We investigate the conductance and zero-frequency shot noise of interacting, multi-level quantum dots coupled to leads. We observe that co-tunneling assisted sequential tunneling (CAST) processes play a dominant role in the transition region from Coulomb blockade to sequential tunneling. We analyze for intermediate coupling strength the dependence of the conductance due to CAST processes on temperature, coupling constant, and gate voltage. Remarkably, the width of the CAST transport feature scales only with temperature, but not with the coupling constant. While the onset of inelastic co-tunneling is associated with a super-Poissonian noise, the noise is even stronger above the threshold for CAST processes
We have investigated the noise properties of the tunneling current through vertically coupled self-assembled InAs quantum dots. We observe super-Poissonian shot noise at low temperatures. For increased temperature this effect is suppressed. The super-Poissonian noise is explained by capacitive coupling between different stacks of quantum dots.
We consider the transition of electron between two quantum dots in which the discrete levels are swept past each other with a constant velocity. If a direct tunneling between the dot levels was allowed, an electron will be transferred between the dots when the levels cross. This transfer is described in terms of the conventional Landau-Zener theory. We assume that direct tunneling between the dots is forbidden. Rather, the transfer is due to the resonant tunneling via a discrete impurity level separating the dots. Then the description of the electron transfer reduces to a threestate (two dots …
By using bosonization method and unitary transformation, we give a general relation between the dimensionless tunneling conductance and the fractional Coulomb blockade conductance peak splitting which is valid both for weak and strong transmission between two quantum dots, and show that the tunneling conductance has a linear temperature dependence in the low energy and low temperature limit.
Assisted hopping effects in magnetic impurities and quantum dots are analyzed. The magnitude of the assisted hopping term in a quantum dot in the limit of large level spacing is comparable to other corrections induced by the electron-electron interactions. Assisted hopping leads to differences between conductance peaks associated to the same level, and, when the effect is sufficiently strong, to local pairing correlations.
The anomalous low-temperature behavior of the spontaneous magnetization of ordered arrays of In_{1-x}Mn_xAs quantum dots is discussed. It is shown that the experimental results can be well understood, within a mean field approximation, assuming collective response through rather strong spin tunneling processes between neighbouring dots.
We investigated the size dependence of the ground state energy in self-assembled InAs quantum dots embedded in resonant tunneling diodes. Individual current steps observed in the current-voltage characteristics are attributed to resonant single-electron tunneling via the ground state of individual InAs quantum dots. The onset voltage of the first step observed is shown to decrease systematically from 200 mV to 0 with increasing InAs coverage. We relate this to a coverage-dependent size of InAs dots grown on AlAs. The results are confirmed by atomic force micrographs and photoluminescence exper…
Molecular beam epitaxy is employed to manufacture self-assembled InAs/AlAs quantum-dot resonant tunneling diodes. Resonant tunneling current is superimposed on the thermal current, and they make up the total electron transport in devices. Steps in current-voltage characteristics and peaks in capacitance-voltage characteristics are explained as electron resonant tunneling via quantum dots at 77K or 300K, and this is the first time that resonant tunneling is observed at room temperature in III-V quantum-dot materials. Hysteresis loops in the curves are attributed to hot electron injection/emissi…
We report on non-linear magneto-tunneling experiments carried out in single GaAs vertical quantum dots. We show that conduction at low voltage bias can be a spectroscopic tool for both the ground state and first excited states of few electrons on the dot. Increasing voltage a large resonant peak is observed and attributed to tunneling across the quasi-continuum of higher excited states.
Molecular beam epitaxy is employed to manufacture self-assembled InAs/GaAs quantum dot Schottky resonant tunneling diodes. By virtue of a thin AlAs insertion barrier, the thermal current is effectively reduced and electron resonant tunneling through quantum dots under both forward and reverse biased conditions is observed at relatively high temperature of 77K. The ground states of quantum dots are found to be at ~0.19eV below the conduction band of GaAs matrix. The theoretical computations are in conformity with experimental data.
We analyze dephasing in single and double quantum dot systems. The decoherence is introduced by the Büttiker model with current conserving fictitious voltage leads connected to the dots. By using the non-equilibrium Green function method, we investigate the dephasing effect on the tunneling current. It is shown that a finite dephasing rate leads to observable effects. The result can be used to measure dephasing rates in quantum dots.
new defect-assisted mechanism of multiphonon intraband carrier relaxation in semiconductor quantum dots, where the carrier is found in a coherent superposition of the initial, final, and defect states, is proposed. It is shown that this mechanism is capable of explaining the observed trends in temperature dependences of the intraband relaxation rates for PbSe and CdSe colloidal nanocrystal quantum dots.
