Current-induced transverse spin wave instability in a thin nanomagnet

We show that an unpolarized electric current incident perpendicular to the plane of a thin ferromagnet can excite a spin-wave instability transverse to the current direction if source and drain contacts are not symmetric. The instability, which is driven by the current-induced ``spin-transfer torque'', exists for one current direction only.Comment: 4 pages, 2 figure

Magnetic-field symmetries of mesoscopic nonlinear conductance

We examine contributions to the dc-current of mesoscopic samples which are non-linear in applied voltage. In the presence of a magnetic field, the current can be decomposed into components which are odd (antisymmetric) and even (symmetric) under flux reversal. For a two-terminal chaotic cavity, these components turn out to be very sensitive to the strength of the Coulomb interaction and the asymmetry of the contact conductances. For both two- and multi-terminal quantum dots we discuss correlations of current non-linearity in voltage measured at different magnetic fields and temperatures.Comment: 9 pages, 4 figure

Mesoscopic fluctuations of nonlinear conductance of chaotic quantum dots

The nonlinear dc conductance of a two-terminal chaotic cavity is investigated. The fluctuations of the conductance (anti)symmetric with respect to magnetic flux inversion through multichannel cavities are found analytically for arbitrary temperature, magnetic field, and interaction strength. For few-channel dots the effect of dephasing is investigated numerically. A comparison with recent experimental data is provided.Comment: 4 pages, 2 figures, v.2-notations correcte

Shot noise of photon-excited electron-hole pairs in open quantum dots

We investigate shot noise of photon-excited electron-hole pairs in open multi-terminal, multi-channel chaotic dots. Coulomb interactions in the dot are treated self-consistently giving a gauge-invariant expression for the finite frequency correlations. The Coulomb interactions decrease the noise, the strong interaction limit coincides with the non-interacting adiabatic limit. Inelastic scattering and dephasing in the dot are described by voltage and dephasing probe models respectively. We find that dephasing leaves the noise invariant, but inelastic scattering decreases correlations eventually down to zero.Comment: 4 pages, 1 figure; minor changes, 3 references adde

Mesoscopic magnetoelectric effect in chaotic quantum dots

The magnitude of the inverse Faraday effect (IFE), a static magnetization due to an ac electric field, can be strongly increased in a mesoscopic sample, sensitive to time-reversal symmetry (TRS) breaking. Random rectification of ac voltages leads to a magnetization flux, which can be detected by an asymmetry of Hall resistances in a multi-terminal setup. In the absence of applied magnetic field through a chaotic quantum dot the IFE scale, quadratic in voltage, is found as an analytic function of the ac frequency, screening, and coupling to the contacts and floating probes, and numerically it does not show any effect of spin-orbit interaction. Our results qualitatively agree with a recent experiment on TRS-breaking in a six-terminal Hall cross.Comment: 4+ pages, 2 figures; v2-published version, small change

Gap theory of rectification in ballistic three-terminal conductors

We introduce a model for rectification in three-terminal ballistic conductors, where the central connecting node is modeled as a chaotic cavity. For bias voltages comparable to the Fermi energy, a strong nonlinearity is created by the opening of a gap in the transport window. Both noninteracting cavity electrons at arbitrary temperature as well as the hot electron regime are considered. Charging effects are treated within the transmission formalism using a self-consistent analysis. The conductance of the third lead in a voltage probe configuration is varied to also model inelastic effects. We find that the basic transport features are insensitive to all of these changes, indicating that the nonlinearity is robust and well suited to applications such as current rectification in ballistic systems. Our findings are in broad agreement with several recent experiments.Comment: 8 pages, 6 figure

Efficient, highly selective laser isotope separation of carbon-13

We recently demonstrated an original approach to highly selective laser isotope separation of carbon-13 that employs vibrational overtone pre-excitation of CF3H together with infrared multiphoton dissociation [O.V. Boyarkin, M. Kowalczyk, T.R. Rizzo, J. Chem. Phys. 118, 93 (2003)]. The practical implementation of this approach was complicated by the long absorption path length needed for the overtone excitation laser beam. In the present work, we employ a low overtone level for the pre-excitation that shortens this pathway, facilitating engineering of the process. We propose an optimal configuration of the isotope separation scheme and consider a realistic example of a separation unit for isotopic enrichment of carbon-13 to 94-98%. The photon energy expenditure of 97eV per separated atom is much lower than that of the current commercial laser technology, making this process economically feasibl

Dynamic Phenomena in Transport through Sub-micron Devices

This thesis considers dynamic phenomena in transport through electronic devices on sub-micron scale. It consists of two closely related parts, the first considering DC current through a quantum dot as a response to a periodic perturbation of its shape and the second, conversely, explores a finite-frequency spin wave in a ferromagnet due to a constant electric current. Both systems are very similar in their theoretical treatment by scattering matrix formalism. The Chapters \ref{chap:2}--\ref{chap:4} consider a charge current induced by a periodic perturbation of an open quantum dot's shape. A dot being mesoscopic, its transport properties strongly fluctuate from sample to sample and therefore knowledge of full sample-to-sample distributions is essential. We consider a ``quantum pumping" regime of reservoirs in equilibrium and periodic variation of the dot's shape by AC voltages applied at the gates. Experimentally measurable first several moments of mesoscopic distribution of charge pumped in one cycle are explored. Chapter \ref{chap:2} considers distributions of adiabatically pumped current $\bar I$ and voltage $\bar V$ and finds that even in a slow weak pumping regime they are not simply related via time-averaged conductance $\bar G$. Moreover, values of $\bar I-\bar V\bar G$ for few-channel dots exhibit strong mesoscopic fluctuations, comparable with those of $\bar I$.% and $\bar V$. Chapter \ref{chap:3} explores mesoscopic distributions of noise and current-to-noise ratio in a weak pumping regime in a wide region of temperatures and pumping frequencies. Fluctuations of noise in the multi-channel limit $N\to\infty$ are found to be small as $1/N$. For a multi-channel system the ensemble-averaged noise is analytically found and calculated for experimentally relevant temperatures, frequencies and pumping strengths. The Chapter \ref{chap:4} concerns the formalism of time-dependent scattering matrix theory and finds correlators of matrix elements up to the fourth order. Our findings allow a systematic treatment of various transport properties, as well as their ensemble-averaged correlations. We also compare our results with results obtained in Hamiltonian approach of Random Matrix theory. The second part, Chapter \ref{chap:5}, considers magneto-transport through a single ferromagnetic layer. Electric current flowing perpendicular to the plane of a thin layer is shown to excite a finite frequency response in form of a spin wave. Unlike the previously known spin-torque due to a polarized current, another mechanism able to induce a destabilizing torque on a local magnetization is found. Spin-diffusion of reflected spins from one point on the normal-ferromagnet boundary to another might excite a spin wave at sufficiently strong currents. We analytically find the critical current value and discuss our results for experimentally relevant parameters.Prof. Brouwer (Cornell), NSF, Center for Nanoscale System

Current induced transverse spin-wave instability in thin ferromagnets: beyond linear stability analysis

A sufficiently large unpolarized current can cause a spin-wave instability in thin nanomagnets with asymmetric contacts. The dynamics beyond the instability is understood in the perturbative regime of small spin-wave amplitudes, as well as by numerically solving a discretized model. In the absence of an applied magnetic field, our numerical simulations reveal a hierarchy of instabilities, leading to chaotic magnetization dynamics for the largest current densities we consider.Comment: 14 pages, 10 figures; revtex