Transport properties of a double quantum dot molecule in the presence of impurity effects

In this Letter, we studied the electronic transport through a parallel-coupled double quantum dot (DQD) molecule including impurity effects at zero temperature. The linear conductance can be calculated by using the Green's function method. An obvious Fano resonance arising from the impurity state in the quantum dot is observed for the symmetric dot-lead coupling structure in the absence of the magnetic flux through the quantum device. When the magnetic flux is presented, two groups of conductance peaks appear in the linear conductance spectra. Each group is decomposed into one Breit-Wigner and one Fano resonances. Tuning the system parameters, we can control effectively the shapes of these conductance peaks. The Aharonov-Bohm (AB) oscillation for the magnetic flux is also studied. The oscillation period of the linear conductance with π, 2π or 4π may be observed by tuning the interdot tunneling coupling or the dot-impurity coupling strengths.     

Anomalous Aharonov-Bohm conductance oscillations from topological insulator surface states

We study Aharonov-Bohm (AB) conductance oscillations arising from the surface states of a topological insulator nanowire, when a magnetic field is applied along its length. With strong surface disorder, these oscillations are predicted to have a component with anomalous period Φ(0)=hc/e, twice the conventional period. The conductance maxima are achieved at odd multiples of 1/2Φ(0), implying that a π AB phase for electrons strengthens the metallic nature of surface states. This effect is special to topological insulators, and serves as a defining transport property. A key ingredient, the surface curvature induced Berry phase, is emphasized here. We discuss similarities and differences from recent experiments on Bi2Se3 nanoribbons, and optimal conditions for observing this effect.     

Surface state density distribution at vacuum-annealed InP(1 0 0) surface as derived from the rigorous analysis of photoluminescence efficiency

The surface state density distribution N_SS(E) and surface Fermi level E_Fs position on a sequentially ultra-high vacuum-annealed n-InP(1 0 0) sample are investigated using rigorous computer analysis of dependences of the room temperature band-edge photoluminescence efficiency (Y_PL) on the photon flux density (Φ). We have found that the minimum density of a U-shaped N_SS(E) distribution as well as the donor-like surface state density are reduced by one order of magnitude after the annealing at 250 and 300 °C. This can be assigned to the decrease in the disorder in the unintentionally formed InP native oxides. On the other hand, we demonstrate that the annealing simultaneously generates discrete surface states probably due to missing group V element (P) in the interface region which may account for electrical interface instabilities observed in InP-based devices. The results are discussed quantitatively and compared to other reports.     

Aharonov-Bohm oscillations in disordered topological insulator nanowires

A direct signature of electron transport at the metallic surface of a topological insulator is the Aharonov-Bohm oscillation observed in a recent study of Bi2Se3 nanowires [Peng, Nature Mater. 9, 225 (2010)] where conductance was found to oscillate as a function of magnetic flux ? through the wire, with a period of one flux quantum ?0=h/e and maximum conductance at zero flux. This seemingly agrees neither with diffusive theory, which would predict a period of half a flux quantum, nor with ballistic theory, which in the simplest form predicts a period of ?0 but a minimum at zero flux due to a nontrivial Berry phase in topological insulators. We show how h/e and h/2e flux oscillations of the conductance depend on doping and disorder strength, provide a possible explanation for the experiments, and discuss further experiments that could verify the theory.     

Spin-polarization-dependent transport in a quantum dot array coupled with an Aharonov-Bohm ring

In this paper the quantum transport in a dot-array coupled with an Aharonov–Bohm (AB) ring is investigated via single-band tight-binding Hamiltonian. It is shown that the output spin current is a periodic function of the magnetic flux in the quantum unit Φ₀. The resonance positions of the total transmission probability do not depend on the size of the AB ring but the electronic spectrum. Moreover, the persistent currents in the AB ring is also spin-polarization dependent and different from the isolated AB ring where the persistent current is independent of spin polarization.     

