Symmetric Superlattice Research Articles

Nonlocal defect solitons in parity-time-symmetric superlattices with defocusing nonlinearity

We report on the nonlocal fundamental defect soliton in parity-time (PT)-symmetric superlattices with defocusing nonlinearity. For positive defect, soliton exist in the second gap, the first gap and the semi-infinite gap. They can be stable in the low power region in the first gap and semi-infinite gap. For negative defect, defect solitons only exist in the second gap, and can also be stable in the low power region. The degree of nonlocality can significantly affect the stability of these defect solitons. We also study the transverse power-flow of these defect solitons in the PT-symmetric superlattices.

-symmetric optical superlattices

The spectral and localization properties of -symmetric optical superlattices, either infinitely extended or truncated at one side, are theoretically investigated, and the criteria that ensure a real energy spectrum are derived. The analysis is applied to the case of superlattices describing a complex (-symmetric) extension of the Harper Hamiltonian in the rational case.

Defect gap solitons in self-focusing Kerr media with parity-time symmetric linear superlattices and modulated nonlinear lattices

We report the existence and stability of defect solitons (DSs) in a dissipative system with a parity-time (PT) symmetric linear superlattice and modulated periodical nonlinearity based on self-focusing Kerr media. It is found that for positive defects, the DSs can exist stably in the semi-infinite gap. For negative defects, the DSs can exist stably in both the semi-infinite and the first gap. The DSs in high power regions are unstable. If the defect strength increases, the stable regions shrink rapidly and eventually disappear. If the PT-symmetry is broken, all DSs are unstable.

Transmission of electrons with flat passbands in finite superlattices

Using the transfer matrix method and the Ben Daniel-Duke equation for variable mass electrons propagation, we calculate the transmittance for symmetric finite superlattices where the width and the height of the potential barriers follow a linear dependence. The width and height of the barriers decreases from the center to the ends of the superlattice. The transmittance presents intervals of stopbands and quite flat passbands.

Strain effect in PbTiO3/PbZr0.2Ti0.8O3 superlattices: From polydomain to monodomain structures

Ferroelectric symmetric superlattices consisting of alternating layers of PbTiO3 and PbZr0.2Ti0.8O3, were grown by pulsed laser deposition on SrTiO3 and SrRuO3-coated SrTiO3 substrates. The superlattices, with wavelengths Λ ranging from 20 Å to 200 Å, were analyzed using x-ray diffraction (θ−2θ diffraction scans, rocking curves, and reciprocal space mapping), high resolution transmission electron microscopy, and piezoforce scanning microscopy. For large-period superlattices, the strain is relieved by the formation of an a/c polydomain structure which propagates through the whole film. We investigate the influence of the wavelength on the a-domain volume fraction, the lattice parameters, the in-plane strain εxx, and the mosaicity of the samples. We show that with decreasing the wavelength, a reduction of the a-domain volume fraction is observed as well as a reduced tensile in-plane strain and a lower mosaicity. A concomitant improvement of the local ferroelectric response is detected. Below a critical wavelength of about 30 Å and a critical sample thickness of 500 Å, the formation of 90° a/c domains is inhibited and the superlattices are completely c oriented. Thus the reduced wavelength induces compressive strain which dominates over the tensile clamping due to the thermal expansion mismatch between the substrate and the superlattice. This compressive strain favors a c-oriented structure in the PbTiO3/PbZr0.2Ti0.8O3 superlattices.

Chaotic transport of a matter-wave soliton in a biperiodically driven optical superlattice

Under the effective particle approximation, we study the temporal ratchet effect for chaotic transport of a matter-wave soliton consisting of an attractive Bose–Einstein condensate held in a quasi-one-dimensional symmetric optical superlattice with biperiodic driving. It is known that chaos can substitute for disorder in Anderson’s scenario [Wimberger S, Krug A, Buchleitner A. Phys Rev Lett 2002;89:263601] and only a higher level of disorder can induce Anderson localization for some special systems [Schwartz T, Bartal G, Fishman S, Segev M. Nature 2007;46:52], and a matter-wave soliton can transit to chaos with high or low probability in a high- or low-chaoticity region [Zhu Q, Hai W, Rong S. Phys Rev E 2009;80:016203]. Here we demonstrate that varying the driving phase to break the time reversal symmetry of the system can increase the size of the high-chaoticity region for low- and moderate-frequency regions. Consequently, the parameter region of the exponential spatial localization increases to the same size, and the low-chaoticity and delocalization region, which includes subregions of the ratchet effect and its inverse effect, correspondingly decreases. The positive dependence of the localization on the driving frequency is also revealed. The results indicate that a high-chaoticity region could replace higher disorder and assists in Anderson localization. From the results we suggest a method for controlling directed motion of a matter-wave soliton by adjusting the driving frequency and amplitude to strengthen or suppress, or even reverse, the temporal ratchet effect.

