States In Superlattices Research Articles

Spatiotemporal pattern formation in parametrically driven two-dimensional Bose–Einstein condensates

We investigate spatiotemporal periodic patterns in harmonically trapped Bose–Einstein condensates (BECs) driven by a periodic modulation of the interaction. Resonant with the breathing mode, we show the emergence of a square lattice pattern containing two orthonormal stripes. We study the time evolutions of the lattice patterns for different driving strengths and dissipations. We find that its spatial periodicity and temporal oscillating frequency match the Bogoliubov dispersion, which is the intrinsic property of the system and relevant to the parametric amplification of elementary excitations. In the circumstances of strong driving strength and low dissipation, we further observe the triad interaction and the resulting superlattice state, which are well explained by the nonlinear amplitude equation for superimposed stripes. These results shed light on unexplored nonlinear spatiotemporal dynamics of two-dimensional patterns in harmonically trapped BECs that can pave the way for engineering exotic patterns by state-of-the-art experiments.

The Nested Topological Band-Gap Structure for the Periodic Domain Walls in a Photonic Super-Lattice

We study the nested topological band-gap structure of one-dimensional (1D) photonic super-lattices. One cell of the super-lattice is composed of two kinds of photonic crystals (PhCs) with different topologies so that there is a domain wall (DW) state at the interface between the two PhCs. We find that the coupling of periodic DWs could form a new band-gap structure inside the original gap. The new band-gap structure could be topologically nontrivial, and a topological phase transition can occur if the structural or material parameters of the PhCs are tuned. Theoretically, we prove that the Hamiltonian of such coupled DWs can be reduced to the simple Su–Schrieffer–Heeger (SSH) model. Then, if two super-lattices carrying different topological phases are attached, a new topological interface state can occur at the interface between the two super-lattices. Finally, we find the nested topological band-gap structure in two-dimensional (2D) photonic super-lattices. Consequently, such nested topological structures can widely exist in complex super-lattices. Our work improves the topological study of photonic super-lattices and provides a new way to realize topological interface states and topological phase transitions in 1D and 2D photonic super-lattices. Topological interface states in super-lattices are sensitive to frequency and have high accuracy, which is desired for high-performance filters and high-finesse cavities.

Exciton states in molecular superlattices

The structure of molecular superlattices consists of van der Waals heterostructures of two-dimensional atomic crystals (ultrathin films) connected to each other with regular and periodic patterns. Inspired by the increasing importance of excitons for optical spectroscopy and the engineering of photonic materials in recent years, we have calculated the energy spectra of excitons in superlattices in this theoretical study. The number of film layers and the ratio between the exciton energy transfer within each motif (film) and the exciton energy transfer between neighboring motifs (between films) are important parameters that determine the behavior in an external electromagnetic field.

Motility-induced phase separation and frustration in active matter swarmalators.

We introduce a two dimensional system of active matter swarmalators composed of elastically interacting run-and-tumble active disks with an internal parameter ϕ_{i}. The disks experience an additional attractive or repulsive force with neighboring disks depending upon their relative difference in ϕ_{i}, making them similar to swarmalators used in robotic systems. In the absence of the internal parameter, the system forms a motility-induced phase separated (MIPS) state, but when the swarmalator interactions are present, a wide variety of other active phases appear depending upon whether the interaction is attractive or repulsive and whether the particles act to synchronize or ant-synchronize their internal parameter values. These phases include a gas-free gel regime, arrested clusters, a labyrinthine state, a regular MIPS state, a frustrated MIPS state for attractive antisynchronization, and a superlattice MIPS state for attractive synchronization.

Adsorption superlattice stabilized by elastic interactions in a soft porous crystal

Elasticity is essential for organizing mesoscale patterns in condensed matter. Here, we numerically show that adsorbed molecules in soft porous crystals form a superlattice (SL) stabilized by elastic interactions. In a mechanically flexible honeycomb lattice model, a long-range ordered 1/3-filling SL state emerges during adsorption. The SL state is robust against thermal fluctuation when the elastic interactions between the next-nearest-neighboring lattice sites are strong. Our results provide a way to switch the functionalities of materials by controlling the distribution of adsorbed molecules. Published by the American Physical Society 2024

Structural and Temperature Dependence of Emergent Electronic States in PbSe Quantum Dot Superlattices.

