Low loss optical waveguides fabricated in LiTaO3 by swift heavy ion irradiation

© 2019 Optical Society of America. Users may use, reuse, and build upon the article, or use the article for text or data mining, so long as such uses are for non-commercial purposes and appropriate attribution is maintained. All other rights are reservedOptical waveguides are fabricated by irradiation of LiTaO 3 with a variety of swift heavy ions that provide increasing levels of both nuclear and electronic damage rates, including C, F and Si ions, in the energy range of 15-40 MeV. A systematic study of the role of the ion fluence has been carried out in the broad range of 1e13-2e15 at/cm 2 . The kinetics of damage is initially of nuclear origin for the lowest fluences and stopping powers and, then, is enhanced by the electronic excitation (for F and Si ions) in synergy with the nuclear damage. Applying suitable annealing treatments, optical propagation losses values as low as 0.1 dB have been achieved. The damage rates found in LiTaO 3 have been compared with those known for the reference LiNbO 3 and discussed in the context of the thermal spike modelV. Tormo-Márquez thanks the CMAM-UAM for their financial support. We thank the Technical staff of the CMAM-UAM center for support with the ion irradiation

Finite size and intrinsic field effect on the polar-active properties of the ferroelectric-semiconductor heterostructures

Using Landau-Ginzburg-Devonshire approach we calculated the equilibrium distributions of electric field, polarization and space charge in the ferroelectric-semiconductor heterostructures containing proper or incipient ferroelectric thin films. The role of the polarization gradient and intrinsic surface energy, interface dipoles and free charges on polarization dynamics are specifically explored. The intrinsic field effects, which originated at the ferroelectric-semiconductor interface, lead to the surface band bending and result into the formation of depletion space-charge layer near the semiconductor surface. During the local polarization reversal (caused by the inhomogeneous electric field induced by the nanosized tip of the Scanning Probe Microscope (SPM) probe) the thickness and charge of the interface layer drastically changes, it particular the sign of the screening carriers is determined by the polarization direction. Obtained analytical solutions could be extended to analyze polarization-mediated electronic transport.Comment: 35 pages, 12 figures, 1 table, 2 appendices, to be submitted to Phys. Rev.

Localization of Carriers and Polarization Effects in Quaternary AlInGaN Multiple Quantum Wells

We report on observing a long-wavelength band in low-temperature photoluminescence(PL)spectrum of quaternary Al0.22In0.02Ga0.76N/Al0.38In0.01Ga0.61N multiple quantum wells(MQWs), which were grown over sapphire substrates by a pulsed atomic-layer epitaxy technique. By comparing the excitation-power density and temperature dependence of the PLspectra of MQWs and bulk quaternary AlInGaN layers, we show this emission band to arise from the carrier and/or exciton localization at the quantum well interface disorders. PL data for other radiative transitions in MQWs indicate that excitation-dependent spectra position is determined by screening of the built-in electric field

Direct numerical simulations of turbulent flow through a stationary and rotating infinite serpentine passage

Serpentine passages are found in a number of engineering applications including turbine blade cooling passages. The design of effective cooling passages for high-temperature turbine blades depends in part on the ability to predict heat transfer, thus requiring an accurate representation of the turbulent flow field. These passages are subjected to strong curvature and rotational effects, and the resulting turbulent flow field is fairly complex. An understanding of the flow physics for flows with strong curvature and rotation is required in order to improve the design of turbine blade cooling passages. Experimental measurements of certain turbulence quantities for such configurations can be challenging to obtain, especially near solid surfaces, making the serpentine passage an ideal candidate for a direct numerical simulation (DNS). A DNS study has been conducted to investigate the coupled effect of strong curvature and rotation by simulating turbulent flow through a fully developed, smooth wall, round-ended, isothermal serpentine channel subjected to orthogonal mode rotation. The geometry investigated has an average radius of curvature Rc/δ=2.0 in the curved section and dimensions 12πδ×2δ×3πδ in the streamwise, transverse, and spanwise directions. The computational domain consists of periodic inflow/outflow boundaries, two solid wall boundaries, and periodic boundaries in the spanwise direction. The simulations were conducted for Reynolds number, Reb=5600, and rotation numbers, Rob,z=0 and 0.32. Differences observed between the stationary and rotating cases are discussed in terms of the mean velocity, secondary flow, and Reynolds stresses

