The interplay of local electron correlations and ultrafast spin dynamics in fcc Ni

The complex electronic structure of metallic ferromagnets is determined by a balance between exchange interaction, electron hopping leading to band formation, and local Coulomb repulsion. The interplay between the respective terms of the Hamiltonian is of fundamental interest, since it produces most, if not all, of the exotic phenomena observed in the solid state. By combining high energy and temporal resolution in femtosecond time-resolved X-ray absorption spectroscopy with ab initio time-dependent density functional theory we analyze the electronic structure in fcc Ni on the time scale of these interactions in a pump-probe experiment. We distinguish transient broadening and energy shifts in the absorption spectra, which we demonstrate to be caused by electron repopulation and correlation-induced modifications of the electronic structure, respectively. Importantly, the theoretical description of this experimental result hence requires to take the local Coulomb interaction into account, revealing a temporal interplay between band formation, exchange interaction, and Coulomb repulsion

Megahertz-rate ultrafast X-ray scattering and holographic imaging at the European XFEL

The advent of X-ray free-electron lasers (XFELs) has revolutionized fundamental science, from atomic to condensed matter physics, from chemistry to biology, giving researchers access to X-rays with unprecedented brightness, coherence and pulse duration. All XFEL facilities built until recently provided X-ray pulses at a relatively low repetition rate, with limited data statistics. Here, results from the first megahertz-repetition-rate X-ray scattering experiments at the Spectroscopy and Coherent Scattering (SCS) instrument of the European XFEL are presented. The experimental capabilities that the SCS instrument offers, resulting from the operation at megahertz repetition rates and the availability of the novel DSSC 2D imaging detector, are illustrated. Time-resolved magnetic X-ray scattering and holographic imaging experiments in solid state samples were chosen as representative, providing an ideal test-bed for operation at megahertz rates. Our results are relevant and applicable to any other non-destructive XFEL experiments in the soft X-ray range

Observation of fluctuation-mediated picosecond nucleation of a topological phase

peer reviewedTopological states of matter exhibit fascinating physics combined with an intrinsic stability. A key challenge is the fast creation of topological phases, which requires massive reorientation of charge or spin degrees of freedom. Here we report the picosecond emergence of an extended topological phase that comprises many magnetic skyrmions. The nucleation of this phase, followed in real time via single-shot soft X-ray scattering after infrared laser excitation, is mediated by a transient topological fluctuation state. This state is enabled by the presence of a time-reversal symmetry-breaking perpendicular magnetic field and exists for less than 300 ps. Atomistic simulations indicate that the fluctuation state largely reduces the topological energy barrier and thereby enables the observed rapid and homogeneous nucleation of the skyrmion phase. These observations provide fundamental insights into the nature of topological phase transitions, and suggest a path towards ultrafast topological switching in a wide variety of materials through intermediate fluctuating states. © 2020, The Author(s), under exclusive licence to Springer Nature Limited.Leibniz Association Grant no. K162/2018 (OptiSPIN

EPIDEMIOLOGY OF MEASLES IN THE CENTRAL REGION OF GHANA: A FIVE-YEAR CASE REVIEW IN THREE DISTRICT HOSPITALS

ABSTRACTObjective: As part of a national accelerated campaign to eliminate measles, we conducteda study, to define the epidemiology of measles in the central region.Design: A descriptive survey, was carried out on retrospective cases of measles.Setting: Patients were drawn from the three district hospitals (Assin, Asikuma andWinneba hospitals) with the highest number of reported cases in the region.Subjects: Records of outpatient and inpatient measles patients attending the selectedhealth facilities between 1996 and 2000. Data on reported measles cases in all healthfacilities in the three study, districts were also analysed.Main outcome measures: The distribution of measles cases in person (age and sex), time(weekly, or monthly, trends) and place (residence), the relative frequency, of cases, andthe outcome of treatment.Results: There was an overall decline in reported cases of measles between 1996 and2000 both in absolute terms and relative to other diseases. Females constituted 48%-52% of the reported 1508 cases in the hospitals. The median age of patients was 36months. Eleven percent of cases were aged under nine months; 66% under five yearsand 96% under 15 years. With some minor variations between districts, the highestand lowest transmission occurred in March and September respectively. Withinhospitals, there were sporadic outbreaks with up to 34 weekly cases.Conclusion: In Ghana, children aged nine months to 14 years could be appropriately,targeted for supplementary, measles immunization campaigns. The best period for thecampaigns is during the low transmission months of August to October. Retrospectivesurveillance can expediently inform decisions about the timing and target age groupsfor such campaigns

Epidemiology of measles in the central region of Ghana: a five-year case review in three district hospitals

Objective: As part of a national accelerated campaign to eliminate measles, we conducted a study, to define the epidemiology of measles in the central region. Design: A descriptive survey, was carried out on retrospective cases of measles. Setting: Patients were drawn from the three district hospitals (Assin, Asikuma and Winneba hospitals) with the highest number of reported cases in the region. Subjects: Records of outpatient and inpatient measles patients attending the selected health facilities between 1996 and 2000. Data on reported measles cases in all health facilities in the three study, districts were also analysed. Main outcome measures: The distribution of measles cases in person (age and sex), time (weekly, or monthly, trends) and place (residence), the relative frequency, of cases, and the outcome of treatment. Results: There was an overall decline in reported cases of measles between 1996 and 2000 both in absolute terms and relative to other diseases. Females constituted 48%- 52% of the reported 1508 cases in the hospitals. The median age of patients was 36 months. Eleven percent of cases were aged under nine months; 66% under five years and 96% under 15 years. With some minor variations between districts, the highest and lowest transmission occurred in March and September respectively. Within hospitals, there were sporadic outbreaks with up to 34 weekly cases. Conclusion: In Ghana, children aged nine months to 14 years could be appropriately, targeted for supplementary, measles immunization campaigns. The best period for the campaigns is during the low transmission months of August to October. Retrospective surveillance can expediently inform decisions about the timing and target age groups for such campaigns. (East African Medical Journal: 2003 80(6): 312-317