Structural and Temperature Dependence of Emergent Electronic States in PbSe Quantum Dot Superlattices.

Topological electronic structure of twin boundaries and twinning superlattices in the SnTe material class

The topological electronic structure of a single twin boundary and coherent twinning superlattices (TSLs) based on the SnTe class of materials is calculated and discussed within a supercell implementation. The superlattices consist of two twin planes (TPs) in the supercell arranged in such a way that each of the boundaries forms a mirror plane for the entire structure. Two types of TP boundary, cationic and anionic, can exist, and so three types of supercells can be constructed. We study the topological phases of each twinning configuration using the tight-binding approximation and calculating the topological invariants. We show that they differ by topological properties and find that all-cationic TSLs are topologically distinct from the anionic case due to the opposite sign of the Berry curvature around the $\mathrm{\ensuremath{\Gamma}}$ point of the TSL's Brillouin zone. Our findings are consistent with a complementary analysis of (111)-oriented slabs with a single twin boundary in the presence of a Zeeman field. They are also consistent with the calculated number of spin-polarized Dirac-like edge states of both superlattices and slabs. We conclude that each type of TP forms a two-dimensional mirror-plane-protected topological crystalline insulator.

Experiment on terahertz metasurfaces of metal split ring resonators with bound state in the continuum

A bound state in the continuum (BIC) is a wave that coexists with a continuous wave but remains localized. In the terahertz band, BIC can design devices with an ultra-high mass factor (Q factor), which is of great value for terahertz science and technology, so we designed a terahertz BIC metasurface structure composed of metal split ring resonators (SRRs). According to the symmetric protection principle of superlattice mode, the leakage process of BIC states to the far field is studied by changing the gap width of SRR. By introducing multiple SRRs and changing their arrangement, we obtain three superlattice modes and BIC states. The leakage of BIC states into the far field is observed experimentally, which means that observable quasi-BIC patterns are formed. We verify a feasible method that allows for flexible design and implementation of BIC.

Domain wall state diagram for SrTiO3/BaTiO3 superlattice structures

The domain wall structure of ferroelectric/ paraelectric superlattices can be much more complex due to the influence of the superlattice stacking structure, the in-plane strain induced by the substrate and environmental temperature. In this study, we employed a phase field model to investigate the domain wall state of the SrTi[Formula: see text]/BaTi[Formula: see text] superlattice structure. The domain wall thickness for the SrTi[Formula: see text]/BaTi[Formula: see text] layer was measured using a hyperbolic function. Based on the simulation results, here, we show a domain wall state diagram to distinguish the hard and soft domain states. The polarization profiles across hard/ soft domain walls were illustrated and analyzed. Our simulation results offer a useful concept for the control of the domain wall state in the ferroelectric superlattice.

Photovoltaics literature survey (No. 181)

Photovoltaics literature survey (No. 181)

Emergence of distinct electronic states in epitaxially-fused PbSe quantum dot superlattices

Quantum coupling in arrayed nanostructures can produce novel mesoscale properties such as electronic minibands to improve the performance of optoelectronic devices, including ultra-efficient solar cells and infrared photodetectors. Colloidal PbSe quantum dots (QDs) that self-assemble into epitaxially-fused superlattices (epi-SLs) are predicted to exhibit such collective phenomena. Here, we show the emergence of distinct local electronic states induced by crystalline necks that connect individual PbSe QDs and modulate the bandgap energy across the epi-SL. Multi-probe scanning tunneling spectroscopy shows bandgap modulation from 0.7 eV in the QDs to 1.1 eV at their necks. Complementary monochromated electron energy-loss spectroscopy demonstrates bandgap modulation in spectral mapping, confirming the presence of these distinct energy states from necking. The results show the modification of the electronic structure of a precision-made nanoscale superlattice, which may be leveraged in new optoelectronic applications.