We review experiments on single electron transport through single quantum dots in the presence of a microwave signal. In the case of a small dot with well-resolved discrete energy states, the applied high-frequency signal allows for inelastic tunnel events that involve the exchange of photons with the microwave field. These photon assisted tunneling (PAT) processes give rise to sideband resonances in addition to the main resonance. Photon absorption can also lead to tunneling via excited states instead of tunneling via the ground state of the quantum dot.
We report on electron transport through an artificial molecule formed by two tunnel coupled quantum dots, which are laterally confined in a two-dimensional electron system of an Al$_x$Ga$_{1-x}$As/GaAs heterostructure. Coherent molecular states in the coupled dots are probed by photon-assisted tunneling (PAT). Above 10 GHz, we observe clear PAT as a result of the resonance between the microwave photons and the molecular states. Below 8 GHz, a pronounced superposition of phonon- and photon-assisted tunneling is observed. Coherent superposition of molecular states persists under excitation of ac…
We study fluctuations of conductance of two connected in series dots in the Coulomb blockade regime. The pattern of the fluctuations turns out to be extremely sensitive to a magnetic field. These conductance fluctuations also provide information about the mesoscopic fluctuations of charge of a partially-opened single quantum dot, which are hard to measure directly.
We investigated the tunneling current peculiarities in the system of two coupled by means of the external field quantum dots (QDs) weakly connected to the electrodes in the presence of Coulomb correlations. It was found that tuning of the external field frequency induces fast multiple tunneling current switching and leads to the negative tunneling conductivity. Special role of multi-electrons states was demonstrated. Moreover we revealed conditions for bistable behavior of the tunneling current in the coupled QDs with Coulomb correlations.
Current steps attributed to resonant tunneling through individual InAs quantum dots embedded in a GaAs-AlAs-GaAs tunneling device are investigated experimentally in magnetic fields up to 28 T. The steps evolve into strongly enhanced current peaks in high fields. This can be understood as a field-induced Fermi-edge singularity due to the Coulomb interaction between the tunneling electron on the quantum dot and the partly spin polarized Fermi sea in the Landau quantized three-dimensional emitter.
We investigate quantum photovoltaic effect in double quantum dots by applying nonequilibrium quantum master equation. The drastic suppression of the photovoltaic current is observed near the open circuit voltage, which leads to the large filling factor. We find that there always exists an optimal inter-dot tunneling that significantly enhances the photovoltaic current. The maximal output power will also be obtained around the optimal inter-dot tunneling. Moreover, the open circuit voltage approximately behaves as the product of the eigen-level gap and the Carnot efficiency. These results sugge…
Reliable detection of single electron tunneling in quantum dots (QD) is paramount to use this category of device for quantum information processing. Here, we report charge sensing in a degenerately phosphorus-doped silicon QD by means of a capacitively coupled single-electron tunneling device made of the same material. Besides accurate counting of tunneling events in the QD, we demonstrate that this architecture can be operated to reveal asymmetries in the transport characteristic of the QD. Indeed, the observation of gate voltage shifts in the detector's response as the QD bias is changed is …
We report measurements of the nonlinear conductance of InAs nanowire quantum dots coupled to superconducting leads. We observe a clear alternation between odd and even occupation of the dot, with sub-gap-peaks at $|V_{sd}|=Δ/e$ markedly stronger(weaker) than the quasiparticle tunneling peaks at $|V_{sd}|=2Δ/e$ for odd(even) occupation. We attribute the enhanced $Δ$-peak to an interplay between Kondo-correlations and Andreev tunneling in dots with an odd number of spins, and substantiate this interpretation by a poor man's scaling analysis.
Pauli exclusion principle can lead to a supression of transport through double quantum dots, even if both quantum dots are connected by a resonant driving ac field. In this work, we study how this effect, known as spin blockade, can be removed by means of the influence of the driving field on the contacts to the leads.
