The authors have studied low-frequency resistance fluctuations in shadow-evaporated Al/ AlO x /Al tunnel junctions. Between 300 and 5 K the spectral density follows a 1 / f law. Below 5 K, individual defects distort the 1 / f shape of the spectrum. The spectral density decreases linearly with temperature between 150 and 1 K and saturates below 0.8 K. At 4.2 K, it is about two orders of magnitude lower than expected from a recent survey ͓D. J. Van 4 Decoherence due to external sources such as the measurement devices has been studied extensively and is by now well understood, 5 permitting qubit dephasing times of up to several microseconds. 6 Future progress in this field of research depends crucially on understanding and controlling decoherence due to defects in the devices. 7-9 Superconducting qubits contain Josephson junctions, whose Josephson energy, E J = ⌽ 0 I C / ͑2 ͒, determines the potential landscape of the qubit ͑I C is the critical current and ⌽ 0 = h /2e is the superconducting flux quantum͒. Due to imperfections of the tunnel barrier, E J fluctuates in time, leading to fluctuations in the qubit potential. Therefore, the qubit energy splitting is not constant during an experiment, which leads to decoherence.

Positron annihilation lifetime spectroscopy and positron-electron momentum density (PEMD) studies on multilayers of PbSe nanocrystals (NCs), supported by transmission electron microscopy, show that positrons are strongly trapped at NC surfaces, where they provide insight into the surface composition and electronic structure of PbSe NCs. Our analysis indicates abundant annihilation of positrons with Se electrons at the NC surfaces and with O electrons of the oleic ligands bound to Pb ad-atoms at the NC surfaces, which demonstrates that positrons can be used as a sensitive probe to investigate the surface physics and chemistry of nanocrystals inside multilayers. Ab initio electronic structure calculations provide detailed insight in the valence and semi-core electron contributions to the positron-electron momentum density of PbSe. Both lifetime and PEMD are found to correlate with changes in the particle morphology characteristic of partial ligand removal

The effect of temperature controlled annealing on the confined valence electron states in CdSe nanocrystal arrays, deposited as thin films, was studied using two-dimensional angular correlation of annihilation radiation. A reduction in the intensity by ϳ35% was observed in a feature of the positron annihilation spectrum upon removal of the pyridine capping molecules above 200°C in a vacuum. This reduction is explained by an increased electronic interaction of the valence orbitals of neighboring nanocrystals, induced by the formation of inorganic interfaces. Partial evaporation of the nanoporous CdSe layer and additional sintering into a polycrystalline thin film were observed at a relatively low temperature of ϳ486°C. © 2009 American Institute of Physics. ͓DOI: 10.1063/1.3094751͔The size and shape of colloidal II-VI semiconductor nanocrystals ͑NCs͒ can be well controlled, leading to a pronounced tunability and variation in their optical and optoelectronic properties.1-3 Their promise for applications in light-emitting diodes, solar cells and other opto-electronic devices was demonstrated in several studies. [4][5][6] For example, recently ultrathin solar cells consisting of sintered nanorods ͑a dual set of CdSe and CdTe͒ were developed. 7 The ͑opto͒electronic properties can be further modified by structural tailoring of nanocrystal superlattices. [8][9][10] The electronic interaction between neighboring NCs is a fundamentally important factor for composite nanocrystal devices, which determines both the transport of electron and hole charge carriers and the electronic structure of the active layers. The coupling can, in principle, be tailored by ligand manipulation or by inorganic tunneling barriers between neighboring nanocrystals. 1,11,12 Thus, innovative heterostructures are created.12 For example, enhanced conductivity has been achieved by removal of pyridine ligands by gentle heating ͑150-175°C͒ in a vacuum at moderate temperatures. 1,8,10 This reduces the average distance between neighboring NCs to less than 2 Å and leads to changes in the optical properties due to strong coupling. 1,8,10,11 A recent in situ electron microscopy study on monolayers of PbSe NCs indicates that this is accompanied by rotations of the NCs and the formation of an inorganic interface between neighboring NCs at slightly higher temperatures. 13Recent studies on semiconductor NCs 14-17 show the potential of positron methods to study the electronic structure of nanocrystal solids since the positron can be used as a sensitive probe for detecting the surface composition of the nanocrystals and the confinement of the upper valence electron states. In the present study, we apply high-resolution depth-sensitive positron methods 15,[17][18][19] to show that the electronic interaction between CdSe NCs, deposited as thin layers, can be monitored through observation of the electron momentum distribution of the valence states. Further, the depth-resolved positron studies provide insights into further sintering of the nanocrystal layers at highe...