Shell filling in closed single-wall carbon nanotube quantum dots

We observe two-fold shell filling in the spectra of closed one-dimensional quantum dots formed in single-wall carbon nanotubes. Its signatures include a bimodal distribution of addition energies, correlations in the excitation spectra for different electron number, and alternation of the spins of the added electrons. This provides a contrast with quantum dots in higher dimensions, where such spin pairing is absent. We also see indications of an additional fourfold periodicity indicative of K-K' subband shells. Our results suggest that the absence of shell filling in most isolated nanotube dots results from disorder or nonuniformity.Comment: 4 pages including 4 figure

Transport phenomena in nanotube quantum dots from strong to weak confinement

We report low-temperature transport experiments on single-wall nanotubes with metallic leads of varying contact quality, ranging from weak tunneling to almost perfect transmission. In the weak tunneling regime, where Coulomb blockade dominates, the nanotubes act as one-dimensional quantum dots. For stronger coupling to the leads the conductance can be strongly enhanced by inelastic cotunneling and the Kondo effect. For open contacts Coulomb blockade is completely suppressed, and the low-temperature conductance remains generally high, although we often see distinct dips in the conductance versus gate voltage which may result from resonant backscattering.Comment: 4 pages including 3 figures, for proceedings of the Moriond meeting 200

Quantum dots in suspended single-wall carbon nanotubes

We present a simple technique which uses a self-aligned oxide etch to suspend individual single-wall carbon nanotubes between metallic electrodes. This enables one to compare the properties of a particular nanotube before and after suspension, as well as to study transport in suspended tubes. As an example of the utility of the technique, we study quantum dots in suspended tubes, finding that their capacitances are reduced owing to the removal of the dielectric substrate

One-dimensional transport in bundles of single-walled carbon nanotubes

We report measurements of the temperature and gate voltage dependence for individual bundles (ropes) of single-walled nanotubes. When the conductance is less than about e^2/h at room temperature, it is found to decrease as an approximate power law of temperature down to the region where Coulomb blockade sets in. The power-law exponents are consistent with those expected for electron tunneling into a Luttinger liquid. When the conductance is greater than e^2/h at room temperature, it changes much more slowly at high temperatures, but eventually develops very large fluctuations as a function of gate voltage when sufficiently cold. We discuss the interpretation of these results in terms of transport through a Luttinger liquid.Comment: 5 pages latex including 3 figures, for proceedings of IWEPNM 99 (Kirchberg

Many-body effects in nonlinear optical responses of 2D layered semiconductors

We performed ultrafast degenerate pump-probe spectroscopy on monolayer WSe2 near its exciton resonance. The observed differential reflectance signals exhibit signatures of strong many-body interactions including the exciton-exciton interaction and free carrier induced band gap renormalization. The exciton-exciton interaction results in a resonance blue shift which lasts for the exciton lifetime (several ps), while the band gap renormalization manifests as a resonance red shift with several tens ps lifetime. Our model based on the many-body interactions for the nonlinear optical susceptibility fits well the experimental observations. The power dependence of the spectra shows that with the increase of pump power, the exciton population increases linearly and then saturates, while the free carrier density increases superlinearly, implying that exciton Auger recombination could be the origin of these free carriers. Our model demonstrates a simple but efficient method for quantitatively analyzing the spectra, and indicates the important role of Coulomb interactions in nonlinear optical responses of such 2D materials

Economic Implications of Patient-Related Factors in Diabetes Care

Diabetes mellitus is a chronic disease characterized by high blood sugar levels with serious complications, particularly if it is not adequately treated. It is increasingly prevalent and burdensome from both a health and economic standpoint globally, amounting to hundreds of billions of dollars in expenditure each year [1,2]. Also increasing are the numbers and types of pharmaceutical interventions being introduced to treat this disease. Over the next decade, numerous diabetes compounds currently in development are expected to be commercialized, making it essential for the most robust and accurate evidence to be available to healthcare decision-makers [3]. Approximately 90% of people with diabetes are diagnosed as Type 2 (T2D), most often in adulthood [4]. In contrast, Type 1 diabetes (T1D) patients are commonly diagnosed as juveniles, but live well into adulthood with effective treatment [4]. Consequently, adult patients with diabetes, namely T2D, are responsible for most of the health and economic burden of this disease. Therefore, much of the effort to improve outcomes and reduce costs in diabetes concentrates on this population