537 research outputs found
Excited-state Forces within a First-principles Green's Function Formalism
We present a new first-principles formalism for calculating forces for
optically excited electronic states using the interacting Green's function
approach with the GW-Bethe Salpeter Equation method. This advance allows for
efficient computation of gradients of the excited-state Born-Oppenheimer
energy, allowing for the study of relaxation, molecular dynamics, and
photoluminescence of excited states. The approach is tested on photoexcited
carbon dioxide and ammonia molecules, and the calculations accurately describe
the excitation energies and photoinduced structural deformations.Comment: 2 figures and 2 table
Coupling of Nonlocal Potentials to Electromagnetic Fields
Nonlocal Hamiltonians are used widely in first-principles quantum
calculations; the nonlocality stems from eliminating undesired degrees of
freedom, e.g. core electrons. To date, attempts to couple nonlocal systems to
external electromagnetic (EM) fields have been heuristic or limited to weak or
long wavelength fields. Using Feynman path integrals, we derive an exact,
closed-form coupling of arbitrary EM fields to nonlocal systems. Our results
justify and clarify the couplings used to date and are essential for systematic
computation of linear and especially nonlinear response.Comment: 1 figure, 1 tabl
The Importance of Worldviews on Women’s Leadership to HRD
Problem: The challenges faced by women in leadership, to some extent, appear throughout the word, across country-based cultures and religious traditions, even where there has been progress. The eight articles that comprise this issue raise questions related to women in leadership, providing a cross-case opportunity to explore what might yet be needed to empower women in leadership roles in business, politics, non-government organizations, academia, and the family. The Solution: There are no easy solutions that emerge from our analysis across these eight articles. Worldviews influence women in leadership; from these articles, we understand the influences better and glimpse opportunities for improving the status of women leaders, globally, as well as within specific countries and religious traditions. We also suggest perspectives that might lead to valuable studies that will help/pave the way for developing future women leaders. Stakeholders: HR scholars and practitioners, potential and current women leaders, and those working with or accommodating women leaders in multiple contexts are the main stakeholders of this issue. Furthermore, because this is the concluding article to this issue, all of the stakeholders listed with each article will be interested in our overall conclusions to this issue
Preparedness planning for pandemic influenza among large US maternity hospitals
The objective of this investigation was to determine the state of pandemic influenza preparedness and to delineate commonly reported challenges among a sample of larger US national maternity hospitals. This was done given the recent emphasis on hospital disaster planning and the disproportionate morbidity and mortality that pregnant women have suffered in previous influenza pandemics. An internet-based survey was sent to all 12 members of the Council of Women's and Infants’ Specialty Hospitals. Questions addressed hospital demographics and overall pandemic preparedness planning, including presence of a pandemic planning committee and the existence of written plans addressing communications, surge capacity, degradation of services, and advance supply planning. Nine of 12 (75%) hospitals responded. All had active pandemic planning committees with identified leadership. The majority (78%) had written formal plans regarding back-up communications, surge/overflow capacity, and degradation of services. However, fewer (44%) reported having written plans in place regarding supply-line/stockpiling of resources. The most common challenges noted were staff and supply coordination, ethical distribution of limited medical resources, and coordination with government agencies. In conclusion, the majority of the Council of Women's and Infants’ Specialty Hospitals maternity hospitals have preliminary infrastructure for pandemic influenza planning, but many challenges exist to optimize maternal and fetal outcomes during the next influenza pandemic
Diameter and Chirality Dependence of Exciton Properties in Carbon Nanotubes
We calculate the diameter and chirality dependences of the binding energies,
sizes, and bright-dark splittings of excitons in semiconducting single-wall
carbon nanotubes (SWNTs). Using results and insights from {\it ab initio}
calculations, we employ a symmetry-based, variational method based on the
effective-mass and envelope-function approximations using tight-binding
wavefunctions. Binding energies and spatial extents show a leading dependence
with diameter as and , respectively, with chirality corrections
providing a spread of roughly 20% with a strong family behavior. Bright-dark
exciton splittings show a leading dependence. We provide analytical
