EJS-01 Valley-selective optical Stark effect in monolayer WS2
Edbert J. Sie, James W. McIver, Yi-Hsien Lee, Liang Fu, Jing Kong, and Nuh Gedik
Breaking space–time symmetries in two-dimensional crystals can markedly influence their macroscopic electronic properties. Monolayer transition metal dichalcogenides (TMDs) are prime examples where the intrinsically broken crystal inversion symmetry permits the generation of valley-selective electron populations, even though the two valleys are energetically degenerate, locked by time-reversal symmetry. Lifting the valley degeneracy in these materials is of great interest because it would allow for valley-specific band engineering and offer additional control in valleytronic applications. Although applying a magnetic field should, in principle, accomplish this task, experiments so far have not shown valley-selective energy level shifts in fields accessible in the laboratory. Here, we show the first direct evidence of lifted valley degeneracy in the monolayer TMD WS2. By applying intense circularly polarized light, which breaks time-reversal symmetry, we demonstrate that the exciton level in each valley can be selectively tuned by as much as 18 meV through the optical Stark effect. These results offer a new way to control the valley degree of freedom, and may provide a means to realize new Floquet topological phases in two-dimensional TMDs.
Nature Materials, 14, 290-294 (2015)
See also the following related articles from the MIT News Office and IEEE Spectrum.

JONP-01 Room-temperature sub-band gap optoelectronic response of hyperdoped silicon
Jonathan P. Mailoa, Austin J. Akey, Christie B. Simmons, David Hutchinson, Jay Mathews, Joseph T. Sullivan, Daniel Recht, Mark T. Winkler, James S. Williams, Jeffrey M. Warrender, Peter D. Persans, Michael J. Aziz, and Tonio Buonassisi
Room-temperature infrared sub-band gap photoresponse in silicon is of interest for telecommunications, imaging and solid-state energy conversion. Attempts to induce infrared response in silicon largely centred on combining the modification of its electronic structure via controlled defect formation (for example, vacancies and dislocations) with waveguide coupling, or integration with foreign materials. Impurity-mediated sub-band gap photoresponse in silicon is an alternative to these methods but it has only been studied at low temperature. Here we demonstrate impurity-mediated room-temperature sub-band gap photoresponse in single-crystal silicon-based planar photodiodes. A rapid and repeatable laser-based hyperdoping method incorporates supersaturated gold dopant concentrations on the order of 1020 cm-3 into a single-crystal surface layer ~150 nm thin. We demonstrate room-temperature silicon spectral response extending to wavelengths as long as 2,200 nm, with response increasing monotonically with supersaturated gold dopant concentration. This hyperdoping approach offers a possible path to tunable, broadband infrared imaging using silicon at room temperature.
Nature Communications, 5:3011 (2014)
See also a few related articles from the
MIT News Office, Novus Light Technologies Today, Photonics Media, as well as the following research highlight in Nature Nanotechnology. An additional link to the paper is given here.

JONP-02 A 2-terminal perovskite/silicon multijunction solar cell enabled by a silicon tunnel junction
Jonathan P. Mailoa, Colin D. Bailie, Eric C. Johlin, Eric T. Hoke, Austin J. Akey, William H. Nguyen, Michael D. McGehee, and Tonio Buonassisi
With the advent of efficient high-bandgap metal-halide perovskite photovoltaics, an opportunity exists to make perovskite/silicon tandem solar cells. We fabricate a monolithic tandem by developing a silicon-based interband tunnel junction that facilitates majority-carrier charge recombination between the perovskite and silicon sub-cells. We demonstrate a 1 cm2 2-terminal monolithic perovskite/silicon multijunction solar cell with a VOC as high as 1.65 V. We achieve a stable 13.7% power conversion efficiency with the perovskite as the current-limiting sub-cell, and identify key challenges for this device architecture to reach efficiencies over 25%.
Applied Physics Letters, 106, 121105 (2015)
See also a few related articles from the
Economist, MIT News Office, Scientific American, IEEE Spectrum, PV Magazine, and Solar Reviews. An additional link to the paper is given here.

NOVA-01 Transcriptional divergence and conservation of human and mouse erythropoiesis
Novalia Pishesha, Prathapan Thiru, Jiahai Shi, Jennifer C. Eng, Vijay G. Sankaran, and Harvey F. Lodish
Mouse models have been used extensively for decades and have been instrumental in improving our understanding of mammalian erythropoiesis. Nonetheless, there are several examples of variation between human and mouse erythropoiesis. We performed a comparative global gene expression study using data from morphologically identical stage-matched sorted populations of human and mouse erythroid precursors from early to late erythroblasts. Induction and repression of major transcriptional regulators of erythropoiesis, as well as major erythroid-important proteins, are largely conserved between the species. In contrast, at a global level we identified a significant extent of divergence between the species, both at comparable stages and in the transitions between stages, especially for the 500 most highly expressed genes during development. This suggests that the response of multiple developmentally regulated genes to key erythroid transcriptional regulators represents an important modification that has occurred in the course of erythroid evolution. In developing a systematic framework to understand and study conservation and divergence between human and mouse erythropoiesis, we show how mouse models can fail to mimic specific human diseases and provide predictions for translating findings from mouse models to potential therapies for human disease.
PNAS, vol.111, No.11, 4103-4108 (2014)