Wider transmission forbidden gaps and tamm state in a complex optoelectronic superlattice based on monolayer MoS2

We have designed a complex optoelectronic superlattice based on monolayer MoS2 consisting of two singly periodic superlattices and investigated its electronic transport properties. It is found that the complex superlattice possesses wider transmission forbidden gaps compared with the two superlattices as its components, and we give a theoretic proof to support this phenomenon. In addition, a Tamm state can be found within the original forbidden gaps, and its tunneling transmission decays greatly with the width of potential regions. These excellent properties imply the complex optoelectronic superlattice proposed here can be applied to electron energy filters and efficient optoelectronic devices.

Methods for controlling the properties of nanoregions formed due to self-organization during electronic phase separation in Eu0.8Ce0.2Mn2O5 multiferroics

Methods for controlling (by magnetic field, changing temperature, and optical pumping) the properties of nanoregions forming due to self-organization during phase separation in Eu0.8Ce0.2Mn2O5 multiferroics are studied. Such nanoregions are superlattices (semiconductor heterostructures). The dynamically equilibrium states of superlattices (their long-lived ground states) are formed during cycling under magnetic field. Such states of superlattices are electrically neutral and consist of ferromagnetic layers. A set of ferromagnetic resonances from superlattices layers make it possible to distinguish the states of superlattices. The optical pumping of various powers allows controlling both the magnetic and electrical properties of superlattices.

Topological states in superlattices of HgTe class of materials for engineering three-dimensional flat bands

In search of materials with three-dimensional flat band dispersions, using ab initio computations we investigate how topological phases evolve as a function of hydrostatic pressure and uniaxial strain in two types of superlattices: HgTe/CdTe and HgTe/HgSe. In short-period HgTe/CdTe superlattices, our analysis unveils the presence of isoenergetic nodal lines, which could host strain-induced three-dimensional flat bands at the Fermi level without requiring doping, when fabricated, for instance, as core-shell nanowires. In contrast, HgTe/HgSe short-period superlattices are found to harbor a rich phase diagram with a plethora of topological phases. Notably, the unstrained superlattice realizes an ideal Weyl semimetal with Weyl points situated at the Fermi level. A small-gap topological insulator with multiple band inversions can be obtained by tuning the volume: under compressive uniaxial strain, the material transitions sequentially into a Dirac semimetal to a nodal-line semimetal, and finally into a topological insulator with a single band inversion.

Biperiodic superlattices and transparent states in graphene

The transmission and transport properties of biperiodic graphene superlattices are studied theoretically. Special attention is paid to the so-called transparent states of biperiodic superlattices. A Dirac-like Hamiltonian is used to describe the charge carriers in graphene. The transfer matrix method and the Landauer–Büttiker formalism are implemented to obtain the transmittance and conductance, respectively. Similar results to those reported for Schrödinger electrons are obtained. However, in the case of Dirac electrons the splitted bands and the transparent states associated to the biperiodicity depend strongly on the angle of incidence as well as the character of the charge carriers. In fact, the dynamic of the splitted bands and transparent states is inverted for holes. The origin of transparent states is unveiled by obtaining an analytic expression for the transmittance. It is found that resonant transmission through single and double barriers gives rise to transparent states. Regarding the transport properties, it is possible to identify the fundamental changes caused by the biperiodicity. In particular, it is found a splitting, shifting, and diminishment of the conductance peaks with respect to the case of regular periodicity. This opens the door to corroborate experimentally the fundamental characteristics of biperiodic gated graphene superlattices through transport measurements.

