Fine structure of excitons in Cu2_2O

Three experimental observations on 1s-excitons in Cu$_2$O are not consistent with the picture of the exciton as a simple hydrogenic bound state: the energies of the 1s-excitons deviate from the Rydberg formula, the total exciton mass exceeds the sum of the electron and hole effective masses, and the triplet-state excitons lie above the singlet. Incorporating the band structure of the material, we calculate the corrections to this simple picture arising from the fact that the exciton Bohr radius is comparable to the lattice constant. By means of a self-consistent variational calculation of the total exciton mass as well as the ground-state energy of the singlet and the triplet-state excitons, we find excellent agreement with experiment.Comment: Revised abstract; 10 pages, revtex, 3 figures available from G. Kavoulakis, Physics Department, University of Illinois, Urban

Theory of optical spectra of polar quantum wells: Temperature effects

Theoretical and numerical calculations of the optical absorption spectra of excitons interacting with longitudinal-optical phonons in quasi-2D polar semiconductors are presented. In II-VI semiconductor quantum wells, exciton binding energy can be tuned on- and off-resonance with the longitudinal-optical phonon energy by varying the quantum well width. A comprehensive picture of this tunning effect on the temperature-dependent exciton absorption spectrum is derived, using the exciton Green's function formalism at finite temperature. The effective exciton-phonon interaction is included in the Bethe-Salpeter equation. Numerical results are illustrated for ZnSe-based quantum wells. At low temperatures, both a single exciton peak as well as a continuum resonance state are found in the optical absorption spectra. By contrast, at high enough temperatures, a splitting of the exciton line due to the real phonon absorption processes is predicted. Possible previous experimental observations of this splitting are discussed.Comment: 10 pages, 9 figures, to appear in Phys. Rev. B. Permanent address: [email protected]

Robust skill of decadal climate predictions

There is a growing need for skilful predictions of climate up to a decade ahead. Decadal climate predictions show high skill for surface temperature, but confidence in forecasts of precipitation and atmospheric circulation is much lower. Recent advances in seasonal and annual prediction show that the signal-to-noise ratio can be too small in climate models, requiring a very large ensemble to extract the predictable signal. Here, we reassess decadal prediction skill using a much larger ensemble than previously available, and reveal significant skill for precipitation over land and atmospheric circulation, in addition to surface temperature. We further propose a more powerful approach than used previously to evaluate the benefit of initialisation with observations, improving our understanding of the sources of skill. Our results show that decadal climate is more predictable than previously thought and will aid society to prepare for, and adapt to, ongoing climate variability and change.D.M.S., A.A.S., N.J.D., L.H. and R.E. were supported by the Met Office Hadley Centre Climate Programme funded by BEIS and Defra and by the European Commission Horizon 2020 EUCP project (GA 776613). L.P.C. was supported by the Spanish MINECO HIATUS (CGL2015-70353-R) project. F.J.D.R. was supported by the H2020 EUCP (GA 776613) and the Spanish MINECO CLINSA (CGL2017-85791-R) projects. W.A. M. and H.P. were supported by the German Ministry of Education and Research (BMBF) under the project MiKlip (grant 01LP1519A). The NCAR contribution was supported by the US National Oceanic and Atmospheric Administration (NOAA) Climate Program Office under Climate Variability and Predictability Program Grant NA13OAR4310138 and by the US National Science Foundation (NSF) Collaborative Research EaSM2 Grant OCE-1243015. The NCAR contribution is also based upon work supported by NCAR, which is a major facility sponsored by the US NSF under Cooperative Agreement No. 1852977. The Community Earth System Model Decadal Prediction Large Ensemble (CESM-DPLE) was generated using computational resources provided by the US National Energy Research Scientific Computing Center, which is supported by the Office of Science of the US Department of Energy under Contract DE-AC02-05CH11231, as well as by an Accelerated Scientific Discovery grant for Cheyenne (https://doi.org/10.5065/D6RX99HX) that was awarded by NCAR’s Computational and Information System Laboratory.Peer ReviewedPostprint (published version

Going Beyond Sarbanes-Oxley Compliance: Five Keys to Creating Value

Discusses the factors involved in implementing Sarbanes-Oxley Act of 2002 for U.S. accounting firms. Appreciation of the goal behind the law; Comprehension of the accounting fraud; Aggressiveness in addressing ethical attitudes and rationalization

How Sales Executives Can Avoid Accounting Fraud Allegations

Is accounting fraud only a concern for CEOs and financial executives? This article discusses recent cases in which the Securities and Exchange Commission (SEC) charged Sales Vice Presidents for their role in accounting fraud. The authors offer suggestions to help sales executives steer clear of accounting fraud allegations

