pinecone/core-2020-05-10-deduplication huggingface dataset:
Rhenium monoselenide:an investigation by density functional theory

The lattice parameters and the relative stability of the hypothetical material ReSe have been calculated within the LDA approximation in the NaCl, NiAs and zincblende structures. The zincblende phase appears to remain metallic even when possible strong Coulomb correlations between Re 5d electrons are taken into account by the introduction of a Hubbard U term. Spin-polarized calculations predict a non-magnetic phase for U up to 7 eV. Calculations of the phonon dispersion of zincblende ReSe point to an instability with respect to a tetragonal distortion and this raises the interesting possibility that it may also be possible to obtain ReSe experimentally in the PbO (FeSe) structure
Thermodynamic stability of mixed Pb:Sn methyl-ammonium halide
  perovskites

Using density functional theory, we investigate systematically mixed
MA(Pb:Sn)X3 perovskites, where MA is CH3NH3+, and X is Cl, Br, or I. Ab initio
calculations of the orthorhombic, tetragonal, and cubic perovskite phases show
that the substitution of lead by tin has a much weaker influence on both
structure and cohesive energies than the substitution of the halogen. The
thermodynamic stability of the MA(Pb:Sn)X3 mixtures at finite, non-zero
temperatures is studied within the Regular Solution Model. We predict that it
will be possible to create MA(Pb:Sn)I3 mixtures at any temperature. Our results
imply that mixing is unlikely for the low-temperature phase of bromide and
chloride compounds, where instead local clusters are more likely to form. We
further predict that in the high-temperature cubic phase, Pb and Sn compounds
will mix for both MA(Pb:Sn)Br3 and MA(Pb:Sn)Cl3 due to the entropy contribution
to the Helmholtz free energy.Comment: 11 pages, 4 figures, 7 table
Optical properties of superconducting EuFe2(As1-xPx)2

We present a broadband optical-conductivity study of superconducting
single-crystalline EuFe2(As1-xPx)2 with three different substitutional levels.
We analyze the normal-state electrodynamics by decomposing the conductivity
spectra using a Drude-Lorentz model with two Drude terms representing two
groups of carriers with different scattering rates. The analysis reveals that
the scattering rate of at least one of the Drude components develops linearly
with temperature for each doping level. This points towards strong
electron-electron correlations and a non-Fermi-liquid behavior in the
P-substituted superconducting Eu-122 pnictides. We also detect a transfer of
the spectral weight from mid-infrared to higher frequencies and assign it to
the Hund's-rule coupling between itinerant and localized carriers. The
conductivity spectra below the superconducting transition show no sharp
features to be associated with the dirty-limit superconducting BCS gaps. We
interpret these results in terms of clean-limit superconductivity in
EuFe2(As1-xPx)2. The best parametrization fit can be achieved using a two-gap
model. We find that the larger gap at the hole pockets of the Fermi surface is
likely to be isotropic, while the smaller gap at the electron pockets is
anisotropic or even nodal.Comment: to be published in Physica Status Solidi B - Special Issue on Iron
  Pnictide
Synthesis, phase stability, structural and physical properties of
  11-type iron chalcogenides

This article reviews recent experimental investigations on two binary
Fe-chalcogenides: FeSe and Fe$_{1+y}$Te. The main focus is on synthesis, single
crystal growth, chemical composition, as well as on the effect of excess iron
on structural, magnetic, and transport properties of these materials. The
structurally simplest Fe-based superconductor Fe$_{1+x}$Se with a critical
temperature $T_c \approx$ 8.5 K undergoes a tetragonal to orthorhombic phase
transition at a temperature $T_s \approx$ 87 K. No long-range magnetic order is
observed down to the lowest measured temperature in Fe$_{1+x}$Se. On the other
hand, isostructural Fe$_{1+y}$Te displays a complex interplay of magnetic and
structural phase transitions in dependence on the tuning parameter such as
excess amount of Fe or pressure, but it becomes a superconductor only when Te
is substituted by a sufficient amount of Se. We summarize experimental evidence
for different competing interactions and discuss related open questions.Comment: 14 pages, 12 figures, Feature Article, Part of Special Issue on
  Iron-Based High Temperature Superconductor
Superconductivity-enhanced Nematicity and '$s+d$' Gap Symmetry in
  Fe(Se$_{1-x}$S$_x$)

