Interacting Quantum Dot Research Articles

Tuning single-electron charging and interactions between compressible Landau level islands in graphene

Interacting and tunable quantum dots (QDs) have been extensively exploited in condensed matter physics and quantum information science. Using a low-temperature scanning tunneling microscope (STM), we both create and directly image a new type of coupled QD system in graphene, a highly interacting quantum relativistic system with tunable density. Using detailed scanning tunneling spectroscopy (STS) measurements, we show that Landau quantization inside a potential well enables novel electron confinement via the incompressible strips between partially filled Landau levels (LLs), forming isolated and concentric LL QDs. By changing the charge density and the magnetic field we can tune continuously between single- and double-concentric LL QD systems within the same potential well. In the concentric QD regime, single-electron charging peaks of the two dots intersect, displaying a characteristic avoidance pattern. At moderate fields, we observe an unconventional avoidance pattern that differs significantly from that observed in capacitively coupled double-QD systems. We find that we can reproduce in detail this anomalous avoidance pattern within the framework of the electrostatic double-QD model by replacing the capacitive interdot coupling with a phenomenological charge-counting system in which charges in the inner concentric dot are counted in the total charge of both islands. The emergence of such strange forms of interdot coupling in a single potential well, together with the ease of producing such charge pockets in graphene and other two-dimensional (2D) materials, reveals an intriguing testbed for the confinement of 2D electrons in customizable potentials.

Neural network representation and optimization of thermoelectric states of multiple interacting quantum dots

We perform quantum master equation calculations and machine learning to investigate the thermoelectric properties of multiple interacting quantum dots (MQD), including electrical conductance, Seebeck coefficient, thermal conductance and the figure of merit (ZT). We show that by learning from the data obtained from the QME, the thermoelectric states of the MQD can be represented well by a two-layer neural network. We also show that after training, the neural network was able to predict the thermoelectric properties of the MQD with much less computational cost compared to the QME approach. Based on the neural network, we further optimize the MQD to achieve a high ZT and power factor. This work presents a powerful route to study, represent, and optimize interacting quantum many-body systems.

Enhanced Negative Nonlocal Conductance in an Interacting Quantum Dot Connected to Two Ferromagnetic Leads and One Superconducting Lead

In this paper, we investigate the electronic transport properties of a quantum dot (QD) connected to two ferromagnetic leads and one superconducting lead in the Kondo regime by means of the finite-U slave boson mean field approach and the nonequilibrium Green function technique. In this three-terminal hybrid nanodevice, we focus our attention on the joint effects of the Kondo correlation, superconducting proximity pairing, and spin polarization of leads. It is found that the superconducting proximity effect will suppress the linear local conductance (LLC) stemming from the weakened Kondo peak, and when its coupling is bigger than the tunnel-coupling of two normal leads, the linear cross conductance (LCC) becomes negative in the Kondo region. Regarding the antiparallel configuration, increasing spin polarization further suppresses LLC but enhances LCC, i.e., causing larger negative values of LCC, since it is beneficial for the emergence of cross Andreev reflection. On the contrary, for the parallel configuration, with increasing spin polarization, the LLC decreases and greatly widens with the appearance of shoulders, and eventually splits into four peaks, while the LCC decreases relatively rapidly to the normal conductance.

Nonlinear optical properties of hybrid associates of Ag2S quantum dots with erythrosine molecules

Decreasing in the self-defocusing threshold at wavelength of 532 nm using 10 ns pulses of Nd:YAG laser second harmonic radiation for hybrid associates of Ag2S (2.0 nm) quantum dots (QDs) with erythrosine molecules (Ery) was shown with use the Z-scan method. The obtained result is attributed to the manifestation of hybrid properties due to the interaction of QDs and Ery molecules in associate. It can be explained using the data on the hybrid associates spectral-luminescent properties. The hybrid association with Ery molecules blocks Ag2S QDs IR luminescence under excitation with radiation from the region of dye absorption. Based on the absence of noticeable changes in the luminescence lifetime in the nanosecond range, as well as position of HOMO–LUMO dye states with respect to the size quantization states of Ag2S QDs, it was concluded that electron is rapidly transferred from Ery to the size quantization states in the conduction band. It leads to change the degree of QDs band filling.

