skip to main content
OSTI.GOV title logo U.S. Department of Energy
Office of Scientific and Technical Information

Title: Percolation Quantum Phase Transitions in Diluted Magnets

Abstract

No abstract prepared.

Authors:
;
Publication Date:
Research Org.:
Ames Laboratory (AMES), Ames, IA
Sponsoring Org.:
USDOE Office of Science and Technology (OST) - (EM-50)
OSTI Identifier:
877164
Report Number(s):
IS-J 7076
Journal ID: ISSN 0031-9007; PRLTAO; TRN: US0601274
DOE Contract Number:
W-7405-Eng-82
Resource Type:
Journal Article
Resource Relation:
Journal Name: Physical Review Letters; Journal Volume: 95; Journal Issue: 23
Country of Publication:
United States
Language:
English
Subject:
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND SUPERFLUIDITY; MAGNETS; PHASE TRANSFORMATIONS; KINETICS

Citation Formats

Thomas Vojta, and Jorg Schmalian. Percolation Quantum Phase Transitions in Diluted Magnets. United States: N. p., 2005. Web. doi:10.1103/PhysRevLett.95.237206.
Thomas Vojta, & Jorg Schmalian. Percolation Quantum Phase Transitions in Diluted Magnets. United States. doi:10.1103/PhysRevLett.95.237206.
Thomas Vojta, and Jorg Schmalian. Fri . "Percolation Quantum Phase Transitions in Diluted Magnets". United States. doi:10.1103/PhysRevLett.95.237206.
@article{osti_877164,
title = {Percolation Quantum Phase Transitions in Diluted Magnets},
author = {Thomas Vojta and Jorg Schmalian},
abstractNote = {No abstract prepared.},
doi = {10.1103/PhysRevLett.95.237206},
journal = {Physical Review Letters},
number = 23,
volume = 95,
place = {United States},
year = {Fri Dec 02 00:00:00 EST 2005},
month = {Fri Dec 02 00:00:00 EST 2005}
}
  • Transition metal halides provide realizations of Ising, XY, and Heisenberg antiferromagnetis in one, two, and three dimensions. The interactions which are of short range, are generally well understood. By dilution with nonmagnetic species such as Zn/sup + +/ or Mg/sup + +/ one is able to prepare site-random alloys which correspond to random systems of particular interest in statistical mechanics. By mixing two magnetic ions such as Fe/sup + +/ and Co/sup + +/ one can produce magnetic crystals with competing interactions- either in the form of competing anisotropies or competing ferromagnetic and antiferromagnetic interactions. In this paper the resultsmore » of a series of neutron scattering experiments on these systems carried out at Brookhaven over the past several years are briefly reviewed. First the critical behavior in Rb/sub 2/Mn/sub 0.5/Ni/sub 0.5/F/sub 4/ and Fe/sub c/Zn/sub 1-c/F/sub 2/ which correspond to two-dimensional and three-dimensional random Ising systems, respectively, are discussed. Percolation phenomena have been studied in Rb/sub 2/Mn/sub c/Mg/sub 1-c/F/sub 4/, Rb/sub 2/Co/sub c/Mg/sub 1-c/F/sub 4/, KMn/sub c/Z/sub 1-c/F/sub 3/, and Mn/sub c/Zn/sub 1-c/F/sub 2/ which correspond to two- and three-dimensional Heisenberg and Ising models, respectively. In these cases c is chosen to be in the neighborhood of the nearest-neighbor percolation concentration. Application of a uniform field to the above systems generates a random staggered magnetic field; this has facilitated a systematic study of the random field problem. As we shall discuss in detail, a variety of novel, unexpected phenomena have been observed.« less
  • We introduce a simple model of quantum percolation and analyze it numerically using transfer matrix methods. A central point of this paper is that 3 both integer and fractional plateau transitions in the quantum Hall effect are due to quantum percolation. Within this model, we obtained the localization length exponent [nu]=2.4[plus minus]0.2, the dynamical exponent [ital z]=1, and the scaling functions for the conductivity tensor for both the integer and the fractional transitions. We show that our results agree extremely well with the experimental results for the integer plateau transition obtained by McEuen [ital et] [ital al].
  • Cited by 12
  • The properties of a disordered granular superconductor consisting of superconducting grains of size comparable to the zero-temperature bulk superconducting coherence length embedded in a nonsuperconducting host are studied by means of a randomly diluted Josephson tunnel junction model near a percolation threshold p/sub c/. A replicated (n..-->..0) continuum Landau-Ginzburg field theory describing the macroscopic properties of this material is derived from first principles. The mean-field phase diagram as a function of temperature T, applied magnetic field H, and grain concentration p exhibits a Meissner phase, an Abrikosov vortex lattice, and a spin-glass phase all arising from the low-temperature phase coherencemore » of the condensate wave function among the grains. For H = 0, in the superconducting phase, the macroscopic superfluid density rho/sub s/--(p-p/sub c/)/sup t/ as p..-->..p/sub c/ where t = 3 in mean-field theory. The spin-glass phase arises from frustration among loops of the percolating network in the presence of an applied magnetic field. Here rho/sub s/ = 0, leading to complete flux penetration on average but with a frozen-in distribution of randomly oriented tunneling supercurrents leading to power-law decaying --x/sup -(//sup d//sup -2)/ local fluctuations in the B field. In the low-temperature limit, vortices are shown to consist of spin-glass cores in a superconducting background.« less
  • From studies of two-phase systems (borosilicate matrices containing ZnSe or CdS quantum dots), it was found that the systems exhibit a specific feature associated with the percolation phase transition of charge carriers (excitons). The transition manifests itself as radical changes in the optical spectra of both ZnSe and CdS quantum dot systems and by fluctuations of the emission band intensities near the percolation threshold. These effects are due to microscopic fluctuations of the density of quantum dots. The average spacing between quantum dots is calculated taking into account their finite dimensions and the volume fraction occupied by the quantum dotsmore » at the percolation threshold. It is shown that clustering of quantum dots occurs via tunneling of charge carriers between the dots. A physical mechanism responsible for the percolation threshold for charge carriers is suggested. In the mechanism, the permittivity mismatch of the materials of the matrix and quantum dots plays an important role in delocalization of charge carriers (excitons): due to the mismatch, 'a dielectric trap' is formed at the external surface of the interface between the matrix and a quantum dot and, thus, surface exciton states are formed there. The critical concentrations of quantum dots are determined, such that the spatial overlapping of such surface states provides the percolation transition in both systems.« less