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The behavior of excitons excited in anatase crystalline materials affects the photocatalytic performance of the materials. Thus, investigating the properties of such excitons is essential for understanding the origin of their influence on photocatalytic performance. Here, angle-resolved electron energy-loss spectroscopy was applied to evaluate the size of the spatial spread of excitons in anatase TiO2. Three kinds of excitons (I–III), which were reported by polarized light absorption experiments, were experimentally identified in the electron energy-loss spectroscopy (EELS) spectra. Exciton size was evaluated from absorption intensity, which was derived by the Kramers–Kronig analysis of the EELS spectra, depending on the momentum transfer (q). The sizes of excitons I, II, and III were evaluated to be 8, 5, and 6 nm, respectively. The larger size of exciton I than that of III was with the same tendency as in the theoretical results. Exciton II, which was evaluated as a delocalized one, was evaluated to have a finite size in this experiment. The largest size of exciton I, approximately 8 nm, is the same order of the exciton diffusion lengths of the material. Therefore, exciton I should significantly influence the photocatalytic activity of anatase. [ABSTRACT FROM AUTHOR]

The bottom-gated graphene field-effect transistors (GFETs) with HfO2 as the dielectric layer are fabricated and the transport behaviors with double Dirac points defined as the right conductance minima (VDirac+) and the left one (VDirac-) are obtained, which exhibit the unique W-shaped transform characteristics rather than the typical V-shape. This observation indicates that the graphene in the channel region shows the different doping behaviors from that underlying the metal contact region. The W-shaped characteristics of GFETs are affected by the ambient environment and the time-dependence of double Dirac points has been also found. After being treated by Poly (ethylene imine) (PEI), the transport behaviors of GFETs could be modulated, especially for VDirac+ showing more obvious negative shift, indicating that PEI has a remarkable n-doping effect on the graphene in the channel region. PEI also screens the environmental influence and could protect the graphene from being p-doped to some extent. At the same time, the annealing effect has been also studied and it is found that the proper annealing process could contribute to the recovery of the graphene properties. [ABSTRACT FROM AUTHOR]

This work is devoted to the investigation of the broad band dielectric spectra of 0.83PbMg1/3Nb2/3O3-0.17PbTiO3 (PMN-17PT) single crystals which have intermediate random fields. The necessity to understand the impact of random fields for the phase transition in heterogeneous perovskite oxides is of central importance. The thorough studies of dielectric properties revealed that the structural phase transition in PMN-17PT has a very complex dynamics. The temperature dependences of dielectric permittivity show that this material has features characteristic of polar nano regions and order-disorder phase transition dynamics. We speculate that this is the fingerprint of the phase transition in such heterogeneous perovskite oxides. [ABSTRACT FROM AUTHOR]

Tunnel junctions are interesting for both studying fundamental physical phenomena and providing new technological applications. Modeling of the tunneling current is important for understanding the tunneling processes and interpreting experimental data. In this work, the tunneling current is modeled using the Tsu-Esaki formulation with numerically calculated transmission. The feasibility of analytical formulae used for fitting experimental results is studied by comparing them with this model. The Tsu-Esaki method with numerically calculated transmission provides the possibility to calculate tunneling currents and fit experimental I-V curves for wide bias voltage and barrier width ranges as opposed to the more restricted analytical formulae. I-V curve features typical of tilted barrier structures are further analyzed to provide insight into the question, which of the phenomena can be explained with this simple barrier model. In particular, a small change in the effective barrier width is suggested as a possible explanation for experimental I-V curve features previously interpreted by a change in the tilt and height of the barrier. [ABSTRACT FROM AUTHOR]

Three-dimensional numerical simulation of equilibrium micromagnetic configurations existing in thin ferromagnetic films with surface anisotropy is carried out taking into account the strong demagnetization field acting on the film magnetization and the true micromagnetic boundary condition on the film surface. The numerical results are obtained in the simplest Néel approximation for surface anisotropy energy, a surface anisotropy constant Ks being a single phenomenological parameter. It is found that the spin canted state has the lowest total energy as compared to various multi-domain configurations in the intermediate range of thickness, Lz,minLz,max, nearly uniform in-plane magnetization, or the vortex has been obtained depending on the film in-plane aspect ratio. On the other hand, different labyrinth domain structures with large in-plane magnetization have been calculated in a thick enough film, Lz>Lz,max, with a sufficiently large surface anisotropy constant. [ABSTRACT FROM AUTHOR]

