pinecone/core-2020-05-10-deduplication huggingface dataset:
Phase separation at the magnetic-superconducting transition in
  La$_{0.7}$Y$_{0.3}$FeAsO$_{1-x}$F$_{x}$

In this paper we report a detailed $\mu^{+}$SR and {}$^{19}$F-NMR study of
the La$_{0.7}$Y$_{0.3}$FeAsO$_{1-x}$F$_{x}$ class of materials. Here, the
diamagnetic La$_{1-y}$Y$_{y}$ substitution increases chemical pressure and,
accordingly, sizeably enhances the optimal superconducting transition
temperature. We investigate the magnetic-superconducting phase transition by
keeping the Y content constant ($y = 0.3$) and by varying the F content in the
range $0.025 \leq x \leq 0.15$. Our results show how magnetism and
superconductivity coexist for $x = 0.065$. Such coexistence is due to
segregation of the two phases in macroscopic regions, resembling what was
observed in LaFeAsO$_{1-x}$F$_{x}$ materials under applied hydrostatic
pressure. This scenario is qualitatively different from the nanoscopic
coexistence of the two order parameters observed when La is fully substituted
by magnetic rare-earth ions like Sm or Ce.Comment: Proceedings of QCNP12 Conferenc
Instability of the chiral phase and electronic ferroelectricity in the
  extended Falicov--Kimball model

We consider a spinless extended Falicov--Kimball model at half-filling, for
the case of opposite-parity bands. Within the Hartree--Fock approach, we
calculate the excitation energies in the chiral phase, which is a possible
mean-field solution in the presence of a hybridisation. It is shown that the
chiral phase is unstable. We then briefly review the accumulated results on
stability and degeneracies of the excitonic insulator phase. Based on these, we
conclude that the presence of both hybridisation and narrow-band hopping is
required for electronic ferroelectricity.Comment: 5 pages, LaTeX, 1 PostScript figure, misprint corrected. Phys. Status
  Solidi B, in pres
Nonequilibrium dynamics of multiorbital correlated electron system under
  time-dependent electric fields

To clarify a key role of orbital degrees of freedom in the response of the
many-body electron state of correlated electrons to an external field, we
investigate the real-time dynamics in an $e_{\rm g}$-orbital Hubbard model
under applied electric fields by exploiting numerical techniques such as a
time-dependent density-matrix renormalization group (DMRG) method. The ground
state without applying an electric field is found to be an
antiferromagnetic/ferro-orbital state. After we switch on an oscillating
electric field, it gives rise to the creation of holon-doublon pairs when the
frequency of the applied electric field exceeds a charge excitation gap. We
argue that a pair-hopping process yields a dynamical deformation of the orbital
configuration.Comment: 4 pages, 4 figures, Proceedings of QCNP2012 (August 26-29, 2012,
  Dresden, Germany
Stability and electronic structure of covalently functionalized graphene
  layers

We present exemplary results of extensive studies of mechanical, electronic
and transport properties of covalent functionalization of graphene monolayers
(GML) with -NH2. We report new results of ab initio studies of covalent
functionalization of GML with -NH2 groups up to 12.5% concentration. Our
studies are performed in the framework of the density functional theory (DFT)
and non-equilibrium Green's function (NEGF). We discuss the stability
(adsorption energy), elastic moduli, electronic structure, band gaps, and
effective electron masses as a function of the density of the adsorbed
molecules. We also show the conductance and I(V) characteristic of these
systems. Generally, the stability of the functionalized graphene layers
decreases with the growing concentration of attachments and we determine the
critical density of the molecules that can be chemisorbed on the surface of
GLs. Because of local deformations of GLs and sp3 rehybridization of the bonds
induced by fragments, elastic moduli decrease with increasing number of groups.
Simultaneously, we observe that the functionalizing molecules stretch the
graphenes lattice, the effect being more pronounced for higher concentration of
adsorbed molecules. We find out that the GLs functionalization leads in many
cases to the opening of the graphene band gap (up to 0.5302 eV for 12.5%
concentration) and can be therefore utilized in graphene devices. The new HOMO
and LUMO originate mostly from the impurity bands induced by the
functionalization and they exhibit parabolic dispersion with electron effective
masses comparable to ones in silicon or gallium nitride.Comment: 3 figures, 1 Tabl
Capturing the re-entrant behaviour of one-dimensional Bose-Hubbard model

