258 research outputs found
A microfluidic device for the study of the orientational dynamics of microrods
We describe a microfluidic device for studying the orientational dynamics of
microrods. The device enables us to experimentally investigate the tumbling of
microrods immersed in the shear flow in a microfluidic channel with a depth of
400 mu and a width of 2.5 mm. The orientational dynamics was recorded using a
20 X microscopic objective and a CCD camera. The microrods were produced by
shearing microdroplets of photocurable epoxy resin. We show different examples
of empirically observed tumbling. On the one hand we find that short stretches
of the experimentally determined time series are well described by fits to
solutions of Jeffery's approximate equation of motion [Jeffery, Proc. R. Soc.
London. 102 (1922), 161-179]. On the other hand we find that the empirically
observed trajectories drift between different solutions of Jeffery's equation.
We discuss possible causes of this orbit drift.Comment: 11 pages, 8 figure
Individualised risk assessment for diabetic retinopathy and optimisation of screening intervals: a scientific approach to reducing healthcare costs.
To access publisher's full text version of this article, please click on the hyperlink in Additional Links field or click on the hyperlink at the top of the page marked Files.
This article is open access.To validate a mathematical algorithm that calculates risk of diabetic retinopathy progression in a diabetic population with UK staging (R0-3; M1) of diabetic retinopathy. To establish the utility of the algorithm to reduce screening frequency in this cohort, while maintaining safety standards.The cohort of 9690 diabetic individuals in England, followed for 2 years. The algorithms calculated individual risk for development of preproliferative retinopathy (R2), active proliferative retinopathy (R3A) and diabetic maculopathy (M1) based on clinical data. Screening intervals were determined such that the increase in risk of developing certain stages of retinopathy between screenings was the same for all patients and identical to mean risk in fixed annual screening. Receiver operating characteristic curves were drawn and area under the curve calculated to estimate the prediction capability.The algorithm predicts the occurrence of the given diabetic retinopathy stages with area under the curve =80% for patients with type II diabetes (CI 0.78 to 0.81). Of the cohort 64% is at less than 5% risk of progression to R2, R3A or M1 within 2 years. By applying a 2 year ceiling to the screening interval, patients with type II diabetes are screened on average every 20 months, which is a 40% reduction in frequency compared with annual screening.The algorithm reliably identifies patients at high risk of developing advanced stages of diabetic retinopathy, including preproliferative R2, active proliferative R3A and maculopathy M1. Majority of patients have less than 5% risk of progression between stages within a year and a small high-risk group is identified. Screening visit frequency and presumably costs in a diabetic retinopathy screening system can be reduced by 40% by using a 2 year ceiling. Individualised risk assessment with 2 year ceiling on screening intervals may be a pragmatic next step in diabetic retinopathy screening in UK, in that safety is maximised and cost reduced by about 40%.Icelandic Research Counci
Schwinger-boson approach to quantum spin systems: Gaussian fluctuactions in the "natural" gauge
We compute the Gaussian-fluctuation corrections to the saddle-point
Schwinger-boson results using collective coordinate methods. Concrete
application to investigate the frustrated J1-J2 antiferromagnet on the square
lattice shows that, unlike the saddle-point predictions, there is a quantum
nonmagnetic phase for 0.53 < J2/J1 < 0.64. This result is obtained by
considering the corrections to the spin stiffness on large lattices and
extrapolating to the thermodynamic limit, which avoids the infinite-lattice
infrared divergencies associated to Bose condensation. The very good agreement
of our results with exact numerical values on finite clusters lends support to
the calculational scheme employed.Comment: 4 pages, Latex, 3 figures included as eps files,minor correction
Novel spin-liquid states in the frustrated Heisenberg antiferromagnet on the honeycomb lattice
Recent experiment on a honeycomb-lattice Heisenberg antiferromagnet (AF)
BiMnO(NO) revealed a novel spin-liquid-like behavior down to
low temperature, which was ascribed to the frustration effect due to the
