Gravitational-wave observatories; phase-change materials; new STEM technique.
India has approved the construction of the world’s third gravitational-wave observatory. This facility will replicate the two Laser Interferometer Gravitational-wave Observatories (LIGOs) in the United States, which recently detected the world’s first gravitational waves.
The Indian project, dubbed LIGO-India, is expected to go online in 2023. The effort brings together three of India’s top research institutes–the Inter-University Centre for Astronomy and Astrophysics (IUCAA); the Raja Ramanna Centre for Advanced Technology (RRCAT); and the Institute for Plasma Research (IPR). The project is managed by the Department of Atomic Energy and the Department of Science and Technology.
The system will be an exact copy of the twin LIGO Observatories, which are located at Hanford, Wash. and Livingston, La. These facilities are operated by the California Institute of Technology and the Massachusetts Institute of Technology (MIT).
Researchers from the LIGO Observatories made big headlines in early February, when they observed gravitational waves. Gravitational waves are distortions or ripples in the fabric of space-time. They are caused by a violent action in the universe. The discovery confirmed a major prediction of Albert Einstein’s 1915 general theory of relativity.
After observing the gravitational waves in February, researchers concluded that they were produced during the final fraction of a second of the merger of two black holes. This, in turn, produced a single, more massive spinning black hole. Researchers estimated that the black holes for this event were about 29 and 36 times the mass of the sun. The event took place 1.3 billion years ago.
Meanwhile, the two LIGO Observatories in the United States are identical. At each observatory, a 4-km long L-shaped interferometer uses laser light split into two beams. The beams travel back and forth down the arms, which are four-foot diameter tubes kept under a near-perfect vacuum.
The beams are used to monitor the distance between mirrors positioned at the ends of the arms. According to Einstein’s theory, the distance between the mirrors will change by an infinitesimal amount when a gravitational wave passes by the detector.
David Reitze, executive director of LIGO and a Caltech research professor, said the precision achieved by LIGO is analogous to being able to measure the distance between our solar system and the sun’s nearest neighbor Alpha Centauri—about 4.4 light-years away—accurately to within a few microns.
The U.S.-based LIGO have built an exact copy of an instrument for the LIGO-India Observatory. LIGO will provide Indian researchers with the components and training to build and run the new detector.
The University of Tsukuba and others used a free electron laser (XFEL) to observe the atomic motion of phase-change memory material in real time and at sub-nanometer resolutions.
Specifically, researchers observed the atomic rearrangements at picosecond timescales of a thin- film, DVD phase-change recording material, based on Ge-Sb-Te.
Phase-change memory has been around for years, but the technology has not taken off. Suddenly, though, phase-change memory is hot. Not long ago, Intel and Micron Technology rolled out a technology called 3D XPoint, a next-generation memory type based on the characteristics of ReRAM and phase-change memory.
Phase-change memory is a chalcogen compound. The materials exhibit large changes in properties, such as optical reflectivity and electrical resistance between crystalline and amorphous states.
To date, the typical time for recording of information using these two states is a nanosecond. But in theory, the transitions can occur on picosecond time scales. If researchers can look at the transitions on this scale, they can better understand phase-change memory.
To advance this goal, researchers used SACLA, a free-electron X-ray laser facility in Japan. In this research project, a pulsed laser was used to excite a Ge2Sb2Te5 epitaxial film. X-ray pulses from the XFEL were used to record the subsequent transient changes.
“These observations revealed that immediately after excitation, bond breaking induced by the excited state resulted in non-thermal local structural rearrangements within a few picoseconds followed by warming of the lattice after 4 picoseconds,” according to researchers on the University of Tsukuba’s Web site.
“The formation of the heretofore unobserved transient structural state was followed by a 2 picometer (= 10-12m) change in lattice spacing after 20 picoseconds as revealed by X-ray diffraction,” according to researchers. “The transient state was observed to persist for over 100 picoseconds, but was found to complete revert to the initial (ground) state after 1.8 nanoseconds.”
The results suggest that the thermally-induced nanosecond order transition of conventional phase-change memory may be used to speed up memory switching to picosecond time scales.
New STEM technique
The scanning transmission electron microscope (STEM) uses a focused beam of electrons to look at tiny features and complex materials.
The problem? The STEM is a destructive technique. So for some specimens, imaging with a STEM requires a very low electron dose.
Seeking to solve the problem, the Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) has developed a new STEM-based imaging technique.
The technique, called MIDI-STEM, stands for matched illumination and detector interferometry STEM. It combines STEM with an optical device called a phase plate. This, in turn, modifies the alternating peak-to-trough, wave-like properties of the electron beam.
It allows changes in a material to be measured, even revealing materials that would be invisible in traditional STEM imaging. “The MIDI-STEM method provides hope for seeing structures with a mixture of heavy and light elements, even when they are bunched closely together,” said Colin Ophus, a project scientist at Berkeley Lab’s Molecular Foundry, on the organization’s Web site.