Spin filling of valley-orbit states in a silicon quantum dot

We report the demonstration of a low-disorder silicon metal-oxide-semiconductor (Si MOS) quantum dot containing a tunable number of electrons from zero to N = 27. The observed evolution of addition energies with parallel magnetic field reveals the spin filling of electrons into valley-orbit states. We find a splitting of 0.10?meV between the ground and first excited states, consistent with theory and placing a lower bound on the valley splitting. Our results provide optimism for the realisation in the near future of spin qubits based on silicon quantum dots.     

Observation of coupling between zero- and two-dimensional semiconductor systems based on anomalous diamagnetic effects

We report the direct observation of coupling between a single self-assembled InAs quantum dot and a wetting layer, based on strong diamagnetic shifts of many-body exciton states using magneto-photoluminescence spectroscopy. An extremely large positive diamagnetic coefficient is observed when an electron in the wetting layer combines with a hole in the quantum dot; the coefficient is nearly one order of magnitude larger than that of the exciton states confined in the quantum dots. Recombination of electrons with holes in a quantum dot of the coupled system leads to an unusual negative diamagnetic effect, which is five times stronger than that in a pure quantum dot system. This effect can be attributed to the expansion of the wavefunction of remaining electrons in the wetting layer or the spread of electrons in the excited states of the quantum dot to the wetting layer after recombination. In this case, the wavefunction extent of the final states in the quantum dot plane is much larger than that of the initial states because of the absence of holes in the quantum dot to attract electrons. The properties of emitted photons that depend on the large electron wavefunction extents in the wetting layer indicate that the coupling occurs between systems of different dimensionality, which is also verified from the results obtained by applying a magnetic field in different configurations. This study paves a new way to observe hybrid states with zero- and two-dimensional structures, which could be useful for investigating the Kondo physics and implementing spin-based solid-state quantum information processing.

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Gate-defined quantum dots in intrinsic silicon

We report the fabrication and measurement of silicon quantum dots with tunable tunnel barriers in a narrow-channel field-effect transistor. Low-temperature transport spectroscopy is performed in both the many-electron ( approximately 100 electrons) regime and the few-electron ( approximately 10 electrons) regime. Excited states in the bias spectroscopy provide evidence of quantum confinement. These results demonstrate that depletion gates are an effective technique for defining quantum dots in silicon.     

Bubbles in Liquid Helium Containing Electrons in Excited States

When an electron enters liquid helium, it forces open a cavity within the liquid. We calculate the size and shape of these electron bubbles for different quantum states of the electron, and determine the negative pressure at which the different bubbles explode.     

Effect of quantum and dielectric confinement on the exciton-exciton interaction energy in type II core/shell semiconductor nanocrystals

We study theoretically two electron-hole pair states (biexcitons) in core/shell hetero-nanocrystals with type II alignment of energy states, which promotes spatial separation of electrons and holes. To describe Coulomb interactions in these structures, we apply first-order perturbation theory, in which we use an explicit form of the Coulomb-coupling operator that takes into account interface-polarization effects. This formalism is used to analyze the exciton-exciton interaction energy as a function of the core and shell sizes and their dielectric properties. Our analysis shows that the combined contributions from quantum and dielectric confinement can result in strong exciton-exciton repulsion with giant interaction energies on the order of 100 meV. Potential applications of strongly interacting biexciton states include such areas as lasing, nonlinear optics, and quantum information.     

Experiments to Study the Effect of Light on Electron Bubbles in Liquid Helium

We give a survey of recent experimental and theoretical work on the effect of light on electron bubbles in liquid helium. The light-induced change in the bubbles is measured using an ugtrasonic technique. In helium at temperatures above about 1.7 K, we are able to produce and detect electron bubbles in the 1P quantum state. The properties of the electron bubbles are in agreement with theoretical expectations. However, the application of light to bubbles at low temperatures (T>1.5 K) resugts in. changes in the properties of the bubbles that are not yet understood.     

