Superlattice Constituents Research Articles

Perspective: Emergent topologies in oxide superlattices

The ability to synthesize high-quality, complex-oxide heterostructures has created a veritable playground in which to explore emergent phenomena and exotic phases which arise from the interplay of spin, charge, orbital, and lattice degrees of freedom. Of particular interest is the creation of artificial heterostructures and superlattices built from two or more materials. Through such approaches, it is possible to observe new phases and phenomena that are not present in the parent materials alone. This is especially true in ferroelectric materials where the appropriate choice of superlattice constituents can lead to structures with complex phase diagrams and rich physics. In this article, we review and explore future directions in such ferroic superlattices wherein recent studies have revealed complex emergent polarization topologies, novel states of matter, and intriguing properties that arise from our ability to manipulate materials with epitaxial strain, interfacial coupling and interactions, size effects, and more. We focus our attention on recent work in (PbTiO3)n/(SrTiO3)n superlattices wherein exotic polar-vortex structures have been observed. We review the history of these observations and highlights of recent studies and conclude with an overview and prospectus of how the field may evolve in the coming years.

Tamm plasmon modes on semi-infinite metallodielectric superlattices

We analyze the fundamental properties of optical waves referred to as Tamm plasmon modes (TPMs) which are tied to the interface of a semi-infinite two-phase metallodielectric superlattice with an arbitrary homogeneous capping medium. Such modes offer new ways of achieving high electromagnetic field localization and spontaneous emission enhancement in the vicinity of the interface in conjunction with absorption loss management, which is crucial for future applications. The homointerface, formed when the capping medium has the same permittivity as one of the superlattice constituents, is found to support a TPM whose dispersion overlaps the single-interface surface plasmon polariton (SPP) dispersion but which has a cut off at the topological transition point. In contrast, a heterointerface formed for an arbitrary capping medium, is found to support multiple TPMs whose origin can be traced by considering the interaction between a single-interface SPP and the homointerface TPM burried under the top layer of the superlattice. By carrying out a systematic comparison between TPMs and single-interface SPPs, we find that the deviations are most pronounced in the vicinity of the transition frequency for superlattices in which dielectric layers are thicker than metallic ones.

Two-colour high-speed asynchronous optical sampling based on offset-stabilized Yb:KYW and Ti:sapphire oscillators.

We present a high-speed asynchronous optical sampling system, based on two different Kerr-lens mode-locked lasers with a GHz repetition rate: An Yb:KYW oscillator and a Ti:sapphire oscillator are synchronized in a master-slave configuration at a repetition rate offset of a few kHz. This system enables two-colour pump-probe measurements with resulting noise floors below 10⁻⁶ at a data aquisition time of 5 seconds. The measured temporal resolution within the 1 ns time window is below 350 fs, including a timing jitter of less than 50 fs. The system is applied to investigate zone-folded coherent acoustic phonons in two different semiconductor superlattices in transmission geometry at a probe wavelength far below the bandgap of the superlattice constituents. The lifetime of the phonon modes with a zero wave vector and frequencies in the range from 100 GHz to 500 GHz are measured at room temperature and compared with previous work.

Improving Multiferroic Functionalities via Interface Engineering

By first-principles calculation, we show that the multiferroic properties of (SrMnO3)1/(SrCoO3)1 superlattices are significantly improved compared with epitaxially strained SrMnO3 and SrCoO3 thin films. A ferroelectric ferromagnetic insulator is achieved at a relatively small tensile strain of +1.80% instead of +3.40% for pure SrMnO3. The mean-field-estimated critical temperature TC is enhanced to above room temperature instead of 92 K for SrMnO3. This is made possible because a rich variety of oxygen octahedral tilting patterns across the SrMnO3/SrCoO3 interfaces facilitate the ferromagnetic superexchange coupling between Co and Mn ions. As Mn and Co ions participate in both magnetic moment formation and polar distortion, strong magnetoelectric coupling is therefore anticipated. Our study suggests that a suitable choice of superlattice constituents not only allows one to optimize the multiferroic functionalities, but also offers a platform to investigate the specific mechanism involved in the multiferroi...

