159 research outputs found
OLED ageing signature characterization under combined thermal and electrical stresses
OLED are appearing at the industrial scale for purposes such as decorative lighting as well as LCD screen back-lighting. One of their main advantages compared to LED is their reduced light intensity: the luminous flux is produced by a much greater surface. Nevertheless there are a number of strong constraints on their allowable operating conditions, which may otherwise dramatically reduce their lifetime. This aspect of OLEDs is up to now not well understood and generally requires the knowledge of the layers and the chemical compounds used to build the device. In this study we submitted commercial OLEDs to limit conditions with respect to their maximum current and temperature. Complete electrical, optical, photometric and thermic characterization was performed on new devices, and repeated at regular intervals during the ageing process. Electrical parameters were measured with a Solartron impedance analyzer and a Keithley Source meter applying current and voltage steps. By the analysis of the indicial response elements of the equivalent linear circuit were extracted. We will illustrate which parameters change the most with the ageing process. The purpose is to predict and quantify the useful lifetime of OLED under electrical and/or thermal stress. Another outcome of importance is related to OLED driver design, namely specifying the end-of-life electrical characteristics. The operating point's significantly drifted from their original values leading sometimes to a doubling of the operating voltage at the same current level
A new approach to strongly correlated fermion systems: the spin-particle-hole coherent-state path integral
We describe a new path integral approach to strongly correlated fermion
systems, considering the Hubbard model as a specific example. Our approach is
based on the introduction of spin-particle-hole coherent states which
generalize the spin-1/2 coherent states by allowing the creation of a hole or
an additional particle. The action of the fermion system
can be expressed as a function of two
Grassmann variables (,) describing
particles propagating in the lower and upper Hubbard bands, and a unit vector
field whose dynamics arises from spin fluctuations. In the strong
correlation limit, can be truncated to quartic
order in the fermionic fields and used as the starting point of a
strong-coupling diagrammatic expansion in ( being the intersite
hopping amplitude and the on-site Coulomb repulsion). We discuss possible
applications of this formalism and its connection to the t-J model and the
spin-fermion model.Comment: 20 pages RevTex, 10 figure
Acoustic resonance detection using statistical methods of voltage envelope characterization in metal halide lamps
Acoustic resonance (AR) phenomenon occurs in metal halide lamps and can cause light flicker, lamp arc bending and rotation, lamp extinction, and in the worst case, arc tube explosion. This study takes place in the context of developing electronic ballasts with robust AR detection and avoidance mechanisms. To this end, a lock-in amplifier is used to measure and characterize lamp voltage root mean square (rms) short-term fluctuations. Statistical criteria based on the standard deviation of this rms value are proposed to assess AR presence and classify its severity. A set of metal halide lamps from different manufacturers and with different powers were tested. The average electrical power and AR level are controlled by adjusting the lamp operating frequency of high-frequency electronic ballast. The proposed criteria enable classifying healthy (without AR) and faulty (with AR) cases based upon either a two-dimensional plane or a boxplot. Regardless of lamp operating power, the results from this study show that the voltage rms variations and the defined criteria are significantly correlated with AR level
A simple high-sensitivity acoustic resonance detection method for metal halide lamps
To detect acoustic resonance (AR) in metal halide (MH) lamps, a simple high-sensitivity method with a multiplier detector is presented in this paper. Voltage envelope variations are measured to evaluate whether AR occurs in MH lamps. The study is focused on improving the sensitivity of an AR detection method. Several manufacturers’ MH lamps are tested in our experiment. In addition, in order to verify the proposed method, the other method of voltage envelope detection is evaluated by a lock-in amplifier with high sensitivity and detection results are analyzed by statistical methods. The results show that the proposed circuit can provide similar sensitivity as the expensive lock-in amplifier to detect AR phenomena and the AR-free and the slight AR level can be easily distinguished
Degradation of the luminance and impedance evolution analysis of an OLED under thermal and electrical stress
Organic light emitting diodes are one of the most innovative light sources. They do not require semiconductor fabrication techniques like the LED family, they are simple to construct and are used in many original applications. The inconvenient of this product is that it does not have a long lasting useful life with more then 10000 hours. Therefore, this paper will present a parametric method to design an aging model of the OLED based on luminance decay and electrical impedance evolution. Accelerated tests using thermal factor and currentdensity will be applied to large warm white OLED panels. A log-normal model for the luminance decay will be merged withdesign of experiments method to include the stress factors as well as impedance characteristics resulting in an effective degradation model that can estimate the lifetime of the OLED
Panton-Valentine Leukocidin and Staphyloccoccal Skin Infections in Schoolchildren1
The Panton-Valentine leukocidin is associated with staphylococcal skin and pulmonary infections. We describe a school outbreak of skin infections and the public health response to it. Nasal carriage of a Panton-Valentine leukocidin–positive Staphylococcus aureus clone was detected only in previously ill classmates and their family members
Moderate to severe acute pain disturbs motor cortex intracortical inhibition and facilitation in orthopedic trauma patients : a TMS study
Objective
Primary motor (M1) cortical excitability alterations are involved in the development and maintenance of chronic pain. Less is known about M1-cortical excitability implications in the acute phase of an orthopedic trauma. This study aims to assess acute M1-cortical excitability in patients with an isolated upper limb fracture (IULF) in relation to pain intensity.
