🥵Thermodynamics Unit 11 – Phase Transitions and Critical Phenomena

Key Concepts and Definitions

• Phase transitions involve changes in the physical properties of a system when it transitions from one phase to another (solid, liquid, gas)
• Critical phenomena occur near the critical point where the distinction between phases disappears
  • Characterized by long-range correlations and diverging susceptibilities
• Order parameters quantify the degree of order in a system undergoing a phase transition
  • Examples include magnetization in ferromagnets and density difference between liquid and gas phases
• Symmetry breaking occurs when a system's symmetry is reduced during a phase transition (cubic to tetragonal in ferroelectric materials)
• Critical exponents describe the behavior of physical quantities near the critical point and are universal for systems with the same dimensionality and symmetry
• Universality classes group systems with the same critical exponents regardless of their microscopic details
• Correlation length represents the average distance over which fluctuations in the system are correlated and diverges at the critical point

Types of Phase Transitions

• First-order phase transitions exhibit discontinuities in the first derivatives of the free energy (volume, entropy)
  • Examples include melting, boiling, and sublimation
  • Characterized by latent heat and phase coexistence during the transition
• Second-order phase transitions have continuous first derivatives but discontinuities in the second derivatives (heat capacity, compressibility)
  • Examples include ferromagnetic and superconducting transitions
  • No latent heat and diverging correlation length at the critical point
• Continuous phase transitions are synonymous with second-order transitions and exhibit critical phenomena
• Liquid-gas transitions demonstrate both first-order (below critical temperature) and second-order (at critical point) behavior
• Structural phase transitions involve changes in the crystal structure of solids (martensitic transformations in shape-memory alloys)
• Quantum phase transitions occur at absolute zero temperature and are driven by changes in quantum fluctuations (superconductor-insulator transition)

Thermodynamic Description of Phase Transitions

• Gibbs free energy $G = U - TS + PV$ determines the stability of phases in a system
  • Phases with the lowest Gibbs free energy are thermodynamically favored
• First-order transitions have a discontinuity in the first derivative of the Gibbs free energy with respect to temperature or pressure
  • Latent heat $L = T \Delta S$ is associated with the entropy change during the transition
• Second-order transitions have a continuous first derivative but a discontinuity in the second derivative of the Gibbs free energy
  • Heat capacity $C_p = -T \left(\frac{\partial^2 G}{\partial T^2}\right)_P$ diverges at the critical point
• Clausius-Clapeyron equation $\frac{dP}{dT} = \frac{L}{T \Delta V}$ describes the slope of the coexistence curve in first-order transitions
• Ehrenfest classification categorizes phase transitions based on the lowest derivative of the Gibbs free energy that is discontinuous
• Landau theory expresses the free energy as a polynomial expansion of the order parameter near the critical point
  • Minimizing the Landau free energy determines the equilibrium state of the system

Order Parameters and Symmetry Breaking

• Order parameters measure the degree of order or symmetry in a system undergoing a phase transition
  • Magnetization $M$ for ferromagnetic transitions
  • Density difference $\rho_l - \rho_g$ for liquid-gas transitions
  • Superconducting gap $\Delta$ for superconducting transitions
• Symmetry breaking occurs when the high-temperature, high-symmetry phase transitions to a low-temperature, low-symmetry phase
  • Ferromagnetic transitions break rotational symmetry as spins align in a preferred direction
  • Liquid-gas transitions break translational symmetry as density fluctuations become long-ranged
• Landau theory introduces a symmetry-breaking field conjugate to the order parameter (magnetic field for ferromagnets)
• Ginzburg-Landau theory extends Landau theory to include spatial variations of the order parameter (superconducting order parameter)
• Spontaneous symmetry breaking occurs when the system chooses a particular ground state among degenerate options (Higgs mechanism in particle physics)

Critical Exponents and Universality

• Critical exponents describe the power-law behavior of physical quantities near the critical point
  • Correlation length $\xi \sim |t|^{-\nu}$, where $t = (T - T_c) / T_c$ is the reduced temperature
  • Order parameter $M \sim |t|^\beta$ for $T < T_c$ and $M = 0$ for $T > T_c$
  • Heat capacity $C \sim |t|^{-\alpha}$
  • Susceptibility $\chi \sim |t|^{-\gamma}$
• Universality implies that critical exponents depend only on the dimensionality and symmetry of the system, not on microscopic details
  • Ising model, liquid-gas transitions, and ferromagnets have the same critical exponents in 3D
• Scaling relations connect different critical exponents (Rushbrooke, Widom, Fisher relations)
• Renormalization group theory provides a framework to understand universality and calculate critical exponents
  • Successive coarse-graining of the system leads to scale-invariant behavior at the critical point

Landau Theory of Phase Transitions

• Landau theory is a phenomenological approach to describe phase transitions using symmetry considerations
• Free energy is expanded as a polynomial in the order parameter $F = F_0 + a(T) \phi^2 + b(T) \phi^4 + \cdots$
  • $a(T) = a_0 (T - T_c)$ changes sign at the critical temperature $T_c$
  • $b(T) > 0$ ensures stability of the system
• Minimizing the Landau free energy with respect to the order parameter determines the equilibrium state
  • Above $T_c$, $a(T) > 0$, and the minimum is at $\phi = 0$ (disordered phase)
  • Below $T_c$, $a(T) < 0$, and the minimum is at $\phi \neq 0$ (ordered phase)
• Landau theory predicts mean-field critical exponents ($\beta = 1/2$, $\gamma = 1$, $\alpha = 0$)
• Ginzburg criterion determines the validity of Landau theory near the critical point
  • Fluctuations become important when the Ginzburg parameter $Gi \sim |t|^{d\nu - 2\beta}$ is large

Experimental Techniques and Observations

• Calorimetry measures the heat capacity and latent heat during phase transitions
  • Differential scanning calorimetry (DSC) and adiabatic calorimetry are common techniques
• Scattering techniques probe the structure and dynamics of systems near phase transitions
  • X-ray, neutron, and light scattering provide information on the correlation length and order parameter
• Magnetic measurements (SQUID, VSM) characterize the magnetization and susceptibility in magnetic transitions
• Electrical transport measurements detect changes in resistivity and conductivity during metal-insulator or superconducting transitions
• Optical techniques (ellipsometry, Raman spectroscopy) are sensitive to structural changes and symmetry breaking
• Critical opalescence is observed in liquid-gas transitions due to increased light scattering near the critical point
• Specific heat and susceptibility measurements near the critical point reveal power-law behavior and critical exponents

Applications and Real-World Examples

• Magnetic phase transitions in ferromagnetic materials (iron, nickel) are used in data storage and sensing applications
• Liquid-gas transitions are crucial in heat engines, refrigeration, and power generation (steam turbines)
• Structural phase transitions in shape-memory alloys (Nitinol) enable applications in aerospace, biomedical, and robotics
  • Martensitic transformations allow the material to recover its original shape after deformation
• Superconducting transitions in materials like niobium and cuprates have applications in high-field magnets, power transmission, and quantum computing
• Ferroelectric transitions in materials like barium titanate are used in capacitors, sensors, and actuators
• Liquid crystal transitions are the basis for display technologies (LCD, OLED)
• Protein folding and denaturation can be described as phase transitions in biological systems
• Quantum phase transitions in ultracold atomic gases provide insights into many-body physics and quantum simulations