Realistic relaxed configurations of triaxially strained graphene quantum dots are obtained from unbiased atomistic mechanical simulations. The local electronic structure and quantum transport characteristics of y-junctions based on such dots are studied, revealing that the quasi-uniform pseudomagnetic field induced by strain restricts transport to Landau level- and edge state-assisted resonant tunneling. Valley degeneracy is broken in the presence of an external field, allowing the selective filtering of the valley and chirality of the states assisting in the resonant tunneling. Asymmetric str…
We derive the quantum rate equations for an Aharonov-Bohm interferometer with two vertically coupled quantum dots embedded in each of two arms by means of the nonequilibrium Green's function in the sequential tunneling regime. Basing on these equations, we investigate time-dependent resonant tunneling under a small amplitude irradiation and find that the resonant photon-assisted tunneling peaks in photocurrent demonstrate a combination behavior of Fano and Lorentzian resonances due to the interference effect between the two pathways in this parallel configuration, which is controllable by thre…
Tunneling spectroscopy of InAs nanocrystals deposited on graphite was measured using scanning tunneling microscopy, in a double-barrier tunnel-junction configuration. The effect of the junction symmetry on the tunneling spectra is studied experimentally and modeled theoretically. When the tip is retracted, we observe resonant tunneling through the nanocrystal states without charging. This is in contrast to previous measurements on similar nanocrystals anchored to gold by linker molecules, where charging took place. Charging is regained upon reducing the tip-nanocrystal distance, making the jun…
Due to the Fermi-Dirac statistics of electrons the temporal correlations of tunneling events in a double barrier setup are typically negative. Here, we investigate the shot noise behavior of a system of two capacitively coupled quantum dot states by means of a Master equation model. In an asymmetric setup positive correlations in the tunneling current can arise due to the bunching of tunneling events. The underlying mechanism will be discussed in detail in terms of the current-current correlation function and the frequency-dependent Fano factor.
We address the question of how to model electron transport through closed Aharonov-Bohm interferometers which contain quantum dots. By explicitly studying interferometers with one and two quantum dots, we establish the connection between a tunneling-Hamiltonian formulation on the one hand and a scattering-matrix approach on the other hand. We prove that, under certain circumstances, both approaches are equivalent, i.e., both types of models can describe the same experimental setups. Furthermore, we analyze how the interplay of the Aharonov-Bohm phase and the orbital phase associated with the l…
Based on the dielectric continuum model, we calculated the phonon assisted tunneling (PAT) current of general double barrier resonant tunneling structures (DBRTSs) including both symmetric and antisymmetric ones. The results indicate that the four higher frequency interface phonon modes (especially the one which peaks at either interface of the emitter barrier) dominate the PAT processes, which increase the valley current and decrease the PVR of the DBRTSs. We show that an asymmetric structure can lead to improved performance.
We present Coulomb Blockade measurements of two few-electron quantum dots in series which are configured such that the electrochemical potential of one of the two dots is aligned with spin-selective leads. The charge transfer through the system requires co-tunneling through the second dot which is $not$ in resonance with the leads. The observed amplitude modulation of the resulting current is found to reflect spin blockade events occurring through either of the two dots. We also confirm that charge redistribution events occurring in the off-resonance dot are detected indirectly via changes in …
We investigated the peculiarities of non-equilibrium charge states and spin configurations in the system of two strongly coupled quantum dots (QDs) weakly connected to the electrodes in the presence of Coulomb correlations. We analyzed the modification of non-equilibrium charge states and different spin configurations of the system in a wide range of applied bias voltage and revealed well pronounced ranges of system parameters where negative tunneling conductivity appears due to the Coulomb correlations.
We review recent work on resonant Andreev tunneling through a strongly interacting quantum dot connected to a normal and to a superconducting lead. We derive a general expression for the current flowing in the structure and discuss the linear and non-linear transport in the nonperturbative regime. New effects associated to the Kondo resonance combined with the two-particle tunneling arise. The Kondo anomaly in the $I-V$ characteristics depends on the relative size of the gap energy and the Kondo temperature.
We consider the photon-assisted tunneling (PAT) and the Landau-Zener-Stueckelberg (LZS) interference for double quantum dots induced electrostatically along a semiconducting carbon nanotube. An atomistic tight-binding approach and the time-dependent configuration interaction method are employed for description of the systems of a few confined electrons and holes. We reproduce the patterns of the LZS interference recently observed for the quantum double dots describing transport across hole-localized states. Moreover, we indicate that for charge configurations for which the ground-state is Paul…
We investigated the electron transport property of the InGaAs/GaAs double quantum dots, the electron g-factors of which are different from each other. We found that in a magnetic field, the resonant tunneling is suppressed even if one of the Zeeman sublevels is aligned. This is because the other misaligned Zeeman sublevels limit the total current. A finite broadening of the misaligned sublevel partially relieves this bottleneck effect, and the maximum current is reached when interdot detuning is half the Zeeman energy difference.
