Sustained RF oscillations from thermally induced spin-transfer torque

We investigate the angular dependence of the spin torque generated when applying a temperature difference across a spin-valve. Our study shows the presence of a non-trivial fixed point in this angular dependence, i.e. the possibility for a temperature gradient to stabilize radio frequency oscillations without the need for an external magnetic field. This so called "wavy" behavior can already be found upon applying a voltage difference across a spin-valve but we find that this effect is much more pronounced with a temperature difference. Our semi-classical theory is parametrized with experimentally measured parameters and allows one to predict the amplitude of the torque with good precision. Although thermal spin torque is by nature less effective than its voltage counterpart, we find that in certain geometries, temperature differences as low as a few degrees should be sufficient to trigger the switching of the magnetization.Comment: 5 pages + 3 pages supplementary material, 3 figure

Dynamical control of interference using voltage pulses in the quantum regime

As a general trend, nanoelectronics experiments are shifting toward frequencies so high that they become comparable to the device's internal characteristic time scales, resulting in new opportunities for studying the dynamical aspects of quantum mechanics. Here we theoretically study how a voltage pulse (in the quantum regime) propagates through an electronic interferometer (Fabry- Perot or Mach-Zehnder). We show that extremely fast pulses provide a conceptually new tool for manipulating quantum information: the possibility to dynamically engineer the interference pattern of a quantum system. Striking physical signatures are associated with this new regime: restoration of the interference in presence of large bias voltages; negative currents with respect to the direction of propagation of the voltage pulse; and oscillation of the total transmitted charge with the total number of injected electrons. The present findings have been made possible by the recent unlocking of our capability for simulating time-resolved quantum nanoelectronics of large systems.Comment: 11 pages, 9 figure

A computational approach to quantum noise in time-dependent nanoelectronic devices

We derive simple expressions that relate the noise and correlation properties of a general time-dependent quantum conductor to the wave functions of the system. The formalism provides a practical route for numerical calculations of quantum noise in an externally driven system. We illustrate the approach with numerical calculations of the noise properties associated to a voltage pulse applied on a one-dimensional conductor. The methodology is however fully general and can be used for a large class of mesoscopic conductors.Comment: 7 pages, 4 figure

Variational wave functions, ground state and their overlap

An intrinsic measure of the quality of a variational wave function is given by its overlap with the ground state of the system. We derive a general formula to compute this overlap when quantum dynamics in imaginary time is accessible. The overlap is simply related to the area under the $E(\tau)$ curve, i.e. the energy as a function of imaginary time. This has important applications to, for example, quantum Monte-Carlo algorithms where the overlap becomes as a simple byproduct of routine simulations. As a result, we find that the practical definition of a good variational wave function for quantum Monte-Carlo simulations, {\it i.e.} fast convergence to the ground state, is equivalent to a good overlap with the actual ground state of the system.Comment: 4 pages, 2 figures, to be published in Phys. Rev. Lett. (revised version

Current induced Spin Torque in a nanomagnet

In a nanomagnet (whose total spin S< 1000), very small polarized currents can lead to magnetic reversal. Treating on the same footing the transport and magnetic properties of a nanomagnet connected to magnetic leads via tunneling barriers, we derive a closed equation for the time evolution of the magnetization. The interplay between Coulomb blockade phenomena and magnetism gives some additional structure to the current induced spin torque. In addition to the possibility of stabilizing uniform spin waves, we find that the system is highly hysteretic: up to three different magnetic states can be simultaneously stable in one region of the parameter (magnetic field and bias voltage) space.Comment: 5 pages, 3 figures to appear in Phys. Rev. Let

Towards Realistic Time-Resolved Simulations of Quantum Devices

We report on our recent efforts to perform realistic simulations of large quantum devices in the time domain. In contrast to d.c. transport where the calculations are explicitly performed at the Fermi level, the presence of time-dependent terms in the Hamiltonian makes the system inelastic so that it is necessary to explicitly enforce the Pauli principle in the simulations. We illustrate our approach with calculations for a flying qubit interferometer, a nanoelectronic device that is currently under experimental investigation. Our calculations illustrate the fact that many degrees of freedom (16,700 tight-binding sites in the scattering region) and long simulation times (80,000 times the inverse Bandwidth of the tight-binding model) can be easily achieved on a local computer.Comment: 8 pages, 6 figure

Two interacting particles in a disordered chain I: Multifractality of the interaction matrix elements

For $N$ interacting particles in a one dimensional random potential, we study the structure of the corresponding network in Hilbert space. The states without interaction play the role of the ``sites''. The hopping terms are induced by the interaction. When the one body states are localized, we numerically find that the set of directly connected ``sites'' is multifractal. For the case of two interacting particles, the fractal dimension associated to the second moment of the hopping term is shown to characterize the Golden rule decay of the non interacting states and the enhancement factor of the localization length.Comment: First paper of a serie of four, to appear in Eur. Phys.

Tunable Magnetic Relaxation In Magnetic Nanoparticles

We investigate the magnetization dynamics of a conducting magnetic nanoparticle weakly coupled to source and drain electrodes, under the assumption that all relaxation comes from exchange of electrons with the electrodes. The magnetization dynamics is characterized by a relaxation time $t_1$, which strongly depends on temperature, bias voltage, and gate voltage. While a direct measure of a nanoparticle magnetization might be difficult, we find that $t_1$ can be determined through a time resolved transport measurement. For a suitable choice of gate voltage and bias voltage, the magnetization performs a bias-driven Brownian motion regardless of the presence of anisotropy.Comment: 4 pages, 2 eps figure