Conduction Band States Research Articles

Probing the wavefunction of the surface states in Bi2Se3 topological insulator: a realistic tight-binding approach

We present results of a microscopic tight-binding modeling of BiSe three-dimensional topological insulator using a sp Slater–Koster Hamiltonian, with parameters calculated from density functional theory. Based on the calculated atomic- and orbital-projections of the wavefunctions associated with valence- and conduction-band states at the center of the Brillouin zone, we propose a real-space description of band inversion for both bulk and a slab of finite thickness. A systematic analysis of the key features of the surface states, in particular the spatial distribution and the spin-character of the surface states wavefunction, is carried out for slabs of different thickness, ranging from one to tens of quintuple layers. We obtain an estimate of the slab thickness at which the energy gap induced by interaction between the top and bottom surface states becomes negligible, based on the present available numerical precision. We anticipate that this finding will be relevant for all microscopic calculations addressing the effect of external perturbations on the surface states near the Dirac point. The modifications in the helical spin-texture of the Dirac-cone surface states, in the form of in-plane and out-of-plane spin projections, are calculated as a function of the slab thickness. These calculations are important for the interpretation of ongoing experiments, which probe the spin-polarization of the surface states in topological insulator thin films.

Exporting superconductivity across the gap: Proximity effect for semiconductor valence-band states due to contact with a simple-metal superconductor

The proximity effect refers to the phenomenon whereby superconducting properties are induced in a normal conductor that is in contact with an intrinsically superconducting material. In particular, the combination of nano-structured semiconductors with bulk superconductors is of interest because these systems can host unconventional electronic excitations such as Majorana fermions when the semiconductor's charge carriers are subject to a large spin-orbit coupling. The latter requirement generally favors the use of hole-doped semiconductors. On the other hand, basic symmetry considerations imply that states from typical simple-metal superconductors will predominantly couple to a semiconductor's conduction-band states and, therefore, in the first instance generate a proximity effect for band electrons rather than holes. In this article, we show how the superconducting correlations in the conduction band are transferred also to hole states in the valence band by virtue of inter-band coupling. A general theory of the superconducting proximity effect for bulk and low-dimensional hole systems is presented. The interplay of inter-band coupling and quantum confinement is found to result in unusual wave-vector dependencies of the induced superconducting gap parameters. One particularly appealing consequence is the density tunability of the proximity effect in hole quantum wells and nanowires, which creates new possibilities for manipulating the transition to nontrivial topological phases in these systems.

Charge transport in colloidal ZnO nanocrystal solids: The significance of surface states

We investigate charge transport behaviour in colloidal ZnO nanocrystal solids with different surface states. Our results show that the logarithm of the conductivity scales with −T−1/4, suggestive of Mott variable-range hopping. Analysis of the density of states at the Femi level suggests that the charge hopping occurs through surface or defect states, rather than by direct hopping between quantum-confined conduction band states of the nanocrystals.

SiN-SiC nanofilm: A nano-functional ceramic with bipolar magnetic semiconducting character

Nowadays, functional ceramics have been largely explored for application in various fields. However, magnetic functional ceramics for spintronics remain little studied. Here, we propose a nano-functional ceramic of sphalerite SiN-SiC nanofilm with intrinsic ferromagnetic order. Based on first principles calculations, the SiN-SiC nanofilm is found to be a ferromagnetic semiconductor with an indirect band gap of 1.71 eV. By mean field theory, the Curie temperature is estimated to be 304 K, close to room temperature. Furthermore, the valence band and conduction band states of the nanofilm exhibit inverse spin-polarization around the Fermi level. Thus, the SiN-SiC nanofilm is a typical bipolar magnetic semiconductor in which completely spin-polarized currents with reversible spin polarization can be created and controlled by applying a gate voltage. Such a nano-functional ceramic provides a possible route for electrical manipulation of carrier's spin orientation.

