Research Outline
Condensed Matter Physics
The research is concentrated on:
• studies of poly
merization and doping of fullerenes and carbon nanotubes under high pressures, primarily to study the basic properties of these new materials but always with an eye open on future applications,
• studies of the relaxation and transport properties of glasses and disordered materials by dielectric and thermal methods,
• studies of the high-pressure properties of energy-related materials, such as hydrides of interest for future hydrogen storage applications,
• high pressure studies of layered porous materials (graphite oxide)
• hydrogen storage in solid materials, measurements of hydrogen adsorption in nanomaterials,
• hydrogenation of nanomaterials at high pressure of hydrogen gas and high temperatures, e.g. Metal Organic Framework materials and nanocarbons
• nanomagnetism in carbon-based nanomaterials, and
• studies of nanomaterials by Atomic/Electrostatic/Magnetic force microscopy.
Over the last ten years our research on carbon nanostructures has been quite successful, and in particular we have been advancing the research front in the area of fullerene polymers, which have been created and studied under high pressures. Work still continues on the basic properties of fullerene compounds and alkali metal doped fullerenes, as well as empty and filled carbon nanotubes and modified graphite materials (e.g. graphite oxide). We also have a long tradition in studies of amorphous materials under high pressure, in particular water and amorphous ice, both of which are currently “hot” international research topics. Hydrogen storage materials are also of large current interest. We have a running program on hydrogenation of carbon materials and the structural properties of alkali metal hydrides at high pressures. A separate project is related to measurements of hydrogen adsorption in nanoporous materials: organic framework materials, clathrates etc.
Organic Electronics
The Organ
ic Photonics & Electronics Group investigates organic (small molecule and polymer) compounds and designs, fabricates and characterizes photonic and electronic devices based on such optimized materials. The current devices under study include light-emitting electrochemical cells (LECs) and field-effect transistors (FETs).
Within the LEC project, we have been able to demonstrate planar devices with extremely large (mm-sized) inter-electrode gaps that emit bright light at low voltage (3-5 V; which is remarkably close to the band-gap potential of the organic polymer). The image to the right visualizes the initial operation of such a planar LEC, specificially the doping formation and progression and the subsequent light emission, with amazing detail. We have made use of this uniquely acquired information for the design of improved devices, and our group was recently able to demonstrate a vertical "sandwich cell" LECs with a record-long lifetime of 1000 h at high brightness.
Within the FET project, our current focus is on the development and efficient fabrication of functional transistors with fullerenes (carbon-cage molecules) as the active material. We are developing a — by us pioneered — photo-induced and resist-free imprinting method, which allows for fast, simple and high-resolution patterning of fullerene films with retained electronic properties. We have recently demonstrated that the method is capable of patterning well-defined micrometer-sized features in a solution-processed fullerene film, and that the patterned film can form the basis for an array of fully functional micrometer-sized transistors.
New Techniques for Ultra-Sensitive Detection of Atoms and Molecules
The Laser Physics Group has activities within the field of “Development of New Techniques for Ultra-Sensitive Detection of Atoms and Molecules”. The long term goal is to develop techniques for sensitive detection of environmentally hazardous and toxic species in gas phase and for trace species analysis. Techniques under development are Wavelength Modulation Diode Laser Atomic Spectrometry (WM-DLAS or TDLAS), Noise Immune Cavity Enhanced Optical Heterodyne Modulation Spectrometry NICE-OHMS),and Ultra-Violet DLAS (UV-DLAS). The first field encompasses, among other things, investigations of the possibility to perform multi-species determination by the use of a new type of widely tunable diode lasers. Of special importance is simultaneous detection of CO, CO2, and H2O at high temperatures for characterization of combustion processes. The NICE-OHMS technique is based upon a combination of an external cavity (for increased sensitivity), frequency modulation (for reduced influence of noise), and Doppler free spectroscopy (for increased selectivity), which gives the technique a number of unique properties. The technique has, for example, previously demonstrated a detection sensitivity down to astonishing 10-13. These properties make the NICE-OHMS technique a very promising candidate for becoming a technique of the future with a superior detectability for detection of a variety of toxic gases. The main aim of our work is to develop NICE-OHMS to a robust and user-friendly technique with superior detectability. Our pilot species are acetylene (C2H2) and ammonia (NH3). In the UV-DLAS project, work is being pursued to detect nitric oxide (NO) down to low ppb levels by direct UV-DLAS absorption and WM-UV-DLAS. A common theme in these two projects is to address the environmentally important problem of how to detect the presence of primarily NO to regulate existing NOx reduction processes (incorporating NH3) at various combustion sites worldwide.
