6,256 research outputs found
Two-Point Entanglement Near a Quantum Phase Transition
In this work, we study the two-point entanglement S(i,j), which measures the
entanglement between two separated degrees of freedom (ij) and the rest of
system, near a quantum phase transition. Away from the critical point, S(i,j)
saturates with a characteristic length scale , as the distance |i-j|
increases. The entanglement length agrees with the correlation length.
The universality and finite size scaling of entanglement are demonstrated in a
class of exactly solvable one dimensional spin model. By connecting the
two-point entanglement to correlation functions in the long range limit, we
argue that the prediction power of a two-point entanglement is universal as
long as the two involved points are separated far enough.Comment: published versio
Magnons in real materials from density-functional theory
We present an implementation of the adiabatic spin-wave dynamics of Niu and
Kleinman. This technique allows to decouple the spin and charge excitations of
a many-electron system using a generalization of the adiabatic approximation.
The only input for the spin-wave equations of motion are the energies and Berry
curvatures of many-electron states describing frozen spin spirals. The latter
are computed using a newly developed technique based on constrained
density-functional theory, within the local spin density approximation and the
pseudo-potential plane-wave method. Calculations for iron show an excellent
agreement with experiments.Comment: 1 LaTeX file and 1 postscript figur
Theoretical model for ultracold molecule formation via adaptive feedback control
We investigate pump-dump photoassociation of ultracold molecules with
amplitude- and phase-modulated femtosecond laser pulses. For this purpose a
perturbative model for the light-matter interaction is developed and combined
with a genetic algorithm for adaptive feedback control of the laser pulse
shapes. The model is applied to the formation of 85Rb2 molecules in a
magneto-optical trap. We find for optimized pulse shapes an improvement for the
formation of ground state molecules by more than a factor of 10 compared to
unshaped pulses at the same pump-dump delay time, and by 40% compared to
unshaped pulses at the respective optimal pump-dump delay time. Since our model
yields directly the spectral amplitudes and phases of the optimized pulses, the
results are directly applicable in pulse shaping experiments
Microscopic theory of vortex dynamics in homogeneous superconductors
Vortex dynamics in fermionic superfluids is carefully considered from the
microscopic point of view. Finite temperatures, as well as impurities, are
explicitly incorporated. To enable readers understand the physical
implications, macroscopic demonstrations based on thermodynamics and
fluctuations- dissipation theorems are constructed. For the first time a clear
summary and a critical review of previous results are given.Comment: Presentations are made more straightforward. A detailed presentation
that why the vortex friction is finite when the geometric phase exists, as
required by referees, though I think it is obviou
Energy spectra, wavefunctions and quantum diffusion for quasiperiodic systems
We study energy spectra, eigenstates and quantum diffusion for one- and
two-dimensional quasiperiodic tight-binding models. As our one-dimensional
model system we choose the silver mean or `octonacci' chain. The
two-dimensional labyrinth tiling, which is related to the octagonal tiling, is
derived from a product of two octonacci chains. This makes it possible to treat
rather large systems numerically. For the octonacci chain, one finds singular
continuous energy spectra and critical eigenstates which is the typical
behaviour for one-dimensional Schr"odinger operators based on substitution
sequences. The energy spectra for the labyrinth tiling can, depending on the
strength of the quasiperiodic modulation, be either band-like or fractal-like.
However, the eigenstates are multifractal. The temporal spreading of a
wavepacket is described in terms of the autocorrelation function C(t) and the
mean square displacement d(t). In all cases, we observe power laws for C(t) and
d(t) with exponents -delta and beta, respectively. For the octonacci chain,
0<delta<1, whereas for the labyrinth tiling a crossover is observed from
delta=1 to 0<delta<1 with increasing modulation strength. Corresponding to the
multifractal eigenstates, we obtain anomalous diffusion with 0<beta<1 for both
systems. Moreover, we find that the behaviour of C(t) and d(t) is independent
of the shape and the location of the initial wavepacket. We use our results to
check several relations between the diffusion exponent beta and the fractal
dimensions of energy spectra and eigenstates that were proposed in the
literature.Comment: 24 pages, REVTeX, 10 PostScript figures included, major revision, new
results adde
Symbolic Logic meets Machine Learning: A Brief Survey in Infinite Domains
The tension between deduction and induction is perhaps the most fundamental
issue in areas such as philosophy, cognition and artificial intelligence (AI).
