443 research outputs found
Electronic structure and magnetic properties of metallocene multiple-decker sandwich nanowires
We present a study of the electronic and magnetic properties of the
multiple-decker sandwich nanowires () composed of cyclopentadienyl (CP)
rings and 3d transition metal atoms (M=Ti to Ni) using first-principles
techniques. We demonstrate using Density Functional Theory that structural
relaxation play an important role in determining the magnetic ground-state of
the system. Notably, the computed magnetic moment is zero in , while in
a significant turn-up in magnetic moment is evidenced. Two compounds
show a half-metallic ferromagnetic ground state with a gap within
minority/majority spin channel. In order to study the effect of electronic
correlations upon the half-metallic ground states in , we introduce a
simplified three-bands Hubbard model which is solved within the Variational
Cluster Approach. We discuss the results as a function of size of the reference
cluster and the strength of average Coulomb and exchange parameters.
Our results demonstrate that for the range of studied parameters and
the half-metallic character is not maintained in the presence of
local Coulomb interactions.Comment: 9 pages, 9 figures, submited to PR
Electric field response of strongly correlated one-dimensional metals: a Bethe-Ansatz density functional theory study
We present a theoretical study on the response properties to an external
electric field of strongly correlated one-dimensional metals. Our investigation
is based on the recently developed Bethe-Ansatz local density approximation
(BALDA) to the density functional theory formulation of the Hubbard model. This
is capable of describing both Luttinger liquid and Mott-insulator correlations.
The BALDA calculated values for the static linear polarizability are compared
with those obtained by numerically accurate methods, such as exact (Lanczos)
diagonalization and the density matrix renormalization group, over a broad
range of parameters. In general BALDA linear polarizabilities are in good
agreement with the exact results. The response of the exact exchange and
correlation potential is found to point in the same direction of the perturbing
potential. This is well reproduced by the BALDA approach, although the fine
details depend on the specific parameterization for the local approximation.
Finally we provide a numerical proof for the non-locality of the exact exchange
and correlation functional.Comment: 8 pages and 8 figure
Urban mobility demand profiles: Time series for cars and bike-sharing use as a resource for transport and energy modeling
The transport sector is currently facing a significant transition, with strong drivers including decarbonization and digitalization trends, especially in urban passenger transport. The availability of monitoring data is at the basis of the development of optimization models supporting an enhanced urban mobility, with multiple benefits including lower pollutants and CO2 emissions, lower energy consumption, better transport management and land space use. This paper presents two datasets that represent time series with a high temporal resolution (five-minute time step) both for vehicles and bike sharing use in the city of Turin, located in Northern Italy. These high-resolution profiles have been obtained by the collection and elaboration of available online resources providing live information on traffic monitoring and bike sharing docking stations. The data are provided for the entire year 2018, and they represent an interesting basis for the evaluation of seasonal and daily variability patterns in urban mobility. These data may be used for different applications, ranging from the chronological distribution of mobility demand, to the estimation of passenger transport flows for the development of transport models in urban contexts. Moreover, traffic profiles are at the basis for the modeling of electric vehicles charging strategies and their interaction with the power grid
A method for characterizing the stability of light sources
We describe a method for measuring small fluctuations in the intensity of a laser source with a resolution of 10⁻⁴. The current signal generated by a PIN diode is passed to a front-end electronics that discriminates the AC from the DC components, which are physically separated and propagated along circuit paths with different gains. The gain long the AC signal path is set one order of magnitude larger than that along the DC signal path in such a way to optimize the measurement dynamic range. We then derive the relative fluctuation signal by normalizing the input-referred AC signal component to its input-referred DC counterpart. In this way the fluctuation of the optical signal waveform relative to the mean power of the laser is obtained. A "Noise-Scattering-Pattern method" and a "Signal-Power-Spectrum method" are then used to analyze the intensity fluctuations from three different solid-state lasers. This is a powerful tool for the characterization of the intensity stability of lasers. Applications are discussed
