Superlattice Effects Research Articles

Modification of Temperature Dependence of Dielectric Properties by Symmetry-Controlled Superlattice Thin Films of BaZrxTi1-xO3

Symmetry-controlled ferroelectric superlattice thin films of BaZrxTi1-xO3 (BZT) were fabricated by a pulse laser deposition (PLD) technique. To reduce the strain effects, we used 5-layer systems of BZT with x=0, 0.25, 0.5, 0.75, and 1; we call these A, B, C, D, and E, respectively. We made two types of superlattice film, namely, ABCDE/ABCDE and ABCDE/EDCBA, where the total concentration of Zr was the same. The former exhibits symmetry breaking as in perovskite structures. In contrast, the latter has symmetry introduced by artificial structures. Superlattices show clear satellite peaks in XRD patterns and the structures are also confirmed by electron microscopy images. ABCDE/ABCDE films have larger relative permittivities than the ABCDE/EDCBA films. This property seems to be the symmetry effects in the superlattice structures. Both films have a weak temperature dependence near room temperature caused by the superlattice effects, which seems to be better for the application of the materials.

Ultrafast Fiske Effect in Semiconductor Superlattices

The current flowing across a semiconductor superlattice in tilted electric and magnetic fields is known to exhibit resonant enhancement, when Landau states of neighboring wells align at certain ratios of the field strengths. We show that the ultrafast version of this effect, in which coherent electron wave packets are involved, has a profound analogy to the Fiske effect in superconductor Josephson junctions and superfluid weak links, in that the coupling of the tunneling-induced charge oscillations (magneto-Bloch versus Josephson oscillations) to another oscillator (in-plane cyclotron oscillations versus external oscillator modes) opens an elastic rectifying transport channel. We explore the superlattice effect both theoretically and experimentally, and find that the transient self-induced current can be adequately modeled if the damping of both types of coupled electron oscillations is properly taken into account.

Electronic structure and optical properties of (ZnSe)n∕(Si2)m (111) superlattices

The electronic properties of (ZnSe)n∕(Si2)m (111) superlattices (SLs) are investigated theoretically in order to clarify the general features of the zone-folding and the band-mixing effects in superlattices composed of an indirect-band-gap semiconductor (Si). The detailed electronic structure of (ZnSe)n∕(Si2)m (111) SLs are studied with the range n=m=10–16, giving special attention to the role of the interface states at the Zn–Si and Se–Si polar interfaces. The presence of the electric field in the SL is totally ignored, i.e., “the zero-field model.” The degeneracy of the energy minima of the conduction band at the M point in the zinc-blende-type bulk material cannot be lifted by the zone-folding effects alone. The band-mixing effect through the interfaces between the two constituent materials plays an important role in determining the overall band lineup throughout the entire Brillouin zone. The states at the conduction- and valence-band edges are confined two dimensionally in the Si layers. Furthermore, we have found two interface bands in the lower and upper regions of the gap. The states of the lower interface band are located at the Zn–Si interface, while those of the upper interface band are located at the Se–Si interface. The energies of the interface states depend on the parameters representing the Zn–Si and Se–Si bond lengths and the valence-band discontinuity between ZnSe and Si, but the interface states do not disappear from the gap with reasonable choices of the parameters. The electronic structure of the superlattice turns out to be quite sensitive to the combination of the well and barrier layer thicknesses. This sensitivity suggests the possibility of designing suitable band structures for device applications. Furthermore, the absorption spectra of the superlattices are calculated and are found to be quite different from those of bulk ZnSe and Si but fairly close to their average. The electronic and optical properties suggest that superlattices composed of indirect-band-gap semiconductors offer great potential for application to optical devices.

Photoelasticity effects in superlattices with quantum wells having a sloping bottom near the interband resonances

An analytical expression is obtained for the linear coefficients of photoelasticity in superlattices with quantum wells having a sloping bottom in the vicinity of the interband resonances. It is shown that these coefficients considerably decrease when the slope of the quantum-well bottom is induced by an electric field applied to the superlattice and remain almost unchanged if the slope is produced by the variband method. A new mechanism providing the emergence of linear photoelasticity near the interband resonances is revealed for superlattices based on piezoelectric crystals. This mechanism is associated with the modulation of the band gap of the quantum-well structure by the generated piezoelectric field. Numerical estimates demonstrate that the contribution to the linear photoelasticity in the region of the interband resonances is substantially greater than the corresponding contribution in the exciton region of the spectrum and is comparable in magnitude to the contribution made by the deformation potential.