An experimentally justified confining potential for electrons in two-dimensional semiconductor quantum dots

We propose a confinement potential for electrons in a two-dimensional (2D) quantum dot that is more physically motivated and better experimentally justified than the commonly used infinite range parabolic potential or few other choices. Because of the specific experimental setup in a 2D quantum dot involving application of gate potentials, an area of electron depletion is created near the gate. The resulting positively charged region can be most simply modeled as a uniformly charged 2D disk of positive background charge. Within this experimental setup, the individual electrons in the dot feel a confinement potential originating from the uniformly positively charged 2D background disk. Differently from the infinitely high parabolic confinement potential, the resulting 2D charged disk potential has a finite depth. The resulting 2D charged disk potential has a form that can be reasonably approximated as a parabolic potential in the central region of the dot (for low energy states of the electrons) and as a Coulomb potential (that becomes zero at large distances). We study the electronic properties of the 2D charged disk confinement potential by means of the numerical diagonalization method and compare the results to the case of 2D quantum dots with a pure parabolic confinement potential.      

Two carriers in vertically coupled quantum dots: magnetic field effect

We study a two-charge-carrier (two holes or two electrons) quantum dot molecule in a magnetic field. In comparison with the electron states in the double quantum dot, the switching between the hole states is achieved by changing both the inter-dot distance and magnetic field. We use harmonic potentials to model the confining of two charge carriers and calculate the energy difference delta E between the two lowest energy states with the Hund-Mulliken technique, including the Coulomb interaction. Introducing the Zeeman effect, we note a ground-state crossing, which can be observed as a pronounced jump in the magnetization at a perpendicular magnetic field of a few Tesla. The ground states of the molecule provide a possible realization for a quantum gate.     

Probing Defect‐Induced Midgap States in MoS2 Through Graphene–MoS2 Heterostructures

Crystalline defects in MoS₂ may induce midgap states, resulting in low carrier mobility. These midgap states are usually difficult to probe by conventional transport measurement. The quantum capacitance of single‐layer graphene is sensitive to defect‐induced states near the Dirac point, at which the density of states is extremely low. It is reported that the hexagonal‐boron nitride/graphene/MoS₂ sandwich structure facilitates the exploration of the properties of those midgap states in MoS₂. Comparative results of the quantum capacitance of pristine graphene indicate the presence of several midgap states with distinct features. Some of these states donate electrons while some states lead to localization of electrons. It is believed that these midgap states originate from intrinsic point defects such as sulfur vacancies, which have a significant impact on the property of the MoS₂/graphene interface. They are responsible for the contact problems of metal/MoS­₂ interfaces.     

Spin states of electrons in quantum dots upon heating. Simulation by the Feynman path integral method. Magnetic properties

Temperature dependences of spin states and spin paramagnetic susceptibility in ellipsoidal quantum dots (QDs) containing two or three electrons are numerically simulated using ab initio calculations based on the Feynman path integral method. Limits of the thermal stability of spin states are estimated. Upon cooling, the pairing of spins of an electron pair is most intense in spherical QDs; notably, prolate QDs hinder the pairing more strongly than the oblate ones. When the spherical shape of a QD is distorted, a characteristic peak in the temperature dependence of the electron-pair magnetic susceptibility shifts to lower temperatures. A spin of the system of three electrons may either increase or decrease upon cooling, depending on the QD shape. In the case of three electrons, strong spatial anisotropy of the electron-confining field causes a relative decrease in the energy of states with large spin values.     

Sb-mediated Ge quantum dots in Ti–oxide–Si diode: negative differential capacitance

Abstract

The negative differential capacitance (NDC) effect is observed on a titanium–oxide–silicon structure, formed on n-type silicon with embedded germanium quantum dots (QDs). The Ge QDs were grown by an Sb-mediated technique. The NDC effect was observed for temperatures below 200 K. We found that approximately six to eight electrons can be trapped in the valence band states of Ge QDs. We explain the NDC effect in terms of the emission of electrons from valence band states in the very narrow QD layer under reverse bias.     