Superconducting iron chalcogenide FeSe has the simplest crystal structure
among all the Fe-based superconductors. Unlike other iron pnictides, FeSe
exhibits no long range magnetic order accompanying the
tetragonal-to-orthorhombic structural distortion, which raises the fundamental
question about the role of magnetism and its associated spin fluctuations in
mediating both nematicity and superconductivity. The extreme sensitivity of
FeSe to external pressure suggests that chemical pressure, induced by
substitution of Se by the smaller ion S, could also a be good tuning parameter
to further study the coupling between superconductivity and nematicity and to
obtain information on both the Fermi-surface changes and the symmetry of the
superconducting state. Here we study the thermodynamic properties of
Fe(Se$_{1-x}$S$_{x}$) for 3 compositions, $x=0$, 0.08 and 0.15, using
heat-capacity and thermal-expansion measurements. With increasing S content we
observe a significant reduction of the tetragonal-to-orthorhombic transition
temperature T$_{s}$. However, this suppression of T$_{s}$ is counterintuitively
accompanied by an enhancement of the orthorhombic distortion $\delta$ below
T$_c$, which clearly indicates that superconductivity favors the nematic state.
In parallel, the superconducting transition temperature T$_{c}$ is sizeably
enhanced, whereas the increase of the Sommerfeld coefficient $\gamma_{n}$ is
quite moderate. In the T$\to$ 0 limit, an unusually large residual density of
states is found for $x>0$ indicative of significant substitution-induced
disorder. We discuss these observations in the context of $s+d$
superconducting-state symmetry.Comment: 7 pages, 3 figures - Ref 23 correcte
Effect of Co doping and hydrostatic pressure on SrFe2As2

We report a pressure study on electron doped SrFe$_{2-x}$Co$_x$As$_2$ by
electrical-resistivity ($\rho$) and magnetic-susceptibility ($\chi$)
experiments. Application of either external pressure or Co substitution rapidly
suppresses the spin-density wave ordering of the Fe moments and induces
superconductivity in SrFe$_2$As$_2$. At $x=0.2$ the broad superconducting (SC)
dome in the $T-p$ phase diagram exhibits its maximum $T_{c,{\rm max}}=20$ K at
a pressure of only $p_{\rm max}\approx 0.75$ GPa. In
SrFe$_{1.5}$Co$_{0.5}$As$_2$ no superconductivity is observed anymore up to 2.8
GPa. Upon increasing the Co concentration the maximum of the SC dome shifts
toward lower pressure accompanied by a decrease in the value of $T_{c,{\rm
max}}$. Even though, superconductivity is induced by both tuning methods, Co
substitution leads to a much more robust SC state. Our study evidences that in
SrFe$_{2-x}$Co$_x$As$_2$ both, the effect of pressure and Co-substitution, have
to be considered in order to understand the SC phase-diagram and further
attests the close relationship of SrFe$_2$As$_2$ and its sister compound
BaFe$_2$As$_2$.Comment: 6 pages, 6 figure
Defect states in LiFeAs as seen by low temperature scanning tunneling
  microscopy and spectroscopy

We present a microscopic investigation of frequently observed
impurity-induced states in stoichiometric LiFeAs using low temperature scanning
tunneling microscopy and spectroscopy (STM/STS). Our data reveal seven distinct
well defined defects which are discernible in topographic measurements.
Depending on their local topographic symmetry, we are able to assign five
defect types to specific lattice sites at the Li, Fe and As positions. The most
prominent result is that two different defect types have a remarkably different
impact on the superconducting state. A specific and quite abundant Fe-defect
with $D_2$-symmetry generates significant impurity-induced additional states
primarily at positive bias voltage with pronounced peaks in the on-site local
density of states (LDOS) at about 4~mV and 12~mV. On the other hand, a
$D_4$-symmetric As-defect causes a significantly enhanced LDOS at both positive
and negative bias voltages. We expect that these findings provide fresh input
for further experimental and theoretical studies on elucidating the nature of
superconductivity in LiFeAs
Impurity effects on spin dynamics in magnetic and superconducting iron
  pnictides and chalcogenides

In this paper we summarize the effects of magnetic and nonmagnetic impurities
on the spin dynamics in Febased superconductors and their parent compounds. The
effects of chemical substitution, vacancies, and disorder on the suppression or
stabilization of superconductivity and spin-density-wave phases are reviewed in
the context of recent neutron-spectroscopy measurements of spin excitations. We
also present new results on the structure of magnetic fluctuations in BaFe2As2
single crystals doped with Mn local moments and discuss them in relationship to
the previously reported (pi, pi) branch of checkerboard magnetic excitations.Comment: Feature article for the "Special Issue on Iron-Based High Temperature
  Superconductors" to appear in Physica Status Solidi (b); 16 pages, 22
  figures, v4: corrected typos, optimized figures, updated reference
Superconductivity and Dirac Fermions in 112-phase Pnictides

