The Exotic Barium Bismuthates

We review the remarkable properties, including superconductivity, charge-density-wave ordering, and metal-insulator transitions, of lead- and potassium-doped barium bismuthate. We discuss some of the early theoretical studies of these systems. Our recent theoretical work, on the negative-U\/, extended-Hubbard model for these systems, is also described. Both the large- and intermediate-U\/ regimes of this model are examined, using mean-field and random-phase approximations, particularly with a view to fitting various experimental properties of these bismuthates. On the basis of our studies, we point out possibilities for exotic physics in these systems. We also emphasize the different consequences of electronic and phonon-mediated mechanisms for the negative U.\/ We show that, for an electronic mechanism, the \secin \,\,phases of these bismuthates must be unique, with their transport properties {\it dominated by charge $\pm 2e$ Cooperon bound states}. This can explain the observed difference between the optical and transport gaps. We propose other experimental tests for this novel mechanism of charge transport and comment on the effects of disorder.Comment: UUencoded LaTex file, 122 pages, figures available on request To appear in Int. J. Mod. Phys. B as a review articl

Investigations of weak and dilute magnetic behaviour in organic and inorganic systems

Muon spin relaxation is an ideal tool with which to study dilute magnetic systems, coupled with tailored bulk magnetic susceptibility measurements it is possible to examine previously unobserved magnetic exchange interactions. Investigations into non-stoichiometric LaCo03 reveals evidence of magnetic excitons in transition metal oxides for the first time. Moreover, data is presented that supports the concept of the defect driven excitons interacting with the stoichiometric LaCo03 which is known to undergo a thermally driven spin state transition. The data suggest the occurrence of more than one possible magnetic interaction of the excitons. Hole doped La1-xSrxCo03 is of interest as it is known to be magnetically and electronically phase separated; by a direct analogy with magnetic excitons it is suggested that the Sr rich ferromagnetic clusters interact with the pure LaCo03 below the metal insulator transition (x = 0.18 ) . It is suggested that it is this interaction observed for the first time that enables the rich phase diagram of La1_xSrxCo03 . The persistent photoconductivity effect on the spin glass transition in the elilute magnetic semiconductor Ccl0.85Mn0.15 Te:In has been investigated using low temperature magnetic susceptibility measurements and for the first time muon spectroscopy. Muon measurements on an Al eloped sample clearly show the spin glass transition, however the presence of the DX centre, which causes PPC when doping with In donors, perturbs the muon response. Particular attention is paid to possibility of the DX centre trapping muonium and preventing the detection of the spin glass transition. PPC does not induce a change in the muon response, however continuous illumination of the sample allows the observation of the spin glass transition, suggesting the presence of multiple DX centres, moreover the centre is found to be diamagnetic. The search for magnetic ordering at room temperature in an organic material has generally neglected polymers. PANiCNQ combines a fully conjugated nitrogen containing backbone with molecular charge transfer side groups. This combination gives rise to a stable polymer with a high density of localised spins, which are expected to give rise to coupling. Magnetic measurements suggest that the polymer is ferri- or ferro- magnetic with a Curie temperature of over 350 K, and a maximum saturation magnetization of 0.1 JT-1 kg-1 . Magnetic force microscopy images support this picture of room temperature magnetic order by providing evidence for domain wall formation and motion

Inductive activation of magnetite filled shape memory polymers

Thermally activated shape memory polymers are a desirable material for use in dynamic structures due to their large strain recovery, light weight, and tunable activation. The addition of ferromagnetic susceptor particles to a polymer matrix provides the ability to heat volumetrically and remotely via induction. Here, remote induction heating of magnetite filler particles dispersed in a thermoset matrix is used to activate shape memory polymer as both solid and foam composites. Bulk material properties and performance are characterized and compared over a range of filler parameters, induction parameters, and packaging configurations. Magnetite filler particles are investigated over a range of power input, in order to understand the effects of particle size and shape on heat generation and flux into the matrix. This investigation successfully activates shape memory polymers in 10 to 20 seconds, with no significant impact of filler particles up to 10wt% on mechanical properties of shape memory foam. Performance of different particle materials is dependent upon the amplitude of the driving magnetic field. There is a general improvement in heating performance for increased content of filler particles. Characterization indicates that heat transfer between the filler nanoparticles and the foam is the primary constraint in improved heating performance. The use of smaller, acicular particles as one way to improve heat transfer, by increasing interfacial area between filler and matrix, is further examined.M.S.Committee Chair: Garmestani, Hamid; Committee Member: Gall, Ken; Committee Member: Thadhani, Nares