Degradation of structure and properties of rail surface layer at long-term operation

The microstructure evolution and properties variation of the surface layer of rail steel after passed 500 and 1000 million tons of gross weight (MTGW) have been investigated. The wear rate increases to 3 and 3.4 times after passed 500 and 1000 MTGW, respectively. The corresponding friction coefficient decreases by 1.4 and 1.1 times. The cementite plates were destroyed and formed the cementite particles of around 10-50 nm in size after passed 500 MTGW. The early stage dynamical recrystallization was observed after passed 1000 MTGW. The mechanisms for these have been suggested. The large number of bend extinction contours is revealed in the surface layer. The internal stress field is evaluated

Electrically tunable GHz oscillations in doped GaAs-AlAs superlattices

Tunable oscillatory modes of electric-field domains in doped semiconductor superlattices are reported. The experimental investigations demonstrate the realization of tunable, GHz frequencies in GaAs-AlAs superlattices covering the temperature region from 5 to 300 K. The orgin of the tunable oscillatory modes is determined using an analytical and a numerical modeling of the dynamics of domain formation. Three different oscillatory modes are found. Their presence depends on the actual shape of the drift velocity curve, the doping density, the boundary condition, and the length of the superlattice. For most bias regions, the self-sustained oscillations are due to the formation, motion, and recycling of the domain boundary inside the superlattice. For some biases, the strengths of the low and high field domain change periodically in time with the domain boundary being pinned within a few quantum wells. The dependency of the frequency on the coupling leads to the prediction of a new type of tunable GHz oscillator based on semiconductor superlattices.Comment: Tex file (20 pages) and 16 postscript figure

Thermodynamics of nanodomain formation and breakdown in Scanning Probe Microscopy: Landau-Ginzburg-Devonshire approach

Thermodynamics of tip-induced nanodomain formation in scanning probe microscopy of ferroelectric films and crystals is studied using the Landau-Ginzburg-Devonshire phenomenological approach. The local redistribution of polarization induced by the biased probe apex is analyzed including the effects of polarization gradients, field dependence of dielectric properties, intrinsic domain wall width, and film thickness. The polarization distribution inside subcritical nucleus of the domain preceding the nucleation event is very smooth and localized below the probe, and the electrostatic field distribution is dominated by the tip. In contrast, polarization distribution inside the stable domain is rectangular-like, and the associated electrostatic fields clearly illustrate the presence of tip-induced and depolarization field components. The calculated coercive biases of domain formation are in a good agreement with available experimental results for typical ferroelectric materials. The microscopic origin of the observed domain tip elongation in the region where the probe electric field is much smaller than the intrinsic coercive field is the positive depolarization field in front of the moving counter domain wall. For infinitely thin domain walls local domain breakdown through the sample depth appears. The results obtained here are complementary to the Landauer-Molotskii energetic approach.Comment: 35 pages, 8 figures, suplementary attached, to be submitted to Phys. Rev.

