arXiv subjects

Piet W. Brouwer

Publications and source records attributed to Piet W. Brouwer.

126 records · Page 7Linked to original sources

arXiv2005

Weak Coulomb blockade effect in quantum dots

We develop the general non-equilibrium theory of transport through a quantum dot, including Coulomb Blockade effects via a 1/N expansion, where N is the number of scattering channels. At lowest order we recover the Landauer formula for the current plus a self-consistent equation for the dot potential. We obtain the leading corrections and compare with earlier approaches. Finally, we show that to leading and next leading order in 1/N there is no interaction correction to the weak localization, in contrast to previous theories, but consistent with experiments by Huibers et al. [Phys. Rev. Lett. 81, 1917 (1998)], where N=4.

cond-mat.mes-hall↗

arXiv2005

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.

cond-mat.mes-hall↗

arXiv2005

Ehrenfest-time dependence of weak localization in open quantum dots

Semiclassical theory predicts that the weak localization correction to the conductance of a ballistic chaotic cavity is suppressed if the Ehrenfest time exceeds the dwell time in the cavity [I. L. Aleiner and A. I. Larkin, Phys. Rev. B {\bf 54}, 14424 (1996)]. We report numerical simulations of weak localization in the open quantum kicked rotator that confirm this prediction. Our results disagree with the `effective random matrix theory' of transport through ballistic chaotic cavities.

cond-mat.mes-hall↗

arXiv2004

Rate equations for Coulomb blockade with ferromagnetic leads

We present a density-matrix rate-equation approach to sequential tunneling through a metal particle weakly coupled to ferromagnetic leads. The density-matrix description is able to deal with correlations between degenerate many-electron states that the standard rate equation formalism in terms of occupation probabilities cannot describe. Our formalism is valid for an arbitrary number of electrons on the dot, for an arbitrary angle between the polarization directions of the leads, and with or without spin-orbit scattering on the metal particle. Interestingly, we find that the density-matrix description may be necessary even for metal particles with unpolarized leads if three or more single-electron levels contribute to the transport current and electron-electron interactions in the metal particle are described by the `universal interaction Hamiltonian'.

cond-mat.mes-hall↗

arXiv2004

Mesoscopic bound on anisotropy in itinerant ferromagnets

We calculate the anisotropy energy of a single-domain ferromagnetic particle in which the only source of anisotropy is the presence of non-magnetic impurities. We find that such anisotropy takes the form of combined easy-axis and easy-plane anisotropies, with random orientations of the axes. Typically the anisotropy energy is of order $N^{1/2} \hbar/tau_{so}$, where $N$ is the number of electrons in the ferromagnetic particle and $τ_{so}$ is the spin-orbit time.

cond-mat.mes-hall↗

arXiv2004

Mesoscopic effects in adiabatic spin pumping

We show that temporal shape modulations (pumping) of a quantum dot in the presence of spin-orbital coupling lead to a finite dc spin current. Depending on the strength of the spin-orbit coupling, the spin current is polarized perpendicular to the plane of the two-dimensional electron gas, or has an arbitrary direction subject to mesoscopic fluctuations. We analyze the statistics of the spin and charge currents in the adiabatic limit for the full cross-over from weak to strong spin-orbit coupling.

cond-mat.mes-hall↗

arXiv2003

Electron correlations in metal nanoparticles with spin-orbit scattering

The combined effect of electron-electron interactions and spin-orbit scattering in metal nanoparticles can be studied by measuring splitting of electron levels in magnetic field ($g$ factors) in tunneling spectroscopy experiments. Using random matrix theory to describe the single-electron states in the metal particle, we find that even a relatively small electron-electron interaction strength (ratio of exchange constant $J$ and mean level spacing $\spacing$ $\simeq 0.3$) significantly increases $g$-factor fluctuations for not-too-strong spin-orbit scattering rates (spin-orbit time $τ_{\rm so} \gtrsim 1/\spacing$). In particular, $g$-factors larger than 2 could be observed. (This is a manifestation of the many-body correlation effects in nanoparticles). While so far measurements only on noble metal (Cu, Ag, Au) and Al samples have been done for which the effects of electron-electron interactions are negligible, we discuss the possibility of observing interaction effects in nanoparticles made of other metals.

cond-mat.mes-hall↗

arXiv2003

Electron-Phonon Scattering in Metallic Single-Walled Carbon Nanotubes

Electron scattering rates in metallic single-walled carbon nanotubes are studied using an atomic force microscope as an electrical probe. From the scaling of the resistance of the same nanotube with length in the low and high bias regimes, the mean free paths for both regimes are inferred. The observed scattering rates are consistent with calculations for acoustic phonon scattering at low biases and zone boundary/optical phonon scattering at high biases.

cond-mat.mes-hall↗

arXiv2003

On the scaling approach to electron-electron interactions in a chaotic quantum dot

A scaling theory is used to study the low energy physics of electron-electron interactions in a double quantum dot. We show that the fact that electrons are delocalized over two quantum dots does not affect the instability criterion for the description of electron-electron interactions in terms of a ``universal interaction Hamiltonian''.

cond-mat.mes-hall↗

arXiv2003

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.

cond-mat.mes-hall↗

arXiv2003

Fluctuations of g-factors in metal nanoparticles: Effects of electron-electron interaction and spin-orbit scattering

