Bose condensation of excitons in two-layer electronic systems in the absence of magnetic field

A high narrow peak of interlayer differential tunnel conductivity is observed at low temperatures in heterostructures with two closely located electronic layers in the absence of magnetic field. The analysis of experimental results suggests that this peak is due to the interlayer phase coherence that arises in the system as a result of the Bose condensation of interlayer excitons (electron-hole pairs) belonging to different layers, in accordance with the recent theoretical predictions.     

Experimental observation of phase coherence in bilayer systems in the absence of a magnetic field

A high narrow peak of differential interlayer tunneling conductance is observed in the absence of a magnetic field at low temperatures in heterostructures with two closely spaced electron layers. The analysis of experimental results suggests that this peak is a consequence of the interlayer phase coherence, which arises in the system owing to the Bose condensation of interlayer excitons, i.e., pairs formed by electrons and holes belonging to different layers, in accordance with the recent theoretical predictions.     

Density of states in the bilayer graphene with the excitonic pairing interaction

In the present paper, we consider the excitonic effects on the single particle normal density of states (DOS) in the bilayer graphene (BLG). The local interlayer Coulomb interaction is considered between the particles on the non-equivalent sublattice sites in different layers of the BLG. We show the presence of the excitonic shift of the neutrality point, even for the noninteracting layers. Furthermore, for the interacting layers, a very large asymmetry in the DOS structure is shown between the particle and hole channels. At the large values of the interlayer hopping amplitude, a large number of DOS at the Dirac’s point indicates the existence of the strong excitonic coherence effects between the layers in the BLG and the enhancement of the excitonic condensation. We have found different competing orders in the interacting BLG. Particularly, a phase transition from the hybridized excitonic insulator phase to the coherent condensate state is shown at the small values of the local interlayer Coulomb interaction.     

Control of polarity and defects in the growth of AlN films on Si (111) surfaces by inserting an Al interlayer

We discuss the role of the Al interlayer in the suppression of pinhole formations and also look at the polarity transition of the AlN layers from N-polarity to Al-polarity when this Al interlayer is present. The AlN layers were grown by molecular beam epitaxy on an AlN nucleation layer. A thin Al interlayer was deposited on the initial nucleated AlN layer after the nitridation of the Al-soaked Si (111) substrates. The AlN layer with an Al interlayer showed a relatively smooth surface with a reduced density of pinholes compared with the AlN layer grown without an Al interlayer. In addition, the AlN layer with an Al interlayer showed some stacking faults in the interface between the Si substrate and the A1N layer. We also identify the polarity change of the AlN layer after the insertion of a thin Al interlayer from N-polarity to Al-polarity by chemical etching. A simple model is constructed to explain the polarity change and the pinhole suppression due to the Al interlayer.     

中间层对三原色白光OLED的影响

基于ITO/NPB/TCTA/Ir（MDQ）₂（acac）：TCTA/FIrpic：TmPyPb/Ir（ppy）₃：TmPyPb/TmPyPb/LiF/Al结构的三原色白光器件,通过分别在蓝光与红光、绿光发光层界面处插入2 nm TCTA与2 nm TmPyPb中间层,研究了中间层的有无对器件性能的影响。结果表明,中间层的引入可以调整激子的分布,影响能量转移。具有双中间层的器件实现了高质量的白光发射,最大发光效率达到22.56 cd/A。     

Interlayer exchange coupling in ferromagnet-semiconductor digital magnetic alloys

The interlayer exchange coupling in ferromagnet-semiconductor digital magnetic alloys in which monolyers (submonolayers) of transition metals are embedded into a semiconductor matrix is studied theoretically. A mechanism of an indirect exchange between ferromagnetic δ layers is proposed; it is based on the confinement of carriers in two-dimensional spin-polarized states inside the energy gap of the semiconductor. These appear due to strong potential and exchange carrier scattering by the δ layers. The interlayer exchange coupling is shown to occur through a nondegenerate semiconductor interlayer because of virtual electron excitations through an energy barrier separating these partly filled two-dimensional spin-polarized states and the edge of the bulk semiconductor band. The interlayer coupling intensity decreases exponentially with increasing distance between neighboring δ layers, and the type of this coupling can change from ferromagnetic into antiferromagnetic or vice versa as the interlayer thickness or the degree of filling the two-dimensional states increases.     

