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The efficiency of organic light-emitting diodes that utilize the principle of thermally activated delayed fluorescence (TADF) depends sensitively on the host material in which the TADF emitter molecules (guests) are embedded. Potential loss processes are "deconfinement,"the transfer of excitons from the guest to the host, and "dissociation,"the formation of intermolecular charge-transfer states. We investigate how both processes can be suppressed by studying the photoluminescence efficiency, emission spectrum, and time-resolved emission intensity of eight thin-film systems in which 5 mol. % of the sky-blue TADF emitter 4-carbazolyl-methylphthalimide (abbreviated here as CzPIMe) is embedded in various host materials. Deconfinement is found to be entirely suppressed if the triplet energy of the host is 0.25 eV or more above that of the guest. For systems allowing for deconfinement, the dependence on the energy difference is consistent with a recent theoretical analysis [C. Hauenstein et al., J. Appl. Phys. 128, 075501 (2020)]. Dissociation, due to hole transfer to a host molecule, is found to be suppressed if the host's highest occupied molecular orbital energy is not more than about 0.2 eV higher than that of the guest. Otherwise, we observe an efficiency loss, a spectral redshift, and the disappearance of distinct prompt and delayed emission regimes. A comprehensive rate-equation model is developed from which we study the sensitivity of these observations to the energy level structure, the intermolecular interaction rates, and the photophysical rates that follow from a fit to the experimental data for the CzPIMe:TCTA [tris (4 - carbazoyl - 9 - ylphenyl) amine] system.

A calculation is presented for the direct conversion of a measured luminescence signal to the implied open circuit voltage. The effects of re-absorption, spectral dependence, and interference with front and back interfaces are all included, so long as the optical properties of the structure are known. The method is validated through a comparison of the terminal open-circuit voltage with the photoluminescence intensity of Cu(In,Ga)Se2 solar cells, each as a function of illumination intensity.

The magnetic and magnetotransport properties of Ni80Fe20/La0.7Sr0.3MnO3 (NiFe/LSMO) bilayers were investigated after bombarding the LSMO surface with low-energy Ar+ or O-2(+)/Ar+ ion beams before the growth of the top NiFe layer. A variety of magnetic properties are revealed, including an asymmetric two-stepped hysteresis loop with an exchange bias loop shift, and alternatively, a symmetric two-stepped hysteresis loop with an enhanced coercivity. Polarized neutron reflectometry measurements provide details of the magnetic depth profile and interface layer magnetism at different temperatures. The LSMO surface modifications determine a complex magnetic and electric NiFe/LSMO interface having a strong effect on the magnetoresistance of the bilayer. Surface engineering based on ion beam bombardment is presented as a promising technique for optimizing the electronic and magnetic properties of NiFe/LSMO junctions for future device applications. Published by AIP Publishing.

We describe an analysis of the dynamics of individual superparamagnetic micro- and nanoparticles in order to quantify their magnetic properties and mobility near a chip surface. The particles are attracted to the chip surface by integrated microscopic current wires. We show that it is possible to accurately analyze particles with a diameter of about 1 µm by the magnetophoretic movement between current wires because of the very high field gradients. This reveals distinct differences in volume susceptibilities of particles with the same outer diameter. Smaller particles are characterized using the technique of confined Brownian motion analysis. By capturing 300 nm particles on a current wire with surface barriers or a focused shape, the magnetization of the particles can be measured with an accuracy better than 10%.

Temperature-dependent photoluminescence measurements and deep level transient spectroscopy have been made to identify defects and deep level traps in InGaAsN alloys lattice-matched to GaAs. The epitaxial layers were grown by molecular beam epitaxy at different substrate temperatures. Defect levels responsible for the quenching of luminescence with increase of temperature have been identified. Several electron trap levels have been identified and characterized in the alloys. The concentrations of all of these increase with N content in the alloy. A dominant center, with an activation energy of ET=0.44eV, is present in all the samples and its concentration is inversely proportional to photoluminescence intensity. H2O and O present in the source nitrogen are believed to be responsible for the presence of this trap, which also has characteristics similar to the E3 and E4 levels in molecular beam epitaxial GaAs.

We report a detailed investigation on the optical transitions of strained type II GaAs0.7Sb0.3/GaAs (100) multiple quantum wells. For the theoretical calculations, both of the elastic deformational potential of intrinsic compressive biaxial strain, and quantum confinement effects are included. The asymmetric photoluminescence spectra reveal the features of excited state transition and quantum confinement Stark effect at high and low temperatures, respectively. The asymmetry features have also been investigated and confirmed by low-temperature photoluminescence experiments under different excitation power. From polarized photoluminescence excitation and photoconductivity spectra, both of the type I and type II optical transitions can also be clearly identified.

