343 research outputs found
Kramers-Wannier Duality and Random Bond Ising Model
We present a new combinatorial approach to the Ising model incorporating
arbitrary bond weights on planar graphs. In contrast to existing methodologies,
the exact free energy is expressed as the determinant of a set of ordered and
disordered operators defined on vertices and dual vertices respectively,
thereby explicitly demonstrating the Kramers-Wannier duality. The implications
of our derived formula for the random bond Ising model are further elucidated
Quantifying Long-Term Scientific Impact
The lack of predictability of citation-based measures frequently used to
gauge impact, from impact factors to short-term citations, raises a fundamental
question: Is there long-term predictability in citation patterns? Here, we
derive a mechanistic model for the citation dynamics of individual papers,
allowing us to collapse the citation histories of papers from different
journals and disciplines into a single curve, indicating that all papers tend
to follow the same universal temporal pattern. The observed patterns not only
help us uncover basic mechanisms that govern scientific impact but also offer
reliable measures of influence that may have potential policy implications
Theory of random packings
We review a recently proposed theory of random packings. We describe the
volume fluctuations in jammed matter through a volume function, amenable to
analytical and numerical calculations. We combine an extended statistical
mechanics approach 'a la Edwards' (where the role traditionally played by the
energy and temperature in thermal systems is substituted by the volume and
compactivity) with a constraint on mechanical stability imposed by the
isostatic condition. We show how such approaches can bring results that can be
compared to experiments and allow for an exploitation of the statistical
mechanics framework. The key result is the use of a relation between the local
Voronoi volume of the constituent grains and the number of neighbors in contact
that permits a simple combination of the two approaches to develop a theory of
random packings. We predict the density of random loose packing (RLP) and
random close packing (RCP) in close agreement with experiments and develop a
phase diagram of jammed matter that provides a unifying view of the disordered
hard sphere packing problem and further shedding light on a diverse spectrum of
data, including the RLP state. Theoretical results are well reproduced by
numerical simulations that confirm the essential role played by friction in
determining both the RLP and RCP limits. Finally we present an extended
discussion on the existence of geometrical and mechanical coordination numbers
and how to measure both quantities in experiments and computer simulations.Comment: 9 pages, 5 figures. arXiv admin note: text overlap with
arXiv:0808.219
Quantum Geometry of Expectation Values
We propose a novel framework for the quantum geometry of expectation values
over arbitrary sets of operators and establish a link between this geometry and
the eigenstates of Hamiltonian families generated by these operators. We show
that the boundary of expectation value space corresponds to the ground state,
which presents a natural bound that generalizes Heisenberg's uncertainty
principle. To demonstrate the versatility of our framework, we present several
practical applications, including providing a stronger nonlinear quantum bound
that violates the Bell inequality and an explicit construction of the density
functional. Our approach provides an alternative time-independent quantum
formulation that transforms the linear problem in a high-dimensional Hilbert
space into a nonlinear algebro-geometric problem in a low dimension, enabling
us to gain new insights into quantum systems
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Built-in self test of RF subsystems
textWith the rapid development of wireless and wireline communications, a variety of new standards and applications are emerging in the marketplace. In order to achieve higher levels of integration, RF circuits are frequently embedded into System on Chip (SoC) or System in Package (SiP) products. These developments, however, lead to new challenges in manufacturing test time and cost. Use of traditional RF test techniques requires expensive high frequency test instruments and long test time, which makes test one of the bottlenecks for reducing IC costs. This research is in the area of built-in self test technique for RF subsystems. In the test approach followed in this research, on-chip detectors are used to calculate circuits specifications, and data converters are used to collect the data for analysis by an on-chip processor. A novel on-chip amplitude detector has been designed and optimized for RF circuit specification test. By using on-chip detectors, both the system performance and specifications of the individual components can be accurately measured. On-chip measurement results need to be collected by Analog to Digital Converters (ADCs). A novel time domain, low power ADC has been designed for this purpose. The ADC architecture is based on a linear voltage controlled delay line. Using this structure results in a linear transfer function for the input dependent delay. The time delay difference is then compared to a reference to generate a digital code. Two prototype test chips were fabricated in commercial CMOS processes. One is for the RF transceiver front end with on-chip detectors; the other is for the test ADC. The 940MHz RF transceiver front-end was implemented with on-chip detectors in a 0.18 [micrometer] CMOS technology. The chips were mounted onto RF Printed Circuit Boards (PCBs), with tunable power supply and biasing knobs. The detector was characterized