Modified gap states in Fe/MgO/SrTiO3 interfaces studied with scanning tunneling microscopy

The geometric and electronic structures of Fe islands on MgO film layers were studied with scanning tunneling microscopy and spectroscopy. The MgO layers were grown on a Nb-doped single crystal SrTiO₃ (100) surface. Deposited Fe atoms aggregate into islands, the height and diameter of which are about 2.5 and 9.4 nm respectively. Fe islands modify the electronic structure of MgO surface; a ring type depression in the scanning tunneling microscope topography appears by lowered local electron density of states around Fe islands. We find that adsorbed Fe atoms reduce the gap states of MgO layers around Fe islands, which is attributed to the reason for the depletion of the electronic density of states.     

Surprising electronic-magnetic properties of closed packed organized organic layers

Cooperative effects generate new electronic and magnetic properties in closed packed organized organic layers. In layers made from chiral molecules, unexpectedly large electronic dichroism is observed, which manifests itself as spin specific electron transmission. For many thiolated molecules self-assembled on gold, a surprisingly large ferromagnetism is observed. All the observations can be rationalized by assuming orbital ferromagnetism of the organic thin layer. This is a new type of magnetism that is caused by the formation of closed packed layers of organic molecules on metal. In particular, charge transfer occurs between the substrate and the adsorbed layer. This charge is responsible for the appearance of magnetism.     

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.     

Quantum size effects in the optical properties of organic superlattices containing 3, 4, 9, 10 perylene tetracarboxylic dianhydride (PTCDA)

A Frenkel exciton model is applied to ultrathin films of PTCDA confined between layers of a material with electronic transitions in a different energetic region. This model accounts for the elongation of an effective internal vibration in the relaxed excited geometry of a molecule and for the transfer of an electronic excitation between different molecular sites. The comparison of the model calculations with experimental findings reveals that due to the modified conditions at the organic interfaces, boundary layers of the PTCDA films show blue-shifted transition energies with respect to inner molecular layers surrounded by the same material. The resulting changes of the lowest vibronic subband contribute further to the distinct absorption line shapes.     

Electronic properties of silicene in BN/silicene van der Waals heterostructures

Silicene is a promising 2D Dirac material as a building block for van der Waals heterostructures(vd WHs). Here we investigate the electronic properties of hexagonal boron nitride/silicene(BN/Si) vd WHs using first-principles calculations.We calculate the energy band structures of BN/Si/BN heterostructures with different rotation angles and find that the electronic properties of silicene are retained and protected robustly by the BN layers. In BN/Si/BN/Si/BN heterostructure, we find that the band structure near the Fermi energy is sensitive to the stacking configurations of the silicene layers due to interlayer coupling. The coupling is reduced by increasing the number of BN layers between the silicene layers and becomes negligible in BN/Si/(BN)_3/Si/BN. In(BN)_n/Si superlattices, the band structure undergoes a conversion from Dirac lines to Dirac points by increasing the number of BN layers between the silicene layers. Calculations of silicene sandwiched by other 2D materials reveal that silicene sandwiched by low-carbon-doped boron nitride or HfO_2 is semiconducting.     

First-principles study of monolayer and bilayer honeycomb structures of group-IV elements and their binary compounds

By using first-principles pseudopotential method, we investigate the structural, vibrational, and electronic properties of monolayer and bilayer honeycomb structures of group-IV elements and their binary compounds. It is found that the honeycomb structures of Si, Ge, and SiGe are buckled for stabilization, while those of binary compounds SiC and GeC containing the first row elements C are planar similar to a graphene sheet. The phonon dispersion relations and electronic band structures are very sensitive to the number of layers, the stacking order, and whether the layers are planar or buckled.     

Band structure of Au layers on the Ru(0001) and graphene/Ru(0001) surfaces

The electronic structures of Au monolayers on the Ru(0001) and graphene-coated Ru(0001) surfaces have been calculated by DFT method using the supercell (repeated-slab) approach. The local densities of states (LDOS) and band structures of the monolayer and bilayer Au films adsorbed on the graphene/Ru(0001) and those of free hexagonal Au layers are found to be very similar. This result indicates that the monolayer graphene almost completely screens the Au layers from the Ru(0001) substrate surface, so that electronic properties of Au films adsorbed on graphene are determined predominantly by the electronic structure of the Au adlayers, essentially independent on the electronic structure of the substrate surface.     

