99 research outputs found
First-principles calculations of hyperfine interaction, binding energy, and quadrupole coupling for shallow donors in silicon
Spin qubits based on shallow donors in silicon are a promising quantum information technology with enormous potential scalability due to the existence of robust silicon-processing infrastructure. However, the most accurate theories of donor electronic structure lack predictive power because of their reliance on empirical fitting parameters, while predictive ab initio methods have so far been lacking in accuracy due to size of the donor wavefunction compared to typical simulation cells. We show that density functional theory with hybrid and traditional functionals working in tandem can bridge this gap. Our first-principles approach allows remarkable accuracy in binding energies (67 meV for bismuth and 54 meV for arsenic) without the use of empirical fitting. We also obtain reasonable hyperfine parameters (1263 MHz for Bi and 133 MHz for As) and superhyperfine parameters. We demonstrate the importance of a predictive model by showing that hydrostatic strain has much larger effect on the hyperfine structure than predicted by effective mass theory, and by elucidating the underlying mechanisms through symmetry analysis of the shallow donor charge density
Mid-infrared interference coatings with excess optical loss below 10 ppm
Low excess optical loss, combined absorption and scatter loss, is a key performance metric for any high-reflectance coating technology and is currently one of the main limiting factors for the application of optical resonators in the mid-infrared spectral region. Here we present high-reflectivity substrate-transferred single-crystal GaAs/AlGaAs interference coatings at a center wavelength of 4.54 µm with record-low excess optical loss below 10 parts per million. These high-performance mirrors are realized via a novel microfabrication process that differs significantly from the production of amorphous multilayers generated via physical vapor deposition processes. This new process enables reduced scatter loss due to the low surface and interfacial roughness, while low background doping in epitaxial growth ensures strongly reduced absorption. We report on a suite of optical measurements, including cavity ring-down, transmittance spectroscopy, and direct absorption tests to reveal the optical losses for a set of prototype mirrors. In the course of these measurements, we observe a unique polarization-orientation-dependent loss mechanism which we attribute to elastic anisotropy of these strained epitaxial multilayers. A future increase in layer count and a corresponding reduction of transmittance will enable optical resonators with a finesse in excess of 100,000 in the mid-infrared spectral region, allowing for advances in high-resolution spectroscopy, narrow-linewidth laser stabilization, and ultrasensitive measurements of various light–matter interactions
Local strain engineering in atomically thin MoS2
Tuning the electronic properties of a material by subjecting it to strain
constitutes an important strategy to enhance the performance of semiconducting
electronic devices. Using local strain, confinement potentials for excitons can
be engineered, with exciting possibilities for trapping excitons for quantum
optics and for efficient collection of solar energy. Two-dimensional materials
are able to withstand large strains before rupture, offering a unique
opportunity to introduce large local strains. Here, we study atomically thin
MoS2 layers with large local strains of up to 2.5% induced by controlled
delamination from a substrate. Using simultaneous scanning Raman and
photoluminescence imaging, we spatially resolve a direct bandgap reduction of
up to 90 meV induced by local strain. We observe a funnel effect in which
excitons drift hundreds of nanometers to lower bandgap regions before
recombining, demonstrating exciton confinement by local strain. The
observations are supported by an atomistic tight-binding model developed to
predict the effect of inhomogeneous strain on the local electronic states in
MoS2. The possibility of generating large strain-induced variations in exciton
trapping potentials opens the door for a variety of applications in atomically
thin materials including photovoltaics, quantum optics and two-dimensional
optoelectronic devices.Comment: Supp.Info. not included here, available following a link included in
the tex
Phosphorene: Fabrication, Properties and Applications
Phosphorene, the single- or few-layer form of black phosphorus, was recently
rediscovered as a twodimensional layered material holding great promise for
applications in electronics and optoelectronics. Research into its fundamental
properties and device applications has since seen exponential growth. In this
Perspective, we review recent progress in phosphorene research, touching upon
topics on fabrication, properties, and applications; we also discuss challenges
and future research directions. We highlight the intrinsically anisotropic
electronic, transport, optoelectronic, thermoelectric, and mechanical
properties of phosphorene resulting from its puckered structure in contrast to
those of graphene and transition-metal dichalcogenides. The facile fabrication
and novel properties of phosphorene have inspired design and demonstration of
new nanodevices; however, further progress hinges on resolutions to technical
obstructions like surface degradation effects and non-scalable fabrication
techniques. We also briefly describe the latest developments of more
sophisticated design concepts and implementation schemes that address some of
the challenges in phosphorene research. It is expected that this fascinating
material will continue to offer tremendous opportunities for research and
development for the foreseeable future.Comment: invited perspective for JPC
A topologically twisted index for three-dimensional supersymmetric theories
We provide a general formula for the partition function of three-dimensional (formula presented) gauge theories placed on S2
7S1 with a topological twist along S2, which can be interpreted as an index for chiral states of the theories immersed in background magnetic fields. The result is expressed as a sum over magnetic fluxes of the residues of a meromorphic form which is a function of the scalar zero-modes. The partition function depends on a collection of background magnetic fluxes and fugacities for the global symmetries. We illustrate our formula in many examples of 3d Yang-Mills-Chern-Simons theories with matter, including Aharony and Giveon-Kutasov dualities. Finally, our formula generalizes to \u3a9-backgrounds, as well as two-dimensional theories on S2 and four-dimensional theories on S2
7 T2. In particular this provides an alternative way to compute genus-zero A-model topological amplitudes and Gromov-Witten invariants
Controlling the luminescence of monolayer MoS2 based on the piezoelectric effect
We report on manipulating the stimulated emission of monolayer molybdenum disulfide (MoS2) with the piezoelectric effect. The analysis is based on quantum mechanics. The stimulated emission of this two-dimensional material has been simulated to establish the relation between the total emission rate and the energy of the photon excitation. We demonstrate that the piezoelectric-induced charges enhance the emission rate by changing the carrier concentration. It is found that the emission intensity is proportional to the carrier density in the low-density range, and eventually reaches a steady value in the high-density region. An externally applied mechanical force also leads to a change in the second harmonic generation of the monolayer MoS2
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