Superlattice System Research Articles

Switchable polar spirals in tricolor oxide superlattices

There are increasing evidences that ferroelectric states at the nanoscale can exhibit fascinating topological structures including polar vortices and skyrmions, akin to those observed in the ferromagnetic systems. Here we report the discovery of a new type of polar topological structure, an ordered array of nanoscale spirals, in the PbTiO3/BiFeO3/SrTiO3 tricolor ferroelectric superlattice system obtained via phase-field simulations. This polar spiral structure is composed of fine ordered semi-vortex arrays with vortex cores forming a wavy distribution. It is demonstrated that the tricolor system has an ultrahigh Curie temperature of ∼1000 K and a temperature of ∼800 K for the phase transformation from spiral structure to in-plane orthorhombic domain structure, demonstrating a great thermal stability. The periodicity phase-diagram is constructed, showing multiple phases from inplane domain, polar spiral, and polar vortex to flux-closure with increasing ferroelectric layer thickness. Moreover, the spiral structure has a net in-plane polarization that could be switched by an experimentally-feasible irrotational in-plane field. The switching process involves a metastable vortex state and is fully reversible. This discovery could open up a new routine to design novel multiferroic topological structures with enhanced stability and tunability towards potential future applications in next-generation electronics.

Dynamical signature of the moiré pattern in a non-Hermitian ladder

We study the dynamical behavior of a non-Hermitian moire superlattice system, which consists of two-coupled SSH chains with staggered imaginary on-site potentials. There are two main spatial regions, in which systems are in unbroken symmetric phases with fully real spectrum, appearing periodically along the ladder. We show that the two quantum phases are dimerized and tetramerized, which determine the distinct dynamical behaviors. Dirac probability can oscillate periodically, increase quadratically and increase exponentially, which correspond to the unbroken phase, exceptional point and the broken phase of the tetramerized region. In comparison, the Dirac probability can exhibit high-frequency oscillation in the dimerized region. These phenomena demonstrate the dynamical signature and provide insightful information of the moire pattern in the non-Hermitian regime.

Inter-valley spiral order in the Mott insulating state of a heterostructure of trilayer graphene-boron nitride

Recent experiment has shown that the ABC-stacked trilayer graphene-boron nitride Moire super-lattice at half-filling is a Mott insulator. Based on symmetry analysis and effective band structure calculation, we propose a valley-contrasting chiral tight-binding model with local Coulomb interaction to describe this Moire super-lattice system. By matching the positions of van Hove points in the low-energy effective bands, the valley-contrasting staggered flux per triangle is determined around π/2. When the valence band is half-filled, the Fermi surfaces are found to be perfectly nested between the two valleys. Such an effect can induce an inter-valley spiral order with a gap in the charge excitations, indicating that the Mott insulating behavior observed in the trilayer graphene-boron nitride Moire super-lattice results predominantly from the inter-valley scattering.

Phase diagrams and magnetic properties of a ferrimagnetic Ising bilayer superlattice: A Monte Carlo study

Monte Carlo simulation has been used to study the magnetic and thermodynamic properties of a ferrimagnetic mixed spin-1 and spin-3/2 Ising bilayer superlattice system on the honeycomb structure. The ground state phase diagrams of the system have been obtained for different single-ion anisotropies and exchange couplings. The system exhibits characteristic critical behaviors such as compensation points, the first- and second-order phase transition points as well as the tricritical points. The effects of the single-ion anisotropies and the exchange couplings on the magnetization, the susceptibility, the internal energy, the specific heat and the hysteresis loops are discussed in detail. Lots of interesting magnetic phenomena have been found, originating from the competitions among the anisotropies, the exchange couplings, the temperature and the external magnetic field. In particular, multiple cycle hysteresis loops behaviors such as the double and triple hysteresis loops have been discovered. Our results are in excellent agreement with previous theoretical studies.

