Lev Davidovich Landau (1908--1968) was a Soviet/Russian physicist who made fundamental contributions to many areas of theoretical physics. He received the 1962 Nobel Prize in Physics for his development of a mathematical theory of superfluidity that accounts for the properties of liquid helium II at a temperature below 2.17 K.

In 1924, he moved from Baku, where he was born to Jewish parents, to the main centre of Soviet physics at that time: the Physics Department of Leningrad State University. He received a doctorate in Physical and Mathematical Sciences in 1934. He was fired from St. Petersburg University for failing all students who, in his opinion, did not understand physics. Therefore, between 1932 and 1937 he headed the Department of Theoretical Physics at the National Scientific Center Kharkov Institute of Physics and Technology and lectured at the University of Kharkov and the Kharkov Polytechnical Institute. Landau developed a famous comprehensive exam called the "Theoretical Minimum" which students were expected to pass before admission to the school. During 27 years, only 43 candidates passed, but those who did later became quite notable theoretical physicists.

Phase transitions is an ubiquitous topic that includes magnets, liquid crystals, superconductors, crystals, amorphous equilibrium solids, and liquid condensation. These transitions occur between equilibrium states as functions of temperature, pressure, magnetic field, etc. A familiar example of such phase transition is water-vapor phase transition. We know the water does not immediately turn into vapor. At a boiling point we are facing a mixture (coexistence) of both phases involved. Liquid phase does not vanish until we provide sufficient amount of energy (heat). Another example gives transition of a disordered water/liquid into a crystalline solid when non apparent structure undergoes a transition to a structure with long range periodic order. Finally, one of the most common phase transitions from every day life is phase separation, which makes it necessary to shake the salad dressing. A phase transition occurs when the equilibrium state of a system changes qualitatively as a function of externally imposed constraints. These constraints could be temperature, pressure, magnetic field, concentration, degree of crosslinking, or any number of other physical quantities.

It is clear that transition between phases possessing different symmetries (e.g. distinct crystal modifications) cannot occur in a continuous manner. Our system will always be found in a state that reflects either one symmetry properties or another. In crystals, a transition between two different modifications can always take place after sudden rearrangement of its constituents, which would result in a discontinuous change of the state. Nonetheless there exist another type of transition affecting only the symmetry properties. Note that in such case there is no discontinuous change in the state of the system, i.e. positions of atoms are changing continuously, but there is a discontinuity in the symmetry at transition point.

To mathematically describe a transition, it is convenient to introduce a quantity called order parameter. In particular, we are interested in a quantity that takes a non-zero value below the critical point and exactly vanishes at the critical point, in the high-symmetry phase. For example, at a critical point, the magnetization is continuous -- as the parameters are tuned closer to the critical point, it gets smaller, becoming zero at the critical point. However, experiments on the liquid-gas phase transition and on three-dimensional magnets both point that even though the magnetization is continuous, its derivative is not.

The first major step toward theoretical understanding came from Landau (1937), and his approach is still called today Landau theory. His paper contains some typos and it took several decades for Nobel Prize winner in Physics 1982 Kenneth G. Wilson (1936--2013) and others to figure out how to fix it. It is important to emphasize that Landau’s original (genius) idea for an effective theory was and remains completely correct. It’s just that the naive computations do not give the right answers. Landau theory is an effective theory of the order parameter. To be precise about it, one first decides what the appropriate order parameter is to describe the phase transition. In one phase, the order parameter is non-vanishing, in another it vanishes. The basic assumption of Landau theory is that at a fixed value of the order parameter, the free energy as a function of the order parameter is analytic. The non-analyticity at a phase transition then comes because in the partition function one must sum over all possible values of the order parameter.

Landau’s theory of phase transitions is based on an expansion of the free energy of a thermodynamic system in terms of an order parameter, which is nonzero in an ordered phase and zero in a disordered phase. For example, the magnetization M of a ferromagnet in zero external field but at finite temperature typically vanishes for temperatures T > T_c, where T_c is the critical temperature, also called the Curie temperature in a ferromagnet. A low order expansion in powers of the order parameter is appropriate sufficiently close to T_c, i.e., at temperatures such that the order parameter, if nonzero, is still small.

