Confinement and deconfinement in the potential of antidot arrays of a massless Dirac electron in magnetic fields

We have investigated the effect of inter-Landau level mixing on confinement/deconfinement in antidot potentials of states with energies less than the potential height of the antidot array. We find that, depending on the ratio between the size of the antidot R and the magnetic length [Formula: see text], probability densities display confinement or deconfinement in antidot potentials (B is the magnetic field). When R/???1 form a nearly degenerate band and their probability densities are independent of k, in contrast to the case of R/??相似文献   

Anisotropic magnetoresistance in electrodeposited cobalt antidot arrays

Nanosphere lithography is a simple and accessible technique for nanostructuring of materials. Combined with electrodeposition, it allows the production of compact, ordered antidot networks. In contrast to other lithographic techniques, the resulting nanostructure shows periodicity also along the growth axis. Interesting results are expected for the magnetoresistive behavior of such structures as function of thickness, due to the confinement of electronic routes and the strong shape anisotropy. We were able to electrodeposit cobalt antidot structures of homogeneous and controlled thickness directly over silicon substrates. Room temperature anisotropic magnetoresistance (AMR) as function of thickness and nanosphere diameter are presented, with the magnetic field applied in plane, transverse to the applied current. An overlap of two effects is observed. At fields lower than 2 kOe typical hysteretic AMR peaks appear around the coercive field, and tend to disappear for thicker films. At higher fields, a reversible contribution, caused by the forced magnetization that rotates the spin away from the local current direction, lowers the magnetoresistance, before it reaches its saturation value.     

Influence of flat milling on vortex matching effect in Bi2Sr2CaCu2O8+y with antidot array

Significant enlargements of antidot diameter by Ar-ion milling were observed in Bi₂Sr₂CaCu₂O_8+y single-crystal films with antidot arrays as well as the thinning of the films. In an original sample with triangular array of antidots, whose diameter is about 200 nm, a few dip structures by the matching effect were observed in the vortex-flow resistance as a function of magnetic field. With increasing the milling time, the number of the dips increases and the appearance of the flow resistance becomes periodic oscillations. These features can be explained mainly by the increase of the antidot diameter.     

Thermoelectric properties of one-dimensional graphene antidot arrays

We investigate the thermoelectric properties of one-dimensional (1D) graphene antidot arrays by nonequilibrium Green?s function method. We show that by introducing antidots to the pristine graphene nanoribbon the thermal conductance can be reduced greatly while keeping the power factor still high, thus leading to an enhanced thermoelectric figure of merit (ZT). Our numerical results indicate that ZT values of 1D antidot graphene arrays can be up to unity, which means the 1D graphene antidot arrays may be promising for thermoelectric applications.     

Localization and antilocalization in InSb and InAs antidot lattices

We report on the observation of localization, antilocalization and Altshuler–Aronov–Spivak (AAS) oscillations in antidot lattices patterned on high-mobility InSb/InAlSb and InAs/AlGaSb heterostructures. In addition, the antidot lattices display ballistic commensurability features. The strength of the localization peak in InSb antidot lattices decreases exponentially with temperature, with a high characteristic temperature of 25 K between 0.4 and 50 K. Analysis of the AAS oscillations enables the extraction of phase and spin coherence lengths in InAs.     

Cobalt antidot arrays on membranes: Fabrication and investigation with transmission X-ray microscopy

We have developed a method to fabricate ferromagnetic antidot arrays on silicon nitride membrane substrates for electron or soft X-ray microscopy with antidot periods ranging from 2 μm down to 200 nm. Observations of cobalt antidot arrays with magnetic soft X-ray microscopy show that for large periods, flux closure states occur between the antidots in the as-grown state and on application of a magnetic field, domain chains are created which show a spin configuration at the chain ends comprising four 90° walls. Pinning of the domain chain ends plays an important role in the magnetization reversal, determining the length of the chains and resulting in preservation of the domain chain configuration on reducing of the applied magnetic field to zero.     

Electron interactions in an antidot in the integer quantum Hall regime

A quantum antidot, a submicron depletion region in a two-dimensional electron system, has been actively studied in the past two decades, providing a powerful tool for understanding quantum Hall systems. In a perpendicular magnetic field, electrons form bound states around the antidot. Aharonov–Bohm resonances through such bound states have been experimentally studied, showing interesting phenomena such as Coulomb charging, h/2e$h/2e$ oscillations, spectator modes, signatures of electron interactions in the line shape, Kondo effect, etc. None of them can be explained by a simple noninteracting electron approach. Theoretical models for the above observations have been developed recently, such as a capacitive-interaction model for explaining the h/2e$h/2e$ oscillations and the Kondo effect, numerical prediction of a hole maximum-density-droplet antidot ground state, and spin-density-functional theory for investigating the compressibility of antidot edges. In this review, we summarize such experimental and theoretical works on electron interactions in antidots.     

