Discrete-charge quantum circuits and electrical resistance

From the theory of quantum LC circuits with discrete charge, and semiclassical considerations, we obtain approximate energy eigenvalues, depending on the parameter . Next, we include electrical resistance for the quantum RLC circuit, obtaining a relation that strongly reminds us of the Landauer formula.     

Quantum RLC circuits: Charge discreteness and resonance

In a recent article [C.A. Utreras-Díaz, Phys. Lett. A 372 (2008) 5059], we have advanced a semiclassical theory of quantum circuits with discrete charge and electrical resistance. In this work, we present a few elementary applications of this theory. For the zero resistance inductive circuit, we obtain the Stark ladder energies in yet another way; for the circuit driven by a combination d.c. plus a.c. electromotive force (emf) we generalize earlier results by Chandía et al. [K. Chandía, J.C. Flores, E. Lazo, Phys. Lett. A 359 (2006) 693]. As a second application, we investigate the effect of electrical resistance and charge discreteness, in the resonance conditions of a series RLC quantum circuit.     

Probing Field Emission from Boron Carbide Nanowires

High density boron carbide nanowires are grown by an improved carbon thermal reduction technique. Transmission electron microscopy and electron energy lose spectroscopy of the sample show that the synthesized nanowires are B4 C with good crystallization. The field emission measurement for an individual boron nanowire is performed by using a Pt tip installed in the focused ion beam system. A field emission current with enhancement factor of 10^6 is observed and the evolution process during emission is also carefully studied. Furthermore, a two-step field emission with stable emission current density is found from the high-density nanowire film. Our results together suggest that boron carbide nanowires are promising candidates for electron emission nanodevices.     

Kicked quantum circuits with charge discreteness: Resonance condition

In this work we study a quantum electrical circuit with charge discreteness perturbed by periodic external kicks. Time evolution equations, for energy and electrical current, are solved analytically. Time evolution fluctuations are also studied and they become bounded. Resonances are well characterized including arbitrary (generic) quantum circuits with charge discreteness.     

Tunnel barrier and noncollinear magnetization effects on shot noise in ferromagnetic/semiconductor/ferromagnetic heterojunctions

Based on the scattering approach, we investigate transport properties of electrons in a one-dimensional waveguide that contains a ferromagnetic/semiconductor/ferromagnetic heterojunction and tunnel barriers in the presence of Rashba and Dresselhaus spin-orbit interactions. We simultaneously consider significant quantum size effects, quantum coherence, Rashba and Dresselhaus spin-orbit interactions and noncollinear magnetizations. It is found that the tunnel barrier plays a decisive role in the transmission coefficient and shot noise of the ballistic spin electron transport through the heterojunction. When the small tunnel barriers are considered, the transport properties of electrons are quite different from those without tunnel barriers.     

Equilibrium spin current induced by spin-orbital interaction in a quantum dot system

We report a theoretical study of the equilibrium spin current flowing in a quantum dot system. Two electrodes are the two-dimensional electron gas with Rashba or Dresselhaus spin-orbital interaction. By using the Keldysh Green's function technique, we demonstrated that a nonzero spin current can flow in the system without bias. At the weak coupling between electrodes and the quantum dot, the spin current is approximately proportional to the cross product of two average pseudo-magnetizations in two electrodes, which agrees with the result of the linear response theory; whereas at the opposite case, the strong coupling between the quantum dot and electrodes can lead to a non-sinusoidal behavior of the equilibrium spin current. These behaviors of the equilibrium spin current are similar to the Josephson current.     

Intradot spin-flip Andreev reflection tunneling through a ferromagnet-quantum dot-superconductor system with ac field

We investigate Andreev reflection (AR) tunneling through a ferromagnet-quantum dot-superconductor (F-QD-S) system in the presence of an external ac field. The intradot spin-flip scattering in the QD is involved. Using the nonequilibrium Green function and BCS quasiparticle spectrum for superconductor, time-averaged AR conductance is formulated. The competition between the intradot spin-flip scattering and photon-assisted tunneling dominates the resonant behaviors of the time-averaged AR conductance. For weak intradot spin-flip scattering strengths, the AR conductance shows a series of equal interval resonant levels. However, the single-peak at main resonant level develops into a well-resolved double-peak resonance at a strong intradot spin-flip scattering strength. Remarkable, multiple-photon-assisted tunneling that generates photonic sideband peaks with a variable interval has been found. In addition, the AR conductance-bias voltage characteristic shows a transition between the single-peak to double-peak resonance as the ratio of the two tunneling strengths varies.     