Spin transport in non-inertial frame

The influence of acceleration and rotation on spintronic applications is theoretically investigated. In our formulation, considering a Dirac particle in a non-inertial frame, different spin related aspects are studied. The spin current appearing due to the inertial spin–orbit coupling (SOC) is enhanced by the interband mixing of the conduction and valence band states. Importantly, one can achieve a large spin current through the k→.p→ method in this non-inertial frame. Furthermore, apart from the inertial SOC term due to acceleration, for a particular choice of the rotation frequency, a new kind of SOC term can be obtained from the spin rotation coupling (SRC). This new kind of SOC is of Dresselhaus type and controllable through the rotation frequency. In the field of spintronic applications, utilizing the inertial SOC and SRC induced SOC term, theoretical proposals for the inertial spin filter, inertial spin galvanic effect are demonstrated. Finally, one can tune the spin relaxation time in semiconductors by tuning the non-inertial parameters.

Strain and nonparabolicity effects in Mid-infrared GaxIn1-xAsySb1-y/AlxGayIn1-x-yAszSb1-z MQW QCL

In this paper, we present a theoretical study of strain and nonparabolicity effects in Mid-infrared (MIR) GaxIn1-xAsySb1-y/AlxGayIn1-x-yAszSb1-z Multiple Quantum Well (MQW) Quantum Cascade Lasers (QCL). We use an envelope function k.P theory model to show the strong effects of bi-axial compressive strain and nonparabolicity on conduction band states of the QCL. These effects are shown to result more particularly in an increase of the electron effective mass near the fundamental laser emission energy transition of the QCL. This in turn is found to have a significant influence on several optical properties of the QCL as a reduction of laser transition energy i.e. an increase of the laser emission wavelength, a decrease of the transit time in the injection region of the QCL, a reduction of the optical dipole matrix element, a reduction of the optical gain of the laser.

Stability and nonlinear optical properties of Cu nanoparticles prepared by femtosecond laser ablation of Cu target in alcohol and water

By using PVP of different concentrations as the protective agent, Cu nanoparticles (NPs) of different mean sizes are fabricated through laser ablation of Cu target in alcohol, water, and their mixtures. As-grown Cu NPs show good stability in alcohol. The nonlinear absorptions of Cu NPs with different mean sizes are investigated. With the increase of laser intensity, the nonlinear absorption of small NPs changes from saturable absorption (SA) to reverse saturable absorption (RSA) and two-photon absorption (TPA), whereas SA dominates the nonlinear absorption of the large NPs. The modulations of occupied and unoccupied density of states (DOS) in conduction band are proposed to be the key factors for the size-dependence of the nonlinear absorption. For the large Cu NPs, there are sufficiently occupied DOS in the ground state and unoccupied DOS in the first excited state to sustain the increase of SA, whereas for the small Cu NPs, the corresponding DOS is insufficient, so the SA is surpassed by the RSA and TPA at high laser excitation intensity.

The effect of electric potential on Landau levels for topological insulator surface states

We study the effect of an electric potential well on the Landau level (LL) spectrum for surface states of a three-dimensional (3D) topological insulator (TI) by using a method of numerical diagonalization for the Hamiltonian in a truncated Hilbert space. It is found that the dispersivity of energy is dependent on the position of electron cyclotron orbit center (COC). A dispersionless energy spectrum like flat LLs appeared as if without the confining potential when the electron COC is far away from the well boundaries (corners). When the electron COC is near the boundaries, however, there exist some interesting interface states propagating along the well due to the presence of electric confinement which causes quantized subband mixing. Moreover, the probability density and the spin polarization of interface states in conduction band oscillate mainly inside the well and decay outside, while those in valence band oscillate outside and decay inside. These findings here may provide a further understanding of 3D TI surface states.

