New insights into electron spin dynamics in the presence of correlated noise

The changes of the spin depolarization length in zinc-blende semiconductors when an external component of correlated noise is added to a static driving electric field are analyzed for different values of field strength, noise amplitude and correlation time. Electron dynamics is simulated by a Monte Carlo procedure which keeps into account all the possible scattering phenomena of the hot electrons in the medium and includes the evolution of spin polarization. Spin depolarization is studied by examinating the decay of the initial spin polarization of the conduction electrons through the D'yakonov-Perel process, the only relevant relaxation mechanism in III-V crystals. Our results show that, for electric field amplitude lower than the Gunn field, the dephasing length shortens with the increasing of the noise intensity. Moreover, a nonmonotonic behavior of spin depolarization length with the noise correlation time is found, characterized by a maximum variation for values of noise correlation time comparable with the dephasing time. Instead, in high field conditions, we find that, critically depending on the noise correlation time, external fluctuations can positively affect the relaxation length. The influence of the inclusion of the electron-electron scattering mechanism is also shown and discussed.Comment: Published on "Journal of Physics: Condensed Matter" as "Fast Track Communications", 11 pages, 9 figure

Voronoi-Delaunay analysis of normal modes in a simple model glass

We combine a conventional harmonic analysis of vibrations in a one-atomic model glass of soft spheres with a Voronoi-Delaunay geometrical analysis of the structure. ``Structure potentials'' (tetragonality, sphericity or perfectness) are introduced to describe the shape of the local atomic configurations (Delaunay simplices) as function of the atomic coordinates. Apart from the highest and lowest frequencies the amplitude weighted ``structure potential'' varies only little with frequency. The movement of atoms in soft modes causes transitions between different ``perfect'' realizations of local structure. As for the potential energy a dynamic matrix can be defined for the ``structure potential''. Its expectation value with respect to the vibrational modes increases nearly linearly with frequency and shows a clear indication of the boson peak. The structure eigenvectors of this dynamical matrix are strongly correlated to the vibrational ones. Four subgroups of modes can be distinguished

Spin injection and spin accumulation in all-metal mesoscopic spin valves

We study the electrical injection and detection of spin accumulation in lateral ferromagnetic metal-nonmagnetic metal-ferromagnetic metal (F/N/F) spin valve devices with transparent interfaces. Different ferromagnetic metals, permalloy (Py), cobalt (Co) and nickel (Ni), are used as electrical spin injectors and detectors. For the nonmagnetic metal both aluminium (Al) and copper (Cu) are used. Our multi-terminal geometry allows us to experimentally separate the spin valve effect from other magneto resistance signals such as the anomalous magneto resistance (AMR) and Hall effects. We find that the AMR contribution of the ferromagnetic contacts can dominate the amplitude of the spin valve effect, making it impossible to observe the spin valve effect in a 'conventional' measurement geometry. In a 'non local' spin valve measurement we are able to completely isolate the spin valve signal and observe clear spin accumulation signals at T=4.2 K as well as at room temperature (RT). For aluminum we obtain spin relaxation lengths (lambda_{sf}) of 1.2 mu m and 600 nm at T=4.2 K and RT respectively, whereas for copper we obtain 1.0 mu m and 350 nm. The spin relaxation times tau_{sf} in Al and Cu are compared with theory and results obtained from giant magneto resistance (GMR), conduction electron spin resonance (CESR), anti-weak localization and superconducting tunneling experiments. The spin valve signals generated by the Py electrodes (alpha_F lambda_F=0.5 [1.2] nm at RT [T=4.2 K]) are larger than the Co electrodes (alpha_F lambda_F=0.3 [0.7] nm at RT [T=4.2 K]), whereas for Ni (alpha_F lambda_F<0.3 nm at RT and T=4.2 K) no spin signal is observed. These values are compared to the results obtained from GMR experiments.Comment: 16 pages, 12 figures, submitted to PR