Dependence of the magnitude and direction of the persistent current on the magnetic flux in superconducting rings

The obtained periodic magnetic-field dependences I _c+(Φ/Φ₀) and I _c?(Φ/Φ₀) of the critical current measured in opposite directions on asymmetric superconducting aluminum rings has made it possible to explain previously observed quantum oscillations of dc voltage as a result of alternating current rectification. It was found that a higher rectification efficiency of both single rings and ring systems is caused by hysteresis of the current-voltage characteristics. The asymmetry of current-voltage characteristics providing the rectification effect is due to the relative shifts of the magnetic dependences I _c?(Φ/Φ₀) = I _c+(Φ/Φ₀ + Δ?) of the critical current measured in opposite directions. This shift means that the position of I _c+(Φ/Φ₀) and I _c?(Φ/Φ₀) minima does not correspond to n + 0.5 magnetic flux Φ quanta, which is in direct contradiction to measured Little-Parks resistance oscillations. Despite this contradiction, the amplitude I _{c, an}(Φ/Φ₀) = I _c+(Φ/Φ₀) ? I _c?(Φ/Φ₀) of critical current anisotropy oscillations and its variations with temperature correspond to the expected amplitude of persistent current oscillations and its variations with temperature.     

AB效应对自旋多端输运的影响

利用紧束缚近似和格林函数方法,研究了AB效应和AB环对电子自旋输运的影响.计算表明,当在AB环的不同位置上连接相同或不同属性的输出端时,在一些能量范围内,由不同的输出端所输出的自旋流的方向是相反的;当固定入射电子的能量时,在同一磁通范围,从两个输出端输出的自旋流属性也是相反的.从而,可以通过控制AB环的结构和环内的磁通在输出端得到不同属性的自旋流. 关键词： 自旋极化输运 量子点 极化率 自旋流     

Neutral and charged excitons in a quantum tube

The optical transition energies of neutral and charged excitons in a quantum tube are calculated as a function of the Aharonov-Bohm magnetic flux Φ. The oscillation amplitude of the ground state energy of the electron-hole relative motion is shown to be larger in a quantum tube than a quantum ring with strong confinement in the axis direction. We find a double maxima structure in the optical transition energy for a quantum tube with radius R = 0.5 in units of the effective Bohr radius because of the difference in the Φ dependencies between the single electron energy and the relative-motion energy of a charged exciton state.     

Spin splitter regime of a mesoscopic Rashba ring

Using the non-equilibrium Greens' function formalism we calculate the spin currents in a one-dimensional ring coupled to three leads and in the presence of perpendicular magnetic flux Φ and Rashba spin-orbit coupling. A finite bias is applied between the input lead and the other two output leads. We show that the spin-orbit coupling allows one to operate this system as a spin splitter, i.e. the output leads deliver spin-polarized currents with different orientations. We find that the spin splitter operation can be tuned at integer multiples of Φ/Φ₀. Its efficiency depends not only on the value of the Rashba coupling but also on the bias applied between the input and output leads. The selected spin orientation of the output leads can be reversed by a slight change of their contact position. We discuss as well the connection between the spin splitter operation and the spectral properties of the ring.     

Messung der kontinuierlichen Phasenschiebung von Elektronenwellen im kraftfeldfreien Raum durch das magnetische Vektorpotential einer Wolfram-Wendel

An electron interferometer with three biprism fibres is described in which a continuously variable magnetic flux in a long solenoid coil of less than 20 μ diameter can be enclosed by two coherent parts of an electron beam. The partial beams can be separated up to distances of 60 μ without violating the coherence condition if the electron line source is made as narrow as about 100 Å units. The continuous phase shifting action of the enclosed flux on electron waves in a field free space can be demonstrated, and it is found that at normal temperature there is no quantization of magnetic flux in quanta of the orderΦ ₀=h/e=4,13·10^?7 Gauss cm². The flux required to cause a phase difference of 2π is measured and found to coincide with the theoretical valueΦ ₀.     