Steady and dynamic magnetic phase transitions in the open interacting quantum dots arrays

We apply the mean-field Hubbard model, non-equilibrium Green’s function (NEGF) theory and the hierarchical equations of motion (HEOM) method to investigate some magnetic phase transitions in the 1D interacting quantum dots arrays (QDA) sandwiched by non-interaction leads. We find that with a weak device-leads coupling, there exists a ferrimagnetic state in the uniform odd-numbered or the staggered-hopping QDA systems. With increasing the coupling strength, temperature or the bias potential, the extra spin-polarized electrons flow out of the QDA, which results in a magnetism-to-nonmagnetism phase transition. With the spin-resolved HEOM method, we also investigate the transient spin dissipation process in these phase transitions. When applying an abrupt large bias potential or a strong device-leads coupling, the spin polarized electrons decay with oscillations from the QDA into electrodes. We believe our results may be helpful in the future quantum computation devices.

Observation of spin\u2013orbit coupling induced Weyl points in a two-electron double quantum dot

Recent years have brought an explosion of activities in the research of topological aspects of condensed-matter systems. Topological phases of matter are accompanied by protected surface states or exotic degenerate excitations such as Majorana modes or Haldane’s localized spinons. Topologically protected degeneracies can, however, also appear in the bulk. An intriguing example is provided by Weyl semimetals, where topologically protected electronic band degeneracies and exotic surface states emerge even in the absence of interactions. Here we demonstrate experimentally and theoretically that Weyl degeneracies appear naturally in an interacting quantum dot system, for specific values of the external magnetic field. These magnetic Weyl points are robust against spin–orbit coupling unavoidably present in most quantum dot devices. Our transport experiments through an InAs double-dot device placed in magnetic field reveal the presence of a pair of Weyl points, exhibiting a robust ground-state degeneracy and a corresponding protected Kondo effect.

Light-mediated effects of CdTe-MSA quantum dots on the autofluorescence of freshwater green microalgae: Spectroscopic studies

The water-soluble semiconductor quantum dots (QDs) serve as optically detectable models of nanoparticles and are commonly applied as photoluminescent markers in biological systems. The unicellular algae represent a popular model system suitable to evaluate pollution-induced effects. There is growing experimental evidence that release of metal ions cannot account for potential toxicity of metal containing nanoparticles, however, the underlying mechanisms are not clearly understood. Surrounding environment and illumination conditions are among the most important factors affecting the stability of QDs as well as the interaction between nanoparticles and cells such as microalgae. The measurements of changes in photoluminescence (PL) of QDs and autofluorescence (AF) of microalgae can thus be used as a non-invasive screening method for detecting mutual effects of nanoparticles and algae cells on each other under natural conditions.In this study, CdTe quantum dots (a peak of PL at 550 nm) capped with a mercaptosuccinic acid (MSA) were introduced into aqueous ionic medium containing wild type green freshwater microalgae Scenedesmus and Chlorella sp. cells under artificial and natural ambient illumination. The spectroscopy and microscopy techniques were applied to observe both the influence of the microalgae on the spectral properties of negatively charged CdTe-MSA quantum dots and the effects of nanoparticles on the microalgae. The presence of algae cells revealed a protecting effect on both medium-dependent and radiation-induced changes in photoluminescence properties of QDs, which could be related with the increased stability of the capping layer. The effects on cellular AF intensity and the interaction of QDs with cellular surface depended on type of microalgae. The observed changes in AF spectral properties and AF induction signals can't be explained only by the photodegradation of QDs and have revealed the ability of nanoparticles to retard the photoadaptation of wild type microalgae under naturally varying illumination conditions.

Quantum Dots-Assisted 2D Fluorescence for Pattern Based Sensing of Amino Acids, Oligopeptides and Neurotransmitters.