The deduction camp concerns itself with questions about the expressiveness of
formal languages for capturing knowledge about the world, together with proof
systems for reasoning from such knowledge bases. The learning camp attempts to
generalize from examples about partial descriptions about the world. In AI,
historically, these camps have loosely divided the development of the field,
but advances in cross-over areas such as statistical relational learning,
neuro-symbolic systems, and high-level control have illustrated that the
dichotomy is not very constructive, and perhaps even ill-formed. In this
article, we survey work that provides further evidence for the connections
between logic and learning. Our narrative is structured in terms of three
strands: logic versus learning, machine learning for logic, and logic for
machine learning, but naturally, there is considerable overlap. We place an
emphasis on the following "sore" point: there is a common misconception that
logic is for discrete properties, whereas probability theory and machine
learning, more generally, is for continuous properties. We report on results
that challenge this view on the limitations of logic, and expose the role that
logic can play for learning in infinite domains
A metallic phase in quantum Hall systems
The electronic eigenstates of a quantum Hall (QH) system are chiral states.
Strong inter-Landau-band mixings among these states can occur when the
bandwidth is comparable to the spacing of two adjacent Landau bands. We show
that mixing of localized states with opposite chirality can delocalize
electronic states. Based on numerical results, we propose the existence of a
metallic phase between two adjacent QH phases and between a QH phase and the
insulating phase. This result is consistent with non-scaling behaviors observed
in recent experiments on quantum-Hall-liquid-to-insulator transition.Comment: 5 pages, 3 figures. Will be published in Phys. Rev. Let
Improved Bond Strength of Cyanoacrylate Adhesives Through Nanostructured Chromium Adhesion Layers
The performance of many consumer products suffers due to weak and inconsistent bonds formed to low surface energy polymer materials, such as polyolefin-based high-density polyethylene (HDPE), with adhesives, such as cyanoacrylate. In this letter, we present an industrially relevant means of increasing bond shear strength and consistency through vacuum metallization of chromium thin films and nanorods, using HDPE as a prototype material and cyanoacrylate as a prototype adhesive. For the as received HDPE surfaces, unmodified bond shear strength is shown to be only 0.20 MPa with a standard deviation of 14 %. When Cr metallization layers are added onto the HDPE at thicknesses of 50 nm or less, nanorod-structured coatings outperform continuous films and have a maximum bond shear strength of 0.96 MPa with a standard deviation of 7 %. When the metallization layer is greater than 50 nm thick, continuous films demonstrate greater performance than nanorod coatings and have a maximum shear strength of 1.03 MPa with a standard deviation of 6 %. Further, when the combination of surface roughening with P400 grit sandpaper and metallization is used, 100-nm-thick nanorod coatings show a tenfold increase in shear strength over the baseline, reaching a maximum of 2.03 MPa with a standard deviation of only 3 %. The substantial increase in shear strength through metallization, and the combination of roughening with metallization, may have wide-reaching implications in consumer products which utilize low surface energy plastics
Quantum information distributors: Quantum network for symmetric and asymmetric cloning in arbitrary dimension and continuous limit
We show that for any Hilbert-space dimension, the optimal universal quantum
cloner can be constructed from essentially the same quantum circuit, i.e., we
find a universal design for universal cloners. In the case of infinite
dimensions (which includes continuous variable quantum systems) the universal
cloner reduces to an essentially classical device. More generally, we construct
a universal quantum circuit for distributing qudits in any dimension which acts
covariantly under generalized displacements and momentum kicks. The behavior of
this covariant distributor is controlled by its initial state. We show that
suitable choices for this initial state yield both universal cloners and
optimized cloners for limited alphabets of states whose states are related by
generalized phase-space displacements.Comment: 10 revtex pages, no figure
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