Direct comparison between potential landscape and local density of states in a disordered two-dimensional electron system

The local density of states (LDOS) of the adsorbate-induced two-dimensional electron system (2DES) on n-InAs(110) is studied by scanning tunneling spectroscopy. In contrast to a similar 3DES, the 2DES LDOS exhibits 20 times stronger corrugations and rather irregular structures. Both results are interpreted as consequences of weak localization. Fourier transforms of the LDOS reveal that the k values of the unperturbed 2DES still dominate the 2DES, but additional lower k values contribute. To clarify the origin of the LDOS patterns, we measure the potential landscape of the 2DES area. We use it to calculate the expected LDOS and find reasonable agreement between calculation and experiment.     

Electronic Resonant Splitting in a Nanostructure with Periodic Magnetic-Electric Barriers

In this letter, the transmission probability and the conductance of the ballistic electron are studied in a nanostructure with the periodic magnetic-electric barriers. We find that the resonant splitting increases with the number of periods increasing, so the number of the resonant peaks increases and the peaks become sharper. For the m-th periodic magnetic-electric barriers tunneling the splitting is (m-1)-fold.     

Laterally modulated 2D electron system in the extreme quantum limit

We report on magnetotransport of a two-dimensional electron system (2DES), located 32 nm below the surface, with a surface superlattice gate structure of periodicity 39 nm imposing a periodic modulation of its potential. For low Landau level fillings nu, the diagonal resistivity displays a rich pattern of fluctuations, even though the disorder dominates over the periodic modulation. Theoretical arguments based on the combined effects of the long-wavelength, strong disorder and the short-wavelength, weak periodic modulation present in the 2DES qualitatively explain the data.     

Optical absorption and magnetotransport in quantum wires in a perpendicular magnetic field

A periodic array of δ function potentials are used to simulate the potential barriers between quantum wires in the presence or absence of lattice site dislocation. The exact eigenenergies and eigenfunctions are found by employing a numerical diagonalization procedure. Based on these results, a self-consistent field theory is derived for the mid-infrared absorption coefficient of the system. The crossover from a cyclotron mode to two tunneling coupled modes and finally to edge and 1D lattice magnetoplasmon modes with increasing modulation strength is investigated. The magnetic field enhanced and suppressed electron tunneling, associated with the evolution to cyclotron modes at strong magnetic fields passing through the formation of tunneling coupled modes, is observed. The edge mode excitation energy oscillates as a function of the electron density. These oscillations correspond to a soft or hard potential wall for which the electron states are extended or localized, respectively. The displacement of the 1D lattice magnetoplasmon modes under strong modulation is found to be periodic and corresponds to the evolution from a complex unit cell which is composed of one narrow and one wide quantum wire to a simple unit cell containing only one quantum wire. The magnetoresistivities and the associated conductivities are also calculated for the lateral surface superlattice. At strong potential modulation there is a giant peak in the Hall conductivity and many peaks in its resistivity in the quantum regime. With strong modulation, the suppression of the transverse conductivity along with oscillations in its resistivity are obtained.     

Time domain capacitance spectroscopy of tunneling electrons in the two-dimensional electron gas

We have developed a technique capable of measuring the tunneling current into both localized and conducting states in a 2D electron system (2DES). The method yields I-V characteristics for tunneling with no distortions arising from low 2D in-plane conductivity. We have used the technique to determine the pseudogap energy spectrum for electron tunneling into and out of a 2D system and, further, we have demonstrated that such tunneling measurements reveal spin relaxation times within the 2DEG. Pseudogap: In a 2DEG in perpendicular magnetic field, a pseudogap develops in the tunneling density of states at the Fermi energy. We resolve a linear energy dependence of this pseudogap at low excitations. The slopes of this linear gap are strongly field dependent. No existing theory predicts the observed behavior. Spin relaxation: We explore the characteristics of equilibrium tunneling of electrons from a 3D electrode into a high mobility 2DES. For most 2D Landau level filling factors, we find that electrons tunnel with a single, well-defined tunneling rate. However, for spin-polarized quantum Hall states (ν=1, 3 and 1/3) tunneling occurs at two distinct rates that differ by up to two orders of magnitude. The dependence of the two rates on temperature and tunnel barrier thickness suggests that slow in-plane spin relaxation creates a bottleneck for tunneling of electrons.     

Imaging transport resonances in the quantum Hall effect

We use a scanning capacitance probe to image transport in the quantum Hall system. Applying a dc bias voltage to the tip induces a ring-shaped incompressible strip (IS) in the 2D electron system (2DES) that moves with the tip. At certain tip positions, short-range disorder in the 2DES creates a quantum dot island in the IS. These islands enable resonant tunneling across the IS, enhancing its conductance by more than 4 orders of magnitude. The images provide a quantitative measure of disorder and suggest resonant tunneling as the primary mechanism for transport across ISs.     

