Photovoltaic effect of double quantum dots under nonuniform light field intensity has been studied theoretically. Comparing with the traditional p-n type photovoltaic effect, the inhomogeneous light field provides asymmetric potential creating polarization of electron number distribution in the neighboring quantum dots and furthermore gives rise to net current. Current density and efficiency of such kind solar cells are estimated to be comparable to the traditional p-n type material based solar cells. Motion of electron is described using quantum master equation around room temperature. The inhomogeneous light photovoltaic effect has potential applications for the gain of more economical solar cells.
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…
We investigate theoretically and experimentally the statistical properties of dc current through an open quantum dot subject to ac excitation of a shape-defining gate. The symmetries of rectification current and photovoltaic current with respect to applied magnetic field are examined. Theory and experiment are found to be in good agreement throughout a broad range of frequency and ac power, ranging from adiabatic to nonadiabatic regimes.
We analyze the photovoltaic current through a double quantum dot system coupled to a high-quality driven microwave resonator. The conversion of photons in the resonator to electronic excitations produces a current flow even at zero bias across the leads of the double quantum dot system. We demonstrate that due to the quantum nature of the electromagnetic field in the resonator, the photovoltaic current exhibits a double peak dependence on the frequency $ω$ of an external microwave source. The distance between the peaks is determined by the strength of interaction between photons in the resonat…
We propose a random matrix theory to describe the influence of a time-dependent external field on electron transport through open quantum dots. We describe the generation of the current by an oscillating field for the dot, connected to two leads with equal chemical potentials. For low frequency fields our results correspond to adiabatic charge pumping. Finite current can be produced if the system goes along a closed loop in parameter space, which covers a finite area. At high frequency finite current is produced even if the loop is a line in parameter space. This result can be explained in the…
Electron transport through a quantum dot chain with two neighboring dots coupled to both leads is theoretically studied. In such a system, it is found that only for the even-numbered quantum dot structure with the same-number quantum dots coupled to each connecting dot, some eigenstates of the quantum dots decouple from the leads. Namely, all odd eigenstates decouple from the leads in the absence of magnetic flux, but all even eigenstates will decouple from the leads when a magnetic flux is introduced. In addition, by adjusting the magnetic fluxes through any subring, some eigenstates decouple…
We used AlGaSb/AlGaAs material system for a theoretical study of photovoltaic performance of the proposed GaAs-based solar cell in which the type-II quantum dot (QDs) absorber is spatially separated from the depletion region. Due to inelastic scattering of photoelectrons on QDs and proper doping of both QDs and their spacers, concentrated sunlight is predicted to quench recombination through QDs. Our calculation shows that 500-sun concentration can increase the Shockley-Queisser limit from 35% to 40% for GaAs single-junction solar cells.
Semiconductor coupled quantum dots provide a unique opportunity of tuning bandgaps by tailoring band offsets, making them ideal for photovoltaic and other applications. Here, we have studied stability, trends in the band gap, band offsets, and optical properties for a series of coupled quantum dots comprised of II-VI semiconductor using a hybrid functional method. We have shown how the quantum confinement and interfacial strain considerably affect the band gap and band offsets for these heterostructures at the nanoscale. We show that the trend in band offsets obtained from our first-principles…
We extend the theory of Coulomb blockade oscillations to quantum dots which are deformed by the confining potential. We show that shape deformations can generate sequences of conductance resonances which carry the same internal wavefunction. This fact may cause strong correlations of neighboring conductance peaks. We demonstrate the relevance of our results for the interpretation of recent experiments on semiconductor quantum dots.
In this work, the influence of Au plasmonics on photocurrent generation in the visible wavelength range in integrated thiol-linked CdSe quantum dot/Au nanogap structures is demonstrated. The plasmonic absorption is sensed utilizing photoacoustic (PA) detection technique. The laser diode is modulated to generate the PA excitation that oscillates the air-filled cell. When light absorption increases, an enhanced acoustic signal is captured by a microphone. The observed enhancement in the PA response is related to plasmonic absorption by the Au layers and the response is further enhanced by about …
The magnitude and phase of the transmission amplitude of a multi-level quantum dot is calculated for the mesoscopic regime of level spacing large compared to level width. The interplay between Kondo correlations and the influence by neighboring levels is discussed. As in the single-level case, the Kondo plateaus of magnitude and phase disappear with increasing temperature. At certain gate voltages, ``stationary'' points are found at which the transmission phase is independent of temperature. Depending on the mesoscopic parameters of the adjacent levels (like relative sign and magnitude of tunn…
Metal halide perovskites quantum dots (MHPQDs) have aroused enormous interesting in the photovoltaic and photoelectric because of their marvelous properties and size characteristics. However, one of key problems that how to systematically analyze charge carriers trapping by different defects is still a challenge task. Here, we study nonradiation multiphonon processes of the charge carrier trapping by various defects in MHPQDs based on the well-known Huang-Rhys model, in which a method of fullconfiguration defect, including different defect species with variable depth and lattice relaxation str…
We study the g-factor of discrete electron states in InAs nanowire based quantum dots. The g values are determined from the magnetic field splitting of the zero bias anomaly due to the spin 1/2-Kondo effect. Unlike to previous studies based on 2DEG quantum dots, the g-factors of neighboring electron states show a surprisingly large fluctuation: g can scatter between 2 and 18. Furthermore electric gate tunability of the g-factor is demonstrated.