Ti-doped MgB2/Cu tapes with Ta as a buffer layer were prepared through the in situ powder in tube method by using Mg, Ti, and B powders. The phase compositions, microstructure features, and superconducting properties were investigated by x-ray diffraction, scanning electron microscope, and superconducting quantum interference device magnetometer. It is found that TiB2 phase was formed in Ti-doped MgB2 tape. Magnetization measurement results show that the critical transition temperature of MgB2/Ta/Cu tape with Ti doping is around 38 K. The irreversibility field Hirr and critical current density Jc can be greatly enhanced by Ti doping. Hirr of the Mg0.9Ti0.1B2 tape is as high as 7.4 T at 10 K. The high critical current density Jc of 1.5×106 A/cm2 (10 K, self field) and 9.3×105 A/cm2 (20 K, self field) are obtained in the Mg0.9Ti0.1B2 tape. In addition, a suitable amount of Ti doping can lead to a high density and fine grain size of MgB2, which may be the reason for high Jc in Ti-doped MgB2 tapes.

A recent article [A. Bejan, A. Almerbati, and S. Lorente, J. Appl. Phys. 121, 044907 (2017)] takes note of a relationship between size and efficiency that appears in the performance data of many engines and power plants. The authors attempt to interpret this phenomenon thermodynamically, concluding that the relationship is predicted by a general thermal power plant model. However, this conclusion is the result of an elementary error. The efficiency of the idealized thermal model in fact stays constant or decreases with increasing size.

CdSeTe/CdMgTe double heterostructures were produced with both n-type and unintentionally doped absorber layers. Measurements of the dependence of photoluminescence intensity on excitation intensity were carried out, as well as measurements of time-resolved photoluminescence decay after an excitation pulse. It was found that decay times under very low photon injection conditions are dominated by a non-radiative Shockley-Read-Hall process described using a recombination center with an asymmetric capture cross section, where the cross section for holes is larger than that for electrons. As a result of the asymmetry, the center effectively extends photoluminescence decay by a hole trapping phenomenon. A reduction in electron capture cross section appeared at doping densities over 1016cm−3. An analysis of the excitation intensity dependence of room temperature photoluminescence revealed a strong relationship with doping concentration. This allows estimates of the carrier concentration to be made through a non-destructive optical method. Iodine was found to be an effective n-type dopant for CdTe, allowing controllable carrier concentrations without an increased rate of non-radiative recombination.

A recent paper [A. Bejan and R. W. Wagstaff, J. Appl. Phys. 119, 094901 (2016)] concludes that bodies of the same size suspended uniformly in space are in a state of high internal tension. The tension is then relieved by rearrangement of the bodies into a non-uniform distribution of mass. In this Comment, it is shown that the conclusions are based upon calculations which are in error, and that the amount of tension is not in fact decreased by such a rearrangement.

High resolution measurements of spectrally resolved cathodoluminescence (CL) decay have been made in several commercial and experimental phosphors doped with Eu and Tb at beam energies ranging from 0.8 to 4 keV. CL emission from the lowest two excited states of both rare-earth activators was compared to the decay of photoluminescence (PL) after pulsed laser excitation. We find that, at long times after the cessation of electron excitation, the CL decay rates are comparable to those measured in PL; at short times, the decay process is considerably faster and has a noticeable dependence on the energy of the electron beam. These beam energy effects are largest for the higher excited states and for phosphors with larger activator concentrations. Measurements of the experimental phosphors over a range of activator fractions from 0.1 to 0.002 show that the beam energy dependence of the steady-state CL efficiency is larger for higher excited states and weakens as the activator concentration is reduced; the latte...

A modified method to measure the lifetime of the excess electron–hole pairs generated by laser beams is proposed. Under the applied bias, excess carriers pass through the sample within the transit time and form the photocurrent Ip. The related white-noise component of the photocurrent can be written as 4egIp, where g is the ratio of the lifetime over the transit time. The lifetime can then be obtained by determining g from the white noise performance. Since the transit time can be adjusted with the applied biases and the sample length, the measured lifetime may be as small as 100 ns with the measurement system even having the analytic frequency range of 10 kHz only. A GaAs sample grown by molecular beam epitaxy is taken as an example to measure the excess carrier’s lifetime. It is concluded from the experiments that the lifetime of excess carriers in our sample is primarily caused by the indirect recombination via the deep trap centers within the band gap. The nonradiative lifetime is not really a constan...

