18 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
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
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
Electric field Assisted Nanostucturing of a Mott Insulator
Accepted for Advanced Functional Materials Published as : Adv Funct Mat Vol 19 p 2800 (2009)International audienceWe report the first experimental evidence for a strong electromechanical coupling in the Mott insulator GaTa4Se8 allowing a highly reproducible nano-writing with a Scanning Tunneling Microscope (STM). The local electric field across the STM junction is observed to have a threshold value above which the clean (100) surface of GaTa4Se8 becomes mechanically instable: At voltage biases V > 1.1V the surface suddenly inflates and comes in contact with the STM tip, resulting in nanometer size craters. The formed pattern can be indestructibly “read” by STM at lower voltage bias, thus allowing a 5 Tdots/inch2 dense writing/reading at room temperature. The discovery of the electromechanical coupling in GaTa4Se8 might give new clues in the understanding of the Electric Pulse Induced Resistive Switching recently observed in this stoechiometric Mott insulator
Signatures of multigap superconductivity in tunneling spectroscopy
International audienceWe considered a two-band superconductor with a nonzero interband quasiparticle coupling and numerically generated partial elementary excitation spectra for each band. These show deviations from the conventional Bardeen, Cooper, and Schrieffer form, resulting in characteristic signatures in the partial tunneling spectra. The total (measurable) tunneling spectra are calculated considering the k selection in the tunneling process. Due to the thermal smearing, the relevant spectral signatures may not be resolved in superconductor-insulator-normal-metal tunneling while they are clearly revealed in superconductor-insulator-superconductor (SIS) geometry. As an example, the excitation spectrum of 2H-NbSe2 is considered in the framework of the developed tunneling model. A remarkable agreement obtained with the experimental SIS data suggests the material to be a two-band superconductor rather than an anisotropic one
Two energy gaps in the tunneling-conductance spectra of the superconducting clathrate Ba8Si46
International audienceWe have studied the quasiparticle excitation spectrum of the superconductor Ba8Si46 by local tunneling spectroscopy. Using high-energy resolution achieved in superconductor-insulator-superconductor junctions we observed tunneling conductance spectra of a nonconventional shape revealing two distinct energy gaps, ΔL=1.3±0.1 meV and ΔS=0.9±0.2 meV. The analysis of tunneling data identified ΔL as the leading superconducting gap in the bulk material. A smaller and more dispersive gap ΔS is interpreted as induced either in reciprocal space, via the quasiparticle interband scattering from the leading superconducting band, or in real space, by the proximity effect to a normal layer at the surface