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A triple point ( 2 ) on a phase diagram is a point at which three phases coexist simultaneously. Triple points exist at a single temperature and pressure.
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Researchers predict that several exotic states of matter can exist in semiconductor structures hosting electrons in one layer and holes in another.
Electron–hole bilayers are semiconductor structures in which electrons and holes—positively charged electron vacancies—are separated into two distinct layers. These structures can host unusual phases of matter owing to the presence of both attraction between opposite charges and repulsion between like charges. However, most studies have focused only on the scenario in which the electron density is equal to the hole density. Now theoretical work by David Dai and Liang Fu at the Massachusetts Institute of Technology has explored the imbalanced case in which this electron–hole density ratio is 2:1 [1]. The researchers’ findings suggest that such an electron–hole bilayer has a remarkably rich phase diagram. The imbalanced case is of particular interest for two key reasons. First, the presence of a net charge density causes the Coulomb interaction between the charges to become stronger than their other interactions, favoring unconventional phases in which the charges are strongly coupled to one another. Second, this setup could facilitate the formation of exotic particles called trions, made of two electrons in the electron layer bound to one hole in the hole layer.
Source: physics.aps.org
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Condensation
The opposite of vaporization, condensation is the phase change that occurs when a gaseous materials transforms into a liquid. It is most often used in terms of water and the water cycle, but applies to the phase transformation of any material. As liquid phases typically have less energy than gaseous phases, a condensation transformation typically requires a mechanism to cool the substance in question, or simply a cooler environment.
Sources/Further Reading: (Image source - serpmedia.org) (Wikipedia) (OpenStax)
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Deposition
The reverse phase transformation of sublimation, deposition is the transition from a gaseous phase to a solid phase. This transformation is an exothermic one, resulting in the release of heat. Deposition phase transformations are exploited in various industries to create thin films and coatings by first evaporating a substance and then letting it solidify on the desired substrate. Frost is an example of deposition in nature.
Sources/Further Reading: (Image source - Wikipedia) (LibreTexts)
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Melting
Melting is the process by which a solid material transition to a liquid material. It occurs when the Gibbs free energy of the liquid becomes lower than that of the solid. This depends on a number of factors, which are themselves linked, including the chemical composition of the substance, the arrangement of the atoms, bonding between atoms, temperature, and pressure. Because historically humans can enact temperature changes more readily and drastically than pressure changes, the melting point of a substance is defined as the temperature at which this transition occurs at a set pressure, typically atmospheric pressure.
Because a substance's melting point depends on its composition and crystal structure (or lack thereof), melting a substance is one technique to test for purity. Pure materials have a singular fixed melting point, whereas mixed materials (composites, alloys with multiple phases, etc.) will have a melting temperature range.
As a side note, melting was also known as fusion, which is why the energy associated with melting is known as the enthalpy of fusion or heat of fusion.
Sources/Further Reading: (Image 1 - Wikipedia) (Introductory Chemistry text) (LibreTexts) (NeoNickel) (1965 article) (2000 article) (Facts.net)
(Image 2)