Voltage-controllable spin-polarized transport through parallel-coupled double quantum dots with Rashba spin-orbit interaction

A spin device, consisting of parallel-coupled double quantum dots and three normal metal leads, is proposed to realize spin-polarized current without the help of magnetic field and magnetic material. Based on the Keldysh nonequilibrium Green function technique and equation of motion method, the spin-dependent current formula in each lead is derived. It is shown that not only a fully polarized current but also a tunable pure spin current can be obtained by modulating the structure parameters, strength of Rashba spin-orbit interaction and bias voltages properly. It further demonstrates the dependence of the spin-polarized current on the strength of the Rashba spin-orbit interaction.     

An alternative mechanism for current-induced antisymmetric lateral edge spin accumulations in ballistic two-dimensional electron gases

In this Letter an alternative mechanism is proposed for current-induced antisymmetric lateral edge spin accumulations in thin strips of ballistic two-dimensional electron gases with intrinsic spin-orbit coupling. In this mechanism, the occurrence of current-induced antisymmetric lateral edge spin accumulations in a semiconductor strip is not due to a transverse spin current but originates from the combined action of the spin-orbit coupling, the boundary confinement on both lateral edges of the strip, and the time-reversal symmetry-breaking caused by the longitudinal charge current circulating through the strip. The results obtained in this Letter indicate that, the occurrence of current-induced antisymmetric lateral edge spin accumulations in a thin strip of a spin-orbit coupled two-dimensional electronic system does not need to be associated necessarily with a transverse spin current in principle.     

Spin-polarized current through a lateral double quantum dot with spin–orbit interaction

We study the spin-polarized current through a vertical double quantum dot scheme. Both the Rashba spin–orbit (RSO) interaction inside one of the quantum dots and the strong intradot Coulomb interactions on the two dots are taken into account by using the second-quantized form of the Hamiltonian. Due to the existence of the RSO interaction, spin-up and spin-down electrons couple to the external leads with different strengths, and then a spin polarized current can be driven out of the middle lead by controlling a set of structure parameters and the external bias voltage. Moreover, by properly adjusting the dot levels and the external bias voltages, a pure spin current with no accompanying charge current can be generated in the weak coupling regime. We show that the difference between the intradot Coulomb interactions strongly influences the spin-polarized currents flowing through the middle lead and is undesirable in the generation of the net spin current. Based on the RSO interaction, the structure we propose can efficiently polarize the electron spin without the usage of any magnetic field or ferromagnetic material. This device can be used as a spin-battery and is realizable using the present available technologies.     

Spin Polarization and Andreev Conductance through a Diluted Magnetic Semiconductor Quantum Wire with Spin--Orbit Interaction

Spin-dependent Andreev reflection and spin polarization through a diluted magnetic semiconductor quantum wire coupled to normal metallic and superconductor electrodes are investigated using scattering theory. When the spin-orbit coupling is considered, more Andreev conductance steps appear at the same Fermi energy. Magnetic semiconductor quantum wire separates the spin-up and spin-down electrons. The Fermi energy, at which different- spin-state electrons begin to separate, becomes lower due to the effect of the spin-orbit interaction. The spin filter effect can be measured more easily by investigating the Andreev conductance than by investigating the normal conductance.     

Antiresonance of electron tunneling through a four-quantum-dot ring with two side-coupled quantum dots

Making use of the equation of motion method and Keldysh Green function technique, we obtain the current formula for a two-terminal four-quantum-dot-ring with two side-coupled quantum dots under a DC bias voltage. Antiresonance and resonance of electron tunneling is studied by numerical calculations. Only when the quantum dots in the ring has the same single electron energy level with that of the side-coupled quantum dots, i.e. and both side-coupling are turned on at the same time, the antiresonance appear exactly at ε₀.     

Surface characterization study of a Pd/Co3O4 methane oxidation catalyst

A surface characterization study using X-ray photoelectron spectroscopy (XPS) and ion scattering spectroscopy (ISS) has been performed on a 5 wt.% Pd/Co₃O₄ methane oxidation catalyst before and after exposure to a mixture of CH₄ and O₂ in N₂ at 250 °C for a period of 6 days. The primary peaks observed in the XPS survey spectra are the Co 2p, Pd 3d, O 1s and C 1s, along with Co, Pd and O Auger peaks. High-resolution Pd 3d spectra reveal that Pd exists on the surface predominantly as PdO, with no apparent change in chemical state during reaction. High-resolution XPS Co 2p and O 1s spectra reveal an accumulation of CoOOH and a depletion of CoO in the near-surface region during reaction. ISS analysis with intermittent 1-keV Ar⁺ sputtering was used to obtain depth profiles from the catalyst before and after reaction. The results indicate that the Pd/Co concentration ratio decreases with sputtering and that this ratio is larger for the as-prepared catalyst indicating that morphological changes occur during reaction. The ISS depth profile spectra obtained from the catalyst after reaction indicates the presence of an oxyhydroxide layer throughout the near-surface region. This observation is consistent with the XPS data indicating accumulation of hydroxide and oxyhydroxide species at the surface during reaction.Based on these data and the results of related studies, a reaction mechanism is proposed. In this mechanism, methane dissociatively chemisorbs to form a surface methoxy species and CoOOH. The remaining hydrogen atoms are stripped from the methoxy species leaving an active adsorbed C species which reacts with surface oxygen and a hydroxyl group to form an adsorbed bicarbonate ion which then decomposes to form CO₂ and a surface hydroxyl group. These hydroxyl groups also react to form H₂O and then more O₂ adsorbs dissociatively at the vacant sites.     