Angle resolved photoemission spectroscopy of Sr_2CuO_2Cl_2 - a revisit

We have investigated the lowest binding-energy electronic structure of the model cuprate Sr_2CuO_2Cl_2 using angle resolved photoemission spectroscopy (ARPES). Our data from about 80 cleavages of Sr_2CuO_2Cl_2 single crystals give a comprehensive, self-consistent picture of the nature of the first electron-removal state in this model undoped CuO_2-plane cuprate. Firstly, we show a strong dependence on the polarization of the excitation light which is understandable in the context of the matrix element governing the photoemission process, which gives a state with the symmetry of a Zhang-Rice singlet. Secondly, the strong, oscillatory dependence of the intensity of the Zhang-Rice singlet on the exciting photon-energy is shown to be consistent with interference effects connected with the periodicity of the crystal structure in the crystallographic c-direction. Thirdly, we measured the dispersion of the first electron-removal states along G->(pi,pi) and G->(pi,0), the latter being controversial in the literature, and have shown that the data are best fitted using an extended t-J-model, and extract the relevant model parameters. An analysis of the spectral weight of the first ionization states for different excitation energies within the approach used by Leung et al. (Phys. Rev. B56, 6320 (1997)) results in a strongly photon-energy dependent ratio between the coherent and incoherent spectral weight. The possible reasons for this observation and its physical implications are discussed.Comment: 10 pages, 8 figure

The Impact of Enterprise Risk Management on the Internal Audit Function

This exploratory study provides evidence about factors associated with the overall impact of enterprise risk management (ERM) on the internal audit function’s activities. Based on responses from 122 organizations in several countries, we find that ERM has the greatest impact on internal audit’s activities when (a) the organization’s ERM process is more completely in place, (b) the CFO and audit committee have called for greater internal audit activity related to ERM, (c) the chief audit executive’s (CAE) tenure is longer, (d) the organization is in the banking industry or is an educational institution, and (e) the internal audit function has provided more ERM leadership. We offer implications and future research directions

Pb0.4Bi1.6Sr2Ca1Cu2O8+xPb_{0.4}Bi_{1.6}Sr_{2}Ca_{1}Cu_{2}O_{8+x} and Oxygen Stoichiometry: Structure, Resistivity, Fermi Surface Topology and Normal State Properties

$Pb_{0.4}Bi_{1.6}Sr_2CaCu_2O_{8+x}$ ($Bi(Pb)-$2212) single crystal samples were studied using transmission electron microscopy (TEM), $ab-$plane ($\rho_{ab}$) and $c-$axis ($\rho_c$) resistivity, and high resolution angle-resolved ultraviolet photoemission spectroscopy (ARUPS). TEM reveals that the modulation in the $b-$axis for $Pb(0.4)-$doped $Bi(Pb)-$2212 is dominantly of $Pb-$type that is not sensitive to the oxygen content of the system, and the system clearly shows a structure of orthorhombic symmetry. Oxygen annealed samples exhibit a much lower $c-$axis resistivity and a resistivity minimum at $80-130$K. He-annealed samples exhibit a much higher $c-$axis resistivity and $d\rho_c/dT<0$ behavior below 300K. The Fermi surface (FS) of oxygen annealed $Bi(Pb)-$2212 mapped out by ARUPS has a pocket in the FS around the $\bar{M}$ point and exhibits orthorhombic symmetry. There are flat, parallel sections of the FS, about 60\% of the maximum possible along $k_x = k_y$, and about 30\% along $k_x = - k_y$. The wavevectors connecting the flat sections are about $0.72(\pi, \pi)$ along $k_x = k_y$, and about $0.80(\pi, \pi)$ along $k_x = - k_y$, rather than $(\pi,\pi)$. The symmetry of the near-Fermi-energy dispersing states in the normal state changes between oxygen-annealed and He-annealed samples.Comment: APS_REVTEX 3.0, 49 pages, including 11 figures, available upon request. Submitted to Phys. Rev. B

On the determination of the Fermi surface in high-Tc superconductors by angle-resolved photoemission spectroscopy

We study the normal state electronic excitations probed by angle resolved photoemission spectroscopy (ARPES) in Bi2201 and Bi2212. Our main goal is to establish explicit criteria for determining the Fermi surface from ARPES data on strongly interacting systems where sharply defined quasiparticles do not exist and the dispersion is very weak in parts of the Brillouin zone. Additional complications arise from strong matrix element variations within the zone. We present detailed results as a function of incident photon energy, and show simple experimental tests to distinguish between an intensity drop due to matrix element effects and spectral weight loss due to a Fermi crossing. We reiterate the use of polarization selection rules in disentangling the effect of umklapps due to the BiO superlattice in Bi2212. We conclude that, despite all the complications, the Fermi surface can be determined unambiguously: it is a single large hole barrel centered about (pi,pi) in both materials.Comment: Expanded discussion of symmetrization method in Section 5, figures remain the sam