Actual SNSF projects
Formative assessment might help to find a better match between instructions and student needs. We have developed a model of formative assessment based on five key strategies defined by Paul Black and Dylan Wiliam. Our model encompasses 4 different tools: Clicker sessions are teaching activities that elicit evidence of student learning and activate students as instructional resources, a monitoring tool that shows students the actual state of their knowledge from their point of view, a diagnostic tool that provides feedback to the student about their knowledge from an independent resource and finally a reflective lesson that allows students to work on their deficits and to catch up.
The goal of our work is to foster concept knowledge in physics. Thus we have developed a project that uses formative assessment in a 15 lessons course of kinematics to improve learning of physics concepts in Swiss gymnasiums. We have designed the four tools described above for the formative assessment and a kinematic test, which can be used as Pre- , Post- and Follow-up Test in order to measure the learning gain. In total 31 teachers were recruited and about 800 students participated in the study. The learning gain in concept knowledge of the experimental group was then compared to two control groups, a traditional teaching group and a frequent testing, which solves the same test as the experimental group but without the formative assessment approach. The results confirm our hypothesis that the experimental group outperforms the two control groups regarding concept knowledge and that there is no difference between all three groups with respect to a conventional test.
For most electronic applications, the charge of the electron is used to manipulate information. So far, the electron's spin is only used in magnetic sensors and read heads of hard disks. Concepts for spintronic devices have been proposed, for example solid-state memory devices based on ferromagnetic junctions, as well as more exotic devices like spin transistors, and domain wall logic devices. In general, these devices have only little technological impact as it is difficult to generate large spin current densities in solids and the spin diffusion length is short. In addition, it is not clear how fast such a device could be operated.
The investigation of ultrafast magneto-dynamical processes leads to novel femtosecond spin current sources. The magnetization of a ferromagnet can be reduced within a time scale of less than one picosecond if exposed to a femtosecond laser pulse. This is surprising, as angular momentum must be transferred from the spin system to the lattice, a process expected to be slower. Recently, a novel transfer process was discovered: In pump-probe experiments, the temperature gradient created by the pump laser on the surface of the sample can generate a spin current, which is sufficiently large to demagnetize the ferromagnet.
Here, we propose to investigate the formation and dynamics of laser generated spin currents and to utilize these currents to manipulate the magnetization of another ferromagnet. We intend to perform three different classes of experiments: 1. In order to quantify the laser induced spin currents we intend to perform all-optical transport experiments. By optical means, we will independently change the average electronic temperature across a magnetic film as well as the temperature gradient. The gradient causes spin currents whereas the average temperature causes demagnetization through spin-orbit coupling and phonon generation. 2. Optical experiments described above provide information about the total spin current. There is a debate, if the spin current is purely diffusive or if there is a super-diffusive component. We intend to answer this question by a spin, energy and time resolved photoemission experiment. 3. The spin currents caused by ultrafast demagnetization are expected to be significantly larger than achievable by electronic means (for example in spin valve structures). We propose a sample architecture, in which the magnetization of a ferromagnetic layer can be manipulated with a laser-induced spin current.
Our proposal aims for bringing the ultrafast time scale to the field of spintronics. It is possible that spintronics on short time scales will play a role in future devices for information processing and storage. One such concept already exists: The next generation of hard disks will employ heat assisted magnetic recording in order to combine long data retention times, short writing times and high storage densities. The proposed experiments will enhance our understanding on spin transport and ultrafast magnetism. In addition, it will offer the opportunity for young scientists to get exposed to ultrafast science and technology. They may become future users of large femtosecond lasers and free electron laser facilities like the SwissFEL.
Completed SNSF projects
In most computer storage devices information is stored by locally changing the orientation of the magnetization. Due to increased demands on storage capacity and data storage rates thermally assisted data writing processes are currently investigated. This new technique requires a fast and local temperature increase in the magnetic storage material. This can potentially be achieved by heating a storage media locally with an ultra short laser pulse. However a short laser pulse does not couple directly to the magnetization of a sample. Till this day the fundamental processes which occur during laser pulse heating are not understood. Resolving the questions around ultrafast energy transfer to a magnetic system represents one of the most fundamental challenges in solid state physics.
The present project addresses the question of ultrafast heating by using new experimental capabilities which were made available by the construction of free electron laser systems. We use the short pulses of such free electron lasers in order to monitor the ultrafast processes which occur during laser pulse heating of a solid. This is done by measuring energy, time and spin resolved photoemission spectra.
With the help of these experiments we hope to clarify the origin of ultrafast processes which were observed but not explained before.