Superlattice Effects Research Articles

Controlling edge state transport in a HgTe topological insulator by superlattice effect

We investigate theoretically the edge state transport in a HgTe topological insulator under periodic electrical modulation. We find constructive interference of the backscattering amplitudes, leading to the formation of superlattice minigaps and hence complete suppression of the edge state transmission. Consequently, the edge channel can be switched on/off by appropriately tuning the modulation amplitude via gate voltages, even for wide Hall bar with a small finite size effect. We also find efficient conversion between spin-up and spin-down edge channels by the gate-induced Rashba spin-orbit interaction.

Synthesis of nano-structured HPPMS CrN/AlN coatings

Nanolaminates are promising coating architectures for tools and components subjected to high loads. Due to the possibility to reach high hardness and low indentation modulus by reason of a superlattice effect or the formation of a solid solution crystallite these types of coatings are widely discussed in the literature. Furthermore, a nanolaminate structure allows simple process design for industrial applications. In this paper high-power pulse magnetron sputtering (HPPMS) was used to deposit CrN/AlN nanolaminates with different bilayer periods by varying the rotational speed of the substrate holders. For bilayer periods of 4.5 nm and a resulting theoretical AlN single layer thickness of about 1.7 nm a hardness of 27.4 GPa can be reached.

An inverse method for extracting the mechanical properties of the constituent materials of a multilayer from nanoindentation data

Multilayer thin films have demonstrated enhanced behaviour relative to monolayers of equivalent thickness (e.g. the superlattice effect). However, multilayer constituents can differ considerably in their microstructures (e.g. from interrupted growth and phase suppression), hence mechanical properties, compared to their monolayer or bulk counterparts. Although methods for extracting effective properties of multilayers are well established, identifying the mechanical properties of the individual constituents of a multilayer required for corresponding finite element models remains very difficult. This paper presents an inverse method for identifying the properties of material constituents in their multilayer states from a combination of finite element simulations and indentation data, which is extensible to multilayers containing any number of constituents. The method is demonstrated using simulations and analysis corresponding to nanoindentation experiments on CrN, TiN and NbN monolayers, and TiN/CrN, NbN/CrN and TiN/NbN multilayers. Constituent properties extracted using the method are presented, and FE models using the extracted properties are employed to assess the applicability of a simple relationship between hardness and yield stress for the constituent materials, and a rule of mixtures for the multilayers.

Large thermoelectric effect in graphene superlattices

We theoretically predict giant thermoelectric effects in graphene superlattices (SLs) containing a defect barrier by using a transfer matrix approach. A great thermoelectric effect occurs near the defect mode, the magnitude of the enhancement being among the largest reported ever. It is found that the magnitude of the enhancement could be controlled by adjusting the structure parameters of graphene SLs.

Observation of strong magnetoelectric effects in Ba0.7Sr0.3TiO3/La0.7Sr0.3MnO3 thin film heterostructures

A robust magnetoelectric (ME) response is essential in multiferroic thin films for potential nonvolatile ME memory elements. We report strong ME effects in bilayers and superlattices of Ba0.7Sr0.3TiO3/La0.7Sr0.3MnO3 composite multiferroics. The low-frequency ME coefficient ranges from 35 to 300 mV/cm Oe, depending on thickness, superlattice periodicity, and bias magnetic field and ac magnetic field orientations. Data on ME coupling are fitted well with a modified free-body model to obtain information on magneto-mechanical coupling in the composites. The strong ME coupling in spite of anticipated substrate clamping is a step forward towards the realization of multistate memories and read-write elements.

Magnetic Proximity Effect inYBa2Cu3O7/La2/3Ca1/3MnO3andYBa2Cu3O7/LaMnO3+δSuperlattices

Using neutron reflectometry and resonant x-ray techniques we studied the magnetic proximity effect (MPE) in superlattices composed of superconducting YBa$_2$Cu$_3$O$_7$ (YBCO) and ferromagnetic-metallic (FM-M) La$_{0.67}$Ca$_{0.33}$MnO$_{3}$ (LCMO) or ferromagnetic-insulating (FM-I) LaMnO$_{3+\delta}$ (LMO). We find that the MPE strongly depends on the electronic state of the manganite layers, being pronounced for the FM-M LCMO and almost absent for FM-I LMO. We also detail the change of the magnetic depth profile due to the MPE and provide evidence for its intrinsic nature.

