671 research outputs found
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 (the Hubbard interaction 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 , 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
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
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.MÄlet med den hÀr rapporten var att undersöka möjligheten att vidare rena spillvattnet pÄ Dows anlÀggning i Landskrona. Dow tillverkar vattenbaserade bindemedel frÄn akryl monomerer. Spillvattnet innehÄller metalliska joner, sÄpor, restmonomerer och restpolymerer. Idag sÄ renas spillvattnet med ultrafiltrering (UF) dÀr permeatet slÀpps ivÀg till det kommunala reningsverket för vidare rening. Eftersom reningsverket Àr hÄrt belastat sÄ Àr Dow intresserade av att undersöka möjligheten att rena sitt processvatten pÄ anlÀggningen. Den hÀr studien har undersökt om nanofiltrering (NF) kan anvÀndas för att producera ett permeat som Àr rent nog att ÄteranvÀnda i processen. PÄ grund av rÄdande vattenbrist i Sverige och runtom i vÀrlden sÄ Àr det viktigt att vara proaktiv inom reningen av spillvatten. Fem olika NF och omvÀnd osmos (RO) membran undersöktes vid en första screening av plattmembran: NF99HF frÄn Alfa Laval, AP frÄn GE, NF270, NF90 och XLE frÄn Dow Filmtec. UF-permeatet frÄn Dow i Landskrona anvÀndes som feed till NF. InnehÄllet i UF och NF-permeaten undersöktes genom att mÀta total organic carbon (TOC) och konduktiviteten. Skillnaden i TOC och konduktivitet bestÀmde retentionen för de olika membranen. PÄverkan av transmembrantryck (TMP) och tvÀrströmshastigheten (CFV) studerades för att bestÀmma de optimala driftsförhÄllandena. FrÄn de studerade membranen sÄ valdes NF90 för vidare studie med ett spiralmembran pÄ grund av dess goda retention och vanliga förekomst inom industrin för rening av industriellt spillvatten. PÄ grund av att Dow varierar vilka produkter de producerar sÄ varierar innehÄllet UF-permeatet vÀldigt mycket. DÀrför mÄste NF membranen klara av att hantera det UF-permeat med högst koncentration av TOC och konduktivitet om allt NF-permeat ska gÄ att ÄteranvÀnda i processen. Trots det varierande innehÄllet i UF-permeatet sÄ lyckades NF producera ett permeat med lÀgre koncentrationer av TOC och konduktivitet Àn stadsvattnet vid flera tillfÀllen. Det bevisar att NF-membranen Àr kapabla att producera ett NF-permeat som kan ÄteranvÀndas i processen. Vid tvÄ tillfÀllen sÄ innehöll UF-permeatet mycket höga koncentrationer av TOC, konduktivitet och oorganiskt kol (IC). Detta fick till följd att koncentrationerna Àven var höga i NF-permeatet och inte lÄg nÀra de lÄga koncentrationerna i stadsvattnet. Vidare studier bör göras om detta gÄr att undvika om Dow vill gÄ vidare med NF. Problemet med fouling gick inte att lösa under studiens gÄng dÄ olika koncentrationer och olika tvÀttmedel inte hade nÄgon pÄverkan pÄ 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 mÄste göras i en större skala innan Dow bestÀmmer sig för om de ska investera i NF.Kan nanofiltrering vara en möjlighet för Dows processvattenrening? DÄ reningsverket redan idag Àr hÄrt belastat sÄ vill Dow undersöka möjligheten att sköta sin processvattenrening internt pÄ anlÀggningen. Den hÀr studien har undersökt om nanofiltrering Àr en möjlighet för att Ästadkomma detta
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
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