Measure of Entanglement States of Two Interacting Electrons in Vertically Coupled Quantum Dots Induced by a Time-Dependent Electric Field

We study the dynamics of two electrons located in two vertically tunnel-coupled quantum dots in the presence of an oscillatory electric field. By solving the time-dependent Schrödinger equation, we predict the dynamical generation of entangled electron states, such as the EPR (Einstein, Podolsky, and Rosen) pairs or Bell states. The Schmidt rank and the von Neumann entropy are evaluated to characterize the degree of entanglement of the two electron states.     

Quantum Cavitation: A Comparison Between Superfluid Helium-4 and Normal Liquid Helium-3

Cavitation has now been studied in superfluid helium-4 and in normal liquid helium-3, both theoretically and experimentally. We compare the two cases and discuss the existence of a crossover from quantum cavitation, where bubbles are nucleated by tunneling, to classical cavitation where their nu-deation is thermally activated. In helium-3, where evidence for quantum nucleation is lacking, the interpretation of the experimental results leads to two related questions. The first one concerns the extrapolation of the properties of this Fermi liquid at negative pressure. The second one concerns the validity of present theories of quantum cavitation in a Fermi liquid.     

Statistics of electron emission from InAs/GaAs quantum dots

A theoretical treatment for thermal and tunneling emission of electrons from InAs/GaAs quantum dots is performed to achieve “effective emission rates” corresponding to experimentally obtained quantities. From these results, Arrhenius graphs are calculated using parameter values for quantum dots with 20/10 nm base/height dimension. Emission from the electron s shell as direct transitions, as two-step transitions from the s to the p shell, as thermal transitions from s to p followed by tunneling and as direct tunneling from the s and the p shell to the GaAs conduction band is taken into account. Due to the varying emission possibilities, Arrhenius graphs appear with complicated shapes depending on quantities originating from structural and electronic properties of the quantum dots.     

Time-resolved detection of single-electron interference

We demonstrate real-time detection of self-interfering electrons in a double quantum dot embedded in an Aharonov-Bohm interferometer, with visibility approaching unity. We use a quantum point contact as a charge detector to perform time-resolved measurements of single-electron tunneling. With increased bias voltage, the quantum point contact exerts a back-action on the interferometer leading to decoherence. We attribute this to emission of radiation from the quantum point contact, which drives noncoherent electronic transitions in the quantum dots.     

Correlation effects in wave function mapping of molecular beam epitaxy grown quantum dots

We investigate correlation effects in the regime of a few electrons in uncapped InAs quantum dots by tunneling spectroscopy and wave function (WF) mapping at high tunneling currents where electron-electron interactions become relevant. Four clearly resolved states are found, whose approximate symmetries are roughly s and p, in order of increasing energy. Because the major axes of the p-like states coincide, the WF sequence is inconsistent with the imaging of independent-electron orbitals. The results are explained in terms of many-body tunneling theory, by comparing measured maps with those calculated by taking correlation effects into account.     

Highly Correlated State of <Emphasis Type="Italic">π</Emphasis> Electrons,Self-organization at Doping,and Superconductivity in Doped Picene

It is shown that superconductivity in doped picene and a number of accompanying effects observed in Mitsuhashi et al. (Nature 464:76 2010) find adequate explanation if we invoke the concept of a highly correlated electron state and the superconductivity model suggested for the high-temperature superconductivity in cuprates that includes formation of a one-dimensional Wigner crystal of electron pairs and its delocalization. It is also shown that the highly correlated state of π electrons in picene results from exchange and Coulomb interactions between the π electrons and leads to formation of a spin-ordered Wigner crystal. Picene doping accompanied by valence electron density redistribution transforms the correlated state of electrons into a highly correlated state of electron pairs manifesting itself in formation of Wigner crystals of electron pairs giving rise to superconductivity of a molecular picene ion. Superconductivity of solid picene includes tunneling of Wigner crystals of pairs through Josephson junctions between molecular picene ions.     

