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Discoveries in condensed matter physics during the
last 15-20 years have revealed many new phenomena and materials
of remarkable richness and diversity. These include the high temperature
superconductors with transition temperatures at least five times
higher than previously known, “heavy fermion” systems,
the “colossal magnetoresistance” (CMR) materials, quantized
conductance in quantum wires, the integer and fractional quantum
Hall effects, fullerene systems, and the whole field of organic
conductors and superconductors.
These phenomena all appear to be manifestations of strong electron
correlation effects, and they present a severe challenge to the
traditional understanding of condensed matter.
Theoretical models exist, at least in basic form, and must form
the foundation of our understanding of these phenomena. The team
of Jaan Oitmaa, Chris Hamer, Oleg Sushkov and Robert Bursill was
awarded an ARC Discovery grant of approximately $750 000 over the
five year period 2003–2007 to determine the properties of
these models, in a manner that is systematic and reliable, and to
match them with experiment.
In addition to Weihong Zheng, who has been part of our group for
many years and is an expert in series expansion methods, we have
appointed two recent German PhD graduates, Jesko Sirker and Alexander
Weisse, to the team.
During the past year we have progressed on a number of fronts:
Oleg Sushkov and Valeri Kotov (a former postdoc in our group, and
now at the University of Lausanne) have obtained an analytic solution,
valid at low doping, for a model of strongly correlated electrons
in 2-dimensions, and have demonstrated the coexistence of spiral
magnetic ordering and superconductivity.
We have investigated models for several recently discovered new
materials, including NaxCoO2.yH2O which was recently discovered
to be superconducting for a range of doping (1/4<x<1/3).
The material TlCuCl3 was discovered a few years ago to have a gap
in the magnetic excitation spectrum, but to undergo a transition
to a field induced magnetically ordered state. This has been interpreted
as a Bose condensation of magnons. We have shown that the proposed
simple BEC scenario gives results in disagreement with experiment.
We have also continued to develop numerical methods for calculating
energies and spectral weights of multi-particle excitations.
Jaan Oitmaa, Chris Hamer,
Oleg Sushkov and Robert Bursill
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| Experimental magnetization curves (Nikuni
et al., Phys. Rev. Lett. 84, 5868 (2000)) and quasiparticle
spectrum (Ruegg. et al., Nature 423, 62 (2003)) compared to
our theoretical calculations (solid lines). |
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| Experimental structure factors for Cu(NO3)2.2.5D2O,
showing both one-magnon and two-magnon contributions, and corresponding
model calculations. |
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