The laser-induced fluorescence spectroscopy of TiF in the ultraviolet region

The laser-induced fluorescence (LIF) spectrum of jet-cooled ⁴⁸TiF has been obtained in the wavelength region of 245-270 nm for the first time. Six pairs of vibronic bands were observed and assigned to two new transitions [37.8]⁴Φ-X⁴Φ and ⁴Δ-X⁴Φ. Rotational analysis was carried out for the (ν′ = 0-3 to ν″ = 0) vibrational bands of the [37.8]⁴Φ_3/2-X⁴Φ_3/2 and [37.8]⁴Φ_5/2-X⁴Φ_5/2 subbands, and also, the (ν′, 0) and (ν′+1, 0) vibrational bands of the ⁴Δ_1/2-X⁴Φ_3/2 and ⁴Δ_3/2-X⁴Φ_5/2 subbands. The effective equilibrium molecular constants for the [37.8]⁴Φ_3/2 and [37.8]⁴Φ_3/2 upper states were determined. In addition, lifetime measurements were carried out for all of the observed bands under collision-free conditions. On the basis of the spectroscopic constants and lifetime measurements, the electronic transitions involved in the observed high-lying electronic states are discussed.     

The [20.4]Φ→(1)Δ Green System of LaF

Electronic bands belonging to the [20.4]³Φ→(1)³Δ system have been observed in the emission spectrum of lanthanum monofluoride at high temperature in the presence of the Ar⁺ 457.9-nm laser line: the (0-0), (1-1), (2-2), and (3-3) bands of the ³Φ₄→³Δ₃ subsystem and the (0-0) bands of the ³Φ₃→³Δ₂ and ³Φ₂→³Δ₁ subsystems. Rotational structures which are well developed in these experimental conditions were analyzed. The data were treated using a polynomial expression for the upper levels term-values, directly at equilibrium for ³Φ₄, whereas a previous Hamiltonian matrix representation of the (1)³Δ state was assumed. A Hund's case (a) representation of the [20.4]³Φ state at v=0 was also considered. Perturbations affecting ³Φ₂ levels at v=0 were observed; the energy and rotational constant of the perturbing state and the interaction parameter were estimated.     

Forward and reverse bias current-voltage characteristics of Au/n-Si Schottky barrier diodes with and without SnO2 insulator layer

The effects of interfacial insulator layer, interface states (N_ss) and series resistance (R_s) on the electrical characteristics of Au/n-Si structures have been investigated using forward and reverse bias current-voltage (I-V) characteristics at room temperature. Therefore, Au/n-Si Schottky barrier diodes (SBDs) were fabricated as SBDs with and without insulator SnO₂ layer to explain the effect of insulator layer on main electrical parameters. The values of ideality factor (n), R_s and barrier height (Φ_Bo) were calculated from ln(I) vs. V plots and Cheung methods. The energy density distribution profile of the interface states was obtained from the forward bias I-V data by taking bias dependence of ideality factor, effective barrier height (Φ_e) and R_s into account for MS and MIS SBDs. It was found that N_ss values increase from at about mid-gap energy of Si to bottom of conductance band edge of both SBDs and the MIS SBD’s N_ss values are 5-10 times lower than those of MS SBD’s. An apparent exponential increase from the mid-gap towards the bottom of conductance band is observed for both SBDs’ (MS and MIS) interface states obtained without taking R_s into account.     

