Disorder Induced Phase Transition in a Random Quantum Antiferromagnet

A two-dimensional Heisenberg model with random antiferromagnetic nearest-neighbor exchange is studied using quantum Monte Carlo techniques. As the strength of the randomness is increased, the system undergoes a transition from an antiferromagnetically ordered ground state to a gapless disordered state. The finite-size scaling of the staggered structure factor and susceptibility is consistent with a dynamic exponent $z = 2$.Comment: Revtex 3.0, 10 pages + 5 postscript figures available upon request, UCSBTH-94-1

Rare region effects at classical, quantum, and non-equilibrium phase transitions

Rare regions, i.e., rare large spatial disorder fluctuations, can dramatically change the properties of a phase transition in a quenched disordered system. In generic classical equilibrium systems, they lead to an essential singularity, the so-called Griffiths singularity, of the free energy in the vicinity of the phase transition. Stronger effects can be observed at zero-temperature quantum phase transitions, at nonequilibrium phase transitions, and in systems with correlated disorder. In some cases, rare regions can actually completely destroy the sharp phase transition by smearing. This topical review presents a unifying framework for rare region effects at weakly disordered classical, quantum, and nonequilibrium phase transitions based on the effective dimensionality of the rare regions. Explicit examples include disordered classical Ising and Heisenberg models, insulating and metallic random quantum magnets, and the disordered contact process.Comment: Topical review, 68 pages, 14 figures, final version as publishe

Viking Age garden plants from southern Scandinavia: diversity, taphonomy and cultural aspects

Plant finds recovered from archaeological sites in southern Scandinavia dated to the Viking Age reflect the diversity of useful plants that were cultivated and collected. This review presents the results of 14 investigations of deposits that are dated between AD 775 and 1050. The site types are categorized as agrarian, urban, military and burials. Garden plants are unevenly distributed, as the greatest diversity is recorded in features from urban contexts. We argue that taphonomic processes played an important role in the picture displayed. Archaeobotanical research results from neighbouring regions suggest that Viking Age horticulture has its roots in older traditions, and that the spectrum of garden plants is influenced by central and north-western European horticultural customs, which were to a great extent shaped by Roman occupation

The Medicine Tree: Unsettling palaeoecological perceptions of past environments and human activity

In this paper, we consider palaeoecological approaches to past landscapes and reflect upon how these are relevant to archaeological themes concerning concepts of environmental change and the role of past and present human communities in these processes. In particular, we highlight the importance of local context in the perception and understanding of landscape. Utilising a case study from Nepal, we look to ‘unsettle’ a conventional palaeoecological interpretation of a pollen record, originally constructed on western ecological principles, and instead draw on an interpretative perspective rooted in local Buddhist ecological knowledge, or a ‘folk taxonomy’, known as ‘The Medicine Tree’. We discuss how the interpretations of patterns and processes of vegetation change from a pollen record are not necessarily absolute. In particular, we outline how the palaeoecological frame of enquiry and reference is rooted in an essentially Eurocentric, Western scientific paradigm, which, in turn, shapes how we perceive and conceive of past landscapes and the role of ‘anthropogenic impact’ on vegetation. The aim of this is not to suggest that scientific approaches to the ‘reconstruction’ of past landscapes are necessarily invalid, but to illustrate how ‘empirical’ scientific methods and interpretations in archaeological science are contingent upon specific social and cultural frames of reference. We discuss the broader relevance of this, such as how we interpret past human activity and perception of landscape change, the ways in which we might look to mobilise research in the context of contemporary problems, issues concerning ‘degraded landscapes’ and how we incorporate local and archaeological perspectives with palaeoecology within an interconnected and iterative process