Dielectric screening in doped Fullerides

For conventional superconductors the electron-electron interaction is strongly reduced by retardation effects, making the formation of Cooper pairs possible. In the alkali-doped Fullerides, however, there are no strong retardation effects. But dielectric screening can reduce the electron-electron interaction sufficiently, if we assume that the random-phase approximation (RPA) is valid. It is not clear, however, if this assumption holds, since the alkali-doped Fullerides are strongly correlated systems close to a Mott transition. To test the validity of the RPA for these systems we have calculated the screening of a test charge using quantum Monte Carlo.Comment: 4 pages, 1 eps figure included; to be published in the proceedings of the International Winterschool on Electronic Properties of Novel Materials, Kirchberg/Tirol, 1998; additional information is available at http://www.mpi-stuttgart.mpg.de/docs/ANDERSEN/fullerene

Metal-Insulator transitions in generalized Hubbard models

We study the Mott transition in Hubbard models with a degenerate band on different 3-dimensional lattices. While for a non-degenerate band only the half-filled system may exhibit a Mott transition, with degeneracy there can be a transition for any integer filling. We analyze the filling dependence of the Mott transition and find that $U_c$ (the Hubbard interaction $U$ at which the transition takes place) decreases away from half-filling. In addition we can change the lattice structure of the model. This allows us to study the influence of frustration on the Mott transition. We find that frustration increases $U_c$, compared to bipartite systems. The results were obtained from fixed-node diffusion Monte Carlo calculations using trial functions which allow us to systematically vary the magnetic character of the system. To gain a qualitative understanding of the results, we have developed simple hopping arguments that help to rationalize the doping dependence and the influence of frustration on the Mott transition. Choosing the model parameters to describe the doped Fullerides, we can make contact with experiment and understand why some of the Fullerides are metals, while others, which according to density functional theory should also be metallic, actually are insulators.Comment: 4 pages LaTeX with 4 eps figures; submitted to Computer Physics Communications, Proceedings of the CPP'99/Centennial Meeting, Atlanta, GA; additional material available at http://www.mpi-stuttgart.mpg.de/docs/ANDERSEN/fullerene

