16,662 research outputs found
Scaling, domains, and states in the four-dimensional random field Ising magnet
The four dimensional Gaussian random field Ising magnet is investigated
numerically at zero temperature, using samples up to size , to test
scaling theories and to investigate the nature of domain walls and the
thermodynamic limit. As the magnetization exponent is more easily
distinguishable from zero in four dimensions than in three dimensions, these
results provide a useful test of conventional scaling theories. Results are
presented for the critical behavior of the heat capacity, magnetization, and
stiffness. The fractal dimensions of the domain walls at criticality are
estimated. A notable difference from three dimensions is the structure of the
spin domains: frozen spins of both signs percolate at a disorder magnitude less
than the value at the ferromagnetic to paramagnetic transition. Hence, in the
vicinity of the transition, there are two percolating clusters of opposite
spins that are fixed under any boundary conditions. This structure changes the
interpretation of the domain walls for the four dimensional case. The scaling
of the effect of boundary conditions on the interior spin configuration is
found to be consistent with the domain wall dimension. There is no evidence of
a glassy phase: there appears to be a single transition from two ferromagnetic
states to a single paramagnetic state, as in three dimensions. The slowing down
of the ground state algorithm is also used to study this model and the links
between combinatorial optimization and critical behavior.Comment: 13 pages, 16 figure
Effects of Disorder on Electron Transport in Arrays of Quantum Dots
We investigate the zero-temperature transport of electrons in a model of
quantum dot arrays with a disordered background potential. One effect of the
disorder is that conduction through the array is possible only for voltages
across the array that exceed a critical voltage . We investigate the
behavior of arrays in three voltage regimes: below, at and above the critical
voltage. For voltages less than , we find that the features of the
invasion of charge onto the array depend on whether the dots have uniform or
varying capacitances. We compute the first conduction path at voltages just
above using a transfer-matrix style algorithm. It can be used to
elucidate the important energy and length scales. We find that the geometrical
structure of the first conducting path is essentially unaffected by the
addition of capacitive or tunneling resistance disorder. We also investigate
the effects of this added disorder to transport further above the threshold. We
use finite size scaling analysis to explore the nonlinear current-voltage
relationship near . The scaling of the current near ,
, gives similar values for the effective exponent
for all varieties of tunneling and capacitive disorder, when the current is
computed for voltages within a few percent of threshold. We do note that the
value of near the transition is not converged at this distance from
threshold and difficulties in obtaining its value in the limit
Mixed mode education: implications for library user services
The Faculty of Information Technology at QUT does not formally carry out distance education for any of its courses. However, it has pursued a number of initiatives that have made it possible for students to carry out an increasing proportion of their coursework off-site. These initiatives include computer-managed learning, World Wide Web and CDROM delivery of administrative and educational materials, and most recently the development of an integrated learning environment (ILE) for electronic delivery. These developments have been complemented and supported by the QUT Library by means of different avenues of access to CDROMs, a regional electronic document delivery service (REDD), and an electronic reserve (E-Reserve) service. Issues associated with the operation and evaluation of such facilities are described, and future library role in educational delivery are discussed
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