Mesoscopic Self-Collimation and Slow Light in All-Positive Index Layered Photonic Crystals

We demonstrate a mesoscopic self-collimation effect in photonic crystal superlattices consisting of a periodic set of all-positive index 2D photonic crystal and homogeneous layers. We develop an electromagnetic theory showing that diffraction-free beams are observed when the curvature of the optical dispersion relation is properly compensated for. This approach allows us to combine slow-light regime together with self-collimation in photonic crystal superlattices presenting an extremely low filling ratio in air.

Modulus and Hardness Change of Silicon and Sapphire Substrates by TiC/VC Multilayer Coatings

The nanohardness H of multilayer specimens TiC/VC@Si and TiC/VC@Sapphire prepared by Pulsed-Laser-Deposition is investigated to check the existence of a superlattice effect as known from TiN/VN multilayers. In the present work the multilayer period thickness λ varies between 1.34 nm and 24.8 nm (total layer thickness t ≈ 200 nm). Unlike Young’s modulus E, H is enhanced, regardless of t, by covering Si as well as sapphire with a TiC/VC multilayer; the relative load carrying capacity being larger for Si. The maximum value of H obtained is 38 GPa for TiC/VC@Sapphire. It is observed for a multilayer thickness of λ ≈ 10 nm. Hardness of TiC/VC@Sapphire obeys, after annealing, a Hall-Petch relation H = 35.25 + 6.945 λ–0.5 (H in GPa und λ≥ 10 nm). From orientation dependent X-ray absorption fine structure and X-ray reflection records, short-range order and layer geometry are derived. These analyses reveal a continuous approach of interatomic distances Ti-C and V-C for deceasing multilayer periods. High-resolution transmission electron microscopy shows that multilayers are nanostructured, i.e., not only TiC/VC phase boundaries but also subgrains represent obstacles against plastic deformation. Dislocations play a major role as sources of internal stress and vehicles of plasticity.

Infrared reflectance and transmission spectra in II-VI alloys and superlattices

Room temperature measurements of the far-infrared (FIR) reflectance spectra are reported for the polar optical phonons in a series of bulk Cd${}_{x}$Zn${}_{1\ensuremath{-}x}$Te (0 \ensuremath{\le} $x$ \ensuremath{\le} 1) and CdSe${}_{x}$Te${}_{1\ensuremath{-}x}$ (0 $x$ \ensuremath{\le} 0.35) crystals grown by Bridgman technique. The composition-dependent spectra exhibited many fundamental aspects of lattice vibrations, while the results of long-wavelength optical modes for the end member binary compounds displayed phonon values in good agreement with the existing inelastic neutron scattering and/or Raman spectroscopy data. Using a standard methodology of multilayer optics with effective-medium dielectric tensors, we simulated the FIR transmission and reflectivity spectra at oblique incidence (Berreman's effect) in many II-VI free-standing thin films, epilayers and superlattices. Berreman's approach provided us with a strong basis for analyzing the infrared experimental data and offered an effective means of estimating the zone-center transverse and longitudinal optical (${\ensuremath{\omega}}_{\mathrm{TO}}$, ${\ensuremath{\omega}}_{\mathrm{LO}}$) phonon frequencies in polar-semiconductor thin films and heterostructure materials of increasing technological importance.

Synthesis and characterization of Ba0.7Sr0.3TiO3, BaZr0.3Ti0.7O3 thin film heterostructures

The paper presents synthesis of Ba0.7Sr0.3TiO3 (BST), Ba0.7Sr0.3TiO3 (BZT) thin films and BZT/BST heterostructures using modified Pechini method. The La0.7Sr0.3MnO3 has been used as a conducting bottom layer to form metal ferroelectric metal capacitor. The thin films are spin coated on SiO2/n-Si (100) substrates. The thin films thus deposited are characterized for crystal structure, morphology, dielectric and complex impedance properties. The results show that BZT/BST heterostructures show reduced loss tangent tan δ and useful value of figure of merit γ in RF range of frequencies. The results on dielectric properties could be analyzed in terms of the Maxwell–Wagner Model and contribution due to superlattice effects.