Multi-vortex versus interstitial vortices scenario in superconducting antidot arrays

In superconducting thin films, engineered lattice of antidots (holes) act as an array of columnar pinning sites for the vortices and thus lead to vortex matching phenomena at commensurate fields guided by the lattice spacing. The strength and nature of vortex pinning is determined by the geometrical characteristics of the antidot lattice (such as the lattice spacing a₀, antidot diameter d, lattice symmetry, and orientation) along with the characteristic length scales of the superconducting thin films, viz., the coherence length (ξ) and the penetration depth (λ). There are at least two competing scenarios: (i) multiple vortices sit on each of the antidots at a higher matching period and (ii) there is nucleation of vortices at the interstitial sites at higher matching periods. Furthermore, it is also possible for the nucleated interstitial vortices to reorder under suitable conditions. We present our experimental results on NbN antidot arrays in the light of the above scenarios.     

Kondo effect in a quantum antidot

We report Kondo-like behavior in a quantum antidot (a submicron depleted region in a two-dimensional electron gas) in the quantum-Hall regime. When both spins of the lowest Landau level are present all around the antidot, the resonances between extended edge states via antidot bound states show an abnormal feature in alternate Coulomb-blockaded regions. The feature becomes suppressed when the temperature or source-drain bias is raised as for Kondo resonances in quantum dots. Although the exact mechanism is unknown, Kondo-like correlated tunneling may arise from a Skyrmion-type edge reconstruction. This observation demonstrates the generality of the Kondo phenomenon.     

Magnetic irreversibility of the Fe antidot arrays film by depositing on the porous alumina templates

An Fe layer was sputter-deposited onto porous alumina templates and Kapton respectively. Fe layer on the porous alumina templates formed an antidot arrays nanostructure, while Fe layer on the Kapton substrate formed a continuous film. Scanning electron microscopy and grazing incidence X-ray diffraction were employed to characterize the morphology and crystal structure of the Fe antidot arrays and continuous film, respectively. The temperature dependence of magnetic properties was shown in the temperature range 2-300 K. The irreversibility of the magnetization of Fe antidot arrays film, as measured in zero-field cooling (ZFC) and field cooling (FC) states, was attributed to the pinning effect of the holes.     

Flux confinement by regular arrays and clusters of antidots in Pb/Cu bilayers

We have studied the magnetoresistance and superconducting–normal phase boundary of superconducting films with an antidot array and mesoscopic antidot clusters of 2 × 2 μm²with only four antidots. For both systems characteristic minima have been observed in the magnetoresistance which are caused by the formation of certain vortex configurations minimizing the free energy. By comparing experimental data with calculations carried out in the London limit of the Ginzburg Landau theory, these vortex configurations have been identified.     

Mechanism of Topology Change of Flat Magnetic Structures

The paper investigates the processes of the magnetization reversal of perforated ferromagnetic films with strong anisotropy of the easy-plane type. The investigations have shown that, influenced by a current impulse passing through an antidot, an inhomogeneous magnetic structure is formed, which is accompanied by the localization of a quasiparticle with the +1 topological charge on the antidot and by an emission of a quasiparticle with a –1 charge. It is established that this scenario of the film magnetization reversal underlies a reformation of its inhomogeneous structure also if two or four antidots are present in the film, irrespective of the fact of through which antidots and in which directions the currents are passed. The results of the research obtained by using two independent methods (solving the Landau–Lifshitz–Gilbert equations and analyzing the lattice model) demonstrated good agreement between the two. It is shown that a magnetic film comprising two or four antidots can be used as a memory cell for recording data in the ternary system.     

Effect of electron–phonon interaction on the energy spectrum of a quantum antidot in the presence of a uniform strong magnetic field

We have studied the effect of electron–phonon interaction for small electron–phonon coupling on the electronic energy spectrum of an electron confined by a parabolic potential and a repulsive antidot potential in the presence of a uniform strong magnetic field and an Aharonov–Bohm flux field by using a variational procedure. We have shown that the presence of the antidot potential removes degeneracy of the Landau levels and electron–phonon interaction has nonnegligible effects on these levels.     

Tunneling through a coherent "Quantum antidot molecule"

We report experiments on resonant tunneling through a quantum antidot in the fractional quantum Hall regime. The envelope of the conductance peaks indicates tunneling via two resonant states, one of them bound on the lithographic antidot, the other on a hill of the disorder potential. Moreover, our analysis indicates that the coherent tunneling rate between the two states is an order of magnitude higher than the phase breaking rate, thus giving evidence for a coherently coupled "antidot molecule."     