Magnetization in lateral superlattices induced by strong dc and ac electric fields

We consider nonlinear electric field effects in lateral superlattices with Rashba spin-orbit coupling. Both strong dc and ac electric fields are applied to the superlattice. The pseudomagnetic field associated with the spin-orbit interaction gives rise to a homogeneous spin polarization under nonequilibrium conditions of a biased superlattice. The application of a strong radiation field in the terahertz domain leads to photon resonances, dynamical localization, and delocalization as well as to a reorientation of the field-induced magnetization. We stress the analogy between the field-induced spinless carrier transport and the spin polarization under intense irradiation.

Structure and quantum-size effects in a surface carbide:W(110)∕C−R(15×3)

Results of the combined investigation of atomic and electronic structure of the $\mathrm{W}(110)∕\mathrm{C}\text{\ensuremath{-}}R(15\ifmmode\times\else\texttimes\fi{}3)$ surface carbide are reported. A variety of experimental techniques has been involved such as scanning tunneling microscopy (STM), low-energy electron diffraction, x-ray photoelectron spectroscopy, and angle-resolved photoemission (ARPES). Distance-dependent STM measurements show a nontrivial geometrical behavior in the topography data, demonstrating five different patterns representing the superstructure at different values of the tip-surface separation. Atomic resolution was achieved at lower tunneling gap resistance. An unexpected spatial asymmetry in the distribution of the local density of states across the surface unit cell has been observed as well. Photoelectron spectroscopy of $\mathrm{C}1s$ and $\mathrm{W}4f$ core levels clarifies the nature of the chemical bonding in the system. The band mapping with ARPES provides information on the wave-vector dependence of the electronic states. Notable quantum size and superlattice effects were discovered in the dispersion of the valence-band states. The experimental data suggests an apparent one-dimensional character of the electronic structure. Lateral quantization and umklapp scattering are proposed as explanation. Finally, based on photoemission and STM measurements, an improved crystallographic model of the tungsten surface carbide is introduced.

Optical and structural properties of InGaN/GaN multiple quantum well structure grown by metalorganic chemical vapor deposition

InGaN/GaN multiple quantum well light emitting diode structures have been grown on sapphire substrates by metalorganic chemical vapor deposition. They are investigated, in this study, by high-resolution X-ray diffraction, high-resolution transmission electron microscopy, photoluminescence, and photoluminescence excitation. HR-XRD showed multiple satellite peaks up to 10th order due to the quantum well superlattice confinement effects. These indicate the high quality of layer interface structures of this sample. Excitation power-dependent photoluminescence shows that both piezoelectric field-induced quantum-confined Stark effect and band filling effect influence the luminescent properties of this sample. Temperature-dependent photoluminescence of this sample has also been studied. The peak position of the PL exhibits a monotonic red-shift and the full width at half maximum of the PL band shows a W-shaped temperature-dependent behavior with increasing temperature. From the photoluminescence excitation results, a large energy difference, so-called Stokes shift, between the band-edge absorption and emission was observed.

Theoretical investigations of epitaxial strain effects in ferroelectric oxide thin films and superlattices

Epitaxial strain in perovskite oxides has been shown to play an important role in determining the structure and properties of thin films and superlattices, especially properties related to polarization. In this short review, we describe theoretical investigations of the epitaxial strain effects in a variety of perovskite oxides, including ferroelectrics, with particular emphasis on first-principles methods, as well as Landau–Devonshire functionals and the phase-field approach.

Electro‐optic investigation of the Coherent Hall Effect in semiconductor superlattices

AbstractAfter the first observation of the Coherent Hall Effect in a GaAs/Al0.3Ga0.7As superlattice by means of THz‐emission spectroscopy, the dynamics of impulsively excited electron wavepackets in crossed electric and magnetic fields has now been investigated in more detail by reflective electro‐optic sampling (REOS). In agreement with the predictions of a semiclassical model, the experiments show the existence of two strictly separated regimes of charge‐carrier motion depending on the ratio E/B of applied electric and magnetic field. Both, the magneto‐Bloch regime for dominating electric field and the cyclotron‐like regime of motion for dominating magnetic field, exhibit a characteristic frequency dependence of the wavepacket oscillations on the external fields. The REOS measurements supplement our results achieved by THz‐emission spectroscopy by covering a much larger range of experimental parameters because REOS is not affected by absorption by the cryostat windows of our set‐up as is the THz radiation at frequencies above 2 THz, and the detection of high oscillation frequencies is possible. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Directing transport by polarized radiation in the presence of chaos and dissipation

We study numerically the dynamics of particles on the Galton board of semi-disk scatters in presence of monochromatic radiation and dissipation. It is shown that under certain conditions the radiation leads to appearance of directed transport linked to an underlining strange attractor. The direction of transport can be efficiently changed by radiation polarization. The experimental realization of this effect in asymmetric antidot superlattices is discussed.