We explore the structural changes in charge the density and the electron configuration of quantum dots caused by the presence of other dots in an array, and the interaction of neighboring dots. We discuss what recent measurements and calculation of the far-infrared absorption reveal about almost isolated quantum dots and investigate some aspects of the complex transition from isolated dots to dots with strongly overlapping electron density. We also address the the effects on the magnetization of such dot array.
We present a self-contained theoretical and computational framework for dynamics following photoexcitation in quantum dots near planar interfaces. A microscopic Hamiltonian parameterized by first principles calculations is merged with a reduced density matrix formalism that allows for the prediction of time-dependent charge and energy transfer processes between the quantum dot and the electrode. While treating charge and energy transfer processes on an equal footing, the non-perturbative effects of sudden charge transitions on the Fermi sea of the electrode are included. We illustrate the form…
The purpose of this work is to look for a practical structure for application of quantum dots (QD) in solar cells in order to enhance sub-band gap photon absorption. We focuse on a stack of strain-compensated GaSb/GaAs type-II QDs. We propose a novel structure with GaSb/GaAs type-II QD absorber embedded in the p-doped region of ideal solar cell, but spatially separated from the depletion region. We developed the model and used the detailed balance principle along with Poisson and continuity equations for calculating of the energy band bending along with the photocurrent and the dark current, a…
We propose a new implementation of a universal set of one- and two-qubit gates for quantum computation using the spin states of coupled single-electron quantum dots. Desired operations are effected by the gating of the tunneling barrier between neighboring dots. Several measures of the gate quality are computed within a newly derived spin master equation incorporating decoherence caused by a prototypical magnetic environment. Dot-array experiments which would provide an initial demonstration of the desired non-equilibrium spin dynamics are proposed.
In the present theoretical work we have considered impurities, either boron or phosphorous, located at different substitutional sites in silicon quantum dots (Si-QDs) with diameters around 1.5\,nm, embedded in a SiO2 matrix. Formation energy calculations reveal that the most energetically-favored doping sites are inside the QD and at the Si/SiO2 interface for P and B impurities, respectively. Furthermore, electron and hole transport calculations show in all the cases a strong reduction of the minimum voltage threshold, and a corresponding increase of the total current in the low-voltage regime…
We measure telegraph noise of current fluctuations in an electrostatically defined quantum dot in bilayer graphene by real-time detection of single electron tunneling with a capacitively coupled neighboring quantum dot. Suppression of the second and third cumulant (related to shot noise) in a tunable graphene quantum dot is demonstrated experimentally. With this method we demonstrate the ability to measure very low current and noise levels. Furthermore, we use this method to investigate the first spin excited state, an essential prerequisite to measure spin relaxation.
