TAILIEUCHUNG - Basic Theoretical Physics: A Concise Overview P40

Basic Theoretical Physics: A Concise Overview P40. This concise treatment embraces, in four parts, all the main aspects of theoretical physics (I . Mechanics and Basic Relativity, II. Electrodynamics and Aspects of Optics, III. Non-relativistic Quantum Mechanics, IV. Thermodynamics and Statistical Physics). It summarizes the material that every graduate student, physicist working in industry, or physics teacher should master during his or her degree course. It thus serves both as an excellent revision and preparation tool, and as a convenient reference source, covering the whole of theoretical physics. It may also be successfully employed to deepen its readers’ insight and. | Molecular Field Theories Mean Field Approaches 407 following identity Sl Sm Si T Sm Sl Q Q Q Q Q S - n i iS l iS l is m S m fib fib fib T IT T Even the factor 2 in front of 2m is derived in this way . from dl m Jm l However neglecting fluctuations is grossly incorrect in the critical region which is often not small . for typical ferromagnets it amounts to the upper 20 of the region below Tc in contrast for conventional 3dsuperconductivity the critical region is negligible . Generally the neglected terms have a drastic influence on the critical exponents. In the critical region the order parameter does not converge to Tc - T 2 as the Landau theory predicts but x Tc T @ where for a d 3-dimensional system the value of 3 is 1 instead of Landau 1 . 2 We also have both above and below Tc with different coefficients X x Tc - T Y with y Y3d 4 3 instead of the value from the Landau theory TLandau 1 Similarly we have e Tc - t with v v3d instead of the Landau value _ 1 vLandau For the twodimensional Ising model a model which only differs from the Heisenberg model by the property that the vectorial spin operators Sl and Sm are replaced by the -components Sz l and Sz m the difference between the critical exponents and the Landau values is even more drastic than in d 3 as detailed in the following. For d 2 the exact values of the critical exponents are actually known 7 2d lsing c 8 7 Y 2d ising 4 and v 2d jsing 1 408 53 Applications I Fermions Bosons Condensation Phenomena and the critical temperature is reduced by the fluctuations by almost 50 compared to the molecular field approximation. For the d 2-dimensional Heisenberg model the phase transition is even completely suppressed by fluctuations . It is no coincidence19 that the quantitative values of the critical exponents and other characteristic quantities of the critical behavior are universal in the sense that they are almost independent of the details of the interactions. For interactions which are sufficiently

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