Optical Manipulation of Biological Objects
The Laser Physics Group also has activities in the field of "Development and Application of Techniques for Non-intrusive Manipulation of Micrometer-sized Biological Objects, Mainly Optical Tweezers. The optical tweezers system is nowadays mainly developed for, and is used for, force measurements in biological systems. The system is used on a regular basis with a measurement range that spans from sub-pN to hundreds of pN. Our force measuring optical tweezers instrumentations have lately been used to characterize the mechanical properties of various types of pili (micrometer long adhesion "tentacles"), primarily P pili, on E. coli, causing urinary tract infections and pyelonephritis. It has been found, among other things, that P pili elongate in three different regions, one elastic, one in which the quaternary structure of the PapA rod of the P Pili unfolds under a constant unfolding force of 27 pN, and one "s"-shaped region in which the linearized PapA rod is elongated further. It has also been found that the unfolding of the PapA rod is elastic, and not plastic, as previously has been hypothesized in the literature. Also dynamic studies of the elongation properties of pili under strain have been done. Additional information about the bio-physical properties of the pili and processes involved is gained by comparisons of the data and various types of bio-physical models of pili elongation under strain/stress, built upon kinetic models of the bond opening and closure and incorporating the effects of entropy as well as sequential and random bond opening, all developed in house. Further investigations of the mechanical and adhesive properties of various types of bacterial pili are under way. The properties of the various pili are also related to their various surroundings in which they exist. The optical tweezers are also used for other purposes, e.g. for characterization of the effects of pilicides. This activity is a part of Umeå Centre for Microbial Research (UCMR).
Theory of Ultra-Cold Atoms
The objective of the research is to develop theoretical .jpg)
models and run numerical simulations to study atoms at ultra-low temperatures (which, in practice, means hundreds of milli-kelvins down to nanokelvins). Topics include laser cooling, motion of atoms in optical lattices, atom-atom collisions, molecule formation, manipulation of quantum information, and quantum phase transitions. Aspects of Bose-Einstein condensates such as vortices and collective modes are studied numerically and analytically. Our research aims at a better under¬standing of the quantum mechanical nature of matter at ultra-low temperatures, as well as extending the methods for manipulation and control of atoms. We collaborate with the local experimental group, as well as with several international groups.
Plasmas and Fluids
The research in classical plasmas and fluids is focused on:
• Non-linear wave coupling phenomena: Analytical studies of non-linear waves in non-uniform fluids and plasmas.
• Non-linear surface waves. In most plasmas there exist transition layers with sharp gradients that separates regions with different values of various background parameters as density, temperature, magnetic field. The amplitudes of waves entering a transition layer can increase significantly, leading to non-linear modifications of the background variables. We investigate such phenomena.
• Freak waves. Large amplitude waves in deep water are studied by means of coupled nonlinear Schrödinger equations. Our results should be useful for understanding the formation and nonlinear propagation characteristics of large amplitude freak waves.
Nonlinear Collective Quantum Electrodynamics and Quantum Plasmas
Photon-photon scattering is a non-classical effect arising in quantum electrodynamics (QED) due to virtual electron-positron pairs in vacuum. Effectively, the interaction between photons and these virtual pairs will result in what is known as photon-photon collisions. Formulated as an effective field theory, using the Heisenberg—Euler Lagrangian, this results in nonlinear corrections to Maxwell's vacuum equations, which to lowest order in the fine structure constant takes the form of cubic nonlinearities in the electromagnetic field. The propagation of photons in strong background electromagnetic fields is non-trivial, and may cause strange effects such as photon splitting and vacuum birefringence. The group is currently studying possible detection techniques, as well as physical implications, of the effects of photon-photon scattering. Quantum effects due to the electromagnetic fields described by QED are often accompanied by quantum effects due to the particles, which are studied by means of quantum kinetic and fluid equations for the plasma particles. This have important applications in the developing field of plasmonics, that utilizes so called surface plasmon polaritons (SPP:s) as a means to send fast signals in nanoscale components, promising to constitute the next generation of electronic devices.
Recently we have also developed quantum models including the spin effects in the fluid equations, which typically have been omitted in previous works. The spin properties are important for low temperature plasmas as well as for strongly magnetized plasmas. Furthermore, it has been shown that certain quantum effects due to the electron spin can survive also in moderate temperature plasmas, that has traditionally been considered as classical.
General Relativity
One project of the group is global solutions for rotating 
stars, i.e., solutions of Einstein's equations describing the gravitational field both inside and outside the star, properly matched on the surface of the star.
The group also works on a method for construction of solutions to the field equations in terms of invariant objects (corresponding to directly measurable objects). The main application is perturbative calculations in cosmology, where we study questions like how much can one tell about the metric of the universe from observations.
Another project is the interaction of gravitational waves with plasmas, which is of relevance for accretion disks in the vicinity of black holes and also for the analysis of data obtained with gravitational wave detectors, like the proposed LISA (Laser Interferometer Space Antenna).
Theory of Flames, Turbulance, Explosions
Flames and related hydrodynamic phenomena are well-known in many physical processes from industrial energy production to stars and Supernovae. The most important applications of combustion are car engines and gas turbines. The Flame Dynamics Group works mostly on hydrodynamic aspects of combustion and focus on flame instabilities and flame interaction with the gas flow, turbulent or laminar. The instabilities and turbulence make a flame front corrugated, thus increasing greatly the flame speed and the rate of energy production. Our research includes also flame interaction with acoustic and shock waves, the possibility of detonation triggering and many other gas-dynamic and combustion phenomena.
