Femtosecond electrons probing currents and atomic structure in nanomaterials

The investigation of ultrafast electronic and structural dynamics in low-dimensional systems like nanowires and two-dimensional materials requires femtosecond probes providing high spatial resolution and strong interaction with small volume samples. Low-energy electrons exhibit large scattering cross sections and high sensitivity to electric fields, but their pronounced dispersion during propagation in vacuum so far prevented their use as femtosecond probe pulses in time-resolved experiments. Employing a laser-triggered point-like source of either divergent or collimated electron wave packets, we developed a hybrid approach for femtosecond point projection microscopy and femtosecond low-energy electron diffraction. We investigate ultrafast electric currents in nanowires with sub-100 femtosecond temporal and few 10 nm spatial resolutions and demonstrate the potential of our approach for studying structural dynamics in crystalline single-layer materials.Comment: 18 pages, 4 figures, includes 8 pages supplementary informatio

Menschenrechtsverletzungen, Unternehmensverantwortung und Transparenz entlang der Lieferkette

Die vorliegende Studie untersucht, wie deutsche Unternehmen im Kupferbereich über die Herkunft ihrer Rohstoffe sowie über ihre Menschenrechtsstandards berichten. Deutschland spielt als international führende Exportnation eine gewichtige Rolle, wenn betroffene Gemeinden und Nichtregierungsorganisationen (NROs) weltweit erhebliche Menschenrechtsverletzungen beim Abbau von Rohstoffen vermelden. Dabei geht es hauptsächlich um Verletzungen von Umwelt- und Sozialstandards, gewaltsame Vertreibungen der Bevölkerung, Kriminalisierung und Unterdrückung sozialer Proteste gegen Rohstoffprojekte oder Korruption (Jäger 2015). Im Global Human Rights Violations Business Index, der von der University of Maastricht erstellt wird und die Anzahl von Menschenrechtsvorwürfen gegen Unternehmen der jeweiligen Länder erfasst, rangiert Deutschland derzeit auf Platz fünf. Die meisten der im Index aufgelisteten Menschenrechtsverstöße, in die deutsche Unternehmen verwickelt sind, werden demzufolge in den Lieferketten des Rohstoffimports und dort vor allem im Abbau dokumentiert (Euractiv 2015). In dieser Studie steht der Rohstoff Kupfer im Mittelpunkt, bei dessen Abbau es immer wieder zu Verletzungen von Menschenrechten und Umweltstandards kommt. Das betrifft auch jene Länder, aus denen Deutschland Kupfer importiert

Competition for the ANC: dominant party losing youth and poorer sections of South African population

South Africa’s governing party, the African National Congress (ANC), will elect a new president in December 2017. This person will also stand for the office of President of South Africa at the beginning of 2019 when President Jacob Zuma reaches the maximum number of two consecutive terms allowed by the country’s constitution. It is currently unclear whether he can finish his final term. Since the dismissal of finance minister, Pravin Gordhan, resistance against Zuma has reached a new climax. A broad alliance of civil society groups, trade unions and party representatives, even some from within his own party, have called for his resignation. Although these protests are aimed directly at Zuma, the causes of the dissatisfaction lie deeper. The ANC has not been able to successfully address the country’s social challenges. In addition, corruption and mismanagement have shaken confidence. For a long time, the former liberation movement was considered the only party to vote for by the black population. In the past four years, however, the ANC has faced serious competition from the Economic Freedom Fighters (EFF). Given the loss of meaning of the ANC, the South African party system is changing. (author's abstract

Properties of tug-of-war model for cargo transport by molecular motors

Molecular motors are essential components for the biophysical functions of the cell. Our current quantitative understanding of how multiple motors move along a single track is not complete; even though models and theories for single motor chemomechanics abound. Recently, M.J.I. M$\ddot{\rm u}$ller {\em et al.} have developed a tug-of-war model to describe the bidirectional movement of the cargo (PNAS(2008) 105(12) P4609-4614). Through Monte Carlo simulations, they discovered that the tug-of-war model exhibits several qualitative different motility regimes, which depend on the precise value of single motor parameters, and they suggested the sensitivity can be used by a cell to regulate its cargo traffic. In the present paper, we carry out a thorough analysis of the tug-of-war model. All the stable, i.e., biophysically observable, steady states are obtained. Depending on several parameters, the system exhibits either uni-, bi- or tristability. Based on the separating boundary of the different stable states and the initial numbers of the different motor species that are bound to the track, the steady state of the cargo movement can be predicted, and consequently the steady state velocity can be obtained. It is found that, the velocity, even the direction, of the cargo movement change with the initial numbers of the motors which are bound to the track and several other parameters

Tug-of-war as a cooperative mechanism for bidirectional cargo transport by molecular motors

Intracellular transport is based on molecular motors that pull cargos along cytoskeletal filaments. One motor species always moves in one direction, e.g. conventional kinesin moves to the microtubule plus end, while cytoplasmic dynein moves to the microtubule minus end. However, many cellular cargos are observed to move bidirectionally, involving both plus-end and minus-end directed motors. The presumably simplest mechanism for such bidirectional transport is provided by a tug-of-war between the two motor species. This mechanism is studied theoretically using the load-dependent transport properties of individual motors as measured in single-molecule experiments. In contrast to previous expectations, such a tug-of-war is found to be highly cooperative and to exhibit seven different motility regimes depending on the precise values of the single motor parameters. The sensitivity of the transport process to small parameter changes can be used by the cell to regulate its cargo traffic.Comment: 17 pages, latex, 11 figures, 4 tables, includes Supporting Informatio

Mozambique still at risk: despite the peace process, a serious crisis looms

In early August 2019 the president of Mozambique and the leader of the largest oppo­si­tion party signed a new peace agreement. This has revived the peace process between the Mozambican National Resistance (RENAMO) and the Mozambique Liberation Front (FRELIMO), which has been in power since 1994. Great challenges remain, such as the disarmament and reintegration of RENAMO fighters. Furthermore, new trouble spots have emerged: Since October 2017, a wave of vio­lence has cost the lives of well over 300 people in Cabo Delgado Province. Although "Islamic State" (IS) has claimed responsibility for some of the attacks, the motives and structures of the group responsible remain unclear. Its occurrence points to pro­found social cleavages and alienation between the population and the political elite. At the same time, the north of Mozambique has become a hub for the illicit economy. Criminal transactions are above all symptoms of state neglect and extensive impunity. Experience from other conflict regions shows that this constellation can have fatal consequences. For this reason, international actors including the German government should press for rapid and far-reaching measures that go beyond the official peace process. (author's abstract

Sense of frustration: the debate on land reform in South Africa

In December 2018, the ruling African National Congress (ANC) and the opposition party, Economic Freedom Fighters (EFF), agreed to draft an amendment to the con­stitution in the South African Parliament. Its intention is to give concrete form to existing options to expropriate land without compensation. The narrative of land reform dis­cussions in South Africa often creates the impression that the expropriation of land owned by white farmers without compensation could solve the country's problem of unequal income distribution. It would, however, take a whole set of politi­cal reforms to create more social justice. Visible successes might help appease those groups that are dis­appointed with South African democracy 25 years after the end of apartheid, but if the reforms fail then this will likely exacerbate the already palpable sense of frustration felt by ordinary South Africans. (Autorenreferat