Electrical and magnetic properties of (SrMnO3)n∕(LaMnO3)2n superlattices

We report the magnetic and transport properties of [(SrMnO3)n∕(LaMnO3)2n]m superlattices grown by molecular-beam epitaxy on (100) SrTiO3 with periodicities n=1, 2, 3, 4, 5, 6, 8, and 16. Although the superlattice constituents, LaMnO3 and SrMnO3, are both antiferromagnetic insulators, for small n (n⩽2) the superlattices behave like the ferromagnetic conductor La0.67Sr0.33MnO3. As n increases, the magnetic properties become dominated by the LaMnO3 layers, but the electronic transport properties continue to be controlled by the interfaces.

Interlayer Coupling in EuS-Based Superlattices (Dependence on the Energy Structure of Non-Magnetic Layer)

The interlayer coupling between ferromagnetic EuS layers separated by spacer layers of diamagnetic insulators, YbSe and SrS, is studied within a 3D tight-binding model. The dependencies of the coupling strength on the energy structure of the spacer, on strains resulting from the lattice mismatch between the superlattice constituents, as well as on an applied hydrostatic pressure and lattice deformations, are presented. The sign and the range of the obtained coupling agree with the behavior of magnetic correlations observed recently in neutron reflectivity spectra of EuS/YbSe superlattices.

Effect ofA-site cation ordering on the magnetoelectric properties in[(LaMnO3)m/(SrMnO3)m]nartificial superlattices

In order to evaluate the effect of charge transfer and/or charge ordering in ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{MnO}}_{3}$ on the magnetic and transport properties, a library of $[({\mathrm{LaMnO}}_{3}{)}_{m}/({\mathrm{SrMnO}}_{3}{)}_{m}{]}_{n}$ superlattices have been fabricated by the combinatorial pulsed laser deposition method. The properties of superlattices were found to depend strongly on the periodicity m; superlattices of $m<~4$ behaved like a solid solution ${\mathrm{La}}_{0.5}{\mathrm{Sr}}_{0.5}{\mathrm{MnO}}_{3}$ film, while the superlattices of $m>4$ were significantly different from the solid solution. These superlattices exhibited a higher resistivity and lower Curie temperature than $m<~4$ superlattices. The results indicate that the properties of the superlattices are dominated not only by the magnetic interaction between the superlattice constituents but also by carrier ${(e}_{g}$ electrons) transfer thorough the superlattice interface. The contribution of carrier transfer is supported by the magnetoelectric properties of another library of $[({\mathrm{La}}_{0.8}{\mathrm{Sr}}_{0.2}{\mathrm{MnO}}_{3}{)}_{8}/({\mathrm{La}}_{0.2}{\mathrm{Sr}}_{0.8}{\mathrm{MnO}}_{3}{)}_{m}{]}_{12}$ superlattices. The A-site cation order can change both effective carrier concentration and magnetic interaction.

Surface electronic structure of a step-well-basis superlattice

Surface electronic structure of a semi-infinite polytype superlattice (SL) with period consisting of three different layers (the so-called step-well basis) is investigated using a transfer-matrix approach within an effective-mass approximation. Explicit analytical formulas for the bulk dispersion relation as well as the energy expression and existence condition for surface states are given for arbitrary terminating medium and their simplification for particular SL terminations by a substrate identical to one of the SL constituents is discussed. Dependence of surface-state properties (i.e., its energy position and the degree of localization) on SL parameters (i.e., layer thicknesses and potential barrier heights) and different surface configurations (depending on a sequence of SL layers approaching the surface as well as on the choice of substrate) is studied for a triple-constituent ${\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}$-based SL. Additionally, surface-state wave functions are plotted in order to examine the associated space-charge distributions.

