226 research outputs found
Ab initio linear scaling response theory: Electric polarizability by perturbed projection
A linear scaling method for calculation of the static {\em ab inito} response
within self-consistent field theory is developed and applied to calculation of
the static electric polarizability. The method is based on density matrix
perturbation theory [Niklasson and Challacombe, cond-mat/0311591], obtaining
response functions directly via a perturbative approach to spectral projection.
The accuracy and efficiency of the linear scaling method is demonstrated for a
series of three-dimensional water clusters at the RHF/6-31G** level of theory.
Locality of the response under a global electric field perturbation is
numerically demonstrated by approximate exponential decay of derivative density
matrix elements.Comment: 4.25 pages in PRL format, 2 figure
Spin Density wave instability in a ferromagnet
Ferromagnetic (FM) and incommensurate spin-density wave (ISDW) states are an
unusual set of competing magnetic orders that are seldom observed in the same
material without application of a polarizing magnetic field. We report, for the
first time, the discovery of an ISDW state that is derived from a FM ground
state through a Fermi surface (FS) instability in FeGa. This was
achieved by combining neutron scattering experiments with first principles
simulations. Neutron diffraction demonstrates that FeGa is in an ISDW
state at intermediate temperatures and that there is a conspicuous re-emergence
of ferromagnetism above 360 K. First principles calculations show that the ISDW
ordering wavevector is in excellent agreement with a prominent nesting
condition in the spin-majority FS demonstrating the discovery of a novel
instability for FM metals; ISDW formation due to Fermi surface nesting in a
spin-polarized Fermi surface.Comment: 6 pages with 4 figures. Supplemental Materials Include
Observation of a mesoscopic magnetic modulation in chiral Mn1/3NbS2
We have investigated the structural, magnetic, thermodynamic, and charge
transport properties of Mn1/3NbS2 single crystals through x-ray and neutron
diffraction, magnetization, specific heat, magnetoresistance, and Hall effect
measurements. Mn1/3NbS2 displays a magnetic transition at TC ~ 45 K with highly
anisotropic behavior expected for a hexagonal structured material. Below TC,
neutron diffraction reveals increased scattering near the structural Bragg
peaks having a wider Q-dependence along the c-axis than the nuclear Bragg
peaks. This indicates helimagnetism with a long pitch length of ~250 nm (or a
wavevector q~0.0025 {\AA}-1) along the c-axis. This q is substantially smaller
than that found for the helimagnetic state in isostructural Cr1/3NbS2 (0.015
{\AA}-1). Specific heat capacity measurements confirm a second-order magnetic
phase transition with a substantial magnetic contribution that persists to low
temperature. The large low-temperature specific heat capacity is consistent
with a large density of low-lying magnetic excitations that are likely
associated with topologically interesting magnetic modes. Changes to the
magnetoresistance, the magnetization, and the magnetic neutron diffraction,
which become more apparent below 20 K, imply a modification in the character of
the magnetic ordering corresponding to the magnetic contribution to the
specific heat capacity. These observations signify a more complex magnetic
structure both at zero and finite fields for Mn1/3NbS2 than for the
well-investigated Cr1/3NbS2.Comment: 22 pages, 7 figure
The KiVa antibullying program in primary schools in Chile, with and without the digital game component: study protocol for a randomized controlled trial.
BACKGROUND: Bullying is a major problem worldwide and Chile is no exception. Bullying is defined as a systematic aggressive behavior against a victim who cannot defend him or herself. Victims suffer social isolation and psychological maladjustment, while bullies have a higher risk for conduct problems and substance use disorders. These problems appear to last over time. The KiVa antibullying program has been evaluated in Finland and other European countries, showing preventive effects on victimization and self-reported bullying. The aims of this study are (1) to develop a culturally appropriate version of the KiVa material and (2) to test the effectiveness of the KiVa program, with and without the online game, on reducing experiences of victimization and bullying behavior among vulnerable primary schools in Santiago (Chile), using a cluster randomized controlled trial (RCT) design with three arms: (1) full KiVa program group, (2) partial KiVa (without online game) program group and (3) control group. METHODS AND DESIGN: This is a three-arm, single-blind, cluster randomized controlled trial (RCT) with a target enrolment of 1495 4th and 5th graders attending 13 vulnerable schools per arm. Students in the full and partial KiVa groups will receive universal actions: ten 2-h lessons delivered by trained teachers during 1Â year; they will be exposed to posters encouraging them to support victims and behave constructively when witnessing bullying; and a person designated by the school authorities will be present in all school breaks and lunchtimes using a visible KiVa vest to remind everybody that they are in a KiVa school. KiVa schools also will have indicated actions, which consist of a set of discussion groups with the victims and with the bullies, with proper follow-up. Only full KiVa schools will also receive an online game which has the aim to raise awareness of the role of the group in bullying, increase empathy and promote strategies to support victimized peers. Self-reported victimization, bullying others and peer-reported bullying actions, psychological and academic functioning, and sense of school membership will be measured at baseline and 12Â months after randomization. DISCUSSION: This is the first cluster RCT of the KiVa antibullying program in Latin America. TRIAL REGISTRATION: ClinicalTrials.gov, Identifier: NCT02898324 . Registered on 8 September 2016
Helical magnetic order and Fermi surface nesting in non-centrosymmetric ScFeGe
An investigation of the structural, magnetic, thermodynamic, and charge
transport properties of non-centrosymmetric hexagonal ScFeGe reveals it to be
an anisotropic metal with a transition to a weak itinerant incommensurate
helimagnetic state below K. Neutron diffraction measurements
discovered a temperature and field independent helical wavevector
\textbf{\textit{k}} = (0 0 0.193) with magnetic moments of 0.53 per
formula unit confined to the {\it ab}-plane. Density functional theory
calculations are consistent with these measurements and find several bands that
cross the Fermi level along the {\it c}-axis with a nearly degenerate set of
flat bands just above the Fermi energy. The anisotropy found in the electrical
transport is reflected in the calculated Fermi surface, which consists of
several warped flat sheets along the -axis with two regions of significant
nesting, one of which has a wavevector that closely matches that found in the
neutron diffraction. The electronic structure calculations, along with a strong
anomaly in the {\it c}-axis conductivity at , signal a Fermi surface
driven magnetic transition, similar to that found in spin density wave
materials. Magnetic fields applied in the {\it ab}-plane result in a
metamagnetic transition with a threshold field of 6.7 T along with a
sharp, strongly temperature dependent, discontinuity and a change in sign of
the magnetoresistance for in-plane currents. Thus, ScFeGe is an ideal system to
investigate the effect of in-plane magnetic fields on an easy-plane magnetic
system, where the relative strength of the magnetic interactions and
anisotropies determine the topology and magnetic structure.Comment: 15 pages, 13 figure
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