14 research outputs found
Robust procedure for creating and characterizing the atomic structure of scanning tunneling microscope tips
Scanning tunneling microscopes (STM) are used extensively for studying and
manipulating matter at the atomic scale. In spite of the critical role of the
STM tip, the control of the atomic-scale shape of STM tips remains a poorly
solved problem. Here, we present a method for preparing tips {\it in-situ} and
for ensuring the crystalline structure and reproducibly preparing tip structure
up to the second atomic layer. We demonstrate a controlled evolution of such
tips starting from undefined tip shapes.Comment: 12 pages preprint-style; 5 figure
Nanofabricated tips for device-based scanning tunneling microscopy
We report on the fabrication and performance of a new kind of tip for
scanning tunneling microscopy. By fully incorporating a metallic tip on a
silicon chip using modern micromachining and nanofabrication techniques, we
realize so-called smart tips and show the possibility of device-based STM tips.
Contrary to conventional etched metal wire tips, these can be integrated into
lithographically defined electrical circuits. We describe a new fabrication
method to create a defined apex on a silicon chip and experimentally
demonstrate the high performance of the smart tips, both in stability and
resolution. In situ tip preparation methods are possible and we verify that
they can resolve the herringbone reconstruction and Friedel oscillations on
Au(111) surfaces. We further present an overview of possible applications
Poor electronic screening in lightly doped Mott insulators observed with scanning tunneling microscopy
The effective Mott gap measured by scanning tunneling microscopy (STM) in the
lightly doped Mott insulator differs
greatly from values reported by photoemission and optical experiments. Here, we
show that this is a consequence of the poor electronic screening of the
tip-induced electric field in this material. Such effects are well known from
STM experiments on semiconductors, and go under the name of tip-induced band
bending (TIBB). We show that this phenomenon also exists in the lightly doped
Mott insulator and that, at doping
concentrations of , it causes the measured energy gap in the sample
density of states to be bigger than the one measured with other techniques. We
develop a model able to retrieve the intrinsic energy gap leading to a value
which is in rough agreement with other experiments, bridging the apparent
contradiction. At doping we further observe circular features
in the conductance layers that point to the emergence of a significant density
of free carriers in this doping range, and to the presence of a small
concentration of donor atoms. We illustrate the importance of considering the
presence of TIBB when doing STM experiments on correlated-electron systems and
discuss the similarities and differences between STM measurements on
semiconductors and lightly doped Mott insulators.Comment: 9 pages, 5 figure
Single-electron charge transfer into putative Majorana and trivial modes in individual vortices
Majorana bound states are putative collective excitations in solids that
exhibit the self-conjugate property of Majorana fermions - they are their own
antiparticles. In iron-based superconductors, zero-energy states in vortices
have been reported as potential Majorana bound states, but the evidence remains
controversial. Here, we use scanning tunneling noise spectroscopy to study the
tunneling process into vortex bound states in the conventional superconductor
NbSe2, and in the putative Majorana platform FeTe0.55Se0.45. We find that
tunneling into vortex bound states in both cases exhibits charge transfer of a
single electron charge. Our data for the zero-energy bound states in
FeTe0.55Se0.45 exclude the possibility of Yu-Shiba-Rusinov states and are
consistent with both Majorana bound states and trivial vortex bound states. Our
results open an avenue for investigating the exotic states in vortex cores and
for future Majorana devices, although further theoretical investigations
involving charge dynamics and superconducting tips are necessary.Comment: 15 pages, 4 figures, and 16 pages for supplementary informatio
Why shot noise does not generally detect pairing in mesoscopic superconducting tunnel junctions
The shot noise in tunneling experiments reflects the Poissonian nature of the
tunneling process. The shot noise power is proportional to both the magnitude
of the current and the effective charge of the carrier. Shot-noise spectroscopy
thus enables - in principle - to determine the effective charge q of the charge
carriers that tunnel. This can be used to detect electron pairing in
superconductors: in the normal state, the noise corresponds to single electron
tunneling (q = 1e), while in the paired state, the noise corresponds to q = 2e,
because of Andreev reflections. Here, we use a newly developed amplifier to
reveal that in typical mesoscopic superconducting junctions, the shot noise
does not reflect the signatures of pairing and instead stays at a level
corresponding to q = 1e. We show that transparency can control the shot noise
and this q = 1e is due to the large number of tunneling channels with each
having very low transparency. At such transparencies, the shot noise in the
junction resembles that of a metallic instead of a superconducting tunnel
junction. Our results indicate that in typical mesoscopic superconducting
junctions one should expect q = 1e noise, and lead to design guidelines for
junctions that allow the detection of electron pairing
Puddle formation, persistent gaps, and non-mean-field breakdown of superconductivity in overdoped (Pb,Bi)2Sr2CuO6+{\delta}
The cuprate high-temperature superconductors exhibit many unexplained
electronic phases, but it was often thought that the superconductivity at
sufficiently high doping is governed by conventional mean-field
Bardeen-Cooper-Schrieffer (BCS) theory[1]. However, recent measurements show
that the number of paired electrons (the superfluid density) vanishes when the
transition temperature Tc goes to zero[2], in contradiction to expectation from
BCS theory. The origin of this anomalous vanishing is unknown. Our scanning
tunneling spectroscopy measurements in the overdoped regime of the
(Pb,Bi)2Sr2CuO6+{\delta} high-temperature superconductor show that it is due to
the emergence of puddled superconductivity, featuring nanoscale superconducting
islands in a metallic matrix[3,4]. Our measurements further reveal that this
puddling is driven by gap filling, while the gap itself persists beyond the
breakdown of superconductivity. The important implication is that it is not a
diminishing pairing interaction that causes the breakdown of superconductivity.
Unexpectedly, the measured gap-to-filling correlation also reveals that
pair-breaking by disorder does not play a dominant role and that the mechanism
of superconductivity in overdoped cuprate superconductors is qualitatively
different from conventional mean-field theory
Spatially dispersing Yu-Shiba-Rusinov states in the unconventional superconductor FeTe<sub>0.55</sub>Se<sub>0.45</sub>
By using scanning tunneling microscopy (STM) we find and characterize dispersive, energy-symmetric in-gap states in the iron-based superconductor FeTe0.55Se0.45, a material that exhibits signatures of topological superconductivity, and Majorana bound states at vortex cores or at impurity locations. We use a superconducting STM tip for enhanced energy resolution, which enables us to show that impurity states can be tuned through the Fermi level with varying tip-sample distance. We find that the impurity state is of the Yu-Shiba-Rusinov (YSR) type, and argue that the energy shift is caused by the low superfluid density in FeTe0.55Se0.45, which allows the electric field of the tip to slightly penetrate the sample. We model the newly introduced tip-gating scenario within the single-impurity Anderson model and find good agreement to the experimental data.</p