Fabrication and ferroelectric properties of BiFeO3/LaNiO3 artificial superlattice structures grown by radio-frequency magnetron-sputtering

Symmetric superlattice structures of multiferroic BiFeO3 and conductive LaNiO3 sublayers were grown on a Nb-doped SrTiO3 substrate with radio-frequency magnetron-sputtering. The formation of a superlattice structure was confirmed from the appearance of satellite lines on both sides of the main line in the X-ray diffraction pattern. X-ray measurements show that these superlattice films become subject to greater tensile stress along the c-axis and to increased compressive stress parallel to the surface plane with a decreasing thickness of the sublayer.The smaller is the thickness of the sublayer, the greater is the crystalline quality and the strain state. The hysteresis loops show a large leakage current at frequencies of 0.5 and 1kHz; the polarization decreases with an increasing frequency.

Transmisión para una superred con modulación lineal de los anchos de las barreras

Usando el método de la matriz de transferencia y la ecuación de Ben Daniel y Duke para propagación de electrones con masa variable, calculamos la transmitancia para una superred lineal finita simétrica que tiene variación lineal del ancho de las barreras de potencial. El ancho de las barreras decrece del centro hacia los extremos de la superred. La dependencia en la energía de la transmitancia presenta intervalos de bandas de rechazo y de bandas bastante planas de transmisión. Realizamos los cálculos de transmisión para varias alturas de barreras y varios anchos de pozos. Comparamos con la transmisión de una superred regular en la cual todas las barreras tienen el mismo ancho. También comparamos con la de una superred lineal invertida en que el ancho de las barreras crece del centro hacia afuera, así como con la de una estructura donde la variación de los anchos de barreras sigue un perfil gaussiano.Using the transfer matrix method and the Ben Daniel-Duke equation for variable mass electrons propagation, we calculate the transmittance for a symmetric finite superlattice which has a linear dependence for the width of the potential barriers. The width of the barriers decreases from the center toward to the ends of the superlattice. The energy dependence of the transmittance presents intervals of stopbands and quite flat passbands. We calculate the transmission for several heights of barriers and widths of wells and compare with the transmission of a regular superlattice where all the barriers have the same width. We also compare with an inverted linear superlattice where the barriers width increases from the center to the ends, as well with the transmission produced by a superlattice with a Gaussian variation of the barriers width.

The contrast between defect solitons in parity—time symmetric superlattice and simple-lattice complex potentials

The existence and stability of defect superlattice solitons in parity—time (PT) symmetric superlattice and simple-lattice complex potentials are reported. Compared with defect simple-lattice solitons in similar potentials, the defect soliton in superlattice has a wider stable range than that in simple-lattice. The solitons' power increases with increasing propagation constant. For the positive defect, the solitons are stable in the whole region where solitons exist in the semi-infinite gap. For the zero defect, the solitons are unstable at the edge of the band. For the negative defect, the solitons propagate with the shape of Y at low propagation constant and propagate stably at the large one.

Self-Assembly of Gold Nanorods into Symmetric Superlattices Directed by OH-Terminated Hexa(ethylene glycol) Alkanethiol

The self-assembly of anisotropic gold nanorods (GNRs) into ordered phases remains a challenge. Herein, we demonstrated the fabrication of symmetric circular- or semicircular-like self-assembled superlattices composed of multilayers of standing GNRs by fine-tuning the repulsive interactions among GNRs. The repulsive force is tailored from electrostatic interaction to steric force by replacing the surface coating of cetyltrimethylammonium bromide (CTAB) (ζ potential of 20-50 mV) with an OH-terminated hexa(ethylene glycol) alkanethiol (here termed as EG(6)OH, ζ potential of -10 mV). The assembly mechanism is discussed via theoretical analyses of the major interactions, and an effective balance between the repulsive steric and attractive depletion interactions is the main driving force for the self-assembly. The real-time observations of solution assembly (UV-vis-NIR absorption spectroscopy) supports the mechanism that we suggested. The superlattices obtained here not only enrich the categories of the self-assembled structures but more importantly deepen the insight of the self-assembly process and pave the way for various potential applications.

