Types of phase transition

• The transitions between the solid, liquid, and gaseous phases of a single component, due to the effects of temperature and/or pressure: » .
  
  A similar behaviour, but with the exponent $u$ instead of $alpha$, applies for the correlation length.
     The exponent $u$ is positive. This is different with $alpha$. Its actual value depends on the type of phase transition we're considering.
     For -1 < α < 0, the heat capacity has a "kink" at the transition temperature. This is the behavior of liquid helium at the "lambda transition" from a normal state to the superfluid state, for which experiments have found α = -0.013±0.003. At least one experiment was performed in the zero-gravity conditions of an orbiting satellite to minimize pressure differences in the sample (see here). This experimental value of α agrees with theoretical predictions based on variational perturbation theory (see here).
     For 0 < α < 1, the heat capacity diverges at the transition temperature (though, since α < 1, the divergence isn't strong enough to produce a latent heat). An example of such behavior is the 3-dimensional ferromagnetic phase transition. In the three-dimensional Ising model for uniaxial magnets, detailed theoretical studies have yielded the exponent α ∼ +0.110.
     Some model systems don't obey a power-law behavior. For example, mean field theory predicts a finite discontinuity of the heat capacity at the transition temperature, and the two-dimensional Ising model has a logarithmic divergence. However, these systems are limiting cases and an exception to the rule. Real phase transitions exhibit power-law behavior.
     Several other critical exponents - β, γ, δ, ν, and η - are defined, examining the power law behavior of a measurable physical quantity near the phase transition. Exponents are related by scaling relations such as , $u=gamma/(2-eta)$. It can be shown that there are only two independent exponents, for example $u$ and $eta$.
     It is a remarkable fact that phase transitions arising in different systems often possess the same set of critical exponents. This phenomenon is known as universality. For example, the critical exponents at the liquid-gas critical point have been found to be independent of the chemical composition of the fluid. More amazingly, but understandable from above, they're an exact match for the critical exponents of the ferromagnetic phase transition in uniaxial magnets. Such systems are said to be in the same universality class. Universality is a prediction of the renormalization group theory of phase transitions, which states that the thermodynamic properties of a system near a phase transition depend only on a small number of features, such as dimensionality and symmetry, and are insensitive to the underlying microscopic properties of the system. Again, the divergency of the correlation length is the essential point.

  Critical slowing down and other phenomena

  There are also other critical phenoma; for example, besides static functions usually there's also the critical dynamics . As a consequence, at a phase transition one may observe critical slowing down or speeding up, respectively. As a consequence, the large static universality classes of a continuous phase transition split into smaller dynamic universality classes. Furthermore, in addition to the critical exponents there are also universal relations for certain static or dynamic functions of the magnetic fields and temperature differences from the critical value.
  

  Phase-change data storage

  Several data-storage technologies use chalcogenide glass, which can be "switched" between two states, crystalline or amorphous, with the application of heat.

  Phase change and optical disc technology

  Phase change technology is also used to write to optical discs, such as CD-RW or DVD-RW discs. This is accomplished by including both a read laser and a more powerful write laser inside the drive. The discs contain a layer of a crystalline material that, when hit by a pulse of laser light from the write laser, changes to an amorphous state, thus changing its reflectivity. A different pulse level will reverse the changes, thus erasing the recorded information. The read laser isn't powerful enough to induce a phase change, but can be used by the drive to tell whether a bit is "on" or "off" based on an area of the disc's reflectivity.

  History of phase change optical disc technology