Methods
Eighty-four (56 IULF patients <14 days post-trauma and 28 healthy controls). IULF patients were divided into two subgroups according to pain intensity (mild versus moderate to severe pain). A single transcranial magnetic stimulation (TMS) session was performed over M1 to compare groups on resting motor threshold (rMT), short-intracortical inhibition (SICI), intracortical facilitation (ICF), and long-interval cortical inhibition (LICI).
Results
Reduced SICI and ICF were found in IULF patients with moderate to severe pain, whereas mild pain was not associated with M1 alterations. Age, sex, and time since the accident had no influence on TMS measures.
Discussion
These findings show altered M1 in the context of acute moderate to severe pain, suggesting early signs of altered GABAergic inhibitory and glutamatergic facilitatory activities
Neel Order and Electron Spectral Functions in the Two-Dimensional Hubbard Model: a Spin-Charge Rotating Frame Approach
Using recently developed quantum SU(2)xU(1) rotor approach, that provides a
self-consistent treatment of the antiferromagnetic state we have performed
electronic spectral function calculations for the Hubbard model on the square
lattice. The collective variables for charge and spin are isolated in the form
of the space-time fluctuating U(1) phase field and rotating spin quantization
axis governed by the SU(2) symmetry, respectively. As a result interacting
electrons appear as composite objects consisting of bare fermions with attached
U(1) and SU(2) gauge fields. This allows us to write the fermion Green's
function in the space-time domain as the product CP^1 propagator resulting from
the SU(2) gauge fields, U(1) phase propagator and the pseudo-fermion
correlation function. As a result the problem of calculating the spectral line
shapes now becomes one of performing the convolution of spin, charge and
pseudo-fermion Green's functions. The collective spin and charge fluctuations
are governed by the effective actions that are derived from the Hubbard model
for any value of the Coulomb interaction. The emergence of a sharp peak in the
electron spectral function in the antiferromagnetic state indicates the decay
of the electron into separate spin and charge carrying particle excitations.Comment: 16 pages, 5 figures, submitted to Phys. Rev.
Antiferromagnetism and single-particle properties in the two-dimensional half-filled Hubbard model: a non-linear sigma model approach
We describe a low-temperature approach to the two-dimensional half-filled
Hubbard model which allows us to study both antiferromagnetism and
single-particle properties. This approach ignores amplitude fluctuations of the
antiferromagnetic (AF) order parameter and is valid below a crossover
temperature which marks the onset of AF short-range order. Directional
fluctuations (spin waves) are described by a non-linear sigma model
(NLM) that we derive from the Hubbard model. At zero temperature and
weak coupling, our results are typical of a Slater antiferromagnet. The AF gap
is exponentially small; there are well-defined Bogoliubov quasi-particles
(QP's) (carrying most of the spectral weight) coexisting with a high-energy
incoherent excitation background. As increases, the Slater antiferromagnet
progressively becomes a Mott-Heisenberg antiferromagnet. The Bogoliubov bands
evolve into Mott-Hubbard bands separated by a large AF gap. A significant
fraction of spectral weight is transferred from the Bogoliubov QP's to
incoherent excitations. At finite temperature, there is a metal-insulator
transition between a pseudogap phase at weak coupling and a Mott-Hubbard
insulator at strong coupling. Finally, we point out that our results
straightforwardly translate to the half-filled attractive Hubbard model, where
the charge and pairing fluctuations combine to
form an order parameter with SO(3) symmetry.Comment: Revtex4, 19 pages, 14 figures; (v2) final version as publishe
Random Matrix Theory and higher genus integrability: the quantum chiral Potts model
We perform a Random Matrix Theory (RMT) analysis of the quantum four-state
chiral Potts chain for different sizes of the chain up to size L=8. Our
analysis gives clear evidence of a Gaussian Orthogonal Ensemble statistics,
suggesting the existence of a generalized time-reversal invariance.
Furthermore a change from the (generic) GOE distribution to a Poisson
distribution occurs when the integrability conditions are met. The chiral Potts
model is known to correspond to a (star-triangle) integrability associated with
curves of genus higher than zero or one. Therefore, the RMT analysis can also
be seen as a detector of ``higher genus integrability''.Comment: 23 pages and 10 figure
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