Evidence for multiple superconducting gaps in optimally doped BaFe1.87_{1.87}Co0.13_{0.13}As2_{2} from infrared spectroscopy

We performed combined infrared reflection and ellipsometry measurements of the in-plane optical reponse of single crystals of the pnictide high temperature superconductor BaFe$_{1.87}$Co$_{0.13}$As$_{2}$ with $T_{c}$ = 24.5 K. We observed characteristic superconductivity-induced changes which provide evidence for at least three different energy gaps. We show that a BCS-model of isotropic gaps with 2$\Delta/k_{B}T_{c}$ of 3.1, 4.7, and 9.2 reproduces the experimental data rather well. We also determine the low-temperature value of the in-plane magnetic penetration depth of 270 nm

Propagation of Exchange Bias in CoFe/FeMn/CoFe Trilayers

CoFe/FeMn, FeMn/CoFe bilayers and CoFe/FeMn/CoFe trilayers were grown in magnetic field and at room temperature. The exchange bias field $H_{eb}$ depends strongly on the order of depositions and is much higher at CoFe/FeMn than at FeMn/CoFe interfaces. By combining the two bilayer structures into symmetric CoFe/FeMn($t_\mathrm{FeMn}$)/CoFe trilayers, $H_{eb}^t$ and $H_{eb}^b$ of the top and bottom CoFe layers, respectively, are both enhanced. Reducing $t_\mathrm{FeMn}$ of the trilayers also results in enhancements of both $H_{eb}^b$ and $H_{eb}^t$. These results evidence the propagation of exchange bias between the two CoFe/FeMn and FeMn/CoFe interfaces mediated by the FeMn antiferromagnetic order

What Is Urban Environmental Stewardship? Constructing a Practitioner-Derived Framework

Agencies and organizations deploy various strategies in response to environmental challenges, including the formulation of policy, programs, and regulations. Citizen-based environmental stewardship is increasingly seen as an innovative and important approach to improving and conserving landscape health. A new research focus on the stewardship of urban natural resources is being launched by the U.S. Forest Service in the Pacific Northwest region. Early scoping efforts are addressing various scales of human systems ranging from individuals to organizations to the entire positive “footprint” of stewardship on the land. This report addresses a fundamental need—to understand and describe civic environmental stewardship in urban settings. Stewardship has been described and defined in diverse ways within a variety of contexts, including the philosophical literature of environmentalism, agency program descriptions, and outreach by sponsoring organizations. Constructing a framework to convey the layered meanings of stewardship will help to focus and guide future research. A cognitive mapping technique was used to elicit responses to the question “What is environmental stewardship?” Semistructured interviews were conducted with representatives of nine Seattle environmental organizations, a group of practitioners who collectively represent over 100 years of experience in the field. Program planners and managers have particularly direct experiences of stewardship. Cognitive mapping enables participants to explore, then display, their particular knowledge and perceptions about an idea or activity. Analysis generated thematic, structural representations of shared concepts. Results show that the practitioners have multilayered perceptions of stewardship, from environmental improvement to community building, and from actions to outcomes. The resulting conceptual framework demonstrates the full extent of stewardship activity and meaning, which can aid stewardship sponsors to improve stewardship programs, leading to better experiences for participants and higher quality outcomes for projects and environments

Inelastic Scattering in Metal-H2-Metal Junctions

We present first-principles calculations of the dI/dV characteristics of an H2 molecule sandwiched between Au and Pt electrodes in the presence of electron-phonon interactions. The conductance is found to decrease by a few percentage at threshold voltages corresponding to the excitation energy of longitudinal vibrations of the H2 molecule. In the case of Pt electrodes, the transverse vibrations can mediate transport through otherwise non-transmitting Pt $d$-channels leading to an increase in the differential conductance even though the hydrogen junction is characterized predominately by a single almost fully open transport channel. In the case of Au, the transverse modes do not affect the dI/dV because the Au d-states are too far below the Fermi level. A simple explanation of the first-principles results is given using scattering theory. Finally, we compare and discuss our results in relation to experimental data.Comment: Accepted in Phys. Rev.

Probing nuclear expansion dynamics with π−/π+\pi^-/\pi^+-spectra

We study the dynamics of charged pions in the nuclear medium via the ratio of differential $\pi^-$- and $\pi^+$-spectra in a coupled-channel BUU (CBUU) approach. The relative energy shift of the charged pions is found to correlate with the pion freeze-out time in nucleus-nucleus collisions as well as with the impact parameter of the heavy-ion reaction. Furthermore, the long-range Coulomb force provides a 'clock' for the expansion of the hot nuclear system. Detailed comparisons with experimental data for $Au + Au$ at 1 GeV/A and $Ni + Ni$ at 2.0 GeV/A are presented.Comment: 21 pages, latex, figures include