When synchrotron radiation is used as an excitation source, the total reflection x-ray fluorescence analysis of surface contamination on silicon wafer has an extremely low background intensity that determines the minimum detection limit. In this article, the background spectrum originating from the photoelectron bremsstrahlung is calculated using the Monte Carlo method. The doubly differential electron bremsstrahlung cross sections obtained from the Born approximation modified by the Elwert factor and with the use of the form factor approach for screening are used instead of empirical formulas. In addition to the bremsstrahlung spectrum produced from the silicon wafer, the bremsstrahlung intensity that photoelectrons, which escape from the silicon wafer, produce in the filter attached to the detector is also calculated in accordance with the usual synchrotron radiation excited total reflection x-ray fluorescence experimental conditions. The calculated photoelectron bremsstrahlung spectra are compared with...

The surface acoustic wave produced electron absorptive and electro-optic modulation in AlGaAs/GaAs quantum well structures are theoretically analyzed. The quantum well structures are optimized by maximizing the optical confinement of the modal field in the active region and the piezoelectric effect of surface acoustic wave on the quantum wells. The effect of penetration depth of the surface acoustic wave on the number (1–25 periods) of quantum wells, serving as the active region, is being studied. For 1–5 period structures, the quantum wells are designed on the top surface so that a strong piezoelectric effect can be obtained. For the 25-period structure, the quantum wells locate at a depth of two-thirds the acoustic-wave wavelength in order to obtain a uniform surface acoustic-wave-induced electric field. The results show that the single and five quantum well devices are suitable for absorptive modulation and optical modulation, respectively, while a general advantage of the 25-period quantum well modulator can shorten the modulation interaction length and increase the modulation bandwidth. The effective index change of these devices are at least ten times larger than the conventional surface acoustic wave devices. These results make the surface acoustic wave quantum well modulators more attractive for the development of acousto-optic device applications.

We report on developing split-gate quantum point contacts (QPCs) that have a tunable length for the transport channel. The QPCs were realized in a GaAs/AlGaAs heterostructure with a two- dimensional electron gas (2DEG) below its surface. The conventional design uses 2 gate fingers on the wafer surface which deplete the 2DEG underneath when a negative gate voltage is applied, and this allows for tuning the width of the QPC channel. Our design has 6 gate fingers and this provides additional control over the form of the electrostatic potential that defines the channel. Our study is based on electrostatic simulations and experiments and the results show that we developed QPCs where the effective channel length can be tuned from about 200 nm to 600 nm. Length-tunable QPCs are important for studies of electron many-body effects because these phenomena show a nanoscale dependence on the dimensions of the QPC channel.

The chemical structure of sodium aluminosilicate glasses is determined by high resolution x‐ray photoelectron spectroscopy (XPS) as silicon is gradually replaced by aluminum. A well‐defined chemical state is found for silicon, aluminum, and sodium atoms, while three different environments are identified for oxygen atoms corresponding to Si–O–Si, Si–O–Al, and Si–O–Na bonds. The binding energy of Na 1s photoelectrons increases significantly with increasing aluminum substitution while that of Al 2p and components of O 1s photoelectrons remains approximately constant. Thus, the ionicity of sodium increases with aluminum amount, but the over all electron density around silicon, aluminum, and different types of oxygen atoms remains unchanged. The dielectric constant of the glasses increases with increasing aluminum substitution. It is analyzed in terms of the polarizabilities of constituent structural units, viz., silicon tetrahedra, nonbridging oxygen–sodium ion pairs, and aluminum tetrahedron–sodium ion pairs...

Phase equilibria were investigated in the Ga–Ni–As ternary system, with particular emphasis on the regions of technological importance to Ni/GaAs electrical contacts. A 600 °C Gibbs isotherm was constructed using x‐ray‐diffraction analysis and electron probe microanalysis of annealed samples. Additionally, three isopleths (NiAs–GaAs, NiGa–NiAs, and NiGa–GaAs) and a partial liquidus projection were established using differential thermal analysis and metallography. These data were utilized to clarify some discrepancies in the literature pertaining to the constitution of the Ga–Ni–As system, particularly questions about the existence of ternary phases. It was demonstrated that at 600 °C, previously reported ternary phases were actually specific compositions of the binary phase, NiAs, which exhibits significant ternary solubility. Additional x‐ray‐diffraction and differential thermal analysis experiments suggested that superlattice structures based on the NiAs structure may become stable at lower temperatures...

GaN films were grown on (100) GaAs substrates by metalorganic chemical vapor deposition and were found to be of (200) cubic or (111) cubic/(0002) hexagonal phase. Their photoluminescence characteristics remained invariant with material phase. We report assignment of band‐edge photoluminescence near 3.36 eV and 3.15–3.31 eV in apparently cubic GaN to intrinsic/bound excitons and phonon‐assisted, donor‐acceptor pair recombination respectively, on the basis of observed temperature and intensity dependences. A free exciton energy of 3.375 eV is deduced at 6.5 K.

