First order 0/π0/\pi quantum phase transition in the Kondo regime of a superconducting carbon nanotube quantum dot

We study a carbon nanotube quantum dot embedded into a SQUID loop in order to investigate the competition of strong electron correlations with proximity effect. Depending whether local pairing or local magnetism prevails, a superconducting quantum dot will respectively exhibit positive or negative supercurrent, referred to as a 0 or $\pi$ Josephson junction. In the regime of strong Coulomb blockade, the 0 to $\pi$ transition is typically controlled by a change in the discrete charge state of the dot, from even to odd. In contrast, at larger tunneling amplitude the Kondo effect develops for an odd charge (magnetic) dot in the normal state, and quenches magnetism. In this situation, we find that a first order 0 to $\pi$ quantum phase transition can be triggered at fixed valence when superconductivity is brought in, due to the competition of the superconducting gap and the Kondo temperature. The SQUID geometry together with the tunability of our device allows the exploration of the associated phase diagram predicted by recent theories. We also report on the observation of anharmonic behavior of the current-phase relation in the transition regime, that we associate with the two different accessible superconducting states. Our results ultimately reveal the spin singlet nature of the Kondo ground state, which is the key process in allowing the stability of the 0-phase far from the mixed valence regime.Comment: 10 pages, 6 figures in main text, 4 figures in appendi

Direct determination of high-order transverse ligand field parameters via µSQUID-EPR in a Et4_{4}N[160^{160}GdPc2_{2}] SMM

The development of quantum technologies requires a thorough understanding of systems possessing quantum effects that can ultimately be manipulated. In the field of molecular magnetism, one of the main challenges is to measure high-order ligand field parameters, which play an essential role in the relaxation properties of SMMs. The development of highly advanced theoretical calculations has allowed the ab-initio determination of such parameters; however, currently, there is a lack of quantitative assessment of how good the ab-initio parameters are. In our quest for technologies that can allow the extraction of such elusive parameters, we develop an experimental technique that combines the EPR spectroscopy and µSQUID magnetometry. We demonstrate the power of the technique by performing EPR-µSQUID measurement of a magnetically diluted single crystal of Et4N[GdPc2], by sweeping the magnetic field and applying a range of multifrequency microwave pulses. As a result, we were able to directly determine the high-order ligand field parameters of the system, enabling us to test theoretical predictions made by state-of-the-art ab-initio methods

On Aharonov-Bohm oscillation in a ferromagnetic ring

Aharonov-Bohm effect in a ferromagnetic thin ring in diffusive regime is theoretically studied by calculating the Cooperon and Diffuson. In addition to the spin-orbit interaction, we include the spin-wave excitation and the spin splitting, which are expected to be dominant sources of dephasing in ferromagnets at low temperatures. The spin splitting turns out to kill the spin-flip channel of Cooperon but leaves the spin-conserving channel untouched. For the experimental confirmation of interference effect (described by Cooperons) such as weak localization and Aharonov-Bohm oscillation with period $h/2e$, we need to suppress the dominant dephasing by orbital motion. To do this we propose experiments on a thin film or thin ring with magnetization and external field perpendicular to the film, in which case the effective field inside the sample is equal to the external field (magnetization does not add up). The field is first applied strong enough to saturate the magnetization and then carrying out the measurement down to zero field keeping the magnetization nearly saturated, in order to avoid domain formations (negative fields may also be investigated if the coercive field is large enough)

Solving rate equations for electron tunneling via discrete quantum states

We consider the form of the current-voltage curves generated when tunneling spectroscopy is used to measure the energies of individual electronic energy levels in nanometer-scale systems. We point out that the voltage positions of the tunneling resonances can undergo temperature-dependent shifts, leading to errors in spectroscopic measurements that are proportional to temperature. We do this by solving the set of rate equations that can be used to describe electron tunneling via discrete quantum states, for a number of cases important for comparison to experiments, including (1) when just one spin-degenerate level is accessible for transport, (2) when 2 spin-degenerate levels are accessible, with no variation in electron-electron interactions between eigenstates, and (3) when 2 spin-degenerate levels are accessible, but with variations in electron-electron interactions. We also comment on the general case with an arbitrary number of accessible levels. In each case we analyze the voltage-positions, amplitudes, and widths of the current steps due to the quantum states.Comment: REVTeX 4, 10 pages, 12 figures, submitted to Phys. Rev. B. Associated programs available at http://www.ccmr.cornell.edu/~ralph

