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Search Results: 1 - 10 of 201225 matches for " P. Kuhn "
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Second layer nucleation and the shape of wedding cakes
J. Krug,P. Kuhn
Physics , 2001,
Abstract: The rate of second layer nucleation -- the formation of a stable nucleus on top of a two-dimensional island -- determines both the conditions for layer-by-layer growth, and the size of the top terrace of multilayer mounds in three-dimensional homoepitaxial growth. It was recently shown that conventional mean field nucleation theory overestimates the rate of second layer nucleation by a factor that is proportional to the number of times a given site is visited by an adatom during its residence time on the island. In the presence of strong step edge barriers this factor can be large, leading to a substantial error in previous attempts to experimentally determine barrier energies from the onset of second layer nucleation. In the first part of the paper simple analytic estimates of second layer nucleation rates based on a comparison of the relevant time scales will be reviewed. In the main part the theory of second layer nucleation is applied to the growth of multilayer mounds in the presence of strong but finite step edge barriers. The shape of the mounds is obtained by numerical integration of the deterministic evolution of island boundaries, supplemented by a rule for nucleation in the top layer. For thick films the shape converges to a simple scaling solution. The scaling function is parametrized by the coverage $\theta_c$ of the top layer, and takes the form of an inverse error function cut off at $\theta_c$. The surface width of a film of thickness $d$ is $\sqrt{(1- \theta_c) d}$. Finally, we show that the scaling solution can be derived also from a continuum growth equation.
Nonlinear optical response of hole-trion systems in quantum dots in tilted magnetic fields
P. Machnikowski,T. Kuhn
Physics , 2010, DOI: 10.1002/pssc.201000809
Abstract: We discuss, from a theoretical point of view, the four wave mixing spectroscopy on an ensemble of p-doped quantum dots in a magnetic field slightly tilted from the in-plane configuration. We describe the system evolution in the density matrix formalism. In the limit of coherent ultrafast optical driving, we obtain analytical formulas for the single system dynamics and for the response of an inhomogeneously broadened ensemble. The results are compared to the previously studied time-resolved Kerr rotation spectroscopy on the same system. We show that the Kerr rotation and four wave mixing spectra yield complementary information on the spin dynamics (precession and damping).
"La biblioteca mundial de Toxicología: la pagina de Espa a"
Guillermo Repetto Kuhn,P. Wexler
Revista de Toxicología , 2009,
Abstract:
Collaborative Governance for Sustainable Development in China  [PDF]
Berthold Kuhn
Open Journal of Political Science (OJPS) , 2016, DOI: 10.4236/ojps.2016.64037
Abstract: The global sustainable development agenda of the United Nations stresses governance and partnerships involving public and private actors as key elements for achieving the sustainable development goals. This paper relates the analysis of China’s growing engagement for sustainable development to the concept of collaborative governance. Based on the analysis of literature, policy documents, participation in conferences as well as interviews with experts, it proposes five factors to explain the promising developments of collaborative governance for sustainable development in China: political leadership, discourses, in-country expertise, institutional density and international cooperation. Against the backdrop of a strong government and tightened political supervision in many policy areas under the Xi Jinping administration, Chinese academics as well as practitioners largely agree that the “green development agenda” stands out in providing opportunities for the business community, think tanks and universities as well as nonprofit organisations to implement projects and gradually influence policies and practices related to the promotion of sustainable development.
Changing Spaces for Civil Society Organisations in China  [PDF]
Berthold Kuhn
Open Journal of Political Science (OJPS) , 2018, DOI: 10.4236/ojps.2018.84030
Abstract: Concerns about shrinking spaces for civil society organisations have risen in China over the past years, in particular among international nonprofit organisations. The third sector in China, however, continued its growth in numbers, diversity and activities of organisations, accounting for more than 700,000 registered organisations. Government’s financial support to nonprofits significantly increased through contracting out of services. The new Charity Law contains provisions for public fundraising activities, including for online platforms. International activities of Chinese nonprofits took off in the context of participation in global conferences. Chinese NGOs also started to engage in delivering humanitarian aid to communities in other countries. Assessments on shrinking or changing spaces for civil society in China much depend on the type of organisations in focus. This paper pays attention to discourses related to NGO development in China and sheds light on changing spaces for different types of nonprofits in China, those that are negatively affected by new regulations and different kinds of restrictions and those benefiting from emerging opportunities in the context of growing cooperation with the government or the business sector. Analysis based on interviews and talks with experts in China and abroad shows that advocacy-oriented organisations and those receiving foreign funding tend to face more difficulties. Larger international nonprofits with a long track-record in China, however, are seen to continuing or even expanding their activities.
