20 research outputs found
The key ingredients of the electronic structure of FeSe
FeSe is a fascinating superconducting material at the frontier of research in
condensed matter physics. Here we provide an overview on the current
understanding of the electronic structure of FeSe, focusing in particular on
its low energy electronic structure as determined from angular resolved
photoemission spectroscopy, quantum oscillations and magnetotransport
measurements of single crystal samples. We discuss the unique place of FeSe
amongst iron-based superconductors, being a multi-band system exhibiting strong
orbitally-dependent electronic correlations and unusually small Fermi surfaces,
prone to different electronic instabilities. We pay particular attention to the
evolution of the electronic structure which accompanies the
tetragonal-orthorhombic structural distortion of the lattice around 90 K, which
stabilizes a unique nematic electronic state. Finally, we discuss how the
multi-band multi-orbital nematic electronic structure has an impact on the
understanding of the superconductivity, and show that the tunability of the
nematic state with chemical and physical pressure will help to disentangle the
role of different competing interactions relevant for enhancing
superconductivity.Comment: 21 pages, 11 figures, to appear in Annual Review of Condensed Matter
Physic
Suppression of superconductivity and enhanced critical field anisotropy in thin flakes of FeSe
FeSe is a unique superconductor that can be manipulated to enhance its superconductivity using different routes, while ist monolayer form grown on different substrates reaches a record high temperature for a two-dimensional system. In order to understand the role played by the substrate and the reduced dimensionality on superconductivity, we examine the superconducting properties of exfoliated FeSe thin flakes by reducing the thickness from bulk down towards 9 nm. Magnetotransport measurements performed in magnetic fields up to 16 T and temperatures down to 2 K help to build up complete superconducting phase diagrams of different thickness flakes. While the thick flakes resemble the bulk behaviour, by reducing the thickness the superconductivity of FeSe flakes is suppressed. The observation of the vortex-antivortex unbinding transition in different flakes provide a direct signature of a dominant two-dimensional pairing channel. However, the upper critical field reflects the evolution of the multi-band nature of superconductivity in FeSe becoming highly two-dimensional and strongly anisotropic only in the thin limit. Our study provides detailed insights into the evolution of the superconducting properties of a multi-band superconductor FeSe in the thin limit in the absence of a dopant substrate
Topological change of the Fermi surface in ternary iron-pnictides with reduced c/a ratio: A dHvA study of CaFe2P2
We report a de Haas-van Alphen effect study of the Fermi surface of CaFe2P2
using low temperature torque magnetometry up to 45 T. This system is a close
structural analogue of the collapsed tetragonal non-magnetic phase of CaFe2As2.
We find the Fermi surface of CaFe2P2 to differ from other related ternary
phosphides in that its topology is highly dispersive in the c-axis, being
three-dimensional in character and with identical mass enhancement on both
electron and hole pockets (~1.5). The dramatic change in topology of the Fermi
surface suggests that in a state with reduced (c/a) ratio, when bonding between
pnictogen layers becomes important, the Fermi surface sheets are unlikely to be
nested
Significant change in the electronic behavior associated with structural distortions in the single crystalline SrAg4As2
We report a combined study of transport and thermodynamic measurements on the
layered pnictide material SrAg4As2. Upon cooling, a drop in electrical and Hall
resistivity, a jump in heat capacity and an increase in susceptibility and
magnetoresistance (MR) are observed around 110 K. These observations suggest
that non-magnetic phase transitions emerge at around 110 K, that are likely
associated with structural distortions. In sharp contrast with the
first-principles calculations based on the crystal structure at room
temperature, quantum oscillations reveal small Fermi pockets with light
effective masses, suggesting a significant change in the Fermi surface topology
caused by the low temperature structural distortion. No superconductivity
emerges in SrAgAs down to 2 K and under pressures up to 2.13 GPa;
instead, the low temperature structural distortion increases linearly with
temperature at a rate of ~13 K/GPa above 0.89 GPa
Strain-tuning of nematicity and superconductivity in single crystals of FeSe
Strain is a powerful experimental tool to explore new electronic states and
understand unconventional superconductivity. Here, we investigate the effect of
uniaxial strain on the nematic and superconducting phase of single crystal FeSe
using magnetotransport measurements. We find that the resistivity response to
the strain is strongly temperature dependent and it correlates with the sign
change in the Hall coefficient being driven by scattering, coupling with the