The Bose Hubbard model (BHM) is an archetypal quantum lattice system
exhibiting a quantum phase transition between its superfluid (SF) and
Mott-insulator (MI) phase. Unlike in higher dimensions the phase diagram of the
BHM in one dimension possesses regions in which increasing the hopping
amplitude can result in a transition from MI to SF and then back to a MI. This
type of re-entrance is well known in classical systems like liquid crystals yet
its origin in quantum systems is still not well understood. Moreover, this
unusual re-entrant character of the BHM is not easily captured in approximate
analytical or numerical calculations. Here we study in detail the predictions
of three different and widely used approximations; a multi-site mean-field
decoupling, a finite-sized cluster calculation, and a real-space
renormalization group (RG) approach. It is found that mean-field calculations
do not reproduce re-entrance while finite-sized clusters display a precursor to
re-entrance. Here we show for the first time that RG does capture the
re-entrant feature and constitutes one of the simplest approximation able to do
so. The differing abilities of these approaches reveals the importance of
describing short-ranged correlations relevant to the kinetic energy of a MI in
a particle-number symmetric way.Comment: To appear in Physica Status Solidi (b). 8 pages, 4 figure
P doped random C20 fullerite - model structure for fullerene-like CPx

Carbon-based amorphous and fullerene-like (FL) thin films\ud
have a great potential due to their mechanical resiliency. TEM\ud
investigation and interpretation of results from FL structures\ud
embedded in bulk phases is not straightforward. Here, a model\ud
is presented for description of the structure of FL-CP0.1 thin\ud
films and to describe all the rings in the electron diffraction\ud
pattern in a self-consistent way. The model structure consists of random close-packed and P-doped C20 clusters resembling a\ud
kind of amorphous fullerite (crystals of fullerene cages). The 10% P doping was implemented by creating C18P2 cages by\ud
replacing two C atoms by P in the C20 cage. The simulated\ud
electron scattering is in agreement with the experimental\ud
electron diffraction pattern and reproduces all the observed\ud
diffuse diffraction rings of FL-CP0.1 at 1.6, 2.6, and 5.9A.\ud
Simulation of HRTEM images confirmed the amorphous\ud
appearance of this nanostructured material
Radiation fields for nanoscale systems

For a group of charged particles obeying quantum mechanics interacting with
an electromagnetic field, the charge and current density in a pure state of the
system are expressed with the many-body wave function of the state. Using these
as sources, the microscopic Maxwell equations can be written down for any given
pure state of a many-body system. By employing semi-classical radiation theory
with these sources, the microscopic Maxwell equations can be used to compute
the strong radiation fields produced by interacting charged quantal particles.
For a charged quantal particle, three radiation fields involve only the vector
potential $\mathbf{A}$. This is another example demonstrating the observability
of vector potential. Five radiation fields are perpendicular to the canonical
momentum of a single charged particle. For a group of charged particles, a new
type of radiation field is predicted to be perpendicular to
$\mathbf{A}(\mathbf{x}_{j},t)\times
\lbrack\nabla\times(\nabla_{j}\Psi^{\prime})]$, where $\Psi^{'}$ is the
many-body wave function. This kind of radiation does not exist for a single
charged particle. The macroscopic Maxwell equations are derived from the
corresponding microscopic equations for a pure state by the Russakoff-Robinson
procedure which requires only a spatial coarse graining. Because the sources of
fields are also the responses of a system to an external field, one also has to
give up the temporal coarse graining of the current density which is often
supposed to be critical in the kinetic approach of conductivity.Comment: 10 pages, physica status solidi (b) accepte
Carbon nanotubes quench singlet oxygen generated by photosynthetic reaction centers

