Scanning probes reveal complex, inhomogeneous patterns on the surface of many condensed matter systems. In some cases, the patterns form self-similar, fractal geometric clusters. In this paper, we advance the theory of criticality as it pertains to those geometric clusters (defined as connected sets of nearest-neighbor aligned spins) in the context of Ising models. We show how data from surface probes can be used to distinguish whether electronic patterns observed at the surface of a material are confined to the surface, or whether the patterns originate in the bulk. Whereas thermodynamic critical exponents are derived from the behavior of Fortuin–Kasteleyn (FK) clusters, critical exponents can be similarly defined for geometric clusters. We find that these geometric critical exponents are not only distinct numerically from the thermodynamic and uncorrelated percolation exponents, but that they separately satisfy scaling relations at the critical fixed points discussed in the text. We furthermore find that the two-dimensional (2D) cross-sections of geometric clusters in the three-dimensional (3D) Ising model display critical scaling behavior at the bulk phase transition temperature. In particular, we show that when considered on a 2D slice of a 3D system, the pair connectivity function familiar from percolation theory displays more robust critical behavior than the spin-spin correlation function, and we calculate the corresponding critical exponent. We discuss the implications of these two distinct length scales in Ising models. We also calculate the pair connectivity exponent in the clean 2D case. These results extend the theory of geometric criticality in the clean Ising universality classes, and facilitate the broad application of geometric cluster analysis techniques to maximize the information that can be extracted from scanning image probe data in condensed matter systems.
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Emerging critical behavior at a first‐order phase transition rounded by disorder
We investigate the two‐dimensional four‐color Ashkin‐Teller model by means of large‐scale Monte‐Carlo simulations. We demonstrate that the first‐order phase transition of the clean system is destroyed by random disorder introduced via site dilution. The critical behavior of the emerging continuous transition belongs to the clean two‐dimensional Ising universality class, apart from logarithmic corrections. These results confirm perturbative renormalization‐group predictions; they also agree with recent findings for the three‐color case, indicating that the critical behavior is universal.
more » « less- NSF-PAR ID:
- 10030724
- Publisher / Repository:
- Wiley Blackwell (John Wiley & Sons)
- Date Published:
- Journal Name:
- Fortschritte der Physik
- Volume:
- 65
- Issue:
- 6-8
- ISSN:
- 0015-8208
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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