Scattering properties of an impedance-matched, ideal, homogeneous, causal "left-handed" sphere

The plane-wave scattering properties of a sphere of material having an ideal, homogeneous, and causal permittivity epsilon(f), and permeability mu(f) were investigated through detailed three-dimensional finite-difference time-domain, method-of-moments, and series-solution simulations. A Lorentzian functional form was chosen for epsilon(f) and mu( f), as it yields causal responses and allows us to study the physics of the left-handed-medium (LHM) regime. Our interest lies mainly in the frequency range where negative refraction [Re(n) < 0] is observed. We found that when operating in the LHM regime, an impedance-matched sphere responds with scattering features strikingly different from those found in ordinary materials. In particular, we found zero back-scattering and forward scattering that exceeds that of a metal sphere of similar size. The equality of E- and H-plane patterns was proved analytically and numerically, and the possibility of internal subwavelength focusing with a zero index sphere is also reported.     

Gate-defined quantum dots in intrinsic silicon

We report the fabrication and measurement of silicon quantum dots with tunable tunnel barriers in a narrow-channel field-effect transistor. Low-temperature transport spectroscopy is performed in both the many-electron ( approximately 100 electrons) regime and the few-electron ( approximately 10 electrons) regime. Excited states in the bias spectroscopy provide evidence of quantum confinement. These results demonstrate that depletion gates are an effective technique for defining quantum dots in silicon.     

Tuning Correlation Effects with Electron–Phonon Interactions

We investigate the effect of tuning the phonon energy on the correlation effects in models of electron–phonon interactions using DMFT. In the regime where itinerant electrons, instantaneous electron–phonon driven correlations and static distortions compete on similar energy scales, we find several interesting results including (1) A crossover from band to Mott behavior in the spectral function, leading to hybrid band/Mott features in the spectral function for phonon frequencies slightly larger than the band width. (2) Since the optical conductivity depends sensitively on the form of the spectral function, we show that such a regime should be observable through the low frequency form of the optical conductivity. (3) The resistivity has a double kondo peak arrangement.     

Causes of Weak-localization of Electrons on Liquid Helium

We have investigated the dephasing processes in a weak localization (WL) experiment of a two-dimensional electron system on liquid helium. From low-field magnetoconductivity measurements we can separate the damping of WL on the dephasing of electrons due to electron–electron interaction and the motion of the helium vapour atoms. We observe an intermediate regime where both damping mechanisms are of comparable importance and determine the cross-over from one dominant regime to the other.     

Time-resolved detection of single-electron interference

We demonstrate real-time detection of self-interfering electrons in a double quantum dot embedded in an Aharonov-Bohm interferometer, with visibility approaching unity. We use a quantum point contact as a charge detector to perform time-resolved measurements of single-electron tunneling. With increased bias voltage, the quantum point contact exerts a back-action on the interferometer leading to decoherence. We attribute this to emission of radiation from the quantum point contact, which drives noncoherent electronic transitions in the quantum dots.     

On the current delivery limit of semiconducting carbon nanotubes

The current carrying capacity of single-walled semiconducting carbon nanotubes (CNTs) is studied by self-consistent quantum simulations using the non-equilibrium Green’s function formalism with the self-consistent Born approximation. The simulation shows that the current carrying capacity depends on the bias regime and is drastically different from that of metallic tubes. For long CNTs (with a length much longer than zone boundary and optical phonon scattering mean free path), the current saturates around 20 μA in the forward bias regime with unipolar transport due to phonon scattering. In ambipolar transport regime, the current delivery limit is still about 20 μA due to recombination of electron and hole currents. In contrast, for short semiconducting CNTs, the current delivery capacity can be above 25 μA in the unipolar transport regime and further double in the ambipolar transport regime. In reverse bias regime, the current of a long CNT can exceed 20 μA due to the second subband conduction and increased electron injection from the drain. The simulation provides a coherent explanation to the dependence of current delivery limit on bias regime and channel length, which is consistent with recent experiments.     

