The Landau-Ginzburg-Wilson Hamiltonian for the Griffiths phase

TL;DR

This paper reexamines the Landau-Ginzburg-Wilson Hamiltonian with randomness, revealing a self-organizing block structure that explains complex critical behaviors and experimental observations in disordered systems near phase transitions.

Contribution

It introduces a multi-generation block framework derived from saddle point solutions, providing a new understanding of emergent variables and critical phenomena in disordered systems.

Findings

Emergent superspins form hierarchical blocks of increasing size.

The effective field on superspins can be greatly amplified by small external fields.

The layered phase space explains diverse experimental and simulation results.

Abstract

The Landau-Ginzburg-Wilson Hamiltonian with random temperature for the phase transition in disordered systems from the Griffiths phase to ferromagnetic phase is reexamined. From the saddle point solutions, especially the excited state solutions, it is shown that the system self-organizes into blocks coupled with their neighbors like superspins, which are emergent variables. Taking the fluctuation around these saddle point solutions into account, we get an effective Hamiltonian, including the emergent superspins of the blocks, the fluctuation around the saddle point solutions, and their couplings. Applying Stratonovich-Hubbard transformation to the part of superspins, we get a Landau-Ginzburg-Wilson Hamiltonian for the blocks. From the saddle point equations for the blocks, we can get the second generation blocks, of which sizes are much larger than the first generation blocks. Repeating this procedure again and again, we get many generations of blocks to describe the asymptotic behavior. If a field is applied, the effective field on the superspins is multiplied greatly and proportional to the block size. For a very small field, the effective field on the higher generation superspins can be so strong to cause the superspins polarizaed radically. This can explain the extra large critical isotherm exponent discovered in the experiments. The phase space of reduced temperature vs. field is divided into many layers , in which different generation blocks dominate the critical behavior. The sizes of the different generation emergent blocks are new relevant length scales. This can explain a lot of puzzles in the experiments and the Monte Carlo simulation.