Pulsed laser deposition was used to grow epitaxial β‐FeSi2 films on Si(111) (1×1) and Si(111) (7×7) with the following epitaxial orientations: β‐FeSi2(001)//Si(111) with β‐FeSi2[010]//Si〈110〉 and three rotational variants. Silicide growth was influenced by substrate temperature and deposition rate, but not by the structure of the starting surface. Films containing both β‐FeSi2 and FeSi were formed at low substrate temperatures and high deposition rates, while films containing only β‐FeSi2 were formed at higher substrate temperatures and lower deposition rates. FeSi grains had the following epitaxial relationship to the Si substrate, FeSi(111)//Si(111) with FeSi(110)//Si(112). The microstructure of the silicide films varied with film thickness, as did the roughness at the silicide/Si interface. These results suggest that an Fe‐rich environment was created during the growth of the silicide films.

A simplified method of calculating thermal diffuse scattering (TDS) for materials at or above their Debye characteristic temperature is presented. This method is based upon the properties of the lattice Green's function and eliminates the need of solving the equations of lattice dynamics. This simplifies the calculation procedure. The general scheme of evaluating the diffuse scattering for polycrystalline materials with a texture is discussed. A sample calculation of the TDS from a polycrystalline material with a fiber texture is given using the simplified procedure just presented. National Science Foundation (U.S.) - Grant No. DMR-8818013

Fe100−xPtx alloy thin films with x=25–67 at. % were prepared by dc magnetron sputtering on naturally oxidized Si substrates. Effects of film composition, annealing temperature (300–650 °C), annealing time (5–120 min), and cooling rate (furnace cooling or ice water quench cooling) on the magnetic properties were investigated. Optimum conditions for saturation magnetization and coercivity of the Fe100−xPtx alloy films were found with x=50 at.%, annealed at 600 °C for 30 min and cooled by ice water quenching. Our experimental data suggests that the magnetic hardening in Fe100−xPtx alloy thin films is mainly due to the fct γ1-FePt phase and the domain wall pinning effect. The domain nucleation mechanism is dominated in samples with furnace cooling; the domain wall pinning mechanism dominates in samples cooled with ice water quenching.

An improved method for measuring the cross sections for carrier trapping at defects in semiconductors is described. This method, a variation of deep level transient spectroscopy (DLTS) used with bipolar transistors, is applied to hot carrier trapping at vacancy-oxygen, carbon-oxygen, and three charge states of divacancy centers (V2) in n- and p-type silicon. Unlike standard DLTS, we fill traps by injecting carriers into the depletion region of a bipolar transistor diode using a pulse of forward bias current applied to the adjacent diode. We show that this technique is capable of accurately measuring a wide range of capture cross sections at varying electric fields due to the control of the carrier density it provides. Because this technique can be applied to a variety of carrier energy distributions, it should be valuable in modeling the effect of radiation-induced generation-recombination currents in bipolar devices.

Epitaxial rare-earth–transition-metal thin films have been grown by magnetron sputtering onto buffered single-crystal MgO substrates. Results are presented for SmFe12(001) and magnetically hard Sm2Co7 and Sm2Co17 (110) films using W (100) buffer layers. The methodology of utilizing epitaxial W buffer layers combined with cosputtering from elemental sources provides a general approach to tailor the orientation and phase of the films.

We present structural and magnetic characterization of epitaxial Fe/Cr(211) and Fe/Cr(100) superlattices grown simultaneously on MgO(110) and MgO(100) substrates by magnetron sputtering. The epitaxial orientation of the Fe/Cr(211) superlattice with the MgO(110) substrate is Fe/Cr[011]//MgO[001] and the orientation of Fe/Cr(100) is Fe/Cr[001]//MgO[011]. We find that for both orientations, that the interlayer coupling oscillates in sign with a period of 18 A, and, furthermore, that the strength and phase of the magnetic coupling are also nearly identical for these two orientations.

The saturation field of sputtered Fe/Nb superlattices oscillates as a function of the Nb thickness with a periodicity of ∼9 A. In contrast to the case of Fe/Cr superlattices, the concurrent magnetoresistance oscillations were found to be very weak. Yet polarized neutron reflection measurements confirm that the Fe/Nb superlattices with high saturation field possess a magnetic ground state of the +−+− type. Neutron and x‐ray measurements indicate that, while the crystalline and antiferromagnetic order is well developed along the thickness of the film, the average lateral size of the crystallites (as well as of the magnetic domains) is quite small. This effect (thought to be related to the gross mismatch of the iron and the niobium crystal lattices) may be the cause of the high overall resistance of the material, and its weak dependence on the magnetization.