Nanowire single-electron memory

We demonstrate storage of electrons in semiconductor nanowires epitaxially grown from Au nanoparticles. The nanowires contain multiple tunnel junctions (MTJs) of InP barriers and InAs quantum dots designed such that the metal seed particles act as storage nodes. By positioning a second nanowire close to the seed particle it is possible to detect tunneling of individual electrons through the MTJ at 4.2 K. A strong memory effect is observed in the detector current when sweeping the writing voltage.     

Electronic Structure in Underdoped Cuprates Due to the Emergence of a Pseudogap

The phenomenological Green’s function developed in the works of Yang, Rice, and Zhang has been very successful in understanding many of the anomalous superconducting properties of the deeply underdoped cuprates. It is based on considerations of the resonating valence bond spin liquid approximation and is designed to describe the underdoped regime of the cuprates. Here, we emphasize the region of doping, x, just below the quantum critical point at which the pseudogap develops. In addition to Luttinger hole pockets centered around the nodal direction, there are electron pockets near the antinodes which are connected to the hole pockets by gapped bridging contours. We determine the contours of nearest approach as would be measured in angular resolved photoemission experiments and emphasize signatures of the Fermi surface reconstruction from the large Fermi contour of Fermi liquid theory (which contains 1+x hole states) to the Luttinger pocket (which contains x hole states). We find that the quasiparticle effective mass renormalization increases strongly toward the edge of the Luttinger pockets beyond which it diverges.     

Photosensitization of ZnO nanowires with CdSe quantum dots for photovoltaic devices

We combine CdSe semiconductor nanocrystals (or quantum dots) and single-crystal ZnO nanowires to demonstrate a new type of quantum-dot-sensitized solar cell. An array of ZnO nanowires was grown vertically from a fluorine-doped tin oxide conducting substrate. CdSe quantum dots, capped with mercaptopropionic acid, were attached to the surface of the nanowires. When illuminated with visible light, the excited CdSe quantum dots injected electrons across the quantum dot-nanowire interface. The morphology of the nanowires then provided the photoinjected electrons with a direct electrical pathway to the photoanode. With a liquid electrolyte as the hole transport medium, quantum-dot-sensitized nanowire solar cells exhibited short-circuit currents ranging from 1 to 2 mA/cm2 and open-circuit voltages of 0.5-0.6 V when illuminated with 100 mW/cm2 simulated AM1.5 spectrum. Internal quantum efficiencies as high as 50-60% were also obtained.     

Optically induced multispin entanglement in a semiconductor quantum well

According to quantum mechanics, a many-particle system is allowed to exhibit non-local behaviour, in that measurements performed on one of the particles can affect a second one that is far away. These so-called entangled states are crucial for the implementation of most quantum information protocols and, in particular, gates for quantum computation. Here we use ultrafast optical pulses and coherent techniques to create and control spin-entangled states in an ensemble of non-interacting electrons bound to donors (at least three) and at least two Mn2+ ions in a CdTe quantum well. Our method, relying on the exchange interaction between localized excitons and paramagnetic impurities, can in principle be applied to entangle an arbitrarily large number of spins.     

Superconducting tunneling without the tunneling Hamiltonian

The theory of superconducting tunneling is extended to treat superconducting junctions with arbitrarily thin, but structureless tunnel barriers. An exact expression for the tunneling current is obtained, using standard, many-body, nonequilibrium Green's function techniques, assuming Fermi distributions in each electrode. The tunneling current result agrees with the recent theory of Blonder, Tinkham, and Klapwijk, but extends their results to treat strong coupling superconductors, proximity effect tunneling, and the effects of tunneling angle. Results for the Josephson critical current in S' INS (superconductor S', insulator I, metal N, superconductor S) junctions, where NS is a proximity effect double layer, are presented for barrier thicknesses ranging from zero to barrier thicknesses for which the tunneling Hamiltonian approach is correct, and for varying N metal thicknesses. Results forI-V curves are presented for normal metal (M)-INS junctions for a similar range of barrier thicknesses and N metal thicknesses. It is shown that the tunneling currentI is the sum of a supercurrentI _SUP carried solely by Cooper pairs through S, and a quasiparticle currentI _QP carried solely by quasiparticles. The influence of leakage on phonon structure observed on tunneling into strong coupling superconductors is described. The nonoscillating portion of the Josephson current is plotted as a function of voltage for the S' INS junction in the tunneling Hamiltonian limit.     

