1,145 research outputs found
Observation of Buried Phosphorus Dopants near Clean Si(100)-(2x1) with Scanning Tunneling Microscopy
We have used scanning tunneling microscopy to identify individual phosphorus
dopant atoms near the clean silicon (100)-(2x1) reconstructed surface. The
charge-induced band bending signature associated with the dopants shows up as
an enhancement in both filled and empty states and is consistent with the
appearance of n-type dopants on compound semiconductor surfaces and passivated
Si(100)-(2x1). We observe dopants at different depths and see a strong
dependence of the signature on the magnitude of the sample voltage. Our results
suggest that, on this clean surface, the antibonding surface state band acts as
an extension of the bulk conduction band into the gap. The positively charged
dimer vacancies that have been observed previously appear as depressions in the
filled states, as opposed to enhancements, because they disrupt these surface
bands.Comment: 4 pages, 3 figures. TeX for OSX from Wierde
The significance of 'the visit' in an English category-B prison: Views from prisoners, prisoners' families and prison staff
A number of claims have been made regarding the importance of prisoners staying in touch with their family through prison visits, firstly from a humanitarian perspective of enabling family members to see each other, but also regarding the impact of maintaining family ties for successful rehabilitation, reintegration into society and reduced re-offending. This growing evidence base has resulted in increased support by the Prison Service for encouraging the family unit to remain intact during a prisoner’s incarceration. Despite its importance however, there has been a distinct lack of research examining the dynamics of families visiting relatives in prison. This paper explores perceptions of the same event – the visit – from the families’, prisoners’ and prison staffs' viewpoints in a category-B local prison in England. Qualitative data was collected with 30 prisoners’ families, 16 prisoners and 14 prison staff, as part of a broader evaluation of the visitors’ centre. The findings suggest that the three parties frame their perspective of visiting very differently. Prisoners’ families often see visits as an emotional minefield fraught with practical difficulties. Prisoners can view the visit as the highlight of their time in prison and often have many complaints about how visits are handled. Finally, prison staff see visits as potential security breaches and a major organisational operation. The paper addresses the current gap in our understanding of the prison visit and has implications for the Prison Service and wider social policy
Weak ferromagnetism with very large canting in a chiral lattice: (pyrimidine)2FeCl2
The transition metal coordination compound (pyrimidine)2FeCl2 crystallizes in
a chiral lattice, space group I 4_1 2 2 (or I4_3 2 2). Combined magnetization,
Mossbauer spectroscopy and powder neutron diffraction studies reveal that it is
a canted antiferromagnet below T_N = 6.4 K with an unusually large canting of
the magnetic moments of 14 deg. from their general antiferromagnetic alignment,
one of the largest reported to date. This results in weak ferromagnetism with a
ferromagnetic component of 1 mu_B. The large canting is due to the interplay
between the antiferromagnetic exchange interaction and the local single-ion
anisotropy in the chiral lattice. The magnetically ordered structure of
(pyrimidine)2FeCl2, however, is not chiral. The implications of these findings
for the search of molecule based materials exhibiting chiral magnetic ordering
is discussed.Comment: 6 pages, 5 figure
Ab initio prediction of Boron compounds arising from Borozene: Structural and electronic properties
Structure and electronic properties of two unusual boron clusters obtained by
fusion of borozene rings has been studied by means of first principles
calculations, based on the generalized-gradient approximation of the density
functional theory, and the semiempirical tight-binding method was used for the
transport calculations. The role of disorder has also been considered with
single vacancies and substitutional atoms. Results show that the pure boron
clusters are topologically planar and characterized by (3c-2e) bonds, which can
explain, together with the aromaticity (estimated by means of NICS), the
remarkable cohesive energy values obtained. Such feature makes these systems
competitive with the most stable boron clusters to date. On the contrary, the
introduction of impurities compromises stability and planarity in both cases.