We reexamine the tunneling density of states in quantum dots and nanoparticles within the model which is extension of the universal Hamiltonian to the case of uniaxial anisotropic exchange. We derive the exact analytical result for the tunneling density of states in the case of arbitrary single-particle energy spectrum. We find that, similar to the case of the isotropic exchange, the tunneling density of states as a function of energy has the maximum due to a finite value of the total spin of the ground state near the Stoner instability. We demonstrate that there are no additional extrema whic…
Single-electron tunneling through a zero-dimensional state in an asymmetric double-barrier resonant-tunneling structure is studied. The broadening of steps in the $I$--$V$ characteristics is found to strongly depend on the polarity of the applied bias voltage. Based on a qualitative picture for the finite-life-time broadening of the quantum dot states and a quantitative comparison of the experimental data with a non-equilibrium transport theory, we identify this polarity dependence as a clear signature of Coulomb interaction.
We investigated the peculiarities of non-equilibrium charge configurations in the system of two strongly coupled quantum dots (QDs) weakly connected to the reservoirs in the presence of Coulomb correlations. We revealed that total electron occupation demonstrates in some cases significant decreasing with increasing of applied bias - contrary to the situation when Coulomb correlations are absent and found well pronounced ranges of system parameters where negative tunneling conductivity appears due to the Coulomb correlations.
We investigate the noise properties of a GaAs-AlAs-GaAs tunneling structure with embedded self-assembled InAs quantum dots in the single-electron tunneling regime. We analyze the dependence of the relative noise amplitude of the shot noise on bias voltage. We observe a non-monotonic behaviour of the Fano-factor $α$ with an average value of $α\approx 0.8$ consistent with the asymmetry of the tunneling barriers. Reproducible fluctuations observed in $α$ can be attributed to the successive participation of more and more InAs quantum dots in the tunneling current.
With downscaling of electronic circuits, components based on semiconductor quantum dots are assuming increasing relevance for future technologies. Their response under external stimuli intrinsically depend on their quantum properties. Here we investigate single-electron tunneling in hard-wall InAs/InP nanowires in the presence of an off-resonant microwave drive. Our heterostructured nanowires include InAs quantum dots (QDs) and exhibit different tunnel-current regimes. In particular, for source-drain bias up to few mV Coulomb diamonds spread with increasing contrast as a function of microwave …
With the aim of improving solar cell efficiency, a structure for realizing electron tunneling from In0.6Al0.4As quantum dots (QDs) through an Al0.4Ga0.6As barrier to AlAs has been grown using molecular beam epitaxy. The photoluminescence decay time decreased from 1.1 ns to 390 ps as the barrier thickness decreased from 4 to 2 nm, which indicates that the photo-excited carriers tunneled from the QDs to the AlAs X energy level for a barrier thickness 2 nm in 0.6 ns, which is significantly longer than the tunneling time of GaAs and InAlAs quantum wells. We expect that this structure will assist i…
Magnetization measurements of a truly axial symmetry Mn12-tBuAc molecular nanomagnet with a spin ground state of S = 10 show resonance tunneling. This compound has the same magnetic anisotropy as Mn12-Ac but the molecules are better isolated and the crystals have less disorder and a higher symmetry. Hysteresis loop measurements at several temperatures reveal a well-resolved step fine-structure which is due to level crossings of excited states. All step positions can be modeled by a simple spin Hamiltonian. The crossover between thermally assisted and pure quantum tunneling can be investigated …
We study the sub-gap spectrum and the transport properties of a double quantum dot coupled to metallic and superconducting leads. The coupling of both quantum dots to the superconducting lead induces a non-local pairing in both quantum dots by the Andreev reflection processes. Additionally, we obtain two channels of Cooper pair tunneling into a superconducting lead. In such a system, the direct tunneling process (by one of two dots) or the crossed tunneling process (by both quantum dots at the same time) is possible. We consider the dependence of the Andreev transmittance on an inter-dot tunne…
We study transport through one or two ultrasmall quantum dots with discrete energy levels to which a time-dependent field is applied (e.g., microwaves). The AC field causes photon-assisted tunneling and also transitions between discrete energy levels of the dot. We treat the problem by introducing a generalization of the rotating-wave approximation to arbitrarily many levels. We calculate the dc-current through one dot and find satisfactory agreement with recent experiments by Oosterkamp et al. . In addition, we propose a novel electron pump consisting of two serially coupled single-level quan…
We systematically investigate electron transport through double quantum dots with particular emphasis on interference induced via multiple paths of electron propagation. By means of the slave-boson mean-field approximation, we calculate the conductance, the local density of states, the transmission probability in the Kondo regime at zero temperature. It is clarified how the Kondo-assisted transport changes its properties when the system is continuously changed among the serial, parallel and T-shaped double dots. The obtained results for the conductance are explained in terms of the Kondo reson…