Nanofiltration of ultrafiltration permeate fom chemical wastewater

The aim of this report was to investigate the possibility of further cleaning of the wastewater at Dow in Landskrona. Dow produces water based adhesives from acrylic monomers. The wastewater contains different metal ions, surfactants, residual monomers and polymers. The wastewater at Dow is treated with ultrafiltration (UF) which produces a permeate that is sent to the municipal waste water treatment plant (WWTP). The municipal WWTP is heavily burdened and it might not be long until they start refusing to treat the wastewater from Dow. This study is a proactive investigation of nanofiltration (NF) as a method to produce a permeate clean enough to be reused in the process at Dow. Being proactive in wastewater treatment is getting more and more important, especially with the current water shortage in Sweden and many other places in the world. Five NF and reverse osmosis (RO) membranes were investigated in the initial screening of flat sheet membranes: NF99HF from Alfa Laval, AP from GE, NF270, NF90 and XLE from Dow Filmtec. The UF permeate from Dow was used as feed for the NF. The total organic carbon (TOC) and conductivity was measured in the feed, the UF permeate, and the NF permeates to determine the retention of the membranes. The influence of transmembrane pressure (TMP) and crossflow velocity (CFV) was studied for all membranes to determine the optimal operation conditions. From the studied membranes, NF90 was chosen for further studying in a spiral wound membrane due to the good retention and common use in industrial wastewater treatment. The contents of the UF permeate varies a lot due to the varying production schedule at Dow. That means that the NF membranes must be equipped to handle the worst-case scenario at Dow, if all the NF permeate is to be reused in the process. Despite the varying content of the UF permeate a NF permeate with concentrations of TOC and conductivity lower than that of the city water in Landskrona has been produced on several occasions. This is proof that NF membranes are capable of producing a permeate clean enough to be reused in the process. However, on two occasions the UF permeate from Dow contained high concentrations of TOC, conductivity and inorganic carbon (IC). The concentrations of the produced NF permeate were nowhere near the low concentrations of the city water. Further investigations should be made if this can be avoided if Dow wants to proceed with the NF system. The fouling issue could not be resolved as it seemed that no combinations and concentrations of cleaning detergent could remove the fouling caused by the filtration. The fouling should be of focus in further studies. NF seems to be a promising method to further clean the wastewater but further studies in a larger scale need to be done before deciding on implementing it at Dow.Målet med den här rapporten var att undersöka möjligheten att vidare rena spillvattnet på Dows anläggning i Landskrona. Dow tillverkar vattenbaserade bindemedel från akryl monomerer. Spillvattnet innehåller metalliska joner, såpor, restmonomerer och restpolymerer. Idag så renas spillvattnet med ultrafiltrering (UF) där permeatet släpps iväg till det kommunala reningsverket för vidare rening. Eftersom reningsverket är hårt belastat så är Dow intresserade av att undersöka möjligheten att rena sitt processvatten på anläggningen. Den här studien har undersökt om nanofiltrering (NF) kan användas för att producera ett permeat som är rent nog att återanvända i processen. På grund av rådande vattenbrist i Sverige och runtom i världen så är det viktigt att vara proaktiv inom reningen av spillvatten. Fem olika NF och omvänd osmos (RO) membran undersöktes vid en första screening av plattmembran: NF99HF från Alfa Laval, AP från GE, NF270, NF90 och XLE från Dow Filmtec. UF-permeatet från Dow i Landskrona användes som feed till NF. Innehållet i UF och NF-permeaten undersöktes genom att mäta total organic carbon (TOC) och konduktiviteten. Skillnaden i TOC och konduktivitet bestämde retentionen för de olika membranen. Påverkan av transmembrantryck (TMP) och tvärströmshastigheten (CFV) studerades för att bestämma de optimala driftsförhållandena. Från de studerade membranen så valdes NF90 för vidare studie med ett spiralmembran på grund av dess goda retention och vanliga förekomst inom industrin för rening av industriellt spillvatten. På grund av att Dow varierar vilka produkter de producerar så varierar innehållet UF-permeatet väldigt mycket. Därför måste NF membranen klara av att hantera det UF-permeat med högst koncentration av TOC och konduktivitet om allt NF-permeat ska gå att återanvända i processen. Trots det varierande innehållet i UF-permeatet så lyckades NF producera ett permeat med lägre koncentrationer av TOC och konduktivitet än stadsvattnet vid flera tillfällen. Det bevisar att NF-membranen är kapabla att producera ett NF-permeat som kan återanvändas i processen. Vid två tillfällen så innehöll UF-permeatet mycket höga koncentrationer av TOC, konduktivitet och oorganiskt kol (IC). Detta fick till följd att koncentrationerna även var höga i NF-permeatet och inte låg nära de låga koncentrationerna i stadsvattnet. Vidare studier bör göras om detta går att undvika om Dow vill gå vidare med NF. Problemet med fouling gick inte att lösa under studiens gång då olika koncentrationer och olika tvättmedel inte hade någon påverkan på foulingen som uppstod av filtreringen. Foulingen borde vara i fokus i en framtida studie. NF verkar vara en lovande metod för att rena Dows processvatten men vidare studier måste göras i en större skala innan Dow bestämmer sig för om de ska investera i NF.Kan nanofiltrering vara en möjlighet för Dows processvattenrening? Då reningsverket redan idag är hårt belastat så vill Dow undersöka möjligheten att sköta sin processvattenrening internt på anläggningen. Den här studien har undersökt om nanofiltrering är en möjlighet för att åstadkomma detta

