Manganese and carbon lines as temperature indicators

Selected hyperfine structure broadened lines of various excitations and the carbon line at 5380 Å are tested as temperature diagnostics for photospheric heterogeneities. This is done by comparing the observed center-to-limb variation of the equivalent widths of these lines with predictions by several proposed models of homogeneous type. Model 10 (Elste, 1968), which also explains the limb-darkening of the continuum in the same spectral range, provides a better basis for the analysis of heterogeneities at different photospheric levels than other recent reference models.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/43705/1/11207_2004_Article_BF00155340.pd

Photospheric network from study of manganese lines

Area scans with the multi-channel magnetograph at the Kitt Peak McMath Telescope provided simultaneous measures of equivalent widths in two Mn lines, continuum intensity and longitudinal magnetic field component. Observations were carried out with apertures of 2.5″ × 3.5″ arc and 1″ × 1″ arc. For the network elements, which were identified using the magnetic field as tracer, we developed relationships between I Continuum , W λ V Doppler and longitudinal field strength. With a minimum of assumptions it is possible to write from these observed relations an expression connecting true temperature perturbation in the line forming layers of the network to true longitudinal field strength. The results provide some constraints upon possible network models.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/43746/1/11207_2004_Article_BF00951834.pd

Resonant and Kondo tunneling through molecular magnets

Transport through molecular magnets is studied in the regime of strong coupling to the leads. We consider a resonant-tunneling model where the electron spin in a quantum dot or molecule is coupled to an additional local, anisotropic spin via exchange interaction. The two opposite regimes dominated by resonant tunneling and by Kondo transport, respectively, are considered. In the resonant-tunneling regime, the stationary state of the impurity spin is calculated for arbitrarily strong molecule-lead coupling using a master-equation approach, which treats the exchange interaction perturbatively. We find that the characteristic fine structure in the differential conductance persists even if the hybridization energy exceeds thermal energies. Transport in the Kondo regime is studied within a diagrammatic approach. We show that magnetic anisotropy gives rise to a splitting of the Kondo peak at low bias voltages.Comment: 13 pages, 5 figures, version as publishe

Dependence of the correlation of small scale photospheric structures upon resolution

Correlations between continuum intensity, velocity, and equivalent widths of two Mn i lines as observed with two different entrance apertures tend to deteriorate with improvement in spatial resolution. The KPNO multichannel magnetograph was used to make area scans at the center of the disk with entrance apertures 3.5″×2.5″ arc and 1″×1″ arc. A coherence analysis shows that this effect is caused by marked differences of fluctuations in temperature and temperature gradients as well as in the velocity structure of photospheric elements of various sizes.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/43749/1/11207_2004_Article_BF00155356.pd

Theory for transport through a single magnetic molecule: Endohedral N@C60

We consider transport through a single N@C60 molecule, weakly coupled to metallic leads. Employing a density-matrix formalism we derive rate equations for the occupation probabilities of many-particle states of the molecule. We calculate the current-voltage characteristics and the differential conductance for N@C60 in a break junction. Our results reveal Coulomb-blockade behavior as well as a fine structure of the Coulomb-blockade peaks due to the exchange coupling of the C60 spin to the spin of the encapsulated nitrogen atom.Comment: 5 pages, 4 figures, v2: version as publishe

Characterististics of plage fragments with photospheric network properties

Using data taken with the multi-channel magnetograph at KPNO, we demonstrate that plage regions surrounding a sunspot have thermal properties found in the photospheric network. These network-like regions existed up to the edge of the penumbra of the sunspot. Temperature gradients inferred from equivalent width fluctuations in our data do not conflict with the requirements of the theory (Parker, 1978) for flux tubes to exist at subphotospheric levels.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/43752/1/11207_2004_Article_BF00146679.pd

Strong and Tunable Nonlinear Optomechanical Coupling in a Low-Loss System

A major goal in optomechanics is to observe and control quantum behavior in a system consisting of a mechanical resonator coupled to an optical cavity. Work towards this goal has focused on increasing the strength of the coupling between the mechanical and optical degrees of freedom; however, the form of this coupling is crucial in determining which phenomena can be observed in such a system. Here we demonstrate that avoided crossings in the spectrum of an optical cavity containing a flexible dielectric membrane allow us to realize several different forms of the optomechanical coupling. These include cavity detunings that are (to lowest order) linear, quadratic, or quartic in the membrane's displacement, and a cavity finesse that is linear in (or independent of) the membrane's displacement. All these couplings are realized in a single device with extremely low optical loss and can be tuned over a wide range in situ; in particular, we find that the quadratic coupling can be increased three orders of magnitude beyond previous devices. As a result of these advances, the device presented here should be capable of demonstrating the quantization of the membrane's mechanical energy.Comment: 12 pages, 4 figures, 1 tabl

Green function techniques in the treatment of quantum transport at the molecular scale

The theoretical investigation of charge (and spin) transport at nanometer length scales requires the use of advanced and powerful techniques able to deal with the dynamical properties of the relevant physical systems, to explicitly include out-of-equilibrium situations typical for electrical/heat transport as well as to take into account interaction effects in a systematic way. Equilibrium Green function techniques and their extension to non-equilibrium situations via the Keldysh formalism build one of the pillars of current state-of-the-art approaches to quantum transport which have been implemented in both model Hamiltonian formulations and first-principle methodologies. We offer a tutorial overview of the applications of Green functions to deal with some fundamental aspects of charge transport at the nanoscale, mainly focusing on applications to model Hamiltonian formulations.Comment: Tutorial review, LaTeX, 129 pages, 41 figures, 300 references, submitted to Springer series "Lecture Notes in Physics

Asymmetries in limb darkening reanalyzed

The cause of the asymmetries in limb darkening reported by Neckel and Labs (1987) is discussed on the basis of new, stray-light-free observations, and found to be of instrumental origin.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/43709/1/11207_2004_Article_BF00158296.pd

Nano-Opto-Electro-Mechanical Systems

A new class of hybrid systems that couple optical, electrical and mechanical degrees of freedom in nanoscale devices is under development in laboratories worldwide. These nano-opto-electro-mechanical systems (NOEMS) offer unprecedented opportunities to dynamically control the flow of light in nanophotonic structures, at high speed and low power consumption. Drawing on conceptual and technological advances from cavity optomechanics, they also bear the potential for highly efficient, low-noise transducers between microwave and optical signals, both in the classical and quantum domains. This Progress Article discusses the fundamental physical limits of NOEMS, reviews the recent progress in their implementation, and suggests potential avenues for further developments in this field.Comment: 27 pages, 3 figures, 2 boxe