5,460 research outputs found
Spin cluster operator theory for the Kagome lattice antiferromagnet
The spin-1/2 quantum antiferromagnet on the Kagome lattice provides a
quintessential example in the strongly correlated electron physics where both
effects of geometric frustration and quantum fluctuation are pushed to their
limit. Among possible non-magnetic ground states, the valence bond solid (VBS)
with a 36-site unit cell is one of the most promising candidates. A natural
theoretical framework for the analysis of such VBS order is to consider quantum
states on a bond connecting the nearest-neighboring sites as fundamental
quantum modes of the system and treat them as effectively independent "bond
particles." While correctly describing the VBS order in the ground state, this
approach, known as the bond operator theory, significantly overestimates the
lowest spin excitation energy. To overcome this problem, we take a next logical
step in this paper to improve the bond operator theory and consider extended
spin clusters as fundamental building blocks of the system. Depending on two
possible configurations of the VBS order, various spin clusters are considered:
(i) in the VBS order with staggered hexagonal resonance, we consider one spin
cluster for a David star and two spin clusters with each composed of a perfect
hexagon and three attached dimers, and (ii) in the VBS order with uniform
hexagonal resonance, one spin cluster composed of a David star and three
attached dimers. It is shown that the majority of low-energy spin excitations
are nearly or perfectly flat in energy. With most of its weight coming from the
David star, the lowest spin excitation has a gap much lower than the previous
value obtained by the bond operator theory, narrowing the difference against
exact diagonalization results.Comment: 24 pages, 10 figures, 6 table
Quantum Hall effect on centimeter scale chemical vapor deposited graphene films
We report observations of well developed half integer quantum Hall effect
(QHE) on mono layer graphene films of 7 mm \times 7 mm in size. The graphene
films are grown by chemical vapor deposition (CVD) on copper, then transferred
to SiO_{2} /Si substrates, with typical carrier mobilities \approx 4000 cm^{2}
/Vs. The large size graphene with excellent quality and electronic homogeneity
demonstrated in this work is promising for graphene-based quantum Hall
resistance standards, and can also facilitate a wide range of experiments on
quantum Hall physics of graphene and practical applications exploiting the
exceptional properties of graphene
Transport optimization on complex networks
We present a comparative study of the application of a recently introduced
heuristic algorithm to the optimization of transport on three major types of
complex networks. The algorithm balances network traffic iteratively by
minimizing the maximum node betweenness with as little path lengthening as
possible. We show that by using this optimal routing, a network can sustain
significantly higher traffic without jamming than in the case of shortest path
routing. A formula is proved that allows quick computation of the average
number of hops along the path and of the average travel times once the
betweennesses of the nodes are computed. Using this formula, we show that
routing optimization preserves the small-world character exhibited by networks
under shortest path routing, and that it significantly reduces the average
travel time on congested networks with only a negligible increase in the
average travel time at low loads. Finally, we study the correlation between the
weights of the links in the case of optimal routing and the betweennesses of
the nodes connected by them.Comment: 19 pages, 7 figure
Effects of laser fluence on silicon modification by four-beam laser interference
This paper discusses the effects of laser fluence on silicon modification by four-beam laser interference. In this work, four-beam laser interference was used to pattern single crystal silicon wafers for the fabrication of surface structures, and the number of laser pulses was applied to the process in air. By controlling the parameters of laser irradiation, different shapes of silicon structures were fabricated. The results were obtained with the single laser fluence of 354 mJ/cm, 495 mJ/cm, and 637 mJ/cm, the pulse repetition rate of 10 Hz, the laser exposure pulses of 30, 100, and 300, the laser wavelength of 1064 nm, and the pulse duration of 7-9 ns. The effects of the heat transfer and the radiation of laser interference plasma on silicon wafer surfaces were investigated. The equations of heat flow and radiation effects of laser plasma of interfering patterns in a four-beam laser interference distribution were proposed to describe their impacts on silicon wafer surfaces. The experimental results have shown that the laser fluence has to be properly selected for the fabrication of well-defined surface structures in a four-beam laser interference process. Laser interference patterns can directly fabricate different shape structures for their corresponding applications
Synthetic Graphene Grown by Chemical Vapor Deposition on Copper Foils
The discovery of graphene, a single layer of covalently bonded carbon atoms,
has attracted intense interests. Initial studies using mechanically exfoliated
graphene unveiled its remarkable electronic, mechanical and thermal properties.