Quantum antidot as an electrometer:Observation of fractional charge

In experiments on resonant tunneling through a quantum antidot in the quantum Hall (QH) regime, we observe periodic conductance peaks both versus magnetic field and a global gate voltage, i.e., electric field. Each conductance peak can be attributed to tunneling through a quantized antidot-bound state. The fact that the variation of the uniform electric field produces conductance peaks implies that the deficiency of the electrical charge on the antidot is quantized in units of charge of quasiparticles of surrounding QH condensate. The period in magnetic field gives the effective area of the antidot state through which tunneling occurs, the period in electric field (obtained from the global gate voltage) then constitutes a direct measurement of the charge of the tunneling particles. We obtain electron charge C in the integer QH regime, and quasiparticle charge C for the QH state.     

The effect of periodic magnetic-electric barriers on electron transport in a nanostructure

Using the transfer matrix method, the transmission probability, the spin polarization and the electron conductance of a ballistic electron are studied in detail in a nanostructure. We observe that these quantities sensitively depend on the number of periodic magnetic-electric barriers. As the number of periods increases, the resonance splitting increases, the number of the resonance peaks increases and the peaks become sharper as well as the spin polarization being enhanced. Surprisingly, a polarization of nearly 100% can be achieved by spin-dependent resonant tunneling in this structure, although the average magnetic field of the structure is zero.     

Imaging of electron potential landscapes on Au(111)

The Hohenberg-Kohn theorem states that the ground state electron density completely determines the external potential acting on an electron system. Inspired by this fundamental theorem, we developed a novel approach to map directly the electron potential in surface systems: linear response theory applied to the total electron density as measured with scanning tunneling microscopy determines the external potential. Potential imaging is demonstrated for the s-p derived surface state on Au(111), where the "herringbone" reconstruction induces a periodic potential modulation, the details of which are revealed by our technique.     

Probing electron-electron interaction in quantum Hall systems with scanning tunneling spectroscopy

Using low-temperature scanning tunneling spectroscopy applied to the Cs-induced two-dimensional electron system (2DES) on p-type InSb(110), we probe electron-electron interaction effects in the quantum Hall regime. The 2DES is decoupled from bulk states and exhibits spreading resistance within the insulating quantum Hall phases. In quantitative agreement with calculations we find an exchange enhancement of the spin splitting. Moreover, we observe that both the spatially averaged as well as the local density of states feature a characteristic Coulomb gap at the Fermi level. These results show that electron-electron interaction can be probed down to a resolution below all relevant length scales.     

Minigap plasmons in a two-dimensional electron gas subjected to a one-dimensional periodic potential

We investigate the plasmon excitations in a two-dimensional electron gas subjected to a one-dimensional weak periodic potential. We derive and discuss the dispersion relations for both intrasubband and intersubband excitations within the framework of Bohm-Pines' random-phase approximation. For such an anisotropic system with spatially modulated charge density, we observe a splitting of the 2D plasmon dispersion. The splitting is caused by the superlattice effect of the charge-density modulation on the collective excitation spectrum. We also discuss how the tunneling and the potential amplitude affect the plasmon excitations.     

Delocalization of phonon-plasmon modes in GaAs/AlAs superlattices with tunnel-thin AlAs barriers

The technique of Raman spectroscopy has been used to investigate doped (n-type) and undoped GaAs/AlAs superlattices with AlAs barrier thicknesses from 17 to 1 monolayers. The peak corresponding to the scattering by a two-dimensional plasmon was found in the Raman spectrum of a doped superlattice with relatively thick barriers. The position of the experimental peak corresponded to the value calculated in the model of plasma oscillations in periodic planes of a two-dimensional electron gas. The electron tunneling effects played an increasingly prominent role as the AlAs barrier thickness decreased. The peaks corresponding to the scattering by coupled phonons with three-dimensional plasmons were found in the Raman spectra for a superlattice with an AlAs thickness of 2 monolayers; i.e., the delocalization of coupled modes was observed. In this case, the folding of acoustic phonons was observed in the superlattice under consideration, indicative of its good periodicity, while the localization of optical phonons in GaAs layers was observed in undoped superlattices with an AlAs thickness of 2 monolayers.     

