317 research outputs found
Double Fe-impurity charge state in the topological insulator BiSe
The influence of individual impurities of Fe on the electronic properties of
topological insulator BiSe is studied by Scanning Tunneling Microscopy.
The microscope tip is used in order to remotely charge/discharge Fe impurities.
The charging process is shown to depend on the impurity location in the
crystallographic unit cell, on the presence of other Fe impurities in the close
vicinity, as well as on the overall doping level of the crystal. We present a
qualitative explanation of the observed phenomena in terms of tip-induced local
band bending. Our observations evidence that the specific impurity neighborhood
and the position of the Fermi energy with respect to the Dirac point and bulk
bands have both to be taken into account when considering the electron
scattering on the disorder in topological insulators.Comment: 10 pages, accepted for publication in Applied Physics Letters, minor
bugs were correcte
Fluctuation Dominated Josephson Tunneling with a Scanning Tunneling Microscope
We demonstrate Josephson tunneling in vacuum tunnel junctions formed between
a superconducting scanning tunneling microscope tip and a Pb film, for junction
resistances in the range 50-300 k. We show that the superconducting
phase dynamics is dominated by thermal fluctuations, and that the Josephson
current appears as a peak centered at small finite voltages. In the presence of
microwave fields (f=15.0 GHz) the peak decreases in magnitude and shifts to
higher voltages with increasing rf power, in agreement with theory.Comment: 4 pages, REVTeX, submitted to PR
Long range coherent magnetic bound states in superconductors
The quantum coherent coupling of completely different degrees of freedom is a
challenging path towards creating new functionalities for quantum electronics.
Usually the antagonistic coupling between spins of magnetic impurities and
superconductivity leads to the destruction of the superconducting order. Here
we show that a localized classical spin of an iron atom immersed in a
superconducting condensate can give rise to new kind of long range coherent
magnetic quantum state. In addition to the well-known Shiba bound state present
on top of an impurity we reveal the existence of a star shaped pattern which
extends as far as 12 nm from the impurity location. This large spatial
dispersion turns out to be related, in a non-trivial way, to the
superconducting coherence length. Inside star branches we observed short scale
interference fringes with a particle-hole asymmetry. Our theoretical approach
captures these features and relates them to the electronic band structure and
the Fermi wave length of the superconductor. The discovery of a directional
long range effect implies that distant magnetic atoms could coherently interact
leading to new topological superconducting phases with fascinating properties
Spectroscopic evidence for strong correlations between local superconducting gap and local Altshuler-Aronov density-of-states suppression in ultrathin NbN films
Disorder has different profound effects on superconducting thin films. For a
large variety of materials, increasing disorder reduces electronic screening
which enhances electron-electron repulsion. These fermionic effects lead to a
mechanism described by Finkelstein: when disorder combined to electron-electron
interactions increases, there is a global decrease of the superconducting
energy gap and of the critical temperature , the ratio
/ remaining roughly constant. In addition, in most films an
emergent granularity develops with increasing disorder and results in the
formation of inhomogeneous superconducting puddles. These gap inhomogeneities
are usually accompanied by the development of bosonic features: a pseudogap
develops above the critical temperature and the energy gap
starts decoupling from . Thus the mechanism(s) driving the appearance of
these gap inhomogeneities could result from a complicated interplay between
fermionic and bosonic effects. By studying the local electronic properties of a
NbN film with scanning tunneling spectroscopy (STS) we show that the
inhomogeneous spatial distribution of is locally strongly correlated
to a large depletion in the local density of states (LDOS) around the Fermi
level, associated to the Altshuler-Aronov effect induced by strong electronic
interactions. By modelling quantitatively the measured LDOS suppression, we
show that the latter can be interpreted as local variations of the film
resistivity. This local change in resistivity leads to a local variation of
through a local Finkelstein mechanism. Our analysis furnishes a purely
fermionic scenario explaining quantitatively the emergent superconducting
inhomogeneities, while the precise origin of the latter remained unclear up to
now.Comment: 11 pages, 4 figure
High tunnel magnetoresistance in spin-polarized scanning tunneling microscopy of Co nanoparticles on Pt(111)