ELECTROPHONON RESONANCE IN DOPED SEMICONDUCTOR SUPERLATTICES

The electrophonon resonance (EPR) effect in doped semiconductor superlattices (DSSL) are investigated by using the quantum kinetic equation (QKE) for electrons to obtain the general analytic expression for optical absorption coefficient. We also obtain an EPR condition as a function of the photon energy and plasma energy. In particular, anomalous behaviors of the EPR effect such as the splitting of EPR peaks for incident photon energy or plasma energy are discussed. This raises a possibility of detecting electric subbands in DSSLs experimentally by utilizing EPR effects.

Observation of hypersonic phononic crystal effects in porous silicon superlattices

Brillouin light scattering experiments were carried out on porous silicon superlattices with modulation wavelengths in the range 37–167 nm. Phonon frequencies deduced from the Brillouin spectra show good agreement with those obtained from an elastic continuum model for a system with one-dimensional periodicity. Evidence for the existence of a hypersonic phononic bandgap and zone-folded longitudinal acoustic phonons is reported.

Pockels effect in short period silicon germanium superlattices

We introduce a simple method of calculating Pockels coefficients in ordered SiGe superlattices and show that the Pockels effect in them can be half as strong as in GaAs, thereby opening a path to efficient CMOS-compatible electro-optic modulators.

Landau Levels and Quantum Hall Effect in Graphene Superlattices

We show that, when graphene is subjected to an appropriate one-dimensional external periodic potential, additional branches of massless fermions are generated with nearly the same electron-hole crossing energy as that at the original Dirac point of graphene. Because of these new zero-energy branches, the Landau levels at charge neutral filling become 4(2N + 1)-fold degenerate (with N = 0, 1, 2, ..., tunable by the potential strength and periodicity) with the corresponding Hall conductivity sigma_{xy} showing a step of size 4(2N + 1)e;{2}/h. These theoretical findings are robust against variations in the details of the external potential and provide measurable signatures of the unusual electronic structure of graphene superlattices.

Concepts for the design of advanced nanoscale PVD multilayer protective thin films

Technological challenges in future surface engineering applications demand continuously new material solutions offering superior properties and performance. Concepts for the design of such advanced multifunctional materials can be systematically evolved and verified by means of physical vapour deposition. The classical multilayer coating concept today is well established and widely used for the design of protective thin films for wear and tribological applications. It has proven great potential for the development of novel thin film materials with tailored properties. In the past decade, the emerging new class of nanoscale coatings has offered to the material scientists an even more powerful toolbox for the engineering thin film design through a combination of the multilayer concept with new nano-coatings. Some examples are the use and integration of low friction carbon-based nanocomposites in advanced multilayer structures or the stabilization of a specific coating in another structure in a nanolaminated multilayer composite. This paper reviews the latest developments in hard, wear-resistant thin films based on the multilayer coating concept. It describes the integration of nanocrystalline, amorphous and nanocrystalline/amorphous composite materials in multilayers and covers various phenomena such as the superlattice effect, stabilization of materials in another, foreign structure, and effects related to coherent and epitaxial growth. Innovative concepts for future, smart multilayer designs based on an extremely fine structural ordering at the nanoscale are presented as well.