High domain wall velocity at zero magnetic field induced by low current densities in spin-valve nanostripes

Current-induced magnetic domain wall motion at zero magnetic field is observed in the permalloy layer of a spin-valve-based nanostripe using photoemission electron microscopy. The domain wall movement is hampered by pinning sites, but in between them high domain wall velocities (exceeding 150 m/s) are obtained for current densities well below 10^{12} \unit{A/m^2}, suggesting that these trilayer systems are promising for applications in domain wall devices in case of well controlled pinning positions. Vertical spin currents in these structures provide a potential explanation for the increase in domain wall velocity at low current densities.Comment: Published version, Applied Physics Express 2, 023003 (2009) http://dx.doi.org/10.1143/APEX.2.02300

Phytoremediation of chrome in tannery effluents using Eichhornia crassipes

El cromo es uno de los mayores contaminantes inorgánicos de las aguas y del suelo en el Paraguay, acarreando serios problemas ambientales. Este metal proviene principalmente de efluentes de curtiembres que son vertidos a cursos de aguas. El objetivo de este trabajo es emplear técnicas de remediación alternativas para el tratamiento de aguas con alto contenido de cromo, utilizando medios naturales (plantas), en un proceso denominado fitorremediación. Ejemplares de Eichhornia crassipes fueron sometidos a una contaminación controlada con Cr3+ durante 20 días y se consideró la máxima concentración tolerada durante todo el periodo del experimento para estudiar la taza y la capacidad de absorción del Cr3+ de la especie. Las constantes cinéticas de este proceso fueron: constante de absorción Kab= 0,10 días -1, y tiempo medio de absorción: t1/2 = 6,8 días. En el efluente final de curtiembre, con 22,4 mg.L-1 de Cr3+, a las 48 horas se produjo una absorción casi completa del cromo suministrado, quedando en el efluente 1,28 mg.L-1 del cromo. Los factores de translocación del cromo para E. crassipes fueron 0,008 y 0,02 en agua de pozo y efluente respectivamente y los factores de bioacumulación 11,6 y 3,2 respectivamente. No se encontraron diferencias significativas entre el contenido de clorofila en los vegetales expuestas en solución de cromo en comparación con el control. Tampoco se encontró oxidación del Cr3+ a Cr6+ medidos espectrofotométricamente.Chromium is one of the largest inorganic pollutants in water and soil in Paraguay, causing serious environmental problems. This metal comes mainly from tannery effluents that are dumped into water courses. The aim of this work was to use alternative remediation techniques for the treatment of waters with high chromium content, using natural media (plants), in a process called phytoremediation. Specimens of Eicchornia crassipes were subjected to controlled contamination with Cr3+ for 20 days and the maximum tolerated concentration during the whole period of the experiment was considered to study the cup and the absorption capacity of Cr3+ from the species. The kinetic constants of this process were: constant absorption: Kab = 0.10 days -1, and absorption half time: t1/2 = 6.8 dys. In the final tannery effluent with 22.4 mg.L-1 of Cr3+, at 48 hours there was almost complete absorption of the chromium supplied, leaving 1.28 mg.L-1 of chromium in the effluent. The chromium translocation factors for E. crassipes were 0.008 and 0.02 in well water and effluent respectively and bioaccumulation factors were 11.6 and 3.2 respectively. No significant differences were found between the chlorophyll content in the plants exposed in chromium solution compared to the control. Neither was oxidation found from Cr3+ to Cr6+ spectrophotometrically measured.Facultad de Ciencias Exacta