Suppression of Giant Magnetoresistance by a superconducting contact
We predict that current perpendicular to the plane (CPP) giant
magnetoresistance (GMR) in a phase-coherent magnetic multilayer is suppressed
when one of the contacts is superconducting. This is a consequence of a
superconductivity-induced magneto-resistive (SMR) effect, whereby the
conductance of the ferromagnetically aligned state is drastically reduced by
superconductivity. To demonstrate this effect, we compute the GMR ratio of
clean (Cu/Co)_nCu and (Cu/Co)_nPb multilayers, described by an ab-initio spd
tight binding Hamiltonian. By analyzing a simpler model with two orbitals per
site, we also show that the suppression survives in the presence of elastic
scattering by impurities.Comment: 5 pages, 4 figures. Submitted to PR
Self-interaction errors in density functional calculations of electronic transport
All density functional calculations of single-molecule transport to date have
used continuous exchange-correlation approximations. The lack of derivative
discontinuity in such calculations leads to the erroneous prediction of
metallic transport for insulating molecules. A simple and computationally
undemanding atomic self-interaction correction greatly improves the agreement
with experiment for the prototype Au/dithiolated-benzene/Au junction.Comment: 4 pages. Also available at http://www.smeagol.tcd.i
Machine Learning Predictions of High-Curie-Temperature Materials
Technologies that function at room temperature often require magnets with a
high Curie temperature, , and can be improved with better
materials. Discovering magnetic materials with a substantial is
challenging because of the large number of candidates and the cost of
fabricating and testing them. Using the two largest known data sets of
experimental Curie temperatures, we develop machine-learning models to make
rapid predictions solely based on the chemical composition of a
material. We train a random forest model and a -NN one and predict on an
initial dataset of over 2,500 materials and then validate the model on a new
dataset containing over 3,000 entries. The accuracy is compared for multiple
compounds' representations ("descriptors") and regression approaches. A random
forest model provides the most accurate predictions and is not improved by
dimensionality reduction or by using more complex descriptors based on atomic
properties. A random forest model trained on a combination of both datasets
shows that cobalt-rich and iron-rich materials have the highest Curie
temperatures for all binary and ternary compounds. An analysis of the model
reveals systematic error that causes the model to over-predict
low- materials and under-predict high- materials.
For exhaustive searches to find new high- materials, analysis of
the learning rate suggests either that much more data is needed or that more
efficient descriptors are necessary.Comment: 9 pages, 11 figures, accepted to Applied Physics Letters, special
issue "Accelerate Materials Discovery and Phenomena
Carrier induced ferromagnetism in diluted magnetic semi-conductors
We present a theory for carrier induced ferromagnetism in diluted magnetic
semi-conductor (DMS). Our approach treats on equal footing quantum fluctuations
within the RPA approximation and disorder within CPA. This method allows for
the calculation of , magnetization and magnon spectrum as a function of
hole, impurity concentration and temperature. It is shown that, sufficiently
close to , and within our decoupling scheme (Tyablicov type) the CPA for
the itinerant electron gas reduces to the Virtual Crystal Approximation. This
allows, in the low impurity concentration and low density of carriers to
provide analytical expression for . For illustration, we consider the case
of and compare our results with available experimental data.Comment: 5 figures included. to appear in Phys. Rev. B (brief report
Spin interactions of interstitial Mn ions in ferromagnetic GaMnAs
The recently reported Rutherford backscattering and particle-induced X-ray
emission experiments have revealed that in low-temperature MBE grown GaMnAs a
significant part of the incorporated Mn atoms occupies tetrahedral interstitial
sites in the lattice. Here we study the magnetic properties of these
interstitial ions. We show that they do not participate in the hole-induced
ferromagnetism. Moreover, Mn interstitial double donors may form pairs with the
nearest substitutional Mn acceptors - our calculations evidence that the spins
in such pairs are antiferromagnetically coupled by the superexchange. We also
show that for the Mn ion in the other, hexagonal, interstitial position (which
seems to be the case in the GaMnBeAs samples) the p-d interactions with the
holes, responsible for the ferromagnetism, are very much suppressed.Comment: 4 pages, 3 figures, submitted to PR
- …