Near-threshold properties of the electronic density of layered quantum-dots

We present a way to manipulate an electron trapped in a layered quantum dot based on near-threshold properties of one-body potentials. We show that potentials with a simple global parameter allows the manipulation of the wave function changing its spatial extent. This phenomenon seems to be fairly general and could be implemented using current quantum-dot quantum wells technologies and materials if a proper layered quantum dot is designed. The layered quantum dot under consideration is similar to a quantum-dot quantum well device, i.e. consists of a spherical core surrounded by successive layers of different materials. The number of layers and the constituent material are chosen to highlight the near-threshold properties. In particular we show that the near-threshold phenomena can be observed using an effective mass approximation model that describes the layered quantum dot which is consistent with actual experimental parameters.Comment: 15 pages, 6 figures, regular articl

Quantum control of a model qubit based on a multi-layered quantum dot

In this work we present a model qubit whose basis states are eigenstates of a multi-layered quantum dot. We show that the proper design of the quantum dot results in qubit states that have excellent dynamical properties when a time-dependent driving is applied to it. In particular, it is shown that a simple sinusoidal driving is sufficient to obtain good quality Rabi oscillations between the qubit states. Moreover, the switching between states can be performed with very low leakage, even under off-resonance conditions. In this sense, the quantum control of the qubit is robust under some perturbations and achieved with simple means.Comment: 19 pages, 8 figure

Intrinsic leakage of the Josephson flux qubit and breakdown of the two-level approximation for strong driving

Solid state devices for quantum bit computation (qubits) are not perfect isolated two-level systems, since additional higher energy levels always exist. One example is the Josephson flux qubit, which consists on a mesoscopic SQUID loop with three Josephson junctions operated at or near a magnetic flux of half quantum. We study intrinsic leakage effects, i.e., direct transitions from the allowed qubit states to higher excited states of the system during the application of pulses for quantum computation operations. The system is started in the ground state and rf- magnetic field pulses are applied at the qubit resonant frequency with pulse intensity $f_p$. A perturbative calculation of the average leakage for small $f_p$ is performed for this case, obtaining that the leakage is quadratic in $f_p$, and that it depends mainly on the matrix elements of the supercurrent. Numerical simulations of the time dependent Schr\"odinger equation corresponding to the full Hamiltonian of this device were also performed. From the simulations we obtain the value of $f_p$ above which the two-level approximation breaks down, and we estimate the maximum Rabi frequency that can be achieved. We study the leakage as a function of the ratio $\alpha$ among the Josephson energies of the junctions of the device, obtaining the best value for minimum leakage ($\alpha\approx0.85$). The effects of flux noise on the leakage are also discussed.Comment: Final improved version. Some figures have changed with new results added. To be published in Phys. Rev.

Derivation of the spin Hamiltonians for Fe in MgO

A method to calculate the effective spin Hamiltonian for a transition metal impurity in a non- magnetic insulating host is presented and applied to the paradigmatic case of Fe in MgO. In a first step we calculate the electronic structure employing standard density functional theory (DFT), based on generalized-gradient approximation (GGA), using plane waves as a basis set. The corresponding basis of atomic-like maximally localized Wannier functions is derived and used to represent the DFT Hamiltonian, resulting in a tight-binding model for the atomic orbitals of the magnetic impurity. The third step is to solve, by exact numerical diagonalization, the N electron problem in the open shell of the magnetic atom, including both effect of spin-orbit and Coulomb repulsion. Finally, the low energy sector of this multi-electron Hamiltonian is mapped into effective spin models that, in addition to the spin matrices S, can also include the orbital angular momentum L when appropriate. We successfully apply the method to Fe in MgO, considering both, the undistorted and Jahn-Teller (JT) distorted cases. Implications for the influence of Fe impurities on the performance of magnetic tunnel junctions based on MgO are discussed.Comment: 10 pages, 7 Figure

Araña en bañera and Terraza de café por la noche, by Emma Cohen

Este trabajo es un acercamiento a la figura de Emma Cohen y a sus dos textos dramáticos para adultos, Araña en bañera y Terraza de café por la noche. Nuestro propósito es comenzar a subsanar la total ausencia de estudios sobre esta artista de curiosidad insaciable cuya creación se diversifica en disciplinas variadas. Las obras a las que nos aproximamos son muy diferentes y, a la vez, representativas ambas del rico universo de Cohen, de sus preferencias literarias, de su estilo. El análisis de estas dos piezas, que no agota esta aportación, nos ofrece un mirador privilegiado para apreciar y entender la creación de la autora.This work is an approach to the figure of Emma Cohen and her two dramatic texts for adults, Araña en bañera and Terraza de café por la noche. Our purpose is to begin rectifying the complete lack of research on this artist whose insatiable curiosity led her to diversify her creation into different disciplines. The plays that we are researching are significantly different and, at the same time, both of them are representative of the rich universe of the writer, her literary preferences, her style. This contribution does not exhaust the study of these two pieces. However, it offers a privileged viewpoint to appreciate and understand the creation of this author

Electronic properties of transition metal atoms on Cu2_2N/Cu(100)

We study the nature of spin excitations of individual transition metal atoms (Ti, V, Cr, Mn, Fe, Co and Ni) deposited on a Cu$_2$N/Cu(100) surface using both spin-polarized density functional theory (DFT) and exact diagonalization of an Anderson model derived from DFT. We use DFT to compare the structural, electronic and magnetic properties of different transition metal adatoms on the surface. We find that the average occupation of the transition metal d shell, main contributor to the magnetic moment, is not quantized, in contrast with the quantized spin in the model Hamiltonians that successfully describe spin excitations in this system. In order to reconcile these two pictures, we build a multi-orbital Anderson Hamiltonian for the d shell of the transition metal hybridized with the p orbitals of the adjacent Nitrogen atoms, by means of maximally localized Wannier function representation of the DFT Hamiltonian. The exact solutions of this model have quantized total spin, without quantized charge at the d shell. We propose that the quantized spin of the models actually belongs to many-body states with two different charge configurations in the d shell, hybridized with the p orbital of the adjacent Nitrogen atoms. This scenario implies that the measured spin excitations are not fully localized at the transition metal.Comment: 12 pages, 14 figures, regular articl