competition between the AF nearest- and next-nearest-neighbor interactions
and . Motivated by the experiment, we study the ordering of the
- frustrated classical Heisenberg AF on a honeycomb lattice both by
a low-temperature expansion and a Monte Carlo simulation. The model has been
known to possess a massive degeneracy of the ground state, which, however,
might be lifted due to thermal fluctuations leading to a unique ordered state,
the effect known as 'order-by-disorder'. We find that the model exhibits an
intriguing ordering behavior, particularly near the AF phase boundary. The
energy scale of the order-by-disorder is suppressed there down to extremely low
temperatures, giving rise to exotic spin-liquid states like a "ring-liquid" or
a "pancake-liquid" state accompanied by the characteristic spin structure
factor and the field-induced antiferromagnetism. We argue that the recent
experimental data are explicable if the system is in such exotic spin-liquid
states
Spin Liquid Phases in 2D Frustrated XY Model
In this paper we consider the classical and quantum 2D XY
model. Spin wave calculations show that a spin liquid phase still exists in the
quantum case as for Heisenberg models. We formulate a semiclassical approach of
these models based on spin wave action and use a variational method to study
the role played by vortices. Liquid and crystal phases of vortex could emerge
in this description. These phases seem to be directly correlated with the spin
liquid one and to its crystalline interpretation.Comment: 16 pages, Latex, 4 figures. To be published in Phys. Rev.
Elevated visual dependency in young adults after chemotherapy in childhood
Chemotherapy in childhood can result in long-term neurophysiological side-effects, which could extend to visual processing, specifically the degree to which a person relies on vision to determine vertical and horizontal (visual dependency). We investigated whether adults treated with chemotherapy in childhood experience elevated visual dependency compared to controls and whether any difference is associated with the age at which subjects were treated. Visual dependency was measured in 23 subjects (mean age 25.3 years) treated in childhood with chemotherapy (CTS) for malignant, solid, non-CNS tumors. We also stratified CTS into two groups: those treated before 12 years of age and those treated from 12 years of age and older. Results were compared to 25 healthy, age-matched controls. The subjective visual horizontal (SVH) and vertical (SVV) orientations was recorded by having subjects position an illuminated rod to their perceived horizontal and vertical with and without a surrounding frame tilted clockwise and counter-clockwise 20° from vertical. There was no significant difference in rod accuracy between any CTS groups and controls without a frame. However, when assessing visual dependency using a frame, CTS in general (p = 0.006) and especially CTS treated before 12 years of age (p = 0.001) tilted the rod significantly further in the direction of the frame compared to controls. Our findings suggest that chemotherapy treatment before 12 years of age is associated with elevated visual dependency compared to controls, implying a visual bias during spatial activities. Clinicians should be aware of symptoms such as visual vertigo in adults treated with chemotherapy in childhood
Quantum Phase Transition in the Frustrated Heisenberg Antiferromagnet
Using the J_1-J_2 model, we present a description of quantum phase transition
from Neel ordered to the spin-liquid state based on the modified spin wave
theory. The general expression for the gap in the spectrum in the spin-liquid
phase is presented.Comment: 8 pages of REVTeX 3.0, one PostScript file appended (Eq. 15
corrected, two recent references added, + some minor changes
Global Phase Diagram of the Kondo Lattice: From Heavy Fermion Metals to Kondo Insulators
We discuss the general theoretical arguments advanced earlier for the T=0
global phase diagram of antiferromagnetic Kondo lattice systems, distinguishing
between the established and the conjectured. In addition to the well-known
phase of a paramagnetic metal with a "large" Fermi surface (P_L), there is also
an antiferromagnetic phase with a "small" Fermi surface (AF_S). We provide the
details of the derivation of a quantum non-linear sigma-model (QNLsM)
representation of the Kondo lattice Hamiltonian, which leads to an effective
field theory containing both low-energy fermions in the vicinity of a Fermi
surface and low-energy bosons near zero momentum. An asymptotically exact
analysis of this effective field theory is made possible through the