Sputtered Fe/Si superlattices were grown to study the magnetic coupling between ferromagnetic Fe layers (30 A thick) for Si spacer‐layer thicknesses (tSi) between 10 and 40 A. The material is ferromagnetical for tSi≤13 A and antiferromagnetically coupled for 13 A≤tSi≤17 A. For tSi≥17 A the Fe layers are uncoupled. X‐ray analysis indicates that the system is well layered, but that the crystal structure remains coherent only for tSi≤17 A. These results, along with our Mossbauer investigation, strongly suggest that the Si layer is crystalline for tSi≤17 A, and is silicide in nature. For thicker spacers, Si becomes amorphous. We propose a model of the layering that is consistent with the known properties of Fe silicide.

We have measured the magnetic anisotropy in a series of Co‐Au and Co‐Cu superlattices prepared by molecular‐beam epitaxy. Significant epitaxial strains give rise to a magnetoelastic contribution and a large crossover thickness (∼19 A) for perpendicular easy magnetization. The results are discussed in the context of a careful analysis of the interfacial strains and coherence determined by in situ. time‐resolved reflection high‐energy electron diffraction techniques and x‐ray scattering.

A self-consistent Ensemble Monte Carlo (EMC) model was developed to simulate the thermionic effect in heterostructure barrier coolers. The model was validated on an InGaAs-InGaAsP heterostructure device of variable barrier height and width, producing good quantitative agreement with previous literature results. The operation of the cooler was found to be a complex and intricate process depending on the field, conduction band and details of barrier structure. When applied to a GaAs-AlGaAs micro-cooler there was good agreement with the experimental results. Importantly, very small alterations in the barrier structure were found to lead to considerable changes in device performance.

By including the generation-recombination process of charge carriers in conduction channel, a model for low-frequency noise in pentacene organic thin-film transistors (OTFTs) is proposed. In this model, the slope and magnitude of power spectral density for low-frequency noise are related to the traps in the gate dielectric and accumulation layer of the OTFT for the first time. The model can well fit the measured low-frequency noise data of pentacene OTFTs with HfO2 or HfLaO gate dielectric, which validates this model, thus providing an estimate on the densities of traps in the gate dielectric and accumulation layer. It is revealed that the traps in the accumulation layer are much more than those in the gate dielectric, and so dominate the low-frequency noise of pentacene OTFTs.

We show that it is possible to obtain large field transmission through a periodic structure at frequencies where the field is lossy in a finite temperature superconductor. The feat is accomplished by using thin superconducting layers. This makes the superconductor photonic crystal useful for transmitting signals over larger distances at higher temperature. Narrow transmission resonances due to surface plasmon effect are damped more quickly with increasing temperature than broader transmission bands. The temperature dependence is useful, particularly for developing optothermal sensors in terahertz and far infrared regimes.

The direct conversion of light-polarization information into electric voltage has been demonstrated using the photoinduced inverse spin-Hall effect in a Pt/GaAs hybrid structure. In the GaAs layer, spin-polarized carriers are generated by the illumination of circularly polarized light, which induces a pure-spin current in the Pt layer through the interface. The pure-spin current is converted into an electromotive force using the inverse spin-Hall effect (ISHE) in the Pt layer. The electromotive force due to the photoinduced ISHE was found to be proportional to the degree of circular polarization of the illuminated light outside the sample in spite of the presence of the Pt top layer, which is consistent with a calculation based on the analysis for light propagation in multilayer structures. This conversion of light-polarization information into electric voltage works at room temperature without bias voltage and magnetic fields, and thus can be used as a spin photodetector. © 2010 American Institute of Physics. 107 11 Cited By :11

Transport and magnetization properties of superconducting/ferromagnetic bilayers YBa2Cu3O7/La0.7Sr0.3MnO3 (YBCO/LSMO) are investigated with the thickness of bottom LSMO layer fixed at 50 nm and that of top YBCO (t) layer varied from 20 to 100 nm. Compared to bulk YBCO, the off set of superconducting temperature TSC-off is suppressed by 10 K for t=20 nm. The suppression rate of TSC versus magnetic field H is obtained as 1.2±0.2 K/T for all bilayers. Considering the polarization effect of LSMO on superconductivity is similar to the field effect, the internal field created by the LSMO in the YBCO(20nm)/LSMO(50nm) bilayer is estimated as 40 T.Transport and magnetization properties of superconducting/ferromagnetic bilayers YBa2Cu3O7/La0.7Sr0.3MnO3 (YBCO/LSMO) are investigated with the thickness of bottom LSMO layer fixed at 50 nm and that of top YBCO (t) layer varied from 20 to 100 nm. Compared to bulk YBCO, the off set of superconducting temperature TSC-off is suppressed by 10 K for t=20 nm. The suppression rate of TSC versus magnetic field H is obtained as 1.2±0.2 K/T for all bilayers. Considering the polarization effect of LSMO on superconductivity is similar to the field effect, the internal field created by the LSMO in the YBCO(20nm)/LSMO(50nm) bilayer is estimated as 40 T.