A detailed investigation of the electronic properties of a multi-layer spherical quantum dot with a parabolic confinement

In this work, we aim a detailed investigation of the electronic properties of a spherical multi-layer quantum dot with and without a hydrogenic impurity. The structure is introduced in the form of core/shell/well/shell layers. The core and well layers are defined by the parabolic electronic potentials. We carry out the effect of the core radius and layer thickness on the energy levels, their wave functions, binding energies of the impurity and the probability distributions. In order to determine the sublevel eigenvalues and eigenfunctions, the Schrödinger equation is solved full numerically by shooting method in the frame of the effective mass approximation. The results are analyzed in detail as a function of the layer thicknesses and their probable physical reasons are tried to be explained. It is found that the electronic properties and impurity binding energies are strongly depending on the layer thicknesses.     

Interface controlled electronic charge inhomogeneities in correlated heterostructures

For heterostructures of ultrathin, strongly correlated copper-oxide films and dielectric perovskite layers, we predict inhomogeneous electronic interface states. Our study is based on an extended Hubbard model for the cuprate film. The interface is implemented by a coupling to the electron and phonon degrees of freedom of the dielectric oxide layer. We find that electronic ordering in the film is associated with a strongly inhomogeneous polaron effect. We propose to consider the interfacial tuning as a powerful mechanism to control the charge ordering in correlated electronic systems.     

1,1-二氨基二硝基乙烯晶体的密度泛函理论研究

对 1,1 二氨基二硝基乙烯晶体进行了DFT B3LYP水平计算 .计算所得晶格能为 -10 5 .81kJ/mol,与文献值相近 .晶体的前线能带较为平坦 ,表明分子轨道能态受分子晶体场的影响较小 .电荷的分布决定了晶体中DADNE以“头 尾”方式通过分子间氢键相连形成层状结构 ,而层与层之间相互作用较弱 .从带隙 4.0eV推知DADNE的导电性介于半导体和绝缘体之间 .前线轨道由C -NO2 的原子轨道所组成 ,说明分子的强共轭性 ,也表明C -NO2 为化学反应活性部位 .C -NO2 键的布居数远小于其它键 ,提示该键为起爆引发键     

Raman spectrum of graphene and graphene layers

Graphene is the two-dimensional building block for carbon allotropes of every other dimensionality. We show that its electronic structure is captured in its Raman spectrum that clearly evolves with the number of layers. The D peak second order changes in shape, width, and position for an increasing number of layers, reflecting the change in the electron bands via a double resonant Raman process. The G peak slightly down-shifts. This allows unambiguous, high-throughput, nondestructive identification of graphene layers, which is critically lacking in this emerging research area.     

Calculation of transport properties of Co–Ag-based multilayered granular alloys

We calculate electronic transport properties of multilayered granular alloys, composed of discontinuous Co layers embedded in Ag alternated with complete Ag layers. We focus our attention on the conductivity dependance on the shape and size of the clusters. The electronic structure is self-consistently calculated using a tight binding hamiltonian which includes a Hubbard term within the unrestricted Hartree–Fock approximation. We obtain different transport regimes depending on the growth conditions and transport direction.     

Electric field tunability of nuclear and electronic spin dynamics due to the hyperfine interaction in semiconductor nanostructures

We present formulas for the nuclear and electronic spin relaxation times due to the hyperfine interaction for nanostructed systems and show that the times depend on the square of the local density of electronic states at the nuclear position. A drastic sensitivity (orders of magnitude) of the electronic and nuclear spin coherence times to small electric fields is predicted for both uniformly distributed nuclear spins and delta-doped layers of specific nuclei. This sensitivity is robust to nuclear spin diffusion.     