This article reviews the status of current research on the 112-phase of
pnictides. The 112-phase has gained augmented attention due to the recent
discovery of high-temperature superconductivity in $\cl$ with a maximum
critical temperature $\tc\sim$ 47\,K upon Sb substitution. The structural,
magnetic, and electronic properties of $\cl$ bear some similarities with other
superconducting pnictide phases, however, the different valence states of the
pnictogen and the presence of a metallic spacer layer are unique features of
the 112-system. Low-temperature superconductivity which coexists with
antiferromagnetic order was observed in transition metal (Ni, Pd) deficient
112-compounds like $\cn$, $\lpb$, $\lps$, $\lns$. Besides superconductivity,
the presence of naturally occurring anisotropic Dirac Fermionic states were
observed in the layered 112-compounds $\smb$, $\cmb$, $\lab$ which are of
significant interest for future nanoelectronics as an alternative to graphene.
In these compounds, the linear energy dispersion resulted in a high
magnetoresistance that stayed unsaturated even at the highest applied magnetic
fields. Here, we describe various 112-type materials systems combining
experimental results and theoretical predictions to stimulate further research
on this less well-known member of the pnictide family.Comment: 18 pages, 20 figure
Ab-initio perspective on structural and electronic properties of
  iron-based superconductors

The discovery of iron pnictides and iron chalcogenides as a new class of
unconventional superconductors in 2008 has generated an enourmous amount of
experimental and theoretical work that identifies these materials as correlated
metals with multiorbital physics, where magnetism, nematicity and
superconductivity are competing phases that appear as a function of pressure
and doping. A microscopic understanding of the appearance of these phases is
crucial in order to determine the nature of superconductivity in these systems.
Here we review our recent theoretical efforts to describe and understand from
first principles the properties of iron pnictides and chalcogenides with
special focus on (i) pressure dependence, (ii) effects of electronic
correlation and (iii) origin of magnetism and superconductivity.Comment: Final report of the authors' contribution to project SPP1458
  (Superconductivity in iron pnictides) of the German Research Foundation;
  Accepted for publication in Physica Status Solidi
Approximation schemes for the study of multi-band Gutzwiller wave
  functions

The minimum of the Gutzwiller energy functional depends on the number of
parameters considered in the variational state. For a three-orbital Hubbard
model we find that the frequently used diagonal Ansatz is very accurate in
high-symmetry situations. For lower symmetry, induced by a crystal-field
splitting or the spin-orbit coupling, the discrepancies in energy between the
most general and a diagonal Gutzwiller Ansatz can be quite significant. We
discuss approximate schemes that may be employed in multi-band cases where a
minimization of the general Gutzwiller energy functional is too demanding
numerically.Comment: 8 pages, 8 figure
Avoided ferromagnetic quantum critical point: Antiferromagnetic ground
  state in substituted CeFePO

We have investigated single crystals of two substitution series
Ce(Ru$_{1-x}$Fe$_{x}$)PO and CeFe(As$_{1-y}$P$_{y}$)O in the vicinity to the
quantum critical material CeFePO by means of magnetic-susceptibility and
specific-heat measurements. We observe an antiferromagnetic ground state in the
vicinity of the quantum critical point, with pronounced metamagnetic
transitions for $H\parallel c$, which is the magnetically hard direction. Our
results verify that a ferromagnetic quantum critical point is avoided in
substituted CeFePO, because we clearly demonstrate that the ferromagnetic
ground state changes into an antiferromagnetic one, when approaching the
quantum critical point
Magnetic Exchange Interaction in the Spin Polarized Electron Gas

The exchange interaction between two magnetic moments embedded in a host
metal is fundamental to the description of the magnetic behavior of solids. In
the standard spin-degenerate electron gas, it leads to the well known
Ruderman-Kittel-Kasuya-Yoshida (RKKY) interaction, which is of the Heisenberg
form $J\vec S_1\cdot \vec S_2$, where the interaction strength $J$ oscillates
and falls off with the distance between the moments in a characteristic way.
Here, we study the more general case of the spin-polarized electron gas both in
two and three dimensions, by evaluating the interaction strength as an
integration over the product of the host Green's functions. We find that in
addition to the Heisenberg term, an additional Ising-like term appears in the
magnetic interaction, so that the net interaction for the spin-polarized gas is
of the form $J_1 \vec S_1\cdot \vec S_2 + J_2 S_{1z} S_{2z}$. The interactions
show a beating pattern as a function of distance, caused by the two different
Fermi momenta for the two spins
Nematicity in LaFeAsO$_{1-x}$F$_x$