Vertical-cavity surface-emitting lasers (VCSELs) have been widely used in short-reach optical communication links and interconnects because of their high modulation speed and low energy consumption. In this paper, the effect of heat dissipation on the modulation bandwidth of highspeed 850-nm VCSELs has been investigated. With increasing bottom distributed Bragg reflector (DBR) diameter from 34 μm to 46 μm, optical output power increased as well as the -3 dB modulation bandwidth. The maximum small-signal modulation bandwidth of 16.2 GHz was achieved with an aperture of 5 μm. Thermal analysis showed that the heat dissipation of the bottom DBR diameter, which determined the differential gain of the quantum wells, has a strong effect on the modulation bandwidth. [ABSTRACT FROM AUTHOR]

We simulate the mixing (demixing) process of a quiescent binary liquid mixture with a composition-dependent viscosity which is instantaneously brought from the two-phase (one-phase) to the one-phase (two-phase) region of its phase diagram. Our theoretical approach follows a standard diffuse-interface model of partially miscible regular binary mixtures wherein convection and diffusion are coupled via a nonequilibrium capillary force, expressing the tendency of the phase-separating system to minimize its free energy. Based on 2D simulation results, we discuss the influence of viscosity ratio on basic statistics of the mixing (segregation) process triggered by a rapid heating (quench), assuming that the ratio of capillary to viscous forces (a.k.a. the fluidity coefficient) is large. We show that, for a phase-separating system, at a fixed value of the fluidity coefficient (with the continuous phase viscosity taken as a reference), the separation depth and the characteristic length of single-phase microdomains decrease monotonically for increasing values of the viscosity of the dispersed phase. This variation, however, is quite small, in agreement with experimental results. On the other hand, as one might expect, at a fixed viscosity of the dispersed phase both of the above statistics increase monotonically as the viscosity of the continuous phase decreases. Finally, we show that for a mixing system the attainment of a single-phase equilibrium state by coalescence and diffusion is retarded by an increase in the viscosity ratio at a fixed fluidity for the dispersed phase. In fact, for large enough values of the viscosity ratio, a thin film of the continuous phase becomes apparent when two drops of the minority phase approach each other, which further retards coalescence. [ABSTRACT FROM AUTHOR]

Al/Pt nanolaminates with a bilayer thickness (tb, width of an Al/Pt pair-layer) of 164 nm were irradiated with single laser pulses with durations of 10ms and 0.5 ms at 189 W/cm² and 1189 W/cm², respectively. The time to ignition was measured for each pulse, and shorter ignition times were observed for the higher power/shorter pulse width. Videographic images of the irradiated area shortly after ignition show a non-uniform radial brightness for the longer pulse, while the shorter pulse shows uniform brightness. A diffusion-limited single step reaction mechanism was implemented in a finite element package to model the progress from reactants to products at both pulse widths. The model captures well both the observed ignition delay and qualitative observations regarding the non-uniform radial temperature. [ABSTRACT FROM AUTHOR]

The structural properties and the spin states of vacancies in ion implanted silicon carbide samples are analyzed by experimental measurements along with first-principles calculations. Different types and dosages of ions (N+, O+, and B+) were implanted in the 4H-silicon carbide single crystal. The Raman spectra, positron annihilation spectroscopy, and magnetization-magnetic field curves of the implanted samples were measured. The fitting results of magnetization-magnetic field curves reveal that samples implanted with 1×1016cm-2 N+ and O+ ions generate paramagnetic centers with various spin states of J = 1 and J=0.7, respectively. While for other implanted specimens, the spin states of the paramagnetic centers remain unchanged compared with the pristine sample. According to the positron annihilation spectroscopy and first-principles calculations, the change in spin states originates from the silicon vacancy carrying a magnetic moment of 3.0 μB in the high dosage N-implanted system and 2.0 μB in the O-doped system. In addition, the ratio of the concentration of implanted N ions and silicon vacancies will affect the magnetic moment of VSi. The formation of carbon vacancy which does not carry a local magnetic moment in B-implanted SiC can explain the invariability in the spin states of the paramagnetic centers. These results will help to understand the magnetic moments of vacancies in ion implanted 4H-SiC and provide a possible routine to induce vacancies with high spin states in SiC for the application in quantum technologies and spintronics. [ABSTRACT FROM AUTHOR]

