20 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
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
Symmetry-forbidden intervalley scattering by atomic defects in monolayer transition-metal dichalcogenides
Intervalley scattering by atomic defects in monolayer transition metal
dichalcogenides (TDMs; MX2) presents a serious obstacle for applications
exploiting their unique valley-contrasting properties. Here, we show that the
symmetry of the atomic defects can give rise to an unconventional protection
mechanism against intervalley scattering in monolayer TMDs. The predicted
defect-dependent selection rules for intervalley scattering can be verified via
Fourier transform scanning tunneling spectroscopy (FT-STS), and provide a
unique identification of, e.g., atomic vacancy defects (M vs X). Our findings
put the absence of the intervalley FT-STS peak in recent experiments in a
different perspective.Comment: 7 pages, 4 figures + supplementary. Published versio
Impurity-induced antiferromagnetic order in Pauli-limited nodal superconductors: application to heavy fermion CeCoIn5
We investigate the properties of the coexistence phase of itinerant
antiferromagnetism and nodal -wave superconductivity (Q-phase) discovered in
heavy fermion CeCoIn5 under applied magnetic field. We solve the minimal model
that includes -wave superconductivity and underlying magnetic correlations
in real space to elucidate the structure of the -phase in the presence of an
externally applied magnetic field. We further focus on the role of magnetic
impurities, and show that they nucleate the Q-phase at lower magnetic fields.
Our most crucial finding is that, even at zero applied field, dilute magnetic
impurities cooperate via RKKY-like exchange interactions to generate a
long-range ordered coexistence state identical to the Q-phase. This result is
in agreement with recent neutron scattering measurements [S. Raymond et al., J.
Phys. Soc. Jpn. {\bf 83}, 013707 (2014)].Comment: 7 pages, 7 figure
Tunable valley Hall effect in gate-defined graphene superlattices
We theoretically investigate gate-defined graphene superlattices with broken
inversion symmetry as a platform for realizing tunable valley dependent
transport. Our analysis is motivated by recent experiments [C. Forsythe et al.,
Nat. Nanotechnol. 13, 566 (2018)] wherein gate-tunable superlattice potentials
have been induced on graphene by nanostructuring a dielectric in the
graphene/patterneddielectric/gate structure. We demonstrate how the electronic
tight-binding structure of the superlattice system resembles a gapped Dirac
model with associated valley dependent transport using an unfolding procedure.
In this manner we obtain the valley Hall conductivities from the Berry
curvature distribution in the superlattice Brillouin zone, and demonstrate the
tunability of this conductivity by the superlattice potential. Finally, we
calculate the valley Hall angle relating the transverse valley current and
longitudinal charge current and demonstrate the robustness of the valley
currents against irregularities in the patterned dielectric.Comment: 12 pages, 11 figure
The Microbial Olympics 2016
Following the success of the inaugural games, the Microbial Olympics return with a new series of events and microbial competitors. The games may have moved to a new hosting venue, but the dedication to training, fitness, competition (and yes, education and humour) lives on