Full configuration interaction approach to the few-electron problem in artificial atoms

We present a new high-performance configuration interaction code optimally designed for the calculation of the lowest energy eigenstates of a few electrons in semiconductor quantum dots (also called artificial atoms) in the strong interaction regime. The implementation relies on a single-particle representation, but it is independent of the choice of the single-particle basis and, therefore, of the details of the device and configuration of external fields. Assuming no truncation of the Fock space of Slater determinants generated from the chosen single-particle basis, the code may tackle regimes where Coulomb interaction very effectively mixes many determinants. Typical strongly correlated systems lead to very large diagonalization problems; in our implementation, the secular equation is reduced to its minimal rank by exploiting the symmetry of the effective-mass interacting Hamiltonian, including square total spin. The resulting Hamiltonian is diagonalized via parallel implementation of the Lanczos algorithm. The code gives access to both wave functions and energies of first excited states. Excellent code scalability in a parallel environment is demonstrated; accuracy is tested for the case of up to eight electrons confined in a two-dimensional harmonic trap as the density is progressively diluted and correlation becomes dominant. Comparison with previous Quantum Monte Carlo simulations in the Wigner regime demonstrates power and flexibility of the method.Comment: RevTeX 4.0, 18 pages, 6 tables, 9 postscript b/w figures. Final version with new material. Section 6 on the excitation spectrum has been added. Some material has been moved to two appendices, which appear in the EPAPS web depository in the published versio

Molecular phases in coupled quantum dots

We present excitation energy spectra of few-electron vertically coupled quantum dots for strong and intermediate inter-dot coupling. By applying a magnetic field, we induce ground state transitions and identify the corresponding quantum numbers by comparison with few-body calculations. In addition to atomic-like states, we find novel "molecular-like" phases. The isospin index characterizes the nature of the bond of the artificial molecule and this we control. Like spin in a single quantum dot, transitions in isospin leading to full polarization are observed with increasing magnetic field.Comment: PDF file only, 28 pages, 3 tables, 4 color figures, 2 appendices. To appear in Physical Review B, Scheduled 15 Feb 2004, Vol. 69, Issue

EIMS Fragmentation and MRM quantification of bacterial metabolites of the sea ice biomarker proxy IP25in Arctic sediments.

RATIONALE: 3,9,13-trimethyl-6-(1,5-dimethylhexyl)-tetradecan-1,2-diol and 2,8,12-trimethyl-5-(1,5-dimethylhexyl)-tridecanoic acid appear to be produced during the bacterial metabolism of IP25, a highly branched isoprenoid lipid often employed for past Arctic sea ice reconstruction. Characterization and quantification of these metabolites in sediments are essential to determine if bacterial degradation may exert a significant influence on IP25-based palaeo sea ice reconstructions. METHODS: EIMS fragmentation pathways of 3,9,13-trimethyl-6-(1,5-dimethylhexyl)-tetradecan-1,2-diol and 2,8,12-trimethyl-5-(1,5-dimethylhexyl)-tridecanoic acid TMS derivatives were investigated. These pathways were deduced by: (i) low energy CID-GC/MS/MS, (ii) accurate mass measurement and (iii) deuterium labelling. RESULTS: CID-MS/MS analyses, accurate mass measurement and deuterium labelling experiments enabled us to elucidate the EIMS fragmentations of 3,9,13-trimethyl-6-(1,5-dimethylhexyl)-tetradecan-1,2-diol and 2,8,12-trimethyl-5-(1,5-dimethylhexyl)-tridecanoic acid TMS derivatives. Some specific fragment ions useful in addition to chromatographic retention times for further characterization could be identified. As an application of some of the described fragmentations, the TMS derivatives of these metabolites were characterized and quantified in MRM mode in different Arctic sediments. CONCLUSIONS: EIMS fragmentations of 3,9,13-trimethyl-6-(1,5-dimethylhexyl)-tetradecan-1,2-diol and 2,8,12-trimethyl-5-(1,5-dimethylhexyl)-tridecanoic acid TMS derivatives exhibit specific fragment ions, which appear to be very useful for the quantification of these bacterial metabolites of the palaeo tracer IP25in sediments

