Chemistry Reference
In-Depth Information
Consequently, there is no latent heat. This type of phase transition is called a
continuous (second-order) transition or critical point.
Phase transitions play an essential role in shaping the world. The large-
scale structure of the universe is the result of phase transitions during the early
stages of its development after the Big Bang. Phase transitions also occur dur-
ing the production of materials, in growth processes, and in chemical reac-
tions. Understanding phase transitions has thus been a prime endeavor of
science. More than a century has gone by from the first (modern) discoveries
of phase transitions in gases and liquids by Andrews
1
and van der Waals
2
until
a consistent picture of the nature of those transitions started to emerge. How-
ever, the theoretical concepts established during this development, viz., scaling
and the renormalization group
3,4
now belong to the central paradigms of mod-
ern physics and chemistry.
The examples of phase transitions mentioned above occur at nonzero
temperature. At these so-called thermal or classical transitions, the ordered
phase (the ice crystal or the ferromagnetic state of iron) is destroyed by ther-
mal fluctuations. In the last two decades or so, considerable attention has
focused on a very different class of phase transitions. These new transitions
occur at zero temperature when a nonthermal parameter such as pressure, che-
mical composition, or magnetic field is changed. The fluctuations that destroy
the ordered phase in these transitions cannot be of a thermal nature. Instead,
they are quantum fluctuations that are a consequence of Heisenberg's uncer-
tainty principle. For this reason, these zero-temperature transitions are called
quantum phase transitions.
As an illustration of this phenomenon, consider the magnetic phase dia-
gram
5
of the compound LiHoF
4
in Figure 1. In zero external magnetic field,
LiHoF
4
undergoes a phase transition from a paramagnet to a ferromagnet
Figure 1
Phase diagram of LiHoF
4
as function of temperature and transverse magnetic
field.