RZP-01 Generation of artificial acoustic-gravity waves and traveling ionospheric disturbances in HF heating experiments
Rezy Pradipta, Min-Chang Lee, Joel A. Cohen, and Brenton J. Watkins
We report the results of our ionospheric HF heating experiments to generate artificial acoustic-gravity waves (AGW) and traveling ionospheric disturbances (TID), which were conducted at the High-frequency Active Auroral Research Program facility in Gakona, Alaska. Based on the data from UHF radar, GPS total electron content, and ionosonde measurements, we found that artificial AGW/TID can be generated in ionospheric modification experiments by sinusoidally modulating the power envelope of the transmitted O-mode HF heater waves. In this case, the modulation frequency needs to be set below the characteristic Brunt-Vaisala frequency at the relevant altitudes. We avoided potential contamination from naturally-occurring AGW/TID of auroral origin by conducting the experiments during geomagnetically quiet time period. We determine that these artificial AGW/TID propagate away from the edge of the heated region with a horizontal speed of approximately 160 m/s.
Earth Moon and Planets, Special Issue on Ionospheric Modification (2015)

RZP-02 GPS observation of continent-size traveling TEC pulsations at the start of geomagnetic storms
Rezy Pradipta, Cesar E. Valladares, and Patricia H. Doherty
We report our experimental observation of continent-size traveling plasma disturbances using GPS measurements of total electron content (TEC) over the North American sector. These plasma disturbances occurred at the beginning of geomagnetic storms, immediately after the shock arrived, and prior to the appearance of large-scale traveling ionospheric disturbances (LSTIDs) from the auroral region. Specifically, these supersize TEC perturbations were observed when the interplanetary magnetic field Bz was oscillating between northward and southward directions. They were found to propagate zonally with a propagation speed of 2-3 km/s. We interpret these TEC pulsations as ion drift waves in the magnetosphere/plasmasphere that propagate azimuthally inside the GPS orbit.
J. Geophys. Res. Space Physics, 119, 6913-6924 (2014)

RHEA-01 Surrogate models and mixtures of experts in aerodynamic performance prediction for aircraft mission analysis
Rhea P. Liem, Charles A. Mader, and Joaquim R.R.A. Martins
The accurate evaluation of aircraft fuel burn over a complete mission is computationally expensive and may require millions of aerodynamic performance evaluations. Thus, it is advantageous to use surrogate models as approximations of high-fidelity aerodynamic or aerostructural models. Conventional surrogate models, such as the radial basis function and kriging, cannot model these functions accurately, especially in the transonic regime. To address this issue, we explore several ways to improve the accuracy of surrogate models. First, we employ an adaptive sampling algorithm to complement a traditional space-filling algorithm. Second, we improve the kriging surrogate performance by including gradient information in the interpolation (a form of gradient-enhanced kriging -- GEK) and by introducing a known trend in the global model component (kriging with a trend). Lastly, we propose a mixture of experts (ME) approach, which is based on the divide-and-conquer principle. We validate our surrogate models using aerodynamic data for conventional and unconventional aircraft configurations, and we assess their performance in predicting the mission ranges by analyzing ten mission profiles. Our results show that the proposed ME approach is superior to the traditional models. Using a mixture of GEK models to approximate the drag coefficients gives approximation errors of less than 5% with fewer than 150 samples, whereas the adaptive sampling fails to converge when training a global model. However, when we have a simple function profile, such as the lift and moment coefficients, using a conventional surrogate model is more efficient than an ME model, because of the added computational complexity in the latter. The range estimation errors associated with the ME models are less than 2% for all the benchmark mission profiles considered, whereas some traditional models yield errors as high as 20–80%. We thus conclude that the ME technique is both necessary and sufficient for modeling the aerodynamic coefficients for surrogate-based mission analysis.
Aerospace Science and Technology, 43, 126-151 (2015)

RHEA-02 Multimission aircraft fuel-burn minimization via multipoint aerostructural optimization
Rhea P. Liem, Gaetan K.W. Kenway, and Joaquim R.R.A. Martins
Aerodynamic shape and aerostructural design optimizations that maximize the performance at a single flight condition may result in designs with unacceptable off-design performance. While considering multiple flight conditions in the optimization improves the robustness of the designs, there is a need to develop a way of choosing the flight conditions and their relative emphases such that multipoint optimizations reflect the true objective function. In addition, there is a need to consider uncertain missions and flight conditions. To address this, a new strategy to formulate multipoint design optimization problems is developed that can maximize the aircraft performance over a large number of different missions. This new strategy is applied to the high-fidelity aerostructural optimization of a long-range twin-aisle aircraft with the objective of minimizing the fuel burn over all the missions it performs in one year. This is accomplished by determining 25 flight conditions and their respective emphases on drag and structural weight that emulate the fuel-burn minimization for over 100,000 missions. The design optimization is based on the computational fluid dynamics of a full aircraft configuration coupled to a detailed finite element model of the wing structure, enabling the simultaneous optimization of wing aerodynamic shape and structural sizing leading to optimal static aeroelastic tailoring. A coupled adjoint method in conjunction with a gradient-based optimizer enable optimization with respect to 311 design variables subject to 152 constraints. Given the high computational cost of the aerostructural analysis, kriging models are used to evaluate the multiple missions. The results show that the multipoint optimized design reduced the total fuel burn by 6.6%, while the single-point optimization reduced it by only 1.7%. This capability to analyze large numbers of flight conditions and missions and to reduce the multimission problem to a multipoint problem could be used with a few modifications to minimize the expected value of any objective function given the probability density functions of the flight conditions.
AIAA Journal, vol.53, No.1, 104-122 (2015)
A complementary conference paper can be found