56 research outputs found
Isolated pairs of Majorana zero modes in a disordered superconducting lead monolayer
Majorana zero modes are fractional quantum excitations appearing in pairs,
each pair being a building block for quantum computation . Some possible
signatures of these excitations have been reported as zero bias peaks at
endpoints of one-dimensional semiconducting wires and magnetic chains. However,
1D systems are by nature fragile to a small amount of disorder that induces
low-energy excitations, hence obtaining Majorana zero modes well isolated in a
hard gap requires extremely clean systems. Two-dimensional systems offer an
alternative route to get robust Majorana zero modes. Indeed, it was shown
recently that Pb/Co/Si(111) could be used as a platform for generating 2D
topological superconductivity with a strong immunity to local disorder. While
2D systems exhibit dispersive chiral edge states, they can also host Majorana
zero modes located on local topological defects. According to predictions, if
an odd number of zero modes are located in a topological domain an additional
zero mode should appear all around the domain's edge. Here we use scanning
tunneling spectroscopy to characterize a disordered superconducting monolayer
of Pb coupled to underlying Co-Si magnetic islands meant to induce a
topological transition. We show that pairs of zero modes are stabilized: one
zero mode positioned at a point in the middle of the magnetic domain and its
zero mode partner extended all around the domain. The zero mode pair is
remarkably robust, it is isolated within a hard superconducting energy gap and
it appears totally immune to the strong disorder present in the Pb monolayer.
Our theoretical scenario supports the protected Majorana nature of this zero
mode pair, highlighting the role of magnetic or spin-orbit coupling textures.
This robust pair of Majorana zero modes offers a new platform for theoretical
and experimental study of quantum computing
Dynamic metastable vortex states in interacting vortex lines
The electron transport in current-biased superconducting nano-bridges is
determined by the motion of the quantum vortex confined in the internal
disorder landscape. Here we consider a simple case of a single or two
neighbouring linear defects crossing a nano-bridge. The strong anharmonicity of
the vortex motion along the defect leads, upon RF-excitation, to fractional
Shapiro steps. In the case of two defects, the vortex motion becomes
correlated, characterized by metastable states that can be locked to a resonant
RF-drive. The lock-unlock process causes sudden voltage jumps and drops in the
voltage-current characteristics observed in experiments. We analyze the
parameters promoting these metastable dynamic states and discuss their
potential applications in quantum devices.Comment: 9 pages, 8 figure
vortex confinés dans des nanostructures de Pb/Si(111) étudiés par microscopie à effet tunnel
Dans les supraconducteurs de type II, le champ magnétique pénètre le matériau sous forme de vortex, tourbillon de courants supraconducteurs circulant autour d'un c¿ur normal. Dans les travaux de cette thèse, nous montrons comment le confinement d'un système à une échelle comparable à la longueur de cohérence ? nanométrique modifie sensiblement ses propriétés supraconductrices.Cette étude est menée dans des nanostructures de plomb déposé in-situ sur un substrat de silicium (111), puis mesurées par spectroscopie à effet tunnel, sous UHV, à 300 mK et sous champ magnétique.En confinement extrême (taille latérale D<<10 ?), le système créé des vortex Géants, objets quantiques prédits théoriquement depuis 45 ans. En confinement plus faible (D~10 ?), les vortex peuvent être piégés, s'organisent ensuite en réseau triangulaire d'Abrikosov, puis s'interpénètrent avec le champ magnétique croissant.Les îlots de Pb cristallins supraconducteurs sont reliés entre eux par une monocouche de mouillage de Pb, ici désordonnée et non supraconductrice. Au voisinage des îlots, cette couche acquiert des caractéristiques spectroscopiques spécifiques qui reflètent la supraconductivité induite par proximité et les interactions entre électrons de type Altshuler-Aronov. L'évolution spatiale des spectres tunnel sont simulés en combinant les équations d'Usadel et la théorie du blocage de Coulomb dynamique.En réduisant la distance entre les îlots, l'effet de proximité autour de chacun se recouvre, ce qui forme une jonction Josephson. Sont étudiés finement le nombre, la position, le spectre et la forme des c¿urs de vortex Josephson sous champ magnétique, à travers une grande panoplie de jonctions.In type II superconductors, the applied magnetic field penetrates the material in quanta of flux called vortices, vortices of superconducting currents circulating around a normal core. In the work of this thesis, we show how the confinement of a system to a scale comparable to the nanometric coherent length ? substantially modifies its superconducting properties.This study is carried out in nanostructures of lead deposited in-situ on a silicon (111) substrate, then studied by scanning tunneling spectroscopy, under UHV, at 300 mK, and under magnetic field. In extremely confinement (lateral size D<<10 ?), systems create Giant vortices, quantum objects predicted 45 years ago. In the weakly confinement (D~10 ?), vortices may be pinned, then are organized in the triangular Abrikosov lattice, finally interpenetrate in surface superconductivity with the increasing magnetic field. Crystalline superconducting Pb islands are here connected by a disordered non-superconducting wetting layer of Pb. In the vicinity of each superconducting island, the wetting layer acquires specific tunnelling characteristics which reflect the interplay between the proximity induced superconductivity and the inherent electron correlations of this ultimate diffusive two-dimensional metal. Spatial evolution of the tunnel spectra are simulated by combining the Usadel equations and the theory of dynamic Coulomb blockade. With reducing the distance between the islands, the proximity effect around each overlaps and forms a Josephson junction. Thanks to the tunneling spectroscopy, number, position, the spectrum and the form of Josephson vortex cores are studied in detail for a large variety of junctions.PARIS-JUSSIEU-Bib.électronique (751059901) / SudocSudocFranceF