Few electron double quantum dots in InAs/InP nanowire heterostructures

We report on fabrication of double quantum dots in catalytically grown InAs/InP nanowire heterostructures. In the few-electron regime, starting with both dots empty, our low-temperature transport measurements reveal a clear shell structure for sequential charging of the larger of the two dots with up to 12 electrons. The resonant current through the double dot is found to depend on the orbital coupling between states of different radial symmetry. The charging energies are well described by a capacitance model if next-neighbor capacitances are taken into account.     

Electroluminescence from a single-nanocrystal transistor

We report the fabrication and characterization of light-emitting transistors incorporating individual cadmium selenide (CdSe) nanocrystals. Electrical measurements conducted at low bias voltage and low temperature show clear evidence of Coulomb blockade behavior, indicating that electrons pass through the nanocrystal by single-electron tunneling. Once the bias voltage exceeds the band gap of CdSe, devices with asymmetric tunnel barriers emit linearly polarized light. Combined analyses of the electrical and optical data indicate that the tunnel couplings between the nanorod and the metallic electrodes change significantly as a function of bias voltage and light emission results from the inelastic scattering of tunneling electrons.     

Effect of electron-vibration interactions on the thermoelectric efficiency of molecular junctions

From first-principles approaches, we investigate the thermoelectric efficiency of a molecular junction where a benzene molecule is connected directly to the platinum electrodes. We calculate the thermoelectric figure of merit ZT in the presence of electron-vibration interactions with and without local heating under two scenarios: linear response and finite bias regimes. In the linear response regime, ZT saturates around the electrode temperature T(e) = 25 K in the elastic case, while in the inelastic case we observe a non-saturated and a much larger ZT beyond T(e) = 25 K attributed to the tail of the Fermi-Dirac distribution. In the finite bias regime, the inelastic effects reveal the signatures of the molecular vibrations in the low-temperature regime. The normal modes exhibiting structures in the inelastic profile are characterized by large components of atomic vibrations along the current density direction on top of each individual atom. In all cases, the inclusion of local heating leads to a higher wire temperature T(w) and thus magnifies further the influence of the electron-vibration interactions due to the increased number of local phonons.     

FFEA电场及电子轨迹模拟讨论

采用有限差分法 ,对有聚焦极的垂直双门结构的场发射阵列进行了轴对称三维模拟计算 ,得到发射微尖附近的电位和电子轨迹分布图。分别讨论了聚焦极电位、聚焦极孔径以及栅极电位对发射电子束的影响。聚焦极电位相对栅极越负 ,聚焦作用越强。减小聚焦极孔径对大发散角的电子有影响 ,但对聚焦作用的影响不如改变电位明显。栅极电位基本上不影响电子的轨迹 ,仅改变了发射电子的多少。以上结论与实验基本一致     

Single- and double-island ferromagnetic single-electron transistors

Electronic transport in a ferromagnetic single-electron transistor has been considered theoretically in the sequential tunneling regime. The device consists of two external leads and one or two islands as the central part, connected to the leads by tunneling barriers. External gates are additionally attached to the islands. Generally, the two external electrodes and the islands can be ferromagnetic with arbitrary orientation of the corresponding magnetic moments. We have carried out detailed theoretical analysis of the current–voltage characteristics and spin-valve magnetoresistance in the limit of fast spin relaxation on the islands. Asymmetry in tunneling probabilities of spin-majority and spin-minority electrons leads to interesting features in the transport characteristics, like for instance magnetoresistance oscillations with the bias and gate voltages, negative differential resistance, and others.     

Electronic transport of lateral PtSi/n/n+-Si Schottky diodes

We investigated the transport properties of a lateral PtSi/n/n(+)-Si Schottky diode prepared on an n-type silicon-on-insulator (SOI) wafer with a special attention on the bipolar transport and the surface effect. With applying a back-gate bias changing from +18 V to -18 V, the unipolar transport behavior switched over to the bipolar one, where an enhanced surface recombination rate due to a high surface-to-volume ratio produced a current density approximately 3 x 10(3) A/cm2 for 2 V bias through a 40 nm-thick and 18 microm-long nanoribbon. The recombination time was estimated to be approximately 1 micros from independent CV measurements, which is much smaller value than that of a bulk. The local Fermi energy level for electrons at the channel center was monitored by an additional voltage probe during each I(D)-V(D) measurement and it revealed the intricate nature of the bipolar transport manifested by the huge asymmetrical hysteretic behavior on a drain bias cycle which is attributed to the charge storage effect and asymmetrical junction profiles.     