Defect solitons in parity-time symmetric superlattices

We study defect solitons (DSs) in a parity-time (PT) symmetric superlattice with focusing Kerr nonlinearity. The properties of the DSs with a PT symmetrical potential are obviously different from those in a superlattice with a real refractive index. Unusual features stemming from PT symmetry can be found. Research results show that the solitons with a zero defect or a positive defect can exist and stably propagate in the semi-infinite gap, but they cannot exist in the first gap. For the case of a negative defect, the soliton can stably exist in both the semi-infinite gap and the first gap.

Cryogenic temperature measurement of THz meta-resonance in symmetric metamaterial superlattice

A symmetric metamaterial superlattice is introduced accommodating a high Q-factor trapped mode. THz time-domain spectroscopy is employed to measure the transmission spectra, identifying the excitation of trapped and open-modes in the meta-resonances. A finite-difference-time- domain calculation showed that the trapped mode excitation is from the cancelation of current densities among the nearest-neighboring meta-particles. A cryogenic temperature THz measurement is carried out to examine the temperature dependence of resonance characteristics of meta-resonances. At low temperatures, the temperature-independent radiative damping is dominant for the open-mode, while the Q-factor of the trapped mode is determined by the temperature-dependent phonon scattering and temperature-independent defect scattering with the radiative damping significantly suppressed. When compared with the room temperature measurement, a 16% increase in Q-factor is observed for the trapped mode, while a 7% increase for the open-mode at the cryogenic temperature.

Structural and Magneto-Electric Properties of Pulsed Laser Deposited Ferroelectric/Ferromagnetic Heterostructure

AbstractAsymmetric superlattices (SLs) with ferromagnetic La0.7Sr0.3MnO3 (LSMO) and ferroelectric Ba0.7Sr0.3TiO3 (BST) as constitutive layers were fabricated on conducting LaNiO3 (LNO) coated (001) oriented MgO substrates using pulsed laser deposition (PLD). The crystallinity, ferroelectric and magnetic properties of the SLs were studied over a wide range of temperatures and frequencies. The structure exhibited ferromagnetic behavior at 300K, and ferroelectric behavior over a range of temperatures between 100K and 300K. The dielectric response as a function of frequency obeys normal behavior below 300 K, whereas it follows Maxwell–Wagner model at elevated temperatures. The effect of ferromagnetic LSMO layers on ferroelectric properties of the SL indicated strong influence of the interfaces. The asymmetric behavior of ferroelectric loop and the capacitance-voltage relationship suggest development of a built field in the SLs due to high strain across the interfaces.

Realization of biferroic properties in La0.6Sr0.4MnO3∕0.7Pb(Mg1∕3Nb2∕3)O3–0.3(PbTiO3) epitaxial superlattices

A set of symmetric and asymmetric superlattices with ferromagnetic La0.6Sr0.4MnO3 (LSMO) and ferroelectric 0.7Pb(Mg1∕3Nb2∕3)O3–0.3(PbTiO3) as the constituting layers was fabricated on LaNiO3 coated (100) oriented LaAlO3 substrates using pulsed laser ablation. The crystallinity, and magnetic and ferroelectric properties were studied for all the superlattices. All the superlattice structures exhibited a ferromagnetic behavior over a wide range of temperatures between 10 and 300K, whereas only the asymmetric superlattices exhibited a reasonably good ferroelectric behavior. Strong influence of an applied magnetic field was observed on the ferroelectric properties of the asymmetric superlattices. Studies were conducted toward understanding the influence of conducting LSMO layers on the electrical responses of the heterostructures. The absence of ferroelectricity in the symmetric superlattice structures has been attributed to their high leakage characteristics. The effect of an applied magnetic field on the ferroelectric properties of the asymmetric superlattices indicated strong influence of the interfaces on the properties. The dominance of the interface on the dielectric response was confirmed by the observed Maxwell-Wagner-type dielectric relaxation in these heterostructures.

