38 research outputs found
Emergence of an incipient ordering mode in FeSe
The structurally simplest Fe-based superconductor FeSe with a critical
temperature 8.5 K displays a breaking of the four-fold
rotational symmetry at a temperature K. We investigated the
electronic properties of FeSe using scanning tunneling microscopy/spectroscopy
(STM/S), magnetization, and electrical transport measurements. The results
indicated two new energy scales (i) 75 K denoted by an onset of
electron-hole asymmetry in STS, enhanced spin fluctuations, and increased
positive magnetoresistance; (ii) 22 - 30 K, marked by opening
up of a partial gap of about 8 meV in STS and a recovery of Kohler's rule. Our
results reveal onset of an incipient ordering mode at and its
nucleation below . The ordering mode observed here, both in spin as
well as charge channels, suggests a coupling between the spins with charge,
orbital or pocket degrees of freedom.Comment: 5 pages, 4 figure
Impurity-induced bound states inside the superconducting gap of FeSe
We investigate the local density of states in the vicinity of a native
dumbbell defect arising from an Fe vacancy in FeSe single crystals. The
tunneling spectra close to the impurity display two bound states inside the
superconducting gap, equally spaced with respect to zero energy but asymmetric
in amplitude. Using spin-polarized density functional theory (DFT) calculations
on realistic slab models with Fe vacancy, we show that such a defect does not
induce a local magnetic moment. Therefore, the dumbbell defect is considered as
non-magnetic. Thus, the in-gap bound states emerging from a non-magnetic
defect-induced pair-breaking suggest a sign changing pairing state in this
material.Comment: 8 pages, 6 figure
Superconducting gap structure of FeSe
The microscopic mechanism governing the zero-resistance flow of current in
some iron-based, high-temperature superconducting materials is not well
understood up to now. A central issue concerning the investigation of these
materials is their superconducting gap symmetry and structure. Here we present
a combined study of low-temperature specific heat and scanning tunnelling
microscopy measurements on single crystalline FeSe. The results reveal the
existence of at least two superconducting gaps which can be represented by a
phenomenological two-band model. The analysis of the specific heat suggests
significant anisotropy in the gap magnitude with deep gap minima. The tunneling
spectra display an overall "U"-shaped gap close to the Fermi level away as well
as on top of twin boundaries. These results are compatible with the anisotropic
nodeless models describing superconductivity in FeSe.Comment: 8 pages, 5 figure
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Mechanism of Skyrmion Attraction in Chiral Magnets near the Ordering Temperatures.
Isolated chiral skyrmions are investigated within the phenomenological Dzyaloshinskii model near the ordering temperatures of quasi-two-dimensional chiral magnets with Cnv symmetry and three-dimensional cubic helimagnets. In the former case, isolated skyrmions (IS) perfectly blend into the homogeneously magnetized state. The interaction between these particle-like states, being repulsive in a broad low-temperature (LT) range, is found to switch into attraction at high temperatures (HT). This leads to a remarkable confinement effect: near the ordering temperature, skyrmions exist only as bound states. This is a consequence of the coupling between the magnitude and the angular part of the order parameter, which becomes pronounced at HT. The nascent conical state in bulk cubic helimagnets, on the contrary, is shown to shape skyrmion internal structure and to substantiate the attraction between them. Although the attracting skyrmion interaction in this case is explained by the reduction of the total pair energy due to the overlap of skyrmion shells, which are circular domain boundaries with the positive energy density formed with respect to the surrounding host phase, additional magnetization "ripples" at the skyrmion outskirt may lead to attraction also at larger length scales. The present work provides fundamental insights into the mechanism for complex mesophase formation near the ordering temperatures and constitutes a first step to explain the phenomenon of multifarious precursor effects in that temperature region
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Quantum chemical insights into hexaboride electronic structures: correlations within the boron p-orbital subsystem
The notion of strong electronic correlations arose in the context of d-metal oxides such as NiO but can be exemplified on systems as simple as the H2 molecule. Here we shed light on correlation effects on B62− clusters as found in MB6 hexaborides and show that the B 2p valence electrons are fairly correlated. B6-octahedron excitation energies computed for CaB6 and YbB6 agree with peak positions found by resonant inelastic x-ray scattering, providing a compelling picture for the latter. Our findings characterize these materials as very peculiar p-electron correlated systems and call for more involved many-body investigations within the whole hexaboride family, both alkaline- and rare-earth compounds, not only for N- but also (N ± 1)-states defining e. g. band gaps
Crystal-field effects competing with spin-orbit interactions in NaCeO
Ce compounds feature a remarkable diversity of electronic properties, which
motivated extensive investigations over the last decades. Inelastic neutron
scattering represents an important tool for understanding their underlying
electronic structures but in certain cases a straightforward interpretation of
the measured spectra is hampered by the presence of strong vibronic couplings.
