Few-electron quantum dot molecules

Semiconductor quantum dots have been studied for nearly two decades with a variety of experimental and theoretical methods. The typical dimensions of these "artificial atoms" are from a hundred nanometers to one micrometer and the number of electrons inside the quantum dots can range from one to several hundred. The tunable size, shape and electron number, as well as the enhanced electron correlation and magnetic field effects, makes quantum dots excellent objects for studying fascinating many-electron quantum physics in a controlled way. In the rapidly growing fields of nanoscience and nanotechnology, quantum dots are promising candidates for future nanoelectronic devices. One of the most intriguing scenarios of quantum dot applications lies in the utilization of an electron spin as a quantum bit in quantum computing. This Thesis deals with the modeling of electron states in two-dimensional coupled quantum dots or quantum-dot molecules. The emphasis is on describing electron correlations properly but concentrating only on a few electrons inside the quantum-dot molecules. In this Thesis we mostly consider two interacting electrons and find remarkably complex behavior as a function of the magnetic field. We find the electron-electron interactions and the shape of the confining potential to have a profound effect on the two-electron quantum states. The main results of this Thesis are the calculations and analysis of the quantum-mechanical states of two electrons in a magnetic field. This includes the ground-state transitions and magnetizations of the system as a function of magnetic field and careful analysis of the wave functions and the calculation and analysis of the far-infrared magneto-optical absorption spectra. We also study classical electrons in a quantum-dot molecule. This is relevant for the Wigner crystallization of electrons in a very high magnetic field or in a very low electron density.reviewe

Strong gate coupling of high-Q nanomechanical resonators

The detection of mechanical vibrations near the quantum limit is a formidable challenge since the displacement becomes vanishingly small when the number of phonon quanta tends towards zero. An interesting setup for on-chip nanomechanical resonators is that of coupling them to electrical microwave cavities for detection and manipulation. Here we show how to achieve a large cavity coupling energy of up to (2 \pi) 1 MHz/nm for metallic beam resonators at tens of MHz. We used focused ion beam (FIB) cutting to produce uniform slits down to 10 nm, separating patterned resonators from their gate electrodes, in suspended aluminum films. We measured the thermomechanical vibrations down to a temperature of 25 mK, and we obtained a low number of about twenty phonons at the equilibrium bath temperature. The mechanical properties of Al were excellent after FIB cutting and we recorded a quality factor of Q ~ 3 x 10^5 for a 67 MHz resonator at a temperature of 25 mK. Between 0.2K and 2K we find that the dissipation is linearly proportional to the temperature.Comment: 6 page

Electronic Refrigeration at the Quantum Limit

We demonstrate quantum limited electronic refrigeration of a metallic island in a low temperature micro-circuit. We show that matching the impedance of the circuit enables refrigeration at a distance, of about 50 um in our case, through superconducting leads with a cooling power determined by the quantum of thermal conductance. In a reference sample with a mismatched circuit this effect is absent. Our results are consistent with the concept of electromagnetic heat transport. We observe and analyze the crossover between electromagnetic and quasiparticle heat flux in a superconductor.Comment: 5 pages, 3 figure

Overestimation of Postpartum Depression Prevalence Based on a 5-item Version of the EPDS:Systematic Review and Individual Participant Data Meta-analysis

Objective:The Maternal Mental Health in Canada, 2018/2019, survey reported that 18% of 7,085 mothers who recently gave birth reported "feelings consistent with postpartum depression" based on scores >= 7 on a 5-item version of the Edinburgh Postpartum Depression Scale (EPDS-5). The EPDS-5 was designed as a screening questionnaire, not to classify disorders or estimate prevalence; the extent to which EPDS-5 results reflect depression prevalence is unknown. We investigated EPDS-5 >= 7 performance relative to major depression prevalence based on a validated diagnostic interview, the Structured Clinical Interview for DSM (SCID).Methods:We searched Medline, Medline In-Process & Other Non-Indexed Citations, PsycINFO, and the Web of Science Core Collection through June 2016 for studies with data sets with item response data to calculate EPDS-5 scores and that used the SCID to ascertain depression status. We conducted an individual participant data meta-analysis to estimate pooled percentage of EPDS-5 >= 7, pooled SCID major depression prevalence, and the pooled difference in prevalence.Results:A total of 3,958 participants from 19 primary studies were included. Pooled prevalence of SCID major depression was 9.2% (95% confidence interval [CI] 6.0% to 13.7%), pooled percentage of participants with EPDS-5 >= 7 was 16.2% (95% CI 10.7% to 23.8%), and pooled difference was 8.0% (95% CI 2.9% to 13.2%). In the 19 included studies, mean and median ratios of EPDS-5 to SCID prevalence were 2.1 and 1.4 times.Conclusions:Prevalence estimated based on EPDS-5 >= 7 appears to be substantially higher than the prevalence of major depression. Validated diagnostic interviews should be used to establish prevalence

Overestimation of Postpartum Depression Prevalence Based on a 5-item Version of the EPDS: Systematic Review and Individual Participant Data Meta-analysis

