Spin Filter Effect in a Parallel Double Quantum Dot

We present a theoretical study of the spectral and the spin-dependent transport properties of a few electron semiconductor parallel double quantum dot (DQD) in the presence of local induced Zeeman splittings at the quantum dots. Working in an extended Hubbard model and treating the coupled QD as a single coherent system, the linear response spin-dependent conductance is calculated at low temperatures. We analyze the conditions such that the device would operate as a bipolar spin filter by only varying the incident electron Fermi energy from non-magnetic leads.     

Spin Filter Effect in a Parallel Double Quantum Dot

We present a theoretical study of the spectral and the spin-dependent transport properties of a few electron semiconductor parallel double quantum dot (DQD) in the presence of local induced Zeeman splittings at the quantum dots. Working in an extended Hubbard model and treating the coupled QD as a single coherent system, the linear response spin-dependent conductance is calculated at low temperatures. We analyze the conditions such that the device would operate as a bipolar spin filter by only varying the incident electron Fermi energy from non-magnetic leads.     

A Ge/Si heterostructure nanowire-based double quantum dot with integrated charge sensor

One proposal for a solid-state-based quantum bit (qubit) is to control coupled electron spins on adjacent semiconductor quantum dots. Most experiments have focused on quantum dots made from III-V semiconductors; however, the coherence of electron spins in these materials is limited by hyperfine interactions with nuclear spins. Ge/Si core/shell nanowires seem ideally suited to overcome this limitation, because the most abundant nuclei in Ge and Si have spin zero and the nanowires can be chemically synthesized defect-free with tunable properties. Here, we present a double quantum dot based on Ge/Si nanowires in which we can completely control the coupling between the dots and to the leads. We also demonstrate that charge on the double dot can be detected by coupling it capacitively to an adjacent nanowire quantum dot. The double quantum dot and integrated charge sensor serve as an essential building block to form a solid-state qubit free of nuclear spin.     

Spin filtering and quantum phase transition in double quantum dots attached to spin-polarized leads

We study the spin filtering and quantum phase transition (QPT) in double quantum dots attached to spin-polarized leads. For spin-independent leads, we observe a Kosterlitz-Thouless transition between the local triplet and doublet. For spin-polarized leads, the above QPT becomes first order, and Kondo splitting, gate-controlled spin reversal and a perfect spin filtering are observed. The breaking of spin-rotation SU(2) symmetry and the interdot transport mediated by the conduction electron are responsible for the fully spin-polarized conductance. Because spin-polarized leads suppress the Kondo effect, in order to obtain a large conductance with perfect spin filtering, one should choose leads with small spin polarization, such as Rashba spin-orbital coupling leads.     

Charging and Spin Effects in Triple Dot Artificial Molecules

We have fabricated artificial molecules consisting of three coupled quantum dots defined in the two-dimensional electron gas of a GaAs/AlGaAs heterostructure using lithographically patterned gates and trenches. The three dots are arranged in a ring structure, where each dot is coupled to the other two dots. We find that, when tuned to the Coulomb blockade regime, the triple quantum dot device acts as a charge rectifier: an electron enters the third dot where it is trapped, producing a jamming effect where no other electron may enter the first dot. Triple quantum dots coupled in a ring will allow for the study of new molecular phases using artificial molecules and may also serve as building blocks of two-dimensional arrays for quantum computation.     

Charging and Spin Effects in Triple Dot Artificial Molecules

We have fabricated artificial molecules consisting of three coupled quantum dots defined in the two-dimensional electron gas of a GaAs/AlGaAs heterostructure using lithographically patterned gates and trenches. The three dots are arranged in a ring structure, where each dot is coupled to the other two dots. We find that, when tuned to the Coulomb blockade regime, the triple quantum dot device acts as a charge rectifier: an electron enters the third dot where it is trapped, producing a jamming effect where no other electron may enter the first dot. Triple quantum dots coupled in a ring will allow for the study of new molecular phases using artificial molecules and may also serve as building blocks of two-dimensional arrays for quantum computation.     

Theoretical and Experimental Studies of Magneto-tunneling in Quantum Dots

We study the magneto-transport in vertical quantum dots boththeoretically and experimentally. We focus our attention onthe non-linear transport regime. We demonstrate that the peakamplitudes in the I–V characteristics show a dramaticallydifferent behaviour as a function of the magnetic field,depending on the value of the angular momentum of the dotstate through which tunnelling occurs. The investigationallows us to probe details of the quantum dot wavefunctionsand to distinguish tunnelling through localised states relatedto impurities or to quantum dots.     

