Joel D. Shore

Hysteresis and hierarchies: Dynamics of disorder-driven first-order phase transformations

We use the zero-temperature random-field Ising model to study hysteretic behavior at first-order phase transitions. Sweeping the external field through zero, the model exhibits hysteresis, the return-point memory effect, and avalanche fluctuations. There is a critical value of disorder at which a jump in the magnetization (corresponding to an infinite avalanche) first occurs. We study the universal behavior at this critical point using mean-field theory, and also present results of numerical simulations in three dimensions.

Highly efficient fluorescent-phosphorescent triplet-harvesting hybrid organic light-emitting diodes

We demonstrate highly efficient white and nonwhite hybrid organic light-emitting diodes (OLEDs) in which singlet and triplet excited states, generated in the recombination zone, are utilized by fluorescence and phosphorescence, respectively. The excited states are formed at a blue fluorescent light-emitting layer (LEL), and the triplets diffuse through a spacer layer to one or more phosphorescent LEL(s). A key feature enabling the triplet diffusion in such OLEDs is the use of a blue fluorescent emitter with triplet energy above, or not much below, that of the fluorescent host. Additional material properties required for triplet harvesting are outlined. At 1000 cd/m2 a blue and yellow harvesting OLED shows 13.6% external quantum efficiency, 3.8 V, 30.1 lm/W, and color characteristics suitable for display application. High-efficiency harvesting R+G+B white, and B+G and B+R nonwhite OLEDs are also demonstrated. The triplet-harvesting mechanism was verified in all devices by physical methods including spectra...

Simulations of the nucleation of AgBr from solution

We use molecular dynamics to study the nucleation of AgBr in water. After first testing our Born–Mayer–Huggins potentials for Ag+ and Br− by looking at bulk AgBr and at AgBr clusters in vacuo, we consider small numbers of Ag+ and Br− ions immersed in water. The system shows the expected qualitative features of nucleation form solution, including a critical cluster size that decreases with increasing concentration. However, we find that for cluster sizes at least as large as Ag18Br18, the most stable cluster is disordered. This is in stark contrast to clusters in vacuo where clusters as small as Ag4Br4 from ordered fragments of the lattice. These results lend some support to the conjecture that nucleation of crystals from solution is a two-stage process with the first stage consisting of the formation of disordered clusters of solute and the second stage involving the nucleation of a crystal from this solute “melt.”

Logarithmically slow domain growth in nonrandomly frustrated systems: Ising models with competing interactions.

We study the growth («coarsening») of domains following a quench from infinite temperature to a temperature T below the ordering transition. The model we consider is an Ising ferromagnet on a square or cubic lattice with weak next-nearest-neighbor antiferromagnetic (AFM) bonds and single-spin-flip dynamics. The AFM bonds introduce free-energy barriers to coarsening and thus greatly slow the dynamics. In two dimensions, the barriers are independent of the characteristic length scale L(t), and therefore the long-time (t→∞) growth of L (t) still obeys the standard t 1/2 law

P-175: Enhanced Outcoupling of Light from Organic Light-Emitting Diodes by Microlens Arrays Based on Breath-Figure Template

We present a bottom-up method of fabricating microlens arrays based on a template of close-packed microvoids created by self-assembly and subsequent evaporation of condensed water droplets on the surface of a polymer solution exposed to humid air. The pattern of microvoids is transferred to an optically transparent, thermally curable elastomer to form the microlens array. The method offers advantages over conventional top-down microfabrication approaches in terms of easy fabrication, high fill factor, and near-optimum shape of the microlenses. Attachment of the microlens array to the glass substrate of an OLED device results in a significant increase in total emission and external quantum efficiency. These experimental results are compared to the predictions of a hybrid ray optics/wave optics model that can be used to gain further understanding and to help optimize the system.

Electromagnetic response of a static vortex line in a type-II superconductor: A microscopic study

The electromagnetic response of a pinned Abrikosov fluxoid is examined in the framework of the Bogoliubov-de Gennes formalism. The matrix elements and the selection rules for both the single photon (emission - absorption) and two photon (Raman scattering) processes are obtained. The results reveal striking asymmetries: light absorption by quasiparticle pair creation or single quasiparticle scattering can occur only if the handedness of the incident radiation is opposite to that of the vortex core states. We show how these effects will lead to nonreciprocal circular birefringence, and also predict structure in the frequency dependence of conductivity and in the differential cross section of the Raman scattering.

