Faasisiirtymät

Faasisiirtymät, or phase transitions, refer to qualitative changes in the state or organization of a system as external conditions such as temperature, pressure, or magnetic field are varied. They are studied in physics, chemistry, and related fields to understand how macroscopic properties change abruptly or gradually. Transitions are often described in terms of an order parameter, a quantity that measures the degree of order in a phase and changes value across the transition.

First-order phase transitions involve a discontinuous change of the order parameter and typically release or absorb latent heat. Classic examples include ice melting to water and water boiling to steam, where density and other properties change abruptly at the transition line.

Continuous, or second-order, phase transitions feature no latent heat and a continuous change of the order parameter, accompanied by diverging correlation lengths and strong fluctuations. Near the critical point, systems exhibit scale invariance and universal behavior independent of microscopic details, described by critical exponents and universality classes. Landau theory provides a phenomenological framework, while the renormalization group explains universality and scaling.

In solids, phase transitions can be structural, magnetic, electronic, or superconducting. Common examples include the ferromagnetic transition at the Curie temperature and the superconducting transition in certain materials. Understanding faasisiirtymät helps in materials design, cryogenics, and the study of critical phenomena across disciplines.