Channel Blockade in a Two-Path Triple-Quantum-Dot System

Electronic transport through a two-path triple-quantum-dot system with two source leads and one drain is studied. By separating the conductance of the two double dot paths, we are able to observe double dot and triple dot physics in transport and study the interaction between the paths. We observe channel blockade as a result of inter-channel Coulomb interaction. The experimental results are understood with the help of a theoretical model which calculates the parameters of the system, the stability regions of each state and the full dynamical transport in the triple dot resonances.Comment: 6 pages, 6 figure

Giant anisotropy of Zeeman splitting of quantum confined acceptors in Si/Ge

Shallow acceptor levels in Si/Ge/Si quantum well heterostructures are characterized by resonant tunneling spectroscopy in the presence of high magnetic fields. In a perpendicular magnetic field we observe a linear Zeeman splitting of the acceptor levels. In an in-plane field, on the other hand, the Zeeman splitting is strongly suppressed. This anisotropic Zeeman splitting is shown to be a consequence of the huge light hole-heavy hole splitting caused by a large biaxial strain and a strong quantum confinement in the Ge quantum well.Comment: 5 figures, 4 page

Thermoelectric and thermal rectification properties of quantum dot junctions

The electrical conductance, thermal conductance, thermal power and figure of merit (ZT) of semiconductor quantum dots (QDs) embedded into an insulator matrix connected with metallic electrodes are theoretically investigated in the Coulomb blockade regime. The multilevel Anderson model is used to simulate the multiple QDs junction system. The charge and heat currents in the sequential tunneling process are calculated by the Keldysh Green function technique. In the linear response regime the ZT values are still very impressive in the small tunneling rates case, although the effect of electron Coulomb interaction on ZT is significant. In the nonlinear response regime, we have demonstrated that the thermal rectification behavior can be observed for the coupled QDs system, where the very strong asymmetrical coupling between the dots and electrodes, large energy level separation between dots and strong interdot Coulomb interactions are required.Comment: 8 page and 14 figure

Electron spin relaxation in bulk III-V semiconductors from a fully microscopic kinetic spin Bloch equation approach

Electron spin relaxation in bulk III-V semiconductors is investigated from a fully microscopic kinetic spin Bloch equation approach where all relevant scatterings, such as, the electron--nonmagnetic-impurity, electron-phonon, electron-electron, electron-hole, and electron-hole exchange (the Bir-Aronov-Pikus mechanism) scatterings are explicitly included. The Elliot-Yafet mechanism is also fully incorporated. This approach offers a way toward thorough understanding of electron spin relaxation both near and far away from the equilibrium in the metallic regime. The dependence of the spin relaxation time on electron density, temperature, initial spin polarization, photo-excitation density, and hole density are studied thoroughly with the underlying physics analyzed. In contrast to the previous investigations in the literature, we find that: (i) In $n$-type materials, the Elliot-Yafet mechanism is {\em less} important than the D'yakonov-Perel' mechanism, even for the narrow band-gap semiconductors such as InSb and InAs. (ii) The density dependence of the spin relaxation time is nonmonotonic and we predict a {\em peak} in the metallic regime in both $n$-type and intrinsic materials. (iii) In intrinsic materials, the Bir-Aronov-Pikus mechanism is found to be negligible compared with the D'yakonov-Perel' mechanism. We also predict a peak in the temperature dependence of spin relaxation time which is due to the nonmonotonic temperature dependence of the electron-electron Coulomb scattering in intrinsic materials with small initial spin polarization. (iv) In $p$-type III-V semiconductors, ...... (the remaining is omitted here due to the limit of space)Comment: 25 pages, 17 figure