We present time-resolved measurements of spin-flip photon-assisted tunneling and spin-flip relaxation in a doubly occupied double quantum dot. The photon-assisted excitation rate as a function of magnetic field indicates that spin-orbit coupling is the dominant mechanism behind the spin-flip under the present conditions. We are able to extract the resulting effective `spin-flip tunneling' energy, which is found to be three orders of magnitude smaller than the regular spin-conserving tunneling energy. We also measure the relaxation and dephasing times of a qubit formed out of two two-electron s…
We study the electronic structure of a single self-assembled InAs quantum dot by probing elastic single-electron tunneling through a single pair of weakly coupled dots. In the region below pinch-off voltage, the non-linear threshold voltage behavior provides electronic addition energies exactly as the linear, Coulomb blockade oscillation does. By analyzing it, we identify the s and p shell addition spectrum for up to six electrons in the single InAs dot, i.e. one of the coupled dots. The evolution of shell addition spectrum with magnetic field provides Fock-Darwin spectra of s and p shell.
We present the first experimental observation of resonance-assisted tunneling, a wave phenomenon, where regular-to-chaotic tunneling is strongly enhanced by the presence of a classical nonlinear resonance chain. For this we use a microwave cavity made of oxygen free copper with the shape of a desymmetrized cosine billiard designed with a large nonlinear resonance chain in the regular region. It is opened in a region, where only chaotic dynamics takes place, such that the tunneling rate of a regular mode to the chaotic region increases the line width of the mode. Resonance-assisted tunneling is…
The transfer Hamiltonian tunneling current is derived in a time-dependent density matrix formulation and is used to examine photon-assisted tunneling. Bardeen's tunneling expression arises as the result of first order perturbation theory in a mean-field expansion of the density matrix. Photon-assisted tunneling from confined electromagnetic fields in the forbidden barrier region occurs due to time-varying polarization and wavefunction overlap in the gap, which leads to a non-zero tunneling current in asymmetric device structures, even in an unbiased state. The photon energy is seen to act as a…
We derive general expressions for the current and shot noise, taking into account non-Markovian memory effects. In generalization of previous approaches our theory is valid for arbitrary Coulomb interaction and coupling strength and is applicable to quantum dots and more complex systems like molecules. A diagrammatic expansion up to second-order in the coupling strength, taking into account co-tunneling processes, allows for a study of transport in a regime relevant to many experiments. As an example, we consider a single-level quantum dot, focusing on the Coulomb-blockade regime. We find supe…
We consider electrical transport through single molecules coupled to metal electrodes via tunneling barriers. Approximating the molecule by the Anderson impurity model as the simplest model which includes Coulomb charging effects, we extend the ``orthodox'' theory to expand current and shot noise systematically order by order in the tunnel couplings. In particular, we show that a combined measurement of current and shot noise reveals detailed information of the system even in the weak-coupling limit, such as the ratio of the tunnel-coupling strengths of the molecule to the left and right elect…
Graphene quantum dots are considered as promising alternatives to quantum dots in III-V semiconductors, e.g., for the use as spin qubits due to their consistency made of light atoms including spin-free nuclei which both imply relatively long spin decoherene times. However, this potential has not been realized in experiments so far, most likely, due to a missing control of the edge configurations of the quantum dots. Thus, a more fundamental investigation of Graphene quantum dots appears to be necessary including a full control of the wave function properties most favorably during transport spe…
We present a theoretical study of the evolution of spatial coherence during relaxation between delocalized exciton states in a pair of vertically stacked semiconductor quantum dots coupled to acoustic phonons. We show that spatial coherence can be transferred to the ground state even in a system of uncoupled non-identical quantum dots if a particular kind of degeneracy between the inter-level energy splittings is present. The phonon-assisted mechanism of coherence transfer leads to a dependence of the amount of the resulting coherence on the inter-dot distance and temperature. We analyze also …
Josephson oscillations generated by a SQUID were used to measure photon assisted tunneling in a superconducting single electron tunneling (SET) transistor. The SQUID was fabricated only a few microns from the SET transistor. The close proximity of the generator to the SET transistor allowed for a continuous sweep of the excitation frequency from 10 GHz to 190 GHz. The amplitude of the Josephson oscillations were tuned by adjusting the flux through the SQUID loop while the frequency was independently adjusted by controlling the bias voltage across the SQUID. Current peaks which could be attribu…
We report both photon- and phonon-assisted tunneling transitions in a linear array of three quantum dots, which can only be understood by considering the full three-dimensionality of the charge stability diagram. Such tunneling transitions potentially contribute to leakage of qubits defined in this system. A detailed understanding of these transitions is important as they become more abundant and complex to analyze as quantum dot arrays are scaled up.