Analysis of polariton dispersion in metal nanocomposite based novel superlattice system

Abstract The influence of metal nanoparticles in tuning the polaritonic gap in a novel piezoelectric superlattice is studied. Dielectric function of the metal nanoparticles is analyzed using Kawabata-Kubo effect and Drude’s theory. The effective dielectric function of the nanocomposite system is studied using Maxwell Garnett approximation. Nanocomposite based LiTaO 3 novel superlattice is formed by arranging the nanocomposite systems in such a way that their orientations are in the opposite direction. Hence there are two additional modes of propagation. The top most modes reflect the metal behavior of the nanoparticles. It is found that these modes of propagation vary with the filling factor. These additional modes of propagations can be exploited in the field of communication.

Magnetic and thermodynamic properties of a ferromagnetic mixed-spin (1/2, 1, 3/2) three-layer film superlattice

Using the Monte Carlo simulation, we have studied the magnetic and thermodynamic properties of a ferromagnetic three-layer film mixed-spin (1/2, 1, 3/2) system. We have discussed the influence of intralayer and interfacial exchange couplings, film thickness, magnetic atom concentration and temperature on the magnetization of the superlattice system, magnetic susceptibility, internal energy and specific heat of the system. The phase diagrams in various parameters planes are obtained. Loads of interesting magnetic behaviors have been found, such as double-peak and triple-peak phenomena in the susceptibility and specific heat curves as well as obvious finite size effects for small layer thickness. Through a comparison, there is qualitatively a good agreement between our results and those of other theoretical and experimental studies.

Flexoelectricity in ATiO3 (A = Sr, Ba, Pb) perovskite oxide superlattices from density functional theory

Flexoelectric coefficients for several bulk and superlattice perovskite systems are determined using a direct approach from first principles density functional theory calculations. A strong enhancement in the longitudinal flexoelectric coefficient has been observed in the 1SrTiO3/1PbTiO3 superlattice with alternating single atomic layers of SrTiO3 and PbTiO3. It was found that atomistic displacement, charge response under strain, and interfaces affect the flexoelectric properties of perovskite superlattice systems. These factors can be used to tune this effect in dielectrics. It was further found that the calculated Born effective charge for an ion under the influence of strain can differ significantly from the bulk value. These insights can be used to help search for more effective flexoelectric materials to be implemented in electromechanical devices.

Theoretical study of native point defects in strained-layer superlattice systems

We developed a theoretical approach that employs first-principles Hamiltonians, tight-binding Hamiltonians, and Green's function techniques to obtain energy levels arising from native point defects (NPDs) in InAs-GaSb and InAs-InAs1-xSbx strained layer superlattice (SLS) systems. In InAs and GaSb regions, we considered four types of NPDs—anion vacancy, cation vacancy, anion anti-site, and cation anti-site—as well as isoelectronic substitution at anion sites (Sb at the As site and As at the Sb site). Additionally, we considered three types of defects—the cation at the second anion site, the second anion at the cation site, and second anion vacancy—in the InAs1-xSbx alloy region of the SLS. For a selected few designs, we studied NPDs both in the bulk region and near the interfaces of the SLS. We have considered 12 designs of InAs-GaSb systems and two designs of InAs-InAs0.7Sb0.3 systems lattice-matched to the GaSb substrate. The calculated defect levels not only agreed well with available measurements, but also revealed the connection between mid-gap levels and specific NPDs. We further calculated defect formation energies both in compounds and in all superlattices considered above. Since the absolute value of defect formation energy depends considerably on growth conditions, we evaluated the formation energies in SLS with respect to their value in the corresponding bulk or alloy. The calculated defect formation energies, together with defect energy level results, allow us to identify a few promising SLS designs for high-performing photodetectors.