There has been a growing need and rapid development in large-area deposition of
graphene film and its applications. Chemical vapour deposition on copper has
emerged as one of the most promising methods in obtaining large-scale graphene
films with quality comparable to exfoliated graphene. In this chapter, we
review the synthesis and characterizations of graphene grown on copper foil
substrates by atmospheric pressure chemical vapour deposition. We also discuss
potential applications of such large scale synthetic graphene.Comment: 23 pages, 4 figure
Concatenating dynamical decoupling with decoherence-free subspaces for quantum computation
A scheme to implement a quantum computer subjected to decoherence and
governed by an untunable qubit-qubit interaction is presented. By concatenating
dynamical decoupling through bang-bang (BB) pulse with decoherence-free
subspaces (DFSs) encoding, we protect the quantum computer from
environment-induced decoherence that results in quantum information dissipating
into the environment. For the inherent qubit-qubit interaction that is
untunable in the quantum system, BB control plus DFSs encoding will eliminate
its undesired effect which spoils quantum information in qubits. We show how
this quantum system can be used to implement universal quantum computation.Comment: 6 pages,2 figures, 1 tabl
Correlated Photons from Collective Excitations of Three-Level Atomic Ensemble
We systematically study the interaction between two quantized optical fields
and a cyclic atomic ensemble driven by a classic optical field. This so-called
atomic cyclic ensemble consists of three-level atoms with Delta-type
transitions due to the symmetry breaking, which can also be implemented in the
superconducting quantum circuit by Yu-xi Liu et al. [Phys. Rev. Lett. 95,
087001 (2005)]. We explore the dynamic mechanisms to creating the quantum
entanglements among photon states, and between photons and atomic collective
excitations by the coherent manipulation of the atom-photon system. It is shown
that the quantum information can be completely transferred from one quantized
optical mode to another, and the quantum information carried by the two
quantized optical fields can be stored in the collective modes of this atomic
ensemble by adiabatically controlling the classic field Rabi frequencies.Comment: 10 pages, 2 figure
Transient dynamics for sequence processing neural networks: effect of degree distributions
We derive a analytic evolution equation for overlap parameters including the
effect of degree distribution on the transient dynamics of sequence processing
neural networks. In the special case of globally coupled networks, the
precisely retrieved critical loading ratio is obtained,
where is the network size. In the presence of random networks, our
theoretical predictions agree quantitatively with the numerical experiments for
delta, binomial, and power-law degree distributions.Comment: 11 pages, 6 figure
A Comparative Study of within QCD Sum Rules with Two Typical Correlators up to Next-to-Leading Order
The B-decay constant is an important component for studying -meson
decays, which can be studied through QCD sum rules. We make a detailed
discussion on from two sum rules, i.e. sum rules I and II, which are
derived from the conventional correlator and the correlator with chiral
currents respectively. It is found that these two sum rules are consistent with
each other. However, the sum rules II has less uncertainty sources than that of
sum rules I, and then it can be more accurate if we know the dimension-four
gluon condensate well. It is found that decreases with the increment of
, and to compare with the Belle experimental data on , both sum rules
prefer smaller pole -quark mass, GeV. By varying all the
input parameters in their reasonable region and adding all the uncertainties
together in quadrature, we obtain MeV for sum rules I and
MeV for sum rules II.Comment: 11 pages, 4 figures, 2 tables. To match the printed version. To be
published in Communications in Theoretical Physic
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