Spin-drift transport and its applications

We study the generation of non-equilibrium spin currents in systems with spatially-inhomogeneous magnetic potentials. For sufficiently high current densities, the spin polarization can be transported over distances significantly exceeding the intrinsic spin-diffusion length. This enables applications that are impossible within the conventional spin-diffusion regime. Specifically, we propose dc measurement schemes for the carrier spin relaxation times, $T_1$ and $T_2$, as well as demonstrate the possibility of spin species separation by driving current through a region with an inhomogeneous magnetic potential.Comment: 4 pages, 2 eps figure

Phase locking the spin precession in a storage ring

This letter reports the successful use of feedback from a spin polarization measurement to the revolution frequency of a 0.97 GeV/$c$ bunched and polarized deuteron beam in the Cooler Synchrotron (COSY) storage ring in order to control both the precession rate ($\approx 121$ kHz) and the phase of the horizontal polarization component. Real time synchronization with a radio frequency (rf) solenoid made possible the rotation of the polarization out of the horizontal plane, yielding a demonstration of the feedback method to manipulate the polarization. In particular, the rotation rate shows a sinusoidal function of the horizontal polarization phase (relative to the rf solenoid), which was controlled to within a one standard deviation range of $\sigma = 0.21$ rad. The minimum possible adjustment was 3.7 mHz out of a revolution frequency of 753 kHz, which changes the precession rate by 26 mrad/s. Such a capability meets a requirement for the use of storage rings to look for an intrinsic electric dipole moment of charged particles

Formation and Evolution of Supermassive Black Holes

The correlation between the mass of supermassive black holes in galaxy nuclei and the mass of the galaxy spheroids or bulges (or more precisely their central velocity dispersion), suggests a common formation scenario for galaxies and their central black holes. The growth of bulges and black holes can commonly proceed through external gas accretion or hierarchical mergers, and are both related to starbursts. Internal dynamical processes control and regulate the rate of mass accretion. Self-regulation and feedback are the key of the correlation. It is possible that the growth of one component, either BH or bulge, takes over, breaking the correlation, as in Narrow Line Seyfert 1 objects. The formation of supermassive black holes can begin early in the universe, from the collapse of Population III, and then through gas accretion. The active black holes can then play a significant role in the re-ionization of the universe. The nuclear activity is now frequently invoked as a feedback to star formation in galaxies, and even more spectacularly in cooling flows. The growth of SMBH is certainly there self-regulated. SMBHs perturb their local environment, and the mergers of binary SMBHs help to heat and destroy central stellar cusps. The interpretation of the X-ray background yields important constraints on the history of AGN activity and obscuration, and the census of AGN at low and at high redshifts reveals the downsizing effect, already observed for star formation. History appears quite different for bright QSO and low-luminosity AGN: the first grow rapidly at high z, and their number density decreases then sharply, while the density of low-luminosity objects peaks more recently, and then decreases smoothly.Comment: 31 pages, 13 figures, review paper for Astrophysics Update

The effects of relativistic bulk motion of X-ray flares in the corona on the iron Kalpha line in Seyfert 1 galaxies