COMMENT ON "FALSIFICATION OF THE ATMOSPHERIC CO2 GREENHOUSE EFFECTS WITHIN THE FRAME OF PHYSICS"

In this journal, Gerhard Gerlich and Ralf D. Tscheuschner claim to have falsified the existence of an atmospheric greenhouse effect.1 Here, we show that their methods, logic, and conclusions are in error. Their most significant errors include trying to apply the Clausius statement of the Second Law of Thermodynamics to only one side of a heat transfer process rather than the entire process, and systematically ignoring most non-radiative heat flows applicable to the Earths surface and atmosphere. They claim that radiative heat transfer from a colder atmosphere to a warmer surface is forbidden, ignoring the larger transfer in the other direction which makes the complete process allowed. Further, by ignoring heat capacity and non-radiative heat flows, they claim that radiative balance requires that the surface cool by 100 K or more at night, an obvious absurdity induced by an unphysical assumption. This comment concentrates on these two major points, while also taking note of some of Gerlich and Tscheuschners other errors and misunderstandings.

Disorder-driven first-order phase transformations: A model for hysteresis

Hysteresis loops in some magnetic systems are composed of small avalanches (manifesting themselves as Barkhausen pulses). Hysteresis loops in other first‐order phase transitions (including some magnetic systems) often occur via one large avalanche. The transition between these two limiting cases is studied, by varying the disorder in the zero‐temperature random‐field Ising model. Sweeping the external field through zero at weak disorder, we get one large avalanche with small precursors and aftershocks. At strong disorder, we get a distribution of small avalanches (small Barkhausen effect). At the critical value of disorder where a macroscopic jump in the magnetization first occurs, universal power‐law behavior of the magnetization and of the distribution of (Barkhausen) avalanches is found. This transition is studied by mean‐field theory, perturbative expansions, and numerical simulation in three dimensions.

White‐emitting OLED devices in an RGBW format with microelement white subpixels

— OLED devices with an RGBW pixel format using an unpatterned white emitter have the potential to provide very good efficiency and color gamut while enabling lower-cost and large-format manufacturing. However, the white subpixel often has unacceptably large color shifts with viewing angle. Furthermore, for some architectures such as top-emitting microcavity devices, it can even be difficult to produce a white subpixel with good on-axis color. In this paper, we describe the use of a white subpixel made up of a combination of differently tuned microelements and demonstrate how such an approach can overcome these problems. By carefully tuning the color and areas of each of the microelements in the white subpixel, we can trade off between better on-axis color, less color change with angle, and higher efficiency. Furthermore, it was demonstrated that an RGBW top-emitter microcavity device with a microelement white subpixel can achieve an increase in both power efficiency and color gamut relative to a conventional RGBW bottom-emitter non-microcavity device.

Microcavity white-emitting OLED devices

Abstract— Microcavity designs for OLED devices with an unpatterned white emitter have the potential to provide greater brightness and larger color gamut than non-microcavity designs while still enabling lower-cost large-format manufacturing. In this paper, such microcavity and non-microcavity designs are compared. Color filters must still be employed to provide an adequate color gamut. Top-emitter structures have somewhat greater on-axis luminance and color gamut, but increased angular change, than bottom-emitter designs. In a single-stack bottom-emitter active-matrix TFT device using an RGBW format, the use of microcavities is estimated to reduce the average power usage by 35% and the peak power by 58%, while increasing the NTSC ratio for color gamut area by about 10%. Angular luminance and color change is likely to be acceptable, especially for hand-held applications. Tandem devices employing multiple emitter stacks increase the lifetime of OLED devices but require larger driving voltages; for such devices, microcavity structures are useful although the percentage reduction obtained in power usage is not quite as large. Generally, tandem devices with microcavities have a slightly stronger cavity effect yielding slightly larger color gamut, but also greater angular color and luminance shift. Therefore, microcavity architectures are less appealing for tandem devices.