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.     

Excited state spectroscopy in carbon nanotube double quantum dots

We report on low-temperature measurements in a fully tunable carbon nanotube double quantum dot. A new fabrication technique has been used for the top-gates in order to avoid covering the whole nanotube with an oxide layer as in previous experiments. The top-gates allow us to form single dots and control the coupling between them, and we observe 4-fold shell filling. We perform inelastic transport spectroscopy via the excited states in the double quantum dot, a necessary step toward the implementation of new microwave-based experiments.     

Scanned probe imaging of quantum dots inside InAs nanowires

We show how a scanning probe microscope (SPM) can be used to image electron flow through InAs nanowires, elucidating the physics of nanowire devices on a local scale. A charged SPM tip is used as a movable gate. Images of nanowire conductance versus tip position spatially map the conductance of InAs nanowires at liquid-He temperatures. Plots of conductance versus backgate voltage without the tip present show complex patterns of Coulomb-blockade peaks. Images of nanowire conductance identify their source as multiple quantum dots formed by disorder along the nanowire--each dot is surrounded by a series of concentric rings corresponding to Coulomb blockade peaks. An SPM image locates the dots and provides information about their size. In this way, SPM images can be used to understand the features that control transport through nanowires. The nanowires were grown from metal catalyst particles and have diameters approximately 80 nm and lengths 2-3 microm.     

Unity quantum yield of photogenerated charges and band-like transport in quantum-dot solids

Solid films of colloidal quantum dots show promise in the manufacture of photodetectors and solar cells. These devices require high yields of photogenerated charges and high carrier mobilities, which are difficult to achieve in quantum-dot films owing to a strong electron-hole interaction and quantum confinement. Here, we show that the quantum yield of photogenerated charges in strongly coupled PbSe quantum-dot films is unity over a large temperature range. At high photoexcitation density, a transition takes place from hopping between localized states to band-like transport. These strongly coupled quantum-dot films have electrical properties that approach those of crystalline bulk semiconductors, while retaining the size tunability and cheap processing properties of colloidal quantum dots.     

Colloidal quantum dot photovoltaics: the effect of polydispersity

The size-effect tunability of colloidal quantum dots enables facile engineering of the bandgap at the time of nanoparticle synthesis. The dependence of effective bandgap on nanoparticle size also presents a challenge if the size dispersion, hence bandgap variability, is not well-controlled within a given quantum dot solid. The impact of this polydispersity is well-studied in luminescent devices as well as in unipolar electronic transport; however, the requirements on monodispersity have yet to be quantified in photovoltaics. Here we carry out a series of combined experimental and model-based studies aimed at clarifying, and quantifying, the importance of quantum dot monodispersity in photovoltaics. We successfully predict, using a simple model, the dependence of both open-circuit voltage and photoluminescence behavior on the density of small-bandgap (large-diameter) quantum dot inclusions. The model requires inclusion of trap states to explain the experimental data quantitatively. We then explore using this same experimentally tested model the implications of a broadened quantum dot population on device performance. We report that present-day colloidal quantum dot photovoltaic devices with typical inhomogeneous linewidths of 100-150 meV are dominated by surface traps, and it is for this reason that they see marginal benefit from reduction in polydispersity. Upon eliminating surface traps, achieving inhomogeneous broadening of 50 meV or less will lead to device performance that sees very little deleterious impact from polydispersity.     

Manipulation of electron orbitals in hard-wall InAs/InP nanowire quantum dots

We present a novel technique for the manipulation of the energy spectrum of hard-wall InAs/InP nanowire quantum dots. By using two local gate electrodes, we induce a strong transverse electric field in the dot and demonstrate the controlled modification of its electronic orbitals. Our approach allows us to dramatically enhance the single-particle energy spacing between the first two quantum levels in the dot and thus to increment the working temperature of our InAs/InP single-electron transistors. Our devices display a very robust modulation of the conductance even at liquid nitrogen temperature, while allowing an ultimate control of the electron filling down to the last free carrier. Potential further applications of the technique to time-resolved spin manipulation are also discussed.     

Conductance and Thermopower of a Quantum Dot with Fano–Rashba Effect

We investigate the conductance and thermopower of a Rashba quantum dot coupled to ferromagnetic leads. We show that the interference of localized electron states with resonant electron states leads to the appearance of the Fano–Rashba effect. This effect occurs due to the interference of bound levels of spin-polarized electrons with the continuum of electronic states with an opposite spin polarization. We obtain an important enhancement of the thermopower due to the Fano–Rashba effect.     