We study the effects of the bulk motion of X-ray flares on the shape and equivalent width of the iron Kalpha line from an untruncated cold disk around a Kerr black hole using fully relativistic calculations. The flares are located above a cold accretion disk -- either on or off the rotation axis. For on- or off-axis flares, the upward/outward bulk motion causes a reduction of the iron Kalpha line width. To a distant observer with a low inclination angle (\theta_o \simlt 30deg.), larger upward/outward bulk velocities decrease the extension of the red wing, with little change in the location of the blue `edge'. In contrast, an observer at a large inclination angle (e.g. \theta_o=60deg.) sees both the red wing and the blue `edge' change with the bulk velocity. The equivalent width of the iron Kalpha line decreases rapidly with increasing bulk velocity of flares. However, the `narrower' line profiles observed in some objects (e.g. IC4329A and NGC4593) are difficult to produce using the out-flowing magnetic flare model with an appropriate equivalent width unless the X-ray emission is concentrated in an outer region with a radius of several tens of r_g=GM/c^2 or more. An important result is that the iron Kalpha line intensity is found to be constant even though the continuum flux varies significantly, which is true for out-flowing magnetic flares with different bulk velocities but similar intrinsic luminosities when located close to the central black hole. We find that fluctuations in the bulk velocities of out-flowing low-height flares located at the inner region (r\simlt 15r_g) can account for a constant iron Kalpha line and significant continuum variation as observered in MCG-6-30-15 and NGC5548. (Abridged)Comment: 30 pages (including 8 figures); minor changes, to appear in ApJ, Nov. 10, 200

Quasars: the characteristic spectrum and the induced radiative heating

Using information on the cosmic X-ray background and the cumulative light of active galactic nuclei at infrared wavelengths, the estimated local mass density of galactic massive black holes (MBHs) and published AGN composite spectra in the optical, UV and X-ray, we compute the characteristic angular-integrated, broad-band spectral energy distribution of the average quasar in the universe. We demonstrate that the radiation from such sources can photoionize and Compton heat the plasma surrounding them up to an equilibrium Compton temperature (Tc) of 2x10^7 K. It is shown that circumnuclear obscuration cannot significantly affect the net gas Compton heating and cooling rates, so that the above Tc value is approximately characteristic of both obscured and unobscured quasars. This temperature is above typical gas temperatures in elliptical galaxies and just above the virial temperatures of giant ellipticals. The general results of this work can be used for accurate calculations of the feedback effect of MBHs on both their immediate environs and the more distant interstellar medium of their host galaxies.Comment: 15 pages, 5 figures. Revised version accepted for publication in MNRA

Anharmonicity, vibrational instability and Boson peak in glasses

We show that a {\em vibrational instability} of the spectrum of weakly interacting quasi-local harmonic modes creates the maximum in the inelastic scattering intensity in glasses, the Boson peak. The instability, limited by anharmonicity, causes a complete reconstruction of the vibrational density of states (DOS) below some frequency $\omega_c$, proportional to the strength of interaction. The DOS of the new {\em harmonic modes} is independent of the actual value of the anharmonicity. It is a universal function of frequency depending on a single parameter -- the Boson peak frequency, $\omega_b$ which is a function of interaction strength. The excess of the DOS over the Debye value is $\propto\omega^4$ at low frequencies and linear in $\omega$ in the interval $\omega_b \ll \omega \ll \omega_c$. Our results are in an excellent agreement with recent experimental studies.Comment: LaTeX, 8 pages, 6 figure

Simulation of dimensionality effects in thermal transport

The discovery of nanostructures and the development of growth and fabrication techniques of one- and two-dimensional materials provide the possibility to probe experimentally heat transport in low-dimensional systems. Nevertheless measuring the thermal conductivity of these systems is extremely challenging and subject to large uncertainties, thus hindering the chance for a direct comparison between experiments and statistical physics models. Atomistic simulations of realistic nanostructures provide the ideal bridge between abstract models and experiments. After briefly introducing the state of the art of heat transport measurement in nanostructures, and numerical techniques to simulate realistic systems at atomistic level, we review the contribution of lattice dynamics and molecular dynamics simulation to understanding nanoscale thermal transport in systems with reduced dimensionality. We focus on the effect of dimensionality in determining the phononic properties of carbon and semiconducting nanostructures, specifically considering the cases of carbon nanotubes, graphene and of silicon nanowires and ultra-thin membranes, underlying analogies and differences with abstract lattice models.Comment: 30 pages, 21 figures. Review paper, to appear in the Springer Lecture Notes in Physics volume "Thermal transport in low dimensions: from statistical physics to nanoscale heat transfer" (S. Lepri ed.