Energy distributions of field emitted electrons from carbon nanosheets: Manifestation of the quantum size effect

We emphasize the importance of experiments with voltage dependent field emission energy distribution analysis in carbon nanosheets. Our analysis shows the crucial influence of the band structure on the energy distribution of field emitted electrons in few-layer graphene. In addition to the main peak we found characteristic sub-peaks in the energy distribution. Their positions strongly depend on the number of layers and the interlayer interaction. The discovery of these peaks in field emission experiments from carbon nanosheets would be a clear manifestation of the quantum size effect in these new materials.     

Effect of the in-plane magnetic field on the recombination radiation spectrum of spatially-separated electron-hole layers

Changes in the recombination radiation spectrum of spatially-separated electron-hole layers has been studied under variation of the in-plane magnetic field and interlayer distance. It has been found that a change in the spectral position of the luminescence line in the low-field limit is proportional to the square of the magnetic field with the proportionality coefficient depending on the interlayer distance. The observed dependence has been shown to agree with the theoretical conceptions, according to which the line shift is quadratic in the magnetic field and interlayer distance and inversely proportional to the sum of the electron and hole masses. This total mass obtained in the experiment has been found to depend on the electric field that separates the layers and may substantially differ from the expected value.     

Quantum interference at the twist boundary in graphene

We explore the consequences of a rotation between graphene layers for the electronic spectrum. We derive the commensuration condition in real space and show that the interlayer electronic coupling is governed by an equivalent commensuration in reciprocal space. The larger the commensuration cell, the weaker the interlayer coupling, with exact decoupling for incommensurate rotations and in the theta-->0 limit. Furthermore, from first-principles calculations we determine that even for the smallest possible commensuration cell the decoupling is effectively perfect, and thus graphene layers will be seen to decouple for all rotation angles.     

Magnetic-state tuning of the rhombohedral graphite film by interlayer spacing and thickness

Based on first-principles total-energy calculations, we systematically investigate how the electronic and magnetic properties of rhombohedral graphite thin films depend on the interlayer spacing and number of layers. Our calculations show that the magnetic ordering of the thin films depends on the interlayer spacing. Thin films under compression normal to the layers possess finite magnetic moments, indicating parallel spin coupling between the two surfaces. We also find that thin graphite films with seven or more atomic layers exhibit magnetic ordering while films with six or less atomic layers are metallic with no magnetic ordering.     

Role of Ge interlayer in the growth of high-quality strain relaxed SiGe layer with low dislocation density

High-quality strain relaxed SiGe layer has been fabricated on Si using a thin Ge interlayer grown at 330 °C. The properties of SiGe layers with and without the low-temperature Ge interlayer are compared. The results indicate that the Ge interlayer plays an important role in the preparation of SiGe layer. The strain relaxed low-temperature Ge interlayer with coalesced island surface, acting as a stable and compliant template, could remove the cross-hatch misfit dislocation lines on surface and promote the strain relaxation in the SiGe layer homogeneously.     

Monotonic growth of interlayer magnetoresistance in strong magnetic field in very anisotropic layered metals

It is shown that the monotonic part of interlayer electronic conductivity strongly decreases in high magnetic field perpendicular to the conducting layers. Only the coherent interlayer tunneling has been considered, and the obtained result strongly contradicts the standard theory. This effect appears in very anisotropic layered quasi-two-dimensional metals, when the interlayer transfer integral is less than the Landau level separation.     

Interlayer superfluidity in bilayer systems of fermionic polar molecules

We consider fermionic polar molecules in a bilayer geometry where they are oriented perpendicularly to the layers, which permits both low inelastic losses and superfluid pairing. The dipole-dipole interaction between molecules of different layers leads to the emergence of interlayer superfluids. The superfluid regimes range from BCS-like fermionic superfluidity with a high Tc to Bose-Einstein (quasi-)condensation of interlayer dimers, thus exhibiting a peculiar BCS-Bose-Einstein condensation crossover. We show that one can cover the entire crossover regime under current experimental conditions.     

