387 research outputs found
Ladder approximation to spin velocities in quantum wires
The spin sector of charge-spin separated single mode quantum wires is
studied, accounting for realistic microscopic electron-electron interactions.
We utilize the ladder approximation (LA) to the interaction vertex and exploit
thermodynamic relations to obtain spin velocities. Down to not too small
carrier densities our results compare well with existing quantum Monte-Carlo
(QMC) data. Analyzing second order diagrams we identify logarithmically
divergent contributions as crucial which the LA includes but which are missed,
for example, by the self-consistent Hartree-Fock approximation. Contrary to
other approximations the LA yields a non-trivial spin conductance. Its
considerably smaller computational effort compared to numerically exact
methods, such as the QMC method, enables us to study overall dependences on
interaction parameters. We identify the short distance part of the interaction
to govern spin sector properties.Comment: 6 pages, 6 figures, to appear in Physical Review
Phase diagram for a class of spin-half Heisenberg models interpolating between the square-lattice, the triangular-lattice and the linear chain limits
We study the spin-half Heisenberg models on an anisotropic two-dimensional
lattice which interpolates between the square-lattice at one end, a set of
decoupled spin-chains on the other end, and the triangular-lattice Heisenberg
model in between. By series expansions around two different dimer ground states
and around various commensurate and incommensurate magnetically ordered states,
we establish the phase diagram for this model of a frustrated antiferromagnet.
We find a particularly rich phase diagram due to the interplay of magnetic
frustration, quantum fluctuations and varying dimensionality. There is a large
region of the usual 2-sublattice Ne\'el phase, a 3-sublattice phase for the
triangular-lattice model, a region of incommensurate magnetic order around the
triangular-lattice model, and regions in parameter space where there is no
magnetic order. We find that the incommensurate ordering wavevector is in
general altered from its classical value by quantum fluctuations. The regime of
weakly coupled chains is particularly interesting and appears to be nearly
critical.Comment: RevTeX, 15 figure
Tomonaga-Luttinger parameters for quantum wires
The low-energy properties of a homogeneous one-dimensional electron system
are completely specified by two Tomonaga-Luttinger parameters and
. In this paper we discuss microscopic estimates of the values of
these parameters in semiconductor quantum wires that exploit their relationship
to thermodynamic properties. Motivated by the recognized similarity between
correlations in the ground state of a one-dimensional electron liquid and
correlations in a Wigner crystal, we evaluate these thermodynamic quantities in
a self-consistent Hartree-Fock approximation. According to our calculations,
the Hartree-Fock approximation ground state is a Wigner crystal at all electron
densities and has antiferromagnetic order that gradually evolves from
spin-density-wave to localized in character as the density is lowered. Our
results for are in good agreement with weak-coupling perturbative
estimates at high densities, but deviate strongly at low
densities, especially when the electron-electron interaction is screened at
long distances. vanishes at small carrier density
whereas we conjecture that when , implying that
should pass through a minimum at an intermediate density.
Observation of such a non-monotonic dependence on particle density would allow
to measure the range of the microscopic interaction. In the spin sector we find
that the spin velocity decreases with increasing interaction strength or
decreasing . Strong correlation effects make it difficult to obtain fully
consistent estimates of from Hartree-Fock calculations. We
conjecture that v_{\sigma}/\vf\propto n/V_0 in the limit where
is the interaction strength.Comment: RevTeX, 23 pages, 8 figures include
Optimum electrode configurations for fast ion separation in microfabricated surface ion traps
For many quantum information implementations with trapped ions, effective
shuttling operations are important. Here we discuss the efficient separation
and recombination of ions in surface ion trap geometries. The maximum speed of
separation and recombination of trapped ions for adiabatic shuttling operations
depends on the secular frequencies the trapped ion experiences in the process.
