315 research outputs found
Spectral properties of Luttinger liquids: A comparative analysis of regular, helical, and spiral Luttinger liquids
We provide analytic expressions for the Green's functions in
position-frequency space as well as for the tunneling density of states of
various Luttinger liquids at zero temperature: the standard spinless and
spinful Luttinger liquids, the helical Luttinger liquid at the edge of a
topological insulator, and the Luttinger liquid that appears either together
with an ordering transition of nuclear spins in a one-dimensional conductor, or
in spin-orbit split quantum wires in an external magnetic field. The latter
system is often used to mimic a helical Luttinger liquid, yet we show here that
it exhibits significantly different response functions and, to discriminate, we
call it the spiral Luttinger liquid. We give fully analytic results for the
tunneling density of state of all the Luttinger liquids as well as for most of
the Green's functions. The remaining Green's functions are expressed by simple
convolution integrals between analytic results.Comment: 17 pages, 4 figures; corresponding to published versio
Magnetic Ordering of Nuclear Spins in an Interacting 2D Electron Gas
We investigate the magnetic behavior of nuclear spins embedded in a 2D
interacting electron gas using a Kondo lattice model description. We derive an
effective magnetic Hamiltonian for the nuclear spins which is of the RKKY type
and where the interactions between the nuclear spins are strongly modified by
the electron-electron interactions. We show that the nuclear magnetic ordering
at finite temperature relies on the (anomalous) behavior of the 2D static
electron spin susceptibility, and thus provides a connection between
low-dimensional magnetism and non-analyticities in interacting 2D electron
systems. Using various perturbative and non-perturbative approximation schemes
in order to establish the general shape of the electron spin susceptibility as
function of its wave vector, we show that the nuclear spins locally order
ferromagnetically, and that this ordering can become global in certain regimes
of interest. We demonstrate that the associated Curie temperature for the
nuclear system increases with the electron-electron interactions up to the
millikelvin range.Comment: 18 pages, 2 figures, final versio
Controlled Radical Polymerization of Vinyl Acetate Mediated by a Bis(imino)pyridine Vanadium Complex
Source type: Prin
Synthesis of Amphiphilic Tadpole-Shaped Linear-Cyclic Diblock Copolymers via Ring-Opening Polymerization Directly Initiating from Cyclic Precursors and Their Application as Drug Nanocarriers
Diminishing catalyst concentration in atom transfer radical polymerization with reducing agents
The concept of initiators for continuous activator regeneration (ICAR) in atom transfer radical polymerization (ATRP) is introduced, whereby a constant source of organic free radicals works to regenerate the Cu(I) activator, which is otherwise consumed in termination reactions when used at very low concentrations. With this technique, controlled synthesis of polystyrene and poly(methyl methacrylate) (M(w)/M(n) < 1.2) can be implemented with catalyst concentrations between 10 and 50 ppm, where its removal or recycling would be unwarranted for many applications. Additionally, various organic reducing agents (derivatives of hydrazine and phenol) are used to continuously regenerate the Cu(I) activator in activators regenerated by electron transfer (ARGET) ATRP. Controlled polymer synthesis of acrylates (M(w)/M(n) < 1.2) is realized with catalyst concentrations as low as 50 ppm. The rational selection of suitable Cu complexing ligands {tris[2-(dimethylamino)ethyl]amine (Me(6)TREN) and tris[(2-pyridyl)methyl]amine (TPMA)} is discussed in regards to specific side reactions in each technique (i.e., complex dissociation, acid evolution, and reducing agent complexation). Additionally, mechanistic studies and kinetic modeling are used to optimize each system. The performance of the selected catalysts/reducing agents in homo and block (co)polymerizations is evaluated
Integrating theory, synthesis, spectroscopy and device efficiency to design and characterize donor materials for organic photovoltaics : a case study including 12 donors
The remarkable improvements in the power conversion efficiency of solution-processable Organic Photovoltaics (OPV) have largely been driven by the development of novel narrow bandgap copolymer donors comprising an electron-donating (D) and an electron-withdrawing (A) group within the repeat unit. Given the large pool of potential D and A units and the laborious processes of chemical synthesis and device optimization, progress on new high efficiency materials can, and has been, slow with a few new efficient copolymers reported every year despite the large number of groups pursuing these materials. In this paper we present an integrated approach toward new narrow bandgap copolymers that uses theory to guide the selection of materials to be synthesized based on their predicted energy levels, and time-resolved microwave conductivity (TRMC) to select the best-performing copolymer–fullerene bulk heterojunction to be incorporated into complete OPV devices. We validate our methodology by using a diverse group of 12 copolymers, including new and literature materials, to demonstrate good correlation between (a) theoretically determined energy levels of polymers and experimentally determined ionization energies and electron affinities and (b) photoconductance, measured by TRMC, and OPV device performance. The materials used here also allow us to explore whether further copolymer design rules need to be incorporated into our methodology for materials selection. For example, we explore the effect of the enthalpy change (ΔH) during exciton dissociation on the efficiency of free charge carrier generation and device efficiency and find that ΔH of −0.4 eV is sufficient for efficient charge generation.12 page(s
Close Packing of Nitroxide Radicals in Stable Organic Radical Polymeric Materials
The relationship between the polymer
network and electronic transport
properties for stable radical polymeric materials has come under investigation
owing to their potential application in electronic devices. For the
radical polymer poly(2,2,6,6-tetramethylpiperidine-4-yl-1-oxyl methacrylate),
it is unclear whether the radical packing is optimal for charge transport
partially because the relationship between radical packing and molecular
structure is not well-understood. Using the paramagnetic nitroxide
radical as a probe of the polymer and synthetic techniques to control
the radical concentration on the methyl methacrylate backbone, we
investigate the dependence of radical concentration on molecular structure.
The electron paramagnetic resonance data indicate that radicals in
the PTMA assume a closest approach distance to each other when more
than 60% of the backbone is populated with radical pendant groups.
Below 60% coverage, the polymer rearranges to accommodate larger radical–radical
spacing. These findings are consistent with theoretical calculations
and help explain some experimentally determined electron-transport
properties
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