240 research outputs found
On the Remarkable Superconductivity of FeSe and Its Close Cousins
Emergent electronic phenomena in iron-based superconductors have been at the forefront of condensed matter physics for more than a decade. Much has been learned about the origins and intertwined roles of ordered phases, including nematicity, magnetism, and superconductivity, in this fascinating class of materials. In recent years, focus has been centered on the peculiar and highly unusual properties of FeSe and its close cousins. This family of materials has attracted considerable attention due to the discovery of unexpected superconducting gap structures, a wide range of superconducting critical temperatures, and evidence for nontrivial band topology, including associated spin-helical surface states and vortex-induced Majorana bound states. Here, we review superconductivity in iron chalcogenide superconductors, including bulk FeSe, doped bulk FeSe, FeTe1−xSex, intercalated FeSe materials, and monolayer FeSe and FeTe1−xSex on SrTiO3. We focus on the superconducting properties, including a survey of the relevant experimental studies, and a discussion of the different proposed theoretical pairing scenarios. In the last part of the paper, we review the growing recent evidence for nontrivial topological effects in FeSe-related materials, focusing again on interesting implications for superconductivity
The Kondo lattice model from strong-coupling viewpoint
We present some preliminary results on the phase diagram of the 2D S=1/2
Kondo lattice model at finite doping. As a starting point the Hamiltonian is
written in terms of local spin and charge excitations, and the interactions
between these modes are subsequently treated in various perturbative schemes.
We find that a paramagnetic-magnetic quantum phase transition does occur, and,
at least on a superficial level, the Kondo effect does not break down at the
critical point. The latter result however might well be a consequence of the
inherent bias of our starting point and/or the level of approximation.Comment: 4 pages, to appear in the Proceedings of SCES 2001, Ann Arbor, August
200
Theory of quasiparticle vortex bound states in Fe-based superconductors: application to LiFeAs
Spectroscopy of vortex bound states can provide valuable information on the
structure of the superconducting order parameter. Quasiparticle wavefunctions
are expected to leak out in the directions of gap minima or nodes, if they
exist, and scanning tunneling spectroscopy (STS) on these low-energy states
should probe the momentum dependence of the gap. Anisotropy can also arise from
band structure effects, however. We perform a quasiclassical calculation of the
density of states of a single vortex in an anisotropic superconductor, and show
that if the gap itself is not highly anisotropic, the Fermi surface anisotropy
dominates, preventing direct observation of superconducting gap features. This
serves as a cautionary message for the analysis of STS data on the vortex state
on Fe-based superconductors, in particular LiFeAs, which we treat explicitly.Comment: 4 pages, 2 figure
Energy Gap Evolution Across the Superconductivity Dome in Single Crystals of (BaK)FeAs
The mechanism of unconventional superconductivity in iron-based
superconductors (IBSs) is one of the most intriguing questions in current
materials research. Among non-oxide IBSs, (BaK)FeAs has
been intensively studied because of its high superconducting transition
temperature and fascinating evolution of the superconducting gap structure from
being fully isotropic at optimal doping (0.4) to becoming nodal at
0.8. Although this marked evolution was identified in several independent
experiments, there are no details of the gap evolution to date because of the
lack of high-quality single crystals covering the entire K-doping range of the
superconducting dome. We conducted a systematic study of the London penetration
depth, , across the full phase diagram for different
concentrations of point-like defects introduced by 2.5 MeV electron
irradiation. Fitting the low-temperature variation with the power law, , we find that the exponent is the highest and
suppression rate with disorder is the smallest at optimal doping, and they
evolve with doping being away from optimal, which is consistent with increasing
gap anisotropy, including an abrupt change around , indicating the
onset of nodal behavior. Our analysis using a self-consistent -matrix
approach suggests the ubiquitous and robust nature of s pairing in IBSs
and argues against a previously suggested transition to a wave state near
in this system
Detection of Single Molecules Illuminated by a Light-Emitting Diode
Optical detection and spectroscopy of single molecules has become an
indispensable tool in biological imaging and sensing. Its success is based on
fluorescence of organic dye molecules under carefully engineered laser
illumination. In this paper we demonstrate optical detection of single
molecules on a wide-field microscope with an illumination based on a
commercially available, green light-emitting diode. The results are directly
compared with laser illumination in the same experimental configuration. The
setup and the limiting factors, such as light transfer to the sample, spectral
filtering and the resulting signal-to-noise ratio are discussed. A theoretical
and an experimental approach to estimate these parameters are presented. The
results can be adapted to other single emitter and illumination schemes.Comment: 7 pages, 5 figure
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