Thermal Conductivity Tensor in YBa2_2Cu3_3O7−x_{7-x}: Effects of a Planar Magnetic Field

We have measured the thermal conductivity tensor of a twinned YBa$_2$Cu$_3$O$_{7-x}$ single crystal as a function of angle $\theta$ between the magnetic field applied parallel to the CuO$_2$ planes and the heat current direction, at different magnetic fields and at T=13.8 K. Clear fourfold and twofold variations in the field-angle dependence of $\kappa_{xx}$ and $\kappa_{xy}$ were respectively recorded in accordance with the d-wave pairing symmetry of the order parameter. The oscillation amplitude of the transverse thermal conductivity $\kappa^0_{xy}$ was found to be larger than the longitudinal one $\kappa^0_{xx}$ in the range of magnetic field studied here ($0 T$$\le B \le 9$$T$). From our data we obtain quantities that are free from non-electronic contributions and they allow us a comparison of the experimental results with current models for the quasiparticle transport in the mixed state.Comment: 9 Figures, Phys. Rev. B(in press

When does teamwork translate into improved team performance? a resource allocation perspective

Drawing on resource allocation theory, the authors examined boundary conditions for the positive effects of two aspects of teamwork (backing up behavior and performance monitoring) on team performance. Participants were 276 undergraduate business students who were organized into 69 teams and who worked on a computer simulation across multiple performance episodes. Approximately half the teams experienced a workload distribution problem. Results indicated that performance monitoring had positive effects on team performance when teams experienced a workload distribution problem. Backing up behavior had positive effects only when teams had both a workload distribution problem and during early performance episodes. The findings of this study suggest that resource allocation theory can provide insights regarding when members should devote and coordinate their own individual resources to assist others in teams. The implications of these findings for future theory and practice regarding teamwork are discussed. © The Author(s) 2010

Does backing up behavior explain the efficacy-performance relationship in teams?

Although the relationship between collective efficacy beliefs and team performance has been well-documented, few studies have explored the causal mechanisms that might explain these effects. In the current study, the authors explore the role of backing up behavior, a specific form of teamwork behavior, in explaining why high efficacy beliefs lead to high levels of team performance. Participants were 416 undergraduate business students who were organized into 104 four-person teams. The teams worked on an interdependent, computerized, decision-making task. Results of the study revealed that collective efficacy beliefs were positively related to team performance and that backing up behaviors largely mediated these effects. The theoretical and practical implications of these findings are discussed. © The Author(s) 2011

Generalized Penner models to all genera

We give a complete description of the genus expansion of the one-cut solution to the generalized Penner model. The solution is presented in a form which allows us in a very straightforward manner to localize critical points and to investigate the scaling behaviour of the model in the vicinity of these points. We carry out an analysis of the critical behaviour to all genera addressing all types of multi-critical points. In certain regions of the coupling constant space the model must be defined via analytical continuation. We show in detail how this works for the Penner model. Using analytical continuation it is possible to reach the fermionic 1-matrix model. We show that the critical points of the fermionic 1-matrix model can be indexed by an integer, $m$, as it was the case for the ordinary hermitian 1-matrix model. Furthermore the $m$'th multi-critical fermionic model has to all genera the same value of $\gamma_{str}$ as the $m$'th multi-critical hermitian model. However, the coefficients of the topological expansion need not be the same in the two cases. We show explicitly how it is possible with a fermionic matrix model to reach a $m=2$ multi-critical point for which the topological expansion has alternating signs, but otherwise coincides with the usual Painlev\'{e} expansion.Comment: 27 pages, PostScrip

Density of states and magnetoconductance of disordered Au point contacts

We report the first low temperature magnetotransport measurements on electrochemically fabricated atomic scale gold nanojunctions. As $T \to 0$, the junctions exhibit nonperturbatively large zero bias anomalies (ZBAs) in their differential conductance. We consider several explanations and find that the ZBAs are consistent with a reduced local density of states (LDOS) in the disordered metal. We suggest that this is a result of Coulomb interactions in a granular metal with moderate intergrain coupling. Magnetoconductance of atomic scale junctions also differs significantly from that of less geometrically constrained devices, and supports this explanation.Comment: 5 pages, 5 figures. Accepted to PRB as Brief Repor

