Non-equilibrium dynamics of Andreev states in the Kondo regime

The transport properties of a quantum dot coupled to superconducting leads are analyzed. It is shown that the quasiparticle current in the Kondo regime is determined by the non-equilibrium dynamics of subgap states (Andreev states) under an applied voltage. The current at low bias is suppressed exponentially for decreasing Kondo temperature in agreement with recent experiments. We also predict novel interference effects due to multiple Landau-Zener transitions between Andreev states.Comment: Revtex4, 4 pages, 4 figure

Decrease of plasma cholesterol with the marine microalga dunaliella tertiolecta in hypercholesterolemic rats

Female Sprague-Dawley rats, whose plasma cholesterol level had been previously increased, were fed on a diet including 22% biomass of the marine microalga Dunaliella tertiolecta. This diet was compared to two control diets, one of casein and the other of soy flour. After 14 days of feeding, the group fed the microalgal diet exhibited the largest decrease in the plasma cholesterol concentration. The plasma cholesterol level in the group fed the microalgal diet was 45.2% of that in the control group (casein diet) and 28.4% lower than the group fed the soy flour diet. These results suggest that the marine microalga Dunaliella tertiolecta has marked anti-hypercholesterolemic activity when incorporated into the diet

Conductance properties of nanotubes coupled to superconducting leads: signatures of Andreev states dynamics

We present a combined experimental and theoretical analysis of the low bias conductance properties of carbon nanotubes coupled to superconducting leads. In the Kondo regime the conductance exhibits a zero bias peak which can be several times larger than the unitary limit in the normal case. This zero bias peak can be understood by analyzing the dynamics of the subgap Andreev states under an applied bias voltage. It is shown that the existence of a linear regime is linked to the presence of a finite relaxation rate within the system. The theory provides a good fitting of the experimental results.Comment: 6 revtex4 pages, 6 figures, to appear in SS

Improvement of growth rate and cell productivity by aeration rate in cultures of the marine microalga Dunaliella tertiolecta

The effect of different aeration rates and CO2 supply regimes on mass cultures of the marine microalga Dunaiella tertiolecta was studied. Eight aeration rates in the range 0-6.51 litres of air min-1 litre of culture-1 were tested. The results were compared to those obtained in non-aerated cultures into which pure CO2 was introduced and to cultures into which air enriched with CO2 was bubbled. The growth rate and final cellular density of D. tertiolecta in a sea-water-based medium increased with the aeration rate in the culture vessel. The maximal cellular density was 12.46 x 106 cells ml-1 under an air flow rate of 6.51 litres min-1 litre of culture-1, but evaporation and salinity increased sharply at this high aeration rate. The final cell density was proportional to the air flow rate and CO2 following the range (figures being litres of air min-1 litre of culture -1): 6.51 = 3.72 > 1.86 > CO2 = (0.93 + CO2) > 0.93 > 0.46 > 0.23 > 0.11 > 0 (Mann-Whitney test p < 0.05). When D. tertiolecta was grown under a CO2 supply within the optimal pH levels but without aeration the cultures reached a cell density of 7 x 106 cells ml-1. D. tertiolecta growth rate was inversely proportional to pH, the upper boundary for maximal growth rate being pH 9.2. Oxygen in the culture media produced by the photosynthetic activity of the microalgae did not inhibit growth. The dissolved CO2 concentration in seawater was the limiting factor for D. tertiolecta growth. At a pH value of 8.3, D. tertiolecta was not able to take up carbon in the form of carbonates dissolved in seawater. The effect of different aeration rates and CO2 supply regimes on mass cultures of the marine microalga Dunaiella tertiolecta was studied. Eight aeration rates in the range 0- 6·51 litres of air min-1 litre of culture-1 were tested. The results were compared to those obtained in non-aerated cultures into which pure CO2 was introduced and to cultures into which air enriched with CO2 was bubbled. The growth rate and final cellular density of D. tertiolecta in a sea-water-based medium increased with the aeration rate in the culture vessel. The maximal cellular density was 12·46×106 cells ml-1 under an air flow rate of 6·51 litres min-1 litre of culture-1, but evaporation and salinity increased sharply at this high aeration rate. The final cell density was proportional to the air flow rate and CO2 following the range (figures being litres of air min-1 litre of culture-1): 6 · 51 = 3 · 72 > 1 · 86 > CO2 = (0 · 93 + CO2) > 0 · 93 > 0 · 46 > 0 · 23 > 0 · 11 > 0 (Mann-Whitney test p < 0 · 05). When D. tertiolecta was grown under a CO2 supply within the optimal pH levels but without aeration, the cultures reached a cell density of 7 × 106 cells ml-1. D. tertiolecta growth rate was inversely proportional to pH, the upper boundary for maximal growth rate being pH 9 · 2. Oxygen in the culture media produced by the photosynthetic activity of the microalgae did not inhibit growth. The dissolved CO2 concentration in seawater was the limiting factor for D. tertiolecta growth. At a pH value of 8·3, D. tertiolecta was not able to take up carbon in the form of carbonates dissolved in seawater

Single-channel transmission in gold one-atom contacts and chains

We induce superconductivity by proximity effect in thin layers of gold and study the number of conduction channels which contribute to the current in one-atom contacts and atomic wires. The atomic contacts and wires are fabricated with a Scanning Tunneling Microscope. The set of transmission probabilities of the conduction channels is obtained from the analysis of the $I(V)$ characteristic curve which is highly non-linear due to multiple Andreev reflections. In agreement with theoretical calculations we find that there is only one channel which is almost completely open.Comment: 4 pages, 2 figures. To be published in Phys. Rev. B, Rapid Communications (2003

Quantum phase transition in a minimal model for the Kondo effect in a Josephson junction