Macromolecular crystal twinning, lattice disorders and multiple crystals1

Macromolecule crystal structure analyses can be severely hampered by cases of twinning or of lattice disorders or of multiple crystals. However, it is increasingly the case that twinning can readily be recognized and accounted for, or remediations found. A review of this topic is given, covering both the less-than-perfect and perfect twinning situations. Remediation of twinning cases is possible via alteration of crystal growth conditions including use, mainly, of chemical additives. The case of multiple crystal growths likewise can hamper crystal structure analysis and, although not necessarily associated with twinning, is a crystal growth situation where similar remediation methods are adopted. There is a nice body of case studies now in the crystallographic literature about macromolecule crystal twinning and multiple crystals situations that make it timely to write this review. The literature on other macromolecule crystal disorders is much smaller but include incommensurate superlattice effects, which are also described. This review concludes with a personal view of the future possible directions. There are new avenues to deal with multiple crystal cases using physical crystallography approaches, such as sample sprays for freezing macromolecule microcrystallites and femtosecond lasers for exactly cutting out crystal fragments. There is a considerable potential for molecular replacement to provide an efficient way for using twinned X-ray data, at least where there is a high amino acid sequence identity, and as ‘protein fold space’ coverage becomes much more complete. 1The terms ‘Macromolecular’ and ‘Macromolecular Crystallography’ sometimes abbreviated ‘MX’, refer to crystals of protein-nucleic acid (DNA or RNA) complexes and/or DNA crystals as well as ‘simply proteins on their own’.

Ultrafast Fiske effect in semiconductor superlattices induced by the coupling of electron Bloch oscillations to longitudinal optical phonons and coherent plasmons

We show that resonant coupling of coherent electron Bloch oscillations to longitudinal optical phonons in a wide-miniband semiconductor superlattice induces a unidirectional transient electron current in the system. This effect has a profound analogy to the DC Fiske effect observed when a superconductor Josephson junction or a superfluid weak link is coupled, respectively, to an electromagnetic or acoustic resonator. In the ultrafast Fiske effect, the coupling opens an elastic rectifying channel which exists only during the coherence time of the electron wave packets involved. In the considered ultrafast coupling effect, the longitudinal optical phonons in the superlattice play the role of the resonator coupled to the free damped electron Bloch oscillations, via both their amplitude and phase. We also show that similar unidirectional transient current, but with the opposite sign, is induced by non-resonant coupling of electron Bloch oscillations to coherent electron plasma oscillations. This unidirectional transient current can probably explain the origin, the sign and the carrier-density dependence of the self-induced coherent unidirectional current, which was observed in an undoped biased semiconductor superlattice with photo-excited carriers.

Metal heterowaveguide superlattices for a plasmonic analog to electronic Bloch oscillations

A metal heterowaveguide superlattice constructed by alternately stacking two metal gap waveguides with gradually changed geometric thicknesses for observing time-resolved plasmon Bloch oscillations (PBOs) is presented. Analytical result from the transfer matrix method reveals the appearance of tilted plasmonic minibands and minigaps, and plasmonic Wannier-Stark ladder in frequency domain, indicating the existence of time-resolved PBO effects in such superlattices. Finite-difference time-domain simulations on the dynamic evolution of plasmon propagation in the superlattices demonstrate the analytical prediction.

ELECTRO-OPTIC EFFECT IN RELAXOR FERROELECTRIC FILMS AND SUPERLATTICES

ABSTRACT Electro-optic (EO) effect in single crystalline thin films and superlattices made from relaxor ferroelectric lead titanate doped lead magnesium niobate (PMN-PT) and rare-earth doped PMN-PT materials have been investigated. Strong in-plane EO anisotropy has been found in both single crystalline thin films and superlattices. A large out-of-plane EO coefficient was observed in PMN-PT superlattices. Such a significant enhancement can be attributed to the spontaneous polarization coupling between the two crystalline phases in superlattices when the stacking period is below 90 nm. When doping with erbium, the EO effect in PMN-PT single crystalline film films decreases as the erbium concentration increases. This research has been motivated by the development of various promising integrated photonic devices such as waveguide EO modulators, waveguide lasers, and photonic bandgap devices.

Study of nanostructured materials by optical and photoelectron spectroscopy

The results of investigations of the electronic energy structure and physicochemical properties of nanostructured systems by optical and photoelectron spectroscopy using synchrotron radiation are briefly reviewed. The quantum-size effect in thin metal films and surface lateral superlattices are analyzed. The effect of structural defects on the chemical state of metal clusters on the surface of oxides in catalytically active systems is discussed. The results of the study of the specific features of chemical bonding in solid-state nanostructures within the quasi-molecular approach are presented. The possibility of using biological structures (proteins) as matrices for the formation of ensembles of nanoclusters is considered.