanotubes grown catalytically in a nanoporous anodic aluminum oxide template by thermal chemical vapor deposition of acetylene. The four-terminal resistance at room temperature scales linearly with the nanotube length indicating diffusive nature of transport. The conductance shows an exp[(−1/T)1/3] dependence on temperature T, suggesting that two-dimensional variable-range hopping is the dominant conduction mechanism. The maximum current density carried by these nanotubes is on the order of 106 A/cm2.

By investigating the polarization-dependent Rabi oscillations using photoluminescence spectroscopy, we determined the respective transition dipole moments of the two excited excitonic states |Ex> and |Ey> of a single self-assembled quantum dot that are nondegenerate due to shape anisotropy. We find that the ratio of the two dipole moments is close to the physical elongation ratio of the quantum dot., 11 pages, 2 figures, MS Word generated PDF file

We have probed the local thermoelectric power of semiconductor nanostructures with the use of ultrahigh-vacuum scanning thermoelectric microscopy. When applied to a p-n junction, this method reveals that the thermoelectric power changes its sign abruptly within 2 nanometers across the junction. Because thermoelectric power correlates with electronic structure, we can profile with nanometer spatial resolution the thermoelectric power, band structures, and carrier concentrations of semiconductor junctions that constitute the building blocks of thermoelectric, electronic, and optoelectronic devices.

An increase of dimer vacancies on the Si(001)-2{times}1 surface after radiation quenching from high temperatures has been observed using STM. After further quenches, vacancies nucleate into chains running perpendicular to the dimer rows. These vacancy chains then connect and develop into vacancy lines (VL`s) that extend for many thousands of A. Each VL consists of mainly two types of dimer vacancies: a divacancy and the combination of a single vacancy and a divacancy separated by an isolated dimer. All the VL`s together with the dimer rows form a 2{times}{ital n} structure with 6{le}{ital n}{le}12. Calculations using the Stillinger-Weber potential have been performed to examine the ordering mechanism of dimer vacancies.

Scaling down semiconductor lasers in all three dimensions holds the key to the development of compact, low-threshold, and ultrafast coherent light sources, as well as integrated optoelectronic and plasmonic circuits. However, the minimum size of conventional semiconductor lasers utilizing dielectric cavity resonators (photonic cavities) is limited by the diffraction limit. To date, surface plasmon amplification by stimulated emission of radiation (spaser)-based plasmonic nanolaser is the only photon and plasmon-emitting device capable of this remarkable feat. Specifically, it has been experimentally demonstrated that the use of plasmonic cavities based on metal-insulator-semiconductor (MIS) nanostructures can indeed break the diffraction limit in all three dimensions. In this review, we present an updated overview of the current status for plasmonic nanolasers using the MIS configuration and other related metal-cladded semiconductor microlasers. In particular, by using composition-varied indium gallium nitride/gallium nitride core-shell nanorods, it is possible to realize all-color, single-mode nanolasers in the full visible wavelength range with ultralow continuous-wave (CW) lasing thresholds. The lasing action in these subdiffraction plasmonic cavities is achieved via a unique auto-tuning mechanism based on the property of weak size dependence inherent in plasmonic nanolasers. As for the choice of metals in the plasmonic structures, epitaxial silver films and giant colloidal silver crystals have been shown to be the superior constituent materials for plasmonic cavities due to their low plasmonic losses in the visible and near-infrared (NIR) spectral regions. In this review, we also provide some perspectives on the challenges and opportunities in this exciting new research frontier.

In this Letter we report a comparative study, in the infrared regime, of surface plasmon polariton (SPP) propagation in epitaxially grown Ag films and in polycrystalline Ag films, all grown on Si substrates. Plasmonic resonance features are analyzed using extraordinary optical transmission (EOT) measurements, and SPP band structures for the two dielectric/metal interfaces are investigated for both types of film. At the Si/Ag interface, EOT spectra show almost identical features for epitaxial and polycrystalline Ag films and are characterized by sharp Fano resonances. On the contrary, at the air/Ag interface, dramatic differences are observed: while the epitaxial film continues to exhibit sharp Fano resonances, the polycrystalline film shows only broad spectral features and much lower transmission intensities. In corroboration with theoretical simulations, we find that surface roughness plays a critical role in SPP propagation for this wavelength range.

We report the measurement of field-field and photon-photon correlations of light scattered by two InAs quantum dots separated by ≈40 μm. Near 4 K a large fraction of photons can be scattered coherently by each quantum dot leading to one-photon interference at a beam splitter (visibility ≈20%). Simultaneously, two-photon interference is also observed (visibility ≈40%) due to the indistinguishability of photons scattered by the two different quantum emitters. We show how spectral diffusion accounts for the reduction in interference visibility through variations in photon flux.

