Crystal structureIn crystallography, crystal structure is a description of the ordered arrangement of atoms, ions, or molecules in a crystalline material. Ordered structures occur from the intrinsic nature of the constituent particles to form symmetric patterns that repeat along the principal directions of three-dimensional space in matter. The smallest group of particles in the material that constitutes this repeating pattern is the unit cell of the structure.
Bragg's lawIn physics and chemistry, Bragg's law, Wulff–Bragg's condition or Laue–Bragg interference, a special case of Laue diffraction, gives the angles for coherent scattering of waves from a large crystal lattice. It encompasses the superposition of wave fronts scattered by lattice planes, leading to a strict relation between wavelength and scattering angle, or else to the wavevector transfer with respect to the crystal lattice. Such law had initially been formulated for X-rays upon crystals.
Precession electron diffractionPrecession electron diffraction (PED) is a specialized method to collect electron diffraction patterns in a transmission electron microscope (TEM). By rotating (precessing) a tilted incident electron beam around the central axis of the microscope, a PED pattern is formed by integration over a collection of diffraction conditions. This produces a quasi-kinematical diffraction pattern that is more suitable as input into direct methods algorithms to determine the crystal structure of the sample.
Scanning electron microscopeA scanning electron microscope (SEM) is a type of electron microscope that produces images of a sample by scanning the surface with a focused beam of electrons. The electrons interact with atoms in the sample, producing various signals that contain information about the surface topography and composition of the sample. The electron beam is scanned in a raster scan pattern, and the position of the beam is combined with the intensity of the detected signal to produce an image.
Dark energyIn physical cosmology and astronomy, dark energy is an unknown form of energy that affects the universe on the largest scales. The first observational evidence for its existence came from measurements of supernovas, which showed that the universe does not expand at a constant rate; rather, the universe's expansion is accelerating. Understanding the universe's evolution requires knowledge of its starting conditions and composition. Before these observations, scientists thought that all forms of matter and energy in the universe would only cause the expansion to slow down over time.
Dark matterDark matter is a hypothetical form of matter thought to account for approximately 85% of the matter in the universe. Dark matter is called "dark" because it does not appear to interact with the electromagnetic field, which means it does not absorb, reflect, or emit electromagnetic radiation and is, therefore, difficult to detect. Various astrophysical observations - including gravitational effects which cannot be explained by currently accepted theories of gravity unless more matter is present than can be seen - imply dark matter's presence.
Dark fluidIn astronomy and cosmology, dark fluid theories attempt to explain dark matter and dark energy in a single framework. The theory proposes that dark matter and dark energy are not separate physical phenomena, nor do they have separate origins, but that they are strongly linked together and can be considered as two facets of a single fluid. At galactic scales, the dark fluid behaves like dark matter, and at larger scales its behavior becomes similar to dark energy.
Rock and rollRock and roll (often written as rock & roll, rock 'n' roll, or rock n roll) is a genre of popular music that evolved in the United States during the late 1940s and early 1950s. It originated from African-American music such as jazz, rhythm and blues, boogie-woogie, gospel, jump blues, as well as country music. While rock and roll's formative elements can be heard in blues records from the 1920s and in country records of the 1930s, the genre did not acquire its name until 1954.
Rock musicRock is a broad genre of popular music that originated as "rock and roll" in the United States in the late 1940s and early 1950s, developing into a range of different styles from the mid-1960s, particularly in the U.S. and the United Kingdom. It has its roots in 1940s and 1950s rock and roll, a style that drew directly from the blues and rhythm and blues genres of African-American music and from country music. Rock also drew strongly from genres such as electric blues and folk, and incorporated influences from jazz, classical, and other musical styles.
Zone plateA zone plate is a device used to focus light or other things exhibiting wave character. Unlike lenses or curved mirrors, zone plates use diffraction instead of refraction or reflection. Based on analysis by French physicist Augustin-Jean Fresnel, they are sometimes called Fresnel zone plates in his honor. The zone plate's focusing ability is an extension of the Arago spot phenomenon caused by diffraction from an opaque disc. A zone plate consists of a set of concentric rings, known as Fresnel zones, which alternate between being opaque and transparent.
Crystal systemIn crystallography, a crystal system is a set of point groups (a group of geometric symmetries with at least one fixed point). A lattice system is a set of Bravais lattices. Space groups are classified into crystal systems according to their point groups, and into lattice systems according to their Bravais lattices. Crystal systems that have space groups assigned to a common lattice system are combined into a crystal family. The seven crystal systems are triclinic, monoclinic, orthorhombic, tetragonal, trigonal, hexagonal, and cubic.
