Artificial Lattices Research Articles

Realization of ground-state artificial skyrmion lattices at room temperature.

The topological nature of magnetic skyrmions leads to extraordinary properties that provide new insights into fundamental problems of magnetism and exciting potentials for novel magnetic technologies. Prerequisite are systems exhibiting skyrmion lattices at ambient conditions, which have been elusive so far. Here, we demonstrate the realization of artificial Bloch skyrmion lattices over extended areas in their ground state at room temperature by patterning asymmetric magnetic nanodots with controlled circularity on an underlayer with perpendicular magnetic anisotropy (PMA). Polarity is controlled by a tailored magnetic field sequence and demonstrated in magnetometry measurements. The vortex structure is imprinted from the dots into the interfacial region of the underlayer via suppression of the PMA by a critical ion-irradiation step. The imprinted skyrmion lattices are identified directly with polarized neutron reflectometry and confirmed by magnetoresistance measurements. Our results demonstrate an exciting platform to explore room-temperature ground-state skyrmion lattices.

Lattice-free models of directed cell motility

Directed cell migration often occurs when individual cells move in response to an external chemical stimulus. Cells can respond by moving in either the direction of increasing (chemoattraction) or decreasing (chemorepulsion) concentration. Many previous models of directed cell migration use a lattice-based framework where agents undergo a lattice-based random walk and the direction of nearest-neighbour motility events is biased in a preferred direction. Such lattice-based models can lead to unrealistic configurations of agents, since the agents always move on an artificial lattice structure which is never observed experimentally. We present a lattice-free model of directed cell migration that incorporates two key features. First, agents move on a continuous domain, with the possibility that there is some preferred direction of motion. Second, to be consistent with experimental observations, we enforce a crowding mechanism so that motility events that would lead to agent overlap are not permitted. We compare simulation data from the new lattice-free model with a more traditional lattice-based model. To provide additional insight into the lattice-free model, we construct an approximate conservation statement which corresponds to a nonlinear advection–diffusion equation in the continuum limit. The solution of this mean-field model compares well with averaged data from the individual-based model.

Triangular Spin-Orbit-Coupled Lattice with Strong Coulomb Correlations: Sn Atoms on a SiC(0001) Substrate.

Two-dimensional (2D) atom lattices provide model setups with Coulomb correlations that induce competing ground states. Here, SiC emerges as a wide-gap substrate with reduced screening. We report the first artificial high-Z atom lattice on SiC(0001) by Sn adatoms, based on experimental realization and theoretical modeling. Density-functional theory of our triangular structure model closely reproduces the scanning tunneling microscopy. Photoemission data show a deeply gapped state (∼2 eV gap), and, based on our calculations including dynamic mean-field theory, we argue that this reflects a pronounced Mott-insulating scenario. We also find indications that the system is susceptible to antiferromagnetic superstructures. Such artificial lattices on SiC(0001) thus offer a novel platform for coexisting Coulomb correlations and spin-orbit coupling, with bearing for unusual magnetic phases and proposed topological quantum states of matter.

Optically reconfigurable magnetic materials

The periodic modulation of the magnetic properties of magnonic crystals controls the flow of spin waves. An optical method is now shown that can produce such modulations by heating, which can be reprogrammed during operation. Structuring of materials is the most general approach for controlling waves in solids. As spin waves—eigen-excitations of the electrons’ spin system—are free from Joule heating, they are of interest for a range of applications, such as processing1,2,3,4,5, filtering6,7,8 and short-time data storage9. Whereas all these applications rely on predefined constant structures, a dynamic variation of the structures would provide additional, novel applications. Here, we present an approach for producing fully tunable, two-dimensionally structured magnetic materials. Using a laser, we create thermal landscapes in a magnetic medium that result in modulations of the saturation magnetization and in the control of spin-wave characteristics. This method is demonstrated by the realization of fully reconfigurable one- and two-dimensional magnonic crystals—artificial periodic magnetic lattices.