Tuning the Mott transition in a Bose-Einstein condensate by multiple photon absorption

We study the time-dependent dynamics of a Bose-Einstein condensate trapped in an optical lattice. Modeling the system as a Bose-Hubbard model, we show how applying a periodic driving field can induce coherent destruction of tunneling. In the low-frequency regime, we obtain the novel result that the destruction of tunneling displays extremely sharp peaks when the driving frequency is resonant with the depth of the trapping potential ("multi-photon resonances"), which allows the quantum phase transition between the Mott insulator and the superfluid state to be controlled with high precision. We further show how the waveform of the field can be chosen to maximize this effect.     

Direct measurement of the local density of states of a disordered one-dimensional conductor

One-dimensional electron systems (1DESs) containing two and one occupied subbands are found below the different types of [11;2] steps of the InAs(110) surface. Using low-temperature scanning tunneling spectroscopy, we determined the subband energies, the disorder potential, and the local density of states of these 1DESs. The rather complete knowledge of the 1DES allowed us to compare the measured LDOS with a single-particle calculation. Surprisingly, we did not find significant deviations from the calculation albeit the electron-electron interaction in the 1DESs is stronger than the electron-disorder interaction.     

Observation of Resonant Photon Tunneling in Photonic Double Barrier Structures

Reflectance and transmittance of 632.8 nm He-Ne laser light for photonic double barrier structures (consisting of a SF10 prism, SiO₂ layer, Al or Al₂O₃ active layer, SiO₂ layer and SF10 prism) were measured as a function of the angle of incidence for both the ρ- and s-polarized incidence. Sharp reflection dips and transmission peaks were observed at angles larger than the critical angle of total reflection. The appearance of the transmission peaks can be attributed to resonant photon tunneling through the photonic double barrier structures analogous to resonant electron tunneling through double potential barrier structures. Resonant tunneling is mediated by the long-range surface plasmon polariton in the case of the Al active layer and the electromagnetic guided modes in the case of the Al₂O₃ layer.This paper was originally presented at the seventh Meeting on Near Field Optics, which was held on July 1, 1998 at Nagoya University, Nagoya, organized by Research Group on Near Field Optics, the Optical Society of Japan, an affiliate of Japan Society of Applied Physics. The authors have won the Near Field Optics Award for their best presentation at the meeting.     

Effects of electron inelastic transport through the potential relief of a spin dimer in a magnetic field

The influence of inelastic effects on electron quantum transport through the potential relief of a dimer system was studied by exact solution of the Schrodinger equation. The nature of this problem is due to the coherent superposition of the different potential profiles through which the spin-polarized electron tunnels. It was found that the low magnetic field initiates new peaks of resonant tunneling. In high magnetic fields, the transport of electrons with opposite spin polarization is qualitatively different.     

电子横向运动对共振隧穿的影响

讨论了电子横纵方向运动耦合时的隧穿现象,对CdSe/Zn1-xCdxSe方形双势垒结构和抛物形双势垒结构的数值计算表明,在零偏压和非零偏压情况下,电子横向运动对共振隧穿的影响是不容忽略的。     

AC field-controlled Andreev tunneling through a serially-coupled double quantum dot system

AC field-controlled Andreev tunneling through two serially-coupled quantum dots are investigated theoretically by using the nonequilibrium Green's function method. The photon-assisted Andreev tunneling is studied in detail. It is found that the average current depends distinctly on the interdot coupling. In the weak interdot coupling case, the average current versus the gate voltage exhibit negative peaks on the left-hand side and positive peaks on the right-hand side of the Fermi level. However, in the strong interdot coupling case, the current exhibit both negative and positive peaks on each side of the Fermi level. Furthermore, the system can function as an electron pump capable of transporting electrons through the resonant photon-assisted Andreev tunneling.     

电子离散发射模型的基础理论

 通过建立电子的离散发射模型并解析求解泊松方程,研究了两平行板电容器之间电子发散注入时电势的分布情况。结果表明：电子注的进入对无电子时两平板之间的电势产生了周期性和非周期性扰动,周期性扰动的平均效果为零,所以电子注产生的扰动主要由非周期性扰动体现。当电子的入射角度在0°～90°时,扰动出现了4个尖峰,除此之外电子注入对电势的扰动基本上为零,这从理论上解释了MICHELL的九粒子发射模型。     

Absence of compressible edge channel rings in quantum antidots

We report resonant tunneling experiments in a quantum antidot sample in the integer quantum Hall regime. In particular, we have measured the temperature T dependence of the peak value of a conductance peak on the i = 2 plateau, where there are two peaks per magnetic flux quantum straight phi(0). We observe a T-1 dependence as expected when tunneling through only one electron state is possible. This result is incompatible with tunneling through a compressible ring of several degenerate states. We also observe, for the first time, three conductance peaks per straight phi(0) on the i = 3 plateau.