We measure electron tunneling in single-molecule transistors made from C_{140}, a molecule with a mass-spring-mass geometry chosen as a model system to study electron-vibration coupling. We observe vibration-assisted tunneling at an energy corresponding to the stretching mode of C_{140}. Molecular modeling provides explanations for why this mode couples more strongly to electron tunneling than the other internal modes of the molecule. We make comparisons between the observed tunneling rates and those expected from the Franck-Condon model.
We theoretically investigate phosphorene zigzag nanorribons as a platform for constriction engineering. In the presence of a constriction at the upper edge, quantum confinement of edge protected states reveals resonant tunnelling Breit-Wigner transmission peaks, if the upper edge is uncoupled to the lower edge. Coupling between edges in thin constrictions gives rise to Fano-like and anti-resonances in the transmission spectrum of the system.
Resonant electron transport through a mesoscopic region (quantum dot or single molecule) with electron-phonon interaction is considered at finite voltage. In this case the standard Landauer-Büttiker approach cannot be applied. Using the nonequilibrium Green function method we show that due to a nonequilibrium distribution function of electrons in the mesoscopic region, the inelastic scattering rate and spectral function of the dot become functions of the voltage and have to be calculated self-consistently.
We investigate the zero-frequency shot noise of electronic tunneling through a single quantum dot (SQD) and coherently coupled quantum dots (CQD) taking into account the Coulomb interaction. Within Hartree-Fock approximation, the analytical expressions of current and zero-frequency shot noise are self-consistently derived in the framework of full counting statistics for the both systems. We demonstrate that the correction term of zero-frequency shot noise induced by the intradot Coulomb interaction is almost negligible compared to the noninteracting shot noise in a SQD, while in a CQD the inte…
We present a TCAD compatible multiscale model of phonon-assisted band-to-band tunneling (BTBT) in semiconductors, that incorporates the non-parabolic nature of complex bands within the bandgap of the material. This model is shown capture the measured current-voltage data in silicon, for current transport along the $[100]$, $[110]$ and $[111]$ directions. Our model will be useful to predict band-to-band tunneling phenomena to quantify on and off currents in Tunnel FETs and in small geometry MOSFETs and FINFETs.
We investigate the optical response of quantum dot molecules coherently driven by polarized laser light. Our description includes the splitting in excitonic levels caused by isotropic and anisotropic exchange interactions. We consider interdot transitions mediated by hole tunneling between states with the same total angular momentum and between bright and dark exciton states, as allowed by spin-flip hopping between the dots in the molecule. Using realistic experimental parameters we demonstrate that the excitonic states coupled by tunneling exhibit a rich and controllable optical response. We …
We perform the investigations of the resonant tunneling via impurities embedded in the AlAs barrier of a single GaAs/AlGaAs heterostructure. In the $I(V)$ characteristics measured at 30mK, the contribution of individual donors is resolved and the fingerprints of phonon assistance in the tunneling process are seen. The latter is confirmed by detailed analysis of the tunneling rates and the modeling of the resonant tunneling contribution to the current. Moreover, fluctuations of the local structure of the DOS (LDOS) and Fermi edge singularities are observed.
We have studied electron transport in clean single-walled carbon nanotube quantum dots. Because of the large number of Coulomb blockade diamonds simultaneously showing both shell structure and Kondo effect, we are able to perform a detailed analysis of tunneling renormalization effects. Thus determining the environment induced level shifts of this artificial atom. In shells where only one of the two orbitals is coupled strongly, we observe a marked asymmetric gate-dependence of the inelastic cotunneling lines together with a systematic gate dependence of the size (and shape) of the Coulomb dia…
