Biomedical subjects

S O Valenzuela

Publications and source records attributed to S O Valenzuela.

5 recordsLinked to original sources

PubMed2006

Coherent quasiclassical dynamics of a persistent current qubit.

A new regime of coherent quantum dynamics of a qubit is realized at low driving frequencies in the strong driving limit. Coherent transitions between qubit states occur via the Landau-Zener process when the system is swept through an energy-level avoided crossing. The quantum interference mediated by repeated transitions gives rise to an oscillatory dependence of the qubit population on the driving-field amplitude and flux detuning. These interference fringes, which at high frequencies consist of individual multiphoton resonances, persist even for driving frequencies smaller than the decoherence rate, where individual resonances are no longer distinguishable. A theoretical model that incorporates dephasing agrees well with the observations.

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PubMed2006

Direct electronic measurement of the spin Hall effect.

The generation, manipulation and detection of spin-polarized electrons in nanostructures define the main challenges of spin-based electronics. Among the different approaches for spin generation and manipulation, spin-orbit coupling--which couples the spin of an electron to its momentum--is attracting considerable interest. In a spin-orbit-coupled system, a non-zero spin current is predicted in a direction perpendicular to the applied electric field, giving rise to a spin Hall effect. Consistent with this effect, electrically induced spin polarization was recently detected by optical techniques at the edges of a semiconductor channel and in two-dimensional electron gases in semiconductor heterostructures. Here we report electrical measurements of the spin Hall effect in a diffusive metallic conductor, using a ferromagnetic electrode in combination with a tunnel barrier to inject a spin-polarized current. In our devices, we observe an induced voltage that results exclusively from the conversion of the injected spin current into charge imbalance through the spin Hall effect. Such a voltage is proportional to the component of the injected spins that is perpendicular to the plane defined by the spin current direction and the voltage probes. These experiments reveal opportunities for efficient spin detection without the need for magnetic materials, which could lead to useful spintronics devices that integrate information processing and data storage.

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PubMed2005

Spin polarized tunneling at finite bias.

A mesoscopic spin valve is used to determine the dynamic spin polarization of electrons tunneling out of and into ferromagnetic (FM) transition metals at finite voltages. The dynamic polarization of electrons tunneling out of the FM slowly decreases with increasing bias but drops faster and even inverts with voltage when electrons tunnel into it. A free-electron model shows that in the former case electrons originate near the Fermi level of the FM with large polarization whereas in the latter, electrons tunnel into hot electron states for which the polarization is significantly reduced. The change in sign is ascribed to the matching of the electron wave function inside and outside the tunnel barrier.

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PubMed2002

Order and mobility of solid vortex matter in oscillatory driving currents.

We study numerically the evolution of the degree order and mobility of the vortex lattice under steady and oscillating applied forces. We show that the oscillatory motion of vortices can favor an ordered structure, even when the motion of the vortices is plastic when the same force is applied in a constant way. Our results relate the spatial order of the vortex lattice with its mobility, and they are in agreement with recent experiments. We predict that, in oscillating applied forces, the lattice orients with a principal axis perpendicular to the direction of motion.

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PubMed2001

Oscillatory dynamics and organization of the vortex solid in YBa2Cu3o7 single crystals.

We report on the degree of order of the vortex solid in YBa2Cu3O7 single crystals observed in ac susceptibility measurements. We show that when vortices are "shaken" by a temporarily symmetric ac field they are driven into an easy-to-move, ordered structure but, on the contrary, when the ac field is temporarily asymmetric, they are driven into a more pinned disordered state. This is characteristic of tearing of the vortex lattice and shows that ordering due to symmetric ac fields is essentially different from an equilibration process or a dynamical crystallization that is expected to occur at high driving currents.

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