development of a renormalization group procedure for mixed fermion-boson
systems. Considerations on how to connect the AF_S and P_L phases lead to a
global phase diagram, which not only puts into perspective the theory of local
quantum criticality for antiferromagnetic heavy fermion metals, but also
provides the basis to understand the surprising recent experiments in
chemically-doped as well as pressurized YbRh2Si2. We point out that the AF_S
phase still occurs for the case of an equal number of spin-1/2 local moments
and conduction electrons. This observation raises the prospect for a global
phase diagram of heavy fermion systems in the Kondo-insulator regime. Finally,
we discuss the connection between the Kondo breakdown physics discussed here
for the Kondo lattice systems and the non-Fermi liquid behavior recently
studied from a holographic perspective.Comment: (v3) leftover typos corrected. (v2) Published version. 32 pages, 4
figures. Section 7, on the connection between the Kondo lattice systems and
the holographic models of non-Fermi liquid, is expanded. (v1) special issue
of JLTP on quantum criticalit
Pairing and Density Correlations of Stripe Electrons in a Two-Dimensional Antiferromagnet
We study a one-dimensional electron liquid embedded in a 2D antiferromagnetic
insulator, and coupled to it via a weak antiferromagnetic spin exchange
interaction. We argue that this model may qualitatively capture the physics of
a single charge stripe in the cuprates on length- and time scales shorter than
those set by its fluctuation dynamics. Using a local mean-field approach we
identify the low-energy effective theory that describes the electronic spin
sector of the stripe as that of a sine-Gordon model. We determine its phases
via a perturbative renormalization group analysis. For realistic values of the
model parameters we obtain a phase characterized by enhanced spin density and
composite charge density wave correlations, coexisting with subleading triplet
and composite singlet pairing correlations. This result is shown to be
independent of the spatial orientation of the stripe on the square lattice.
Slow transverse fluctuations of the stripes tend to suppress the density
correlations, thus promoting the pairing instabilities. The largest amplitudes
for the composite instabilities appear when the stripe forms an antiphase
domain wall in the antiferromagnet. For twisted spin alignments the amplitudes
decrease and leave room for a new type of composite pairing correlation,
breaking parity but preserving time reversal symmetry.Comment: Revtex, 28 pages incl. 5 figure
Finite-Size Scaling of the Ground State Parameters of the Two-Dimensional Heisenberg Model
The ground state parameters of the two-dimensional S=1/2 antiferromagnetic
Heisenberg model are calculated using the Stochastic Series Expansion quantum
Monte Carlo method for L*L lattices with L up to 16. The finite-size results
for the energy E, the sublattice magnetization M, the long-wavelength
susceptibility chi_perp(q=2*pi/L), and the spin stiffness rho_s, are
extrapolated to the thermodynamic limit using fits to polynomials in 1/L,
constrained by scaling forms previously obtained from renormalization group
calculations for the nonlinear sigma model and chiral perturbation theory. The
results are fully consistent with the predicted leading finite-size corrections
and are of sufficient accuracy for extracting also subleading terms. The
subleading energy correction (proportional to 1/L^4) agrees with chiral
perturbation theory to within a statistical error of a few percent, thus
providing the first numerical confirmation of the finite-size scaling forms to
this order. The extrapolated ground state energy per spin, E=-0.669437(5), is
the most accurate estimate reported to date. The most accurate Green's function
Monte Carlo (GFMC) result is slightly higher than this value, most likely due
to a small systematic error originating from ``population control'' bias in
GFMC. The other extrapolated parameters are M=0.3070(3), rho_s = 0.175(2),
chi_perp = 0.0625(9), and the spinwave velocity c=1.673(7). The statistical
errors are comparable with those of the best previous estimates, obtained by
fitting loop algorithm quantum Monte Carlo data to finite-temperature scaling
forms. Both M and rho_s obtained from the finite-T data are, however, a few
error bars higher than the present estimates. It is argued that the T=0
extrapolations performed here are less sensitive to effects of neglectedComment: 16 pages, RevTex, 9 PostScript figure
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