31 research outputs found
Creating and Probing Electron Whispering Gallery Modes in Graphene
Designing high-finesse resonant cavities for electronic waves faces
challenges due to short electron coherence lengths in solids. Previous
approaches, e.g. the seminal nanometer-sized quantum corrals, depend on careful
positioning of adatoms at clean surfaces. Here we demonstrate an entirely
different approach, inspired by the peculiar acoustic phenomena in whispering
galleries. Taking advantage of graphene's unique properties, namely
gate-tunable light-like carriers, we create Whispering Gallery Mode (WGM)
resonators defined by circular pn-junctions, induced by a scanning tunneling
probe. We can tune the resonator size and the carrier concentration under the
probe in a back-gated graphene device over a wide range, independently and in
situ. The confined modes, revealed through characteristic resonances in the
tunneling spectrum, originate from Klein scattering at pn junction boundaries.
The WGM-type confinement and resonances are a new addition to the quantum
electron-optics toolbox, paving the way to real-world electronic lenses and
resonators
Quantum Simulation of an Extended Fermi-Hubbard Model Using a 2D Lattice of Dopant-based Quantum Dots
The Hubbard model is one of the primary models for understanding the
essential many-body physics in condensed matter systems such as Mott insulators
and cuprate high-Tc superconductors. Recent advances in atomically precise
fabrication in silicon using scanning tunneling microscopy (STM) have made
possible atom-by-atom fabrication of single and few-dopant quantum dots and
atomic-scale control of tunneling in dopant-based devices. However, the complex
fabrication requirements of multi-component devices have meant that emulating
two-dimensional (2D) Fermi-Hubbard physics using these systems has not been
demonstrated. Here, we overcome these challenges by integrating the latest
developments in atomic fabrication and demonstrate the analog quantum
simulation of a 2D extended Fermi-Hubbard Hamiltonian using STM-fabricated 3x3
arrays of single/few-dopant quantum dots. We demonstrate low-temperature
quantum transport and tuning of the electron ensemble using in-plane gates as
efficient probes to characterize the many-body properties, such as charge
addition, tunnel coupling, and the impact of disorder within the array. By
controlling the array lattice constants with sub-nm precision, we demonstrate
tuning of the hopping amplitude and long-range interactions and observe the
finite-size analogue of a transition from Mott insulating to metallic behavior
in the array. By increasing the measurement temperature, we simulate the effect
of thermally activated hopping and Hubbard band formation in transport
spectroscopy. We compare the analog quantum simulations with numerically
simulated results to help understand the energy spectrum and resonant tunneling
within the array. The results demonstrated in this study serve as a launching
point for a new class of engineered artificial lattices to simulate the
extended Fermi-Hubbard model of strongly correlated materials
Better Pumps: Promoting Reliable Water Infrastructure for Everyone
Approximately 90 million people in Africa lack access to safe drinking water, despite having water infrastructure installed in their community. The India Mark II and the Afridev handpumps are among the most widely used handpumps in the world. Sadly, studies show that approximately 30% of these handpumps are non-operational due to failures of the bearings, seals, head flange, and other common components. The Better Pumps team of the Collaboratory provides engineering support for partners who are working to improve handpump sustainability. We have partnered with Tony Beers and AlignedWorks to validate a bearing test methodology for the India Mark II handpump. By modifying the loading conditions in our handpump test machine, we were able to replicate failures observed by AlignedWorks in a field trial of their bearing design. However, these modifications caused our test machine tabletop to noticeably deflect, so we made modifications to stiffen the tabletop. We partnered with Matt Schwiebert and Living Water International to test new seal designs for the India Mark II and Afridev handpumps. Seal performance data collected by the team was used to validate a new design in advance of field trials by Living Water International. We developed and performed clear cylinder testing on the seals to visualize the leak paths. A new Afridev testing apparatus is being designed to test the longevity of the Afridev bearings and seals. Test methodologies and results are reported.