Objective:The Maternal Mental Health in Canada, 2018/2019, survey reported that 18% of 7,085 mothers who recently gave birth reported "feelings consistent with postpartum depression" based on scores >= 7 on a 5-item version of the Edinburgh Postpartum Depression Scale (EPDS-5). The EPDS-5 was designed as a screening questionnaire, not to classify disorders or estimate prevalence; the extent to which EPDS-5 results reflect depression prevalence is unknown. We investigated EPDS-5 >= 7 performance relative to major depression prevalence based on a validated diagnostic interview, the Structured Clinical Interview for DSM (SCID).Methods:We searched Medline, Medline In-Process & Other Non-Indexed Citations, PsycINFO, and the Web of Science Core Collection through June 2016 for studies with data sets with item response data to calculate EPDS-5 scores and that used the SCID to ascertain depression status. We conducted an individual participant data meta-analysis to estimate pooled percentage of EPDS-5 >= 7, pooled SCID major depression prevalence, and the pooled difference in prevalence.Results:A total of 3,958 participants from 19 primary studies were included. Pooled prevalence of SCID major depression was 9.2% (95% confidence interval [CI] 6.0% to 13.7%), pooled percentage of participants with EPDS-5 >= 7 was 16.2% (95% CI 10.7% to 23.8%), and pooled difference was 8.0% (95% CI 2.9% to 13.2%). In the 19 included studies, mean and median ratios of EPDS-5 to SCID prevalence were 2.1 and 1.4 times.Conclusions:Prevalence estimated based on EPDS-5 >= 7 appears to be substantially higher than the prevalence of major depression. Validated diagnostic interviews should be used to establish prevalence

Comparison of major depression diagnostic classification probability using the SCID, CIDI, and MINI diagnostic interviews among women in pregnancy or postpartum: An individual participant data meta‐analysis

OBJECTIVES: A previous individual participant data meta-analysis (IPDMA) identified differences in major depression classification rates between different diagnostic interviews, controlling for depressive symptoms on the basis of the Patient Health Questionnaire-9. We aimed to determine whether similar results would be seen in a different population, using studies that administered the Edinburgh Postnatal Depression Scale (EPDS) in pregnancy or postpartum. METHODS: Data accrued for an EPDS diagnostic accuracy IPDMA were analysed. Binomial generalised linear mixed models were fit to compare depression classification odds for the Mini International Neuropsychiatric Interview (MINI), Composite International Diagnostic Interview (CIDI), and Structured Clinical Interview for DSM (SCID), controlling for EPDS scores and participant characteristics. RESULTS: Among fully structured interviews, the MINI (15 studies, 2,532 participants, 342 major depression cases) classified depression more often than the CIDI (3 studies, 2,948 participants, 194 major depression cases; adjusted odds ratio [aOR] = 3.72, 95% confidence interval [CI] [1.21, 11.43]). Compared with the semistructured SCID (28 studies, 7,403 participants, 1,027 major depression cases), odds with the CIDI (interaction aOR = 0.88, 95% CI [0.85, 0.92]) and MINI (interaction aOR = 0.95, 95% CI [0.92, 0.99]) increased less as EPDS scores increased. CONCLUSION: Different interviews may not classify major depression equivalently

Seawater carbonate chemistry and carbonate chemistry in the microenvironment within cyanobacterial aggregates

Photosynthesis and respiration cause distinct chemical microenvironments within cyanobacterial aggregates. Here, we used microsensors and a diffusion–reaction model to characterize gradients in carbonate chemistry and investigate how these are affected by ocean acidification in Baltic vs. Pacific aggregates (Nodularia and Dolichospermum vs. Trichodesmium). Microsensor measurements of O2 and pH were performed under in situ and expected future pCO2 levels on Nodularia and Dolichospermum aggregates collected in the Baltic Sea. Under in situ conditions, O2 and pH levels within the aggregates covered ranges of 80–175% air saturation and 7.7–9.4 in dark and light, respectively. Carbon uptake in the light was predicted to reduce HCO3− by 100–150 μmol/L and CO2 by 3–6 μmol/L in the aggregate center compared to outside, inducing strong CO2 depletion (down to 0.5 μmol/L CO2 remaining in the center) even when assuming that HCO3− covered 80–90% of carbon uptake. Under ocean acidification conditions, enhanced CO2 availability allowed for significantly lower activity of carbon concentrating mechanisms, including a reduction of the contribution of HCO3− to carbon uptake by up to a factor of 10. The magnification of proton gradients under elevated pCO2 that was predicted based on a lower buffer capacity was observed in measurements despite a concurrent decrease in photosynthetic activity. In summary, we provide a quantitative image of the inorganic carbon environment in cyanobacterial aggregates under present-day and expected future conditions, considering both the individual and combined effects of the chemical and biological processes that shape these environments

Microsensor measurements of oxygen concentration profiles, and computer-simulated concentration profiles of oxygen, ammonium and nitrate oxygen in Trichodesmium colonies

Trichodesmium is an important dinitrogen (N~2~)-fixing cyanobacterium in marine ecosystems. Recent nucleic acid analyses indicate that Trichodesmium colonies with their diverse epibionts support various nitrogen (N) transformations beyond N~2~-fixation. However, rates of these transformations and concentration gradients of N-compounds in Trichodesmium colonies remain largely unresolved. We combined isotope-tracer incubations, micro-profiling, and numeric modelling to explore carbon fixation, N-cycling processes, as well as oxygen, ammonium and nitrate concentration gradients in individual field-sampled Trichodesmium colonies. Colonies were net-autotrophic, with carbon and N~2~-fixation occurring mostly at day-time. Ten percent of the fixed N was released as ammonium after 12-hour incubations. Nitrification was not detectable but nitrate consumption was high when nitrate was added. The consumed nitrate was partly reduced to ammonium, while denitrification was insignificant. Thus, the potential N-transformation network was characterized by fixed N gain and recycling processes rather than denitrification. Oxygen concentrations within colonies were 60–200% air-saturation. Moreover, our modelling predicted steep concentration gradients, with up to 6-fold higher ammonium concentrations, and nitrate depletion in the colony centre compared to the ambient seawater. These gradients created a chemically heterogeneous microenvironment, presumably facilitating diverse microbial metabolisms in millimetre-sized Trichodesmium colonies