655 research outputs found
Revisiting Quasiparticle Scattering Interference in High-Temperature Superconductors: The Problem of Narrow Peaks
We revisit the interpretation of quasiparticle scattering interference in
cuprate high- superconductors. This phenomenon has been very successful in
reconstructing the dispersions of d-wave Bogoliubov excitations, but the
successful identification and interpretation of QPI in scanning tunneling
spectroscopy (STS) experiments rely on theoretical results obtained for the
case of isolated impurities. We introduce a highly flexible technique to
simulate STS measurements by computing the local density of states using
real-space Green's functions defined on two-dimensional lattices with as many
as 100,000 sites. We focus on the following question: to what extent can the
experimental results be reproduced when various forms of distributed disorder
are present? We consider randomly distributed point-like impurities, smooth
"Coulombic" disorder, and disorder arising from random on-site energies and
superconducting gaps. We find an apparent paradox: the QPI peaks in the
Fourier-transformed local density of states appear to be sharper and better
defined in experiment than those seen in our simulations. We arrive at a no-go
result for smooth-potential disorder since this does not reproduce the QPI
peaks associated with large-momentum scattering. An ensemble of point-like
impurities gets closest to experiment, but this goes hand in hand with impurity
cores that are not seen in experiment. We also study the effects of possible
measurement artifacts (the "fork mechanism"), which turn out to be of
relatively minor consequence. It appears that there is an unknown mechanism at
work which renders the QPI peaks much sharper than they are based on present
theoretical understanding.Comment: 23 pages, 25 figures, published version, includes minor change
Creating better superconductors by periodic nanopatterning
The quest to create superconductors with higher transition temperatures is as
old as superconductivity itself. One strategy, popular after the realization
that (conventional) superconductivity is mediated by phonons, is to chemically
combine different elements within the crystalline unit cell to maximize the
electron-phonon coupling. This led to the discovery of NbTi and Nb3Sn, to name
just the most technologically relevant examples. Here, we propose a radically
different approach to transform a `pristine' material into a better (meta-)
superconductor by making use of modern fabrication techniques: designing and
engineering the electronic properties of thin films via periodic patterning on
the nanoscale. We present a model calculation to explore the key effects of
different supercells that could be fabricated using nanofabrication or
deliberate lattice mismatch, and demonstrate that specific pattern will enhance
the coupling and the transition temperature. We also discuss how numerical
methods could predict the correct design parameters to improve
superconductivity in materials including Al, NbTi, and MgB
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
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
Tunable self-assembly of one-dimensional nanostructures with orthogonal directions
High-temperature exposure of a Mo(110) surface to borazine (HBNH)3leads to the formation of two distinctly different self-assembling nanostructures. Depending on the substrate temperature during preparation, either well-aligned, ultra-thin boron nanowires or a single-layer stripe structure of hexagonal boron nitride forms. Both structures show one-dimensional (1D) characteristics, but in directions perpendicular to each other. It is also possible to grow the two phases in coexistence. The relative weights are controlled by the sample temperature during preparation
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
Wind Energy and the Turbulent Nature of the Atmospheric Boundary Layer
Wind turbines operate in the atmospheric boundary layer, where they are
exposed to the turbulent atmospheric flows. As the response time of wind
turbine is typically in the range of seconds, they are affected by the small
scale intermittent properties of the turbulent wind. Consequently, basic
features which are known for small-scale homogeneous isotropic turbulence, and
in particular the well-known intermittency problem, have an important impact on
the wind energy conversion process. We report on basic research results
concerning the small-scale intermittent properties of atmospheric flows and
their impact on the wind energy conversion process. The analysis of wind data
shows strongly intermittent statistics of wind fluctuations. To achieve
numerical modeling a data-driven superposition model is proposed. For the
experimental reproduction and adjustment of intermittent flows a so-called
active grid setup is presented. Its ability is shown to generate reproducible
properties of atmospheric flows on the smaller scales of the laboratory
conditions of a wind tunnel. As an application example the response dynamics of
different anemometer types are tested. To achieve a proper understanding of the
impact of intermittent turbulent inflow properties on wind turbines we present
methods of numerical and stochastic modeling, and compare the results to
measurement data. As a summarizing result we find that atmospheric turbulence
imposes its intermittent features on the complete wind energy conversion
process. Intermittent turbulence features are not only present in atmospheric
wind, but are also dominant in the loads on the turbine, i.e. rotor torque and
thrust, and in the electrical power output signal. We conclude that profound
knowledge of turbulent statistics and the application of suitable numerical as
well as experimental methods are necessary to grasp these unique features (...)Comment: Accepted by the Journal of Turbulence on May 17, 201
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