We employ variable-temperature spin-polarized scanning tunneling microscopy in constant current mode to read the magnetic state of monodomain cobalt nanoparticles on Pt(111). In order to avoid stray fields we use in situ prepared antiferromagnetically (Cr) coated W tips. The contrast in apparent height between nanoparticles with opposite magnetization is typically Delta z=0.20 +/- 0.05 A, but can reach up to 1.1 A, indicating 80% spin-polarization of the nanoparticles and 850% magnetoresistance of the tip-sample tunnel junction with tip and sample at 300 K and 160 K, respectively. There is no zero-bias anomaly. These results suggest state-selective tunneling which is expected to lead to very high magnetoresistance values. (C) 2005 American Institute of Physics
Power spectrum of many impurities in a d-wave superconductor
Recently the structure of the measured local density of states power spectrum
of a small area of the \BSCCO (BSCCO) surface has been interpreted in terms of
peaks at an "octet" of scattering wave vectors determined assuming weak,
noninterfering scattering centers. Using analytical arguments and numerical
solutions of the Bogoliubov-de Gennes equations, we discuss how the
interference between many impurities in a d-wave superconductor alters this
scenario. We propose that the peaks observed in the power spectrum are not the
features identified in the simpler analyses, but rather "background" structures
which disperse along with the octet vectors. We further consider how our
results constrain the form of the actual disorder potential found in this
material.Comment: 5 pages.2 figure
Interlayer tunneling spectroscopy of BiSrCaCuO: a look from inside on the doping phase diagram of high superconductors
A systematic, doping dependent interlayer tunneling spectroscopy of Bi2212
high superconductor is presented. An improved resolution made it possible
to simultaneously trace the superconducting gap (SG) and the normal state
pseudo-gap (PG) in a close vicinity of and to analyze closing of the PG
at . The obtained doping phase diagram exhibits a critical doping point
for appearance of the PG and a characteristic crossing of the SG and the PG
close to the optimal doping. This points towards coexistence of two different
and competing order parameters in Bi2212. Experimental data indicate that the
SG can form a combined (large) gap with the PG at and that the
interlayer tunneling becomes progressively incoherent with decreasing doping.Comment: 5 pages, 5 figure
Electronic Structure of the Cuprate Superconducting and Pseudogap Phases from Spectroscopic Imaging STM
We survey the use of spectroscopic imaging STM to probe the electronic
structure of underdoped cuprates. Two distinct classes of electronic states are
observed in both the d-wave superconducting (dSC) and the pseudogap (PG)
phases. The first class consists of the dispersive Bogoliubov quasiparticle
excitations of a homogeneous d-wave superconductor, existing below a lower
energy scale E=Delta0. We find that the Bogoliubov quasiparticle interference
signatures of delocalized Cooper pairing are restricted to a k-space arc which
terminates near the lines connecting k=\pm(pi/a0,0) to k=\pm(pi/a0). This arc
shrinks continuously with decreasing hole density such that Luttinger's theorem
could be satisfied if it represents the front side of a hole-pocket which is
bounded behind by the lines between k=\pm(pi/a0,0) and k=\pm(0,pi/a0). In both
phases the only broken symmetries detected for the |E|<Delta0 states are those
of a d-wave superconductor. The second class of states occurs proximate to the
pseudogap energy scale E=Delta1. Here the non-dispersive electronic structure
breaks the expected 90o-rotational symmetry of electronic structure within each
unit cell, at least down to 180o-rotational symmetry. This Q=0 electronic
symmetry breaking was first detected as an electronic inequivalence at the two
oxygen sites within each unit cell by using a measure of nematic (C2) symmetry.
Incommensurate non-dispersive conductance modulations, locally breaking both
rotational and translational symmetries, coexist with this intra-unit-cell
electronic symmetry breaking at E=Delta1. Their characteristic wavevector Q is
determined by the k-space points where Bogoliubov quasiparticle interference
terminates and therefore changes continuously with doping. The distinct broken
electronic symmetry states (Q=0 and finite Q) coexisting at E~Delta1 are found
to be indistinguishable in the dSC and PG phases.Comment: 32 pages with 10 figure
Role of hydrogen in giant spin polarization observed on magnetic nanostructures
We demonstrate that the giant spin contrast observed by scanning tunneling microscopy for double-layer Coislands on Pt(111) is caused by adsorbates at the apex of the Cr-coated W tip. The most likely candidate, in ab initio simulations, is hydrogen. Here, the electron charge is highly polarized by the adjacent Cr layers. The hydrogen adsorption site is shown to change from hollow to on top due to the electric field at the tip apex, created by the tunnel voltage
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