380 research outputs found
Size-Dependent Surface Plasmon Dynamics in Metal Nanoparticles
We study the effect of Coulomb correlations on the ultrafast optical dynamics
of small metal particles. We demonstrate that a surface-induced dynamical
screening of the electron-electron interactions leads to quasiparticle
scattering with collective surface excitations. In noble-metal nanoparticles,
it results in an interband resonant scattering of d-holes with surface
plasmons. We show that this size-dependent many-body effect manifests itself in
the differential absorption dynamics for frequencies close to the surface
plasmon resonance. In particular, our self-consistent calculations reveal a
strong frequency dependence of the relaxation, in agreement with recent
femtosecond pump-probe experiments.Comment: 8 pages + 4 figures, final version accepted to PR
The role of occupied d states in the relaxation of hot electrons in Au
We present first-principles calculations of electron-electron scattering
rates of low-energy electrons in Au. Our full band-structure calculations
indicate that a major contribution from occupied d states participating in the
screening of electron-electron interactions yields lifetimes of electrons in Au
with energies of above the Fermi level that are larger than
those of electrons in a free-electron gas by a factor of . This
prediction is in agreement with a recent experimental study of ultrafast
electron dynamics in Au(111) films (J. Cao {\it et al}, Phys. Rev. B {\bf 58},
10948 (1998)), where electron transport has been shown to play a minor role in
the measured lifetimes of hot electrons in this material.Comment: 4 pages, 2 figures, to appear in Phys. Rev.
Electron-lattice relaxation, and soliton structures and their interactions in polyenes
Density matrix renormalisation group calculations of a suitably parametrised
model of long polyenes (polyacetylene oligomers), which incorporates both long
range Coulomb interactions and adiabatic lattice relaxation, are presented. The
triplet and 2Ag states are found to have a 2-soliton and 4-soliton form,
respectively, both with large relaxation energies. The 1Bu state forms an
exciton-polaron and has a very small relaxation energy. The relaxed energy of
the 2Ag state lies below that of the 1Bu state. The soliton/anti-soliton pairs
are bound.Comment: RevTeX, 5 pages, 4 eps figures included using epsf. To appear in
Physical Review Letters. Fig. 1 fixed u
Excited states of linear polyenes
We present density matrix renormalisation group calculations of the Pariser-
Parr-Pople-Peierls model of linear polyenes within the adiabatic approximation.
We calculate the vertical and relaxed transition energies, and relaxed
geometries for various excitations on long chains. The triplet (3Bu+) and even-
parity singlet (2Ag+) states have a 2-soliton and 4-soliton form, respectively,
both with large relaxation energies. The dipole-allowed (1Bu-) state forms an
exciton-polaron and has a very small relaxation energy. The relaxed energy of
the 2Ag+ state lies below that of the 1Bu- state. We observe an attraction
between the soliton-antisoliton pairs in the 2Ag+ state. The calculated
excitation energies agree well with the observed values for polyene oligomers;
the agreement with polyacetylene thin films is less good, and we comment on the
possible sources of the discrepencies. The photoinduced absorption is
interpreted. The spin-spin correlation function shows that the unpaired spins
coincide with the geometrical soliton positions. We study the roles of
electron-electron interactions and electron-lattice coupling in determining the
excitation energies and soliton structures. The electronic interactions play
the key role in determining the ground state dimerisation and the excited state
transition energies.Comment: LaTeX, 15 pages, 9 figure
From policy to practice: exploring practitioners' perspectives on social enterprise policy claims
Tuning ultrafast electron thermalization pathways in a van der Waals heterostructure
Ultrafast electron thermalization - the process leading to Auger
recombination, carrier multiplication via impact ionization and hot carrier
luminescence - occurs when optically excited electrons in a material undergo
rapid electron-electron scattering to redistribute excess energy and reach
electronic thermal equilibrium. Due to extremely short time and length scales,
the measurement and manipulation of electron thermalization in nanoscale
devices remains challenging even with the most advanced ultrafast laser
techniques. Here, we overcome this challenge by leveraging the atomic thinness
of two-dimensional van der Waals (vdW) materials in order to introduce a highly
tunable electron transfer pathway that directly competes with electron
thermalization. We realize this scheme in a graphene-boron nitride-graphene
(G-BN-G) vdW heterostructure, through which optically excited carriers are
transported from one graphene layer to the other. By applying an interlayer
bias voltage or varying the excitation photon energy, interlayer carrier