Observation of room temperature negative differential resistance in multi-layer heterostructures of quantum dots and conducting polymers

Multi-layer heterostructure negative differential resistance devices based on poly-[2-methoxy-5-(2'-ethyl-hexyloxy)-1,4-phenylenevinylene] (MEH-PPV) conducting polymer and CdSe quantum dots is reported. The conducting polymer MEH-PPV acts as a barrier while CdSe quantum dots form the well layer. The devices exhibit negative differential resistance (NDR) at low voltages. For these devices, strong negative differential resistance is observed at room temperature. A maximum value of 51 for the peak-to-valley ratio of current is reported. Tunneling of electrons through the discrete quantum confined states in the CdSe quantum dots is believed to be responsible for the multiple peaks observed in the I-V measurement. Depending on the observed NDR signature, operating mechanisms are explored based on resonant tunneling and Coulomb blockade effects.     

s-Wave vs.d -Wave Superconductivity in the Two-Dimensional Hubbard Model

The anisotropic superconductivity has been considered in the two-dimensional Hubbard model. We allow for on and off-site intralayer Cooper pairs. Interlayer momentum-conserving Josephson tunneling which couples adjacent layers has been taken into account. The meanfield solution for the superconducting transition temperature demonstrates the competition between local Coulomb repulsion and interlayer tunneling in thes-wave channel. This competition leads to the mixedsd superconducting state at low doping. We also demonstrate thatc-phonon-assisted transitions between intra and interlayer states can effectively give rise to momentum-conserving Josephson tunneling.     

Tunneling Conductance in Strained Graphene-Based Superconductor: Effect of Asymmetric Weyl–Dirac Fermions

Based on the BTK theory, we investigate the tunneling conductance in uniaxially strained graphene-based normal metal (NG)/barrier (I)/superconductor (SG) junctions. In the present model, we assume that by depositing the conventional superconductor on the top of the uniaxially strained graphene, normal graphene may turn to superconducting graphene with the Cooper pairs formed by the asymmetric Weyl–Dirac electrons, the massless fermions with direction-dependent velocity. The highly asymmetrical velocity, v _y /v _x ≫1, may be created by strain in the zigzag direction near the transition point between gapless and gapped graphene. In the case of highly asymmetrical velocity, we find that the Andreev reflection strongly depends on the direction of strain, and the current perpendicular to the direction of strain can flow through the junction as if there were no barrier. Also, the current parallel to the direction of strain anomalously oscillates as a function of the gate voltage with very high frequency. Our predicted result is quite different from the feature of the quasiparticle tunneling in the unstrained graphene-based NG/I/SG conventional junction. This is because of the presence of the direction-dependent-velocity quasiparticles in the highly strained graphene system.     

The Modulation and Enhancement of Thermopower in a Luttinger liquid

We investigate the thermoelectric transport properties of the Luttinger liquid coupled to the quantum dot by tunnel junctions. A general thermopower expression is derived by using nonequilibrium Green function method. At low temperature the thermopower is of linear temperature dependence and conductance is of a temperature-dependent power-law behavior. There is a peak in the curve of the thermopower vs. gate voltage. Intralead electron interaction enhances thermopower at low temperatures and the large slope is interpreted as a character of the Luttinger liquid. In the limit of strong electron interaction thermopower S(T) at low temperature can be represented by the noninteracting electrons S ₀(T) as: S(T)=3S ₀(T)/(2g).     