with measurements which show that the detector keeps linear performance over a wide input amplitude range of 500mV. Preliminary simulation and measurements show accurate transceiver performance prediction under process variations. A 300MS/s 6 bit ADC was designed using the novel time domain architecture in a 0.13 [micrometer] standard digital CMOS process. The simulation results show 36.6dB Signal to Noise Ratio (SNR), 34.1dB Signal to Noise and Distortion Ratio (SNDR) for 99MHz input, Differential Non-Linearity (DNL)<0.2 Least Significant Bit (LSB), and Integral Non-Linearity (INL)<0.5LSB. Overall chip power is 2.7mW with a 1.2V power supply. The built-in detector RF test was extended to a full transceiver RF front end test with a loop-back setup, so that measurements can be made to verify the benefits of the technique. The application of the approach to testing gain, linearity and noise figure was investigated. New detector types are also evaluated. In addition, the low-power delay-line based ADC was characterized and improved to facilitate gathering of data from the detector. Several improved ADC structures at the system level are also analyzed. The built-in detector based RF test technique enables the cost-efficient test for SoCs.Electrical and Computer Engineerin
Cavity method for force transmission in jammed disordered packings of hard particles
The force distribution of jammed disordered packings has always been
considered a central object in the physics of granular materials. However, many
of its features are poorly understood. In particular, analytic relations to
other key macroscopic properties of jammed matter, such as the contact network
and its coordination number, are still lacking. Here we develop a mean-field
theory for this problem, based on the consideration of the contact network as a
random graph where the force transmission becomes a constraint optimization
problem. We can thus use the cavity method developed in the last decades within
the statistical physics of spin glasses and hard computer science problems.
This method allows us to compute the force distribution for random
packings of hard particles of any shape, with or without friction. We find a
new signature of jamming in the small force behavior , whose exponent has attracted recent active interest: we find a
finite value for , along with . Furthermore, we relate
the force distribution to a lower bound of the average coordination number of jammed packings of frictional spheres with
coefficient . This bridges the gap between the two known isostatic limits
(in dimension ) and by extending the naive Maxwell's counting argument to
frictional spheres. The theoretical framework describes different types of
systems, such as non-spherical objects in arbitrary dimensions, providing a
common mean-field scenario to investigate force transmission, contact networks
and coordination numbers of jammed disordered packings
Modeling and Predicting Popularity Dynamics via Reinforced Poisson Processes
An ability to predict the popularity dynamics of individual items within a
complex evolving system has important implications in an array of areas. Here
we propose a generative probabilistic framework using a reinforced Poisson
process to model explicitly the process through which individual items gain
their popularity. This model distinguishes itself from existing models via its
capability of modeling the arrival process of popularity and its remarkable
power at predicting the popularity of individual items. It possesses the
flexibility of applying Bayesian treatment to further improve the predictive
power using a conjugate prior. Extensive experiments on a longitudinal citation
dataset demonstrate that this model consistently outperforms existing
popularity prediction methods.Comment: 8 pages, 5 figure; 3 table
From Micro to Macro: Uncovering and Predicting Information Cascading Process with Behavioral Dynamics
Cascades are ubiquitous in various network environments. How to predict these
cascades is highly nontrivial in several vital applications, such as viral
marketing, epidemic prevention and traffic management. Most previous works
mainly focus on predicting the final cascade sizes. As cascades are typical
dynamic processes, it is always interesting and important to predict the
cascade size at any time, or predict the time when a cascade will reach a
certain size (e.g. an threshold for outbreak). In this paper, we unify all
these tasks into a fundamental problem: cascading process prediction. That is,
given the early stage of a cascade, how to predict its cumulative cascade size
of any later time? For such a challenging problem, how to understand the micro
mechanism that drives and generates the macro phenomenons (i.e. cascading
proceese) is essential. Here we introduce behavioral dynamics as the micro
mechanism to describe the dynamic process of a node's neighbors get infected by
a cascade after this node get infected (i.e. one-hop subcascades). Through
data-driven analysis, we find out the common principles and patterns lying in
behavioral dynamics and propose a novel Networked Weibull Regression model for
behavioral dynamics modeling. After that we propose a novel method for
predicting cascading processes by effectively aggregating behavioral dynamics,
and propose a scalable solution to approximate the cascading process with a
theoretical guarantee. We extensively evaluate the proposed method on a large
scale social network dataset. The results demonstrate that the proposed method
can significantly outperform other state-of-the-art baselines in multiple tasks
including cascade size prediction, outbreak time prediction and cascading
process prediction.Comment: 10 pages, 11 figure
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