Charge fluctuations in open chaotic cavities

We present a discussion of the charge response and the charge fluctuations of mesoscopic chaotic cavities in terms of a generalized Wigner-Smith matrix. The Wigner-Smith matrix is well known in investigations of time-delay of quantum scattering. It is expressed in terms of the scattering matrix and its derivatives with energy. We consider a similar matrix but instead of an energy derivative we investigate the derivative with regard to the electric potential. The resulting matrix is then the operator of charge. If this charge operator is combined with a self-consistent treatment of Coulomb interaction, the charge operator determines the capacitance of the system, the non-dissipative ac-linear response, the RC-time with a novel charge relaxation resistance, and in the presence of transport a resistance that governs the displacement currents induced into a nearby conductor. In particular these capacitances and resistances determine the relaxation rate and dephasing rate of a nearby qubit (a double quantum dot). We discuss the role of screening of mesoscopic chaotic detectors. Coulomb interaction effects in quantum pumping and in photon assisted electron-hole shot noise are treated similarly. For the latter we present novel results for chaotic cavities with non-ideal leads.Comment: 29 pages, 13 figures;v.2--minor changes; contribution for the special issue of J. Phys. A on "Trends in Quantum Chaotic Scattering

Nonequilibrium Josephson effect in mesoscopic ballistic multiterminal SNS junctions

We present a detailed study of nonequilibrium Josephson currents and conductance in ballistic multiterminal SNS-devices. Nonequilibrium is created by means of quasiparticle injection from a normal reservoir connected to the normal part of the junction. By applying a voltage at the normal reservoir the Josephson current can be suppressed or the direction of the current can be reversed. For a junction longer than the thermal length, $L\gg\xi_T$, the nonequilibrium current increases linearly with applied voltage, saturating at a value equal to the equilibrium current of a short junction. The conductance exhibits a finite bias anomaly around $eV \sim \hbar v_F/L$. For symmetric injection, the conductance oscillates $2\pi$-periodically with the phase difference $\phi$ between the superconductors, with position of the minimum ($\phi=0$ or $\pi$) dependent on applied voltage and temperature. For asymmetric injection, both the nonequilibrium Josephson current and the conductance becomes $\pi$-periodic in phase difference. Inclusion of barriers at the NS-interfaces gives rise to a resonant behavior of the total Josephson current with respect to junction length with a period $\sim \lambda_F$. Both three and four terminal junctions are studied.Comment: 21 pages, 19 figures, submitted to Phys. Rev.

Magnetic properties of gold nanoparticles: A room-temperature quantum e ffect

International audienceGold nanoparticles elicit a huge research activity in view oftheir applications in diagnostics,[1, 2] therapy,[3] drug or gene delivery,[4] sensing[5, 6, 7] and imaging.[8] Gold nanoparticles also displayinteresting catalytic[9, 10] and optical[11, 12, 13, 14] properties.This Communication focuses on the least understood and sofar unused property of gold: its becoming magnetic when preparedin the form of nanoparticles. All these desirable properties,bound together in one nanometric piece of matter, possiblyself-organized thanks to its ligands, make functionalizedgold nanoparticles a treasurable entity for nanosciences. Theex nihilo magnetic properties of functionalized gold (and otherdiamagnetic metals, such a Ag or Cu) nanoparticles, that is,their ferromagnetic-like behavior, are well documented,though still poorly understood.[15] This unexpected propertyopens new perspectives in materials science, in particular forthe design of metamaterials. One may also envisage applicationsin information storage and processing: nanometric magneticparticles with no obvious temperature limitation and possiblyself-organizing are currently much sought-after by thecomputer industry and developing a room-temperature magnetic semiconductor is paramount for the realization of spintronicstechnologies.Herein, we wish to present the results of our own investigationsinto the magnetic properties of functionalized gold nanoparticles.We have made attempts at understanding this magneticbehavior using both traditional techniques (e.g. superconductingquantum interference device, SQUID, magnetometry)and other methods less common in this field, such as zerofield197Au NMR (nuclear magnetic resonance) and SANS (smallangleneutron scattering). We also directly probed the localmagnetic field at the surface of gold nanoparticles using paramagneticTEMPO [(2,2,6,6-tetramethylpiperidin-1-yl)oxyl] radicalsand ESR (electron spin resonance) spectrometry. Surprisingly,none of these experiments provided a clearer picture infine. These “negative” results led us to pondering whether ornot the explanation could be elsewhere. Our hypothesis is thatthe magnetism of gold (and possibly other metals) could verywell originate in self-sustained persistent currents. We shalldemonstrate hereafter that this hypothesis is indeed very plausibleand would actually reconcile all of the experimental datareported to date.Striking results are often obtained when SQUID magnetometryis performed on functionalized Au nanoparticles, such asdodecanethiol-coated ones. Rather than being diamagnetic, asexpected, the nanoparticles can be found to be para- or ferromagneticat room temperature and above. When hysteresis isobserved, the magnetization curve looks like that of a soft ferromagnetand exhibits a remnant magnetization MR and a coercivefield HC, though both are rather weak. These parametershave been observed to have values that vary by orders ofmagnitude from sample to sample[15] (see Figure ESI-1 of theSupporting Information). Very often, the magnetization doesnot saturate. Diamagnetic samples are more diamagnetic thanthe bulk metal. Also, the magnetic observables show little dependenceon temperature between 2 and 400 K. The measurementsreported so far have been performed by totally independentgroups, on systems that were synthesized usingknown chemical procedures. Figure 1 compares the magnetizationof bulk gold with that of two diamagnetic samples ofgold nanoparticles. It can be seen that nanoparticles havea much larger absolute diamagnetic susceptibility than massivegold.Figure 2 compares two samples of gold nanoparticles, exhibitinga paramagnetic behavior and a ferromagnetic-like one.There is a weak but clear hysteresis, and the magnetizationdoes not really saturate even at high field values