Orbital ordering has recently emerged as another important state in iron
based superconductors, and its role for superconductivity as well as its
connection to magnetic order and orthorhombic lattice distortion are heavily
debated. In order to search for signatures of this so-called nematic phase in
oxypnictides, we revisit the normal state properties of the pnictide
superconductor LaFeAsO$_{1-x}$F$_x$ with a focus on resistivity, Nernst effect,
thermal expansion, and $^{75}$As NMR data. The transport properties at the
underdoped level $x=0.05$ exhibit pronounced anomalies at about the same
temperature where undoped LaFeAsO develops long-range nematic ordering, i.e. at
about 160 K. Furthermore, the $^{75}$As-NMR spin-lattice relaxation rate
$1/(T_1T)$ reveals a progressive slowing down of spin fluctuations. Yet,
long-range magnetic order and also a detectable orthorhombic lattice distortion
are absent. Thus, we conclude from the data that short-range orbital-nematic
ordering or a slowly fluctuating form of it sets in near 160 K. Remarkably, all
anomalies in the transport and also the indications of slow spin fluctuations
disappear close to optimal doping $x=0.1$ which suggests that in
LaFeAsO$_{1-x}$F$_x$ the nematic phase actually competes with
superconductivity
Towards carbon nanotube growth into superconducting microwave resonator
  geometries

The in-place growth of suspended carbon nanotubes facilitates the observation
of both unperturbed electronic transport spectra and high-Q vibrational modes.
For complex structures integrating, e.g., superconducting rf elements on-chip,
selection of a chemically and physically resistant material that survives the
chemical vapor deposition (CVD) process provides a challenge. We demonstrate
the implementation of molybdenum-rhenium coplanar waveguide resonators that
exhibit clear resonant behaviour at cryogenic temperatures even after having
been exposed to nanotube growth conditions. The properties of the MoRe devices
before and after CVD are compared to a reference niobium device.Comment: 6 pages, 4 figures, IWEPNM conference proceedin
Facile preparation of Au(111)/MICA substrates for high-quality graphene nanoribbon synthesis

A setup for the preparation of oriented Au(111)/mica thin\ud
films of ∼100 nm thickness as an active substrate for graphene nanoribbon growth is shown. Low-energy electron diffraction shows that the Au(111) thin films have a single crystallographic orientation over the whole surface area. Graphene nanoribbons are synthesized by the bottom-up approach via surface polymerization of precursor molecules that are evaporated onto the catalytically active Au(111)/mica substrate. The graphene nanoribbons are investigated by a combination of resonance Raman,\ud
X-ray photoemission spectroscopy, and near-edge X-ray absorption fine structure spectroscopy confirming a nanoribbon quality equal to nanoribbons grown on commercially available single-crystal Au(111) substrates. The present work, therefore, establishes the insitu preparation of Au(111)/mica as an inexpensive and simple method to prepare substrates of desired shape and thickness for surface polymerization reactions which are of interest to a growing community of researchers working on graphene nanoribbons
Electronic properties of CVD graphene: The role of grain boundaries, atmospheric doping, and encapsulation by ALD

Grain boundaries and unintentional doping can have profound
effects on graphene-based devices. Here we study these in
detail for CVD grown poly-crystalline monolayer graphene
with two significantly different grain size distributions
centered around 10–25 mm and 100–400 mm. Although the
two types of graphene are processed under identical
conditions after growth, they show distinct transport
properties in field effect transistor devices. While all asfabricated
samples showed similar p-type doping, the smaller
grain size type graphene with larger number of grain
boundaries exhibit lower average mobility. In order to
separate out the effects of grain boundaries and doping from
ambient exposure on the transport properties, the devices
were encapsulated with Al2O3 by atomic layer deposition.
The encapsulation of large grain samples thereby showed
drastic improvements in the performance with negligible
doping while the small grain samples are largely intolerant
to this process. We discuss the implications of our data
for the integrated manufacturing of graphene-based device
platforms.We acknowledge funding from
EPSRC (grant EP/K016636/1, GRAPHTED). Z.A.V.V. acknowledges
funding from ESPRC grant EP/L016087/1. J.A.A.-W.
acknowledges a Research Fellowship from Churchill College,
Cambridge.This is the final version of the article. It first appeared from Wiley at http://dx.doi.org/10.1002/pssb.201600255
Line Shape of the Raman 2D Peak of Graphene in Van Der Waals
  Heterostructures

