768 research outputs found
Role of nematicity in controlling spin fluctuations and superconducting Tc in bulk FeSe
Bulk FeSe superconducts inside a nematic phase, that sets in through an
orthorhombic distortion of the high temperature tetragonal phase. Bulk
non-alloy tetragonal superconducting FeSe does not exist as yet. This raises
the question whether nematicity is fundamental to superconductivity. We employ
an advanced ab-initio ability and show that bulk tetragonal FeSe can, in
principle, superconduct at almost the same Tc as the orthorhombic phase had
that been the ground state. Further, we perform rigorous benchmarking of our
theoretical spin susceptibilities against experimentally observed data over all
energies and relevant momentum direction. We show that susceptibilities
computed in both the tetragonal and orthorhombic phases already have the
correct momentum structure at all energies, but not the desired intensity. The
enhanced nematicity that simulates the correct spin fluctuation intensity can
only lead to a maximum 10-15% increment in the superconducting Tc . Our results
suggest while nematicity may be intrinsic property of the bulk FeSe, is not the
primary force driving the superconducting pairing.Comment: 5 page, 4 figure
First-Principles Correlated Approach to the Normal State of Strontium Ruthenate
The interplay between multiple bands, sizable multi-band electronic
correlations and strong spin-orbit coupling may conspire in selecting a rather
unusual unconventional pairing symmetry in layered SrRuO. This
mandates a detailed revisit of the normal state and, in particular, the
-dependent incoherence-coherence crossover. Using a modern first-principles
correlated view, we study this issue in the actual structure of
SrRuO and present a unified and quantitative description of a range
of unusual physical responses in the normal state. Armed with these, we propose
that a new and important element, that of dominant multi-orbital charge
fluctuations in a Hund's metal, may be a primary pair glue for unconventional
superconductivity. Thereby we establish a connection between the normal state
responses and superconductivity in this system.Comment: 8 pages, 4 figure
Evening out the spin and charge parity to increase T in unconventional superconductor Sr_{2}RuO_{4}
Unconventional superconductivity in SrRuO has been intensively
studied for decades. The origin and nature of the pairing continues to be
widely debated, in particular, the possibility of a triplet origin of Cooper
pairs. However, complexity of SrRuO with multiple low-energy
scales, involving subtle interplay among spin, charge and orbital degrees of
freedom, calls for advanced theoretical approaches which treat on equal footing
all electronic effects. Here we develop a novel approach, a detailed \emph{ab
initio} theory, coupling quasiparticle self-consistent \emph{GW} approximation
with dynamical mean field theory (DMFT), including both local and non-local
correlations. We report that the superconducting instability has multiple
triplet and singlet components. In the unstrained case the triplet eigenvalues
are larger than the singlets. Under uniaxial strain, the triplet eigenvalues
drop rapidly and the singlet components increase. This is concomitant with our
observation of spin and charge fluctuations shifting closer to wave-vectors
favoring singlet pairing in the Brillouin zone. We identify a complex mechanism
where charge fluctuations and spin fluctuations co-operate in the even-parity
channel under strain leading to increment in , thus proposing a novel
mechanism for pushing the frontier of in unconventional `triplet'
superconductors.Comment: 30 pages, 9 figure, 2 table
Metal-insulator transition in copper oxides induced by apex displacements
High temperature superconductivity has been found in many kinds of compounds
built from planes of Cu and O, separated by spacer layers. Understanding why
critical temperatures are so high has been the subject of numerous
investigations and extensive controversy. To realize high temperature
superconductivity, parent compounds are either hole-doped, such as
{LaCuO} (LCO) with Sr (LSCO), or electron doped, such as
{NdCuO} (NCO) with Ce (NCCO). In the electron doped cuprates, the
antiferromagnetic phase is much more robust than the superconducting phase.
However, it was recently found that the reduction of residual out-of-plane
apical oxygens dramatically affects the phase diagram, driving those compounds
to a superconducting phase. Here we use a recently developed first principles
method to explore how displacement of the apical oxygen (A-O) in LCO affects
the optical gap, spin and charge susceptibilities, and superconducting order
parameter. By combining quasiparticle self-consistent GW (QS\emph{GW}) and
dynamical mean field theory (DMFT), that LCO is a Mott insulator; but small
displacements of the apical oxygens drive the compound to a metallic state
through a localization/delocalization transition, with a concomitant maximum
-wave order parameter at the transition. We address the question whether NCO
can be seen as the limit of LCO with large apical displacements, and elucidate
the deep physical reasons why the behaviour of NCO is so different than the
hole doped materials. We shed new light on the recent correlation observed
between T and the charge transfer gap, while also providing a guide towards
the design of optimized high-Tc superconductors. Further our results suggest
that strong correlation, enough to induce Mott gap, may not be a prerequisite
for high-Tc superconductivity
Controlling T through band structure and correlation engineering in collapsed and uncollapsed phases of iron arsenides
Recent observations of selective emergence (suppression) of superconductivity
in the uncollapsed (collapsed) tetragonal phase of LaFeAs has rekindled
interest in understanding what features of the band structure control the
superconducting T. We show that the proximity of the narrow Fe-d
state to the Fermi energy emerges as the primary factor. In the uncollapsed
phase this state is at the Fermi energy, and is most strongly correlated and
source of enhanced scattering in both single and two particle channels. The
resulting intense and broad low energy spin fluctuations suppress magnetic
ordering and simultaneously provide glue for Cooper pair formation. In the
collapsed tetragonal phase, the d state is driven far below the Fermi
energy, which suppresses the low-energy scattering and blocks
superconductivity. A similar source of broad spin excitation appears in
uncollapsed and collapsed phases of CaFeAs. This suggests
controlling coherence provides a way to engineer T in unconventional
superconductors primarily mediated through spin fluctuations
Interplay between band structure and Hund's correlation to increase T in FeSe
FeSe is classed as a Hund's metal, with a multiplicity of bands near the
Fermi level. Correlations in Hund's metals mostly originate from the exchange
parameter \emph{J}, which can drive a strong orbital selectivity in the
correlations. The Fe-chalcogens are the most strongly correlated of the
Fe-based superconductors, with the most correlated orbital. Yet little
is understood whether and how such correlations directly affect the
superconducting instability in Hund's systems.
By applying a recently developed high-fidelity \emph{ab initio} theory, we
show explicitly the connections between correlations in and the
superconducting critical temperature . Starting from the \emph{ab
initio} results as a reference, we consider various kinds of excursions in
parameter space around the reference to determine what controls . We
show small excursions in can cause colossal changes in .
Additionally we consider changes in hopping by varying the Fe-Se bond length in
bulk, in the free standing monolayer M-FeSe, and M-FeSe on a SrTiO
substrate (M-FeSe/STO). The twin conditions of proximity of the state
to the Fermi energy, and the strength of emerge as the primary criteria for
incoherent spectral response and enhanced single- and two-particle scattering
that in turn controls . Using constrained RPA, we show further that FeSe
in monolayer form (M-FeSe) provides a natural mechanism to enhance . We
explain why M-FeSe/STO has a high , whereas M-FeSe in isolation should
not.
Our study opens a paradigm for a unified understanding what controls
in bulk, layers, and interfaces of Hund's metals by hole pocket and electron
screening cloud engineering
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