Structural and electrical characterizations of nanocrystalline Zn1−xCdxS (0≤x≤0.9) prepared by low cost dip coating

Thin films of nanocrystalline Zn1−xCdxS (0≤x≤0.9) were deposited by a dip coating method on glass substrates from aqueous solution containing cadmium acetate, zinc acetate and thiourea at 200°C. The morphological, structural and electrical properties of the deposited Zn1−xCdxS thin films were studied by atomic force microscopy (AFM), X-ray diffractometer (XRD), selected area electron diffraction (SAED) and photoluminescence (PL). To understand the predominant conduction mechanism of the nanocrystalline Zn1−xCdxS (0≤x≤0.9) thin films, DC electrical conductivity was measured in the temperature range of 300–420K. These measurements revealed that the DC behavior of the films can be described by a one-dimensional variable range hopping (VRH) model in the entire temperature range instead of a three-dimensional variable range hopping (VRH) model. The current density–voltage (J–V) characteristics of Zn1−xCdxS (0≤x≤0.9) thin films shows that the current conduction is ohmic type at the low-voltage region while the charge transport phenomenon appears to be space charge limited current (SCLC) at the higher-voltage regions. The latter conduction is attributed to the presence of a discrete trapping level. Various electrical parameters were determined and studied as a function of Cd-content such as electron mobility (μ0), density of states in conduction band (Nc), Fermi energy (EF), trap energy (Et) and trap electron density (Nt).

Effect of Sensitizer Structure and TiO2 Protonation on Charge Generation in Dye-Sensitized Solar Cells

We report a joint theoretical and experimental investigation on the effect of TiO2 protonation on the interfacial electronic coupling and injection rates in organic dye-sensitized solar cells (DSCs). We model the electronic structure of different organic dye-sensitized TiO2 cluster models at different degrees of surface protonation and experimentally show the enhancement in the photocurrent generation upon the acidic treatment of the substrate. By merging theory and experiments, we elucidate the role of TiO2 protonation on the relative alignment and electronic coupling (injection rates) between the dye’s lowest unoccupied molecular orbital and the semiconductor conduction band states, also in relation to the different electronic structure of the anchored dye (length of conjugation, conjugated vs not conjugated anchoring group). The photocurrent enhancement observed with TiO2 protonation is attributed to a combined effect of both red-shifted absorption of the protonated TiO2 films and to an overall improvement in the interfacial charge generation

Electronic band gap reduction and intense luminescence in Co and Mn ion-implanted SiO2

Cobalt and manganese ions are implanted into SiO2 over a wide range of concentrations. For low concentrations, the Co atoms occupy interstitial locations, coordinated with oxygen, while metallic Co clusters form at higher implantation concentrations. For all concentrations studied here, Mn ions remain in interstitial locations and do not cluster. Using resonant x-ray emission spectroscopy and Anderson impurity model calculations, we determine the strength of the covalent interaction between the interstitial ions and the SiO2 valence band, finding it comparable to Mn and Co monoxides. Further, we find an increasing reduction in the SiO2 electronic band gap for increasing implantation concentration, due primarily to the introduction of Mn- and Co-derived conduction band states. We also observe a strong increase in a band of x-ray stimulated luminescence at 2.75 eV after implantation, attributed to oxygen deficient centers formed during implantation.

The electronic structure of lanthanide doped compounds with 3d, 4d, 5d, or 6d conduction band states

The chemical shift model of electronic binding energies will be applied to the lanthanides in T O2 and MT O3 compounds where T is the cation Ti4+, Zr4+, Ce4+, Hf4+, or Th4+ and M is the alkaline earth cation Ba2+, Sr2+, or Ca2+. As input, data from lanthanide spectroscopy will be used to generate the binding energies of electrons in all lanthanide impurity states and in the valence band and conduction band states of the host compound. In these compounds the bottom of the conduction band has a strong nd-orbital character (n=3, 4, 5, and 6 for titanates, zirconates, hafnates, and thorates, respectively). Electronic structure diagrams are determined that show the valence band and conduction band energy together with all lanthanide impurity level energies relative to the vacuum level. They reveal clear trends when n increases that has profound consequences for the lanthanide luminescence properties.