Near infrared emission spectra of CoH and CoD

New high resolution emission spectra of CoH and CoD molecules have been recorded in the 640 nm to 3.5 μm region using a Fourier transform spectrometer. The bands were excited in a carbon tube furnace by the reaction of cobalt metal vapor and a mixture of H₂ or D₂ with He at a temperature of about 2600 °C. Eight bands were observed for the A′³Φ₄-X³Φ₄ electronic transition of CoD, and five bands for the corresponding transition of CoH. The (0, 0) bands of the A′³Φ₃-X³Φ₃ system were also recorded for both isotopologues, although one of the parity components in the X³Φ₃ sub-state of CoH was found to be perturbed. The A′³Φ₃-X³Φ₄ transition was also observed in our spectrum of CoH. In addition, a new [13.3]4 electronic state was found by observing [13.3]4-X³Φ₃ and [13.3]4-X³Φ₄ transitions in the spectrum of CoD. Analysis of the transitions with ΔΩ = 0, ± 1 provided more accurate values of spin-orbit splittings between Ω = 4 and Ω = 3 components. The ground-state data for both molecules were fitted both to band-constant and Dunham-expansion expressions, and a combined-isotopologue analysis of the X³Φ₄ spin component was carried out using the data for CoH and CoD. The upper states were represented by term values in these analyses because of perturbations, but estimated band constants for them were obtained in separate fits in which ground-state constants were held fixed.     

The transition from surface to bulk oxide growth on Pt(1 0 0): Precursor-mediated kinetics

We investigated the kinetics governing the transition from surface (2D) to bulk (3D) oxide growth on Pt(1 0 0) in ultrahigh vacuum as a function of the surface temperature and the incident flux of an oxygen atom beam. For the incident fluxes examined, the bulk oxide formation rate increases linearly with incident flux (Φ_O) as the oxygen coverage increases to about 1.7 ML (monolayer) and depends only weakly on the surface temperature in the limit of low surface temperature (T_S < 475 K). In contrast, in the high temperature limit (T_S > 525 K), the bulk oxide formation rate increases with for oxygen coverages as high as 1.6 ML, and decreases with increasing surface temperature. We show that the measured kinetics is quantitatively reproduced by a model which assumes that O atoms adsorb on top of the 2D oxide, and that this species acts as a precursor that can either associatively desorb or react with the 2D oxide to form a 3D oxide particle. According to the model, the observed change in the flux and surface temperature dependence of the oxidation rate is due to a change in the rate-controlling steps for bulk oxide formation from reaction at low temperature to precursor desorption at high temperature. From analysis of flux-dependent uptake data, we estimate that the formation rate of a bulk oxide nucleus has a fourth-order dependence on the precursor coverage, which implies a critical configuration for oxide nucleus formation requiring four precursor O atoms. Considering the similarities in the development of surface oxides on various transition metals, the precursor-mediated transition to bulk oxide growth reported here may be a general feature in the oxidation of late transition metal surfaces.     

Commensurate photon-irradiated Fano-Kondo tunneling through a quantum dot embedded Aharonov-Bohm interferometer

The commensurate photon-irradiated mesoscopic transport in a strongly correlated quantum dot (QD) embedded Aharonov-Bohm (AB) interferometer has been investigated. We focus our investigation on the dynamic Kondo and Fano cooperated effect affected by the double commensurate MWFs with q=ω₂/ω₁ being an arbitrary integer, where ω₁ and ω₂ are the two frequencies of the fields. The general tunneling current formula is derived by employing the nonequilibrium Green's function technique, and the different photon absorption and emission processes induced nonlinear properties have been studied to compare with the single-field system where q=0. Our numerical calculations are performed for the special cases with two commensurate fields possessing q=1,2. The Kondo peak can be suppressed to be a Kondo valley for the case where the commensurate number q=1, and the Fano asymmetric structure exhibits in the differential conductance quite evidently. Different commensurate number q contributes different photon absorption and emission effects. However, the conductance for the case of q=2 possesses more peaks and heavier asymmetric structure than the situations of q=0,1. The enhancement of satellite peaks behaves quite differently for the two cases with q=1, and q=2. The asymmetric peak-valley structure is adjusted by the gate voltage, commensurate MWFs, AB flux, source-drain bias, and non-resonant tunneling strength to form novel Fano and Kondo resonant tunneling.     