Influence of the multilayer HTS-cable conductor design on the current distribution

The work presents the basic principles of the multilayer cable conductor design to achieve the maximum current-carrying capacity and the minimum losses in a superconductor and constructive cable elements. The multilayer conductors of two to ten layers are analyzed. The results show that the traditional core design with alternative winding directions from layer to layer is useful only for two-layer conductor. The conductor with more layers must have either the layers wound in one direction but with different pitch lengths or two layer groups wound with different pitch lengths. Only for these cases, the balanced design can be realized and current distribution will be uniform. In such balanced design, the interlayer electrical voltage and as a result, the coupling losses, are absent and interlayer electrical insulation is not needed. The recommendations to achieve the maximum critical current as a function of conductor dimensions are derived.     

Calculated angular dependence of the interlayer couplings in Fe/Cr superlattices having imperfect interfaces

We present calculations of the non-collinear magnetic structure in Fe/Cr superlattices having imperfect interfaces modeled by considering atomic steps in the Cr layers and Fe/Cr interfacial ordered compounds. The interlayer couplings are obtained directly from self-consistent tight binding band structure calculations. We show that the bilinear–biquadratic expression for the coupling energy fits nicely the calculated interlayer couplings curves.     

Evidence for a finite-temperature phase transition in a bilayer quantum Hall system

We study the Josephson-like interlayer tunneling signature of the strongly correlated nuT=1 quantum Hall phase in bilayer two-dimensional electron systems as a function of the layer separation, temperature, and interlayer charge imbalance. Our results offer strong evidence that a finite temperature phase transition separates the interlayer coherent phase from incoherent phases which lack strong interlayer correlations. The transition temperature is dependent on both the layer spacing and charge imbalance between the layers.     

不同基底石墨烯涂层的层间滑移减磨性能研究

石墨烯薄膜作为一种二维材料,是提高微/纳机电系统(MEMS/NEMS)摩擦力学性能的优异润滑剂.为了探究基底材料和石墨烯层数对其减磨性能的影响,本文通过在不同基底制备了不同层数的石墨烯涂层,利用原子力显微镜(AFM)实验和分子动力学(MD)仿真结合的方法,研究了石墨烯层数对减磨效应的影响.并且通过建立不同层数石墨烯涂层的摩擦性能分析模型,探究出石墨烯层间滑移是产生减磨的主要因素.结果表明:在不同载荷下,石墨烯涂层对硅基底和铜基底均有优异的减磨效果,摩擦力随着石墨烯层数的增加逐渐降低,当石墨烯层数大于10层时,达到最优99.3%的减磨效果.通过仿真分析发现,随着层数增加,石墨烯与基底的干摩擦转变为石墨烯的层间摩擦,并产生层间剪切滑移,石墨烯层间滑移是导致多层石墨烯优异减磨性能的主要因素.     

Carrier spin polarization in ferromagnet-semiconductor (Ga,Mn)As/GaAs structures

The effect of ferromagnetic layers on the spin polarization of holes and electrons in ferromagnet-semiconductor superlattices with a fixed Mn δ-layer thickness of 0.11 nm and different GaAs interlayer thicknesses varying in the range from 2.5 to 14.4 nm and a fixed number of periods (40) is studied by means of hot-electron photoluminescence (HPL). Here, our study of the HPL demonstrates that the holes in δ-layers of (Ga,Mn)As DMS occupy predominantly the Mn acceptor impurity band. The width of the impurity band decreases with the increase of the interlayer distance. We also found that an increase in the GaAs interlayer thickness softens the magnetic properties of the ferromagnetic layers as well as reduces the carrier polarization. It is demonstrated that the hole spin polarization in the DMS layers and spin polarization of electrons in nonmagnetic GaAs are proportional to the sample magnetization.     

Interlayer exchange between impurity states in iron/silicon multilayers

A model is proposed for exchange coupling between ferromagnetic metal layers through a nondegenerate semiconductor interlayer with point defects. The asymptotics of the exchange integrals is calculated. It is shown that the interlayer exchange can reverse sign depending on the position and occupation of impurity states in the interlayer. The results provide a qualitative explanation of the experimental data obtained for iron/silicon multilayers.