We propose a minimal model for the Josephson current through a quantum dot in a Kondo regime. We start with the model that consists of an Anderson impurity connected to two superconducting (SC) leads with the gaps $\Delta_{\alpha}=|\Delta_{\alpha}| e^{i \theta_{\alpha}}$, where $\alpha = L, R$ for the lead at left and right. We show that, when one of the SC gaps is much larger than the others $|\Delta_L| \gg |\Delta_R|$, the starting model can be mapped exactly onto the single-channel model, which consists of the right lead of $\Delta_R$ and the Anderson impurity with an extra onsite SC gap of $\Delta_d \equiv \Gamma_L e^{i \theta_L}$. Here $\theta_L$ and $\Gamma_L$ are defined with respect to the starting model, and $\Gamma_L$ is the level width due to the coupling with the left lead. Based on this simplified model, we study the ground-state properties for the asymmetric gap, $|\Delta_L| \gg |\Delta_R|$, using the numerical renormalization group (NRG) method. The results show that the phase difference of the SC gaps $\phi \equiv \theta_R -\theta_L$, which induces the Josephson current, disturbs the screening of the local moment to destabilize the singlet ground state typical of the Kondo system. It can also drive the quantum phase transition to a magnetic doublet ground state, and at the critical point the Josephson current shows a discontinuous change. The asymmetry of the two SC gaps causes a re-entrant magnetic phase, in which the in-gap bound state lies close to the Fermi level.Comment: 23 pages, 13 figures, typos are correcte

Evaluation of polymeric nanofiltration membranes on metal valorisation from acidic mine waters

Postprint (author's final draft

Revealing the electronic structure of a carbon nanotube carrying a supercurrent

Carbon nanotubes (CNTs) are not intrinsically superconducting but they can carry a supercurrent when connected to superconducting electrodes. This supercurrent is mainly transmitted by discrete entangled electron-hole states confined to the nanotube, called Andreev Bound States (ABS). These states are a key concept in mesoscopic superconductivity as they provide a universal description of Josephson-like effects in quantum-coherent nanostructures (e.g. molecules, nanowires, magnetic or normal metallic layers) connected to superconducting leads. We report here the first tunneling spectroscopy of individually resolved ABS, in a nanotube-superconductor device. Analyzing the evolution of the ABS spectrum with a gate voltage, we show that the ABS arise from the discrete electronic levels of the molecule and that they reveal detailed information about the energies of these levels, their relative spin orientation and the coupling to the leads. Such measurements hence constitute a powerful new spectroscopic technique capable of elucidating the electronic structure of CNT-based devices, including those with well-coupled leads. This is relevant for conventional applications (e.g. superconducting or normal transistors, SQUIDs) and quantum information processing (e.g. entangled electron pairs generation, ABS-based qubits). Finally, our device is a new type of dc-measurable SQUID

The neuronal ischemic tolerance is conditioned by the Tp53 Arg72Pro polymorphism

Cerebral preconditioning (PC) confers endogenous brain protection after stroke. Ischemic stroke patients with a prior transient ischemic attack (TIA) may potentially be in a preconditioned state. Although PC has been associated with the activation of prosurvival signals, the mechanism by which preconditioning confers neuroprotection is not yet fully clarified. Recently, we have described that PC-mediated neuroprotection against ischemic insult is promoted by p53 destabilization, which is mediated by its main regulatorMDM2. Moreover, we have previously described that the human Tp53 Arg72Pro single nucleotide polymorphism (SNP) controls susceptibility to ischemia-induced neuronal apoptosis and governs the functional outcome of patients after stroke. Here, we studied the contribution of the human Tp53 Arg72Pro SNP on PC-induced neuroprotection after ischemia. Our results showed that cortical neurons expressing the Pro72-p53 variant exhibited higher PC-mediated neuroprotection as compared with Arg72-p53 neurons. PC prevented ischemia-induced nuclear and cytosolic p53 stabilization in Pro72-p53 neurons. However, PC failed to prevent mitochondrial p53 stabilization, which occurs in Arg72-p53 neurons after ischemia. Furthermore, PC promoted neuroprotection against ischemia by controlling the p53/active caspase-3 pathway in Pro72-p53, but not in Arg72-p53 neurons. Finally, we found that good prognosis associated to TIA within 1 month prior to ischemic stroke was restricted to patients harboring the Pro72 allele. Our findings demonstrate that the Tp53 Arg72Pro SNP controls PC-promoted neuroprotection against a subsequent ischemic insult bymodulatingmitochondrial p53 stabilization and then modulates TIA-induced ischemic tolerance.This work was funded by The Instituto de Salud Carlos III grants CP14/00010 (M.D.-E.); PI15/00473 and RD12/0014/ 0007 (A.A.); CM14/00096 (ME.R.-A.); RD16/0019/0018 (C.R.); and Junta de Castilla y Leon grant BIO/SA35/15 (M.D.-E.), and the European Regional Development Fund (R.V.) was funded by the FPU program (Ministerio de Educación)

Spontaneous Spin Polarized Currents in Superconductor-Ferromagnetic Metal Heterostructures

We study a simple microscopic model for thin, ferromagnetic, metallic layers on semi-infinite bulk superconductor. We find that for certain values of the exchange spliting, on the ferromagnetic side, the ground states of such structures feature spontaneously induced spin polarized currents. Using a mean-field theory, which is selfconsistent with respect to the pairing amplitude $\chi$, spin polarization $\vec{m}$ and the spontaneous current $\vec{j}_s$, we show that not only there are Andreev bound states in the ferromagnet but when their energies $E_n$ are near zero they support spontaneous currents parallel to the ferromagnetic-superconducting interface. Moreover, we demonstrate that the spin-polarization of these currents depends sensitively on the band filling.Comment: 4 pages, 5 Postscript figures (included