In monolayers of transition metal dichalcogenides the nonlocal nature of the effective dielectric screening leads to large binding energies of excitons. Additional lateral confinement gives rise to exciton localization in quantum dots. By assuming parabolic confinement for both the electron and the hole, we derive model wave functions for the relative and the center-of-mass motions of electron-hole pairs, and investigate theoretically resonant energy transfer among excitons localized in two neighboring quantum dots. We quantify the probability of energy transfer for a direct-gap transition by …
Deterministically integrating semiconductor quantum emitters with plasmonic nano-devices paves the way towards chip-scale integrable, true nanoscale quantum photonics technologies. For this purpose, stable and bright semiconductor emitters are needed, which moreover allow for CMOS-compatibility and optical activity in the telecommunication band. Here, we demonstrate strongly enhanced light-matter coupling of single near-surface ($<10\,nm$) InAs quantum dots monolithically integrated into electromagnetic hot-spots of sub-wavelength sized metal nanoantennas. The antenna strongly enhances the emi…
The effect of the surface curvature on the magnetic moment and persistent currents in two-dimensional (2D) quantum rings and dots is investigated. It is shown that the surface curvature decreases the spacing between neighboring maxima of de Haas -- van Alphen (dHvA) type oscillations of the magnetic moment of a ring and decreases the amplitude and period of Aharonov -- Bohm (AB) type oscillations. In the case of a quantum dot, the surface curvature reduces the level degeneracy at zero magnetic fields. This leads to a suppression of the magnetic moment at low magnetic fields. The relation betwe…
Recently lead halide nanocrystals (quantum dots) have been reported with potential for photovoltaic and optoelectronic applications due to their excellent luminescent properties. Herein excitonic photoluminescence (PL) excited by two-photon absorption in perovskite CsPbBr3 quantum dots (QDs) have been studied across a broad temperature range from 80K to 380K. Two-photon absorption has been investigated with absorption coefficient up to 0.085 cm/GW at room temperature. Moreover, the photoluminescence excited by two-photon absorption shows a linear blue-shift (0.25meV/K) below temperature of ~22…
In semiconductors, photoexcited electrons and holes (carriers) initially occupy high-energy states, but quickly lose energy to phonons and relax to the band edge within a picosecond [1]. Increasing the lifetime of carriers in light-absorbing materials is necessary to improve open-circuit voltage in photovoltaics [2], charge separation in organic solar cells [3], and charge transfer in photodetection de vices [4]. Here we demonstrate long lifetimes over one hundred picoseconds for electron-hole pairs in graphene quantum dots (GQDs) due to large transition energies and weak coupling to excitonic…
Many advanced concepts for high-efficiency photovoltaic devices exploit the peculiar optoelectronic properties of semiconductor nanostructures such as quantum wells, wires and dots. While the optics of such devices is only modestly affected due to the small size of the structures, the optical transitions and electronic transport can strongly deviate from the simple bulk picture known from conventional solar cell devices. This review article discusses the challenges for an adequate theoretical description of the photovoltaic device operation arising from the introduction of nanostructure absorb…
We present the photoluminescence excitation study of the self-assembled CdTe/ZnTe quantum dots doped with manganese ions. We demonstrate the identification method of spontaneously coupled quantum dots pairs containing single Mn2+ ions. As the result of the coupling, the resonant absorption of the photon in one quantum dot is followed by the exciton transfer into a neighboring dot. It is shown that the Mn2+ ion might be present in the absorbing, emitting or both quantum dots. The magnetic properties of the Mn2+ spin are revealed by a characteristic sixfold splitting of the excitonic line. The s…
We review the random matrix description of electron transport through open quantum dots, subject to time-dependent perturbations. All characteristics of the current linear in the bias can be expressed in terms of the scattering matrix, calculated for a time-dependent Hamiltonian. Assuming that the Hamiltonian belongs to a Gaussian ensemble of random matrices, we investigate various statistical properties of the direct current in the ensemble. Particularly, even at zero bias the time-dependent perturbation induces current, called photovoltaic current. We discuss dependence of the photovoltaic c…
Liquid He-4 is free from magnetic defects, making it an ideal substrate for electrons with long-lived spin states. Such states can serve as qubit states. Here we consider the spin states of electrons electrostatically localized in quantum dots on a helium surface. Efficient gate operations in this system require spin-orbit coupling. It can be created by a nonuniform magnetic field from a current-carrying wire, can be turned on and off, and allows one to obtain large electro-dipole moment and comparatively fast coupling of spins in neighboring dots. Of central importance is to understand the sp…
We study bistability in the electron transport through a ring of N coupled quantum dots with two orbitals in each dot. One orbital is localized (called b orbital) and coupling of the b orbitals in any two dots is negligible; the other is delocalized in the plane of the ring (called d orbital), due to coupling of the d orbitals in the neighboring dots, as described by a tight-binding model. The d orbitals thereby form a band with finite width. The b and d orbitals are connected to the source and drain electrodes with a voltage bias V, allowing the electron tunnelling. Tunnelling current is calc…
Highly accurate quantum electron dynamics calculations demonstrate that energy can be efficiently transferred between quantum dots. Specifically, in a double quantum dot an incoming electron is captured by one dot and the excess energy is transferred to the neighboring dot and used to remove an electron from this dot. This process is due to long-range electron correlation and shown to be operative at rather large distances between the dots. The efficiency of the process is greatly enhanced by preparing the double quantum dot such that the incoming electron is initially captured by a two-electr…
We have measured the Zeeman splitting of quantum levels in few-electron quantum dots (QDs) formed in narrow bandgap InSb nanowires via the Schottky barriers at the contacts under application of different spatially orientated magnetic fields. The effective g-factor tensor extracted from the measurements is strongly anisotropic and level-dependent, which can be attributed to the presence of strong spin-orbit interaction (SOI) and asymmetric quantum confinement potentials in the QDs. We have demonstrated a successful determination of the principal values and the principal axis orientations of the…
Motivated by recent experiments of Zajac et al. [arXiv:1708.03530], we theoretically describe high-fidelity two-qubit gates using the exchange interaction between the spins in neighboring quantum dots subject to a magnetic field gradient. We use a combination of analytical calculations and numerical simulations to provide the optimal pulse sequences and parameter settings for the gate operation. We present a novel synchronization method which avoids detrimental spin flips during the gate operation and provide details about phase mismatches accumulated during the two-qubit gates which occur due…
Single electron spins in quantum dots are attractive for quantum communication because of their expected long coherence times. We propose a method to create entanglement between two remote spins based on the coincident detection of two photons emitted by the dots. Local nodes of several qubits can be realized using the dipole-dipole interaction between trions in neighboring dots and spectral addressing, allowing the realization of quantum repeater protocols. We have performed a detailed feasibility study of our proposal based on tight-binding calculations of quantum dot properties.
