The properties of quasi-two-dimensional semiconductor quantum dots are reviewed. Experimental techniques for measuring the electronic shell structure and the effect of magnetic fields are briefly described. The electronic structure is analyzed in terms of simple single-particle models, density-functional theory, and "exact" diagonalization methods. The spontaneous magnetization due to Hund's rule, spin-density wave states, and electron localization are addressed. As a function of the magnetic field, the electronic structure goes through several phases with qualitatively different properties. The formation of the so-called maximum-density droplet and its edge reconstruction is discussed, and the regime of strong magnetic fields in finite dot is examined. In addition, quasi-one-dimensional rings, deformed dots, and dot molecules are considered. (Less)

/pdf/electronic-structure-of-quantum-dots-c4uhdr9l0b.pdf

Electronic structure of quantum dots

The conductive polymer poly(3,4-ethylenedioxythiophene) (PEDOT), and especially its complex with poly(styrene sulfonate) (PEDOT:PSS), is perhaps the most well-known example of an organic conductor. It is highly conductive, largely transmissive to light, processible in water, and highly flexible. Much recent work on this ubiquitous material has been devoted to increasing its deformability beyond flexibility-a characteristic possessed by any material that is sufficiently thin-toward stretchability, a characteristic that requires engineering of the structure at the molecular- or nanoscale. Stretchability is the enabling characteristic of a range of applications envisioned for PEDOT in energy and healthcare, such as wearable, implantable, and large-area electronic devices. High degrees of mechanical deformability allow intimate contact with biological tissues and solution-processable printing techniques (e.g., roll-to-roll printing). PEDOT:PSS, however, is only stretchable up to around 10%. Here, the strategies that have been reported to enhance the stretchability of conductive polymers and composites based on PEDOT and PEDOT:PSS are highlighted. These strategies include blending with plasticizers or polymers, deposition on elastomers, formation of fibers and gels, and the use of intrinsically stretchable scaffolds for the polymerization of PEDOT.

Stretchable Conductive Polymers and Composites Based on PEDOT and PEDOT:PSS.

The Kondo effect—a many-body phenomenon in condensed-matter physics involving the interaction between a localized spin and free electrons—was discovered in metals containing small amounts of magnetic impurities, although it is now recognized to be of fundamental importance in a wide class of correlated electron systems1,2 In fabricated structures, the control of single, localized spins is of technological relevance for nanoscale electronics3,4 Experiments have already demonstrated artificial realizations of isolated magnetic impurities at metallic surfaces5,6, nanoscale magnets7, controlled transitions between two-electron singlet and triplet states8, and a tunable Kondo effect in semiconductor quantum dots9,10,11,12 Here we report an unexpected Kondo effect in a few-electron quantum dot containing singlet and triplet spin states, whose energy difference can be tuned with a magnetic field We observe the effect for an even number of electrons, when the singlet and triplet states are degenerate The characteristic energy scale is much larger than in the ordinary spin-1/2 case

Kondo effect in an integer-spin quantum dot

Nanomechanical oscillators are at the heart of ultrasensitive detectors of force, mass and motion. As these detectors progress to even better sensitivity, they will encounter measurement limits imposed by the laws of quantum mechanics. If the imprecision of a measurement of the displacement of an oscillator is pushed below a scale set by the standard quantum limit, the measurement must perturb the motion of the oscillator by an amount larger than that scale. Here we show a displacement measurement with an imprecision below the standard quantum limit scale. We achieve this imprecision by measuring the motion of a nanomechanical oscillator with a nearly shot-noise limited microwave interferometer. As the interferometer is naturally operated at cryogenic temperatures, the thermal motion of the oscillator is minimized, yielding an excellent force detector with a sensitivity of 0.51 aN Hz(-1/2). This measurement is a critical step towards observing quantum behaviour in a mechanical object.