Laser field with superintensity beyond 1029 W/cm2 can be generated by coherent superposition of multiple 100 fs laser pulses in circular geometry setup upon retroreflection by a ring mirror. We have found the criteria for attaining such intensities using broadband ring mirror within the practical damage threshold and paraxial focusing regime. Simple expressions for the intensity enhancement factor are obtained, providing insight for achieving unlimited laser intensity. Higher intensities can be achieved by using few-cycle laser pulses.

Hard and refractory particles with various shapes have been added to ceramic materials to enhance creep resistance. Also, many of the particles in food emulsions, magnetic particle suspensions, and other industrial systems may have nonspherical shapes. Hence, it is important to understand the effects of particle shape on the viscosity of suspensions. While the shape of many particles can be approximated as prolate or oblate spheroids, the purpose of the present study is to model the effective viscosity of semidilute suspensions of rigid ellipsoids. Closed-form expressions are obtained; however, their formulations are formidable. For the special cases of rodlike or disklike particles, simple closed-form expressions can be obtained. The present solutions are compared with existing solutions.

Te-doped and ZnTe-doped diluted ZnSeTe samples were grown by using metal-organic chemical vapor deposition. The green lights emitted from these two samples in the photoluminescence (PL) measurement have different peak emission energies and can be attributed to different emission mechanisms. For the Te-doped sample, the PL emission band is excitation laser power independent, the lifetime of the PL emission is short, and the PL emission could be attributed to the emission originated from Te isoelectronic centers. For the ZnTe-doped sample, the energy of the green emission band is laser power dependent, the PL lifetime is much longer than the Te-doped sample, and the PL emission could be attributed to the emission from type-II ZnTe∕ZnSe quantum dots. The results demonstrate that there are two different mechanisms responsible for the green PL emission in ZnSeTe system and our growth methods provide a way of selecting one of the two mechanisms for the green light emission in this system.

We have performed rf-skin depth (complex-conductivity) and magnetoresistance measurements of antiferromagnetic UTX compounds (T=Ni and X=Al, Ga, Ge) in applied magnetic fields up to 60 T applied parallel to the easy directions. The rf penetration depth was measured by coupling the sample to the inductive element of a resonant tank circuit and then, measuring the shifts in the resonant frequency Δf of the circuit. Shifts in the resonant frequency Δf are known to be proportional to the skin depth of the sample and we find a direct correspondence between the features in Δf and magnetoresistance. Several first-order metamagnetic transitions, which are accompanied by a drastic change in Δf, were observed in these compounds. In general, the complex-conductivity results are consistent with magnetoresistance data.

X-ray absorption near-edge structure (XANES) and x-ray emission spectroscopy (XES) measurements were used to investigate the effect of Mg doping in ZnO nanorods. The intensities of the features in the O K-edge XANES spectra of Zn{sub 1-x}Mg{sub x}O nanorods are lower than those of pure ZnO nanorods, suggesting that Mg doping increases the negative effective charge of O ions. XES and XANES spectra of O 2p states indicate that Mg doping raises (lowers) the conduction-band-minimum (valence-band-maximum) and increases the bandgap. The bandgap is found to increase linearly with the Mg content, as revealed by photoluminescence and combined XANES and XES measurements.

A dual-shell structure consisting of a silica buffer shell and a metallic outer shell is studied and designed for highly efficient and tunable fluorescence. The enhanced Purcell effect due to the plasmonic resonance and the dependence of the resonance on the structural parameters are analyzed and discussed through a classical electromagnetic approach. With the present dual-shell structure a fivefold enhancement of the fluorescence efficiency can be achieved for a nanofluorophore comprised of multiple tetramethylrhodamine isothiocyanate dye molecules, as compared to the nanofluorophore within a single silica shell. Furthermore, the peak wavelength of the fluorescence spectrum can be tuned easily (due to the variation of the surface plasmon resonance) by over 50 nm when the refractive index of the ambient material varies from 1.3 to 1.6.