Jackiw-Rebbi-type bound state carrying fractional fermion parity

We find the coexistence of two kinds of fractionalized modes, Majorana modes at the geometric ends and Jackiw–Rebbi-type bound state (JRBS) at a domain-wall, in a new topological superconducting phase in one-dimensional (1D) systems. Each localized JRBS carries a new fractional quantity, half of the parity of the fermion number. This induces a topological protected crossing at the zero energy for its eigenenergy. For a chain embedded with a JRBS, it is possible to switch between the occupied and the empty states of the Majorana zero energy state (MZES) by varying the strength of the external magnetic field across that crossing point. This enables a way to encode a quantum qubit into one MZES without breaking parity conservation. We propose that such JRBS and Majorana fermions may appear in two 1D models. One may be implemented in an artificial lattice with staggered hopping, staggered spin-orbital interaction and staggered superconducting pairing in cold fermion atoms. The other is a 1D semiconductor chain sandwiched between s-wave superconductor and antiferromagnet.

The rare two-dimensional materials with Dirac cones

Abstract Inspired by the great development of graphene, more and more research has been conducted to seek new two-dimensional (2D) materials with Dirac cones. Although 2D Dirac materials possess many novel properties and physics, they are rare compared with the numerous 2D materials. To provide explanation for the rarity of 2D Dirac materials as well as clues in searching for new Dirac systems, here we review the recent theoretical aspects of various 2D Dirac materials, including graphene, silicene, germanene, graphynes, several boron and carbon sheets, transition-metal oxides (VO2)n/(TiO2)m and (CrO2)n/(TiO2)m, organic and organometallic crystals, so-MoS2, and artificial lattices (electron gases and ultracold atoms). Their structural and electronic properties are summarized. We also investigate how Dirac points emerge, move, and merge in these systems. The von Neumann–Wigner theorem is used to explain the scarcity of Dirac cones in 2D systems, which leads to rigorous requirements on the symmetry, parameters, Fermi level, and band overlap of materials to achieve Dirac cones. Connections between existence of Dirac cones and the structural features are also discussed.

Protecting trapped animals from heat exposure: the influence of shading on temperature within small metal box traps

Animal research guidelines recognise that trapped animals can potentially be subjected to heat stress, but recommendations are limited to avoiding exposing traps to weather extremes. Intuitively, shading metal box traps should be beneficial in warm to hot climates. Temperatures inside metal box traps were measured within traps located in direct sunlight concurrently with ones covered with artificial shading. Additionally, comparisons were conducted in continuous shade cast by ground-cover vegetation, and in stippled sunlight under an artificial lattice. Deep continuous shade from vegetation was most effective and able to keep traps within 5°C of ambient. Artificial shading using dark shade cloth could actually elevate trap temperatures above that in uncovered traps when exposed to direct sunshine. Reflective foil insulation cover not only provided rain protection for traps but was the most effective shading method trialled for reducing both the rate at which traps heated after sunrise and the maximum temperature reached during the day when exposed to direct sunshine. The use of foil sleeves can provide an extra level of protection from excessive heat exposure to trapped animals, keeping trap temperatures as much as 15°C lower than unprotected traps.

Current-induced magnetization switching at low current densities in current-perpendicular-to-plane structural Fe<sub>3</sub>Si/FeSi<sub>2</sub> artificial lattices with various cross-sectional areas

Current-induced magnetization switching at low current densities in current-perpendicular-to-plane structural Fe<sub>3</sub>Si/FeSi<sub>2</sub> artificial lattices with various cross-sectional areas

Observation of valley-dependent beams in photonic graphene.

Valley-dependent propagation of light in an artificial photonic hexagonal lattice, akin to electrons in graphene, is investigated in microwave regime. Both numerical and experimental results show that the valley degeneracy in the photonic graphene is broken when the frequency is away from the Dirac point. The peculiar anisotropic wave transport property due to distinct valleys is analyzed using the equifrequency contours. More interestingly, the valley-dependent self-collimation and beam splitting phenomena are experimentally demonstrated with the armchair and zigzag interfaces, respectively. Our results confirm that there are two inequivalent Dirac points that lead to two distinct valleys in photonic graphene, which could be used to control the flow of light and might be used to carry information in valley polarized beam splitter, collimator or guiding device.

Matter waves in two-dimensional arbitrary atomic crystals

We present a general scheme to realize a cold-atom quantum simulator of bidimensional atomic crystals. Our model is based on the use of two independently trapped atomic species: the first one, subject to a strong in-plane confinement, constitutes a two-dimensional matter wave which interacts only with atoms of the second species, deeply trapped around the nodes of a two-dimensional optical lattice. By introducing a general analytic approach we show that the system Green function can be exactly determined, allowing for the investigation of the matter-wave transport properties. We propose some illustrative applications to both Bravais (square, triangular) and non-Bravais (graphene, kagom\'e) lattices, studying both ideal periodic systems and experimental-size and disordered ones. Some remarkable spectral properties of these atomic artificial lattices are pointed out, such as the emergence of single and multiple gaps, flat bands, and Dirac cones. All these features can be manipulated via the interspecies interaction, which proves to be widely tunable due to the interplay between scattering length and confinements.