Matrix elements of intraband transitions in quantum dot intermediate band solar cells: the influence of quantum dot presence on the extended-state electron wave-functions

The intraband transitions which are essential for quantum dot intermediate band solar cells (QD IBSCs) are theoretically investigated by estimating the matrix elements from a ground bound state, which is often regarded as an intermediate band (IB), to conduction band (CB) states for a structure with a quantum dot (QD) embedded in a matrix (a QD/matrix structure). We have found that the QD pushes away the electron envelope functions (probability densities) from the QD region in almost all quantum states above the matrix CB minimum. As a result, the matrix elements of the intraband transitions in the QD/matrix structure are largely reduced, compared to those calculated assuming the envelope functions of free electrons (i.e., plane-wave envelope functions) in a matrix structure as the final states of the intraband transitions. The result indicates the strong influence of the QD itself on the intraband transitions from the IB to the CB states in QD IBSC devices. This work will help in better understanding the problem of the intraband transitions and give new insight, that is, engineering of quantum states is indispensable for the realization of QD IBSCs with high solar energy conversion efficiencies.

First‐principles study on the conduction band electron states of GaAsN alloys

AbstractWe have analyzed the electronic states of the lowest conduction band of GaAsN alloys based on the first‐principles calculation. In order to study the nitrogen concentration dependence of the electron effective mass, we calculated the electronic structure of GaAsN alloys using a full‐potential density functional theory program. The calculated effective mass of GaAsN with a dilute nitrogen concentration is about twice as heavy as that of GaAs. In addition, the effective mass is found to decrease with increasing nitrogen concentration although it is always heavier than that of GaAs. In addition, we calculated the density of states of GaAsN to examine what the cause of the decrease in the effective mass is. Our calculations suggest the discontinuity of the physical property from GaAs to GaAsN and provide an important insight into the formation of the conduction band states of GaAsN alloys. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Nodal ground states and orbital textures in semiconductor quantum dots

Conventional understanding implies that the ground state of a nonmagnetic quantum mechanical system should be nodeless. While this notion also provides a valuable guidance in understanding the ordering of energy levels in semiconductor nanostructures, there are reports that nodal ground states for holes are possible. However, the existence of such nodal states has been debated and even viewed merely as an artifact of a $\mathbit{k}\ifmmode\cdot\else\textperiodcentered\fi{}\mathbf{p}$ model. Using complementary approaches of both $\mathbit{k}\ifmmode\cdot\else\textperiodcentered\fi{}\mathbf{p}$ and tight-binding models, further supported by an effective Hamiltonian for a continuum model, we reveal that nodal ground states in quantum dots are not limited to a specific approach. Remarkably, the emergence of nodal hole states at the top of the valence band can be attributed to the formation of orbital vortex textures through competition between the hole kinetic energy and the coupling to the conduction band states. We suggest an experimental test for our predictions of the reversed energy ordering and the existence of nodal ground states. We discuss how our findings and the studies of orbital textures could be also relevant for other materials systems.

Direct Quantitative Electrical Measurement of Many-Body Interactions in Exciton Complexes in InAs Quantum Dots

We present capacitance-voltage spectra for the conduction band states of InAs quantum dots obtained under continuous illumination. The illumination leads to the appearance of additional charging peaks that we attribute to the charging of electrons into quantum dots containing a variable number of illumination-induced holes. By this we demonstrate an electrical measurement of excitonic states in quantum dots. Magnetocapacitance-voltage spectroscopy reveals that the electron always tunnels into the lowest electronic state. This allows us to directly extract, from the highly correlated many-body states, the correlation energy. The results are compared quantitatively to state of the art atomistic configuration interaction calculations, showing very good agreement for a lower level of excitations and also limitations of the approach for an increasing number of particles. Our experiments offer a rare benchmark to many-body theoretical calculations.