expressions for the binding energies, sizes, and splittings that should be
useful to guide future experiments
Theory and it ab initio calculation of radiative lifetime of excitons in semiconducting carbon nanotubes
We present theoretical analysis and first-principles calculation of the
radiative lifetime of excitons in semiconducting carbon nanotubes. An intrinsic
lifetime of the order of 10 ps is computed for the lowest optically active
bright excitons. The intrinsic lifetime is however a rapid increasing function
of the exciton momentum. Moreover, the electronic structure of the nanotubes
dictates the existence of dark excitons nearby in energy to each bright
exciton. Both effects strongly influence measured lifetime. Assuming a thermal
occupation of bright and dark exciton bands, we find an effective lifetime of
the order of 10 ns at room temperature, in good accord with recent experiments.Comment: 12 pages, 3 figure
Excitonic Effects and Optical Spectra of Single-Walled Carbon Nanotubes
Many-electron effects often dramatically modify the properties of reduced
dimensional systems. We report calculations, based on an many-electron Green's
function approach, of electron-hole interaction effects on the optical spectra
of small-diameter single-walled carbon nanotubes. Excitonic effects
qualitatively alter the optical spectra of both semiconducting and metallic
tubes. Excitons are bound by ~ 1 eV in the semiconducting (8,0) tube and by ~
100 meV in the metallic (3,3) tube. These large many-electron effects explain
the discrepancies between previous theories and experiments.Comment: 6 pages, 3 figures, 2 table
Qudit versions of the qubit "pi-over-eight" gate
When visualised as an operation on the Bloch sphere, the qubit
"pi-over-eight" gate corresponds to one-eighth of a complete rotation about the
vertical axis. This simple gate often plays an important role in quantum
information theory, typically in situations for which Pauli and Clifford gates
are insufficient. Most notably, when it supplements the set of Clifford gates
then universal quantum computation can be achieved. The "pi-over-eight" gate is
the simplest example of an operation from the third level of the Clifford
hierarchy (i.e., it maps Pauli operations to Clifford operations under
conjugation). Here we derive explicit expressions for all qudit (d-level, where
d is prime) versions of this gate and analyze the resulting group structure
that is generated by these diagonal gates. This group structure differs
depending on whether the dimensionality of the qudit is two, three or greater
than three. We then discuss the geometrical relationship of these gates (and
associated states) with respect to Clifford gates and stabilizer states. We
present evidence that these gates are maximally robust to depolarizing and
phase damping noise, in complete analogy with the qubit case. Motivated by this
and other similarities we conjecture that these gates could be useful for the
task of qudit magic-state distillation and, by extension, fault-tolerant
quantum computing. Very recent, independent work by Campbell, Anwar and Browne
confirms the correctness of this intuition, and we build upon their work to
characterize noise regimes for which noisy implementations of these gates can
(or provably cannot) supplement Clifford gates to enable universal quantum
computation.Comment: Version 2 changed to reflect improved distillation routines in
arXiv:1205.3104v2. Minor typos fixed. 12 Pages,2 Figures,3 Table
Local Quantum Measurement and No-Signaling Imply Quantum Correlations
We show that, assuming that quantum mechanics holds locally, the finite speed
of information is the principle that limits all possible correlations between
distant parties to be quantum mechanical as well. Local quantum mechanics means
that a Hilbert space is assigned to each party, and then all local
positive-operator-valued measurements are (in principle) available; however,
the joint system is not necessarily described by a Hilbert space. In
particular, we do not assume the tensor product formalism between the joint
systems. Our result shows that if any experiment would give nonlocal
correlations beyond quantum mechanics, quantum theory would be invalidated even
locally.Comment: Published version. 5 pages, 1 figure
Selection Rules for One- and Two-Photon Absorption by Excitons in Carbon Nanotubes
Recent optical absorption/emission experiments showed that the lower energy
optical transitions in carbon nanotubes are excitonic in nature, as predicted
by theory. These experiments were based on the symmetry aspects of free
electron-hole states and bound excitonic states. The present work shows,
however, that group theory does not predict the selection rules needed to
explain the two photon experiments. We obtain the symmetries and selection
rules for the optical transitions of excitons in single-wall carbon nanotubes
within the approach of the group of the wavevector, thus providing important
information for the interpretation of theoretical and experimental optical
spectra of these materials.Comment: 4 pages, 1 figure, 1 tabl
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