transport can be controlled to occur faster or slower than the intralayer
scattering events, thus effectively tuning the electron thermalization pathways
in graphene. Our findings, which demonstrate a novel means to probe and
directly modulate electron energy transport in nanoscale materials, represent
an important step toward designing and implementing novel optoelectronic and
energy-harvesting devices with tailored microscopic properties.Comment: Accepted to Nature Physic
E‐transparency and government budgetary corruption: A social marketing and transformation case from Nigeria
This article shows how Information and Communication Technology (ICT), incorporating social media, can lead to accountability and transparency in a government's budget. Specifically, it examined how a Non-Governmental Organization (NGO) used ICT to foster citizenship engagement in the Nigerian government budgetary process. The article, using abductive reasoning, presents four citizen empowerment stages and four social marketing transition stages through which government budget transparency can be improved and corruption reduced. A model was also inferred that can help lessen the exclusivity around the government budget to encourage dialog and openness around the government budget in similar contexts. Furthermore, this article shows that the social transformative role for NGOs using ICT to increase government budget transparency and reduce corruption is a process that happens over time
The functional capacity and quality of life of women with advanced breast cancer
The rehabilitation needs of patients with metastatic breast cancer (MBC) are poorly studied. The primary aim of the study was to evaluate the functional capacity of women with MBC and quality of life (QoL). The present study is an open, non-randomized, prospective cross-sectional observation study. The functional capacity of 128 MBC patients with ongoing cancer treatments, were studied in Helsinki University Hospital (HUS): Peak expiratory flow (PEF), dynamic and static balance, 6 minute walking distance (6MWD), 10 meter walking, sit-to-stand test, repeated squat, grip strength, shoulder movement, pain, and QoL by Beck's depression scale (BDI), health assessment questionnaire (HAQ), RAND SF-36 and EORTC QLQ-30 items. The walking capacity was compromised in half and the strength of the lower extremities in one-third of the patients. PEF was below the normal reference in 55 %, static balance in 62 % and dynamic balance in 73 % (= 61 year olds). The grip power was lowered in 44/30 % of the patients (right/left) and the shoulder movement was restricted in 30 %. Some disability in physical functioning experienced 55 % (HAQ) and 37 % felt depressive (BDI). The QoL (RAND SF-36) was poor especially in the field of physical, role and social functioning and bodily pain (<0.001). Pain, depression, and a poor 6MWD results independently determined the physical component of QoL (p <0.001). The functional capacity of patients with MBC was significantly lowered. This, in association with distressing symptoms like pain and depression causes a vicious circle further leading to functional disabilities and impaired QoL.Peer reviewe
Validation of the PHQ-9 as a screening instrument for depression in diabetes patients in specialized outpatient clinics
Background. For the treatment of depression in diabetes patients, it is important that depression is recognized at an early stage. A screening method for depression is the patient health questionnaire (PHQ-9). The aim of this study is to validate the 9-item Patient Health Questionnaire (PHQ-9) as a screening instrument for depression in diabetes patients in outpatient clinics. Methods. 197 diabetes patients from outpatient clinics in the Netherlands filled in the PHQ-9. Within 2 weeks they were approached for an interview with the Mini Neuropsychiatric Interview. DSM-IV diagnoses of Major Depressive Disorder (MDD) were the criterion for which the sensitivity, specificity, positive- and negative predictive values and Receiver Operator Curves (ROC) for the PHQ-9 were calculated. Results. The cut-off point of a summed score of 12 on the PHQ-9 resulted in a sensitivity of 75.7% and a specificity of 80.0%. Predictive values for negative and positive test results were respectively 93.4% and 46.7%. The ROC showed an area under the curve of 0.77. Conclusions. The PHQ-9 proved to be an efficient and well-received screening instrument for MDD in this sample of diabetes patients in a specialized outpatient clinic. The higher cut-off point of 12 that was needed and somewhat lower sensitivity than had been reported elsewhere may be due to the fact that the patients from a specialized diabetes clinic have more severe pathology and more complications, which could be recognized by the PHQ-9 as depression symptoms, while instead being diabetes symptom
Intravenous immunoglobulin (IVIg) dampens neuronal toll-like receptor-mediated responses in ischemia
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