2,256 research outputs found
Theoretical study of impurity-induced magnetism in FeSe
Experimental evidence suggests that FeSe is close to a magnetic instability,
and recent scanning tunneling microscopy (STM) measurements on FeSe multilayer
films have revealed stripe order locally pinned near defect sites. Motivated by
these findings, we perform a theoretical study of locally induced magnetic
order near nonmagnetic impurities in a model relevant for FeSe. We find that
relatively weak repulsive impurities indeed are capable of generating
short-range magnetism, and explain the driving mechanism for the local order by
resonant eg-orbital states. In addition, we investigate the importance of
orbital-selective self-energy effects relevant for Hund's metals, and show how
the structure of the induced magnetization cloud gets modified by orbital
selectivity. Finally, we make concrete connection to STM measurements of
iron-based superconductors by symmetry arguments of the induced magnetic order,
and the basic properties of the Fe Wannier functions relevant for tunneling
spectroscopy.Comment: 10 pages, 4 figure
Study of the educational and service facilities in the Providence junior high schools in comparison with accepted standards
Thesis (M.A.)--Boston University, 1933. This item was digitized by the Internet Archive
Universality of scanning tunneling microscopy in cuprate superconductors
We consider the problem of local tunneling into cuprate superconductors,
combining model based calculations for the superconducting order parameter with
wavefunction information obtained from first principles electronic structure.
For some time it has been proposed that scanning tunneling microscopy (STM)
spectra do not reflect the properties of the superconducting layer in the
CuO plane directly beneath the STM tip, but rather a weighted sum of
spatially proximate states determined by the details of the tunneling process.
These "filter" ideas have been countered with the argument that similar
conductance patterns have been seen around impurities and charge ordered states
in systems with atomically quite different barrier layers. Here we use a
recently developed Wannier function based method to calculate topographies,
spectra, conductance maps and normalized conductance maps close to impurities.
We find that it is the local planar Cu Wannier function,
qualitatively similar for many systems, that controls the form of the tunneling
spectrum and the spatial patterns near perturbations. We explain how, despite
the fact that STM observables depend on the materials-specific details of the
tunneling process and setup parameters, there is an overall universality in the
qualitative features of conductance spectra. In particular, we discuss why STM
results on BiSrCaCuO and CaNaCuOCl are
essentially identical
Robustness of Quasiparticle Interference Test for Sign-changing Gaps in Multiband Superconductors
Recently, a test for a sign-changing gap function in a candidate multiband
unconventional superconductor involving quasiparticle interference data was
proposed. The test was based on the antisymmetric, Fourier transformed
conductance maps integrated over a range of momenta corresponding to
interband processes, which was argued to display a particular resonant form,
provided the gaps changed sign between the Fermi surface sheets connected by
. The calculation was performed for a single impurity, however, raising
the question of how robust this measure is as a test of sign-changing pairing
in a realistic system with many impurities. Here we reproduce the results of
the previous work within a model with two distinct Fermi surface sheets, and
show explicitly that the previous result, while exact for a single nonmagnetic
scatterer and also in the limit of a dense set of random impurities, can be
difficult to implement for a few dilute impurities. In this case, however,
appropriate isolation of a single impurity is sufficient to recover the
expected result, allowing a robust statement about the gap signs to be made.Comment: 9 pages, 12 figure
Collective motion and nonequilibrium cluster formation in colonies of gliding bacteria
We characterize cell motion in experiments and show that the transition to
collective motion in colonies of gliding bacterial cells confined to a
monolayer appears through the organization of cells into larger moving
clusters. Collective motion by non-equilibrium cluster formation is detected
for a critical cell packing fraction around 17%. This transition is
characterized by a scale-free power-law cluster size distribution, with an
exponent , and the appearance of giant number fluctuations. Our
findings are in quantitative agreement with simulations of self-propelled rods.
This suggests that the interplay of self-propulsion of bacteria and the
rod-shape of bacteria is sufficient to induce collective motion
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