Engineered Biferroic 0.7Pb(Mg1/3Nb2/3)O3-0.3PbTiO3/La0.6Sr0.4MnO3 Epitaxial Superlattices

AbstractSymmetric and asymmetric superlattices (SLs) composed of ferromagnetic La0.6Sr0.4MnO3 (LSMO) and ferroelectric 0.7Pb(Mg1/3Nb2/3)O3 – 0.3PbTiO3 (PMN-PT) with different periodicities have been fabricated on LaNiO3 (LNO) coated LaAlO3 (100) (LAO) substrates by pulsed laser ablation deposition. Structural, ferromagnetic and ferroelectric properties have been studied for all the SLs. All the heterostructures exhibited good ferromagnetic response over a wide range of temperatures (10K – 300K), whereas only the asymmetric SLs exhibited reasonably good ferroelectric behaviour. Ferromagnetic and ferroelectric hysteresis loops observed in the asymmetric SLs confirmed their biferroic nature. Studies were conducted towards understanding the influence of LSMO layers on the electrical responses of the heterostructures. Absence of ferroelectricity in the symmetric SL structures has been attributed to their high leakage characteristics. Strong influence of an applied magnetic field of 1.2T was observed on the ferroelectric properties of the asymmetric SLs. The effect of magnetic field on the ferroelectric properties of the SLs indicated possibility of strong interfacial effect.

Localized electronic states in [formula omitted]-layer-based superlattices with structural defects

Using an effect-barrier height method, we study the properties of the localized electronic states in an N -layer-based superlattice with structural defects within the framework of effective-mass theory. The coupling effect between normal and lateral degrees of freedom of an electron on the localized electronic states in both symmetric and asymmetric triple layer superlattices with structural defects has been considered numerically. The results show that the localized states display different behaviors in both symmetric and asymmetric structures in spite of the minibands being not influenced by the structural symmetry. Moreover, the coupling effect causes the minibands, minigaps and localized electron levels to depend on the transverse wave number k xy . A brief physical analysis is given.

金属ナノ粒子の自在配列制御とナノデバイスへの応用

Physical properties of metal nanoparticle superlattices are dependent on the particle size, interparticle spacing, superlattice periodicity and symmetry, among which the control of superlattice symmetry is most difficult to achieve. Various symmetric superlattices of ligand-protected metal nanoparticles, including 2-fold chain, 3-fold quasi-honeycomb, 4-fold square, 6-fold hexagonal and network structures, can be fabricated through interligand interactions and by template methods. The electronic and magnetic properties of 6-fold hexagonal superlattices of metal nanoparticles for the future nanodevice applications are also presented.

Thickness dependence on the dielectric properties of BaTiO 3/SrTiO 3-multilayers

We prepared highly textured SrTiO 3/BaTiO 3-multilayers with artificial lattice structure by pulsed laser deposition in off-axis geometry on SrTiO 3:Nb-single crystals. In this paper we compare the structural and dielectric properties of symmetric superlattice structures of SrTiO 3/BaTiO 3 layers with (Ba 0.5 Sr 0.5)TiO 3 solid solutions of the same thickness. The dielectric constant (DC) of the multilayers is lower in comparison to the solid solutions, the thickness dependence is weaker and the influence of an external bias field on the multilayer films is weaker as well. The temperature dependence on the DC of BaTiO 3/SrTiO 3-superlattices shows a broader transition peak and the Curie temperature is shifted slightly to lower values.

Scattering of electrons at impurity ions at low temperatures in a superlattice with doped quantum wells

The longitudinal and transverse mobilities of electrons in a superlattice with doped quantum wells, limited by scattering of the electrons at impurity ions, are calculated. The case of low temperatures and low densities of charge carriers is considered. The Conwell-Weisskopf hypothesis of the minimum scattering angle is used to solve the Boltzmann equation for a nondegenerate electron gas in the region of weak screening of the Coulomb potential of impurity ions. The numerical calculations are performed for the symmetric superlattice GaAs/Al0.36Ga0.64As with a period of 10 nm and an electron concentration of 1014 cm−3 at T=20 K.

Quasi-phase-matched second-harmonic generation in a GaAs/AlAs superlattice waveguide by ion-implantation-induced intermixing.

We report type I second-harmonic generation by use of first-order quasi-phase matching in a GaAs/AlAs symmetric superlattice structure with femtosecond fundamental pulses at 1.55 microm. Periodic spatial modulation of the bulklike second-order susceptibility chi(zxy)(2) was achieved with quantum-well intermixing for which the group III vacancies were created by As+-ion implantation. A narrow second-harmonic bandwidth of approximately 0.9 nm (FWHM) with an average power of approximately 1.5 microW was detected, corresponding to an internal conversion efficiency of approximately 0.06%, which was considerably limited by the spectral bandwidth of the fundamental.