Symmetry of surface states in graphene superlattices terminated with magnetic cap layers

Symmetry of surface states in graphene superlattices terminated with magnetic cap layers

Spatial order in a two-dimensional spin–orbit-coupled spin-1/2 condensate: superlattice, multi-ring and stripe formation

We demonstrate the formation of stable spatially-ordered states in a uniform and also trapped quasi-two-dimensional (quasi-2D) Rashba or Dresselhaus spin–orbit (SO) coupled pseudo spin-1/2 Bose–Einstein condensate using the mean-field Gross–Pitaevskii equation. For weak SO coupling, one can have a circularly-symmetric (0, +1)- or (0, −1)-type multi-ring state with intrinsic vorticity, for Rashba or Dresselhaus SO coupling, respectively, where the numbers in the parentheses denote the net angular momentum projection in the two components, in addition to a circularly-asymmetric degenerate state with zero net angular momentum projection. For intermediate SO couplings, in addition to the above two types, one can also have states with stripe pattern in component densities with no periodic modulation in total density. The stripe state continues to exist for large SO coupling. In addition, a new spatially-periodic state appears in the uniform system: a superlattice state, possessing some properties of a supersolid, with a square-lattice pattern in component densities and also in total density. In a trapped system the superlattice state is slightly different with multi-ring pattern in component density and a square-lattice pattern in total density. For an equal mixture of Rashba and Dresselhaus SO couplings, in both uniform and trapped systems, only stripe states are found for all strengths of SO couplings. In a uniform system all these states are quasi-2D solitonic states.

Structural behaviour of BiFeO3/SrRuO3 superlattices: An X-ray diffraction and Raman spectroscopy investigation

Epitaxial BiFeO 3 /SrRuO 3 superlattices have been grown by pulsed laser deposition on a (001) oriented LaAlO 3 substrate and probed by X-ray diffraction and Raman spectroscopy . To investigate the structural competition between rhombohedral BiFeO 3 and orthorhombic SrRuO 3 the total thickness of all SLs was kept constant and the bilayer thickness (period) Λ was varied. The interlayer strain effects are therefore tuned from large strain effects (short Λ period) to quasi-relaxed structure (large Λ). A complementary investigation using X-ray diffraction and phonon dynamics hints to change from a rhombohedral to a tetragonal structure in the superlattices with the increase of the interlayer strain effect. • In depth structural resolution of BiFeO3/SrRuO3 superlattices. • Group theoretical analysis superlattices raman spectra. • Strain induced tetragonal state in superlattices.

Mott insulating state and d+id superconductivity in an ABC graphene trilayer

Motivated by the recently experimental reported signatures of the tunable Mott insulating state and superconductivity in an ABC graphene trilayer superlattice, we investigate the charge compressibility, the spin correlation, and the superconducting instability within the Hubbard model on a three-layer honeycomb lattice. It is found that an antiferromagnetically ordered Mott insulator emerges beyond a critical $U_c$ at half-filling, and the electronic correlation drives a $d+id$ superconducting pairing to be dominant over other pairing patterns in a wide doped region. The effective pairing interaction with $d+id$ pairing symmetry is strongly enhanced with the increasing of on-site interaction, and suppressed as the interlayer coupling strength increases. Our intensive numerical results demonstrate that the insulating state and superconductivity in an ABC graphene trilayer are driven by strong electric correlation, and it may offer an attractive systems to explore rich correlated behaviors.

Optical Control of Superlattices States Formed Due to Electronic Phase Separation in Multiferroic Eu0.8Ce0.2Mn2O5.

The effect of optical pumping and magnetic field on properties of the electronic phase separation regions, which are the multiferroic semiconductor heterostructures in the form of superlattices, have been studied in Eu0.8Ce0.2Mn2O5. These superlattices are formed due to self-organization in a dielectric crystal matrix as a result of the competing internal interactions balance and occupy a small crystal volume. The dynamical equilibrium states of superlattices are initially formed during cycling of as-grown samples in a magnetic field. The superlattices in such states are ferromagnetic and electrically neutral. Sets of ferromagnetic resonances were observed from individual layers of superlattices. Their features give rise information on properties of these layers and of a superlattice as a whole. The differences in the parameters of these resonances were due to different distributions of Mn3+ and Mn4+ ions in individual superlattices layers. It has been found that optical pumping having different powers allows us to control of multiferroic properties of superlattices layers by changing their magnetic and electric properties. It is shown that, under certain conditions, it is possible to significantly increase the temperatures at which multiferroic heterostructures exist.