Funding for this work provided by The Collaboratory for Strategic Partnerships and Applied Research.https://mosaic.messiah.edu/engr2022/1000/thumbnail.jp
Effective elastic properties of a van der Waals molecular monolayer at a metal surface
Adsorbing anthracene on a Cu(111) surface results in a wide range of complex and intriguing superstructures spanning a coverage range from 1 per 17 to 1 per 15 substrate atoms. In accompanying first-principles density-functional theory calculations we show the essential role of van der Waals interactions in estimating the variation in anthracene adsorption energy and height across the sample. We can thereby evaluate the compression of the anthracene film in terms of continuum elastic properties, which results in an effective Young\u27s modulus of 1.5 GPa and a Poisson ratio approximate to 0.1. These values suggest interpretation of the molecular monolayer as a porous material-in marked congruence with our microscopic observations
An On/Off Berry Phase Switch in Circular Graphene Resonators
The phase of a quantum state may not return to its original value after the
system's parameters cycle around a closed path; instead, the wavefunction may
acquire a measurable phase difference called the Berry phase. Berry phases
typically have been accessed through interference experiments. Here, we
demonstrate an unusual Berry-phase-induced spectroscopic feature: a sudden and
large increase in the energy of angular-momentum states in circular graphene
p-n junction resonators when a small critical magnetic field is reached. This
behavior results from turning on a -Berry phase associated with the
topological properties of Dirac fermions in graphene. The Berry phase can be
switched on and off with small magnetic field changes on the order of 10 mT,
potentially enabling a variety of optoelectronic graphene device applications
Factors Associated with Revision Surgery after Internal Fixation of Hip Fractures
Background: Femoral neck fractures are associated with high rates of revision surgery after management with internal fixation. Using data from the Fixation using Alternative Implants for the Treatment of Hip fractures (FAITH) trial evaluating methods of internal fixation in patients with femoral neck fractures, we investigated associations between baseline and surgical factors and the need for revision surgery to promote healing, relieve pain, treat infection or improve function over 24 months postsurgery. Additionally, we investigated factors associated with (1) hardware removal and (2) implant exchange from cancellous screws (CS) or sliding hip screw (SHS) to total hip arthroplasty, hemiarthroplasty, or another internal fixation device. Methods: We identified 15 potential factors a priori that may be associated with revision surgery, 7 with hardware removal, and 14 with implant exchange. We used multivariable Cox proportional hazards analyses in our investigation. Results: Factors associated with increased risk of revision surgery included: female sex, [hazard ratio (HR) 1.79, 95% confidence interval (CI) 1.25-2.50; P = 0.001], higher body mass index (fo
Dynamics and Interactions of Organic Molecules Bound to the Cu(111) Surface
The progress of modern technology is dominated by shrinking component size (with a goal of reaching angstrom scale resolution), particularly with respect to electronics and optimization of common industrial processes such as heterogeneous catalysis. Understanding of such systems lies at the upper end of applicability for first-principles calculations and existing theoretical models, and a scientific framework is needed to understand, predict, and control these systems at the molecular level. My research has focused on organic molecules adsorbed on a Cu(111) surface as model systems, studied experimentally by means of scanning tunneling microscopy (STM) as well as theoretically by density functional theory (DFT) and development of simplified explanatory models. Results of this investigation show that: 1) on Cu (111) full mono-layer coverages of acetylene undergo long-range ordering which at short-range is driven by a need to minimize localized stress induced in the upper substrate layers by adsorption while at longer ranges DFT finds an oscillatory interaction that correlates well with the surface state, 2) long-range ordered networks of anthraquinone (AQ) are found to mold the surface state into optimized quantum dots - this need for optimization under the constraint that neighbors must form H-bonds drives formation of the network at the precise size and shape observed in STM, 3) CO molecules co-adsorbed into the AQ network's pores titrate the surface state quantum dots and experience increased mobility, 4) the adsorption of anthracene modified with chalcogen linkers onto Cu (111) when viewed within a molecular orbital theory framework yields a chemical explanation for the diffusion behavior observed in STM. In combination, these observations and derived explanatory models help to characterize and quantify the fundamental physics underlying the interactions of adsorbates with one another and with the Cu (111) substrate, in a broader context acting as a model for other confined surface systems where the same kinds of interactions play a dominant role
Chemical insight from density functional modeling of molecular adsorption: Tracking the bonding and diffusion of anthracene derivatives on Cu(111) with molecular orbitals
We present a method of analyzing the results of density functional modeling of molecular adsorption in terms of an analogue of molecular orbitals. This approach permits intuitive chemical insight into the adsorption process. Applied to a set of anthracene derivates (anthracene, 9,10-anthraquinone, 9,10-dithioanthracene, and 9,10-diselenonanthracene), we follow the electronic states of the molecules that are involved in the bonding process and correlate them to both the molecular adsorption geometry and the speciesâ diffusive behavior. We additionally provide computational code to easily repeat this analysis on any system.https://doi.org/10.1063/1.490604