Properties of Molecules in Weak and Strong Magnetic Fields

Magnetic fields alter the properties of molecules, affecting the electron distribution, the electron configuration and the molecular geometry. In weak magnetic fields, the changes are subtle. Electrons as charged particles placed in magnetic field start following specific pathways, giving rise to magnetically induced ring currents. They follow the contour of the molecule, as well as form vortices around certain molecular rings and chemical bonds. Strong ring currents arise near atomic nuclei due to the core electrons. Magnetically induced currents are a unique fingerprint of the molecular structure but they also serve as an indicator for electron delocalisation, aromatic properties and applicability in optoelectronics. Various organic molecules were investigated using the gauge-including magnetically-induced current density approach. It has been demonstrated that heteroatoms alter the ring-current pathways and the current strength, and thereby affect molecular aromaticity.The topology of Möbius systems has been shown to depend both on the twist of the molecular rings of a series of [40]annulenes, as well as on their spatial folding (writhe). The investigation of a series of toroidal carbon nanotubes showed helical current flow in one of the chiral molecules in the study, which is a pre-requisite for the generation of anapole moment when the molecule is placed in a magnetic field. Very strong magnetic fields beyond achievable on Earth cause major changes in the electron configuration of atoms and molecules. Orbitals with high angular momentum and high-spin configurations become lower in energy than the typical zero-field occupation. Weak magnetic fields can be studied as a perturbation to the zero-field Hamiltonian. However, as the field strength increases, the magnetic interaction becomes equally strong as the electrostatic one. The explicit treatment of the magnetic field strength involves the angular momentum operator in the \schr, thus leading to complex orbitals. Therefore, new quantum chemistry software is necessary. A benchmark study for the performance of a traditional implementation based on Gaussian-type orbitals versus a fully numerical code has been done at the \acl{hf} level. After determining the accuracy of the method, small hydrocarbon molecules have been investigated, which showed that they exist as bound molecules in high-spin configurations where only the core electrons of the carbon atom are paired.Kuten tiedämme, magneetti vetää rautaesineitä puoleensa. Magneettien avulla kiinnitetään lappuja jääkaapin oveen, suljetaan kaappien ovet ja älypuhelimen kotelo. Lääkärit tutkivat potilaita vahvan magneettikentän avulla, magnetisoitunut neula kompassissa osoittaa pohjoiseen, ja tietokoneen kovalevy lukee sille talletetut tiedot magneetin avulla. Näiden ilmiöiden salaisuus piilee elektronien ja magneettikentän välisissä vuorovaikutuksissa. Molekyylitasolla magnetismi aikaansaa elektroniliikkeen molekyylin ympäri. Elektronit kiertävät myös tiettyjä atomiryhmiä renkaanmuotoisilla poluilla. Koska jokaisella molekyylillä on omanlainen elektronijakauma, niin näitä elektronipolkuja tutkimalla saadaan tietoja molekyylin ominaisuuksista. Polut kertovat mm. molekyylin soveltuvuudesta aurinkokenno- ja akkukäyttöön. Väitöskirjassa on tutkittu erilaisia orgaanisia molekyylejä sekä toroidimaisia – eli renkaankaltaisia – hiilinanoputkia. Laitetta, joka suoraan pystyisi mittaamaan elektroniliikettä magneettikentässä ei ainakaan vielä ole olemassa, joten tutkimus on suoritettu teoreettisen mallinnuksen avulla, kvanttikemiallisia menetelmiä käyttäen. Laboratoriossa valmistetut magneetit voivat olla jopa miljoona kertaa maapallon omaa magneettikenttää voimakkaampia. Sellaista ainetta, joka kestäisi sitä valtavaa voimaa, jolla vahva magneettikenttä vaikuttaa kappaleeseen, ei ole olemassa. Maailmankaikkeudesta, tiettyjen tähtien läheisyydessä, löytyy kuitenkin jopa miljardikertaisesti vahvempia magneettikenttiä. Elämänsä loppuvaiheessa tähti voi kutistua pieneksi, erittäin tiheäksi kappaleeksi – niin sanotuksi valkoiseksi kääpiöksi. Mikäli alkuperäinen tähti on ollut riittävän iso, lopuksi jää kappale, joka on niin tiheä, että atomitkin hajoavat. Tällaista taivaankappaletta kutsutaan neutronitähdeksi. Erittäin vahva magneettikenttä aiheuttaa huomattavia muutoksia molekyylien elektronirakenteissa, mikä puolestaan johtaa uusiin ja pääosin arvaamattomiin ominaisuuksiin. Näiden ominaisuuksien tutkiminen onkin väitöskirjan toinen aihe. Perinteiset kvanttikemian ohjelmistot eivät pysty mallintamaan magneettikentän aiheuttamia muutoksia elektronirakenteessa. Väitöskirjassa tutkittiin uudentyyppisten ohjelmistojen tarkkuutta vahvassa magneettikentässä olevien molekyylien mallinnuksessa. Tutkimuksen kohteena oli pienten molekyylien elektronikonfiguraatio, geometria ja sidosten vahvuus; ominaisuuksia, joita ei aikaisemmin juurikaan ole tutkittu

Investigation of toroidal magnetic moments in Dy-based molecular nanomagnets.

Molecular nanomagnets which incorporate a triangle of exchange coupled, magnetic Dy ions can support toroidal magnetic moments. In this work, a Al2Dy3 molecule is investigated with regards to magnetic susceptibility, magnetization curves, and inelastic neutron scattering (INS) spectra. The available experimental data for magnetic susceptibility and magnetization curves are fitted and simulated using full and effective Hamiltonian models in order to extract the model parameters, especially the tilt angles of the anisotropy axes. The inelastic neutron scattering spectra are simulated in order to develop experimental schemes for directly observing toroidal magnetic moments

A 2D rhomboidal system of manganese(II) [Mn(3-MeC6H4COO)2(H2O)2]n with spin canting: rationalization of the magnetic exchange

The crystal structure of Mn(II) carboxylate with 3-methylbenzoate as a bridging ligand [Mn(3-MeC6H4COO)2(H2O)2]n shows a rhomboidal layer, where each pair of neighbor Mn(II) ions are bridged through only one carboxylate group with a syn-anti conformation. The magnetic exchange between neighbor ions is weakly antiferromagnetic (J = −0.52 cm−1, g = 2.04), and at low temperature the system shows spin canting with TB = 3.8 K. Computational studies, based on periodic calculations of the energies of the significant spin states on the magnetic cell and some higher supercells, corroborate the weak AF interaction between the adjacent Mn(II) ions and preclude the negligible effect of frustration caused by very weak interactions between the non-adjacent ions in the magnetic response of the system. The results provide compelling evidence that the observed spin canting is due to the local coordination geometry of the manganese ions leading to two antiferromagnetically coupled subnets with different axial vectors