Electron spins in an array of tunnel-coupled quantum dots

Mechanisms of electron spin relaxation in semiconductor arrays with tunnel-coupled quantum dots are reviewed. The contribution for anisotropic exchange interaction is shown for asymmetrical quantum dots having no inversion axis relative to their plane. The configuration of vertically coupled double Ge/Si quantum dots is found where anisotropic exchange coupling does not contribute to spin decoherence. It could be a basic configuration of spin-based quantum computation schemes.     

Flux-dependent tunnel magnetoresistance in parallel-coupled double quantum dots

The tunnel magnetoresistance (TMR) in an Aharonov–Bohm interferometer with two quantum dots inserted in its arms, which is attached to ferromagnetic leads with parallel and antiparallel magnetic configurations, is theoretically studied by means of the nonequilibrium Green’s function technique. We pay particular attention to the influence of an applied magnetic flux on the characteristics of the TMR. In the linear response regime (the external bias voltage V→0) and when the electrons are free from intradot Coulomb interaction, the magnetic flux only changes the peak or dip positions of the TMR. But in the presence of intradot Coulomb repulsion, its peak or dip positions, signs and magnitude are tuned by the magnetic flux. For the nonlinear response regime (V≠0), the TMR is symmetric with respect to zero bias voltage and the magnetic flux can influence its magnitude, signs and the peak positions regardless of the existence of intradot Coulomb interaction. The behavior of the TMR is interpreted in terms of the quantum interference (Fano) effect.     

Giant hysteresis of magnetoresistance in the quantum hall effect regime

A simple system consisting of a two-dimensional electron gas with a narrow conducting wire is studied. In this system, a giant hysteresis of both longitudinal and Hall magnetoresistances in the quantum Hall effect regime is observed for even and odd filling factors v of the Landau levels. At v = 1 and v = 2, the giant hysteresis occurs in the background of the zero-resistance plateau, and the width of the hysteresis loop in a magnetic field is comparable to the plateau width. At the entry to the hysteresis region, the magnetoresistance varies in a threshold manner; i.e., a magnetically induced breakdown of the quantum Hall effect takes place. It is shown that the system under study reflects the relaxation processes in the two-dimensional electron gas adjacent to the wire and, therefore, represents an effective instrument for investigating the hysteresis phenomena in the two-dimensional electron gas itself. An unusual “anticoercive” behavior of the hysteresis is revealed. A comparative analysis of the results obtained and the experimental data on the long relaxation of eddy currents and on the ferromagnetic state of the quantum Hall liquid indicates the common physical origin of these effects.     

Giant optical anisotropy in single InAs quantum dots

We present the experimental evidence of giant optical anisotropy in single InAs QDs. Polarization-resolved photoluminescence spectroscopy in single QDs reveals a linear polarization ratio which fluctuates, from one dot to another, in sign and in magnitude with absolute values up to 82%. We do not observe any dependence of the linear polarization on incident power and temperature.     

Large magnetoresistance induced by quantum charge fluctuations in magnetic double dots

We study electron tunnelling through two small ferromagnetic dots. Quantum charge fluctuations and interdot coupling cause each Coulomb peak of conductance at zero interdot coupling to split. The interdot tunnel coupling depends on the relative orientation of magnetizations of the two dots, leading to different splitting energies of the Coulomb peaks in parallel and antiparallel magnetization alignments. As a result, a very large tunnelling magnetoresistance occurs near the Coulomb peaks, and its sign may be either positive or negative.     

Relaxation of mechanical stresses in an array of Ge quantum dots obtained in Si

Raman scattering on optical phonons in Si/Ge/Si structures with Ge quantum dots grown by molecular beam epitaxy at low temperatures 200–300°C has been investigated. A pseudomorphic state of an array of Ge quantum dots to a Si matrix with an ideally sharp interface has been obtained. Features associated with the inelastic relaxation of mechanical stresses have been revealed in the Raman spectrum. Two mechanisms of stress relaxation are separated. It has been shown that the spectrum of the electronic states of the array differs significantly from the set of the discrete levels of a single quantum dot, because the relaxation is inhomogeneous.     

Electronic transport through a ring-shaped array of quantum dots

Making use of the equation of motion method and Keldysh Green function technique, we have developed a calculation method for the ring-shaped array of quantum dots with arbitrary dots. A general formula for the current under dc bias is obtained; the transmission probability and the differential conductance are numerically studied.     

