Quantum Dots In Regime Research Articles

G-tensor control in bent carbon nanotube quantum dots

We demonstrate gate control of the electronic g-tensor in single and double quantum dots formed along a bend in a carbon nanotube. From the dependence of the single-dot excitation spectrum on magnetic field magnitude and direction, we extract spin-orbit coupling, valley coupling, spin and orbital magnetic moments. Gate control of the g-tensor is measured using the splitting of the Kondo peak in conductance as a sensitive probe of Zeeman energy. In the double quantum dot regime, the magnetic field dependence of the position of cotunneling lines in the two dimensional charge stability diagram is used to infer the positions of the two dots along the nanotube.

Optical and Structural Investigation of CdSe Quantum Dots Dispersed in PVA Matrix and Photovoltaic Applications

CdSe quantum dots (QDs) dispersed in polyvinyl alcohol (PVA) matrix with their sizes within the quantum dot regime have been synthesized via a simple heat induced thermolysis technique. The effect of the concentrations of the cadmium source on the optical properties of CdSe/PVA thin films was investigated through UV-Vis absorption spectroscopy. The structural analysis and particle size determination as well as morphological studies of the CdSe/PVA nanocomposite thin films were done with the help of X-ray diffraction (XRD) and transmission electron microscopy (TEM). The XRD analysis reveals that CdSe/PVA nanocomposite thin film has a hexagonal (wurtzite) structure. A prototype thin film solar cell of CdSe/CdTe has been synthesized and its photovoltaic parameters were measured.

Single charge sensing and transport in double quantum dots fabricated from commercially grown Si/SiGe heterostructures

We perform quantum Hall measurements on three types of commercially available modulation-doped Si/SiGe heterostructures to determine their suitability for depletion gate defined quantum dot devices. By adjusting the growth parameters, we are able to achieve electron gases with charge densities 1–3 × 1011/cm2 and mobilities in excess of 100 000 cm2/Vs. Double quantum dot devices fabricated on these heterostructures show clear evidence of single charge transitions as measured in dc transport and charge sensing and exhibit electron temperatures of 100 mK in the single quantum dot regime.

Two-phonon processes of intraband relaxation in the terahertz regime in quantumdots

We theoretically investigate the intraband relaxation of quantum dots in the terahertzregime due to two acoustic phonon scattering by applying a lattice relaxationapproach based on the deformation potential coupling between electrons andacoustic phonons. In particular, we find that the relaxation time depends stronglyon the ratio of two acoustic phonons. The influences of the energy separationbetween the ground and first excited state, the quantum dot height, and the latticetemperature on the relaxation time are also discussed. Our theoretical results not onlygive a reasonable explanation for the current experimental measurement but alsoprovide some insight into two-phonon intraband relaxation in quantum dots.

Critical and excess current through an open quantum dot: Temperature and magnetic-field dependence

We present measurements of temperature and magnetic-field dependence of the critical current and excess current in a carbon nanotube Josephson quantum dot junction. The junction is fabricated in a controlled environment which allows for extraction of the full critical current. The measurements are performed in the open quantum dot regime and fitted to theory with good qualitative agreement. We also show how to extract level spacing, level broadening, and charging energy of an open quantum dot from a bias spectroscopy plot.

Universal interacting crossover regime in two-dimensional quantum dots

Interacting electrons in quantum dots with large Thouless number $g$ in the three classical random matrix symmetry classes are well understood. When a specific type of spin-orbit coupling known to be dominant in two-dimensional semiconductor quantum dots is introduced, we show that an interacting quantum critical crossover energy scale emerges and low-energy quasiparticles generically have a decay width proportional to their energy. The low-energy physics of this system is an example of a universal interacting crossover regime.