Photosensitizers may convert molecular oxygen into reactive oxygen species (ROS) including, e.g., singlet oxygen (1O2), superoxide anion (O2-•), and hydroxyl radicals (•OH), chemicals with extremely high cyto- and potential genotoxicity. Photodynamic ROS reactions are determinative in medical photodynamic therapy (cancer treatment with externally added photosensitizers) and in reactions damaging the photosynthetic apparatus of plants (via internal pigments). The primary events of photosynthesis take place in the chlorophyll containing reaction center protein complex (RC), where the energy of light is converted into chemical potential. 1O2 is formed by both bacterial bacteriochlorophylls and plant RC triplet chlorophylls in high light and if the quenching of 1O2 is impaired. In plant physiology, reducing the formation of the ROS and thus lessening photooxidative membrane damage (including the RC protein) and increasing the efficiency of the photochemical energy conversion is of special interest. Carbon nanotubes, in artificial systems, are also known to react with singlet oxygen. To investigate the possibility of 1O2 quenching by carbon nanotubes in a biological system, we studied the effect of carbon nanotubes on 1O2 photogenerated by photosynthetic RCs purified from purple bacteria. 1,3-Diphenylisobenzofuran (DPBF), a dye responding to oxidation by 1O2 with absorption decrease at 420nm was used to measure 1O2 concentrations. 1O2 was produced either from a photosensitizer (methylene blue) or from triplet photosynthetic RCs and the antioxidant capacity of carbon nanotubes was assessed. Less 1O2 was detected by DPBF in the presence of carbon nanotubes, suggesting that these are potential quenchers of this ROS. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Symmetry breaking and low energy conformational fluctuations in
  amorphous graphene

Recently, the prospects for amorphous phases of graphene (a-G) have been
explored computationally. Initial models were flat, and contained odd-member
rings, while maintaining three-fold coordination and sp2 bonding. Upon
relaxation, puckering occurs, and may be traced to the existence of pentagons,
in analogy with the situation for fullerenes. In this work, we systematically
explore the inherent structures with energy close to the flat starting
structure. As expected, the planar symmetry can be broken in various ways,
which we characterize for 800-atom model of a-G, always using local basis
density functional techniques. The classical normal modes of various structural
models are discussed, with an emphasis on imaginary modes indicating the
evolution from flat to puckered. We also discuss very low energy conformational
fluctuations akin to those seen previously in amorphous silicon, and reflect on
the nature of the amorphous "ground state" within a network of fixed topology.
For completeness, high energy modes were also computed, and are found to be
associated with strained parts of the network
Kondo effect in the Kohn-Sham conductance of multiple levels quantum
  dots

At zero temperature, the Landauer formalism combined with static density
functional theory is able to correctly reproduce the Kondo plateau in the
conductance of the Anderson impurity model provided that an
exchange-correlation potential is used which correctly exhibits steps at
integer occupation. Here we extend this recent finding to multi-level quantum
dots described by the constant-interaction model. We derive the exact
exchange-correlation potential in this model for the isolated dot and deduce an
accurate approximation for the case when the dot is weakly coupled to two
leads. We show that at zero temperature and for non-degenerate levels in the
dot we correctly obtain the conductance plateau for any odd number of electrons
on the dot. We also analyze the case when some of the levels of the dot are
degenerate and again obtain good qualitative agreement with results obtained
with alternative methods. As in the case of a single level, for temperatures
larger than the Kondo temperature, the Kohn-Sham conductance fails to reproduce
the typical Coulomb blockade peaks. This is attributed to {\em dynamical}
exchange-correlation corrections to the conductance originating from
time-dependent density functional theory.Comment: 8 pages, 4 figures, submitted to special issue of pss(b
Introduction and annealing of primary defects in proton-bombarded n-GaN

We report on in situ space charge spectroscopy measure-ments on low-temperature 1.6 MeV proton-bombarded n-type gallium nitride
thin film samples. The scope of this study was to investigate the introduction and annealing dynamics of radiation-induced lattice
damage. Using optical excitation allowed for the detection of electronic defect states in the entire GaN bandgap and to detect unstable
primary defects that would have been invisible in thermal space charge spectroscopic measurements. The introduction of
compensating acceptor-like primary defects by the bombardment was observed and manifested as a decrease in the sample
capacitance. After the bombardment the concentrations of deep-levels and acceptor states were monitored by deep-level transient
spectroscopy and photo-capacitance measurements while the temperature was increased. It was found that annealing and reactions of
primary bombardment-induced defects occurs even below room-temperature which might account for the radiation-hardness of GaN.University of Pretoria and the National Research Foundation (NRF).http://onlinelibrary.wiley.com/journal/10.1002/(ISSN)1521-3951hb201
Quantitatively Accurate Calculations of Conductance and Thermopower of
  Molecular Junctions