Nature of contrast in Ge/Si(111) layers in scanning tunneling microscopy in the presence of Bi and Sb surfactants

It is known that the use of Bi surfactant (unlike Sb) upon the growth of Ge layers on Si(111) increases the contrast between Ge and Si atoms in a scanning tunneling microscope. This makes it possible to distinguish the Ge and Si surfaces. This effect is studied using computer simulation based on the density functional theory. To explain the observed difference between the Ge and Si layers, both structural and electronic effects are considered. The local density of electronic states, as well as the corresponding decay length to vacuum, has been calculated for each of the surfaces. The simulation results have been compared to the previous scanning tunneling microscopy data.     

Suppression of Anderson localization in a graphene sheet applied by a random voltage pattern

We theoretically study the transport of electronic waves through a graphene sheet applied by a random voltage pattern in which the magnitudes and/or the widths of the voltages are random. When the magnitudes of the voltages exceed the electronic energy, the applied region can be considered as left-handed (LH) layers. Compared to the disordered structures with right-handed (RH) layers only, the spectra of the (average) density of states and the localization lengths in mixed random structures with RH and LH layers all show the suppression of Anderson localization, owing to the phase compensation effect of LH layers that reduces the long-range interference in the random system.     

Enhanced superconductivity in superlattices of high-Tc cuprates

The electronic properties of multilayers of strongly correlated models for cuprate superconductors are investigated using cluster dynamical mean-field techniques. We focus on combinations of underdoped and overdoped layers and find that the superconducting order parameter in the overdoped layers is enhanced by the proximity effect of the strong pairing scale originating from the underdoped layers. The enhanced order parameter can even exceed the maximum value in uniform systems. This behavior is well reproduced in slave-boson mean-field calculations which also find higher transition temperatures than in the uniform system.     

Electronic states on the surface of graphite

Graphite consists of graphene layers in an AB (Bernal) stacking arrangement. The introduction of defects can reduce the coupling between the top graphene layers and the bulk crystal producing new electronic states that reflect the degree of coupling. We employ low temperature high magnetic field scanning tunneling microscopy (STM) and spectroscopy (STS) to access these states and study their evolution with the degree of coupling. STS in magnetic field directly probes the dimensionality of electronic states. Thus two-dimensional states produce a discrete series of Landau levels while three-dimensional states form Landau bands providing a clear distinction between completely decoupled top layers and ones that are coupled to the substrate. We show that the completely decoupled layers are characterized by a single sequence of Landau levels with square-root dependence on field and level index indicative of massless Dirac fermions. In contrast weakly coupled bilayers produce special sequences reflecting the degree of coupling, and multilayers produce sequences reflecting the coexistence of massless and massive Dirac fermions. In addition we show that the graphite surface is soft and that an STM tip can be quite invasive when brought too close to the surface and that there is a characteristic tip-sample distance beyond which the effect of sample-tip interaction is negligible.     

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.     

Visible and near ultraviolet photocurrent generation in carbon nanotubes

Multiwall carbon nanotubes are found to generate photocurrent in the visible and near ultra violet spectral range using a photoelectrochemical technique. Peaks in the photocurrent are observed at excitation energies in the visible region. Their electron energy loss spectra exhibit the π plasmon feature, typical of graphite layers, and a peak at lower energy. Features at energies between 0 and 4 eV have been already observed for single wall carbon nanotubes and ascribed to interband electronic transitions due to the reduced dimensionality of these systems. The present measurements suggest that the usual identification of multiwall carbon nanotubes electronic density of states with that of graphite layers is not sufficient and more theoretical investigations are necessary to shed light on this point.     

Two waveguide layers in lithium niobate crystal formed by swift heavy Kr ion irradiation

We report the formation of two waveguide layers in a lithium niobate crystal by irradiation with swift heavy Kr ions with high(Ge V) energies and ultralow fluences. The micro-Raman spectra are measured at different depths in the irradiated layer and show that the high electronic energy loss can cause lattice damage along the ion trajectory, while the nuclear energy loss causes damage at the end of the ion track. Two waveguide layers are formed by confinement with two barriers associated with decreases in the refractive index that are caused by electronic and nuclear energy losses, respectively.