155 research outputs found
Charge Order in the Falicov-Kimball Model
We examine the spinless one-dimensional Falicov-Kimball model (FKM) below
half-filling, addressing both the binary alloy and valence transition
interpretations of the model. Using a non-perturbative technique, we derive an
effective Hamiltonian for the occupation of the localized orbitals, providing a
comprehensive description of charge order in the FKM. In particular, we uncover
the contradictory ordering roles of the forward-scattering and backscattering
itinerant electrons: the latter are responsible for the crystalline phases,
while the former produces the phase separation. We find an Ising model
describes the transition between the phase separated state and the crystalline
phases; for weak-coupling we present the critical line equation, finding
excellent agreement with numerical results. We consider several extensions of
the FKM that preserve the classical nature of the localized states. We also
investigate a parallel between the FKM and the Kondo lattice model, suggesting
a close relationship based upon the similar orthogonality catastrophe physics
of the associated single-impurity models.Comment: 39 pages, 6 figure
Photoconductivity in BiFeO3 thin films
The optical properties of epitaxial BiFeO3 thin films have been characterized in the visible range. Variable temperature spectra show an absorption onset near 2.17 eV, a direct gap (2.667±0.005 eV at 300 K), and charge transfer excitations at higher energy. Additionally, we report photoconductivity in BiFeO3 films under illumination from a 100 mW/cm2 white light source. A direct correlation is observed between the magnitude of the photoconductivity and postgrowth cooling pressure. Dark conductivities increased by an order of magnitude when comparing films cooled in 760 and 0.1 Torr. Large increases in photoconductivity are observed in light
Phase separation due to quantum mechanical correlations
Can phase separation be induced by strong electron correlations? We present a
theorem that affirmatively answers this question in the Falicov-Kimball model
away from half-filling, for any dimension. In the ground state the itinerant
electrons are spatially separated from the classical particles.Comment: 4 pages, 1 figure. Note: text and figure unchanged, title was
misspelle
Fragmentation of pooled PCR products for highly multiplexed TILLING
Improvements to massively parallel sequencing have allowed the routine recovery of natural
and induced sequence variants. A broad range of biological disciplines have benefited from this, ranging
from plant breeding to cancer research. The need for high sequence coverage to accurately recover single
nucleotide variants and small insertions and deletions limits the applicability of whole genome approaches.
This is especially true in organisms with a large genome size or for applications requiring the screening of
thousands of individuals, such as the reverse-genetic technique known as TILLING. Using PCR to target and
sequence chosen genomic regions provides an attractive alternative as the vast reduction in interrogated
bases means that sample size can be dramatically increased through amplicon multiplexing and multidimensional
sample pooling while maintaining suitable coverage for recovery of small mutations. Direct
sequencing of PCR products is limited, however, due to limitations in read lengths of many next generation
sequencers. In the present study we show the optimization and use of ultrasonication for the simultaneous
fragmentation of multiplexed PCR amplicons for TILLING highly pooled samples. Sequencing performance
was evaluated in a total of 32 pooled PCR products produced from 4096 chemically mutagenized Hordeum
vulgare DNAs pooled in three dimensions. Evaluation of read coverage and base quality across amplicons
suggests this approach is suitable for high-throughput TILLING and other applications employing highly
pooled complex sampling schemes. Induced mutations previously identified in a traditional TILLING screen
were recovered in this dataset further supporting the efficacy of the approach
Tunneling electroresistance effect in ferroelectric tunnel junctions at the nanoscale
Stable and switchable polarization of ferroelectric materials opens a
possibility to electrically control their functional behavior. A particularly
promising approach is to employ ferroelectric tunnel junctions where the
polarization reversal in a ferroelectric barrier changes the tunneling current
across the junction. Here, we demonstrate the reproducible tunneling
electroresistance effect using a combination of Piezoresponse Force Microscopy
(PFM) and Conducting Atomic Force Microscopy (C-AFM) techniques on
nanometer-thick epitaxial BaTiO3 single crystal thin films on SrRuO3 bottom
electrodes. Correlation between ferroelectric and electronic transport
properties is established by the direct nanoscale visualization and control of
polarization and tunneling current in BaTiO3 films. The obtained results show a
change in resistance by about two orders of magnitude upon polarization
reversal on a lateral scale of 20 nm at room temperature. These results are
promising for employing ferroelectric tunnel junctions in non-volatile memory
and logic devices, not involving charge as a state variable.Comment: 18 pages, 4 figure
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