An abnormal structure of the ZnSe nanowheels composed of teethlike extended patterns on nanoring bases has been successfully synthesized by thermal evaporation method. It is interesting to note that the as-synthesized ZnSe nanowheels are metastable wurtzite phase with the dominant exposed surfaces of ±(21¯1¯0) while the stable ZnSe is typically zinc blende phase. A full picture of the growth mechanism of the metastable wurtzite phase ZnSe nanostructures will be proposed from the thermodynamic point of view. Meanwhile, the formation of the nanowheels is also explained by a two-stage mechanism. In the first stage, the base of the nanowheel begins to form by vapor-solid mechanism, while in the second stage, the teethlike extended structures grow through the self-catalyzed growth process. The cathodoluminescence spectrum of ZnSe nanowheel exhibited a band edge transition at about 460nm and a strong self-activated luminescence at 610nm. It is important to note that the discussions of the nanostructure thermody...

int 0 of 0.25, the maximum zero-degree luminance and EL efficiency can be achieved by locating the emitters around the first antinode of the microcavity while for phosphorescent material with int 0 =1.0, the maximum zero-degree luminance and EL efficiency are around the second antinode. Through relaxing the efficiency by 10%‐20%, the angular intensity distribution can be even better than the Lambertian distribution; meanwhile, the color shows only a small variation over an angle range of 150°. Our results, which are in good agreement with experiments, show that the Purcell effect on TOLED performances is significant and should be carefully examined in studying TOLEDs. © 2007 American Institute of Physics. DOI: 10.1063/1.2739220

Our micromagnetic calculation demonstrates that the dominant coercivity mechanism is self-pinning in most exchange-coupled permanent and composite magnets. Such a pinning is attributed to the change of the intrinsic parameters associated with the phase change at the interface. From this self-pinning some more specific formulas on pinning field can be derived. In particular, for sufficiently large soft grains/defects, the pinning field can be expressed as HP=αHK, where HK=2k∕MS is the anisotropy field and α depends on the material parameters and micromagnetic structures. For an exchange-coupled Nd2Fe14B–αFe system with abrupt change of parameters in the interface, α≈0.1. Reducing the size of the soft grain will increase the coercivity, while the smooth change of the parameters in the interface will lead to a reduction in the coercivity. Comparison with experimental data justifies our calculation.

Carbon and titanium concurrently doped MgB2 alloys with a composition of Mg1−xTixB2−yCy have been prepared by an in situ solid state reaction method. The combination effects of carbon and Ti doping on structure, microstructure, irreversibility field (Hirr), and critical current density (Jc) of MgB2 have been investigated. It is found that doping Ti in MgB2 does not reduce the solubility of carbon in MgB2, but, on the contrary, it eliminates the porosity present in the carbon-doped MgB2, resulting in an improved intergrain connectivity. As a result, Jc and Hirr of MgB2 have been significantly improved, sharply contrasted to the situation of C-only-doped or Ti-only-doped MgB2 bulks. Our results clearly reveal that carbon and Ti codoping are largely cooperative in improving the performance of MgB2 in the high magnetic fields (>3T) and at high temperature (∼20K).

Excimer laser crystallization processing of thin silicon films on amorphous silicon oxide substrates was simulated by means of phase field modeling. The quantitative phase field model was derived from the Gibbs-Thompson equation coupled with energy conservation. Because the adaptive mesh scheme was adopted, the present calculations could accommodate both two-dimensional superlateral growth (SLG) phenomena and the realistic interface thickness (in the order of 10−10m). The vertical growth of fine-grained nucleation structures was simulated using one-dimensional calculations, and the results are consistent with those obtained in previous experiments. Two cases of SLG were also simulated, and the evolution of the interface and thermal fields was determined. Based on our simulation results, we conclude that SLG crystallization does not achieve steady growth because of the extremely fast heat dissipation from the substrate. To obtain very uniform electric characteristics for device fabrication, the layout desi...

The effects of microstructural variations with depth on Barkhausen effect (BE) signals in surface-modified ferrous materials have been studied through measurements and simulations based on a hysteretic-stochastic model. The BE signals measured from an unhardened sample show a peak near zero field. In contrast, the BE signals in surface-hardened samples with different case depths exhibit low-amplitude pulses near zero field and a peak at a high reverse field, which are attributed to irreversible magnetization processes in the soft core and the hardened surface layer, respectively. Theoretical analysis showed that the model parameters k and ξ, which describe the domain-wall pinning strength and the range of interaction of a domain wall with pinning sites, respectively, are related to each other as ξ = a ρ−1/2 = b/k via the pinning site density ρ, where a and b are constants. The relationship was used to simulate BE signals of the surface-hardened samples as a sum of signals generated at different depths by taking into account signal attenuation due to eddy current shielding. The simulated results were found to exhibit the general features observed in the experimental results.