We recently predicted that the interatomic Coulombic electron capture (ICEC) process, a long-range electron correlation driven capture process, is achievable in gated double quantum dots (DQDs). In ICEC an incoming electron is captured by one QD and the excess energy is used to remove an electron from the neighboring QD. In this work we present systematic full three-dimensional electron dynamics calculations in quasi-one dimensional model potentials that allow for a detailed understanding of the connection between the DQD geometry and the reaction probability for the ICEC process. We derive an…
We present a theory for understanding the exchange interaction between electron spins in neighboring quantum dots, either by changing the detuning of the two quantum dots or independently tuning the tunneling barrier between quantum dots. The Hubbard model and a more realistic confining-potential model are used to investigate how the tilting and barrier control affect the effective exchange coupling and thus the gate fidelity in both the detuning and symmetric regimes. We show that the exchange coupling is less sensitive to the charge noise through tunnel barrier control (while allowing for ex…
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, anti-crossings between indirectly coupled levels and higher order cotunneling are discussed.
A two-dimensional arrangement of quantum dots with finite inter-dot tunnel coupling provides a promising platform for studying complicated spin correlations as well as for constructing large-scale quantum computers. Here, we fabricate a tunnel-coupled triangular triple quantum dot with a novel gate geometry in which three dots are defined by positively biasing the surface gates. At the same time, the small area in the center of the triangle is depleted by negatively biasing the top gate placed above the surface gates. The size of the small center depleted area is estimated from the Aharonov-Bo…
We prepare a triple quantum dot with a separate contact lead to each dot to study Pauli spin blockade in the tunnel-coupled three dots in a row. We measure the tunneling current flowing between the center dot and either the left or right dot with the left and right leads as a common source and the center lead as a drain. In the biased stability diagram, we establish Pauli spin blockade in the respective neighboring dots, with features similarly obtained in double quantum dot systems. We further realize Pauli spin blockade with two different conditions by tuning the inter-dot coupling gates: st…
With the increasing energy crisis and the prevalent concept of green sustainability, quantum dot materials have become a hot spot in the academic and industrial fields of chemistry. Due to unique, tailor-made photovoltaic properties based on marked quantum-confined effects, it's necessary to identify the QD-based charge transfer process connected with a lifetime of stimulated excitons. Additionally, inorganic nanoparticles with a continuum of electron states contribute to the consistency between electron dynamics and their function through complexation with QDs. Ultrafast spectroscopy can be w…
We propose a scalable scheme to implement quantum computation in graphene nanoribbon. It is shown that electron or hole can be naturally localized in each zigzag region for a graphene nanoribbon with a sequence of Z-shaped structure without exploiting any confined gate. An one-dimensional graphene quantum dots chain is formed in such graphene nanoribbon, where electron or hole spin can be encoded as qubits. The coupling interaction between neighboring graphene quantum dots is found to be always-on Heisenberg type. Applying the bang-bang control strategy and decoherence free subspaces encoding …
Nanoscale dielectric resonators and quantum-confined semiconductors have enabled unprecedented control over light absorption and excited charges, respectively. In this work, we embed luminescent silicon nanocrystals (Si-NCs) into a 2D array of SiO2 nanocylinders, and experimentally prove a powerful concept: the resulting metamaterial preserves the radiative properties of the Si-NCs and inherits the spectrally-selective absorption properties of the nanocylinders. This hierarchical approach provides increased photoluminescence (PL) intensity obtained without utilizing any lossy plasmonic compone…
We report on realization and transport spectroscopy study of single quantum dots (QDs) made from InSb nanowires grown by molecular beam epitaxy (MBE). The nanowires employed are 50-80 nm in diameter and the QDs are defined in the nanowires between the source and drain contacts on a Si/SiO$_2$ substrate. We show that highly tunable QD devices can be realized with the MBE-grown InSb nanowires and the gate-to-dot capacitance extracted in the many-electron regimes is scaled linearly with the longitudinal dot size, demonstrating that the devices are of single InSb nanowire QDs even with a longitudi…