锯齿形石墨烯反点网络加工与输运性质研究

具有特定边界的石墨烯纳米结构在纳电子学、自旋电子学等研究领域表现出良好的应用前景.然而石墨烯加工成纳米结构时,无序的边界不可避免地会降低其载流子迁移率.氢等离子体各向异性刻蚀技术是加工具备完美边界石墨烯微纳结构的一项关键技术,刻蚀后的石墨烯呈现出规则的近原子级平整的锯齿形边界.本文研究了氮化硼上锯齿形边界石墨烯反点网络的磁输运性质,低磁场下可以观测到载流子围绕着一个空位缺陷运动时的公度振荡磁阻峰.随着磁场的增大,朗道能级简并度逐渐增大,载流子的磁输运行为从Shubnikov-de Haas振荡逐渐向量子霍尔效应转变.在零磁场附近可以观测到反点网络周期性空位缺陷的边界散射所导致的弱局域效应.研究结果表明,在氮化硼衬底上利用氢等离子体刻蚀技术加工锯齿形边界石墨烯反点网络,其样品质量会明显提高,这种简单易行的方法为后续高质量石墨烯反点网络的输运研究提供了新思路.     

Triangular antidot array to honeycomb wire network – Role of interstitial vortices

We present results on interesting vortex matching effects that arise out of the interplay between the geometric shape of the vortex and the geometry of the underlying nano-engineered antidot lattice in Nb thin films. The antidots are in the shape of honeycomb with gradually increasing area such that one essentially goes over from a triangular antidot array to a honeycomb wire network. The fractional matching at 1/2 is absent in the antidot array limit hinting towards a possible role of the interstitial vortices.     

Low-Temperature Magnetic Properties of Co Antidot Array

Cobalt antidot arrays with different thicknesses are fabricated by rf magnetron sputtering onto porous alumina substrates. Scanning electron microscopy and grazing incidence x-ray diffraction are employed to characterize the morphology and crystal structure of the antidot array, respectively. The temperature dependence of magnetic properties shows that in the temperature range 5K--300K, coercivity and squareness increase firstly, reach their maximum values, then decrease. The anomalous temperature dependences of coercivity and squareness are discussed by considering the pinning effect of the antidot and the magnetocrystalline anisotropy.     

Magnetization distribution in submicrometer iron-nickel antidot arrays patterned by a focused ion beam

Antidots of size 0.5 μm are prepared by patterning iron-nickel films with a focused ion beam. The magnetization distribution in antidot arrays is examined with Lorentz transmission electron microscopy. It is shown that one side of the array makes an angle of about 20° with the easy magnetic axis of the film. Magnetization reversal in the direction close to the easy magnetic axis starts with domain nucleation at the antidot edges that are perpendicular to the applied field and adjacent to the unpatterned region of the film, and propagates as the domain walls move. Magnetization reversal in the direction close to the hard magnetic axis starts with magnetization rotation outside the patterned region at the antidot edges and propagates as the domain walls execute a complicated motion. It is demonstrated that some areas between the edges of adjacent antidots can carry information bits. Results obtained are explained in terms of competition between the demagnetizing energy, energy of internal anisotropy, and misorientation effect. The feasibility of such structures as high-density storage elements is discussed.     

Characterization and Magnetic Properties of Iron-Based Alloy Antidot Arrays

Fe29Cerl and Fe19Ni81 antidot arrays, with different dimensions, are prepared with the rf magnetron sputtering method onto the porous alumina substrate. The size and shape of antidot arrays are characterized by scanning electron microscopy. The glancing angle x-ray diffraction patterns of Fe29C071 and FelgNi81 antidot arrays indicate the bcc and fcc structures, respectively. The coercivities of both the alloys show abnormal thickness dependence, which are discussed qualitatively by considering the pinning and the thickness effect to the films.     

Polaronic effects in one-dimensional quantum antidot arrays

We study the effect of polaronic corrections arising from theelectron-longitudinal optical phonon interaction on the energyspectrum of a two-dimensional electron system with a one-dimensionalperiodic antidot array geometry created by a weak electrostaticmodulation potential, and subjected to a weak magnetic fieldmodulation as well as a uniform strong perpendicular staticmagnetic field. To incorporate the effects of electron-phononinteractions within the framework of Fröhlich polaron theory, wefirst apply a displaced-oscillator type unitary transformation todiagonalise the relevant Fröhlich Hamiltonian, and we thendetermine the parameters of this transformation together with theparameter included in the electronic trial wave function . On thebasis of this technique, it has been shown that the polaroniccorrections have non-negligible effects on the electronic spectrumof a two-dimensional electron system with a quantum antidot array,since switching such an interaction results in shifting thedegeneracy restoring points of Landau levels wherein the flatbandcondition is fulfilled, thus suppressing the Weiss oscillations.