Growth of GaInAsP quantum wire heterostructures using the strain-induced lateral-layer ordering process

Abstract We present a generalized strain-balance theory on the strain-induced lateral-layer ordering (SILO) process, where various combinations of short-period superlattice (SPS) constituents are used to fabricate novel quantum wire (QWR) heterostructures. Based on the theory, we have fabricated two sets of QWRs in the GaInP and GaInAsP material systems with emission wavelengths of 6200 A and 1 μm, respectively. Growth of ( GaP ) 2 ( InP ) 1 SPS was used to create QWRs on GaAs0.66P0.34-based substrates. Also, QWRs were formed using ( GaP ) 2.2 ( InAs ) 1 SPS on GaAs substrates. In both these cases, the monolayer pairings of the SPS constituents were unequal. Transmission electron microscopy and polarized photoluminescence spectroscopy confirm the formation of QWRs. These results support the validity of the generalized theory of the SILO process.

Electron paramagnetic resonance of Mn 2+ in strained-layer semiconductor superlattices

We report giant crystal field splittings of Mn 2+ electron paramagnetic resonance (EPR) lines observed in ZnTe MnTe and ,nu> CdTe MnTe superlattices. The EPR spectra of isolated Mn 2+ ions diffused into the ZnTe or the CdTe layers provide a direct and precise measure of both the magnitude and the uniformity of strain produced by lattice mismatch between the superlattice constituents.

Investigation of Strain in II-VI Semiconductor Superlattices Using Electron Paramagnetic Resonance of Mn++

We explore the possibility of using electron paramagnetic resonance (EPR) of Mn++ for measuring uniaxial strain in II-VI superlattices. This work is motivated by the fact that the EPR spectrum of Mn++ is very strongly affected by crystalline fields. Changes in a crystalline field which arise from strain are thus automatically expected to have a profound effect on the EPR spectrum. Consistent with this expectation, we have observed giant crystal field splittings of Mn++ EPR lines in ZnTe/MnTe, CdTe/MnTe, and ZnTe/MnSe superlattices. The EPR spectra observed in these systems are ascribed to isolated Mn++ ions diffused into the ZnTe or the CdTe layers from the respective MnTe or MnSe layers. In addition to providing precise information oii the magnitude and the sign of strain produced by lattice mismatch between the superlattice constituents, we show that the EPR spectrum also provides a direct measure of strain fluctuations in the layered medium. PACS numbers: 78.66.-w, 71.55.Gs

Raman scattering by optical phonons in isotopic 70(Ge)n74(Ge)n superlattices.

We present Raman spectra of a series of isotopic $^{70}$(Ge${)}_{\mathit{n}}^{74}$(Ge${)}_{\mathit{n}}$ superlattices with 2\ensuremath{\le}n\ensuremath{\le}32. In these novel structures only the phonons are subject to a periodic isotopic mass modulation, whereas the electrons retain their bulklike character. This differs substantially from conventional superlattices where both electronic and vibronic properties are affected by the reduction in dimensionality. Our results show clear evidence for optic phonon confinement in each of the two superlattice constituents. Observed frequencies, relative intensities, and mode mixing compare favorably with calculations based on a planar bond-charge model and the bond polarizability approach.

Band offsets and electron localization in semiconductor interfaces and superlattices

The long standing problem of determining band offsets at semiconductor interfaces is readdressed. We show that the valence‐band discontinuity can be extracted from a knowledge of the local density of states across the interface. This method can also be used to determine the band offsets in superlattice structures. The concept of band offsets in short period superlattices is examined. It is shown that this is intimately connected with the localization of bands in one or other of the superlattice constituents.

Strain mapping in [111] and [001] InGaAs/GaAs superlattices

Raman area maps measuring the strain in lattice-mismatched [111] and [001] oriented InxGa1−xAs/GaAs superlattices (x=0.1, 0.17) are presented and compared with independent x-ray rocking curve studies of the average strain in the same samples. We find that the LO phonon frequency, but not the TO frequency, is a valid measure of strain for [111] oriented superlattices exhibiting one-mode behavior. This is explained by the lack of compensation between the effects of alloying and strain for the TO mode in InxGa1−xAs. The capability to nondestructively map small growth variations in superlattice and buffer layer constituents is demonstrated.