Supramolecular spin valves

Magnetic molecules are potential building blocks for the design of spintronic devices. Moreover, molecular materials enable the combination of bottom-up processing techniques, for example with conventional top-down nanofabrication. The development of solid-state spintronic devices based on the giant magnetoresistance, tunnel magnetoresistance and spin-valve effects has revolutionized magnetic memory applications. Recently, a significant improvement of the spin-relaxation time has been observed in organic semiconductor tunnel junctions, single non-magnetic molecules coupled to magnetic electrodes have shown giant magnetoresistance and hybrid devices exploiting the quantum tunnelling properties of single-molecule magnets have been proposed. Herein, we present an original spin-valve device in which a non-magnetic molecular quantum dot, made of a single-walled carbon nanotube contacted with non-magnetic electrodes, is laterally coupled through supramolecular interactions to TbPc(2) single-molecule magnets (Pc=phthalocyanine). Their localized magnetic moments lead to a magnetic field dependence of the electrical transport through the single-walled carbon nanotube, resulting in magnetoresistance ratios up to 300% at temperatures less than 1 K. We thus demonstrate the functionality of a supramolecular spin valve without magnetic leads. Our results open up prospects of new spintronic devices with quantum properties.     

A triple quantum dot in a single-wall carbon nanotube

A top-gated single-wall carbon nanotube is used to define three coupled quantum dots in series between two electrodes. The additional electron number on each quantum dot is controlled by top-gate voltages allowing for current measurements of single, double, and triple quantum dot stability diagrams. Simulations using a capacitor model including tunnel coupling between neighboring dots captures the observed behavior with good agreement. Furthermore, anticrossings between indirectly coupled levels and higher order cotunneling are discussed.     

Spin-polarized light-emitting diodes with Mn-doped InAs quantum dot nanomagnets as a spin aligner

We have fabricated and characterized surface-emitting, spin-polarized light-emitting diodes with a Mn-doped InAs dilute magnetic quantum dot spin-injector and contact region grown by low-temperature molecular beam epitaxy, and an In(0.4)Ga(0.6)As quantum dot active region. Energy-dispersive X-ray and electron energy loss spectroscopies performed on individual dots indicate that the Mn atoms incorporate within the dots themselves. Circularly polarized light is observed up to 160 K with a maximum degree of circular polarization of 5.8% measured at 28 K, indicating high-temperature spin injection and device operation.     

Scanning gate imaging of quantum dots in 1D ultra-thin InAs/InP nanowires

We use a scanning gate microscope (SGM) to characterize one-dimensional ultra-thin (diameter ≈ 30 nm) InAs/InP heterostructure nanowires containing a nominally 300 nm long InAs quantum dot defined by two InP tunnel barriers. Measurements of Coulomb blockade conductance versus backgate voltage with no tip present are difficult to decipher. Using the SGM tip as a charged movable gate, we are able to identify three quantum dots along the nanowire: the grown-in quantum dot and an additional quantum dot near each metal lead. The SGM conductance images are used to disentangle information about individual quantum dots and then to characterize each quantum dot using spatially resolved energy-level spectroscopy.     

Excitonic quantum interference in a quantum dot chain with rings

We demonstrate excitonic quantum interference in a closely spaced quantum dot chain with nanorings. In the resonant dipole-dipole interaction model with direct diagonalization method, we have found a peculiar feature that the excitation of specified quantum dots in the chain is completely inhibited, depending on the orientational configuration of the transition dipole moments and specified initial preparation of the excitation. In practice, these excited states facilitating quantum interference can provide a conceptual basis for quantum interference devices of excitonic hopping.     

Electronic excited states in bilayer graphene double quantum dots

We report tunneling spectroscopy experiments on a bilayer graphene double quantum dot device that can be tuned by all-graphene lateral gates. The diameter of the two quantum dots are around 50 nm and the constrictions acting as tunneling barriers are 30 nm in width. The double quantum dot features additional energies on the order of 20 meV. Charge stability diagrams allow us to study the tunable interdot coupling energy as well as the spectrum of the electronic excited states on a number of individual triple points over a large energy range. The obtained constant level spacing of 1.75 meV over a wide energy range is in good agreement with the expected single-particle energy spacing in bilayer graphene quantum dots. Finally, we investigate the evolution of the electronic excited states in a parallel magnetic field.