/pdf/nanomechanical-motion-measured-with-an-imprecision-below-the-59vq4yks02.pdf

Nanomechanical motion measured with an imprecision below the standard quantum limit

Carbon nanotubes are a versatile material in which many aspects of condensed matter physics come together. Recent discoveries have uncovered new phenomena that completely change our understanding of transport in these devices, especially the role of the spin and valley degrees of freedom. This review describes the modern understanding of transport through nanotube devices. Unlike in conventional semiconductors, electrons in nanotubes have two angular momentum quantum numbers, arising from spin and valley freedom. The interplay between the two is the focus of this review. The energy levels associated with each degree of freedom, and the spin-orbit coupling between them, are explained, together with their consequences for transport measurements through nanotube quantum dots. In double quantum dots, the combination of quantum numbers modifies the selection rules of Pauli blockade. This can be exploited to read out spin and valley qubits and to measure the decay of these states through coupling to nuclear spins and phonons. A second unique property of carbon nanotubes is that the combination of valley freedom and electron-electron interactions in one dimension strongly modifies their transport behavior. Interaction between electrons inside and outside a quantum dot is manifested in SU(4) Kondo behavior and level renormalization. Interaction within a dot leads to Wigner molecules and more complex correlated states. This review takes an experimental perspective informed by recent advances in theory. As well as the well-understood overall picture, open questions for the field are also clearly stated. These advances position nanotubes as a leading system for the study of spin and valley physics in one dimension where electronic disorder and hyperfine interaction can both be reduced to a low level.

/pdf/quantum-transport-in-carbon-nanotubes-482jlbi7aa.pdf

Quantum transport in carbon nanotubes

Great improvement in the output power density of diamond FETs on a diamond homoepitaxial layer grown using a high-purity source gas is reported. For a device with a gate length of 0.1 µm and gate width of 100 µm, at 1 GHz, maximum output power density of 2.1 W/mm, maximum power gain of 10.9 dB, and power added efficiency of 31.8% were obtained.

2 W/mm output power density at 1 GHz for diamond FETs

Addition energy spectra at 0 T of circular and ellipsoidally deformed few-electron vertical quantum dots are measured and compared to results of model calculations within spin-density-functional theory. Because of the rotational symmetry of the lateral harmonic confining potential, circular dots show a pronounced shell structure. With the lifting of the single-particle level degeneracies, even a small deformation is found to radically alter the shell structure leading to significant modifications in the addition energy spectra. Breaking the circular symmetry with deformation also induces changes in the total spin. This ``piezomagnetic'' behavior of quantum dots is discussed, and the addition energies for a set of realistic deformation parameters are provided. For the case of the four-electron ground state at 0 T, a spin-triplet to spin-singlet transition is predicted, i.e., Hund's first rule no longer applies. Application of a magnetic field parallel to the current confirms that this is the case, and also suggests that the anisotropy of an elliptical dot, in practice, may be higher than that suggested by the geometry of the device mesa in which the dot is located.

Ellipsoidal deformation of vertical quantum dots

High RF output power for H-terminated diamond FETs

We report the successful demonstration of all-optical switching using a sub-wavelength InP/InAsP nanowire integrated in a silicon photonic crystal at telecommunication wavelengths. In this work, we...

All-optical InAsP/InP nanowire switches integrated in a Si photonic crystal

We report shot noise spectroscopy on a semiconductor quantum dot in a cotunneling regime. The dc conductance measurements show clear signatures of both elastic and inelastic cotunneling transport inside a Coulomb diamond. We observed Poissonian shot noise with the Fano factor $F\ensuremath{\approx}1$ in the elastic cotunneling regime, and super-Poissonian Fano factor $1lFl3$ in the inelastic cotunneling regime. The differences in the value of the Fano factor between elastic and inelastic processes reveal the microscopic mechanisms involved in the cotunneling transport.

Satoshi Sasaki

Papers

2 W/mm output power density at 1 GHz for diamond FETs

Ellipsoidal deformation of vertical quantum dots

High RF output power for H-terminated diamond FETs

All-optical InAsP/InP nanowire switches integrated in a Si photonic crystal

Shot noise spectroscopy on a semiconductor quantum dot in the elastic and inelastic cotunneling regimes