Statistical Physics and Jamming
There are many systems where the solid phase is about as disordered as a liquid. One common example is a glass and another example is ordinary sand which can behave as a fluid (flow) or as a solid (support the weight of a car) with no clear change in structure. In some of these systems the approach to jamming is governed by the lowering of the temperature whereas in others it is controlled by the increase in density. In spite of such differences it has been suggested that the behaviour of many such systems could be described by a common theory, and that this is related to a certain "critical point".
To study this jamming transition we are examining a simple model with a number of disks within a rectangular frame. The disks are of different sizes to avoid crystallization. We examine what happens if the frame is sheared with a given (usually very low) shear rate. We are interested both in the shear stress that opposes the motion and in the motion of the particles. Not surprisingly, the behaviour of this system depends on the density of disks. At low densities the shear stress is weak, and it grows when the density of particles increases.
Our approach is to perform computer simulations on such a model. One advantage with a computer simulation before a real experiment is that we have total control over all parameters. Our simulations give good evidence that the jamming transition is a "critical phenomenon"; the rapid increase of the shear viscosity as the density increases is related to a growing dynamic correlation length.
Quantum Physics
The advancement in device manufacturing has entered a phase where quantum mechanics has become fundamental for understanding the device functioning. The development was originally driven by miniaturization of transistors in order to accommodate an increasing number of functions on a single chip. This has lead to the emergence of the importance of studying systems on the length scale where a system exhibits quantum coherence, the mesoscopic scale, which is inter¬mediate between the macroscopic and atomic scales. Present day technology makes it possible to construct structures atom by atom leading to completely new possibilities for tailor-made nano-scale devices behaviour of which is dominated by the laws of quantum mechanics.
The group studies physics on nano-scales where the quantum effects are of primary importance. The research deals with general questions of quantum transport and quantum coherence, as well as mechanisms of decoherence in realistic systems interacting with the environment. We study quantum statistics and quantum noise, for instance, we study the full counting statistics of charge transfers in a quantum point contact. We maintain close contacts with the Cold Atoms group and work on physics of ultra-cold atoms and Bose-Einstein condensates.
One of the directions of research in the group is related to the emerging quantum information technology. A quantum computer has the potential of massive parallelism, and will be capable of executing computational tasks which cannot be performed by any classical computer, however fast. Our research is concerned with physics of readout of quantum bits and in a broader aspect, with the problem of quantum measurement.
Complex Networks
The whole is greater than the sum of its parts — it is the sum of its parts plus the interactions. In our interdisciplinary research, we use tools from statistical mechanics to understand how macroscopic phenomena emerge from interactions between the units of the system on the microscopic level. In the interface between physics and ecology, the units can be species and the interactions can be trophic (who eats whom); in the interface between physics and cell biology, the units can be proteins and the interactions can say whether the genes of proteins regulate each other; in the interface between physics and epidemiology, the units are individuals and the interactions represent possibilities of disease transmission; and so on. In many systems the interactions take place in a network that is neither random nor completely structured. From such complex networks, we can extract information about the systems’ behaviour and function.
In collaborative research across the life sciences, we map and model real world networks to better understand the feedback between dynamics on the network and the system’s evolution to, for example, develop protocols to vaccinate a population efficiently, better understand signalling in protein networks, identify drug-targets in biochemical networks, and simplify and highlight important structures and their evolution in large systems such as citation networks, disease networks, and the World Wide Web.
Space Physics
The presence of charged particles (a plasma) together with electric and magnetic fields in space makes this region a very fascinating and challenging world to explore. The interaction between the Earth´s magnetic field and the solar wind (a stream of charged particles dragging the sun's magnetic field with it) creates a cavity surrounding the Earth known as the magnetosphere. Analogously, magnetospheres also arise around other magnetized planets (e.g. Jupiter). Moreover, similar cavities, so called induced magnetospheres, also appear around non-magnetized planets such as Mars and Venus.
The colourful northern lights (auroras), often seen in Umeå, are visible signs of the many complex processes that take place high above our heads in the Earth's magnetosphere. Corresponding auroral phenomena also appear in other planetary magnetospheres, for example at Mars and Jupiter.
The Space Physics Group is trying to describe and explain what is taking place within the magnetospheres of the Earth and other Earth-like planets. To achieve this it is often necessary to combine theoretical work with computer simulations and analysis of satellite data.
Physics Education
The group in Physics Education works with research questions relevant to physics education in universities and upper secondary schools. The research is focused on student active teaching methods, especially group discussions in problem solving and how these affect the students' interest in physics. The gender issue is important. Another area of interest is the students' experience and learning during laboratory work in physics.
Only students that have taken some courses in a teacher education are admitted to Master´s projects in physics directed towards physics education.