Strain-induced fundamental optical transition in (In,Ga)As/GaP quantum dots

The nature of the ground optical transition in an (In,Ga)As/GaP quantum dot is thoroughly investigated through a million atoms supercell tight-binding simulation. Precise quantum dot morphology is deduced from previously reported scanning-tunneling-microscopy images. The strain field is calculated with the valence force field method and has a strong influence on the confinement potentials, principally, for the conduction band states. Indeed, the wavefunction of the ground electron state is spatially confined in the GaP matrix, close to the dot apex, in a large tensile strain region, having mainly Xz character. Photoluminescence experiments under hydrostatic pressure strongly support the theoretical conclusions.

Correlation between electrochromism and electronic structures of tungsten oxide films

The in-situ X-ray absorption spectroscopy of three tungsten oxide films was performed to study the electronic and atomic structures following repeated cycles of coloration and bleaching processes. The transparent tungsten oxide films become deep blue upon intercalation of Li+ ions in the WO6 octahedra when an external electrical bias was applied. These films reverted to transparent when a reverse external electrical bias was applied. W L3-edge X-ray absorption near-edge structure (XANES) measurements of the nanocrystalline and crystalline tungsten oxide films revealed that the intensity of the white-line feature decreases after coloration and recoverably increases after bleaching owing to the filling and unfilling of the W 5d–O 2p conduction band states. The second derivative of the W L3-edge XANES spectra indicated an increase in structural disordering following repeated cycles of coloration and bleaching. However, the extended X-ray absorption fine structure analysis showed that the nearest-neighbor W–O bond distances in the samples overall remain unchanged by coloration and bleaching. The nanocrystalline tungsten oxide film exhibited more effective recovery (∼97% after first cycle) of the electronic structures than the other two crystalline samples in terms of the filling and unfilling of the W 5d–O 2p conduction band states after repeated coloration and bleaching. These results show that the nanocrystalline tungsten oxide sample has superior electrochromic properties to the crystalline samples.

In situ electronic structural study of VO2 thin film across the metal—insulator transition

The in situ valence band photoemission spectrum (PES) and X-ray absorption spectrum (XAS) at V LII — LIII edges of the VO2 thin film, which is prepared by pulsed laser deposition, are measured across the metal—insulator transition (MIT) temperature (TMIT = 67 °C). The spectra show evidence for changes in the electronic structure depending on temperature. Across the TMIT, pure V 3d characteristic d‖ and O 2p-V 3d hybridization characteristic πpd, σpd bands vary in binding energy position and density of state distributions. The XAS reveals a temperature-dependent reversible energy shift at the V LIII-edge. The PES and XAS results imply a synergetic energy position shift of occupied valence bands and unoccupied conduction band states across the phase transition. A joint inspection of the PES and XAS results shows that the MIT is not a one-step process, instead it is a process in which a semiconductor phase appears as an intermediate state. The final metallic phase from insulating state is reached through insulator—semiconductor, semiconductor—metal processes, and vice versa. The conventional MIT at around the TMIT = 67 °C is actually a semiconductor—insulator transformation point.

Surface State Recombination and Passivation in Nanocrystalline TiO2 Dye-Sensitized Solar Cells

The relative role of surface state recombination in dye-sensitized solar cells is not fully understood, yet reductions in the recombination rate are frequently attributed to the passivation of surface states. We have investigated reports of trap state passivation using an Al2O3-coated TiO2 photoanode achieved through atomic layer deposition (ALD). Electrochemical characterization, performed through impedance measurements and intensity modulated photovoltage spectroscopy (IMVS), data showed that the Al2O3 deposition successfully blocked electron recombination and that the chemical capacitance of the film was unchanged after the ALD treatment. A theoretical model outlining the recombination kinetics was applied to the experimental data to obtain charge transfer rates from conduction band states, exponentially distributed traps, and monoenergetic traps. The determined electron transfer rates showed that the deposited Al2O3 coating did not selectively passivate trap states at the nanoparticle surface but redu...