Degradation state of organic matter in surface sediments from the Southern Beaufort Sea: a lipid approach

For the next decades significant climatic changes should occur in the Arctic zone. The expected destabilisation of permafrost and its consequences for hydrology and plant cover should increase the input of terrigenous carbon to coastal seas. Consequently, the relative importance of the fluxes of terrestrial and marine organic carbon to the seafloor will likely change, strongly impacting the preservation of organic carbon in Arctic marine sediments. Here, we investigated the lipid content of surface sediments collected on the Mackenzie basin in the Beaufort Sea. Particular attention was given to biotic and abiotic degradation products of sterols and monounsaturated fatty acids. By using sitosterol and campesterol degradation products as tracers of the degradation of terrestrial higher plant inputs and brassicasterol degradation products as tracers of degradation of phytoplanktonic organisms, it could be observed that autoxidation, photooxidation and biodegradation processes act much more intensively on higher plant debris than on phytoplanktonic organisms. Examination of oxidation products of monounsaturated fatty acids showed that photo- and autoxidation processes act more intensively on bacteria than on phytodetritus. Enhanced damages induced by singlet oxygen (transferred from senescent phytoplanktonic cells) in bacteria were attributed to the lack of an adapted antioxidant system in these microorganisms. The strong oxidative stress observed in the sampled sediments resulted in the production of significant amounts of epoxy acids and unusually high proportions of monounsaturated fatty acids with a <i>trans</i> double bond. The formation of epoxy acids was attributed to peroxygenases (enzymes playing a protective role against the deleterious effects of fatty acid hydroperoxides in vivo), while <i>cis/trans</i> isomerisation was probably induced by thiyl radicals produced during the reaction of thiols with hydroperoxides. Our results confirm the important role played by abiotic oxidative processes in the degradation of marine bacteria and do not support the generally expected refractory character of terrigenous material deposited in deltaic systems

Biotic and abiotic degradation of the sea ice diatom biomarker IP 25 and selected algal sterols in near-surface Arctic sediments

International audienceThe organic geochemical IP 25 (Ice Proxy with 25 carbon atoms) has been used as a proxy for Arctic sea ice in recent years. To date, however, the role of degradation of IP 25 in Arctic marine sediments and the impact that this may have on palaeo sea ice reconstruction based on this biomarker have not been investigated in any detail. Here, we show that IP 25 may be susceptible to autoxidation in near-surface oxic sediments. To arrive at these conclusions, we first subjected a purified sample of IP 25 to autoxidation in the laboratory and characterised the oxidation products using high resolution gas chromatography-mass spectrometric methods. Most of these IP 25 oxidation products were also detected in near-surface sediments collected from Barrow Strait in the Canadian Arctic, although their proposed secondary oxidation and the relatively lower abundances of IP 25 in other sediments probably explain why we were not able to detect them in material from other parts of the region. A rapid decrease in IP 25 concentration in some near-surface Arctic marine sediments, including examples presented here, may potentially be attributed to at least partial degradation, especially for sediment cores containing relatively thick oxic layers representing decades or centuries of deposition. An increase in the ratio of two common phytoplanktonic ster-ols-epi-brassicasterol and 24-methylenecholesterol-provides further evidence for such autoxidation reactions given the known enhanced reactivity of the latter to such processes reported previously. In addition, we provide some evidence that biodegradation processes also act on IP 25 in Arctic sediments. The oxidation products identified in the present study will need to be quantified more precisely in down-core records in the future before the effects of degradation processes on IP 25-based palaeo sea ice reconstruction can be fully understood. In the meantime, a brief overview of some previous investigations of IP 25 in relatively shallow Arctic marine sediments suggests that overlying climate conditions were likely dominant over degradation processes, as evidenced from often increasing IP 25 concentration downcore, together with positive relationships to known sea ice conditions