The energy gap values indicate that these clusters possess a semiconducting
character, while when the larger system is considered, zero-values of the
density of states are found exclusively within the HOMO-LUMO gap. Electron
transport calculations within the Landauer formalism confirm these indications,
showing semiconductor-like low bias differential conductance for these
stuctures. Differences and similarities with Carbon clusters are highlighted in
the discussion.Comment: 10 pages, 2 tables, 5 figure
Excited States of Ladder-type Poly-p-phenylene Oligomers
Ground state properties and excited states of ladder-type paraphenylene
oligomers are calculated applying semiempirical methods for up to eleven
phenylene rings. The results are in qualitative agreement with experimental
data. A new scheme to interpret the excited states is developed which reveals
the excitonic nature of the excited states. The electron-hole pair of the
S1-state has a mean distance of approximately 4 Angstroem.Comment: 24 pages, 21 figure
Ab-Initio Calculation of Molecular Aggregation Effects: a Coumarin-343 Case Study
We present time-dependent density functional theory (TDDFT) calculations for
single and dimerized Coumarin-343 molecules in order to investigate the quantum
mechanical effects of chromophore aggregation in extended systems designed to
function as a new generation of sensors and light-harvesting devices. Using the
single-chromophore results, we describe the construction of effective
Hamiltonians to predict the excitonic properties of aggregate systems. We
compare the electronic coupling properties predicted by such effective
Hamiltonians to those obtained from TDDFT calculations of dimers, and to the
coupling predicted by the transition density cube (TDC) method. We determine
the accuracy of the dipole-dipole approximation and TDC with respect to the
separation distance and orientation of the dimers. In particular, we
investigate the effects of including Coulomb coupling terms ignored in the
typical tight-binding effective Hamiltonian. We also examine effects of orbital
relaxation which cannot be captured by either of these models
Structure-dependent exchange in the organic magnets Cu(II)Pc and Mn(II)Pc
We study exchange couplings in the organic magnets copper(II) phthalocyanine
(Cu(II)Pc) and manganese(II) phthalocyanine (Mn(II)Pc) by a combination of
Green's function perturbation theory and \textsl{ab initio} density-functional
theory (DFT). Based on the indirect exchange model our perturbation-theory
calculation of Cu(II)Pc qualitatively agrees with the experimental
observations. DFT calculations performed on Cu(II)Pc dimer show a very good
quantitative agreement with exchange couplings that we extract by using a
global fitting for the magnetization measurements to a spin-1/2 Bonner-Fisher
model. These two methods give us remarkably consistent trends for the exchange
couplings in Cu(II)Pc when changing the stacking angles. The situation is more
complex for Mn(II)Pc owing to the competition between super-exchange and
indirect exchange.Comment: 13 pages,10 figures. To appear in Physical Review
‘It was just like we were a family again’: play as a means to maintain family ties for children visiting an imprisoned parent
Children can find the process of visiting a prison traumatic and as a result of parental incarceration may experience a range of adverse outcomes. When children stay in contact with their imprisoned parent through prison visiting, however, this seems to be a protective factor. This paper reports on a play visits service based at Her Majesty's Prison Leeds, UK. The service provides supervised play work provision for children visiting their father. Data were derived from prisoners and prisoners' families and were triangulated as a means of achieving a level of validity. The findings reveal that play visits do produce positive outcomes for children and play visits are effective in maintaining and strengthening family ties. These effects may be stronger when compared to standard prison visits, but further research is needed to confirm this
All-electron magnetic response with pseudopotentials: NMR chemical shifts
A theory for the ab initio calculation of all-electron NMR chemical shifts in
insulators using pseudopotentials is presented. It is formulated for both
finite and infinitely periodic systems and is based on an extension to the
Projector Augmented Wave approach of Bloechl [P. E. Bloechl, Phys. Rev. B 50,
17953 (1994)] and the method of Mauri et al [F. Mauri, B.G. Pfrommer, and S.G.
Louie, Phys. Rev. Lett. 77, 5300 (1996)]. The theory is successfully validated
for molecules by comparison with a selection of quantum chemical results, and
in periodic systems by comparison with plane-wave all-electron results for
diamond.Comment: 25 pages, 4 tables, submitted to Physical Review
High-throughput identification of genotype-specific cancer vulnerabilities in mixtures of barcoded tumor cell lines.
Hundreds of genetically characterized cell lines are available for the discovery of genotype-specific cancer vulnerabilities. However, screening large numbers of compounds against large numbers of cell lines is currently impractical, and such experiments are often difficult to control. Here we report a method called PRISM that allows pooled screening of mixtures of cancer cell lines by labeling each cell line with 24-nucleotide barcodes. PRISM revealed the expected patterns of cell killing seen in conventional (unpooled) assays. In a screen of 102 cell lines across 8,400 compounds, PRISM led to the identification of BRD-7880 as a potent and highly specific inhibitor of aurora kinases B and C. Cell line pools also efficiently formed tumors as xenografts, and PRISM recapitulated the expected pattern of erlotinib sensitivity in vivo
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