116 research outputs found
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 and voltage and finds that
even in a slow weak pumping regime they are not simply related via
time-averaged conductance . Moreover, values of for few-channel dots exhibit strong mesoscopic
fluctuations, comparable with those of .% and .
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 are
found to be small as . 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
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