Quantum phase diagram of granular superconducting quantum dots array

We study the quantum phase diagram of granular superconducting quantum dots (GSQD) array. We implement the physics of granularity by considering site dependent Josephson couplings, on-site charging energies and the intersite interactions. We predict dimer density wave and staggered phases at the insulating state for higher order commensurability. Several parts of the quantum phase diagram of GSQD are in contrast with the clean superconducting quantum dots array. We also obtain the superconducting phase of GSQD. We develop the theory for weak tunneling conductance and the Coulomb energy is smaller than the superconducting gap.     

Direct observation of charge-carrier capture in an array of self-assembled InAs/GaAs quantum dots

In this work, charge-carrier capture by an array of self-assembled InAs/GaAs quantum dots was directly observed for the first time by capacitance recharge. It is proposed to process the obtained transient-capture data by a similar method to that used for emission, by the box-car method. The capture activation energies are determined and compared with the emission activation energies.     

Optimized Peltier cooling via an array of quantum dots with stair-like ground-state energy configuration

With the advancement in fabrication and scaling technology, the rising temperature in nano devices has attracted special attention towards thermoelectric or Peltier cooling. In this paper, I propose optimum Peltier cooling by employing an array of connected quantum dots with stair-like ground-state eigen energy configuration. The difference in ground state eigen energy between two adjacent quantum dots in the stair-like configuration is chosen to be identical with the optical phonon energy for efficient absorption of lattice heat. I show that in the proposed configuration, for a given optical phonon energy, one can optimize the cooling power by tuning the number of stages in the array of quantum dots. A further analysis demonstrates that the maximum cooling power at a given potential bias under optimal conditions does not depend strongly on the optical phonon energy or the number of stages at which the maximum cooling power is achieved, provided that the optical phonon energy is less than kT. The proposed concept can also be applied to $2?D$ or bulk resonant tunnel and superlattice structures with stair-like resonant energy configuration.     

Polariton excitations in a non-ideal array of microcavities with quantum dots

The polariton spectrum of a one-dimensional non-ideal array of coupled microcavities containing quantum dots has been studied. The specific features of the dispersion of electromagnetic excitations, which are induced in this system both by a variation in the distance between the adjacent microcavities and by a variation in the composition of the quantum dots, have been investigated using the numerical simulation within the framework of the virtual crystal approximation. The density of states of the quasiparticles under consideration has been determined.     

Extraordinary electron transmission through a periodic array of quantum dots

We study electron transmission through a periodic array of quantum dots (QD) sandwiched between doped semiconductor leads. When the Fermi wavelength of tunneling electron exceeds the array lattice constant, the off-resonant per QD conductance is enhanced by several orders of magnitude relative to the single-QD conductance. The physical mechanism of the enhancement is delocalization of a small fraction of system eigenstates caused by coherent coupling of QDs via the electron continuum in the leads.     

Giant backscattering resonances in the magneto-resistance of GaAs/AlGaAs quantum dots

We discuss the observation of large resonant features, superimposed upon the quantum Hall plateaux of gated GaAs/AlGaAs quantum dots. The resonances correspond to a magnetically induced increase in the edge state backscattering, and under certain conditions can imply a complete reflection of the applied current. We demonstrate that the resonances are correlated to the depopulation of bulk Landau levels, and suggest they result from an increase in backscatterlng via confined Landau levels, as the latter depopulate in a magnetic field. The resonances are therefore analogous to the Shubnikov-de Haas oscillations, observed in two dimensional electron gas systems, and their temperature dependence is found to take the same functional form. We argue that the resonances are an intrinsic feature of edge state transport in quantum dots, since they result from scattering via Landau levels, controllably confined within the dot, and discuss our results in relation to recent theoretical and experimental studies, of edge state transport in small wires and dots.     

Giant magnetoresistance of coiling polymers

We examine the magnetoresistance (MR) of conducting polymers with interest paid on the role of structural flexibility. Through Monte Carlo simulation and Green function method, we evaluate the electric transmission for a variety of polymer configurations. It is found that for a single polymer the transmission displays a complex oscillation and also a parity-dependent periodicity. For an ensemble of polymers the averaged transmission yields the nonlinear behavior of MR under varying magnetic fields. Interestingly, more flexible polymers are shown to achieve higher MR, depending on the population and the size of the loops.     

Giant magnetoresistance in a ferromagnet-polymer system

Giant magnetoresistance in a ferromagnet-electroactive polymer-nonmagnetic metal structure is studied. The insulator-to-metal transition in the conductivity of the system is attributed to the change in the shape of the current-voltage characteristics of the sample in a magnetic field. It is concluded that the giant magnetoresistance observed in the experiment may be of injection nature, and this is why the effect is realized at the ferromagnet-electroactive polymer interface alone.