A single assumption is made about the dynamic correlation function, that close to the critical point and for large r and t, it is a generalized homogeneous function. After proving and using certain simple theorems on this class of functions, contact is made with previous formulations of dynamic and static scaling. It is found that 1. this single assumption is sufficient to obtain all previous results of both the dynamic and static scaling hypotheses, 2. simple expressions, which can be written down ‘by inspection’ for all critical point exponents, enable all the exponent relations to be derived immediately, 3. we have additional ways of plotting data in the form of ‘scaling functions’, 4. it becomes possible to easily and accurately calculate the scaling function for models (such as the Heisenberg model) for which no low‐temperature series expansions exist, and 5. the two‐exponent relations (e. g., dν=2−α) are not consequences of our approach unless we make an additional assumption, an assumption made implicitlyby many other workers. If we don't make this additional assumption, the two‐exponent relations are found to have correction terms which transform them into ordinary thermodynamic scaling laws.
A single assumption is made about the dynamic correlation function, that close to the critical point and for large r and t, it is a generalized homogeneous function. After proving and using certain simple theorems on this class of functions, contact is made with previous formulations of dynamic and static scaling. It is found that 1. this single assumption is sufficient to obtain all previous results of both the dynamic and static scaling hypotheses, 2. simple expressions, which can be written down ‘by inspection’ for all critical point exponents, enable all the exponent relations to be derived immediately, 3. we have additional ways of plotting data in the form of ‘scaling functions’, 4. it becomes possible to easily and accurately calculate the scaling function for models (such as the Heisenberg model) for which no low‐temperature series expansions exist, and 5. the two‐exponent relations (e. g., dν=2−α) are not consequences of our approach unless we make an additional assumption, an assumption made implicitlyby many other workers. If we don't make this additional assumption, the two‐exponent relations are found to have correction terms which transform them into ordinary thermodynamic scaling laws.
A function f(x1,x2,…,xn) is a generalized homogeneous function (GHF) if we can find numbers a1,a2,…,an such that for all values of the positive number $\ensuremath{\lambda}$, $f({\ensuremath{\lambda}}^{{a}_{1}}{x}_{1}, {\ensuremath{\lambda}}^{{a}_{2}}{x}_{2}, \dots{}, {\ensuremath{\lambda}}^{{a}_{n}}{x}_{n})={\ensuremath{\lambda}}^{{a}_{f}}f({x}_{1}, {x}_{2}, \dots{}, {x}_{n})$. We organize the properties of GHFs in four theorems. These are used to systematically examine the consequences of various scaling hypotheses. An advantage of this approach is that the same formalism may be used to treat thermodynamic functions, static correlation functions, dynamic correlation functions, and "universality." The simple case of thermodynamic scaling (two independent variables) is first generalized to static and dynamic correlation functions (three and four variables), and then to scaling with a parameter (for which the critical subspace becomes higher dimensional). In this last case, where a second GHF hypothesis is made, the necessity of crossover lines is demonstrated. The assumption of homogeneity is clearly separated from any extra assumptions that may also be called scaling (or "strong scaling"), but are independent of and different from that of homogeneity. One practical insight gained from the present approach is that all experimentally measured exponents are expressible as the ratio of two scaling powers, af (which refers to the function) and aj (which refers to the path of approach to the critical point). A second practical advantage is that, since a GHF can be scaled with respect to any of its arguments, one can immediately write a variety of scaling functions for each type of scaling hypothesis. The GHF approach thereby permits data to be plotted in a variety of convenient fashions, and is found to facilitate computation of the relevant scaling functions (in particular, the GHF approach led directly to the recent calculation of the Heisenberg model scaling function by Milos\ifmmode \breve{}\else \u{}\fi{}evi\ifmmode \acute{c}\else \'{c}\fi{} and Stanley).
The static scaling laws may be generalized to dynamic phenomena by making assumptions on the behavior of time-dependent correlation functions near the magnetic critical point. At any temperature different from Tc, the correlation functions are assumed to reflect the ``hydrodynamic'' behavior of the system, for sufficiently long wavelengths and low frequencies. As the critical temperature is approached, however, the range ξ of spatial correlations in the system diverges, and the domain of applicability of hydrodynamics is reduced to very small wavenumbers, satisfying the inequality kξ≪1. The dynamic scaling assumptions relate the behavior of the time-dependent correlation functions in the ``hydrodynamic'' (kξ≪1) and ``critical'' (kξ≫1) regions. It is thus possible to predict certain features of the dynamic behavior at Tc, from a knowledge of the behavior away from Tc. Application is made to the spin-correlation function of the isotropic Heisenberg antiferromagnet, which is directly measurable by inelastic neutron scattering. Detailed predictions are presented on the form of the cross section as a function of energy and momentum transfer, and of temperature near Tc.
