Description

2009/07/14, Thomas Vojta (Missouri University of Science and Technology, Rolla, United States)

Place: Sala de Seminarios del Departamento de Física Teórica e Historia de la Ciencia
Time: 10h.
Title: Phase transitions and disorder: Harris criterion, Griffiths singularities, and smearing
Abstract
Phase transitions are fascinating phenomena in nature with
consequences ranging from the large scale structure of the
universe to exotic quantum phases at low temperatures. Many
realistic systems contain impurities, defects and other forms
of quenched disorder. This talk explores the consequences
of such randomness on the properties of phase transitions.
At zero-temperature quantum phase transitions, randomness
can have particularly peculiar and strong effects. Often, rare
strong disorder fluctuations and the rare spatial regions that
support them dominate the physics close to the transition.
They give rise to strong singularities in the free energy, the so-called quantum-Griffiths singularities, In some systems
such as metallic magnets, the effects of rare fluctuations
can be even stronger, leading to a destruction of the phase
transition by smearing. We suggest a classification of these
rare region effects based on the effective dimensionality of
the defects, and we illustrate it using examples from classical,
quantum, and nonequilibrium phase transitions.