Analytical approach to semiconductor Bloch equations

Although semiconductor Bloch equations have been widely used for decades to address ultrafast optical phenomena in semiconductors, they have a few important drawbacks: (i) Coulomb terms between free electron-hole pairs require Hartree-Fock treatment which, in its usual form, preserves excitonic poles but loses biexcitonic resonances. (ii) Solving the resulting coupled differential equations imposes heavy numerics which completely hide the physics. This can be completely avoided if, instead of free electron-hole pairs, we use correlated pairs, i.e., excitons. Their interactions are easy to handle through the recently constructed composite-exciton many-body theory, which allows us to \emph{analytically} obtain the time evolution of the polarization induced by a laser pulse. This polarization comes from Coulomb interactions between virtual excitons, but also from Coulomb-free fermion exchanges, which are dominant at large detuning

Two path transport measurements on a triple quantum dot

We present an advanced lateral triple quantum dot made by local anodic oxidation. Three dots are coupled in a starlike geometry with one lead attached to each dot thus allowing for multiple path transport measurements with two dots per path. In addition charge detection is implemented using a quantum point contact. Both in charge measurements as well as in transport we observe clear signatures of states from each dot. Resonances of two dots can be established allowing for serial transport via the corresponding path. Quadruple points with all three dots in resonance are prepared for different electron numbers and analyzed concerning the interplay of the simultaneously measured transport along both paths.Comment: 4 pages, 4 figure

Kinetics of four-wave mixing for a 2D magneto-plasma in strong magnetic fields

We investigate the femtosecond kinetics of an optically excited 2D magneto-plasma at intermediate and high densities under a strong magnetic field perpendicular to the quantum well (QW). We assume an additional weak lateral confinement which lifts the degeneracy of the Landau levels partially. We calculate the femtosecond dephasing and relaxation kinetics of the laser pulse excited magneto-plasma due to bare Coulomb potential scattering, because screening is under these conditions of minor importance. In particular the time-resolved and time-integrated four-wave mixing (FWM) signals are calculated by taking into account three Landau subbands in both the valance and the conduction band assuming an electron-hole symmetry. The FWM signals exhibit quantum beats mainly with twice the cyclotron frequency. Contrary to general expectations, we find no pronounced slowing down of the dephasing with increasing magnetic field. On the contrary, one obtains a decreasing dephasing time because of the increase of the Coulomb matrix elements and the number of states in a given Landau subband. In the situation when the loss of scattering channels exceeds these increasing effects, one gets a slight increase at the dephasing time. However, details of the strongly modulated scattering kinetics depend sensitively on the detuning, the plasma density, and the spectral pulse width relative to the cyclotron frequency.Comment: 13 pages, in RevTex format, 10 figures, Phys. Rev B in pres

Combined atomic force microscope and electron-beam lithography used for the fabrication of variable-coupling quantum dots

We have combined direct nanofabrication by local anodic oxidation with conventional electron-beam lithography to produce a parallel double quantum dot based on a GaAs/AlGaAs heterostructure. The combination of both nanolithography methods allows to fabricate robust in-plane gates and Cr/Au top gate electrodes on the same device for optimal controllability. This is illustrated by the tunability of the interdot coupling in our device. We describe our fabrication and alignment scheme in detail and demonstrate the tunability in low-temperature transport measurements.Comment: 4 pages, 3 figure

Multi-subband effect in spin dephasing in semiconductor quantum wells

Multi-subband effect on spin precession and spin dephasing in $n$-type GaAs quantum wells is studied with electron-electron and electron-phonon scattering explicitly included. The effects of temperature, well width and applied electric field (in hot-electron regime) on the spin kinetics are thoroughly investigated. It is shown that due to the strong inter-subband scattering, the spin procession and the spin dephasing rate of electrons in different subbands are almost identical despite the large difference in the D'yakonov-Perel' (DP) terms of different subbands. It is also shown that for quantum wells with small well width at temperatures where only the lowest subband is occupied, the spin dephasing time increases with the temperature as well as the applied in-plane electric field until the contribution from the second subband is no longer negligible. For wide quantum wells the spin dephasing time tends to decrease with the temperature and the electric field.Comment: 6 pages, 4 figures in eps forma