Coherent transport in homojunction between excitonic insulator and semimetal

From the solution of a two-band model, we predict that the thermal and electrical transport across the junction of a semimetal and an excitonic insulator will exhibit high resistance behavior and low entropy production at low temperatures, distinct from a junction of a semimetal and a normal semiconductor. This phenomenon, ascribed to the dissipationless exciton flow which dominates over the charge transport, is based on the much longer length scale of the change of the effective interface potential for electron scattering due to the coherence of the condensate than in the normal state.Comment: RevTeX 4.0, 13 pages, 5 b/w figures, 1 colour figure, 1 table. Version modified with respect to the original, which will appear in Physical Review Letters. This version includes the supplementary (EPAPS) material as an Appendix, and it is slightly longer than the accepted version (more text and references

Seasonal survey of the composition and degradation state of particulate organic matter in the Rhône River using lipid tracers

International audienceLipid tracers including fatty acids, hydroxyacids, n-alkanols, sterols and triterpenoids were used to determine the origin and fate of suspended particulate organic matter (POM) collected in the Rhône River (France). This seasonal survey (April 2011 to May 2013) revealed a year-round strong terrestrial higher-plant contribution to the particulate organic matter (POM), with significant algal inputs observed in March and attributed to phytoplanktonic blooms likely dominated by diatoms. Major terrigenous contributors to our samples are gymnosperms, and more precisely their roots and stems, as evidenced by the presence of high proportions of ω-hydroxydocosanoic acid (a suberin biomarker). The high amounts of coprostanol detected clearly show that the Rhône River is significantly affected by sewage waters. Specific sterol degradation products were quantified and used to assess the part of biotic and abiotic degradation of POM within the river. Higher-plant-derived organic matter appears to be mainly affected by photo-oxidation and au-toxidation (free radical oxidation), while organic matter of mammal or human origin, evidenced by the presence of co-prostanol, is clearly more prone to bacterial degradation. Despite the involvement of an intense autoxidation-inducing ho-molytic cleavage of peroxy bonds, a significant proportion of hydroperoxides is still intact in higher plant debris. These compounds could affect the degradation of terrestrial material by inducing an intense autoxidation upon its arrival at sea

Coulomb correlation effects in semiconductor quantum dots: The role of dimensionality

We study the energy spectra of small three-dimensional (3D) and two-dimensional (2D) semiconductor quantum dots through different theoretical approaches (single-site Hubbard and Hartree-Fock hamiltonians); in the smallest dots we also compare with exact results. We find that purely 2D models often lead to an inadequate description of the Coulomb interaction existing in realistic structures, as a consequence of the overestimated carrier localization. We show that the dimensionality of the dots has a crucial impact on (i) the accuracy of the predicted addition spectra; (ii) the range of validity of approximate theoretical schemes. When applied to realistic 3D geometries, the latter are found to be much more accurate than in the corresponding 2D cases for a large class of quantum dots; the single-site Hubbard hamiltonian is shown to provide a very effective and accurate scheme to describe quantum dot spectra, leading to good agreement with experiments.Comment: LaTeX 2.09, RevTeX, 25 pages, 9 Encapsulated Postscript figures. To be published in Physical Review

Dynamics of cold bosons in optical lattices: Effects of higher Bloch bands

The extended effective multiorbital Bose-Hubbard-type Hamiltonian which takes into account higher Bloch bands, is discussed for boson systems in optical lattices, with emphasis on dynamical properties, in relation with current experiments. It is shown that the renormalization of Hamiltonian parameters depends on the dimension of the problem studied. Therefore, mean field phase diagrams do not scale with the coordination number of the lattice. The effect of Hamiltonian parameters renormalization on the dynamics in reduced one-dimensional optical lattice potential is analyzed. We study both the quasi-adiabatic quench through the superfluid-Mott insulator transition and the absorption spectroscopy, that is energy absorption rate when the lattice depth is periodically modulated.Comment: 23 corrected interesting pages, no Higgs boson insid