Quantum dots (QDs) are very attractive nanomaterials for analytical chemistry, due to high photostability, large surface area featuring numerous ways of bioconjugation with biomolecules, usually high quantum yield and long decay times. Their broad absorption spectra and narrow, sharp emission spectra of size-tunable fluorescence make them ideal tools for pattern-based sensing. However, almost always they are applied for specific sensing with zero-dimensional (0D) signal reporting (only peak heights or peak shifts are considered), without taking advantage of greater amount of information hidden in 1D signal (emission spectra), or huge amount of information hidden in 2D fluorescence maps (Excitation-Emission Matrixes, EEMs). Therefore, in this work we propose opposite strategy—non-specific interactions of QDs, which are usually avoided and regarded as their disadvantage, were exploited here for 2D fluorescence fingerprinting. Analyte-specific multivariate fluorescence response of QDs is decoded with the use of Partial Least Squares—Discriminant Analysis. Even though only one type of QDs is studied, the proposed pattern-based method enables to obtain satisfactory accuracy for all studied compounds—various neurotransmitters, amino-acids and oligopeptides. This is a proof of principle of the possibility of the identification of various bioanalytes by such fluorescence fingerprinting with the use of QDs.

Fluctuation-driven Coulomb drag in interacting quantum dot systems

Coulomb drag between nanoscale conductors is of both fundamental and practical interest. Here, we theoretically study drag in a double quantum-dot (QD) system consisting of a biased drive QD and an unbiased drag QD coupled via a direct interdot Coulomb interaction. We demonstrate that the Coulomb drag is driven by the charge fluctuations in the drive QD, and show how the properties of the associated quantum noise allow to distinguish it from, e.g., shot-noise driven drag in circuits of weakly interacting quantum conductors. In the strong-interaction regime exhibiting an orbital ("pseudospin") Kondo effect, the drag is governed by charge fluctuations induced by pseudospin-flip cotunneling processes. The quenching of pseudospin-flip processes by Kondo correlations are found to suppress the drag at low bias and introduce a zero-bias anomaly in the second-order differential transconductance. Finally, we show that the drag is maximized for values of the interdot interaction matching the lead couplings. Our findings are relevant for the understanding of drag in QD systems and provide experimentally testable predictions in different transport regimes.

Exact Evaluation of Statistical Moments in Superradiant Emission

Superradiance describes the coherent collective radiation caused by the interaction between many emitters, mediated by a shared electromagnetic field. Recent experiments involving Bose–Einstein condensates coupled to high-finesse cavities and interacting quantum dots in condensed-matter have attracted attention to the superradiant regime as a fundamental step to create quantum technologies. Here, we consider a simplified description of superradiance that allows the evaluation of statistical moments. A correspondence with the classical birthday problem recovers the statistical moments for discrete time and an arbitrary number of emitters. In addition, the correspondence provides a way to calculate the degeneracy of the problem.

Exploring the conformational changes in fibrinogen by forming protein corona with CdTe quantum dots and the related cytotoxicity.

This study describes synthesis of N‑acetyl‑l‑cysteine-capped CdTe quantum dots (QDs) and investigates their interaction with plasma protein fibrinogen (FIB) and the structural changes of FIB. It is shown that the interaction of QDs with FIB is a spontaneous process and the major driving forces are van der Waals forces and hydrogen bonds. Multi-spectroscopic measurements show that the intrinsic fluorescence of FIB was quenched and secondary and tertiary structures were altered due to the interaction with QDs. In addition, the aggregation state of FIB was altered in the presence of QDs. Furthermore, the formed complexes of FIB with QDs reduced the cytotoxicity of QDs. The coating of FIB on QDs could lower intracellular QDs uptake and therefore result in less released cadmium ions and ROS productions. This study, therefore, might be helpful to the comprehensive understanding of QDs toxicity and provide evidence for assessing the safe application of nanoparticles.

Thermoelectric transport through a finite-U quantum dot side-coupled to Majorana bound state

We study the thermoelectric transport through a single-level quantum dot (QD) coupled to two normal metallic leads and side-coupled to Majorana bound state (MBS). The Coulomb interaction in QD is considered. To investigate only the influence of MBS on thermoelectric transport, we focus on the relatively high temperature region ($T\gg T_K$), where Kondo effect does not appear. The electric and thermal conductance and thermopower as a function of gate voltage (i.e. QD level) are completely different whether the coupling between MBSs is zero or not. When the coupling between MBSs is finite, all thermoelectric characteristics are similar to the transport without MBS. However, for zero MBSs' coupling, the electric and thermal conductance peaks are reduced by 3/4. Especially, in the case of QD without MBS, the sign of thermopower changes three times, however, in the case of QD strongly side-coupled to ideal and isolated MBS, the sign of thermopower changes 9 or 5 times. It can be used for detecting of the signature of MBS. It has actual possibilities when the nanowire is long enough and pure without any defects.