With Landau’s assumption at fixed value of the order parameter is analytic, the free energy can be expanded in a Taylor series around the critical point. The symmetries of the theory determine the form of this expansion. The simplest example is the quartic free energy,

where m is a dimensionless measure of the magnetization density, and where f₀, a, and b are all functions of the dimensionless temperature θ, which in a ferromagnet is the ratio \( \theta = k_B T/ {\cal J} , \) where \( {\cal J} = \sum_j J_{ij} , \) is a sum over the couplings. Let us assume b > 0, which is necessary if the free energy is to be bounded from below. Otherwise, b < 0, when b is negative, we’d have to consider higher order terms in the Landau expansion. The equation of state, has three solutions in the complex m plane: (i) θ = 0, (ii) \( m = \sqrt{-a/b} , \) and (iii) \( m = -\sqrt{-a/b} . \) The latter two solutions lie along the (physical) real axis if a < 0. We assume that \( a(\theta ) \) is monotonically increasing, and that there exists a unique temperature θ_c where \( a(\theta_c ) = 0. \) Minimizing f, we find The free energy is continuous at θ_c since \( a(\theta_c ) = 0. \) he specific heat, however, is discontinuous across the transition, with The presence of a magnetic field h breaks the symmetry \( m \,\mapsto \, -m . \) The free energy becomes and the mean field equation is This is a cubic equation for m with real coefficients, and as such it can either have three real solutions or one real solution and two complex solutions related by complex conjugation. Clearly we must have a < 0 in order to have three real roots because \( b\, m^3 + a\,m \) is monotonically increasing otherwise. The boundary between these two class es of solution sets occurs when two roots coincide, which means \( f'' (m) =0 \) as well as \( f' (m) =0 . \) simultaneously solving these two equations, we find or, equivalently, If, for fixed h, we have \( a < a^{\ast} (h) , \) then there will be three real solutions to the mean field equation \( f' (m) =0 , \) one of which is a global minimum (the one for which \( m \cdot h > 0 \) ). For \( a > a^{\ast} (h) , \) there is only a single global minimum, at which m also has the same sign as h. If we solve the mean field equation perturbatively in h/a, we find

 

The phase transition of chocolate

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As is well known, chocolate is a common foodstuff in daily life, as well as an important industrial material. For example, it is used as the raw material for a recent 3D food printer. the main ingredient in chocolate is cocoa butter, which has six crystalline states with different melting points. The crystalline state is related to the chemical composition of the cocoa butter, which, in turn, depends on the country of production and the harvest period. Therefore, the polymorphism of cocoa butter renders chocolate a complex structure.

1. Rullong Ren et al, Hysteresis in the phase transition of chocolate, European Journal of Physics, 37, 2016.
2. http://iypt.org/Tournaments/Shrewsbury
3. Loisel C., Keller G., Bourgaux C., and Ollivon M. Phase transition and polymorphism of cocoa butter. Journal of the American Oil Chemists' Society, 75, issue 4, 425--439, 1998.
4. Yeomans J.M., Statistical Mechanics of Phase Transitions, New York, Clarendon, 1992.
5. Dovonshire A.F., Theory of ferroelectrics. Advances in Physics, 3, no. 10, 85--130, 1954.

Chocolate is usually solid at room temperature (20^oC -- 25^oC) and melts when the temperature rises to body temperature (36^oC -- 37.5^oC); that is, the phase transition occurs during solidification and melting. in the re-cooling process, however, chocolate remains a liquid when the temperature drops to room temperature (so it exhibits hysteresis). In fact, hysteresis behaviors widely exist in the phase transitions for ferromagnetic or ferroelectric materials.

According to Landau theory, the free energy of the system can be expressed as

where η is the order parameter, T is the temperature, φ₀ is the initial energy, T_c is the characteristic temperature. Meanwhile a, B, D, V, and h are the parameters irrespective of temperature. The stabilization condition for the free energy is given by For convenience, the above equation can be rewritten as The experimental data fit the theoretical curve well when