Photoemission from surface-localized structures on vicinal surfaces: Initial- or final-state superlattice effect?

Strong superlattice effects are reported on E ( k ‖ ) band dispersions measured by angle-resolved photoemission on W(3 3 1) and W(5 5 1), and graphene nanostripes on Ni(7 7 1). A splitting of the dispersions with reciprocal superlattice vector G = 2 π / L is observed for a surface resonance on W(3 3 1) and W(5 5 1) and for π -states of graphene/Ni(7 7 1). Photon-energy-dependent measurements and comparison to LEED show that electrons are confined to the one-dimensional terraces and stripes, respectively, and that the observed superlattice effects are due to diffraction in the final state of the photoemission transitions.

Quantum-well states and lateral superlattice effect in Ag and Au stripes on stepped W(1 4 5)

Au and Ag on the stepped W(1 4 5) surface with step width L ∼ 11.6 Å form quantum stripes up to a thickness of 2 and 3 monolayers (ML), respectively. The 2 ML stripes show a one-dimensional E ( k ‖ ) dispersion which perpendicular to the stripes is determined by a repetition with the reciprocal superlattice vector G = 2 π / L .

Photoemission from Stepped W(110): Initial or Final State Effect?

The electronic structure of the (110)-oriented terraces of stepped W(331) and W(551) is compared to the one of flat W(110) using angle-resolved photoemission. We identify a surface-localized state which develops perpendicular to the steps into a repeated band structure with the periodicity of the step superlattices. It is shown that a final-state diffraction process rather than an initial-state superlattice effect is the origin of the observed behavior and why it does not affect the entire band structure.

Couplings and interface effects in binary superlattices

By employing the high-temperature series expansion technique, we have analyzed the phase transition and the critical phenomena of the two-component superlattice, through three models: Ising, XY and Heisenberg. The reduced critical temperature of the system is studied as function of the thickness of the constituents in the unit cell and the exchange interactions in each material and within the interface. A critical value of the interface exchange interaction above which the interface magnetism appears is obtained. A number of characteristic behaviours such as the possibility of existence of a critical length of the unit cell thickness at which the critical temperature of the system remains insensitive to the exchange coupling within interface are reported. The values of the critical exponent γ, associated with the magnetic susceptibility are comparable to the universal ones, in the limit cases when the system is still comparable to an infinite simple cubic lattice. The asymmetry of the structure and the competition of the effects of the exchange couplings, are important for the magnetic properties of the system.

Theory of minigap plasmons in a two-dimensional electron gas subjected to a one-dimensional periodic potential

We investigate the plasmon excitations in a two-dimensional (2D) electron gas subjected to a one-dimensional (1D) weak periodic potential. We derive and discuss the dispersion relations for both intrasubband and intersubband excitations within the framework of Bohm-Pines' random-phase approximation. For such an anisotropic system with spatially modulated charge density, we observe a splitting of the 2D plasmon dispersion. The splitting is caused by the superlattice effect of the charge-density modulation on the collective excitation spectrum. Several illustrative examples are presented on the computed plasmon excitation energy as a function of the propagation vector as well as the Bloch vector. We also discuss how the tunneling and the potential amplitude influence the minigaps and the plasmon excitations.

Minigap plasmons in a two-dimensional electron gas subjected to a one-dimensional periodic potential

We investigate the plasmon excitations in a two-dimensional electron gas subjected to a one-dimensional weak periodic potential. We derive and discuss the dispersion relations for both intrasubband and intersubband excitations within the framework of Bohm–Pines' random-phase approximation. For such an anisotropic system with spatially modulated charge density, we observe a splitting of the 2D plasmon dispersion. The splitting is caused by the superlattice effect of the charge-density modulation on the collective excitation spectrum. We also discuss how the tunneling and the potential amplitude affect the plasmon excitations.

Dynamic quasi-energy-band modulation and exciton effects in biased superlattices driven by a two-color far-infrared field: Disappearance of dynamic localization