Thermopower measurements of molecular junctions have recently gained interest
as a characterization technique that supplements the more traditional
conductance measurements. Here we investigate the electronic conductance and
thermopower of benzenediamine (BDA) and benzenedicarbonitrile (BDCN) connected
to gold electrodes using first-principles calculations. We find excellent
agreement with experiments for both molecules when exchange-correlation effects
are described by the many-body GW approximation. In contrast, results from
standard density functional theory (DFT) deviate from experiments by up to two
orders of magnitude. The failure of DFT is particularly pronounced for the
n-type BDCN junction due to the severe underestimation of the lowest unoccupied
molecular orbital (LUMO). The quality of the DFT results can be improved by
correcting the molecular energy levels for self-interaction errors and image
charge effects. Finally, we show that the conductance and thermopower of the
considered junctions are relatively insensitive to the metal-molecule bonding
geometry. Our results demonstrate that electronic and thermoelectric properties
of molecular junctions can be predicted from first-principles calculations when
exchange-correlation effects are taken properly into account.Comment: Submitted to special issue of pss (b
Heat, molecular vibrations, and adiabatic driving in non-equilibrium
  transport through interacting quantum dots

In this article we review aspects of charge and heat transport in interacting
quantum dots and molecular junctions under stationary and time-dependent
non-equilibrium conditions due to finite electrical and thermal bias. In
particular, we discuss how a discrete level spectrum can be beneficial for
thermoelectric applications, and investigate the detrimental effects of
molecular vibrations on the efficiency of a molecular quantum dot as an energy
converter. In addition, we consider the effects of a slow time-dependent
modulation of applied voltages on the transport properties of a quantum dot and
show how this can be used as a spectroscopic tool complementary to standard
dc-measurements. Finally, we combine time-dependent driving with
thermoelectrics in a double-quantum dot system - a nanoscale analogue of a
cyclic heat engine - and discuss its operation and the main limitations to its
performance.Comment: Review article submitted to PSS (b) for the special issue "Quantum
  transport at the molecular scale
Influencing the conductance in biphenyl-like molecular junctions with
  THz radiation

We investigate the torsional vibrations in biphenyl-like molecular junctions
and transport properties in the presence of an external THz field. Ab-initio
calculations including external electric fields show that the torsional angle
{\phi} of a thiolated biphenyl junction exhibits virtually no response.
However, if functional groups are added to the molecule, creating a dipole
moment in each of the rings, an external field becomes more effective for
changing {\phi}. A model based on the cos2{\phi} dependence of the current is
proposed for the biphenyl-like molecular junctions in presence of an external
THz field including 2,2'-bipyridine, 3,3'-bipyridine and 2,2',4,4'-
tetramethyl-3,3'-bipyridine. The current through these molecules is shown to
change if the THz frequency gets in resonance to the torsional vibration mode.Comment: 9 pages, 10 figures, submitted to PSS
Influence of Dielectric Environment on Role of Spin-Orbit Interaction
  for Image Potentials

We present a formalism for calculating the image potential for a
two-dimensional electron gas (2DEG) with Rashba spin-orbit interaction (SOI) as
well as for a 2D topological insulator (TI). The formalism is further
generalized for including the Coulomb coupled multiple layers. Roles of broken
inversion symmetry near the surface and the dielectric environment are
investigated by using a surface-response function. The insignificant role of
SOI in 2DEG is dramatically enhanced in TI by selecting a small relative
permittivity $\epsilon_b$ for the dielectric environment. Manipulating
$\epsilon_b$ is proven to provide an efficient way to drive electrons with
opposite spins into two different integral quantum Hall states. The prediction
made in this paper is expected to be experimentally observable for a 2DTI
system, such as Bi$_2$Se$_3$, with a helical spin behavior and a dominant
linear Rashba SOI-like term in the energy dispersion.Comment: 3 figure
Spin disorder scattering in a ferromagnetic insulator-on-graphene structure

