Quantum Monte Carlo Study of High Pressure Solid Molecular Hydrogen

We use the diffusion quantum Monte Carlo (DMC) method to calculate the ground state phase diagram of solid molecular hydrogen and examine the stability of the most important insulating phases relative to metallic crystalline molecular hydrogen. We develop a new method to account for finite-size errors by combining the use of twist-averaged boundary conditions with corrections obtained using the Kwee-Zhang-Krakauer (KZK) functional in density functional theory. To study band-gap closure and find the metallization pressure, we perform accurate quasi-particle many-body calculations using the $GW$ method. In the static approximation, our DMC simulations indicate a transition from the insulating Cmca-12 structure to the metallic Cmca structure at around 375 GPa. The $GW$ band gap of Cmca-12 closes at roughly the same pressure. In the dynamic DMC phase diagram, which includes the effects of zero-point energy, the Cmca-12 structure remains stable up to 430 GPa, well above the pressure at which the $GW$ band gap closes. Our results predict that the semimetallic state observed experimentally at around 360 GPa [Phys. Rev. Lett. {\bf 108}, 146402 (2012)] may correspond to the Cmca-12 structure near the pressure at which the band gap closes. The dynamic DMC phase diagram indicates that the hexagonal close packed $P6_3/m$ structure, which has the largest band gap of the insulating structures considered, is stable up to 220 GPa. This is consistent with recent X-ray data taken at pressures up to 183 GPa [Phys. Rev. B {\bf 82}, 060101(R) (2010)], which also reported a hexagonal close packed arrangement of hydrogen molecules

The Evaluation of Repetitive Overhead Movements in Heating Ventilation and Air Conditioning Technicians

Air conditioning technicians are the most non-fatally injured amongst construction workers. It has been identified that the shoulder is one of joints most commonly affected areas in industrial workers. While repetitive overhead movements have been evaluated in construction workers, it has not been described in air conditioning (AC) technicians. PURPOSE: To describe repetitive arm movement performed by AC technicians using inertial measurement unit (IMU) system, throughout a full work day. METHODS: An IMU is a small portable motion detection monitor that combines information from triaxial accelerometers, gyroscopes, and magnetometers to track translational movements. Six AC technicians participated on six separate days. Four IMU monitors were secured 2cm below the spinous process of the 7th cervical vertebrae (C7), the flat broad portion of the acromion process on the dominant arm, the posterior aspect of the dominant arm just superior of the elbow, and on the dorsal aspect of the dominant-side wrist to track multi-axial joint kinematics. An investigator then accompanied each participant to their work place to capture data and log when the participants were performing work-specific (e.g., mounting ceiling vents, installing and repairing air conditioning units), tasks and non-work-specific tasks. The arm activity was described separately and compared between times when the technicians performed work-specific and non-work specific tasks. Wilcoxon signed rank tests were used to compare the variables between times spent performing work-specific tasks and non-work-specific tasks. RESULTS: It was found that they spent 3.9±1.4 hours (41.6±15.6 %) performing work-specific tasks and 4.9±1.1 hours (53.8±11.9 %) performing non-work-specific tasks. The participants’ arm were elevated above 60° for 1.7±0.7 hours (18.5±8.0 %), and above 90° for 0.4±0.2 hours (4.2±2.6 %). It was also found that the participants spent 7.1 – 18.5% of their work shift with arm above 60° for more than 5 seconds and 1.0 – 4.9% above 90° for more than 5 seconds. CONCLUSION: These findings were similar to what has been reported in previous studies on electricians. Previous studies on manual laborers identified arm exposure as a risk factor for shoulder musculoskeletal disorders. Specifically, the greater time spent with arm maintained above 60° for more than 5 seconds has been correlated with shoulder pain. Our group of participants spent 7.1 – 18.5% of the work shift maintaining arm above 60° for more than 5 seconds, which is comparable to the ranges reported in a previous longitudinal study (2 – 21%). Considering that 27% of the participants in the previous study developed shoulder pain within 2.5 years of the follow up period, similar proportion of AC technicians may be at risk for developing pain and additional dysfunctional pathology

Unconventional phase III of high-pressure solid hydrogen

We reassess the phase diagram of high-pressure solid hydrogen using mean-field and many-body wave function based approaches to determine the nature of phase III of solid hydrogen. To discover the best candidates for phase III, density functional theory calculations within the meta-generalized gradient approximation by means of the strongly constrained and appropriately normed (SCAN) semilocal density functional are employed. We study eleven molecular structures with different symmetries, which are the most competitive phases, within the pressure range of 100 to 500~GPa. The SCAN phase diagram predicts that the $C2/c-24$ and $P6_122-36$ structures are the best candidates for phase III with an energy difference of less than 1~meV/atom. To verify the stability of the competitive insulator structures of $C2/c-24$ and $P6_122-36$, we apply the diffusion Monte Carlo (DMC) method to optimise the percentage $\alpha$ of exact-exchange in the trial many-body wave function. We found that the optimised $\alpha$ equals to $40 \%$, and denote the corresponding exchange and correlation functional as PBE1. The energy gain with respect to the well-known hybrid functional PBE0, where $\alpha = 25\%$, varies with density and structure. The PBE1-DMC enthalpy-pressure phase diagram predicts that the $P6_122-36$ structure is stable up to 210~GPa, where it transforms to the $C2/c-24$. Hence, we predict that the phase III of high-pressure solid hydrogen is polymorphic.Comment: Accepted for publication in Phys. Rev.