Hot Charge Carriers in Quantum Dots: Generation, Relaxation, Extraction, and Applications

AbstractThis article discusses the hot charge carriers in semiconductor quantum dots (QDs): their generation, relaxation, extraction and applications in different technologically relevant areas. It has been reported that the most common ways to generate hot charge carriers are photo‐excitation by energy more than the band gap energy. However, recently the other means to generate hot charge carriers such as doping, and semiconductor plasmon interaction have also been evolved and their advantages over the conventional method have been discussed. Relaxation dynamics of hot charge carriers and different processes related to this such as multiple exciton generation, Auger recombination, phonon vibrations and ligand assisted charge carrier relaxation are mentioned. It was shown that the relaxation mechanism of hot charge carriers (both hot electron and hot hole) follow different pathways and either of the mechanism is playing a role depending on the QDs structure and the surrounding conditions. Studies pertaining to interaction of QDs with other semiconductors, metal nanoparticles or with different molecular adsorbates suggest that the hot charge carrier extraction is possible in these heterojunctions with extraction time in sub‐ps scale. Application of hot charge carriers in different fields such as photovoltaics, photo‐catalysis, infrared detectors, and H2 detection have been shown and it was observed that the efficiency of the above‐mentioned processes is much higher when the hot charge carriers are involved, suggesting their importance in these areas.

Photon blockade induced tunable source of one/two photon in a double quantum dot-semiconductor microcavity system

We propose a scheme to implement a strong photon blockade with two interacting quantum dots, optically coupled to a semiconductor microcavity. The photon blockade effect is enhanced by simultaneously driving the microcavity and one of the two quantum dots with two classical laser fields. Quantum interference effect with different transition paths is the mechanism responsible with photon blockade. This system has been found to be a tunable source of single or two-photon with applications in solid state quantum computation and quantum information processing.

Properties of the Majorana-state tunneling Josephson junction mediated by an interacting quantum dot

We consider a model of a Josephson junction of two topological superconducting wires mediated by an interacting quantum dot. An additional normal electrode coupled to the dot from the top allows to probe its density of states. The Majorana states adjacent to the dot hybridize across the junction and from a bound state in the dot. The dot is subjected to the effective magnetic field arising from the superposition of the fields driving each wire into topological states, which, dependent on the angle between the fields, introduces variable Zeeman splitting of the dot active level. We show that electron interactions in the dot diminish the characteristic for Majoranas zero bias peak arising in the transverse conductance through the dot and introduce an overall asymmetry of the conductance. They also renormalize the hybridization between the end-state Majoranas in shorter wires. The Majorana spin polarization is determined by the effective magnetic field in the dot. Phase-biased Josephson current exhibits spin polarization in thermal equilibrium, which possesses characteristic periodicity, and its sign can be switched when an unpaired Majorana state is present in the junction. We also observe spin-dependent Majorana state ‘leaking’, which can be controlled by the position of the dot level in energy scale.

Catalog of Andreev spectra and Josephson effects in structures with time-reversal-invariant topological superconductor wires

We study all the possible different two terminal configurations of Josephson junctions containing wires of time-reversal invariant topological superconductors (TRITOPS) and ordinary superconductors, including combinations with an interacting quantum dot between both wires in the junction. We introduce simple effective Hamiltonians which explain the different qualitative behaviors obtained. We analyze a wide range of phenomena, including occurrence and quenching of the so called $0-\pi$ transition, anomalous periodicity and jumps of the Josephson current as a function of the phase difference, and finite Josephson current in the absence of magnetic flux.