Resistive switching induced by electronic avalanche breakdown in GaTaSeTe narrow gap Mott Insulators
Mott transitions induced by strong electric fields are receiving a growing
interest. Recent theoretical proposals have focused on the Zener dielectric
breakdown in Mott insulators, however experimental studies are still too scarce
to conclude about the mechanism. Here we report a study of the dielectric
breakdown in the narrow gap Mott insulators GaTaSeTe. We find
that the I-V characteristics and the magnitude of the threshold electric field
(E) do not correspond to a Zener breakdown, but rather to an avalanche
breakdown. E increases as a power law of the Mott Hubbard gap (E),
in surprising agreement with the universal law E E
reported for avalanche breakdown in semiconductors. However, the delay time for
the avalanche that we observe in Mott insulators is over three orders of
magnitude longer than in conventional semiconductors. Our results suggest that
the electric field induces local insulator-to-metal Mott transitions that
create conductive domains which grow to form filamentary paths across the
sample
Yu-Shiba-Rusinov bound states versus topological edge states in Pb/Si(111)
There is presently a tremendous activity around the field of topological
superconductivity and Majorana fermions. Among the many questions raised, it
has become increasingly important to establish the topological or
non-topological origin of features associated with Majorana fermions such as
zero-bias peaks. Here, we compare in-gap features associated either with
isolated magnetic impurities or with magnetic clusters strongly coupled to the
atomically thin superconductor Pb/Si(111). We study this system by means of
scanning tunneling microscopy and spectroscopy (STM/STS). We take advantage of
the fact that the Pb/Si(111) monolayer can exist either in a crystal-ordered
phase or in an incommensurate disordered phase to compare the observed
spectroscopic features in both phases. This allows us to demonstrate that the
strongly resolved in-gap states we found around the magnetic clusters in the
disordered phase of Pb have a clear topological origin.Comment: 11 pages, 5 figures. To be published in European Physical Journal
Special Topics.dedicated to the conference FQMT'1
Two-dimensional topological superconductivity in Pb/Co/Si(111)
Just like insulators can host topological Dirac states at their edges,
superconductors can also exhibit topological phases characterized by Majorana
edge states. Remarkable zero-energy states have been recently observed at the
two ends of proximity induced superconducting wires, and were interpreted as
localized Majorana end states in one-dimensional (1D) topological
superconductor. By contrast, propagating Majorana states should exist at the 1D
edges of two-dimensional (2D) topological superconductors. Here we report the
direct observation of dispersive in-gap states surrounding topological
superconducting domains made of a single atomic layer of Pb covering magnetic
islands of Co/Si(111). We interpret the observed continuous dispersion across
the superconducting gap in terms of a spatial topological transition
accompanied by a chiral edge mode and residual gaped helical edge states. Our
experimental approach enables the engineering and control of a large variety of
novel quantum phases. This opens new horizons in the field of quantum materials
and quantum electronics where the magnetization of the domains could be used as
a control parameter for the manipulation of topological states.Comment: 12 pages, 3 figure
Quantum confinement effects in Pb Nanocrystals grown on InAs
In the recent work of Ref.\cite{Vlaic2017-bs}, it has been shown that Pb
nanocrystals grown on the electron accumulation layer at the (110) surface of
InAs are in the regime of Coulomb blockade. This enabled the first scanning
tunneling spectroscopy study of the superconducting parity effect across the
Anderson limit. The nature of the tunnel barrier between the nanocrystals and
the substrate has been attributed to a quantum constriction of the electronic
wave-function at the interface due to the large Fermi wavelength of the
electron accumulation layer in InAs. In this manuscript, we detail and review
the arguments leading to this conclusion. Furthermore, we show that, thanks to
this highly clean tunnel barrier, this system is remarkably suited for the
study of discrete electronic levels induced by quantum confinement effects in
the Pb nanocrystals. We identified three distinct regimes of quantum
confinement. For the largest nanocrystals, quantum confinement effects appear
through the formation of quantum well states regularly organized in energy and
in space. For the smallest nanocrystals, only atomic-like electronic levels
separated by a large energy scale are observed. Finally, in the intermediate
size regime, discrete electronic levels associated to electronic wave-functions
with a random spatial structure are observed, as expected from Random Matrix
Theory.Comment: Main 12 pages, Supp: 6 page
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
Electric pulse induced electronic patchwork in the Mott insulator GaTa4Se8
Following a recent discovery of the Insulator-to-Metal Transition induced by electric field in GaTa4Se8, we performed a detailed Scanning Tunneling Microscopy/Spectroscopy study of both pristine (insulating) and transited (conducting) crystals of this narrow gap Mott insulator. The spectroscopic maps show that pristine samples are spatially homogeneous insulators while the transited samples reveal at nanometer scale a complex electronic pattern that consists of metallic and superinsulating patches immersed in the pristine insulating matrix. Surprisingly, both kinds of patches are accompanied by a strong local topographic inflation, thus evidencing for a strong electron-lattice coupling involved in this metal-insulator transition. Finally, using a strong electric field generated across the STM tunneling junction, we demonstrate the possibility to trig the metal-insulator transition locally even at room temperature
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