lattice and multiband phenomena. Band structure calculations suggest that under
strain the electron pockets develop a large in-plane anisotropy as compared
with the hole pocket. Magnetotransport studies at low temperatures indicate
that the mobility of the dominant carriers increases with tensile strain. Close
to the critical temperature, all resistivity curves at constant strain cross in
a single point, indicating a universal critical exponent linked to a
strain-induced phase transition. Our results indicate that the superconducting
state is enhanced under compressive strain and suppressed under tensile strain,
in agreement with the trends observed in FeSe thin films and overdoped
pnictides, whereas the nematic phase seems to be affected in the opposite way
by the uniaxial strain. By comparing the enhanced superconductivity under
strain of different systems, our results suggest that strain on its own cannot
account for the enhanced high superconductivity of FeSe systems.Comment: 11 pages, 8 figure
Resurgence of superconductivity and the role of dxy hole band in FeSeTe
Iron-chalcogenide superconductors display rich phenomena caused by orbital-dependent band shifts and electronic correlations. Additionally, they are potential candidates for topological superconductivity due to the band inversion between the Fe d bands and the chalcogen p band. Here we present a detailed study of the electronic structure of the nematic superconductors FeSeTe (0 < x < 0.4) using angle-resolved photoemission spectroscopy to understand the role of orbital-dependent band shifts, electronic correlations and the chalcogen band. We assess the changes in the effective masses using a three-band low energy model, and the band renormalization via comparison with DFT band structure calculations. The effective masses decrease for all three-hole bands inside the nematic phase, followed by a strong increase for the band with d orbital character. Interestingly, this nearly-flat d band becomes more correlated as it shifts towards the Fermi level with increasing Te concentrations and as the second superconducting dome emerges. Our findings suggests that the d hole band, which is very sensitive to the chalcogen height, could be involved in promoting an additional pairing channel and increasing the density of states to stabilize the second superconducting dome in FeSeTe. This simultaneous shift of the d hole band and enhanced superconductivity is in contrast with FeSeS
Unconventional localization of electrons inside of a nematic electronic phase
The magnetotransport behaviour inside the nematic phase of bulk FeSe reveals
unusual multiband effects that cannot be reconciled with a simple two-band
approximation proposed by surface-sensitive spectroscopic probes. In order to
understand the role played by the multiband electronic structure and the degree
of two-dimensionality we have investigated the electronic properties of
exfoliated flakes of FeSe by reducing their thickness. Based on
magnetotransport and Hall resistivity measurements, we assess the mobility
spectrum that suggests an unusual asymmetry between the mobilities of the
electrons and holes with the electron carriers becoming localized inside the
nematic phase. Quantum oscillations in magnetic fields up to 38 T indicate the
presence of a hole-like quasiparticle with a lighter effective mass and a
quantum scattering time three times shorter, as compared with bulk FeSe. The
observed localization of negative charge carriers by reducing dimensionality
can be driven by orbitally-dependent correlation effects, enhanced interband
spin-fluctuations or a Lifshitz-like transition which affect mainly the
electron bands. The electronic localization leads to a fragile two-dimensional
superconductivity in thin flakes of FeSe, in contrast to the two-dimensional
high-Tc induced with electron doping via dosing or using a suitable interface.Comment: 22 pages, 14 figure
Competing pairing interactions responsible for the large upper critical field in a stoichiometric iron-based superconductor, CaKFeAs
The upper critical field of multiband superconductors is an important
quantity that can reveal the details about the nature of the superconducting
pairing. Here we experimentally map out the complete upper critical field phase
diagram of a stoichiometric superconductor, CaKFeAs, up to 90T for
different orientations of the magnetic field and at temperatures down to 4.2K.
The upper critical fields are extremely large, reaching values close to ~3
at the lowest temperature, and the anisotropy decreases dramatically with
temperature leading to essentially isotropic superconductivity at 4.2K. We find
that the temperature dependence of the upper critical field can be well
described by a two-band model in the clean limit with band coupling parameters
favouring intraband over interband interactions. The large Pauli paramagnetic
effects together with the presence of the shallow bands is consistent with the
stabilization of an FFLO state at low temperatures in this clean
superconductor.Comment: to appear in Physical Review B (2020); 13 pages, 9 figure