We reply to the comment by Guerin and Guvarc’h concerning the phase equilibria of the Ga–Ni–As ternary system. We refute their position that the quench rate of the phase diagram samples is not important and that the solid-state phase equilibria of the Ga–Ni–As system does not change significantly between the temperatures of 25 and 800 °C. It is also demonstrated that the occurrence of ordered superlattice structures is not inconsistent with our work. We conclude that an adequate number of samples were prepared in our study to justify the modifications proposed to the Ga–Ni–As isothermal section and it is an accurate representation of the phase equilibria at 600 °C.

The effect of Pt alloy in Ni(Pt∼5 and 10 at. %) on the agglomeration and Ge out diffusion in nickel germanosilicide formed on Si0.75Ge0.25(100) has been studied. A remarkable improvement in the agglomeration behavior with increasing Pt atomic percentage is observed by sheet resistance measurements while still maintaining Ni(Pt) monogermanosilicide phase between 400 and 800 °C. Ge out diffusion from the monogermanosilicide grains has been suppressed up to a temperature of 700 °C with the addition of Pt, evident by x-ray diffraction and micro-Raman spectroscopy. In addition, that improvement of surface morphology and suppression of Ge out diffusion with increasing Pt atomic percent is also confirmed by Rutherford backscattering and cross-sectional transmission electron microscopy. The improved morphology and agglomeration are explained by a grain-boundary model which includes kinetic effects. The suppression of Ge out diffusion from the germanosilicide grains is attributed to reduced atomic diffusion and th...

Vertically aligned carbon nanofibers (VACNFs) were synthesized on Ni-coated Si substrates using a dc plasma-enhanced chemical-vapor deposition system. The size of the Ni islands used as catalyst to grow the VACNFs was formed by both thermal annealing and laser processing on thin metal layers. It was observed that the diameter of the carbon nanofibers is strongly dependent on the initial Ni island dimension. By varying the laser power from 228 to 279mJ∕cm2, the size of these Ni islands can be controlled independent of the initial Ni film thickness. Electron field-emission results show that the emission threshold field is dependent on both the height and radius of these VACNFs and also field shielding effects. Threshold fields as low as 2V∕μm was obtained from the sample with the largest height over radius ratio.

N,N′-di(naphthalene-1-yl)-N,N′-diphenyl-benzidine samples exhibiting interesting nano/microstructure were fabricated by thermal evaporation in a tube furnace under Ar gas flow. We investigated the influence of the substrate type, substrate temperature, source temperature, and the gas flow rate on the obtained morphology. The deposited material was investigated using scanning electron microscopy, x-ray diffraction, and photoluminescence. We found that the substrate temperature was the factor which significantly affected the obtained morphology, while other factors such as substrate type, source temperature, and gas flow mainly affected the size distribution of the features but not the type of morphology observed.

Formation energies and vibration frequencies for H in wurtzite GaN were calculated from density-functional theory and used to predict equilibrium state occupancies and solid solubilities at elevated temperatures for p-type, intrinsic, and n-type material. The solubility of deuterium (D) was measured in p-type, Mg-doped GaN at 600, 700, and 800 °C as a function of D2 pressure and compared with theory. Agreement was obtained by reducing the H formation energies 0.22 eV from ab initio theoretical values. The predicted stretch-mode frequency for H bound to the Mg acceptor lies 5% above an observed infrared absorption attributed to this complex. More limited solubility measurements were carried out for nominally undoped material rendered n-type by donors provisionally identified as O impurities, and results agree well with theory after the aforementioned adjustment of formation energies. It is concluded that currently recognized H states and physical processes can account for the equilibrium, elevated-temperat...

We present the effects of antiferromagnetic (AF) order of the Cr spacers in Fe/Cr(001) superlattices on the interlayer coupling of the Fe layers. AF order of the Cr spacers is suppressed for layer thicknesses less than 42 A. For ≳42 A of Cr, the Neel temperature (TN) increases rapidly and asymptotically approaches the bulk value for thick Cr spacers as characterized by a transition‐temperature shift exponent λ=1.4±0.3. Neutron diffraction confirms both the AF order of the Cr layers in superlattices with 62, 100, and 200 A thick Cr layers, and the existence of the incommensurate, transverse spin‐density‐wave magnetic structure whose nesting wave vector is equal to that of bulk Cr. The AF ordering of the Cr results in anomalies in a variety of magnetic properties, including the interlayer coupling, remanent magnetization, coercivity, and magnetoresistance. Most strikingly, the 90° or ‘‘biquadratic’’ coupling of the Fe layers observed for T≳TN is suppressed below TN as confirmed by polarized neutron reflecti...