California Institute of Technology

About: California Institute of Technology is a education organization based out in Pasadena, California, United States. It is known for research contribution in the topics: Galaxy & Redshift. The organization has 57649 authors who have published 146691 publications receiving 8620287 citations. The organization is also known as: Caltech & Cal Tech.

Materials for thermoelectric energy conversion

Abstract: The field of thermoelectric energy conversion is reviewed from both a theoretical and an experimental standpoint. The basic theory is introduced and the thermodynamic and solid state views are compared. An overview of the development of thermoelectric materials is presented with particular emphasis being placed on the most recent developments in high-temperature semiconductors. A number of possible device applications are discussed and the successful use and suitability of these devices for space power is manifest.

Nitrogenase MoFe-protein at 1.16 A resolution: a central ligand in the FeMo-cofactor.

Abstract: A high-resolution crystallographic analysis of the nitrogenase MoFe-protein reveals a previously unrecognized ligand coordinated to six iron atoms in the center of the catalytically essential FeMo-cofactor. The electron density for this ligand is masked in structures with resolutions lower than 1.55 angstroms, owing to Fourier series termination ripples from the surrounding iron and sulfur atoms in the cofactor. The central atom completes an approximate tetrahedral coordination for the six iron atoms, instead of the trigonal coordination proposed on the basis of lower resolution structures. The crystallographic refinement at 1.16 angstrom resolution is consistent with this newly detected component being a light element, most plausibly nitrogen. The presence of a nitrogen atom in the cofactor would have important implications for the mechanism of dinitrogen reduction by nitrogenase.

An optical lattice clock with accuracy and stability at the 10 −18 level

Abstract: In the search for stable and accurate atomic clocks, many-atom lattice clocks have shown higher precision than clocks based on single trapped ions, but have been less accurate; here, a stable many-atom clock is demonstrated that has accuracy better than single-ion clocks. Whether for the definition of SI units, testing the laws of physics or for applications yet to be dreamt of, scientists will always want more stability and more accuracy in their atomic clocks. Many-atom lattice clocks have achieved better precision than clocks based on single trapped ions, but their accuracy has so far been relatively poor. This study from the National Institute of Standards and Technology (NIST) demonstrates a many-atom clock that achieves better accuracy than single-ion-based clocks, and at the same time reduces the required measurement time by two orders of magnitude. Based on thousands of neutral strontium atoms trapped in a laser beam, this new 'optical lattice' clock has the stability, reproducibility and accuracy that make it a prime contender for consideration as a primary standard. It would neither gain nor lose one second in about 5 billion years — although the Earth is unlikely to last that long. Progress in atomic, optical and quantum science1,2 has led to rapid improvements in atomic clocks. At the same time, atomic clock research has helped to advance the frontiers of science, affecting both fundamental and applied research. The ability to control quantum states of individual atoms and photons is central to quantum information science and precision measurement, and optical clocks based on single ions have achieved the lowest systematic uncertainty of any frequency standard3,4,5. Although many-atom lattice clocks have shown advantages in measurement precision over trapped-ion clocks6,7, their accuracy has remained 16 times worse8,9,10. Here we demonstrate a many-atom system that achieves an accuracy of 6.4 × 10−18, which is not only better than a single-ion-based clock, but also reduces the required measurement time by two orders of magnitude. By systematically evaluating all known sources of uncertainty, including in situ monitoring of the blackbody radiation environment, we improve the accuracy of optical lattice clocks by a factor of 22. This single clock has simultaneously achieved the best known performance in the key characteristics necessary for consideration as a primary standard—stability and accuracy. More stable and accurate atomic clocks will benefit a wide range of fields, such as the realization and distribution of SI units11, the search for time variation of fundamental constants12, clock-based geodesy13 and other precision tests of the fundamental laws of nature. This work also connects to the development of quantum sensors and many-body quantum state engineering14 (such as spin squeezing) to advance measurement precision beyond the standard quantum limit.

Slow Electrons in a Polar Crystal

Abstract: A variational principle is developed for the lowest energy of a system described by a path integral. It is applied to the problem of the interaction of an electron with a polarizable lattice, as idealized by Frohlich. The motion of the electron, after the phonons of the lattice field are eliminated, is described as a path integral. The variational method applied to this gives an energy for all values of the coupling constant. It is at least as accurate as previously known results. The effective mass of the electron is also calculated, but the accuracy here is difficult to judge.

Temporal structure in neuronal activity during working memory in macaque parietal cortex

Abstract: Many cortical structures have elevated firing rates during working memory, but it is not known how the activity is maintained. To investigate whether reverberating activity is important, we studied the temporal structure of local field potential (LFP) activity and spiking from area LIP in two awake macaques during a memory-saccade task. Using spectral analysis, we found spatially tuned elevated power in the gamma band (25-90 Hz) in LFP and spiking activity during the memory period. Spiking and LFP activity were also coherent in the gamma band but not at lower frequencies. Finally, we decoded LFP activity on a single-trial basis and found that LFP activity in parietal cortex discriminated between preferred and anti-preferred direction with approximately the same accuracy as the spike rate and predicted the time of a planned movement with better accuracy than the spike rate. This finding could accelerate the development of a cortical neural prosthesis.