Terahertz ionization of highly charged quantum posts in a perforated electron gas

"Quantum posts" are roughly cylindrical semiconductor nanostructures that are embedded in an energetically shallower "matrix" quantum well of comparable thickness. We report measurements of voltage-controlled charging and terahertz absorption of 30 nm thick InGaAs quantum wells and posts. Under flat-band (zero-electric field) conditions, the quantum posts each contain approximately six electrons, and an additional ~2.4 × 10(11) cm(-2) electrons populate the quantum well matrix. In this regime, absorption spectra show peaks at 3.5 and 4.8 THz (14 and 19 meV) whose relative amplitude depends strongly on temperature. These peaks are assigned to intersubband transitions of electrons in the quantum well matrix. A third, broader feature has a temperature-independent amplitude and is assigned to an absorption involving quantum posts. Eight-band k·p calculations incorporating the effects of strain and Coulomb repulsion predict that the electrons in the posts strongly repel the electrons in the quantum well matrix, "perforating" the electron gas. The strongest calculated transition, which has a frequency close to the center of the quantum post related absorption at 5 THz (20 meV), is an ionizing transition from a filled state to a quasi-bound state that can easily scatter to empty states in the quantum well matrix.     

Non-reciprocal Interferometers for Matter Waves

We propose non-reciprocal interferometers for matter waves and explore them by means of modeling. These interferometers may be implemented as asymmetric quantum rings with broken time-inversion symmetry. Our preliminary analyses lead us to predict that these devices will feature asymmetric transport properties for particles such as electrons, for which the interferometers’ ground states act as directional filters. As a function of the electron velocity, the filters seem to let electrons preferentially pass in one direction rather than in the reverse, thereby reducing the entropy of the electron systems in the contacts. Mechanisms are discussed that are candidates to prevent directional filtering if the filters are operating in thermal equilibrium with incoherent electron reservoirs.     

Fission of Multielectron Bubbles in Liquid Helium Under Electric Fields

Multielectron bubbles (MEBs) are cavities in liquid helium which contain a layer of electrons trapped within few nanometres from their inner surfaces. These bubbles are promising candidates to probe a system of interacting electrons in curved geometries, but have been subjected to limited experimental investigation. Here, we report on the observation of fission of MEBs under strong electric fields, which arises due to fast rearrangement of electrons inside the bubbles, leading to their deformation and eventually instability. We measured the electrons to be distributed unequally between the daughter bubbles which could be used to control the charge density inside MEBs.     

Fano-Rashba effect in quantum dots

We consider the electronic transport through 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 investigate this Fano-Rashba effect as a function of the applied magnetic field and Rashba spin-orbit coupling.     

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.     

Conservation of Angular Momentum in a Flux Qubit

Oscillations of superconducting current between clockwise and counterclockwise directions in a flux qubit do not conserve the angular momentum of the qubit. To compensate for this effect the solid containing the qubit must oscillate in unison with the current. This requires entanglement of quantum states of the qubit with quantum states of a macroscopic body. The question then arises whether slow decoherence of quantum oscillations of the current is consistent with fast decoherence of quantum states of a macroscopic solid. This problem is analyzed within an exactly solvable quantum model of a qubit embedded in an absolutely rigid solid and for the elastic model that conserves the total angular momentum. We show that while the quantum state of a flux qubit is, in general, a mixture of a large number of rotational states, slow decoherence is permitted if the system is macroscopically large. Practical implications of entanglement of qubit states with mechanical rotations are discussed.     

Electronic transient processes and optical spectra in quantum dots for quantum computing

Quantum dot systems are studied theoretically from the point of view of realization of quantum bit using the orbital state of electronic motion in a quantum dot. Attention is paid to several effects which can influence significantly the application of quantum dot electronic orbital states in quantum computing, for example, the effect of upconversion of the population and the incomplete depopulation of electronic orbital states, the effect of optical line broadening and presence of continuous background in optical spectra. Attention is also paid to the effect of upconversion of electronic population to the wetting-layer or above-barrier electronic states in quantum dot samples. Interaction of electrons with the longitudinal optical phonons is shown to play a significant role in these effects. Possible impact of these phenomena on the realization of quantum bit based on electronic orbital states in quantum dots is discussed.