Modelling of the Sb and N distribution in type II GaAsSb/GaAsN superlattices for solar cell applications

GaAsSbN dilute nitrides are potential candidates for integration in high-performance multi-junction solar cells due to the bandgap tunability in the 1.0–1.15 eV range and the possibility to match the lattice constant to the GaAs substrates. Recently, the use of GaAsSb/GaAsN superlattices (SLs) has been shown as an effective way to enhance photovoltaic efficiency as compared to the quaternary counterparts. Here we apply a combination of HR-XRD and cross-sectional (S)TEM techniques together with theoretical calculations to analyse the compositional distribution in GaAsSb/GaAsN SLs with different periodicity. The measurements of compositional profiles indicate that Sb is strongly segregated into the GaAsN layers while N remains confined where it is deposited. We demonstrate that the Sb profiles run as a shark-fin waveform that can be precisely described with a one-dimensional model where segregation of the supplied Sb is promoted by a three-layer fluid exchange mechanism. Moreover, the role played by the periodicity in the effectiveness of the Sb incorporation adds a new level of complexity. The modelling of Sb segregation in the GaAsSb/GaAsN SLs system could be used to carry out more precise pseudopotential calculations on the band structure in order to understand and predict their electrical and optical behaviour.

Interfacial Antiferromagnetic Coupling and Dual-Exchange Bias in Tetragonal SrRuO3-PrMnO3 Superlattices.

The functional properties of oxide heterostructures depend on the interfaces accommodating ions, their spins, and structural mismatches. Here, by stabilizing tetragonal symmetry, we achieve the in-plane antiferromagnetic (AFM) ordering and dual-exchange bias in the superlattices consisting of two ferromagnets SrRuO3 (SRO) and PrMnO3 (PMO). The tetragonal symmetry of this superlattice system achieved after the octahedral rotations yield an elongation of the c-axis parameter with Ru-O-Mn bond angle close to 180°, induces an interfacial antiferromagnetic ordering, which is suppressed as the ferromagnetic (FM) ordering in the PMO layer increases. The 0.1 T in-plane cooling field (Hcool) leads to the shift (ca. -0.04 T) of minor hysteresis loop along the negative field axis due to the presence of -0.87 erg/cm2 AFM interfacial exchange coupling energy density (ERu,Mn) at 20 K. The exchange bias field (HEB) switches from negative to positive value with the increase in Hcool. For 5 T Hcool, the HEB is positive, but the ERu,Mn is -1.25 erg/cm2 for n ≤ 8 (n = number of unit cells of PMO) and 1.52 erg/cm2 for n ≥ 8. The HEB and its switching from negative to positive with the increase in Hcool are explained by the interplay of strong antiferromagnetic coupling energy and Zeeman energy at the interfaces. The results demonstrate that the SRO-PMO superlattice could be a model system for the investigation of the interfacial exchange coupling in functional oxides.

Atomic Mechanism of Interfacial-Controlled Quantum Efficiency and Charge Migration in InAs/GaSb Superlattice.

A series of systematic electron microscopy imaging evidence are illustrated to prove that a high-quality interface is vital for enhancing quantum efficiency from 23 to 50% effectively, because improved crystal quality of each layer can suppress the disordered atom arrangement and enhance the carrier lifetime via decreasing the overall residual strain. The distribution width of charge rises and then falls as bias increasing, revealing the existence of an optimum operating voltage, which could be attributed to the proper energy band bending. Our results provide new insights into the understanding of the association between macro-property and microstructure of the superlattice system.

Coefficient of Interband Light Absorption by InAs/GaхIn1-хAs Quantum Dot Superlattice at Low Temperatures

In the paper the InAs/GaxIn1-xAs superlattice system of small size cubic QDs (10 nm) has been considered. Dispersion relations for electron and hole subbands have been calculated for superlattices of different dimensionality. The dependences of the interband absorption coefficient on light frequency, quantum dot size and interdot distance have been researched.It is shown, that the dimension of the superlattice has influence on the shape of the absorption bands and the increasingof the distance between quantum dots is followed by narrowing of the absorption peaks for all three superlatticetypes.

