128 research outputs found
Infrared anomalous Hall effect in SrRuO: Evidence for crossover to intrinsic behavior
The origin of the Hall effect in many itinerant ferromagnets is still not
resolved, with an anomalous contribution from the sample magnetization that can
exhibit extrinsic or intrinsic behavior. We report the first mid-infared (MIR)
measurements of the complex Hall (), Faraday (), and Kerr
() angles, as well as the Hall conductivity () in a
SrRuO film in the 115-1400 meV energy range. The magnetic field,
temperature, and frequency dependence of the Hall effect is explored. The MIR
magneto-optical response shows very strong frequency dependence, including sign
changes. Below 200 meV, the MIR changes sign between 120 and 150
K, as is observed in dc Hall measurements. Above 200 meV, the temperature
dependence of is similar to that of the dc magnetization and the
measurements are in good agreement with predictions from a band calculation for
the intrinsic anomalous Hall effect (AHE). The temperature and frequency
dependence of the measured Hall effect suggests that whereas the behavior above
200 meV is consistent with an intrinsic AHE, the extrinsic AHE plays an
important role in the lower energy response.Comment: The resolution of figures is improve
Electronic structure of ferromagnetic semiconductor Ga1-xMnxAs probed by sub-gap magneto-optical spectroscopy
We employ Faraday and Kerr effect spectroscopy in the infrared range to
investigate the electronic structure of Ga1-xMnxAs near the Fermi energy. The
band structure of this archetypical dilute-moment ferromagnetic semiconductor
has been a matter of controversy, fueled partly by previous measurements of the
unpolarized infrared absorption and their phenomenological impurity-band
interpretation. The infrared magneto-optical effects we study arise directly
from the spin-splitting of the carrier bands and their chiral asymmetry due to
spin-orbit coupling. Unlike the unpolarized absorption, they are intimately
related to ferromagnetism and their interpretation is much more microscopically
constrained in terms of the orbital character of the relevant band states. We
show that the conventional theory of the disordered valence band with dominant
As p-orbital character and coupled by kinetic-exchange to Mn local moments
accounts semi-quantitatively for the overall characteristics of the measured
infrared magneto-optical spectra.Comment: 4 pages 3 figure
Review of the ELI-NP-GBS low level rf and synchronization systems
The Gamma Beam System (GBS) of ELI-NP is a linac based gamma-source in construction at Magurele (RO) by the European consortium EuroGammaS led by INFN. Photons with tunable energy and with intensity and brilliance well beyond the state of the art will be produced by Compton back-scattering between a high quality electron beam (up to 740 MeV) and a 515 nm intense laser pulse. Production of very intense photon flux with narrow bandwidth requires multi-bunch operation at 100 Hz repetition rate. A total of 13 klystrons, 3 S-band (2856 MHz) and 10 C-band (5712 MHz) will power a total of 14 Travelling Wave accelerating sections (2 S-band and 12 C-band) plus 3 S-band Standing Wave cavities (a 1.6 cell RF gun and 2 RF deflectors). Each klystron is individually driven by a temperature stabilized LLRF module, for a maximum flexibility in terms of accelerating gradient, arbitrary pulse shaping (e.g. to compensate beam loading effects in multi-bunch regime) and compensation of long-term thermal drifts. In this paper, the whole LLRF system architecture and bench test results, the RF reference generation and distribution together with an overview of the synchronization system will be described
Terahertz dynamics of a topologically protected state: quantum Hall effect plateaus near cyclotron resonance in a GaAs/AlGaAs heterojunction
We measure the Hall conductivity of a two-dimensional electron gas formed at
a GaAs/AlGaAs heterojunction in the terahertz regime close to the cyclotron
resonance frequency by employing a highly sensitive Faraday rotation method
coupled with electrical gating of the sample to change the electron density. We
observe clear plateau-and step-like features in the Faraday rotation angle vs.
electron density and magnetic field (Landau-level filling factor), which are
the high frequency manifestation of quantum Hall plateaus - a signature of
topologically protected edge states. The results are compared to a recent
dynamical scaling theory.Comment: 18 pages, 3 figure
Spin-dynamic field coupling in strongly THz driven semiconductors : local inversion symmetry breaking
We study theoretically the optics in undoped direct gap semiconductors which
are strongly driven in the THz regime. We calculate the optical sideband
generation due to nonlinear mixing of the THz field and the near infrared
probe. Starting with an inversion symmetric microscopic Hamiltonian we include
the THz field nonperturbatively using non-equilibrium Green function
techniques. We find that a self induced relativistic spin-THz field coupling
locally breaks the inversion symmetry, resulting in the formation of odd
sidebands which otherwise are absent.Comment: 8 pages, 6 figure
Terahertz Magneto Optical Polarization Modulation Spectroscopy
We report the development of new terahertz techniques for rapidly measuring
the complex Faraday angle in systems with broken time-reversal symmetry using
the cyclotron resonance of a GaAs two-dimensional electron gas in a magnetic
field as a system for demonstration of performance. We have made polarization
modulation, high sensitivity (< 1 mrad) narrow band rotation measurements with
a CW optically pumped molecular gas laser, and by combining the distinct
advantages of terahertz (THz) time domain spectroscopy and polarization
modulation techniques, we have demonstrated rapid broadband rotation
measurements to < 5 mrad precision.Comment: 25 pages including 7 figures, introduces use of rotating polarizer
with THz TDS for Complex Faraday Angle determinatio
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Seasonal cycle of precipitation variability in South America on intraseasonal timescales
The seasonal cycle of the intraseasonal (IS) variability of precipitation in South America is described through the analysis of bandpass filtered outgoing longwave radiation (OLR) anomalies. The analysis is discriminated between short (10--30 days) and long (30--90 days) intraseasonal timescales. The seasonal cycle of the 30--90-day IS variability can be well described by the activity of first leading pattern (EOF1) computed separately for the wet season (October--April) and the dry season (May--September). In agreement with previous works, the EOF1 spatial distribution during the wet season is that of a dipole with centers of actions in the South Atlantic Convergence Zone (SACZ) and southeastern South America (SESA), while during the dry season, only the last center is discernible. In both seasons, the pattern is highly influenced by the activity of the Madden--Julian Oscillation (MJO). Moreover, EOF1 is related with a tropical zonal-wavenumber-1 structure superposed with coherent wave trains extended along the South Pacific during the wet season, while during the dry season the wavenumber-1 structure is not observed. The 10--30-day IS variability of OLR in South America can be well represented by the activity of the EOF1 computed through considering all seasons together, a dipole but with the stronger center located over SESA. While the convection activity at the tropical band does not seem to influence its activity, there are evidences that the atmospheric variability at subtropical-extratropical regions might have a role. Subpolar wavetrains are observed in the Pacific throughout the year and less intense during DJF, while a path of wave energy dispersion along a subtropical wavetrain also characterizes the other seasons. Further work is needed to identify the sources of the 10--30-day-IS variability in South America
Measuring anisotropic scattering in the cuprates
A simple model of anisotropic scattering in a quasi two-dimensional metal is
studied. Its simplicity allows an analytic calculation of transport properties
using the Boltzmann equation and relaxation time approximation. We argue that
the c-axis magnetoresistance provides the key test of this model of transport.
We compare this model with experiments on overdoped Tl-2201 and find reasonable
agreement using only weak scattering anisotropy. We argue that optimally doped
Tl-2201 should show strong angular-dependent magnetoresistance within this
model and would provide a robust way of determining the in-plane scattering
anisotropy in the cuprates.Comment: 12 pages, 8 figures, typset in REVTeX 4. Version 2; added references
and corrected typo
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