Higher secular frequencies during the transportation processes can be achieved
by optimising trap geometries. We show how two different arrangements of
segmented static potential electrodes in surface ion traps can be optimised for
fast ion separation or recombination processes. We also solve the equations of
motion for the ion dynamics during the separation process and illustrate
important considerations that need to be taken into account to make the process
adiabatic
State-of-the-Art in Weighted Finite-State Spell-Checking
Proceeding volume: 2The following claims can be made about finite-state methods for spell-checking: 1) Finite-state language models provide support for morphologically complex languages that word lists, affix stripping and similar approaches do not provide; 2) Weighted finite-state models have expressive power equal to other, state-of-the-art string algorithms used by contemporary spell-checkers; and 3) Finite-state models are at least as fast as other string algorithms for lookup and error correction. In this article, we use some contemporary non-finite-state spell-checking methods as a baseline and perform tests in light of the claims, to evaluate state-of-the-art finite-state spell-checking methods. We verify that finite-state spell-checking systems outperform the traditional approaches for English. We also show that the models for morphologically complex languages can be made to perform on par with English systems.Peer reviewe
A high-precision rf trap with minimized micromotion for an In+ multiple-ion clock
We present an experiment to characterize our new linear ion trap designed for
the operation of a many-ion optical clock using 115-In^+ as clock ions. For the
characterization of the trap as well as the sympathetic cooling of the clock
ions we use 172-Yb^+. The trap design has been derived from finite element
method (FEM) calculations and a first prototype based on glass-reinforced
thermoset laminates was built. This paper details on the trap manufacturing
process and micromotion measurement. Excess micromotion is measured using
photon-correlation spectroscopy with a resolution of 1.1nm in motional
amplitude, and residual axial rf fields in this trap are compared to FEM
calculations. With this method, we demonstrate a sensitivity to systematic
clock shifts due to excess micromotion of |({\Delta}{\nu}/{\nu})| = 8.5x10^-20.
Based on the measurement of axial rf fields of our trap, we estimate a number
of twelve ions that can be stored per trapping segment and used as an optical
frequency standard with a fractional inaccuracy of \leq 1x10^-18 due to
micromotion.Comment: 19 pages with 14 picture
A robust microparticle platform for a STING-targeted adjuvant that enhances both humoral and cellular immunity during vaccination
Most FDA-approved adjuvants for infectious agents boost humoral but not cellular immunity, and have poorly-understood mechanisms. Stimulator of interferon genes (STING, also known as MITA, MPYS, or ERIS) is an exciting adjuvant target due to its role in cyclic dinucleotide (CDN)-driven anti-viral immunity; however, a major hindrance is STING's cytosolic localization which requires intracellular delivery of its agonists. As a result, STING agonists administered in a soluble form have elicited suboptimal immune responses. Delivery of STING agonists via particle platforms has proven a more successful strategy, but the opportunity for improved formulations and bioactivity remains. In this study we evaluated the adjuvant activity of the potent STING agonist, CDN 3′3′-cGAMP (cGAMP), encapsulated in acid-sensitive acetalated dextran (Ace-DEX) polymeric microparticles (MPs) which passively target antigen-presenting cells for intracellular release. This formulation was superior to all particle delivery systems evaluated and maintained its bioactivity following a sterilizing dose of gamma irradiation. Compared to soluble cGAMP, the Ace-DEX cGAMP MPs enhanced type-I interferon responses nearly 1000-fold in vitro and 50-fold in vivo, caused up to a 104-fold boost in antibody titers, increased Th1-associated responses, and expanded germinal center B cells and memory T cells. Furthermore, the encapsulated cGAMP elicited no observable toxicity in animals and achieved protective immunity against a lethal influenza challenge seven months post-immunization when using CDN adjuvant doses up to 100-fold lower than previous reports. For these reasons, Ace-DEX MP-encapsulated cGAMP represents a potent vaccine adjuvant of humoral and cellular immunity
Evidence for a mixed mass composition at the `ankle' in the cosmic-ray spectrum
We report a first measurement for ultra-high energy cosmic rays of the
correlation between the depth of shower maximum and the signal in the water
Cherenkov stations of air-showers registered simultaneously by the fluorescence
and the surface detectors of the Pierre Auger Observatory. Such a correlation
measurement is a unique feature of a hybrid air-shower observatory with
sensitivity to both the electromagnetic and muonic components. It allows an
accurate determination of the spread of primary masses in the cosmic-ray flux.
Up till now, constraints on the spread of primary masses have been dominated by
systematic uncertainties. The present correlation measurement is not affected
by systematics in the measurement of the depth of shower maximum or the signal
in the water Cherenkov stations. The analysis relies on general characteristics
of air showers and is thus robust also with respect to uncertainties in
hadronic event generators. The observed correlation in the energy range around
the `ankle' at differs significantly from
expectations for pure primary cosmic-ray compositions. A light composition made
up of proton and helium only is equally inconsistent with observations. The
data are explained well by a mixed composition including nuclei with mass . Scenarios such as the proton dip model, with almost pure compositions, are
thus disfavoured as the sole explanation of the ultrahigh-energy cosmic-ray
flux at Earth.Comment: Published version. Added journal reference and DOI. Added Report
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