Filling dependence of the Mott transition in the degenerate Hubbard model

Describing the doped Fullerenes using a generalized Hubbard model, we study the Mott transition for different integer fillings of the t_1u band. We use the opening of the energy-gap E_g as a criterion for the transition. E_g is calculated as a function of the on-site Coulomb interaction U using fixed-node diffusion Monte Carlo. We find that for systems with doping away from half-filling the Mott transitions occurs at smaller U than for the half-filled system. We give a simple model for the doping dependence of the Mott transition.Comment: 7 pages RevTeX with 10 eps figures, additional material available at http://www.mpi-stuttgart.mpg.de/docs/ANDERSEN/fullerene

Efficient Security Protocols for Constrained Devices

During the last decades, more and more devices have been connected to the Internet.Today, there are more devices connected to the Internet than humans.An increasingly more common type of devices are cyber-physical devices.A device that interacts with its environment is called a cyber-physical device.Sensors that measure their environment and actuators that alter the physical environment are both cyber-physical devices.Devices connected to the Internet risk being compromised by threat actors such as hackers.Cyber-physical devices have become a preferred target for threat actors since the consequence of an intrusion disrupting or destroying a cyber-physical system can be severe.Cyber attacks against power and energy infrastructure have caused significant disruptions in recent years.Many cyber-physical devices are categorized as constrained devices.A constrained device is characterized by one or more of the following limitations: limited memory, a less powerful CPU, or a limited communication interface.Many constrained devices are also powered by a battery or energy harvesting, which limits the available energy budget.Devices must be efficient to make the most of the limited resources.Mitigating cyber attacks is a complex task, requiring technical and organizational measures.Constrained cyber-physical devices require efficient security mechanisms to avoid overloading the systems limited resources.In this thesis, we present research on efficient security protocols for constrained cyber-physical devices.We have implemented and evaluated two state-of-the-art protocols, OSCORE and Group OSCORE.These protocols allow end-to-end protection of CoAP messages in the presence of untrusted proxies.Next, we have performed a formal protocol verification of WirelessHART, a protocol for communications in an industrial control systems setting.In our work, we present a novel attack against the protocol.We have developed a novel architecture for industrial control systems utilizing the Digital Twin concept.Using a state synchronization protocol, we propagate state changes between the digital and physical twins.The Digital Twin can then monitor and manage devices.We have also designed a protocol for secure ownership transfer of constrained wireless devices. Our protocol allows the owner of a wireless sensor network to transfer control of the devices to a new owner.With a formal protocol verification, we can guarantee the security of both the old and new owners.Lastly, we have developed an efficient Private Stream Aggregation (PSA) protocol.PSA allows devices to send encrypted measurements to an aggregator.The aggregator can combine the encrypted measurements and calculate the decrypted sum of the measurements.No party will learn the measurement except the device that generated it

Mott Transition in Degenerate Hubbard Models: Application to Doped Fullerenes

The Mott-Hubbard transition is studied for a Hubbard model with orbital degeneracy N, using a diffusion Monte-Carlo method. Based on general arguments, we conjecture that the Mott-Hubbard transition takes place for U/W \propto \sqrt{N}, where U is the Coulomb interaction and W is the band width. This is supported by exact diagonalization and Monte-Carlo calculations. Realistic parameters for the doped fullerenes lead to the conclusion that stoichiometric A_3 C_60 (A=K, Rb) are near the Mott-Hubbard transition, in a correlated metallic state.Comment: 4 pages, revtex, 1 eps figure included, to be published in Phys.Rev.B Rapid Com

Mott-Hubbard insulators for systems with orbital degeneracy

We study how the electron hopping reduces the Mott-Hubbard band gap in the limit of a large Coulomb interaction U and as a function of the orbital degeneracy N. The results support the conclusion that the hopping contribution grows as roughly \sqrt{N}W, where W is the one-particle band width, but in certain models a crossover to a \sim NW behavior is found for a sufficiently large N.Comment: 7 pages, revtex, 6 figures more information at http://www.mpi-stuttgart.mpg.de/dokumente/andersen/fullerene