Su(4) mixed valence regime in carbon nanotube quantum dots

We study the evolution of conductance regimes in carbon nanotubes with doubly degenerate orbitals (“shells”) by controlling the contact transparency within the same sample. For sufficiently open contacts, Kondo behavior is observed for 1, 2, and 3 electrons in the topmost shell. As the contacts are opened more, the sample enters the “mixed valence” regime, where different charge states are strongly hybridized by electron tunneling. Here, the conductance as a function of gate voltage shows pronounced modulations with a period of four electrons, and all single-electron features are washed away at low temperature. We successfully describe this behavior by a simple formula with no fitting parameters. Finally, we find a surprisingly small energy scale that controls the temperature evolution of conductance and the tunneling density of states in the mixed valence regime.

A Bit per Particle: Electrostatic Assembly of CdSe Quantum Dots as Memory Elements

Two-dimensional arrays of CdSe nanoparticles are fabricated via electrostatic assembly. The size of the particles ranges down to the quantum dot regime. Individual particles and arrays of particles exhibit electrical bistability and associated memory phenomenon. The bistability, which is due to charge confinement in the particles, results in experimental evidence of quantum dots as memory elements storing a bit per particle.

Spin‐dependent electron tunnelling and spin relaxation in quantum dots in regime with filling factor of around two

AbstractSpin‐dependent electron tunnelling and spin relaxation were studied for a quantum dot in the regime with a filling factor between two and four. In this regime, the electronic configuration of a quantum dot undergoes transitions between a spin singlet and triplet states for an even number of electrons, and between two doublet states for an odd number of electrons. These transitions were clearly distinguished by using quantum wires as spin filtering contact leads to the dot. In addition, the temporal behaviour of electron tunnelling was studied for a quantum dot in a similar filling factor regime, using a quantum point contact as a charge sensor. Electron tunnelling through the dot in a spin singlet state could be well distinguished from that in a triplet state using the fact that the tunnelling rate was much larger for the triplet state. The difference in the tunnelling rate was also used to derive a triplet‐to‐singlet‐state relaxation time. The obtained relaxation time agreed fairly well with that predicted by the theory of spin‐orbit interaction. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Pn-Junction Rectifiers Based on p-ZnO and n-ZnO Nanoparticles

We form pn-junctions by electrostatic binding of a p-type and an n-type ZnO nanoparticle. Current−voltage characteristics of pn-junctions are rectifying in nature. Individual components of the junction do not show any rectification. An np-junction, formed by reversing the binding sequence of the two types of nanoparticles, shows rectification in the reverse bias direction. By controlling the type of dopants in the ZnO nanoparticles, pn+ (and n+p) and nn+ (and n+n) junctions are formed that exhibit rectification. Current rectification in a junction between two nanoparticles shows that a depletion layer may have formed even in the quantum dot regime.

Asymmetry of Nonlinear Transport and Electron Interactions in Quantum Dots

The symmetry properties of transport beyond the linear regime in chaotic quantum dots are investigated experimentally. A component of differential conductance that is antisymmetric in both applied source-drain bias V and magnetic field B, absent in linear transport, is found to exhibit mesoscopic fluctuations around a zero average. Typical values of this component allow a measurement of the electron interaction strength.

Ballistic chaotic quantum dots with interactions: A numerical study of the Robnik-Berry billiard

In previous work we have found a regime in ballistic quantum dots where interelectron interactions can be treated asymptotically exactly as the Thouless number $g$ of the dot becomes very large. However, this work depends on some assumptions concerning the renormalization group and various properties of the dot obeying Random Matrix Theory predictions at scales of the order of the Thouless energy. In this work we test the validity of those assumptions by considering a particular ballistic dot, the Robnik-Berry billiard, numerically. We find that almost all of our predictions based on the earlier work are borne out, with the exception of fluctuations of certain matrix elements of interaction operators. We conclude that, at least in the Robnik-Berry billiard, one can trust the results of our previous work at a qualitative and semi-quantitative level.

Quantum Dots and Their Tunnel Barrier in Semiconducting Single-Wall Carbon Nanotubes with a p-Type Behavior

Electrical transport properties have been measured in an individual p-type semiconducting single-wall carbon nanotube at the temperature range from 1.5 K to 160 K. The quantum dot transport has been observed, which depends on the gate voltage range. In the small positive gate voltage range, the single quantum dot behavior was observed, while the multi-dot behavior was observed in the further positive gate voltage. In the sufficiently large positive gate voltage range, no current flowed. The barrier height in the single quantum dot regime was estimated to be ∼10 meV by measuring the temperature dependence of the current–voltage curve at a Coulomb peak position.