Quantum Noise of Electron–Phonon Heat Current

We analyze heat current fluctuations between electrons and phonons in a metal. In equilibrium we recover the standard result consistent with the fluctuation–dissipation theorem. Here we show that heat current noise at finite frequencies remains non-vanishing down to zero temperature. From the experimental point of view, it is a small effect and up to now elusive. We briefly discuss the impact of electron–phonon heat current fluctuations on calorimetry, particularly in the regime of single microwave-photon detection.     

Correlation effects in wave function mapping of molecular beam epitaxy grown quantum dots

We investigate correlation effects in the regime of a few electrons in uncapped InAs quantum dots by tunneling spectroscopy and wave function (WF) mapping at high tunneling currents where electron-electron interactions become relevant. Four clearly resolved states are found, whose approximate symmetries are roughly s and p, in order of increasing energy. Because the major axes of the p-like states coincide, the WF sequence is inconsistent with the imaging of independent-electron orbitals. The results are explained in terms of many-body tunneling theory, by comparing measured maps with those calculated by taking correlation effects into account.     

A new regime for operating capacitive micromachined ultrasonic transducers

We report on a new operation regime for capacitive micromachined ultrasonic transducers (cMUTs). Traditionally, cMUTs are operated at a bias voltage lower than the collapse voltage of their membranes. In the new proposed operation regime, first the cMUT is biased past the collapse voltage. Second, the bias voltage applied to the collapsed membrane is reduced without releasing the membrane. Third, the cMUT is excited with an ac signal at the bias point, keeping the total applied voltage between the collapse and snapback voltages. In this operation regime, the center of the membrane is always in contact with the substrate. Our finite element methods (FEM) calculations reveal that a cMUT operating in this new regime, between collapse and snapback voltages, possesses a coupling efficiency (k/sub T//sup 2/) higher than a cMUT operating in the conventional regime below its collapse voltage. This paper compares the simulation results of the coupling efficiencies of cMUTs operating in conventional and new operation regimes.     

Detection of spin reversal and nutations through current measurements

The dynamics of a single spin embedded in a tunnel junction between ferromagnetic contacts is strongly affected by the exchange coupling to the tunneling electrons. Moment reversal of the local spin induced by the bias voltage across the junction is shown to have a measurable effect on the tunneling current. Furthermore, the frequency of a harmonic bias voltage is picked up by the local spin dynamics and transferred back to the current, generating a double frequency component.     

Exception for the zero-forward-scattering theory

Studies on single scattering of electromagnetic waves by magnetic particles were reported in the 1980s by Kerker et al. [J. Opt. Soc. Am.73, 765 (1983)]. They obtained that very small spherical particles with electric permittivity and magnetic permeability values such that epsilon=(4-mu)/(2mu+1) do not produce forward scattering. We show here that this condition contains an interesting exception at (epsilon=-2, mu=-2) when electric and magnetic resonances are present and around which the scattered field distribution is computed and described showing a polarization-insensitive behavior at the point (epsilon=-2, mu=-2).     

Determination of optical parameters of very thin (lambda/50) films

A straightforward approach for estimation of thickness (d), real (epsilon(1)) and imaginary parts (epsilon(2)) of the complex permittivity of very thin films from spectrophotometric measurements is presented. The uncertainties in epsilon(1), epsilon(2), and d due to methodical error and the uncertainties in the measured quantities are investigated. It is shown that the influence of these factors is considerable when epsilon(1), epsilon(2), and d are obtained simultaneously for each wavelength. The accuracy of epsilon(1), epsilon(2), and d is significantly increased if the value of d is evaluated first, its value is kept constant over the whole spectral region, and then epsilon(1) and epsilon(2) are calculated for each wavelength.