Quantum diffusion in solids

The lectures are devoted to the problems connected with the tunneling motion of particles in a solid in the presence of strong interaction with phonon and electron excitations. We examine the quantum diffusion in insulators, metals, and superconductors. The interaction with the barrier fluctuations as well as the intrawell interaction are taken into account. Special attention is paid to the role of adiabaticity, playing an important part in tunneling phenomena, revealing the decisive influence of nonadiabatic excitations. We consider the entire temperature interval: coherent diffusion at low T, suppression of the coherent phase correlations and the transition to the incoherent quantum diffusion, and then with the further increase in T the crossover to the tunneling diffusion induced by excitations. The detailed analysis of the quantum diffusion in imperfect crystals and the problems of localization, in particular, caused by the interparticle interaction is given. We discuss the wide spectrum of experimental results displaying the quantum diffusion in different systems.     

A Tunneling Dielectric Layer Free Floating Gate Nonvolatile Memory Employing Type‐I Core–Shell Quantum Dots as Discrete Charge‐Trapping/Tunneling Centers

A nonvolatile memory with a floating gate structure is fabricated using ZnSe@ZnS core–shell quantum dots as discrete charge‐trapping/tunneling centers. Systematical investigation reveals that the spontaneous recovery of the trapped charges in the ZnSe core can be effectively avoided by the type‐I energy band structure of the quantum dots. The surface oleic acid ligand surrounding the quantum dots can also play a role of energy barrier to prevent unintentional charge recovery. The device based on the quantum dots demonstrates a large memory window, stable retention, and good endurance. What is more, integrating charge‐trapping and tunneling components into one quantum dot, which is solution synthesizable and processible, can largely simplify the processing of the floating gate nonvolatile memory. This research reveals the promising application potential of type‐I core–shell nanoparticles as the discrete charge‐trapping/tunneling centers in nonvolatile memory in terms of performance, cost, and flexibility.     

Voltage-controlled optical precursors in quantum dot molecules

Abstract

Optical precursors generated in a quantum-dot-molecule system by an incident square-modulated pulse are theoretically investigated. Voltage-controlled electron tunneling couples two quantum dots instead of a laser field, and a narrow transparency window appears with normal dispersion. The main pulse is delayed due to slow-light effect and the precursor pulses are obtained. We examine the linear susceptibility and find that its one part exhibits normal dispersion and the other one presents anomalous dispersion. Competition between the two parts leads to normal dispersion for the linear susceptibility, which contributes to the above results. Voltage-controlled precursors may be useful to design novel optical devices for generating precursors.     

Ge quantum dot memory structure with laterally ordered highly dense arrays of Ge dots

This work was devoted to the development of a Ge quantum dot memory structure of a MOSFET type with laterally ordered Ge quantum dots within the gate dielectric stack. Lateral ordering of the Ge dots was achieved by the combination of the following technological steps: (a) use of a focused ion beam (FIB) to create ordered two-dimensional arrays of regular holes on a field oxide on the silicon substrate, (b) chemical cleaning and restoring of the Si surface in the holes, (c) further oxidation to transfer the pattern from the field oxide to the silicon substrate, (d) removal of the field oxide and thermal re-oxidation of the sample in order to create a tunneling oxide of homogeneous thickness on the patterned silicon surface, and (e) self-assembly of the two-dimensional arrays of Ge dots on the patterned tunneling oxide. The charging properties of the obtained memory structure were characterized by electrical measurements. Charging of the Ge quantum dot layer by electrons injected from the substrate resulted in a large shift in the capacitance-voltage curves of the MOS structure. Charges were stored in deep traps in the charging layer, and consequently the erasing process was difficult, resulting in a limited memory window. The advantages of controlled positioning of the quantum dots in the charging layer will be discussed.     

Theory of nuclear spin relaxation in a two-dimensional disordered HTcS in the normal state

The nuclear spin relaxation rateT ₁ ^–1 is calculated for a disordered two dimensional highcritical temperature superconductor taking into consideration the inelastic scattering of the electrons on the impurities. The deviation from the Korringa law of the formT ₁ ^–1 =AT+ B has been obtained if the quantum correction to the transport is dominated by the magnetic correlations.