A theoretical study of the optical and electronic properties of semiconductor superlattices in ac-dc fields, termed the dynamic Wannier-Stark ladder (DWSL), is done. The biased superlattices are driven by two far-infrared fields with different frequencies and relative phase of $\ensuremath{\delta}.$ Here, the frequency of the first laser is equal to the Bloch frequency ${\ensuremath{\omega}}_{B}$ of the system under study, while that of the second laser is equal to $2{\ensuremath{\omega}}_{B}.$ Quasienergies of the DWSL are calculated based on the Floquet theorem, and the associated linear photoabsorption spectra are evaluated. For $\ensuremath{\delta}=0,$ a gourd-shaped quasi-energy structure characteristic of both dynamic localization (DL) and delocalization (DDL), similar to the usual DWSL driven by a single laser, appears. By changing the ratio of the two laser strengths, however, the width of the quasi-energy band and the locations of both DL and DDL vary noticeably. As for $\ensuremath{\delta}\ensuremath{\ne}0,$ on the other hand, band collapse and the associated DL do not necessarily follow. In fact, DL vanishes and the quasi-energy degeneracy is lifted in a certain range of $\ensuremath{\delta}.$ Just DDL remains over the entire range of the laser strength, eventually resulting in a plateaulike band structure in the linear absorption spectra. The basic physics underlying this phenomenon, which can be readily interpreted in terms of a closed analytical expression, is that all quasi-energies for given crystal momenta are out of phase with each other as a function of laser strength without converging to a single point of energy. This is a feature of this DWSL which sharply distinguishes it from a conventional DWSL generated using a single laser to drive it. Furthermore, an exciton effect is incorporated with the above noninteracting problem, so that exciton dressed states are formed. It is found that this effect gives rise to more involved quasi-energy structures and a more pronounced release of the energy degeneracy of DL, leading again to the formation of a band structure in the absorption spectra. These are exclusively due to unevenly spaced exciton energy levels.

ACTIVE PHOTONIC CRYSTAL NANO-ARCHITECTURES

The development of nano-scaled photonic crystal structures has resulted in many new devices exhibiting non-classical optical behavior. Typically, in these structures a photonic band gap and associated defect mode are used to create waveguides, resonators, couplers and filters. In this paper we propose that the functionality of these structures can be significantly enhanced by the infiltration of the photonic crystal with other classes of materials, particularly highly nonlinear liquid crystals and electro-optical materials. The properties of conventional 2D PC slab waveguides were simulated by the finite difference time domain method and shown to exhibit very large refraction and dispersion, and significant tunable effects under bias when infiltrated with liquid crystal. In particular, a new superlattice photonic crystal concept is proposed and shown to exhibit up to ~50° tunability in the angle of refraction when alternate liquid crystal infiltrated pixel rows were modulated from their aligned to unaligned state. This modulation corresponds to index changes from 1.5 to 2.1; it is assumed that a refractive index change of up to approximately Δn=0.6 can be achieved. The superlattice effect was also demonstrated to induce new switching and out-coupling effects that were strongly dependent on the direction of propagation and index modulation. These simulations demonstrate the potential of a new class of optically-active photonic crystal architectures to tune giant refraction and dispersion characteristics and to enable new switching phenomena.

Coupled third harmonic generations and multiple mode effects in aperiodic optical superlattices with a finite lateral width

We study the coupled third harmonic generations (CTHGs) from quasi-one-dimensional aperiodic optical superlattices (AOSs) with a finite lateral width. We design the special AOS that can implement multiple wavelength CTHG with identical effective nonlinear coefficient at the preassigned wavelengths of incident light with the use of simulated annealing method. We reveal the effects of mode-mode coupling and the lateral width of the AOSs on the CTHG for the two cases: One case corresponds to the incident light beam of a plane wave profile and other case to the incident light beam of Gaussian profile. The results show that the characteristics of the CTHG from the AOSs are closely related to the profiles of the incident fundamental frequency light beam. For the incidental plane wave beam, the dependence of the effective nonlinear coefficient on the width of the AOS is quite weak. On the contrary, for the incident Gaussian profile beam, this dependence exhibits a rapid decrease at the beginning and then tends to a constant. We also display the variation of the effective nonlinear coefficients with the optical wave propagating distance from the impinging surface of incident light beam in the sample illuminating by the plane wave light beam and find that this variation exhibits basically increasing behavior. This clearly infers that the contribution of every individual block to optical CTHG process is a constructive addition. When increasing the number of the modes, the effective nonlinear coefficient of CTHG is rapidly decreased at the beginning and it then tends to a saturation value for the incident plane wave light beam. However, for the incident light beam of Gaussian profile, similar behavior is observed only when the lateral width of the AOS is smaller than that of the Gaussian profile beam; in contrast, the Nc dependence of the effective nonlinear coefficient of the CTHG is quite weak.

Highly efficient direct third-harmonic generation based on control of the electro-optic effect in quasi-periodic optical superlattices

We present a method for controlling the efficiency of direct third-harmonic generation in quasi-periodic optical superlattices by means of the electro-optic effect. The calculated results manifest that this method is extremely efficient for achieving high efficiency of direct third-harmonic generation in predesigned quasi-phase-matched coupled parametric processes and is feasible at any given fundamental intensity. In addition, we demonstrate that the electro-optic control approach is significantly better than the temperature-control method.