We analyze the light scattered by a single InAs quantum dot interacting with a resonant continuous-wave laser. High resolution spectra reveal clear distinctions between coherent and incoherent scattering, with the laser intensity spanning over four orders of magnitude. We find that the fraction of coherently scattered photons can approach unity under sufficiently weak or detuned excitation, ruling out pure dephasing as a relevant decoherence mechanism. We show how spectral diffusion shapes spectra, correlation functions, and phase-coherence, concealing the ideal radiatively-broadened two-level system described by Mollow., to appear in PRB 85, 235315

Quantum wires, as a smallest electronic conductor, are expected to be a fundamental component in all quantum architectures. The electronic conductance in quantum wires, however, is often dictated by structural instabilities and electron localization at the atomic scale. Here we report on the evolutions of electronic transport as a function of temperature and interwire coupling as the quantum wires of GdSi(2) are self-assembled on Si(100) wire-by-wire. The correlation between structure, electronic properties, and electronic transport are examined by combining nanotransport measurements, scanning tunneling microscopy, and density functional theory calculations. A metal-insulator transition is revealed in isolated nanowires, while a robust metallic state is obtained in wire bundles at low temperature. The atomic defects lead to electron localizations in isolated nanowire, and interwire coupling stabilizes the structure and promotes the metallic states in wire bundles. This illustrates how the conductance nature of a one-dimensional system can be dramatically modified by the environmental change on the atomic scale.

We spectrally resolve the light scattered by a single InAs semiconductor quantum dot and analyze the relative contribution of elastic and inelastic scattering processes.

Successful development of epitaxial Ag as a new plasmonic platform has enabled low-threshold, continuous-wave (CW) operation of a SPASER-enabled nanolaser comprised of a subdiffraction plasmonic cavity and a single InGaN@GaN core-shell nanorod gain medium.

Semiconductor heterojunctions (HJs) have played a critical role in many modern electronic and photonic devices. The emergence of transition metal dichalcogenides (TMDs) as a new class of two-dimensional semiconducting materials creates exciting new opportunities to push semiconductor heterostructures toward a new frontier. Vertically stacked van der Waals heterostructures have been quickly recognized as a powerful platform to create atomically thin heterostructures.1 Recently, attention has been directed to the creation of monolayer ‘lateral heterojunctions’ with a line interface between two different TMDs, pushing semiconductor HJs to a new dimension. In late 2014, three papers published simultaneously, demonstrating direct growth of lateral heterojunctions using chemical vapor deposition.2, 3, 4 The papers demonstrated growth including both lattice matched and mismatched HJs; however, the interfaces were not chemically abrupt (that is, diffuse interface). In particular, the lattice mismatched system (for example, WS2-WSe2) displayed a compositional transition width of 40 nm. Presumably, the gradual composition alleviates the abrupt change of the lattice constant, similar to the epitaxial growth of conventional heterostructures with a large lattice mismatch. Conventional wisdom suggests a chemically abrupt lateral interface in a lattice mismatched system would be difficult to achieve. In contrast, in 2015 by using a two-step process, Li et al.5 reported the successful growth of lattice-mismatched lateral HJs of WSe2–MoS2 with an atomically abrupt compositional interface as shown schematically in Figure 1a. This system is coherently strained judging by the high Photoluminescence efficiency of individual TMDs. This scenario is similar to the growth of coherently strained InAs quantum dots in GaAs, although in this case, compositional mixing at the interface has been difficult to eliminate. Thus, an atomically abrupt compositional interface in a system with such a large lattice mismatch comes as a pleasant surprise.

We report on the development of a new two‐dimensional micropositioning device, or walker, which is capable of moving across very large distances (in principle unlimited) and with a very small step size (as small as 100 A/step) in both directions. Based on a unique tracking design, the motion is extremely orthogonal with very little crosstalk between the two directions. Additionally, there is no detectable backlash in either direction. The walker performance has been extensively tested by using a position‐sensitive proximitor probe. Tests have been done between 77 and 300 K. However, we project that the walker will be able to operate at temperatures as low as 4 K. This walker system has shown extremely reliable performance in a UHV environment for use with scanning tunneling microscopy and has been especially useful for cross‐sectional scanning tunneling microscopy and spectroscopy studies of semiconductor hetero‐ and homostructures. We show one example of results on the (AlGa)As/GaAs heterostructure system.

Structural and electronic properties of Al0.3Ga0.7As/GaAs heterojunction and GaAs pn junction systems are investigated by cross‐sectional scanning tunneling microscopy and spectroscopy (XSTM/S). The cross‐sectional samples were prepared by passivating ex situ with a sulfide [(NH4)2S] solution and were transferred into an ultra‐high vacuum system for STM/S studies. It is found that passivated samples are advantageous for the measurements of scanning tunneling spectroscopy. The STM/S results and the experimental details are reported.