We investigate the combined effect of spin-orbit scattering and electron-electron interactions on the probability distribution of $g$-factors of metal nanoparticles. Using random matrix theory, we find that even a relatively small interaction strength %(ratio of exchange constant $J$ and mean level %spacing $\spacing$ $\simeq 0.3$) significantly increases $g$-factor fluctuations for not-too-strong spin-orbit scattering (ratio of spin-orbit rate and single-electron level spacing $1/τ_{\rm so} \spacing \lesssim 1$), and leads to the possibility to observe $g$-factors larger than two.

cond-mat.mes-hall↗

arXiv2003

Conductance Peak Height Correlations for a Coulomb-Blockaded Quantum Dot in a Weak Magnetic Field

We consider statistical correlations between the heights of conductance peaks corresponding to two different levels in a Coulomb-blockaded quantum dot. Correlations exist for two peaks at the same magnetic field if the field does not fully break time-reversal symmetry as well as for peaks at different values of a magnetic field that fully breaks time-reversal symmetry. Our results are also relevant to Coulomb-blockade conductance peak height statistics in the presence of weak spin-orbit coupling in a chaotic quantum dot.

cond-mat.mes-hall↗

arXiv2003

Weak localization and conductance fluctuations in a quantum dot with parallel magnetic field and spin-orbit scattering

In the presence of both spin-orbit scattering and a magnetic field the conductance of a chaotic GaAs quantum dot displays quite a rich behavior. Using a Hamiltonian derived by Aleiner and Falko [Phys. Rev. Lett. 87, 256801 (2001)] we calculate the weak localization correction and the covariance of the conductance, as a function of parallel and perpendicular magnetic field and spin-orbit coupling strength. We also show how the combination of an in-plane magnetic field and spin-orbit scattering gives rise to a component to the magnetoconductance that is anti-symmetric with respect to reversal of the perpendicular component of the magnetic field and how spin-orbit scattering leads to a "magnetic-field echo" in the conductance autocorrelation function. Our results can be used for a measurement of the Dresselhaus and Bychkov-Rashba spin-orbit scattering lengths in a GaAs/GaAlAs heterostructure.

cond-mat.mes-hall↗

arXiv2002

Enhanced mesoscopic fluctuations in the crossover between random matrix ensembles

In random-matrix ensembles that interpolate between the three basic ensembles (orthogonal, unitary, and symplectic), there exist correlations between elements of the same eigenvector and between different eigenvectors. We study such correlations, using a remarkable correspondence between the interpolating ensembles late in the crossover and a basic ensemble of finite size. In small metal grains or semiconductor quantum dots, the correlations between different eigenvectors lead to enhanced fluctuations of the electron-electron interaction matrix elements which become parametrically larger than the non-universal fluctuations.

cond-mat.mes-hall↗

arXiv2001

Spin torque in a magnetic trilayer coupled to a superconductor

A ferromagnet-normal-metal-ferromagnet trilayer does not conserve spin current and as a result, the conduction electrons can create a torque on the magnetic moments. When the trilayer is connected to a superconducting electrode, the spin torque created by a current flowing perpendicularly to the trilayer can drive the system to a configuration where the two magnetic moments are perpendicular to each other. Here we argue that in contrast to the non-equilibrium torque, the equilibrium torque (or magnetic exchange interaction) does not single out the perpendicular configuration. We calculate the equilibrium torque for a one-dimensional lattice model and find that it is sensitive to the presence of the superconductivity.

cond-mat.supr-con↗

arXiv2001

Magnetic exchange interaction induced by a Josephson current

We show that a Josephson current flowing through a ferromagnet-normal-metal-ferromagnet trilayer connected to two superconducting electrodes induces an equilibrium exchange interaction between the magnetic moments of the ferromagnetic layers. The sign and magnitude of the interaction can be controlled by the phase difference between the order parameters of the two superconductors. We present a general framework to calculate the Josephson current induced magnetic exchange interaction in terms of the scattering matrices of the different layers. The effect should be observable as the periodic switching of the relative orientation of the magnetic moments of the ferromagnetic layers in the ac Josephson effect.

cond-mat.mes-hall↗

arXiv2001

Current induced switching of magnetic domains to a perpendicular configuration

In a ferromagnet--normal-metal--ferromagnet trilayer, a current flowing perpendicularly to the layers creates a torque on the magnetic moments of the ferromagnets. When one of the contacts is superconducting, the torque not only favors parallel or antiparallel alignment of the magnetic moments, as is the case for two normal contacts, but can also favor a configuration where the two moments are perpendicular. In addition, whereas the conductance for parallel and antiparallel magnetic moments is the same, signalling the absence of giant magnetoresistance in the usual sense, the conductance is greater in the perpendicular configuration. Thus, a negative magnetoconductance is predicted, in contrast with the usual giant magnetoresistance.

cond-mat.mes-hall↗

arXiv2000

The Role of Spin-Dependent Interface Scattering in Generating Current-Induced Torques in Magnetic Multilayers

We present a calculation of current-induced torques in metallic magnetic multilayers derived from the spin-dependent transmission and reflection properties of the magnetic layers. A scattering formalism is employed to calculate the torques in a magnetic-nonmagnetic-magnetic trilayer, for currents perpendicular to the layers, in both the ballistic and diffusive regimes.

cond-mat.mes-hall↗