Decoherence in a Josephson junction qubit

The zero-voltage state of a Josephson junction biased with constant current consists of a set of metastable quantum energy levels. We probe the spacings of these levels by using microwave spectroscopy to enhance the escape rate to the voltage state. The widths of the resonances give a measurement of the coherence time of the two states involved in the transitions. We observe a decoherence time shorter than that expected from dissipation alone in resonantly isolated 20 um x 5 um Al/AlOx/Al junctions at 60 mK. The data is well fit by a model including dephasing effects of both low-frequency current noise and the escape rate to the continuum voltage states. We discuss implications for quantum computation using current-biased Josephson junction qubits, including the minimum number of levels needed in the well to obtain an acceptable error limit per gate.Comment: 4 pages, 6 figure

Magnetic field effect on the dielectric constant of glasses: Evidence of disorder within tunneling barriers

The magnetic field dependence of the low frequency dielectric constant $e_r$(H) of a structural glass a - SiO2 + xCyHz was studied from 400 mK to 50 mK and for H up to 3T. Measurement of both the real and the imaginary parts of $e_r$ is used to eliminate the difficult question of keeping constant the temperature of the sample while increasing H: a non-zero $e_r$(H) dependence is reported in the same range as that one very recently reported on multicomponent glasses. In addition to the recently proposed explanation based on interactions, the reported $e_r$(H) is interpreted quantitatively as a consequence of the disorder lying within the nanometric barriers of the elementary tunneling systems of the glass.Comment: latex Bcorrige1.tex, 5 files, 4 figures, 7 pages [SPEC-S02/009

Optical properties of the pseudogap state in underdoped cuprates

Recent optical measurements of deeply underdoped cuprates have revealed that a coherent Drude response persists well below the end of the superconducting dome. In addition, no large increase in optical effective mass has been observed, even at dopings as low as 1%. We show that this behavior is consistent with the resonating valence bond spin-liquid model proposed by Yang, Rice, and Zhang. In this model, the overall reduction in optical conductivity in the approach to the Mott insulating state is caused not by an increase in effective mass, but by a Gutzwiller factor, which describes decreased coherence due to correlations, and by a shrinking of the Fermi surface, which decreases the number of available charge carriers. We also show that in this model, the pseudogap does not modify the low-temperature, low-frequency behavior, though the magnitude of the conductivity is greatly reduced by the Gutzwiller factor. Similarly, the profile of the temperature dependence of the microwave conductivity is largely unchanged in shape, but the Gutzwiller factor is essential in understanding the observed difference in magnitude between ortho-I and -II YBa$_2$Cu$_3$O$_y$.Comment: 9 pages, 6 figures, submitted to Eur. Phys. J.

Which blazars are neutrino loud?

Protons accelerated in the cores of active galactic nuclei can effectively produce neutrinos only if the soft radiation background in the core is sufficiently high. We find restrictions on the spectral properties and luminosity of blazars under which they can be strong neutrino sources. We analyze the possibility that neutrino flux is highly beamed along the rotation axis of the central black hole. The enhancement of neutrino flux compared to GeV gamma-ray flux from a given source makes the detection of neutrino point sources more probable. At the same time the smaller open angle reduces the number of possible neutrino-loud blazars compared to the number of gamma-ray loud ones. We present the table of 15 blazars which are the most likely candidates for the detection by future neutrino telescopes.Comment: 9 pages, 5 figures, version to be published in PR

Point-contact spectroscopy of the nickel borocarbide superconductor YNi2B2C in the normal and superconducting state

Point-contact (PC) spectroscopy measurements of YNi2B2C single crystals in the normal and superconducting (SC) state (T_c=15.4K) for the main crystallographic directions are reported. The PC study reveals the electron-phonon interaction (EPI) spectral function with dominant phonon maximum around 12 meV and further weak structures (hump or kink) at higher energy at about 50 meV. No "soft" modes below 12 meV are resolved in the normal state. The PC EPI spectra are qualitatively similar for the different directions. Contrary, directional study of the SC gap results in \Delta_[100]=1.5 meV for the a direction and \Delta_[001]=2.3 meV along the c axis; however the critical temperature T_c in PC in all cases is near to that in the bulk sample. The value 2\Delta_[001]/kT_c=3.6 is close to the BCS value of 3.52, and the temperature dependence \Delta_[001](T) is BCS-like, while the for small gap \Delta_[100](T) is below BCS behavior at T>T_c/2 similarly as in the two-gap superconductor MgB2. It is supposed that the directional variation \Delta can be attributed to a multiband nature of the SC state in YNi2B2C.Comment: 9 pages, 10 figures, to be published in a special issue of J. Low Temp. Phys. in honour of Prof. H. von Loehneyse