We present a cross-sectional scanning tunneling microscopy (STM) study of heterogeneous-droplet-epitaxy (HDE)-grown InGaAs quantum dots (QDs). We found that the structural properties of HDE-grown QDs such as size, shape, etc., are quite different from that of Stranski–Krastanov (SK)-grown InGaAs QDs. HDE-grown InGaAs QDs exhibit a reverse trapezoidal shape, opposite to the SK-grown QDs. In addition, the In concentration within individual HDE QDs is rather uniform, contrary to the case in SK QDs. These HDE QDs also show large size fluctuation. However, we found that there is a size dependence in the In concentration within the QD—the larger QD has lower In concentration, suggesting a self-compensation effect which gives rise to a sharp photoluminescence linewidth.

Electron transport measurements on thin films reveal whether two-dimensional metals support macroscopic supercurrents.

We systematically address superconductivity of Pb nano-islands with different thicknesses and lateral sizes via a scanning tunneling microscopy/spectroscopy (STM/STS). Reduction of the superconducting gap (\ensuremath{\Delta}) is observed even when the island is larger than the bulk coherence length (\ensuremath{\xi}) and becomes very fast below \ensuremath{\sim}50-nm lateral size. The suppression of \ensuremath{\Delta} with size depends to a good approximation only on the volume of the island and is independent of its shape. Theoretical analysis indicates that the universal quenching behavior is primarily manifested by the mean number of electronic orbitals within the pairing energy window.

By using the prototypical ${\mathrm{Al}}_{0.3}$${\mathrm{Ga}}_{0.7}$As/GaAs system, we demonstrate the unique capability of scanning tunneling microscopy to directly map out detailed electronic structure across heterojunctions. Three novel applications are reported: (1) precise determination of band offsets, (2) measurement of asymmetrical electronic transition widths between the normal and inverted interfaces, and (3) mapping of multiple-valley band structures. Important implications of these results are discussed.

In this article scanning tunneling microscopy studies of atomic motion of defects on the GaAs(110) surface at room temperature are reported. The slow dynamic behavior of vacancies and As adatoms can be resolved within a time scale of about one minute. The vacancies and As adatoms are observed to move preferably along the [110] direction, corresponding to a large anisotropy in the two‐dimensional diffusion coefficients. We attribute such a large anisotropy to the energetics of atom migration in the presence of the GaAs(110) surface zigzag chain structure. Observations of As adatoms also indicate that they form linear chains along the zigzag chain. Finally, the fast dynamic behavior of atomic flip‐flop action at a dual divacancy is discussed. The characteristic flip‐flop time is estimated from the scanning speed and sampling frequency to be on the order of milliseconds.

In the absence of tip-sample mechanical interaction, graphite images show very different behaviors from those of typical images. Contrary to the current interpretation of site asymmetry, where protrusions in images are considered as B-site atoms, we have found that one can selectively image A- or B-site atoms depending on the bias polarity. Furthermore, the corrugation amplitude is extremely small. If measurements are performed under tip-sample mechanical interaction, one observes larger corrugations and in-phase images at opposite polarities. A new interpretation of site asymmetry is proposed.

Semiconductor nanostructures such as quantum dots (QDs) have offered unique opportunities to investigate quantum optical effects in solid-state systems. These include quantum interference, Rabi oscillations, as well as photon antibunching, and were previously observable only in isolated atoms or ions. In addition, QDs can be integrated into optical microcavities, making them attractive for applications in quantum information processing and high efficiency quantum light sources. Despite much progress towards these goals, one area that was little explored is coherent control of such solid-state quantum emitters in cavities. The main technical hurdle lies in overcoming the laser background scattering. By using a sample structure in which QDs are embedded in a planar Fabry-Perot cavity and by using an orthogonal excitation geometry, we have achieved a nearly complete elimination of laser background scattering. This in turn allows us to show resonantly controlled light emission of quantum dots in the cavity including (a) Rabi flopping using pulse control, (b) direct observation of Mollow triplets in the frequency domain, and (c) simultaneously measured first-order and second order photon-photon correlations.

Using scanning tunneling microscopy and spectroscopy, we demonstrate that the adsorption of a minute amount of Cs on a Pb mesa grown in the quantum regime can induce dramatic morphological changes of the mesa, characterized by the appearance of populous monatomic-layer-high Pb nano-islands on top of the mesa. The edges of the Pb nano-islands are decorated with Cs adatoms, and the nano-islands preferentially nucleate and grow on the quantum mechanically unstable regions of the mesa. Furthermore, first-principles calculations within density functional theory show that the Pb atoms forming these nano-islands were expelled by the adsorbed Cs atoms via a kinetically accessible place exchange process when the Cs atoms alloyed into the top layer of the Pb mesa., Comment: Corrected misc. typos