A glass with the mol. % compositions 51.7 B2O3/9.3 K2O/1 P2O5/27.6 Li2O/10.4 Fe2O3 was crystallized at temperatures in the range from 400 to 540 °C for different periods of time (2-12 h). X-ray diffraction showed cubic Li ferrite with a spinel structure, LiFe5O8, with small crystallites with sizes in the range from 3 to 31 nm. While the samples crystallized at 440, 480, and 500 °C reveal a disordered phase, crystallization at 540 °C indicates a phase transformation to the ordered state. Magnetization curves showed that the samples crystallized at 440 °C are superparamagnetic with very low maximum magnetization, while the samples crystallized at 480 and 500 °C show thin clear S-shaped magnetization curves with zero coercivity and hence are also superparamagnetic. By contrast, the sample crystallized at 540 °C shows a coercive field of 40 Oe and thus is ferrimagnetic. The magnetic properties can be tailored by the size of the ferrite crystals and hence by the crystallization conditions. [ABSTRACT FROM AUTHOR]

We recently demonstrated a magnetic parallel dipole line (PDL) system that serves as a unique diamagnetic trap with a fascinating one-dimensional camelback potential along its longitudinal axis. The system can be realized with a pair of transversely magnetized cylindrical magnets and a cylindrical graphite rod as the trapped object. We present more detailed experimental and theoretical studies of the finite size effect of the rod and its impact on the stability and oscillation dynamics of the trap. We show that the camelback potential effect only occurs when the length of the PDL system is beyond certain critical length (LC). The length of the trapped rod determines the "effective camelback potential" and is subject to maximum and minimum values for the trap to be stable. Both length and radius of the rod determine the damping dynamics or the quality factor of the oscillator. These characteristics are important for designing the PDL trap system for various sensing applications, for example, we demonstrated a PDL trap gas viscometer system through measurement of the oscillation damping time constant. [ABSTRACT FROM AUTHOR]

H+ ion-induced damage of multilayer graphene (MLG) is investigated using Molecular Dynamics simulations as H2 plasmas could provide a possible route to pattern graphene. Low-energy (5-25 eV) H+ cumulative bombardment of ABA-stacked MLG samples shows an increase of the hydrogenation rate with the ion dose and ion energy. At 5 eV, the H coverage grows with the ion fluence only on the upper-side of the top layer but saturates around 35%. Hydrogenation of multilayers and carbon etching are observed at higher energies. Layer-by-layer peeling/erosion of the MLG sample is observed at 10 eV and occurs in two phases: the MLG sample is first hydrogenated before carbon etching starts via the formation of CHx (∼60%) and C2Hx (∼30%) by-products. A steady state is reached after an ion dose of ∼5×1016 H+/cm², as evidenced by a constant C etch yield (∼0.02 C/ion) and the saturation of the hydrogenation rate. At 25 eV, an original etching mechanism—lifting-off the entire top layer—is observed at low fluences due to the accumulation of H2 gas in the interlayer space and the absence of holes/vacancies in the top layer. However, as the underneath layers contain more defects and holes, this Smartcut-like mechanism cannot be not repeated and regular ion-assisted chemical etching is observed at higher fluences, with a yield of ∼0.05 C/ion. [ABSTRACT FROM AUTHOR]