Charge and spin currents tunnelling through a toroidal carbon nanotube side-coupled with a quantum dot

We have investigated the mesoscopic transport through the system with a quantum dot (QD) side-coupled to a toroidal carbon nanotube (TCN) in the presence of spin-flip effect. The coupled QD contributes to the mesoscopic transport significantly through adjusting the gate voltage and Zeeman field applied to the QD. The compound TCN-QD microstructure is related to the separate subsystems, the applied external magnetic fields, as well as the combination of subsystems. The spin current component I_z^s is independent on time, while the spin current components I_x^s and I_y^s evolve with time sinusoidally. The rotating magnetic field induces novel levels due to the spin splitting and photon absorption procedures. The suppression and enhancement of resonant peaks, and semiconductor-metal phase transition are observed by studying the differential conductance through tuning the source-drain bias and photon energy. The magnetic flux induces Aharonov-Bohm oscillation, and it controls the tunnelling behavior due to adjusting the flux. The Fano type of multi-resonant behaviors are displayed in the conductance structures by adjusting the gate voltage V_g and the Zeeman field applied to the QD.     

Studying of barrier height and ideality factor relation in the nano sized Au-n type Si Schottky diodes

The results of formation of the operating potential barrier height (Φ_в) of inhomogeneous Schottky diodes (SD) in view of an additional electric field in the near contact region of the semiconductor and features of its dependence on the external applied voltage are presented. A correlation, between SD heterogeneity and dependence between potential barrier height (Φ_в) and ideality factor (n), is presented. Using conducting probe atomic force microscope (CP-AFM) techniques, it is shown that Au/n-Si diodes consist of sets of parallel-connected and cooperating nano diodes with the contact surfaces sizes in the order of 100-200 nm. The effective Φ_в and ideality factors of the SD have been obtained from the current-voltage (I-V) characteristics, which were measured using a CP-AFM along a contact surface. It was experimentally shown that the forward and reverse part of I-V characteristics and their effective Φ_в and ideality factors of the identically fabricated nano-SD differ from diode to diode. The Φ_в for the nano-SD has ranged from 0.565 to 0.723 eV and ideality factor from 1.11 to 1.98. No correlation can be found between the Φ_в and ideality factor. The Φ_в distribution obtained from the I-V characteristics has been fitted by a Gaussian function but the ideality factor distribution could not be fitted by a Gaussian function.     

STM spectroscopy of an organic superconductor

Electron tunneling spectroscopy of the organic superconductor κ-(BEDT-TTF)₂Cu(NCS)₂using low temperature scanning tunneling microscope (STM) is reported. The tunneling differential conductance in the superconducting phase was obtained in theb–cplane of a single crystal, by varying the tip position on the sample surface. The differential conductance is reduced near zero bias voltage and enhanced at the gap edge, associated with the superconducting gap structure below[formula] K. The gap width differs slightly from sample to sample, while the overall functional shape of the conductance is sample-independent. The tunneling conductance is reduced to almost zero near zero bias voltage, while it is finite inside the gap edge. The curve obtained cannot be fit to the BCS density of states withs-wave pairing symmetry, even if the life-time broadening of one-electron levels is taken into account. Finite conductance inside the gap edge suggests anisotropy of the gap. However, the conductance curve obtained is not explained by a simpled-wave symmetry for Δ(k). The reduced conductance near zero bias voltage suggests a finite gap. An anisotropic model with a finite gap, in which Δ(k) varies depending on the direction ink-space, is examined. The tunneling conductance in the low-energy region is almost fit by the model with Δ_min = 2 meV and Δ_max = 6 meV. The finite conductance is explained by introducing a small effect of life time broadening. We conclude that the gap is anisotropic and is finite (at least Δ_min = 2 meV) on the entire Fermi surface.