8,843 research outputs found
Interplay of disorder and geometrical frustration in doped Gadolinium Gallium Garnet
The geometrically-frustrated, triangular antiferromagnet GGG exhibits a rich
mix of short-range order and isolated quantum states. We investigate the
effects of up to 1% Neodymium substitution for Gallium on the ac magnetic
response at temperatures below 1 K in both the linear and nonlinear regimes.
Substitutional disorder actually drives the system towards a more perfectly
frustrated state, apparently compensating for the effect of imperfect
Gadolinium/Gallium stoichiometry, while at the same time more closely
demarcating the boundaries of isolated, coherent clusters composed of hundreds
of spins. Optical measurements of the local Nd environment substantiate the
picture of an increased frustration index with doping.Comment: 5 pages, 5 figure
Conductivity of Metallic Si:B near the Metal-Insulator Transition: Comparison between Unstressed and Uniaxially Stressed Samples
The low-temperature dc conductivities of barely metallic samples of p-type
Si:B are compared for a series of samples with different dopant concentrations,
n, in the absence of stress (cubic symmetry), and for a single sample driven
from the metallic into the insulating phase by uniaxial compression, S. For all
values of temperature and stress, the conductivity of the stressed sample
collapses onto a single universal scaling curve. The scaling fit indicates that
the conductivity of si:B is proportional to the square-root of T in the
critical range. Our data yield a critical conductivity exponent of 1.6,
considerably larger than the value reported in earlier experiments where the
transition was crossed by varying the dopant concentration. The larger exponent
is based on data in a narrow range of stress near the critical value within
which scaling holds. We show explicitly that the temperature dependences of the
conductivity of stressed and unstressed Si:B are different, suggesting that a
direct comparison of the critical behavior and critical exponents for stress-
tuned and concentration-tuned transitions may not be warranted
High Resolution Study of Magnetic Ordering at Absolute Zero
High fidelity pressure measurements in the zero temperature limit provide a
unique opportunity to study the behavior of strongly interacting, itinerant
electrons with coupled spin and charge degrees of freedom. Approaching the
exactitude that has become the hallmark of experiments on classical critical
phenomena, we characterize the quantum critical behavior of the model,
elemental antiferromagnet chromium, lightly doped with vanadium. We resolve the
sharp doubling of the Hall coefficient at the quantum critical point and trace
the dominating effects of quantum fluctuations up to surprisingly high
temperatures.Comment: 5 pages, 4 figure
Critical Behavior of the Conductivity of Si:P at the Metal-Insulator Transition under Uniaxial Stress
We report new measurements of the electrical conductivity sigma of the
canonical three-dimensional metal-insulator system Si:P under uniaxial stress
S. The zero-temperature extrapolation of sigma(S,T -> 0) ~\S - S_c\^mu shows an
unprecidentedly sharp onset of finite conductivity at S_c with an exponent mu =
1. The value of mu differs significantly from that of earlier stress-tuning
results. Our data show dynamical sigma(S,T) scaling on both metallic and
insulating sides, viz. sigma(S,T) = sigma_c(T) F(\S - S_cT^y) where sigma_c(T)
is the conductivity at the critical stress S_c. We find y = 1/znu = 0.34 where
nu is the correlation-length exponent and z the dynamic critical exponent.Comment: 5 pages, 4 figure
Baby-Step Giant-Step Algorithms for the Symmetric Group
We study discrete logarithms in the setting of group actions. Suppose that
is a group that acts on a set . When , a solution
to can be thought of as a kind of logarithm. In this paper, we study
the case where , and develop analogs to the Shanks baby-step /
giant-step procedure for ordinary discrete logarithms. Specifically, we compute
two sets such that every permutation of can be
written as a product of elements and . Our
deterministic procedure is optimal up to constant factors, in the sense that
and can be computed in optimal asymptotic complexity, and and
are a small constant from in size. We also analyze randomized
"collision" algorithms for the same problem
Inference with interference between units in an fMRI experiment of motor inhibition
An experimental unit is an opportunity to randomly apply or withhold a
treatment. There is interference between units if the application of the
treatment to one unit may also affect other units. In cognitive neuroscience, a
common form of experiment presents a sequence of stimuli or requests for
cognitive activity at random to each experimental subject and measures
biological aspects of brain activity that follow these requests. Each subject
is then many experimental units, and interference between units within an
experimental subject is likely, in part because the stimuli follow one another
quickly and in part because human subjects learn or become experienced or
primed or bored as the experiment proceeds. We use a recent fMRI experiment
concerned with the inhibition of motor activity to illustrate and further
develop recently proposed methodology for inference in the presence of
interference. A simulation evaluates the power of competing procedures.Comment: Published by Journal of the American Statistical Association at
http://www.tandfonline.com/doi/full/10.1080/01621459.2012.655954 . R package
cin (Causal Inference for Neuroscience) implementing the proposed method is
freely available on CRAN at https://CRAN.R-project.org/package=ci
Variational Approach to Gaussian Approximate Coherent States: Quantum Mechanics and Minisuperspace Field Theory
This paper has a dual purpose. One aim is to study the evolution of coherent
states in ordinary quantum mechanics. This is done by means of a Hamiltonian
approach to the evolution of the parameters that define the state. The
stability of the solutions is studied. The second aim is to apply these
techniques to the study of the stability of minisuperspace solutions in field
theory. For a theory we show, both by means of perturbation
theory and rigorously by means of theorems of the K.A.M. type, that the
homogeneous minisuperspace sector is indeed stable for positive values of the
parameters that define the field theory.Comment: 26 pages, Plain TeX, no figure
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