Matter waves in atomic artificial graphene

We present a new model to realize artificial 2D lattices with cold atoms investigating the atomic artificial graphene: a 2D confined matter wave is scattered by atoms of a second species trapped around the nodes of a honeycomb optical lattice. The system allows an exact determination of the Green function, hence of the transport properties. The inter-species interaction can be tuned via the interplay between scattering length and confinements. Band structure and density of states of a periodic lattice are derived for different values of the interaction strength. Emergence and features of Dirac cones are pointed out, together with the appearance of multiple gaps and a non-dispersive and isolated flat band. Robustness against finite-size and vacancies effects is numerically investigated.

Interfacial structure of multi-layered thin-films produced by pulsed laser deposition for use in small-scale ceramic capacitors

The aim of this study was to develop thin film capacitors with superior properties that could provide an alternative to materials currently used in conventional multi-layer ceramic capacitors fabricated by sintering. To this end, an artificial dielectric super lattice technique, incorporating pulsed laser deposition, was applied to improving the dielectric properties of thin film capacitors. This method permits the A-site atoms of a perovskite ABO3 structure to be selected layer by layer at a nanoscopic scale; consequently, multi-layer BaTiO3- SrTiO3 thin films were produced on Pt(111)/Ti/SiO2/Si(100) and SrTiO3(111) substrates. Hetero-epitaxial grain growth was observed between BaTiO3 and SrTiO3, with the lattice mismatch between them introducing a compressive residual strain at the interface. The dielectric properties of these multi-layer thin-film capacitors were found to be superior to those of conventional solid-solution thin films once the thickness of the layers and the ratio of the two oxides were optimized.

Generation of tunable plasma photonic crystals in meshed dielectric barrier discharge

Tunable superlattice plasma photonic crystals are obtained in a meshed dielectric barrier discharge. These plasma photonic crystals are composed of thin artificial lattices and thick self-organized lattices, and can be tuned easily by adjusting the applied voltage. A plasma photonic crystal with self-organized hexagonal lattice coupled to artificial square lattice is first realized. The dispersion relations of the square sublattices with different radii, which are recorded by an intensified charge-coupled device camera, are calculated. The results show that the thick square sublattice has the higher band edge frequencies and wider band widths. Band gaps of superlattice plasma photonic crystals are actually temporal integrations of those of transient sublattices.

Microscopic structure and electrical transport property of sputter-deposited amorphous indium-gallium-zinc oxide semiconductor films

We report on microscopic structures and electrical and optical properties of sputter-deposited amorphous indium-gallium-zinc oxide (a-IGZO) films. From electron microscopy observations and an x-ray small angle scattering analysis, it has been confirmed that the sputtered a-IGZO films consist of a columnar structure. However, krypton gas adsorption measurement revealed that boundaries of the columnar grains are not open-pores. The conductivity of the sputter-deposited a-IGZO films shows a change as large as seven orders of magnitude depending on post-annealing atmosphere; it is increased by N2-annealing and decreased by O2-annealing reversibly, at a temperature as low as 300°C. This large variation in conductivity is attributed to thermionic emission of carrier electrons through potential barriers at the grain boundaries, because temperature dependences of the carrier density and the Hall mobility exhibit thermal activation behaviours. The optical band-gap energy of the a-IGZO films changes between before and after annealing, but is independent of the annealing atmosphere, in contrast to the noticeable dependence of conductivity described above. For exploring other possibilities of a-IGZO, we formed multilayer films with an artificial periodic lattice structure consisting of amorphous InO, GaO, and ZnO layers, as an imitation of the layer-structured InGaZnO4 homologous phase. The hall mobility of the multilayer films was almost constant for thicknesses of the constituent layer between 1 and 6 Å, suggesting rather small contribution of lateral two-dimensional conduction It increased with increasing the thickness in the range from 6 to 15 Å, perhaps owing to an enhancement of two-dimensional conduction in InO layers.