Solving the time- and frequency-multiplexed problem of constrained radiofrequency induced hyperthermia

Targeted radiofrequency (RF) heating induced hyperthermia has a wide range of applications, ranging from adjunct anti-cancer treatment to localized release of drugs. Focal RF heating is usually approached using time-consuming nonconvex optimization procedures or approximations, which significantly hampers its application. To address this limitation, this work presents an algorithm that recasts the problem as a semidefinite program and quickly solves it to global optimality, even for very large (human voxel) models. The target region and a desired RF power deposition pattern as well as constraints can be freely defined on a voxel level, and the optimum application RF frequencies and time-multiplexed RF excitations are automatically determined. 2D and 3D example applications conducted for test objects containing pure water (r(target) = 19 mm, frequency range: 500–2000 MHz) and for human brain models including brain tumors of various size (r(1) = 20 mm, r(2) = 30 mm, frequency range 100–1000 MHz) and locations (center, off-center, disjoint) demonstrate the applicability and capabilities of the proposed approach. Due to its high performance, the algorithm can solve typical clinical problems in a few seconds, making the presented approach ideally suited for interactive hyperthermia treatment planning, thermal dose and safety management, and the design, rapid evaluation, and comparison of RF applicator configurations

HMDB: A Large Video Database for Human Motion Recognition

With nearly one billion online videos viewed everyday, an emerging new frontier in computer vision research is recognition and search in video. While much effort has been devoted to the collection and annotation of large scalable static image datasets containing thousands of image categories, human action datasets lag far behind. Current action recognition databases contain on the order of ten different action categories collected under fairly controlled conditions. State-of-the-art performance on these datasets is now near ceiling and thus there is a need for the design and creation of new benchmarks. To address this issue we collected the largest action video database to-date with 51 action categories, which in total contain around 7,000 manually annotated clips extracted from a variety of sources ranging from digitized movies to YouTube. We use this database to evaluate the performance of two representative computer vision systems for action recognition and explore the robustness of these methods under various conditions such as camera motion, viewpoint, video quality and occlusion.United States. Defense Advanced Research Projects Agency. Information Processing Techniques OfficeUnited States. Defense Advanced Research Projects Agency. System Science Division. Defense Sciences OfficeNational Science Foundation (U.S.) (NSF-0640097)National Science Foundation (U.S.) (NSF-0827427)United States. Air Force Office of Scientific Research (FA8650-05- C-7262)Adobe SystemsKing Abdullah University of Science and TechnologyNEC ElectronicsSony CorporationEugene McDermott FoundationBrown University. Center for Computing and VisualizationRobert J. and Nancy D. Carney Fund for Scientific InnovationUnited States. Defense Advanced Research Projects Agency (DARPA-BAA-09-31)United States. Office of Naval Research (ONR-BAA-11-001)Ministry of Science, Research and the Arts of Baden Württemberg, German

Top-down Attention Recurrent VLAD Encoding for Action Recognition in Videos

Most recent approaches for action recognition from video leverage deep architectures to encode the video clip into a fixed length representation vector that is then used for classification. For this to be successful, the network must be capable of suppressing irrelevant scene background and extract the representation from the most discriminative part of the video. Our contribution builds on the observation that spatio-temporal patterns characterizing actions in videos are highly correlated with objects and their location in the video. We propose Top-down Attention Action VLAD (TA-VLAD), a deep recurrent architecture with built-in spatial attention that performs temporally aggregated VLAD encoding for action recognition from videos. We adopt a top-down approach of attention, by using class specific activation maps obtained from a deep CNN pre-trained for image classification, to weight appearance features before encoding them into a fixed-length video descriptor using Gated Recurrent Units. Our method achieves state of the art recognition accuracy on HMDB51 and UCF101 benchmarks.Comment: Accepted to the 17th International Conference of the Italian Association for Artificial Intelligenc

In vivo potassium MRI of the human heart

PURPOSE: Potassium ions (K(+)) play a critical role in cardiac electrophysiology, and changes in their concentration reflect pathophysiological processes related to cardiovascular diseases. Here, we investigated the feasibility of in vivo (39)K MRI of the human heart. To achieve this, we developed, evaluated, and applied a (39)K/(1)H RF coil, which is tailored for (39)K MRI of human heart at 7.0T. METHODS: The performance of the (39)K/(1)H RF coil was evaluated by electromagnetic field and specific absorption ratio simulations using 2 (male/female) human voxel models. The RF coil was evaluated at the bench and applied in an in vivo proof-of-principle study involving 7 healthy volunteers. The experiments were performed using a 7.0T whole-body MR system in conjunction with a 3D density-adapted projection reconstruction imaging technique. RESULTS: For in vivo (39)K MRI of the human heart, a nominal spatial resolution of 14.5 × 14.5 × 14.5 mm(3) within a total scan time of 30 min was achieved. The average SNR within the heart was 9.6 ± 2.4. CONCLUSION: This work validates the design of a (39)K/(1)H RF coil for cardiac MR at 7.0T and demonstrates for the first time in vivo the feasibility of (39)K MRI of the human heart

Continuous lasing for perovskites

Optically generated local phase changes in methylammonium lead iodide produce a transient quantum well structure with robust optical gain. The result is a perovskite laser that supports continuous-wave lasing under optical pumping.PostprintNon peer reviewe