Direct measurements of the adiabatic temperature change (DTAD) of Ni50Mn35In14.5B0.5 have been done using an adiabatic magnetocalorimeter in a temperature range of 250-350 K, and with magnetic field changes up to DH=1.8 T. The initial susceptibility in the low magnetic field region drastically increases with temperature starting at about 300 K. Magnetocaloric effects parameters, adiabatic temperature changes, and magnetic entropy changes were found to be a linear function of H2/3 in the vicinity of the second order transitions (SOT), whereas the first order transitions do not obey the H2/3 law due to the discontinuity of the transition. The relative cooling power based on the adiabatic temperature change for a magnetic field change of 1.8 T has been estimated. Maximum values of DTAD=-2.6K and 1.7K were observed at the magnetostructural transition (MST) and SOT for DH=1.8 T, respectively. The observed DTAD at the MST exceeds the DTAD for Ni50Mn35In14X with X=In, Al, and Ge by more than 20% and is larger than the Gd based Heusler alloys. [ABSTRACT FROM AUTHOR]

e compare the plasma emission and harmonic generation spectra in the cases of resonance enhancement of single harmonic using various laser-produced plasmas. The analysis of these spectra showed that the enhancement of harmonics depended on the oscillator strengths of the nearby ionic transitions rather than the plasma emission transitions. To prove it, we used the tunable source of mid-infrared radiation from an optical parametric amplifier (1200-1600 nm) and the fixed wavelength source (810 nm) of femtosecond pulses. The comparison of resonantly enhanced and featureless plateaulike harmonic spectra from different laser-produced plasmas, as well as their plasma emissions, confirmed this assumption. [ABSTRACT FROM AUTHOR]

High quality hybrid perovskite films are grown by the pulsed laser deposition (PLD) process through target stoichiometry and gas ambient control in the commonly used on-axis geometry which is compatible with PLD of other materials such as metal oxides. The use of an off-stoichiometric target and a momentum softening gas mixture of argon and hydrogen (90%:10%) is shown to yield dense films in the on-axis geometry with excellent stoichiometry and optical quality. The band gap can be easily tuned from 1.6 to 2.3 eV with either mixed halide composite target or successive depositions of interdiffusing MAPbBr3 and MAPbI3 layers. A photosensor based on PLD grown MAPbI3 is shown to render impressive performance over the broad range of wavelengths from 375 to 800 nm at a voltage of 5V bias. A planar heterojunction solar cell based on the dry-processed on-axis PLD grown film exhibits a champion conversion efficiency of 10.9%. A heterostructure comprising in situ pulsed laser deposited Cu2O (p-type) followed by the hybrid perovskite film shows a rectifying characteristic and photoresponse. [ABSTRACT FROM AUTHOR]

We present a novel terahertz (THz) quantum cascade laser (QCL) design where C-valley states are used for lasing transition and X-valley states—in particular, Xz-states—are used as injector subbands. Since the lasing states in our proposed structure are populated and depopulated mainly through the interface roughness assisted C-Xz electron scattering, we present a model to describe this intervalley carrier transport. In the injector region of the proposed THz QCL, we use a quaternary AlGaAsP material to introduce tensile strain, which plays a crucial role in increasing the gain. To compensate the strain per period, we propose to grow the periodic heterostructure on a GaAs0.94P0.06 virtual substrate. To simulate the carrier transport and hence calculate the gain and lasing performance of the proposed THz QCL, we use a simplified density matrix formalism that considers resonant tunneling, dephasing, and the important intersubband scattering mechanisms. Since electron temperature significantly varies from lattice temperature for QCLs, we take their difference into account using the kinetic energy balance method. We show that the proposed structure is capable of lasing up to a maximum lattice temperature of ∼119K at 4.8 THz. For future improvements of the device, we identify major performance-degrading factors of the proposed design. [ABSTRACT FROM AUTHOR]

We studied the optical properties of periodic InGaN/GaN multiple quantum well systems with different numbers of periods. A resonant increase in the optical reflection and simultaneous suppression of the optical absorption have been revealed experimentally at room temperature when the Bragg and exciton resonances were tuned to each other. Numerical modeling with a single set of parameters gave a quantitatively accurate fit of the experimental reflection and transmission spectra in a wide wavelength range and various angles of the light incidence. The model included both exciton resonance and non-resonant band-to-band transitions in the InGaN quantum wells, as well as Rayleigh light scattering in the GaN buffer layer. The analysis also involved x-ray diffraction and photoluminescence data. It allowed us to determine the key parameters of the structure. In particular, the radiative broadening of the InGaN QW excitons was evaluated as 0.20 ± 0.02 meV. [ABSTRACT FROM AUTHOR]