Electronic properties of transition metal atoms on Cu2_2N/Cu(100)

We study the nature of spin excitations of individual transition metal atoms (Ti, V, Cr, Mn, Fe, Co and Ni) deposited on a Cu$_2$N/Cu(100) surface using both spin-polarized density functional theory (DFT) and exact diagonalization of an Anderson model derived from DFT. We use DFT to compare the structural, electronic and magnetic properties of different transition metal adatoms on the surface. We find that the average occupation of the transition metal d shell, main contributor to the magnetic moment, is not quantized, in contrast with the quantized spin in the model Hamiltonians that successfully describe spin excitations in this system. In order to reconcile these two pictures, we build a multi-orbital Anderson Hamiltonian for the d shell of the transition metal hybridized with the p orbitals of the adjacent Nitrogen atoms, by means of maximally localized Wannier function representation of the DFT Hamiltonian. The exact solutions of this model have quantized total spin, without quantized charge at the d shell. We propose that the quantized spin of the models actually belongs to many-body states with two different charge configurations in the d shell, hybridized with the p orbital of the adjacent Nitrogen atoms. This scenario implies that the measured spin excitations are not fully localized at the transition metal.Comment: 12 pages, 14 figures, regular articl

Large Amplitude Harmonic Driving of Highly Coherent Flux Qubits

The device for the Josephson flux qubit (DJFQ) can be considered as a solid state artificial atom with multiple energy levels. When a large amplitude harmonic excitation is applied to the system, transitions at the energy levels avoided crossings produce visible changes in the qubit population over many driven periods that are accompanied by a rich pattern of interference phenomena. We present a Floquet treatment of the periodically time-dependent Schr\"odinger equation of the strongly driven qubit beyond the standard two levels approach. For low amplitudes, the average probability of a given sign of the persistent current qubit exhibits, as a function of the static flux detuning and the driving amplitude, Landau-Zener-St\"uckelberg interference patterns that evolve into complex diamond-like patterns for large amplitudes. In the case of highly coherent flux qubits we find that the higher order diamonds can not be simply described relying on a two-level approximations. In addition we propose a new spectroscopic method based on starting the system in the first excited state instead of in the ground state, which can give further information on the energy level spectrum and dynamics in the case of highly coherent flux qubits. We compare our numerical results with recent experiments that perform amplitude spectroscopy to probe the energy spectrum of the artificial atom.Comment: 12 Pages and 12 Figures Phys. Rev. B (in press

Illuminating the evolution of bioluminescence in sharks

The evolutionary context in which shark bioluminescence originated is poorly understood, despite it being critical to uncovering influential factors in the evolutionary history and diversity of living chondrichthyans as well as the mechanisms of deep-water colonization by vertebrates. This study provides the first joint reconstruction of the habitats, lifestyles, and occurrence of bioluminescence in the evolution of squalomorph sharks using ancestral state estimation analysis to resolve the timing of deep-sea colonization, the evolutionary origin of bioluminescence and the ancestral ecologies of this group. The results suggest that most squalomorphs originated in neritic environments from where they colonized deep waters on several independent occasions during the Late Jurassic and Early Cretaceous, predating most of the previous estimates of the timing of this event. The colonization of the deep sea took place via the benthic zone, in contrast to the view that an intermediate mesopelagic stage occurred during this ecological transition. Finally, the analyses accounting for uncertainty of the presence of bioluminescence strongly support that this trait evolved only once among sharks in a bathydemersal ancestor. This study reveals that shark bioluminescence evolved in a complex scenario that combines elements of several previous proposals, and enriches our perspective on the sequence of events that characterized the vertebrate conquest of the deep sea

Effect of the diurnal heating on urban street canyons: a CFD study

In this study Computational Fluid Dynamics (CFD) are used to predict the air flow structure and pollutant retention inside urban canyons taking into account thermal effects due to solar radiation in order to imitate diurnal heating over a city. The diurnal heating induces buoyancy therefore the flow structure and pollutant concentration will be completely different depending on the daytime. The presented method departs from a 2D domain comprised by eight urban canyons of aspect ratio 1 and different 2D transient RANS (Reynolds-Averaged Navier-Stokes) simulations that have been conducted using OpenFoam. Isothermal, single-surface heating and multi-surface heating cases are considered. For all of them, the results obtained have been post-processed using ParaView and Matlab to be able to analyse all the information related with velocity, pressure, temperature, pollutant concentration, streamlines, etc. These results have been also contrasted and validated with data obtained from wind-tunnel experiments and other CFD simulations which have been conducted using a 2D model and conditions similar to the ones used in this thesis. Finally, the conclusions obtained for an airflow with a Reynolds number of 12000 are that for most cases, a single main vortex is formed which is strengthen for the cases where leeward or ground surfaces are heated up compared to the vortex formed in isothermal case. In all cases where the windward side is heated up, a second counter-rotating vortex appears due to buoyancy and displaces the main one from the center of the canyon. Also, when the windward side is heated, the concentration of pollutants increases at the pedestrian level which may be considered as the worst case. Further improvements may be considered to continue with this work such as improving the mesh quality, considering an additional heat source, using another turbulence modelling (LES) or even considering a 3D model since the buoyant flows have an unsteady and 3D characteristc that require complex numerical models to obtain more accurate result