Strong vortex matching effects in YBCO films with periodic modulations of the superconducting order parameter fabricated by masked ion irradiation

We report on measurements of the magnetoresistance and of the critical current in thin films of the high-temperature superconductor YBa2Cu3O7-δ (YBCO). A square array of regions with suppressed superconducting order parameter has been created in these films by introducing point defects via irradiation with He+ ions through a silicon stencil mask. In such a structure distinct peaks of the critical current can be observed at commensurate arrangements of magnetic flux quanta with the artificial defect lattice. Concurrently, the magnetoresistance shows pronounced minima. Both observations demonstrate that the strong intrinsic pinning in YBCO can be overcome by a periodic array of ion-damage columns with 300nm spacing.

Accidental degeneracy of double Dirac cones in a phononic crystal

Artificial honeycomb lattices with Dirac cone dispersion provide a macroscopic platform to study the massless Dirac quasiparticles and their novel geometric phases. In this paper, a quadruple-degenerate state is achieved at the center of the Brillouin zone in a two-dimensional honeycomb lattice phononic crystal, which is a result of accidental degeneracy of two double-degenerate states. In the vicinity of the quadruple-degenerate state, the dispersion relation is linear. Such quadruple degeneracy is analyzed by rigorous representation theory of groups. Using method, a reduced Hamiltonian is obtained to describe the linear Dirac dispersion relations of this quadruple-degenerate state, which is well consistent with the simulation results. Near such accidental degeneracy, we observe some unique properties in wave propagating, such as defect-insensitive propagating character and the Talbot effect.

Dirac Cones, Topological Edge States, and Nontrivial Flat Bands in Two-Dimensional Semiconductors with a Honeycomb Nanogeometry

New two-dimensional materials artificially engineered to have unusual electronic properties will broaden the material basis for our quest for ever-smaller, more versatile electronic devices. Theoretical investigation of an artificial honeycomb lattice of zinc-blende semiconductor nanocrystals reveals a rich electronic structure that is part graphene-like and part topological-insulator-like, indicating a new direction of electronic-materials engineering.

Current-induced magnetization switching at low current densities in current-perpendicular-to-plane structural Fe3Si/FeSi2 artificial lattices

Current-perpendicular-to-plane (CPP) junctions of Fe3Si/FeSi2 were fabricated from Fe3Si/FeSi2 artificial lattice films, which were prepared by facing-target direct-current sputtering, by employing a focused ion beam (FIB) technique. CPP structurization was confirmed by scanning electron microscopy. The CPP junctions, in which antiferromagnetic interlayer coupling is induced between the Fe3Si layers, exhibited a clear hysteresis loop in the electrical resistance for current injection, which is probably due to current-induced magnetization switching. The critical current density for it is approximately 3.3 × 101 A/cm2, which is at least four orders smaller than the values that have ever been reported.

Temperature‐dependent interlayer coupling in Fe3Si/FeSi2 artificial lattices

Fe3Si/FeSi2 artificial lattices, wherein ferromagnetic (F)/antiferromagnetic (AF) interlayer coupling between the Fe3Si layers were induced by controlling the thickness of FeSi2 layers, were prepared on Si(111) substrates by facing targets direct‐current sputtering. The interlayer couplings were investigated at different temperatures by measuring the magnetization curves. The AF coupling at room temperature was gradually weakened with a decrease in the temperature, and it finally became ferromagnetic or noncoupled at temperatures lower than 77 K. We consider that the FeSi2 layers act as semiconductors and their change in the electric state for the temperature induces the interlayer coupling switching.

Magnetoresistance effects in current‐perpendicular‐to‐plane structures based on Fe3Si/FeSi2 artificial lattices

AbstractFrom Fe3Si/FeSi2 artificial lattices, wherein Fe3Si layers are partially epitaxially grown from the first layer on Si(111) up to the top layer across the FeSi2 layers with the same orientation relationship as that in the first layer, current‐perpendicular‐to‐plane (CPP) structures that partially contain current‐in‐to‐plane (CIP) structural portions were fabricated by a mask method. It was confirmed that the antiparallel alignment of magnetization at a zero magnetic field due to antiferromagnetic interlayer coupling is changed to parallel alignment with the applied magnetic field by polarized neutron reflectometry. The MR ratio was comparable to that of CIP structural films. It might be because a current should predominantly flow in the CIP portion since the CIP structural portion is dimensionally so large as compared with that of the CPP portion. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)