Bounds on gravitational wave backgrounds from large distance clock comparisons

Our spacetime is filled with gravitational wave backgrounds that constitute a fluctuating environment created by astrophysical and cosmological sources. Bounds on these backgrounds are obtained from cosmological and astrophysical data but also by analysis of ranging and Doppler signals from distant spacecraft. We propose here a new way to set bounds on those backgrounds by performing clock comparisons between a ground clock and a remote spacecraft equipped with an ultra-stable clock, rather than only ranging to an onboard transponder. This technique can then be optimized as a function of the signal to be measured and the dominant noise sources, leading to significant improvements on present bounds in a promising frequency range where different theoretical models are competing. We illustrate our approach using the SAGAS project which aims to fly an ultra stable optical clock in the outer solar system.Comment: 10 pages, 8 figures, minor amendment

Isotropic AdS/CFT fireball

We study the AdS/CFT thermodynamics of the spatially isotropic counterpart of the Bjorken similarity flow in d-dimensional Minkowski space with d>=3, and of its generalisation to linearly expanding d-dimensional Friedmann-Robertson-Walker cosmologies with arbitrary values of the spatial curvature parameter k. The bulk solution is a nonstatic foliation of the generalised Schwarzschild-AdS black hole with a horizon of constant curvature k. The boundary matter is an expanding perfect fluid that satisfies the first law of thermodynamics for all values of the temperature and the spatial curvature, but it admits a description as a scale-invariant fluid in local thermal equilibrium only when the inverse Hawking temperature is negligible compared with the spatial curvature length scale. A Casimir-type term in the holographic energy-momentum tensor is identified from the threshold of black hole formation and is shown to take different forms for k>=0 and k<0.Comment: 20 pages. v3: typos corrected. Published versio

Pressure Effect and Specific Heat of RBa2Cu3Ox at Distinct Charge Carrier Concentrations: Possible Influence of Stripes

In YBa2Cu3Ox, distinct features are found in the pressure dependence of the transition temperature, dTc/dp, and in DeltaCp*Tc, where DeltaCp is the jump in the specific heat at Tc: dTc/dp becomes zero when DeltaCp*Tc is maximal, whereas dTc/dp has a peak at lower oxygen contents where DeltaCp*Tc vanishes. Substituting Nd for Y and doping with Ca leads to a shift of these specific oxygen contents, since oxygen order and hole doping by Ca influences the hole content nh in the CuO2 planes. Calculating nh from the parabolic Tc(nh) behavior, the features coalesce for all samples at nh=0.11 and nh=0.175, irrespective of substitution and doping. Hence, this behavior seems to reflect an intrinsic property of the CuO2 planes. Analyzing our results we obtain different mechanisms in three doping regions: Tc changes in the optimally doped and overdoped region are mainly caused by charge transfer. In the slightly underdoped region an increasing contribution to dTc/dp is obtained when well ordered CuO chain fragments serve as pinning centers for stripes. This behavior is supported by our results on Zn doped NdBa2Cu3Ox and is responsible for the well known dTc/dp peak observed in YBa2Cu3Ox at x=6.7. Going to a hole content below nh=0.11 our results point to a crossover from an underdoped superconductor to a doped antiferromagnet, changing completely the physics of these materials.Comment: 6 pages, 5 figures Proccedings of the 'Stripes 2000' Conference, Rome (2000

A catalogue of the Chandra Deep Field South with multi-colour classification and photometric redshifts from COMBO-17

We present the COMBO-17 object catalogue of the Chandra Deep Field South for public use, covering a field which is 31.5' x 30' in size. This catalogue lists astrometry, photometry in 17 passbands from 350 to 930 nm, and ground-based morphological data for 63,501 objects. The catalogue also contains multi-colour classification into the categories 'Star', 'Galaxy' and 'Quasar' as well as photometric redshifts. We include restframe luminosities in Johnson, SDSS and Bessell passbands and estimated errors. The redshifts are most reliable at R<24, where the sample contains approximately 100 quasars, 1000 stars and 10000 galaxies. We use nearly 1000 spectroscopically identified objects in conjunction with detailed simulations to characterize the performance of COMBO-17. We show that the selection of quasars, more generally type-1 AGN, is nearly complete and minimally contaminated at z=[0.5,5] for luminosities above M_B=-21.7. Their photometric redshifts are accurate to roughly 5000 km/sec. Galaxy redshifts are accurate to 1% in dz/(1+z) at R<21. They degrade in quality for progressively fainter galaxies, reaching accuracies of 2% for galaxies with R~222 and of 10% for galaxies with R>24. The selection of stars is complete to R~23, and deeper for M stars. We also present an updated discussion of our classification technique with maps of survey completeness, and discuss possible failures of the statistical classification in the faint regime at R>24.Comment: submitted to Astronomy & Astrophysics, public data set available at http://www.mpia.de/COMBO/combo_index.htm

Direct frequency comb spectroscopy of trapped ions

Direct frequency comb spectroscopy of trapped ions is demonstated for the first time. It is shown that the 4s^2S_(1/2)-4p^2P_(3/2) transition in calcium ions can be excited directly with a frequency comb laser that is upconverted to 393 nm. Detection of the transition is performed using a shelving scheme to suppress background signal from non-resonant comb modes. The measured transition frequency of f=761 905 012.7(0.5) MHz presents an improvement in accuracy of more than two orders of magnitude.Comment: 4 pages, 5 figur