Laser excess noise and interferometric effects in frequency-modulated diode-laser spectrometers

Tunable Diode-Laser Absorption Spectroscopy (TDLAS) is increasingly being used to measure trace-gas concentrations down to low part-per-billion levels (1 ppbv = 10^–9 volume mixing ratio). Semiconductor lead-salt diode lasers give access to the mid-infrared spectral region and the application of high-Frequency Modulation (FM) schemes can further improve the sensitivity and detection speed of modern instrumentation. Several factors influence or even limit spectrometer performance. The central elements in such spectrometers are lead-salt diode lasers. Experimental data will be presented, which demonstrate that high-frequency excess-noise contributions above several MHz can be attributed to mode hopping and mode partition noise during multimode laser operation. Additionally it will be discussed how a FM-TDLAS spectrometer can be interpreted as an optimized Michelson interferometer for absolute distance measurements and, therefore, is extremely sensitive towards drift effects. The higher the modulation frequency, the higher is the drift sensitivity of the spectrometer due to interferometric effects. These drift effects are a second factor affecting ultrasensitive measurements. While wideband-laser noise characteristics call for high modulation frequencies, the aforementioned interferometric effects in the spectrometer require low modulation frequencies.     

Parallel keyed hash function construction based on chaotic maps

Recently, a variety of chaos-based hash functions have been proposed. Nevertheless, none of them works efficiently in parallel computing environment. In this Letter, an algorithm for parallel keyed hash function construction is proposed, whose structure can ensure the uniform sensitivity of hash value to the message. By means of the mechanism of both changeable-parameter and self-synchronization, the keystream establishes a close relation with the algorithm key, the content and the order of each message block. The entire message is modulated into the chaotic iteration orbit, and the coarse-graining trajectory is extracted as the hash value. Theoretical analysis and computer simulation indicate that the proposed algorithm can satisfy the performance requirements of hash function. It is simple, efficient, practicable, and reliable. These properties make it a good choice for hash on parallel computing platform.     

Effects of Magnet Size and Geometry on Magnetic Levitation Force

We obtain analytical relations for the levitation force as a function of dimensions of the superconductor-magnet system. The force has been calculated on the basis of the dipole-dipole interaction model. The effect of thickness of the superconductor on the levitation force is investigated. The results show that the influence of geometry and thickness of the magnet becomes significantly large at small levitation distances. Furthermore, approximating the permanent magnet as a point dipole results in an inaccurate estimation of the levitation force.     

Frequency- and wavelength-modulation spectroscopies: Comparison of experimental methods using an AlGaAs diode laser

Low-wavelength modulation (1 kHz), high-wavelength modulation (100 MHz) and two-tone frequency modulation (390±5 MHz) spectroscopies are systematically compared by measuring the minimum detectable absorption achieved using an AlGaAs diode laser tuned on a third-overtone methane transition at 886 nm. From the S/N behavior has been extrapolated a minimum relative absorption (1 Hz of bandwidth) of 4.5(1)×10^–7 for the LMW, 9.7(3)×10^–8 for the HWM and 6.4(2)×10^–8 for the TTFM. In the LWM case the detection-limit value is represented by the laser amplitude 1/f excess noise, while for the high-frequency detection techniques this contribution is negligible with respect to other noise sources. These detection limits well agree with the calculated quantum limited values based on measured laser power, modulation index, noise figure of the electronic components, and other parameters of the apparatus.     

Cryptanalysis on a parallel keyed hash function based on chaotic maps

This Letter analyzes the security of a novel parallel keyed hash function based on chaotic maps, proposed by Xiao et al. to improve the efficiency in parallel computing environment. We show how to devise forgery attacks on Xiao's scheme with differential cryptanalysis and give the experiment results of two kinds of forgery attacks firstly. Furthermore, we discuss the problem of weak keys in the scheme and demonstrate how to utilize weak keys to construct collision.     

Effects of disorder on tunnel magnetoresistance in Fe/MgO/Fe junctions

Effects of lattice distortion and oxygen vacancy on tunnel magnetoresistance in Fe/MgO/Fe junctions are theoretically investigated. By treating the distortion in MgO as the random potential and performing numerical simulations based on the Kubo–Landauer formula, it is shown that the magnetoresistance ratio decreases with increasing randomness. Moreover, first-principles calculations within the density functional theory show that the defect states in the Fe/MgO cluster containing an oxygen vacancy induce no significant shift in the Fermi level.