Multiple Chromosomal Rearrangements Structured the Ancestral Vertebrate Hox-Bearing Protochromosomes

While the proposal that large-scale genome expansions occurred early in vertebrate evolution is widely accepted, the exact mechanisms of the expansion—such as a single or multiple rounds of whole genome duplication, bloc chromosome duplications, large-scale individual gene duplications, or some combination of these—is unclear. Gene families with a single invertebrate member but four vertebrate members, such as the Hox clusters, provided early support for Ohno's hypothesis that two rounds of genome duplication (the 2R-model) occurred in the stem lineage of extant vertebrates. However, despite extensive study, the duplication history of the Hox clusters has remained unclear, calling into question its usefulness in resolving the role of large-scale gene or genome duplications in early vertebrates. Here, we present a phylogenetic analysis of the vertebrate Hox clusters and several linked genes (the Hox “paralogon”) and show that different phylogenies are obtained for Dlx and Col genes than for Hox and ErbB genes. We show that these results are robust to errors in phylogenetic inference and suggest that these competing phylogenies can be resolved if two chromosomal crossover events occurred in the ancestral vertebrate. These results resolve conflicting data on the order of Hox gene duplications and the role of genome duplication in vertebrate evolution and suggest that a period of genome reorganization occurred after genome duplications in early vertebrates

Molecular evolution of vertebrate sex-determining genes

Y-linked Dmy (also called dmrt1bY) in the teleost fish medaka, W-linked Dm-W in the African clawed frog (Xenopus laevis), and Z-linked Dmrt1 in the chicken are all sex chromosome-linked Dmrt1 homologues required for sex determination. Dmy and Dm-W both are Dmrt1 palalogues evolved through Dmrt1 duplication, while chicken Dmrt1 is a Z-linked orthologue. The eutherian sex-determining gene, Sry, evolved from an allelic gene, Sox3. Here we analyzed the exon–intron structures of the Dmrt1 homologues of several vertebrate species through information from databases and by determining the transcription initiation sites in medaka, chicken, Xenopus, and mouse. Interestingly, medaka Dmrt1 and Dmy and Xenopus Dm-W and Dmrt1 have a noncoding-type first exon, while mouse and chicken Dmrt1 do not. We next compared the 5′-flanking sequences of the Dmrt1 noncoding and coding exons 1 of several vertebrate species and found conservation of the presumptive binding sites for some transcription factors. Importantly, based on the phylogenetic trees for Dmrt1 and Sox3 homologues, it was implied that the sex-determining gene Dmy, Dm-W, and Sry have a higher substitution rate than thier prototype genes. Finally, we discuss the evolutionary relationships between vertebrate sex chromosomes and the sex-determining genes Dmy/Dm-W and Sry, which evolved by neofunctionalization of Dmrt1 and Sox3, respectively, for sex determining function. We propose a coevolution model of sex determining gene and sex chromosome, in which undifferentiated sex chromosomes easily allow replacement of a sex-determining gene with another new one, while specialized sex chromosomes are restricted a particular sex-determining gene

Estudios de propiedad intelectual

Transcurrida la primera década del siglo XXI, uno de los principales retos que continúa afrontando el derecho es determinar los mecanismos más adecuados para proteger la expresión de las ideas. Mientras algunos opinan que no deberían existir regulaciones que impidan acceder al conocimiento, otros afirman que se trata de un derecho que les asiste a los creadores y a quienes participan en el proceso creativo, razón por la cual las normas deben ser cada vez más protectoras e, incluso, sancionatorias. Son múltiples los tópicos que suscita la mencionada controversia, aplicados a diferentes aspectos de la cotidianeidad, enmarcados principalmente en temas relacionados con la investigación y el desarrollo, el comercio, la cultura y la industria del entretenimiento. Transcurrida la primera década del siglo XXI, uno de los principales retos que continúa afrontando el derecho es determinar los mecanismos más adecuados para proteger la expresión de las ideas