Vacuum-assisted closure of the open abdomen in a resource-limited setting
AM Campbell, WP Kuhn, P Barker
South African Journal of Surgery , 2010,
Abstract: Aim. We describe our experience of developing a modified vacuum-assisted closure (VAC) dressing for open abdomens. Background. We see a high volume of trauma in our department. Massive delays in presentation of patients with acute abdomen are common. Closure at initial laparotomy is not possible in many cases, either because the patient has or will develop abdominal compartment syndrome, or because several re-look laparotomies will be required. A significant proportion of our patients who have undergone laparotomy therefore spend some of their stay in hospital with an open abdomen. The management of these patients is particularly labour intensive for nursing staff. The Opsite sandwich or Bogota bag invariably leaks, and sometimes needs changing daily. If a patient also has a temporary ileostomy, application can be difficult. The commercial VAC dressing is an improvement on the Opsite sandwich, but is prohibitively expensive. Financial constraints and the volume of abdominal trauma and sepsis we see mean that commercial VAC dressings for laparostomy are not affordable in our setting. Methods/results. We describe our adapted VAC dressing. It is inexpensive and easy to apply, has made a big difference in the nursing of patients with an open abdomen, and has enabled us to increase the rate of delayed primary closure (i.e. we have reduced the rate of ventral hernia). Conclusion. The modified VAC dressing is now our department’s method of choice for temporary abdominal closure.
Utilisation and outcomes of cervical cancer prevention services among HIV-infected women in Cape Town
P Batra, L Kuhn, L Denny
South African Medical Journal , 2010,
Abstract: Objective. An audit of outcomes of cervical cancer screening and prevention services for HIV-positive women in Cape Town, South Africa. Design. Retrospective review of clinic registers, patient records and pathology databases at three HIV primary health clinics and a tertiary colposcopy referral centre. Subjects. Women recently diagnosed with HIV at three primary health clinics between 2006 and 2008 (N=2 240); new patients seen for colposcopy at a tertiary referral centre between 2006 and 2009 (N=2 031). Outcome measures. The proportion of women undergoing cervical cancer screening after HIV diagnosis at primary health clinics, demographic characteristics of women referred for colposcopy at a tertiary centre, and outcomes of therapy for precancerous lesions of the cervix. Results. The proportion of women undergoing at least one Pap smear at HIV primary health clinics after HIV diagnosis was low (13.1%). Women referred for colposcopy tended to be HIV-positive and over the age of 30 years, and in most (70.2%) cytological examination revealed high-grade cervical dysplasia. HIV-positive women treated with excision for precancerous lesions of the cervix were significantly more likely than their HIV-negative counterparts to undergo incomplete excision, experience persistent cervical disease after treatment, and be lost to follow-up. Conclusion. Cervical cancer screening efforts must be scaled up for women with HIV. Treatment and surveillance guidelines for cervical intraepithelial neoplasia in HIV-positive women may need to be revised and new interventions developed to reduce incomplete treatment and patient default.
Excitons in quantum dot molecules: Coulomb coupling, spin-orbit effects and phonon-induced line broadening
J. M. Daniels,P. Machnikowski,T. Kuhn
Physics , 2013, DOI: 10.1103/PhysRevB.88.205307
Abstract: Excitonic states and the line shape of optical transitions in coupled quantum dots (quantum dot molecules) are studied theoretically. For a pair of electrically tunable, vertically aligned quantum dots we investigate the coupling between spatially direct and indirect excitons caused by different mechanisms such as tunnel coupling, Coulomb coupling, coupling due to the spin-orbit interaction and coupling induced by a structural asymmetry. The peculiarities of the different types of couplings are reflected in the appearance of either crossings or avoided crossings between direct and indirect excitons, the latter ones being directly visible in the absorption spectrum. We analyze the influence of the phonon environment on the spectrum by calculating the line shape of the various optical transitions including contributions due to both pure dephasing and phonon-induced transitions between different exciton states. The line width enhancement due to phonon-induced transitions is particularly pronounced in the region of an anticrossing and it strongly depends on the energy splitting between the two exciton branches.
Reducing decoherence of the confined exciton state in a quantum dot by pulse-sequence control
V. M. Axt,P. Machnikowski,T. Kuhn
Physics , 2004, DOI: 10.1103/PhysRevB.71.155305
Abstract: We study the phonon-induced dephasing of the exciton state in a quantum dot excited by a sequence of ultra-short pulses. We show that the multiple-pulse control leads to a considerable improvement of the coherence of the optically excited state. For a fixed control time window, the optimized pulsed control often leads to a higher degree of coherence than the control by a smooth single Gaussian pulse. The reduction of dephasing is considerable already for 2-3 pulses.
Spin Structure of the Nucleon - Status and Recent Results
Kuhn, S. E.;Chen, J. -P.;Leader, E.
High Energy Physics - Phenomenology , 2008, DOI: 10.1016/j.ppnp.2009.02.001
Abstract: After the initial discovery of the so-called "spin crisis in the parton model" in the 1980's, a large set of polarization data in deep inelastic lepton-nucleon scattering was collected at labs like SLAC, DESY and CERN. More recently, new high precision data at large x and in the resonance region have come from experiments at Jefferson Lab. These data, in combination with the earlier ones, allow us to study in detail the polarized parton densities, the Q^2 dependence of various moments of spin structure functions, the duality between deep inelastic and resonance data, and the nucleon structure in the valence quark region. Together with complementary data from HERMES, RHIC and COMPASS, we can put new limits on the flavor decomposition and the gluon contribution to the nucleon spin. In this report, we provide an overview of our present knowledge of the nucleon spin structure and give an outlook on future experiments. We focus in particular on the spin structure functions g_1 and g_2 of the nucleon and their moments.
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