The latter may give rise to extra spectral features, which complicates the
mapping of experimental data onto standard multiplet diagrams. To benchmark the
performance of embedded-cluster quantum chemical computational schemes for the
case of systems, we here address the Ce 4 multiplet structure of
NaCeO, an antiferromagnet with magnetic-site symmetry for which
neutron scattering measurements indicate only weak vibronic effects. Very good
agreement with the experimental results is found in the computations, which
validates our computational approach and confirms NaCeO as a 4 magnet in
the intermediate coupling regime with equally strong 4-shell spin-orbit and
crystal-field interactions
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Resonating holes vs molecular spin-orbit coupled states in group-5 lacunar spinels
The valence electronic structure of magnetic centers is one of the factors that determines the characteristics of a magnet. This may refer to orbital degeneracy, as for jeff = 1/2 Kitaev magnets, or near-degeneracy, e.g., involving the third and fourth shells in cuprate superconductors. Here we explore the inner structure of magnetic moments in group-5 lacunar spinels, fascinating materials featuring multisite magnetic units in the form of tetrahedral tetramers. Our quantum chemical analysis reveals a very colorful landscape, much richer than the single-electron, single-configuration description applied so far to all group-5 GaM4X8 chalcogenides, and clarifies the basic multiorbital correlations on M4 tetrahedral clusters: while for V strong correlations yield a wave-function that can be well described in terms of four V4+V3+V3+V3+ resonant valence structures, for Nb and Ta a picture of dressed molecular-orbital jeff = 3/2 entities is more appropriate. These internal degrees of freedom likely shape vibronic couplings, phase transitions, and the magneto-electric properties in each of these systems
Luxuriant correlation landscape in lacunar spinels: multiconfiguration expansions in molecular-orbital basis vs resonant valence structures
The valence structure of magnetic centers is one of the factors that
determines the characteristics of a magnet. It may pertain to orbital
degeneracy, as for Kitaev magnets, or near-degeneracy, e.g.
-, in cuprate superconductors. Here we explore the inner structure of
magnetic moments in group-5 lacunar spinels, fascinating materials featuring
multisite magnetic units in the form of tetrahedral tetramers. Our analysis
reveals a very colorful landscape, much richer than the generic (...)
single-configuration description applied so far to all group-5 Ga
chalcogenides, and clarifies the basic multiorbital correlations on
units: while for V ions strong correlations yield a wave-function that can be
well described in terms of four VVVV resonant
valence structures, for Nb and Ta a picture of dressed molecular-orbital-like
entities is more appropriate. These internal degrees of
freedom likely shape vibronic couplings, phase transitions, and
magneto-electric properties in each of these systems.Comment: 6 pages, 3 figure
Nematic state of the FeSe superconductor
We study the crystal structure of the tetragonal iron selenide FeSe and its
nematic phase transition to the low-temperature orthorhombic structure using
synchrotron x-ray and neutron scattering analyzed in both real and reciprocal
space. We show that in the local structure the orthorhombic distortion
associated with the electronically driven nematic order is more pronounced at
short length scales. It also survives up to temperatures above 90 K where
reciprocal-space analysis suggests tetragonal symmetry. Additionally, the
real-space pair distribution function analysis of the synchrotron x-ray
diffraction data reveals a tiny broadening of the peaks corresponding to the
nearest Fe-Fe, nearest Fe-Se, and the next-nearest Fe-Se bond distances as well
as the tetrahedral torsion angles at a short length scale of 20 angstr\"om.
This broadening appears below 20 K and is attributed to a pseudogap. However,
we did not observe any further reduction in local symmetry below orthorhombic
down to 3 K. Our results suggest that the superconducting gap anisotropy in
FeSe is not associated with any symmetry-lowering short-range structural
correlations.Comment: 9 pages, 6 figure