2,854 research outputs found
Fractional Power-Law Spectral Response of CaCu3Ti4O12 Dielectric: Many-Body Effects
Spectral character of dielectric response in CaCu3Ti4O12 across 0.5Hz-4MHz
over 45-200K corresponding to neither the Debyean nor the KWW relaxation
patterns rather indicates a random-walk like diffusive dynamics of moments.
Non-linear relaxation here is due to the many body dipole-interactions, as
confirmed by spectral-fits of our measured permittivity to the Dissado-Hill
behaviour. Fractional power-laws observed in {\epsilon}*({\omega})
macroscopically reflect the fractal microscopic configurations. Below ~100K,
the power-law exponent m (n) steeply decreases (increases), indicating finite
length-scale collective response of moment-bearing entities. At higher
temperatures, m gradually approaches 1 and n falls to low values, reflecting
tendency towards the single-particle/Debyean relaxation.Comment: 10 pages, 3 figures, 22 reference
Discovery of Strange Kinetics in Bulk Material: Correlated Dipoles in CaCu3Ti4O12
Dielectric spectroscopy of CaCu3Ti4O12 was performed spanning broad ranges of
temperature (10-300K) and frequency (0.5Hz-2MHz). We attribute the permittivity
step-fall to the evolution of Kirkwood-Fr\"oehlich dipole-correlations;
reducing the moment-density due to anti-parallel orienting dipoles, with
decreasing temperature. Unambiguous sub-Arrhenic dispersion of the associated
loss-peak reveals the prime role of strange kinetics; used to describe
nonlinearity-governed meso-confined/fractal systems, witnessed here for the
first time in a bulk material. Effective energy-scale is seen to follow thermal
evolution of the moment density, and the maidenly estimated correlation-length
achieves mesoscopic scale below 100K. Temperature dependence of correlations
reveals emergence of a new, parallel-dipole-orientation branch below 85K. Novel
features observed define a crossover temperature window connecting the
single-dipoles regime and the correlated moments. Conciling known results, we
suggest a fractal-like self-similar configuration of Ca/Cu-rich sub-phases;
resultant heterogeneity endowing CaCu3Ti4O12 its peculiar electrical behaviour.Comment: 19 pages, 5 figures, 44 reference
Reentrant Adhesion Behavior in Nanocluster Deposition
We simulate the collision of atomic clusters with a weakly attractive surface
using molecular dynamics in a regime between soft-landing and fragmentation,
where the cluster undergoes large deformation but remains intact. As a function
of incident kinetic energy, we find a transition from adhesion to reflection at
low kinetic energies. We also identify a second adhesive regime at intermediate
kinetic energies, where strong deformation of the cluster leads to an increase
in contact area and adhesive energy.Comment: 7 pages, 6 figure
t-SURFF: Fully Differential Two-Electron Photo-Emission Spectra
The time dependent surface flux (t-SURFF) method is extended to single and
double ionization of two electron systems. Fully differential double emission
spectra by strong pulses at extreme UV and infrared wave length are calculated
using simulation volumes that only accommodate the effective range of the
atomic binding potential and the quiver radius of free electrons in the
external field. For a model system we find pronounced dependence of shake-up
and non-sequential double ionization on phase and duration of the laser pulse.
Extension to fully three-dimensional calculations is discussed
Label optimal regret bounds for online local learning
We resolve an open question from (Christiano, 2014b) posed in COLT'14
regarding the optimal dependency of the regret achievable for online local
learning on the size of the label set. In this framework the algorithm is shown
a pair of items at each step, chosen from a set of items. The learner then
predicts a label for each item, from a label set of size and receives a
real valued payoff. This is a natural framework which captures many interesting
scenarios such as collaborative filtering, online gambling, and online max cut
among others. (Christiano, 2014a) designed an efficient online learning
algorithm for this problem achieving a regret of , where
is the number of rounds. Information theoretically, one can achieve a regret of
. One of the main open questions left in this framework
concerns closing the above gap.
In this work, we provide a complete answer to the question above via two main
results. We show, via a tighter analysis, that the semi-definite programming
based algorithm of (Christiano, 2014a), in fact achieves a regret of
. Second, we show a matching computational lower bound. Namely,
we show that a polynomial time algorithm for online local learning with lower
regret would imply a polynomial time algorithm for the planted clique problem
which is widely believed to be hard. We prove a similar hardness result under a
related conjecture concerning planted dense subgraphs that we put forth. Unlike
planted clique, the planted dense subgraph problem does not have any known
quasi-polynomial time algorithms.
Computational lower bounds for online learning are relatively rare, and we
hope that the ideas developed in this work will lead to lower bounds for other
online learning scenarios as well.Comment: 13 pages; Changes from previous version: small changes to proofs of
Theorems 1 & 2, a small rewrite of introduction as well (this version is the
same as camera-ready copy in COLT '15
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