We theoretically study the transport properties of a single graphene layer between two insulating materials, i.e., a ferromagnetic EuO thin film and a nonmagnetic SiC substrate. An exchange interaction between the charge carrier spins in graphene and the localized magnetic moments in the ferromagnetic insulator is assumed. This proximity effect and the large spin fluctuations at temperatures close to the ferromagnetic transition temperature TC lead to spin disorder scattering, which is calculated using a Green's function technique. Numerical results indicate that at temperatures close to TC the contribution of the spin disorder scattering to the total electron mobility is clearly observable even in the case of a weak exchange interaction and a low background mobility of the graphene layer. This enables the experimental determination of the exchange interaction parameter using the present model and a simple resistivity measurement.Peer reviewe
Dynamics of the polarization of a pinned domain wall in a magnetic
  nanowire

We consider the dynamics of polarization of a single domain wall in a
magnetic nanowire, which is strongly pinned by impurities. In this case the
equation of motion for the polarization parameter does not include any other
dynamical variables and is nonlinear due to magnetic anisotropy. We calculated
numerically the magnetization dynamics for different choices of parameters
under short current pulses inducing polarization switching. Our results show
that the switching is most effective for very rapid current pulses. Damping
also enhances the switching probability.Comment: 4 pages, 4 figure
Spin Hot Spots in Single-Electron GaAs-based Quantum Dots

Spin relaxation of a single electron in a weakly coupled double quantum dot
is calculated numerically. The phonon assisted spin flip is allowed by the
presence of the linear and cubic spin-orbit couplings and nuclear spins. The
rate is calculated as a function of the interdot coupling, the magnetic field
strength and orientation, and the dot bias. In an in-plane magnetic field, the
rate is strongly anisotropic with respect to the magnetic field orientation,
due to the anisotropy of the spin-orbit interactions. The nuclear spin
influence is negligible. In an out-of-plane field, the nuclear spins play a
more important role due selection rules imposed on the spin-orbit couplings.
Our theory shows a very good agreement with data measured in [Srinivasa, et
al., PRL 110, 196803 (2013)], allowing us to extract information on the linear
spin-orbit interactions strengths in that experiment. We estimate that they
correspond to spin-orbit lengths of about 5-15 $\mu$m.Comment: 7 pages, 9 figure
Unambiguous determination of spin dephasing times in ZnO

Time-resolved magneto-optics is a well-established optical pump probe
technique to generate and to probe spin coherence in semiconductors. By this
method, spin dephasing times T_2^* can easily be determined if their values are
comparable to the available pump-probe-delays. If T_2^* exceeds the laser
repetition time, however, resonant spin amplification (RSA) can equally be used
to extract T_2^*. We demonstrate that in ZnO these techniques have several
tripping hazards resulting in deceptive values for T_2^* and show how to avoid
them. We show that the temperature dependence of the amplitude ratio of two
separate spin species can easily be misinterpreted as a strongly temperature
dependent T_2^* of a single spin ensemble, while the two spin species have
T_2^* values which are nearly independent of temperature. Additionally,
consecutive pump pulses can significantly diminish the spin polarization, which
remains from previous pump pulses. While this barely affects T_2^* values
extracted from delay line scans, it results in seemingly shorter T_2^* values
in RSA.Comment: 11 pages, 10 figure
Influence of vibrations on electron transport through nanoscale contacts

In this article we present a novel semi-analytical approach to calculate
first-order electron-vibration coupling constants within the framework of
density functional theory. It combines analytical expressions for the
first-order derivative of the Kohn-Sham operator with respect to nuclear
displacements with coupled-perturbed Kohn-Sham theory to determine the
derivative of the electronic density matrix. This allows us to efficiently
compute accurate electron-vibration coupling constants. We apply our approach
to describe inelastic electron tunneling spectra of metallic and molecular
junctions. A gold junction bridged by an atomic chain is used to validate the
developed method, reproducing established experimental and theoretical results.
For octane-dithiol and octane-diamine single-molecule junctions we discuss the
influence of the anchoring group and mechanical stretching on the inelastic
electron tunneling spectra.Comment: 13 pages, 5 figure