Quasiperiodicity and suppression of multistability in nonlinear dynamical systems

It has been known that noise can suppress multistability by dynamically connecting coexisting attractors in the system which are otherwise in separate basins of attraction. The purpose of this mini-review is to argue that quasiperiodic driving can play a similar role in suppressing multistability. A concrete physical example is provided where quasiperiodic driving was demonstrated to eliminate multistability completely to generate robust chaos in a semiconductor superlattice system.

Experimental verification of orbital engineering at the atomic scale: Charge transfer and symmetry breaking in nickelate heterostructures

Epitaxial strain, layer confinement and inversion symmetry breaking have emerged as powerful new approaches to control the electronic and atomic-scale structural properties in complex metal oxides. Nickelate heterostructures, based on RENiO$_3$, where RE is a trivalent rare-earth cation, have been shown to be relevant model systems since the orbital occupancy, degeneracy, and, consequently, the electronic/magnetic properties can be altered as a function of epitaxial strain, layer thickness and superlattice structure. One such recent example is the tri-component LaTiO$_3$-LaNiO$_3$-LaAlO$_3$ superlattice, which exhibits charge transfer and orbital polarization as the result of its interfacial dipole electric field. A crucial step towards control of these parameters for future electronic and magnetic device applications is to develop an understanding of both the magnitude and range of the octahedral network's response towards interfacial strain and electric fields. An approach that provides atomic-scale resolution and sensitivity towards the local octahedral distortions and orbital occupancy is therefore required. Here, we employ atomic-resolution imaging coupled with electron spectroscopies and first principles theory to examine the role of interfacial charge transfer and symmetry breaking in a tricomponent nickelate superlattice system. We find that nearly complete charge transfer occurs between the LaTiO$_3$ and LaNiO$_3$ layers, resulting in a Ni$^{2+}$ valence state. We further demonstrate that this charge transfer is highly localized with a range of about 1 unit cell, within the LaNiO$_3$ layers. The results presented here provide important feedback to synthesis efforts aimed at stabilizing new electronic phases that are not accessible by conventional bulk or epitaxial film approaches.

Fate of Majorana Modes in CeCoIn_{5}/YbCoIn_{5} Superlattices: A Test Bed for the Reduction of Topological Classification.

In this paper, we propose CeCoIn_{5}/YbCoIn_{5} superlattice systems as a test bed for the reduction of topological classification in free fermions. We find that the system with a quadlayer of CeCoIn_{5} shows a topological crystalline superconducting phase with the mirror Chern number eight at the noninteracting level. Furthermore, we demonstrate that in the presence of two-body interactions, gapless edge modes are no longer protected by the symmetry in the system with a quadlayer, but are protected in the system with a bilayer or trilayer. This clearly exemplifies the reduction of topological classification from Z⊕Z to Z⊕Z_{8}.

Probing the Origin of Interfacial Carriers in SrTiO3–LaCrO3 Superlattices

Emergent phenomena at complex oxide interfaces could provide the basis for a wide variety of next-generation devices, including photovoltaics and spintronics. To date, detailed characterization and computational modeling of interfacial defects, cation intermixing, and film stoichiometry have helped to explain many of the novel behaviors observed at a single heterojunction. Unfortunately, many of the techniques employed to characterize a single heterojunction are less effective for a superlattice made up of a repeating series of interfaces that induce collective interfacial phenomena throughout a film. These repeating interfaces present an untapped opportunity to introduce an additional degree of complexity, such as confined electric fields, that cannot be realized in a single heterojunction. In this work, we explore the properties of SrTiO3–LaCrO3 superlattices to understand the role of defects, including variations in cation stoichiometry of individual layers of the superlattice, intermixing across interfaces, and interfacial oxygen vacancies. Using X-ray photoelectron spectroscopy (XPS) and scanning transmission electron microscopy electron energy-loss spectroscopy (STEM-EELS), we quantify the stoichiometry of individual layers of the superlattice and determine the degree of intermixing in these materials. By comparing these results to both density functional theory (DFT) models and STEM-EELS measurements of the Ti and Cr valence in each layer of the superlattice, we correlate different types of defects with the associated materials properties of the superlattice. We show that a combination of ab initio modeling and complementary structural characterization methods can offer unique insight into structure–property relationships in many oxide superlattice systems.