Enhanced antiferroelectric-like relaxor ferroelectric characteristic boosting energy storage performance of (Bi0.5Na0.5)TiO3-based ceramics via defect engineering

Lead-free (Bi0.5Na0.5)TiO3 (BNT)-based relaxor ferroelectric (RFE) ceramics have attracted a lot of attention due to their high power density and rapid charge-discharge capabilities, as well as their potential application in pulse power capacitors. However, because of the desire for smaller electronic devices, their energy storage performance (ESP) should be enhanced even further. We describe a defect engineering strategy for enhancing the antiferroelectric-like RFE feature of BNT-based ceramics by unequal substitution of rare-earth La3+ in this paper. The ESP of La3+-doped samples is raised by 25% with the same synthetic procedure and thickness, due to an increase in the critical electric field (E-field) and saturated E-field during polarization response, which is induced by a modification in the energy barrier between the lattice torsion. More impressively, an ultrahigh recoverable energy storage density Wrec of 8.58 J/cm3 and a high energy storage efficiency η of 94.5% are simultaneously attained in 3 at.% La3+-substituted 0.6(Bi0.5Na0.4K0.1)1-1.5xLaxTiO3-0.4[2/3SrTiO3-1/3Bi(Mg2/3Ni1/3)O3] RFE ceramics with good temperature stability (Wrec = 4.6 ± 0.2 J/cm3 and higher η of ≥90% from 30 °C to 120 °C), frequency stability, and fatigue resistance. The significant increase in ESP achieved through defect engineering not only proves the effectiveness of our strategy, but also presents a novel dielectric material with potential applications in pulse power capacitors. © 2022 The Chinese Ceramic SocietyNational Natural Science Foundation of China, NSFC: 52172127; Xi’an Jiaotong University, XJTU; National Key Research and Development Program of China, NKRDPC: 2021YFE0115000, SQ2021YFB380003202; Fundamental Research Funds for the Central UniversitiesThis work was supported by the National Natural Science Foundation of China (Grant No. 52172127 ) the National Key R&D Program of China (Grant Nos. 2021YFE0115000 and SQ2021YFB380003202 ) and the Fundamental Research Funds for the Central Universities (XJTU). The SEM work was done at International Center for Dielectric Research (ICDR), Xi’an Jiaotong University, Xi'an, China

Statics and Dynamics of Ferroelectric Domains in Molecular Multiaxial Ferroelectric (Me3NOH)2[KCo(CN)6]

The recent emergence of multiaxial molecular ferroelectrics opens up a new route toward technological evolution in the next-generation flexible/wearable device applications. However, a fundamental understanding of multiaxial ferroelectricity and polarization switching at the microscopic level in these materials is still missing. Herein, we study a high-temperature multiaxial perovskite ferroelectric (Me3NOH)2[KCo(CN)6] (TMC-4) that exhibits a bond-switching phase transition at 417 K with notable piezoelectricity and spontaneous polarization in the ferroelectric phase. The cleavage and reformation of coordination bonds and hydrogen bonds during the bond-switching transition all contribute to a large entropy change of 178.79 J K-1 kg-1 at the phase transition. Using piezoresponse force microscopy (PFM), we observed diverse ferroelectric domain structures and provide evidence for both 180° and non-180° domain switching and their possible effect on the functional properties of molecular ferroelectrics. The results provide an insight into the multiaxial ferroelectricity of TMC-4 at the microscopic level enabling its further use in device applications. © 2021 The Royal Society of Chemistry.This work was supported by the NSFC (22071273, 21805312, and 21821003), and Local Innovative and Research Teams Project of Guangdong Pearl River Talents Program (2017BT01C161). This work (including the grant to W.-J. X.) was also developed within the scope of the project CICECO-Aveiro Institute of Materials, UIDB/50011/2020 & UIDP/50011/2020, PTDC/CTM-CTM/4044/ 2020 financed by national funds through the FCT/MEC and when appropriate co-financed by FEDER under the PT2020 Partnership Agreement. The equipment of the Ural Center for Shared Use ‘‘Modern Nanotechnology’’ UrFU was used. The work was supported by the Ministry of Science and Higher Education of the Russian Federation (state task FEUZ-2020-0054). It is also funded by National Funds (OE), through FCT–Fundação para a Ciência e a Tecnologia, I.P., in the scope of the framework contract foreseen in the numbers 4, 5 and 6 of the Article 23, of the Decree-Law 57/2016, of August 29, changed by Law 57/2017, of July 19