The usual static scaling laws are generalized to nonequilibrium phenomena by making assumptions on the behavior of time-dependent correlation functions near the critical point of second-order phase transitions. At any temperature different from Tc, the correlation functions are assumed to reflect the hydrodynamic behavior of the system, for sufficiently long wavelengths and low frequencies. As the critical temperature is approached, however, the range of spatial correlations in the system diverges, and the domain of applicability of hydrodynamics is reduced to a vanishingly small region of wavelengths and frequencies. The dynamic-scaling assumptions lead to predictions for the behavior of the hydrodynamic parameters near Tc, as well as for the form of the correlation functions for macroscopic distances and times, outside the hydrodynamic range. In particular, singularities are predicted to occur in the temperature dependence of transport coefficients, and anomalies are expected in the frequency spectrum of certain operators, which are observable by inelastic scattering of neutrons or light. A distinction is made between the restricted dynamic-scaling hypothesis, which refers to the order parameter only, and extended dynamic scaling, which applies to other operators and involves stronger assumptions. Applications are discussed to antiferromagnets, ferromagnets, the gas-liquid critical point, and the $\ensuremath{\lambda}$ transition in superfluid helium. Specific experiments are suggested to test the scaling assumptions, and existing experimental evidence is briefly reviewed. Finally, a comparison is made with other theories of dynamical behavior near critical points.
An argument is presented for a certain asymptotic form for the correlation functions of two local dynamical variables when the system is near its critical point and in the one phase region. If A(r) and B(r) are local dynamical variables and $\ensuremath{\omega}$ stands for the variables specifying the thermodynamic state, it is found that for $\ensuremath{\omega}$ near the critical point ${\ensuremath{\omega}}_{c}$ and for r large enough, $〈A(\mathbf{r})B(0)〉\ensuremath{-}〈A(\mathbf{r})〉〈B(0)〉\ensuremath{\simeq}\frac{{d}_{A}(\ensuremath{\omega}){d}_{B}(\ensuremath{\omega}){e}^{\ensuremath{-}\ensuremath{\kappa}(\ensuremath{\omega})}}{{r}^{b(\ensuremath{\omega})}}$, where $\ensuremath{\kappa}(\ensuremath{\omega})$ and $b(\ensuremath{\omega})$ are the same for a large class of A and B and $\ensuremath{\kappa}(\ensuremath{\omega})\ensuremath{\rightarrow}0$, $b(\ensuremath{\omega})\ensuremath{\rightarrow}b\ensuremath{\ge}0 \mathrm{as} \ensuremath{\omega}\ensuremath{\rightarrow}{\ensuremath{\omega}}_{c}$. For the Ising model of any dimension in zero field and below the critical temperature, the additional assumption of a single correlation length implies the relationship ${\ensuremath{\gamma}}^{\ensuremath{'}}+2\ensuremath{\beta}=2\ensuremath{-}{\ensuremath{\alpha}}^{\ensuremath{'}}$, which is often conjectured for the critical exponents.
This chapter explains the static scaling hypothesis, and shows how it leads to scaling laws. It discusses the topic of dynamical scaling, in which time-dependent correlations in equilibrium near the critical point exhibit scaling. The chapter introduces the topic of scaling in non-equilibrium systems. Scaling and scaling laws near the critical point of equilibrium systems follow from the renormalisation group theory, which is capable of calculating the values of the critical exponents and the form of scaling functions when used in combination with approximate methods such as perturbation theory. The static scaling hypothesis, written in the form appropriate for a magnetic system, is an attempt to encode two experimental results in one equation. Equilibrium statistical mechanics is primarily concerned with static quantities. Nevertheless, time-dependent fluctuations of a system in equilibrium fall within the scope of equilibrium statistical mechanics, through the use of the fluctuation dissipation theorem.
Originally, the term ‘dynamic critical phenomena’ was coined for time-dependent properties near second-order phase transitions in thermal equilibrium . The kinetics of phase transitions in magnets, at the gas–liquid transition, and at the normal- to superfluid phase transition in helium 4 were among the prominent examples investigated already in the 1960s. The dynamic scaling hypothesis , generalizing the scaling ansatz for the static correlation function and introducing an additional dynamic critical exponent, successfully described a variety of these experiments. Yet only the development of the systematic renormalization group (RG) approach for critical phenomena in the subsequent decade provided a solid conceptual foundation for phenomenological scaling theories. Supplemented with exact solutions for certain idealized model systems, and guided by invaluable input from computer simulations in addition to experimental data, the renormalization group now provides a general framework to explore not only the static and dynamic properties near a critical point, but also the large-scale and low-frequency response in stable thermodynamic phases. Scaling concepts and the renormalization group have also been successfully applied to phase transitions at zero temperature driven by quantum rather than thermal fluctuations. It is to be hoped that RG methods may help to classify the strikingly rich phenomena encountered in far-from-equilibrium systems as well. Recent advances in studies of simple reaction-diffusion systems, active to absorbing state phase transitions, driven lattice gases, and scaling properties of moving interfaces and growing surfaces, among others, appear promising in this respect.