Characterization of the Nb-B superlattice system

We study the growth, stacking and superconducting properties of Nb and B thin films and superlattices. The interest in these resides in their possible use in transition edge neutron sensors. The samples were grown by magnetron sputtering over Si (1 0 0) substrates. The X-ray diffraction patterns for all Nb containing samples show a Nb (1 1 0) preferential orientation. From the low-angle X-ray reflectivity we obtain information on the superlattice structure. The superconducting transition temperatures of the superlattices, obtained from the temperature dependence of the magnetization, are higher than those of single Nb films of similar thickness. The temperature dependence of the perpendicular and parallel upper critical fields indicate that the superlattices behave as an array of decoupled superconducting Nb layers.

The dependence of the tunneling characteristic on the electronic energy bands and the carrier’s states of Graphene superlattice

Using the transfer matrix method, the carrier tunneling properties in graphene superlattice generated by the Thue–Morse sequence and Kolakoski sequence are investigated. The positions and strength of the transmission can be modulated by the barrier structures, the incident energy and angle, the height and width of the potential. These carriers tunneling characteristic can be understood from the energy band structures in the corresponding superlattice systems and the carrier’s states in well/barriers. The transmission peaks above the critical incident angle rely on the carrier’s resonance in the well regions. The structural diversity can modulate the electronic and transport properties, thus expanding its applications.

Amorphous/epitaxial superlattice for thermoelectric application

An amorphous/epitaxial superlattice system is proposed for application to thermoelectric devices, and the superlattice based on a PbGeTeS system was prepared by the alternate deposition of PbS and GeTe using a hot wall epitaxy technique. The structure was analyzed by high-resolution transmission electron microscopy (HRTEM) and X-ray analysis, and it was found that the superlattice consists of an epitaxial PbTe-based layer and a GeS-based amorphous layer by the reconstruction of the constituents. A reduction in thermal conductivity due to the amorphous/epitaxial system was confirmed by a 2ω method. Electrical and thermoelectric properties were measured for the samples.

Multistability, chaos, and random signal generation in semiconductor superlattices.

Historically, semiconductor superlattices, artificial periodic structures of different semiconductor materials, were invented with the purpose of engineering or manipulating the electronic properties of semiconductor devices. A key application lies in generating radiation sources, amplifiers, and detectors in the "unusual" spectral range of subterahertz and terahertz (0.1-10 THz), which cannot be readily realized using conventional radiation sources, the so-called THz gap. Efforts in the past three decades have demonstrated various nonlinear dynamical behaviors including chaos, suggesting the potential to exploit chaos in semiconductor superlattices as random signal sources (e.g., random number generators) in the THz frequency range. We consider a realistic model of hot electrons in semiconductor superlattice, taking into account the induced space charge field. Through a systematic exploration of the phase space we find that, when the system is subject to an external electrical driving of a single frequency, chaos is typically associated with the occurrence of multistability. That is, for a given parameter setting, while there are initial conditions that lead to chaotic trajectories, simultaneously there are other initial conditions that lead to regular motions. Transition to multistability, i.e., the emergence of multistability with chaos as a system parameter passes through a critical point, is found and argued to be abrupt. Multistability thus presents an obstacle to utilizing the superlattice system as a reliable and robust random signal source. However, we demonstrate that, when an additional driving field of incommensurate frequency is applied, multistability can be eliminated, with chaos representing the only possible asymptotic behavior of the system. In such a case, a random initial condition will lead to a trajectory landing in a chaotic attractor with probability 1, making quasiperiodically driven semiconductor superlattices potentially as a reliable device for random signal generation to fill the THz gap. The interplay among noise, multistability, and chaos is also investigated.