Thermoelectric transport through interacting quantum dots in graphene

We study the thermoelectric properties of an electronic localized level coupled to two graphene electrodes. Graphene band structure shows a pseudogap density of states (DOS) that strongly affects the charge transport. We focus on the Coulomb blockade regime and derive the expression for the Onsager matrix that relates the charge and heat currents to the voltage and temperature biases in the linear response regime. The elements of the Onsager matrix are functions of the transmission coefficient, which depends on the dot Green’s function. Our self-consistent calculation of the Green’s function is based on the equation-of-motion technique. We find a double-peak structure for the electric and thermal responses as the dot level is tuned with an external gate terminal, in accordance with the Coulomb blockade phenomenon. Remarkably enough, the thermal conductance is much smaller than its electric counterpart, giving rise to a high thermoelectric figure of merit for certain values of the gate voltage. Finally, we discuss a large departure from the Wiedemann–Franz law caused mainly by the pseudogap DOS in the contacts and weakly affected by interactions.

Fluorescence intensity modulation of CdSe/ZnS quantum dots assesses reactive oxygen species during chemotherapy and radiotherapy for cancer cells.

Quantum dots (QDs) are semiconductor nanoparticles ranging in size from 2 to 10 nm. QDs are increasingly being developed for biomedical imaging, targeted drug delivery and green energy technology. These have led to much research on QD interactions with various physical, chemical and biological systems. For biological systems, research has focused on the biocompatibility/cytotoxicity of QDs in the context of imaging/therapy. However, there is a paucity of work on how biological systems and bioactive molecules might be used to alter the optoelectronic properties of QDs. Here, it is shown that these properties can be altered by reactive oxygen species (ROS) from chemotherapeutic media and biological cells following controlled changes in cellular activities. Using CdSe/ZnS core-shell QDs, spectroscopic analysis of optically excited QDs with HL60, K562 and T98G cancer cell lines is performed. Our results show statistically significant (P < 0.0001) modulation of the fluorescence emission spectra of the QDs due to the ROS produced by common chemotherapeutic drugs, daunorubicin and doxorubicin and by cells following chemotherapy/radiotherapy. This optical modulation, in addition to assessing ROS generation, will possibly enhance applications of QDs in simultaneous diagnostic imaging and nanoparticle-mediated drug delivery as well as simultaneous ROS assessment and radiosensitization for improved outcomes in cancer treatments. Reactive molecular species produced by biological cells and chemotherapeutic drugs can create electric fields that alter the photophysical properties of QDs, and this can be used for concurrent monitoring of cellular activities, while inducing changes in those cellular activities.

Dual functionalized, stable and water dispersible CdTe quantum dots: Facile, one-pot aqueous synthesis, optical tuning and energy transfer applications

We report a facile one-pot synthesis of aqueously dispersible CdTe quantum dots (QDs) with tunable optical properties by using dual capping agents namely MPA and Citric acid. Optimum precursor ratio and pH > 8 at 80 °C facilitated particle size tunability and exceptional optical properties. QDs are ∼3.7 nm as seen from HR-TEM image; highly-luminescent showing absolute quantum-yield up to 54% and bear good photostability. CdTe QDs are highly stable in water with large negative Zeta potential. Optimum ratio of Cd2+:MPA:TeO32−:NaBH4:citric acid was 1:2:0.2:3.2:5.2, respectively. The pH dependent nucleation and growth of QDs subsequently lead to quasi CdTe/CdS structure as indicated by XRD, FTIR and particle size analysis. QDs showed excitation independent emission with tunable peak width. QDs can be stored over 6-months at 4 °C without causing agglomeration. Interaction of QDs with Rhodamine-6G and plasmonic silver nanoparticles via resonance energy transfer (RET) imply their suitability for sensors and biomedical applications.

Charge susceptibility and conductances of a double quantum dot

We calculate the charge susceptibility and the linear and differential conductances of a double quantum dot coupled to two metallic reservoirs both at equilibrium and when the system is driven away from equilibrium. This work is motivated by recent progress in the realization of solid state spin qubits. The calculations are performed by using the Keldysh nonequilibrium Green function technique. In the noninteracting case, we give the analytical expression for the electrical current and deduce from there the linear conductance as a function of the gate voltages applied to the dots, leading to a characteristic charge stability diagram. We determine the charge susceptibility which also exhibits peaks as a function of gate voltages. We show how the study can be extended to the case of an interacting quantum dot.