The Raman 2D line of graphene is widely used for device characterization and
during device fabrication as it contains valuable information on e.g. the
direction and magnitude of mechanical strain and doping. Here we present
systematic asymmetries in the 2D line shape of exfoliated graphene and graphene
grown by chemical vapor deposition. Both graphene crystals are fully
encapsulated in van der Waals heterostructures, where hexagonal boron nitride
and tungsten diselenide are used as substrate materials. In both material
stacks, we find very low doping values and extremely homogeneous strain
distributions in the graphene crystal, which is a hall mark of the outstanding
electronic quality of these samples. By fitting double Lorentzian functions to
the spectra to account for the contributions of inner and outer processes to
the 2D peak, we find that the splitting of the sub-peaks, $6.6 \pm 0.5$
cm$^{-1}$(hBN-Gr-WSe2) and $8.9 \pm 1.0$ cm$^{-1}$ (hBN-Gr-hBN), is
significantly lower than the values reported in previous studies on suspended
graphene
Full characterization of a carbon nanotube parallel double quantum dot

We have measured the differential conductance of a parallel carbon nanotube
(CNT) double quantum dot (DQD) with strong inter-dot capacitance and inter-dot
tunnel coupling. Nominally, the device consists of a single CNT with two
contacts. However, we identify two sets of Coulomb blockade (CB) diamonds that
do not block transport individually, which suggest that two quantum dots (QDs)
are contacted in parallel. We find strong and periodic anti-crossings in the
gate and bias dependence, which are only possible if the QDs have similar
characteristics. We discuss qualitatively the level spectrum and the involved
transport processes in this device and extract the DQD coupling parameters.
These results lead us to believe that clean and undoped QDs are formed parallel
to the CNT axis, possibly on the outer and inner shells of a multi-wall CNT, or
in a double-stranded CNT bundle.Comment: 5 pages, 3 figure
Superconductivity and fluctuation in Ba$_{1-p}$K$_p$Fe$_2$As$_2$ and
  Ba(Fe$_{1-n}$Co$_n$)$_2$As$_2$

We study the interplay of fluctuations and superconductivity in
BaFe$_2$As$_2$ (Ba-122) compounds with Ba and Fe substituted by K ($p$ doping)
and Co ($n$ doping), respectively. To this end we measured electronic Raman
spectra as a function of polarisation and temperature. We observe gap
excitations and fluctuations for all doping levels studied. The response from
fluctuations is much stronger for Co substitution and, according to the
selection rules and the temperature dependence, originates from the exchange of
two critical spin fluctuations with characteristic wave vectors $(\pm\pi, 0)$
and $(0,\pm\pi)$. At 22\% K doping ($p=0.22$), we find the same selection rules
and spectral shape for the fluctuations but the intensity is smaller by a
factor of 5. Since there exists no nematic region above the orthorhombic
spin-density-wave (SDW) phase the identification of the fluctuations via the
temperature dependence is not possible. The gap excitations in the
superconducting state indicate strongly anisotropic near-nodal gaps for Co
substitution which make the observation of collective modes difficult. The
variation with doping of the spectral weights of the $A_{1g}$ and $B_{1g}$ gap
features does not support the influence of fluctuations on Cooper pairing.
Therefore, the observation of Bardasis-Schrieffer modes inside the nearly clean
gaps on the K-doped side remains the only experimental evidence for the
relevance of fluctuations for pairing.Comment: 10 pages, 9 figures, accepted for publication in Phys. Status Solidi
Anisotropic Elliott-Yafet Theory and Application to KC$_8$ Potassium
  Intercalated Graphite

We report Electron Spin Resonance (ESR) measurements on stage-I potassium
intercalated graphite (KC$_8$). Angular dependent measurements show that the
spin-lattice relaxation time is longer when the magnetic field is perpendicular
to the graphene layer as compared to when the magnetic field is in the plane.
This anisotropy is analyzed in the framework of the Elliott-Yafet theory of
spin-relaxation in metals. The analysis considers an anisotropic spin-orbit
Hamiltonian and the first order perturbative treatment of Elliott is reproduced
for this model Hamiltonian. The result provides an experimental input for the
first-principles theories of spin-orbit interaction in layered carbon and thus
to a better understanding of spin-relaxation phenomena in graphene and in other
layered materials as well