How to avoid non‐radiative escape of excitons from quantum dots?

AbstractWe investigated the transition between two different exciton recombination regime in quantum dots depending on temperature. When temperature is raised above a threshold value that we determine experimentally non‐radiative recombination channels, characterized by their activation energy, dominate over radiative recombination. The analyses of time resolved photoluminescence spectroscopy versus temperature shows that the maximum temperature for dominant radiative recombination scales linearly with the activation energy of the non‐radiative channels over a large range of values (10–60 meV) measured for various kind of II–VI‐based quantum dots: CdTe/ZnTe, CdTe/ZnMgTe, CdSe/ZnSe. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Observation of Thermopower Oscillations in the Coulomb Blockade Regime in a Semiconducting Carbon Nanotube

We have measured the thermoelectric power (TEP) and Coulomb charging characteristics of an individual p-type semiconducting single-wall carbon nanotube (SWNT) in the quantum dot regime. The TEP mea...

Collapse of the spin-singlet phase in quantum dots.

We present experimental and theoretical results on a new regime in quantum dots in which the filling factor two-singlet state is replaced by new spin polarized phases. We make use of spin blockade spectroscopy to identify the transition to this new regime as a function of the number of electrons. The key experimental observation is a reversal of the phase in the systematic oscillation of the amplitude of Coulomb blockade peaks as the number of electrons is increased above a critical number. It is found theoretically that correlations are crucial to the existence of the new phases.

Energy spectra of quantum rings.

Quantum mechanical experiments in ring geometries have long fascinated physicists. Open rings connected to leads, for example, allow the observation of the Aharonov-Bohm effect, one of the best examples of quantum mechanical phase coherence. The phase coherence of electrons travelling through a quantum dot embedded in one arm of an open ring has also been demonstrated. The energy spectra of closed rings have only recently been studied by optical spectroscopy. The prediction that they allow persistent current has been explored in various experiments. Here we report magnetotransport experiments on closed rings in the Coulomb blockade regime. Our experiments show that a microscopic understanding of energy levels, so far limited to few-electron quantum dots, can be extended to a many-electron system. A semiclassical interpretation of our results indicates that electron motion in the rings is governed by regular rather than chaotic motion, an unexplored regime in many-electron quantum dots. This opens a way to experiments where even more complex structures can be investigated at a quantum mechanical level.

Semiconductor quantum rings: Shallow-donor levels

A theoretical study of shallow-donor states in toroidal GaAs-(Ga,Al)As quantum rings, within the effective-mass approximation and following a variational calculation, is presented. The particular topology of quantum rings is shown to produce interesting confining effects on the impurity states, namely, the effective dimensionality of a quantum ring may increase as it shrinks. The density of impurity states is calculated, and quantum-well wire and quantum-dot regimes are discussed. Also, the effects of an axial magnetic field on free-electron and shallow-donor states are analyzed.

Coulomb blockade oscillations in semiconductor nanostructures

A resume is given of recent theoretical and experimental work on single-electron tunneling in the size quantized regime in semiconductor quantum dots and wires. In addition, the quantum Hall effect regime is discussed.

Fabrication of coupled quantum dot arrays with a 100–150 nm period

This paper demonstrates a novel method for fabricating two-dimensional lateral surface superlattices. The technique involves etching a grid into a GaAs/GaAlAs modulation-doped heterostructure using a mask consisting of an array of 50-nm diam Ni/Au dots with a 100–150 nm period. Bias applied to a Schottky gate deposited over the structured surface varies the coupling strength between dots. The devices can be tuned to operate in the superlattice (weak and strong coupling) or the quantum dot (no coupling) regimes. Low-temperature characterization of the superlattices fabricated specifically for transport and capacitance measurements reveal novel magnetic field effects.