Kondo effect in an integer-spin quantum dot

The Kondo effect is a key many-body phenomenon in condensed matter physics. It concerns the interaction between a localised spin and free electrons. Discovered in metals containing small amounts of magnetic impurities, it is now a fundamental mechanism in a wide class of correlated electron systems. Control over single, localised spins has become relevant also in fabricated structures due to the rapid developments in nano-electronics. Experiments have already demonstrated artificial realisations of isolated magnetic impurities at metallic surfaces, nanometer-scale magnets, controlled transitions between two-electron singlet and triplet states, and a tunable Kondo effect in semiconductor quantum dots. Here, we report an unexpected Kondo effect realised in a few-electron quantum dot containing singlet and triplet spin states whose energy difference can be tuned with a magnetic field. This effect occurs for an even number of electrons at the degeneracy between singlet and triplet states. The characteristic energy scale is found to be much larger than for the ordinary spin-1/2 case.Comment: 12 page

Driven coherent oscillations of a single electron spin in a quantum dot

The ability to control the quantum state of a single electron spin in a quantum dot is at the heart of recent developments towards a scalable spin-based quantum computer. In combination with the recently demonstrated exchange gate between two neighbouring spins, driven coherent single spin rotations would permit universal quantum operations. Here, we report the experimental realization of single electron spin rotations in a double quantum dot. First, we apply a continuous-wave oscillating magnetic field, generated on-chip, and observe electron spin resonance in spin-dependent transport measurements through the two dots. Next, we coherently control the quantum state of the electron spin by applying short bursts of the oscillating magnetic field and observe about eight oscillations of the spin state (so-called Rabi oscillations) during a microsecond burst. These results demonstrate the feasibility of operating single-electron spins in a quantum dot as quantum bits.Comment: Total 25 pages. 11 pages main text, 5 figures, 9 pages supplementary materia

Interface effects, band overlap and the semimetal to semiconductor transition in InAs/GaSb interband resonant tunnelling diodes

We report some of the highest 77 K peak to valley ratios (PVRs) for single heterojunction InAs(n)/GaSb(p) resonant conduction diodes. The devices were grown with low background doping (n ≈ p ≈ 1016 cm-3) on (100) oriented substrates by atmospheric pressure MOVPE, and were prepared by switching the precursors in a predetermined order, to have "InSb" or "GaAs" like interfaces. We observe a stronger resonance with a weaker temperature dependence when the interface is "GaAs" like. In all samples, the voltage of the resonance and the peak current both decrease with hydrostatic pressure due to the pressure induced decrease of band overlap. Our results are consistent with a shift of -10 meV/kbar, and overlaps of 120 ± 20 meV and 250 ± 50 meV respectively for "InSb" like and "GaAs" like interfaces, and are in agreement with high pressure parallel transport results in superlattices with "InSb" like interfaces. © 1994

Vertical diatomic artificial molecule in the intermediate-coupling regime in a parallel and perpendicular magnetic field

We present experimental results for the ground-state electrochemical potentials of a few electron semiconductor artificial molecule made by vertically coupling two quantum dots, in the intermediate-coupling regime, in perpendicular and parallel magnetic fields up to Bsimilar to 5 T. We perform a quantitative analysis based on local-spin density functional theory. The agreement between theoretical and experimental results is good, and the phase transitions are well reproduced

Thermal assessment of heat mitigation strategies : the case of Portland State University, Oregon, USA

Courtyard vegetation, high albedo surfaces, and courtyard ponds were investigated as potential heat mitigation strategies using field measurements and simulations in a university campus environment. The investigation was performed during a summer period in the temperate climate of Portland, Oregon, USA. In a comparison of seven locations on the campus, the maximum park cooling island effect recorded was 5.8 °C between the heavily treed campus park and a nearby parking lot with asphalt pavement. Simulations of courtyards with vegetation and a water pond showed 1.6 °C and 1.1 °C air temperature reduction, respectively. Changing the albedo of the pavement in a bare courtyard from 0.37 (black) to 0.91 (white) led to 2.9 °C increase of mean radiant temperature and 1.3 °C decrease of air temperature

Spin dependence of electron effective masses in InGaAs/InAlAs quantum well

The effective masses for spin-up and spin-down electrons of a partially spin-polarized Fermi liquid are theoretically expected to be different. We extract the spin-up and spin-down effective masses from magnetotransport measurements at different temperatures for a two-dimensional electron gas in an In(0.65)Ga(0.35)As/In(0.52)Al(0.48)As quantum well exhibiting zero-field spin splitting. We apply two analytical methods, one involving the simultaneous fitting of fast Fourier transform (FFT) spectra and the other involving inverse FFT analysis. Both methods confirm that the effective masses for spin-up and spin-down are different, consistent with theoretical expectations. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3633509