We present experimental and theoretical studies of the effects of quenched disorder on one-dimensional crystal ordering in three dimensions. This fragile smectic liquid crystal layering, the material with the simplest positional order, is also the most easily deformed periodic structure and is, therefore, profoundly affected by disorder, introduced here by confinement in silica aerogel. Theory and experiment combine to characterize this system to an extraordinary degree, their close accord producing a coherent picture: crystal ordering is lost, giving way to extended short-range correlations that exhibit universal structure and scaling, anomalous layer elasticity, and glassy dynamics.

https://science.sciencemag.org/content/sci/294/5544/1074.full.pdf

Universality and Scaling in the Disordering of a Smectic Liquid Crystal

The random field Ising model

Relaxor ferroelectricity is one of the most widely investigated but the least understood material classes in the condensed matter physics. This is largely due to the lack of experimental tools that decisively confirm the existing theoretical models. In spite of the diversity in the models, they share the core idea that the observed features in relaxors are closely related to localized chemical heterogeneity. Given this, this review attempts to overview the existing models of importance chronologically, from the diffuse phase transition model to the random-field model and to show how the core idea has been reflected in them to better shape our insight into the nature of relaxor-related phenomena. Then, the discussion will be directed to how the models of a common consensus, developed with the so-called canonical relaxors such as Pb(Mg1/3Nb2/3)O3 (PMN) and (Pb, La)(Zr, Ti)O3 (PLZT), are compatible with phenomenological explanations for the recently identified relaxors such as (Bi1/2Na1/2)TiO3 (BNT)-based lead-free ferroelectrics. This review will be finalized with a discussion on the theoretical aspects of recently introduced 0−3 and 2−2 ferroelectric/relaxor composites as a practical tool for strain engineering.

A brief review on relaxor ferroelectrics and selected issues in lead-free relaxors

Belief networks represent a powerful approach to problems involving probabilistic inference, but much of the work in this area is software based utilizing standard deterministic hardware based on the transistor which provides the gain and directionality needed to interconnect billions of them into useful networks. This paper proposes a transistor like device that could provide an analogous building block for probabilistic networks. We present two proof-of-concept examples of belief networks, one reciprocal and one non-reciprocal, implemented using the proposed device which is simulated using experimentally benchmarked models.

A building block for hardware belief networks.

Symmetry, dimensionality, and disorder play a pivotal role in critical phenomena. The atomic imaging capabilities of the scanning tunneling microscope were used to directly visualize the interaction between charge density oscillations and lattice defects in a two-dimensional charge density wave (CDW) system. Point defects act as nucleation centers of the CDW, which, as the temperature is lowered, results in the formation of pinned CDW domains that are separated by atomically abrupt charge boundaries. Incomplete freezing of substitutional disorder at low temperature indicates a novel CDW-mediated hopping of pinning centers.

https://science.sciencemag.org/content/sci/285/5436/2107.full.pdf

Defect-Mediated Condensation of a Charge Density Wave

Much of the enormous increase in our understanding of collective phenomena during the past few decades has arisen from the study of magnetic systems. The formulation of concepts such as universality, broken symmetry, and scaling near continuous phase transitions, as well as the development of the powerful ideas of the renormalization group, have been strongly influenced by research in magnetism. This is due in part to the availability of a host of experimentally accessible magnetic systems, and in part to the remarkable fact that simple models of magnetism capture the essential physics of the phases and ordering transitions in more complicated systems.

Geoffrey M. Grinstein

Papers

Theory of Random Magnets