Magnetic noise from randomly fluctuating nuclear spin ensembles is the dominating source of decoherence for many multi-quantum-dot multielectron spin qubits. Here we investigate in detail the effect of a DC electric current on the coupled electron-nuclear spin dynamics in double and triple quantum dots tuned to the regime of Pauli spin blockade. We consider both systems with and without significant spin-orbit coupling and find that in all cases the flow of electrons can induce a process of dynamical nuclear spin polarization that effectively suppresses the nuclear polarization gradients over n…
Finding out how symmetry encodes optical polarization information into the selection rule in molecules and materials is important for their optoelectronic applications including spectroscopic analysis, display technology and quantum computation. Here, we extend the polarization-dependent selection rules from atoms to solid systems with point group descriptions via rotational operator for circular polarization and $2$-fold rotational operator (or reflection operator) for linear polarization. As a variant of graphene quantum dot (GQD), twisted bilayer graphene quantum dot (TBGQD) certainly inher…
While electron spins in silicon heterostructures make attractive qubits, little is known about the coherence of electrons at the Si/SiO2 interface. We report spin relaxation (T1) and coherence (T2) times for mobile electrons and natural quantum dots at a 28Si/SiO2 interface. Mobile electrons have short T1 and T2 of 0.3 us at 5 K. In line with predictions, confining electrons and cooling increases T1 to 0.8 ms at 350 mK. In contrast, T2 for quantum dots is around 10 us at 350 mK, increasing to 30 us when the dot density is reduced by a factor of two. The quantum dot T2 is shorter than T1, indic…
We describe a mechanism of spin transfer between individual quantum dots that does not require tunneling. Incident circularly-polarized photons create inter-band excitons with non-zero electron spin in the first quantum dot. When the quantum-dot pair is properly designed, this excitation can be transferred to the neighboring dot via the Coulomb interaction with either {\it conservation} or {\it flipping} of the electron spin. The second dot can radiate circularly-polarized photons at lower energy. Selection rules for spin transfer are determined by the resonant conditions and by the strong spi…
The Heisenberg exchange interaction between neighboring quantum dots allows precise voltage control over spin dynamics, due to the ability to precisely control the overlap of orbital wavefunctions by gate electrodes. This allows the study of fundamental electronic phenomena and finds applications in quantum information processing. Although spin-based quantum circuits based on short-range exchange interactions are possible, the development of scalable, longer-range coupling schemes constitutes a critical challenge within the spin-qubit community. Approaches based on capacitative coupling and ca…
Tuning a material to the cusp between two distinct ground states can produce exotic physical properties, unlike those in either of the neighboring phases. The prospect of designing a model experimental system to capture such behavior is tantalizing. An array of tunnel-coupled quantum dots, each hosting a local spin, should have an appropriately complex phase diagram, but scaling up from individual dots to uniform clusters or lattices has proven difficult: though each site can be tuned to the same occupancy, each has a different set of localized wavefunctions whose couplings to neighboring site…
We demonstrate quantum dot (QD) formation in three-dimensional Dirac semimetal Cd$_{3}$As$_{2}$ nanowires using two electrostatically tuned p$-$n junctions with a gate and magnetic fields. The linear conductance measured as a function of gate voltage under high magnetic fields is strongly suppressed at the Dirac point close to zero conductance, showing strong conductance oscillations. Remarkably, in this regime, the Cd$_{3}$As$_{2}$ nanowire device exhibits Coulomb diamond features, indicating that a clean single QD forms in the Dirac semimetal nanowire. Our results show that a p$-$type QD can…
Quantum dots are nanoscopic systems, where carriers are confined in all three spatial directions. Such nanoscopic systems are suitable for fundamental studies of quantum mechanics and are candidates for applications such as quantum information processing. It was also proposed that linear arrangements of quantum dots could be used as quantum cascade laser. In this work we study the impact of electron-electron interactions on transport in a spinful serial triple quantum dot system weakly coupled to two leads. We find that due to electron-electron scattering processes the transport is enabled bey…
